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Independent and Simultaneous Unilateral Occlusion of the Pulmonary Artery and Extra-Pulmonary Primary Bronchus in Broilers1
Independent and Simultaneous Unilateral Occlusion of the Pulmonary Artery and Extra-Pulmonary Primary Bronchus in Broilers1 ROBERT F. WIDEMAN JR.,*,2 YVONNE KOCHERA KIRBY,* CAYE D. TACKETT,* NATHAN E. MARSON,* CORINNA J. TRESSLER,* and RONALD W. MCNEW+ *Department of Poultry Science and +Agricultural Statistics Laboratory, University of Arkansas, Fayetteville, Arkansas 72701 snares triggered arterial blood hypoxemia, acidosis, and hypercapnia similar to or greater in magnitude than the responses obtained by tightening the pulmonary artery snare independently. Tightening either snare independently or both snares simultaneously caused pulmonary arterial pressure to increase (pulmonary hypertension), and permanent obstruction of one bronchus in a separate experiment caused an increase in the righttotal ventricular weight ratio, which is indicative of chronic pulmonary hypertension. The mean systemic arterial pressure decreased when the pulmonary artery snare was tightened independently or in combination with the bronchial snare, but not when the bronchial snare was tightened independently. The respiratory rate increased and the heart rate decreased when the pulmonary artery snare was tightened independently, but not when the bronchial snare was tightened independently or in combination with the pulmonary artery snare. These results demonstrate that the V / Q mismatch caused by forcing all the CO to perfuse one lung cannot be attenuated by simultaneously directing the entire respiratory minute volume toward the same lung.
(Key words: ascites, pulmonary hypertension, respiration, blood pressure, acidosis, ventilation-perfusion inequality) 1996 Poultry Science 75:1417-1427
INTRODUCTION The reduced oxygen content of systemic arterial blood (hypoxemia) during the onset of pulmonary hypertension syndrome (PHS, ascites) in broilers may be symptomatic of an imbalance between ventilation (V) of the lungs with air and perfusion (Q) of the lungs with blood (Peacock et al, 1989, 1990; Reeves et al, 1991; Wideman and Kirby, 1995b). A ventilation-perfusion
Received for publication April 1, 1996. Accepted for publication July 26, 1996. JPublished as A r k a n s a s Agricultural Experiment Station manuscript Number 96032 with the approval of the Experiment Station Director. 2 To whom correspondence should be addressed: Department of Poultry Science, O-402 Poultry Science Center, University of Arkansas, Fayetteville, AR 72701.
(V/Q) mismatch theoretically would trigger hypoxemia in susceptible broilers if a high cardiac output (CO) caused blood to flow too rapidly through the lungs to permit adequate oxygen uptake, or if an excessively high blood flow rate was necessary to convey a normal CO through a pulmonary vascular bed having an anatomically inadequate capacity (Powell et al., 1985; Wideman and Bottje, 1993; Wideman and Kirby, 1995a,b). Unilateral pulmonary artery obstruction has been used as an experimental model for effectively doubling the volume of blood flow through the unobstructed lung in birds (Burton et ah, 1968; Powell et ah, 1985). Evidence obtained using chronic and acute adaptations of this model supports the hypothesis that a proportionally high CO coupled with a low capacity, noncompliant pulmonary vasculature can lead to a V / Q mismatch, hypoxemia, and pulmonary hypertension in
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ABSTRACT Acutely tightening a snare around one pulmonary artery previously was shown to trigger a reversible ventilation-perfusion (V/Q) mismatch in broilers, as reflected by decreases in the partial pressure of oxygen in arterial blood (hypoxemia), accompanied by increases in the hydrogen ion concentration (acidosis) and partial pressure of carbon dioxide (hypercapnia). In the present study, snares were loosely implanted around the right pulmonary artery and the right extrapulmonary primary bronchus in anesthetized male broilers. These snares were tightened and released independently and then simultaneously to evaluate the possibility that directing the entire respiratory minute volume toward the left lung might attenuate the V / Q mismatch caused by forcing the entire cardiac output (CO) through the left lung. Fully reversible arterial blood hypoxemia, acidosis, and hypercapnia occurred when either snare was tightened independently. Presumably, tightening the bronchial snare restricted ventilation but not blood flow to the right lung, thereby permitting blood to perfuse poorly ventilated gas exchange surfaces. Simultaneously tightening both
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WIDEMAN JR. ET AL.
MATERIALS AND METHODS Experiment 1 Male by-product chicks of the Hubbard3 breeder pullet line were reared on fresh wood shavings litter in an environmental chamber (8 m 2 floor space). They were brooded at 32 and 30 C during Weeks 1 and 2, respectively, and thereafter the temperature was maintained at 24 C. They were fed corn-soybean meal-based broiler rations formulated to meet or exceed the minimum NRC (1984) standards for all ingredients. Feed and water were provided for ad libitum consumption. The surgical protocol was described previously (Wideman and Kirby, 1995b). When the birds were 6 to 7 wk of age, 10 of the largest clinically healthy individuals were anesthetized to a surgical plane by injecting allobarbital (5,5-diallyl-barbituric acid; 4 25 mg/mL, 50 m g / k g body mass) into the Pectoralis muscle. They were fastened in dorsal recumbency on a heated surgical board that was thermostatically regulated to maintain a surface temperature of 35 C. The end of the surgical board was elevated to a 20 C head-up angle. A midline incision was made through the skin of the thoracic inlet, and the connective tissues surrounding the right pulmonary artery were separated to allow a snare, consisting of a loop of braided 2-0 silk suture thread 5 passed through a 6-cm length of PE290 polyethylene tubing, 6 to be placed loosely around
3Hubbard Farms, Walpole, N H 03608. 4 Sigma Chemical Co., St. Louis, MO 63178-9916. 5 Arista Surgical Supply Co., Inc., New York, NY 10010-1898. 6 Intramedic Co., Parsippany, NJ 07054. 7 Miltex-Instruments Co., Lake Success, NY 11042. 8 Dow Corning Corp., Midland, MI 48686-0994. 9 World Precision Instruments, Sarasota, FL 34230. 10 Biopac Systems, Inc., Goleta, CA 93117. n Criticare Systems, Inc., Milwaukee, WI 53226. 12 Pilot studies demonstrated that more prolonged tightening of the bronchus snare caused severe arterial blood acidosis (pH < 7.15) and death (R. F. Wideman, personal observations).
the pulmonary artery. A similar snare was placed loosely around the right extra-pulmonary primary bronchus. The skin of the thoracic inlet was sealed with surgical wound clips. 7 The left cutaneous ulnar vein was cannulated with Silastic® Laboratory Tubing (0.012 internal diameter x 0.025 outside diameter) 8 filled with 0.8% sodium chloride containing 200 IU heparin/mL. 4 The distal end of the cannula tubing was attached to a BLPR blood pressure transducer 9 interfaced through a Transbridge™ preamplifier9 to a Biopac MP 100 data acquisition system using AcqKnowledge software. 10 The cannula tubing was advanced slowly through the right atrium and ventricle into a pulmonary artery using continuous monitoring of characteristic pulse pressures to identify the location of the tip of the cannula, as described by Owen et al. (1995). If the cannula was in the right pulmonary artery, as assessed by loss of pulse pressure when the snare was briefly tightened, then the cannula was withdrawn into the right ventricle and repositioned until it entered the left pulmonary artery. The left brachial artery and left anterior tibial vein were cannulated with PE50 polyethylene tubing 6 filled with heparinized saline. The arterial cannula was advanced to a position near the descending aorta and, except when used for arterial blood sampling, was attached to a blood pressure transducer for continuous monitoring of systemic arterial pressure. A solution of 25 g mannitol 4 /L of water was infused at a constant rate of 0.05 m L / m i n through the venous cannula to hydrate the birds throughout the remainder of the experiment. The probe of a pulse oximeterii was positioned on the right wing to illuminate the tissue between the radius and ulna for noninvasive measurements of percentage saturation of hemoglobin with oxygen (Peacock et al, 1990; Julian and Mirsalimi, 1992; Wideman and Kirby, 1995a,b). When surgical preparations were complete and a stabilization period of 20 to 30 min had elapsed, control data were collected for 15 min, and two arterial blood samples were withdrawn 5 and 10 min into the control period (Samples A and B). The pulmonary artery snare was tightened to fully occlude the right pulmonary artery for 10 min, and an arterial blood sample was withdrawn 5 min after tightening the snare (Sample C). The pulmonary artery snare was released, and arterial blood samples were collected 5 and 10 min into the 15-min recovery period (Samples D and E). The bronchial snare was gently tightened to fully occlude the right extra-pulmonary primary bronchus for 4 min, and an arterial blood sample was collected 2 min after tightening the bronchial snare (Sample F). 12 The bronchial snare was released, and arterial blood samples were collected 5 and 10 min into the 15-min recovery period (Samples G and H). Both the pulmonary artery and bronchial snares were tightened simultaneously for 4 min, and an arterial blood sample was collected 2 min after both snares were tightened (Sample I). Both snares were released, and arterial Samples "J" and "K" were collected 5 and 10 min into the subsequent 15-min recovery period.
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fast-growing broilers (Wideman and Kirby, 1995a,b, 1996). However, these physiological interpretations are confounded by the likelihood that the pulmonary air flow remained fairly evenly distributed between both lungs during unilateral pulmonary artery occlusion (Wideman and Kirby, 1995a). This potential ventilatory mismatch raises the possibility that effective hypoventilation contributes significantly to the hypoxemic response observed during acute unilateral pulmonary artery occlusion. In the present study, snares implanted around the right pulmonary artery and the right extrapulmonary primary bronchus in male broilers were tightened and released independently and then simultaneously to evaluate the possibility that forcing the entire respiratory minute volume toward the left lung may attenuate the V / Q mismatch caused by forcing the entire CO through the left lung.
OCCLUSION OF THE PULMONARY ARTERY AND PRIMARY BRONCHUS
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the expiratory wave excursions for three consecutive respiratory cycles preceding and three respiratory cycles following each blood sampling interval.
Experiment
2
Male by-product chicks were reared as described in Experiment 1. When the chicks reached 16 to 18 d of age, they were anesthetized to a surgical plane, the thoracic inlet was opened, and a silver clip was positioned to fully obstruct the left pulmonary artery (PA-Clamp Group; 17 chicks) or the left extra-pulmonary primary bronchus (Bronchus Clamp Group; 14 chicks). A detailed description of the surgical procedures has been provided previously (Wideman and Kirby, 1995b). Sham-operated chicks were anesthetized and the thoracic inlet was opened; however, no clamps were inserted on the pulmonary artery or bronchus (Sham Group; 12 chicks). Chicks in the Control Group (19 chicks) were not anesthetized and did not undergo surgical procedures. Following recovery from surgery, the chicks were reared together in the same environmental chamber and were fed the same diets until they reached 28 d of age (10 to 12 d postsurgery). Then they were killed with an overdose of anesthetic, and the heart was removed, dissected, and weighed for calculation of the RV:TV ratio.
Statistical
Analysis
Because pH is a logarithmic function, all blood p H values were converted to hydrogen ion concentrations ([H+])for statistical analysis. In Experiment 1, data were analyzed using the General Linear Models procedure of SAS® (SAS Institute, 1982) using repeated measures ANOVA for comparisons over time. Differences between two sample intervals were confirmed by a paired t test. Responses obtained when a snare was tightened were considered significant when they differed (P < 0.05) from both sample intervals immediately preceding tightening of the snare. In Experiment 2, means were analyzed using ANOVA followed by t tests for pairwise comparisons; a chi-square test was used to compare the proportions of ascites among the four treatment groups.
RESULTS The 10 male broilers used in Experiment 1 weighed 2,246 ± 92 g (mean + SEM), had a RV:TV ratio of 0.257 ± 0.009, and their lung weights averaged 7.20 ± 0.41 g (left lung) and 6.87 + 0.32 g (right lung). Hematocrits averaged 31 ± 1% during Sample Intervals A to F, decreased to 30 + 1 % during Sample Intervals G to I, and decreased to 29 + 1% during Sample Intervals J to K. As shown in Figure 1, the P02 of arterial blood averaged between 97 and 100 mm Hg during all periods when the pulmonary artery and bronchial snares were loose (Sample Intervals A and B, D and E, G and H, and J and K), and decreased by 20 mm Hg or more when the
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All arterial blood samples were 1 mL each. No saline or blood replacement was administered. All samples were withdrawn anaerobically with a syringe, as described previously (Wideman and Kirby, 1995a), and were immediately injected into a Radiometer ABL330 AcidBase Laboratory. 13 Appropriate function of the blood gas analyzer was assessed by periodically injecting Blood Gas Qualicheck® reference standards, as described previously (Wideman and Buss, 1985). The primary arterial blood values for p H and partial pressures of oxygen (P02) a n d carbon dioxide (Pco2) were generated by the ABL330 operating at a sample chamber temperature of 37 C, and were recalculated by the ABL330 for a temperature of 41C to match the normal body temperature of domestic fowl (Lutz et al, 1974; Fedde, 1986). A portion of the arterial blood was used for duplicate hematocrit (HCT) determinations using heparinized capillary tubes and a microhematocrit centrifuge. Birds were killed with an overdose of anesthetic at the end of the experiment; the heart was removed, dissected, and weighed for calculation of the righfctotal ventricular weight ratio (RV:TV ratio), which serves as a reliable index of pulmonary hypertension (Burton and Smith, 1967; Burton et ah, 1968); and the lungs were removed and weighed. The Biopac MP 100 data acquisition system was used to continuously collect data from three channels throughout each experiment, including systemic arterial pressure (millimeters of Hg), pulmonary arterial pressure (PAP, millimeters of Hg), and percentage saturation of hemoglobin with oxygen (%HbC>2). Heart rate (HR, beats per minute) was obtained by counting systolic peaks over time in the PAP recording. Respiratory rate (RR, breaths per minute) was obtained by counting the wave cycles associated with respiratory movement that comprise an integral part of the pulmonary and systemic arterial pressure recordings (Sturkie, 1986). Average values for these parameters were measured electronically during the 30-s time intervals immediately preceding and following withdrawal of each arterial blood sample, then the separate pre- and postsampling measurements were averaged to yield a single value representative of the data collected throughout the blood sampling interval. Values for these parameters also were averaged over an approximately 20- to 40-s interval coincident with the maximum ("Peak") increase in pulmonary artery pressure attained before collecting arterial blood Samples C, F, and I. The measurements thus obtained for systemic arterial pressure and PAP incorporated multiple respiratory wave cycles in the overall estimation of mean arterial pressure (MAP) and mean PAP. To evaluate possible influences of the respiratory excursions on blood pressure measurements, data from each bird were analyzed separately to obtain discrete values for systolic and diastolic systemic arterial pressure and PAP during the maximum of the inspiratory wave excursions and during the minimum of
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pulmonary artery and bronchial snares were tightened independently (Sample Intervals C and F) or simultaneously (Sample Interval I). The hypoxemic P02 values during Sample Intervals F and I did not differ from each other but were lower than those of Sample Interval G. Pulse oximetry measurements indicated that the percentage saturation of hemoglobin with oxygen decreased from average values of 85% when the pulmonary artery
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FIGURE 1. Partial pressure of oxygen (mean ± SEM, n = 10) in arterial blood at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). Blood samples were not obtained for blood gas analysis during the "Peak" intervals. Asterisks designate significant differences (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare(s).
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FIGURE 3. Partial pressure of carbon dioxide (mean ± SEM, n = 10) in arterial blood at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). Blood samples were not obtained for blood gas analysis during the "Peak" intervals. Asterisks designate significant differences (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare(s).
The Pco2 values during Sample Intervals C, F, and I did not differ from one another. Significant acidosis also developed whenever the pulmonary artery or bronchial snares were tightened independently or simultaneously (Figure 4). The p H and [H+] values during Sample Intervals C, F, and I did not differ from one another. The respiratory rate increased and the heart rate decreased when the pulmonary artery snare was
tightened alone, but not when the bronchial snare was tightened alone or in combination with the pulmonary artery snare (Figures 5 and 6). Tightening the pulmonary artery and bronchial snares independently or simultaneously caused PAP to increase regardless of whether PAP was measured as a mean over several respiratory cycles (Figure 7A), or was evaluated as discrete systolic and diastolic pressures at the peak of
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WIDEMAN JR. ET AL.
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FIGURE 5. Respiratory rate (mean ± SEM, n = 10) at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). "Peak" values were measured over a 20- to 40-s interval bracketing the maximum (Peak) increase in pulmonary artery pressure during the first 5 min after tightening the pulmonary artery snare, or within the first 2 min after tightening the bronchial snare. Asterisks designate significant difference (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare.
inspiration (Figure 7B) and at the trough of expiration (Figure 7C). The increase in the mean PAP was greater when the pulmonary artery snare was tightened alone or in combination with the bronchial snare than when the bronchial snare was tightened alone. Due to the gradual onset of pulmonary hypertension when the bronchial snare was tightened, the "Peak" PAP values measured during the 2-min interval immediately after tightening the snare tended to be lower than the PAP values bracketing the collection interval for blood Sample F. When the systemic arterial pressure was
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Sample Intervals FIGURE 6. Heart rate (mean ± SEM, n = 10) at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). "Peak" values were measured over a 20- to 40-s interval bracketing the maximum (Peak) increase in pulmonary artery pressure during the first 5 min after tightening the pulmonary artery snare, or within the first 2 min after tightening the bronchial snare. Asterisks designate significant difference (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare.
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WIDEMAN JR. ET AL.
group) led to a reduced Day 28 body weight, an increased right ventricle weight, a decreased left ventricle plus septum weight, and an increased RV:TV ratio. Chronically clamping one pulmonary artery (PA-Clamp group) triggered larger reductions in the body weight and left ventricle plus septum weight, and a larger increase in . the RV:TV ratio when compared with chronically clamping one bronchus. The incidence of ascites by Day 28 was higher in the PA-Clamp group than in the Control, Sham, and Bronchus Clamp groups (Table 1).
DISCUSSION
Independently tightening the bronchial snare also initiated a fully reversible V / Q mismatch accompanied by a modest pulmonary hypertensive response. In this case, tightening the snare around the right extrapulmonary primary bronchus directed the respiratory minute volume toward the left lung, whereas pulmonary arterial blood presumably continued to perfuse both lungs. Consequently, the blood exiting the poorly ventilated right lung would essentially remain "venous" in its gas composition, leading to the observed rapid onset of systemic arterial hypoxemia, hypercapnia, and acidosis. Unexpectedly, these alterations in systemic arterial blood composition failed to stimulate an increased respiratory rate when the bronchial snare was tightened independently or simultaneously along with the pulmonary artery snare. Perhaps the increased resistance to air flow through the conducting airways, caused by obstructing an extra-pulmonary primary
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As demonstrated previously (Wideman and Kirby, 1995a), acutely tightening a snare around one pulmonary artery in healthy male broilers triggered an immediate pulmonary hypertension accompanied by systemic arterial hypoxemia, hypercapnia, and acidosis. These symptoms of a V / Q mismatch developed in spite of an increased respiratory rate throughout the period when the pulmonary artery snare was independently tightened. In the absence of direct measurements of CO and pulmonary vascular resistance (PVR), these results indicate that halving the pulmonary vascular capacity by tightening the pulmonary artery snare effectively doubled PVR and forced the right ventricle to substantially increase PAP in order to propel the entire CO through the unobstructed lung. Pressure and flow characteristics of this nature are to be expected if the pulmonary vasculature of broilers has a low compliance and little reserve capacity for capillary recruitment (Burton et al, 1968; Powell et ah, 1985; Wideman and Bottje, 1993; Wideman and Kirby, 1995a,b). Within these limitations, even moderate increases in CO must dictate correspondingly proportional increases in PAP and in the rate at which blood flows past the gas exchange surfaces, making broilers highly susceptible to pulmonary hypertension and a perfusion-induced V / Q mismatch.
bronchus, forced any increase in pulmonary ventilation to be accomplished through amplification of the tidal volume rather than by accelerating the respiratory rate. Tidal volume was not measured in the present study; however, the pulmonary and systemic arterial pressure wave excursions associated with intra-thoracic and intra-abdominal pressure changes during the respiratory cycle did not appreciably widen when the bronchial snare was tightened, as would have been expected if tidal volume increased substantially while a constant respiratory rate was maintained (Figures 7 A to C and 8 A to B). Alternatively, hypoxia coupled with tightening of the bronchial snare may have introduced contradictory input from various stretch and chemoreceptors that regulate avian respiration (Butler, 1967; Fedde, 1986). In either case, the unilateral bronchial snare technique provides a model of avian V / Q mismatch induced by effective hypopnea. The modest pulmonary hypertension triggered by independently tightening the bronchial snare probably reflects the increased right ventricular work required to overcome pulmonary vasoconstriction (increased PVR) triggered by local hypoxia and hypoxemia within the right lung, or systemic hypoxemia affecting the vasculature of both lungs (Burton et al., 1968; Besch and Kadono, 1978; Peacock et al, 1989; Owen et al., 1995). The unilateral bronchus clamp technique should provide an experimental model for mimicking field outbreaks of PHS thought to be induced by respiratory damage associated with disease, dust, and poor air quality. Hypoventilation previously was discounted as a cause for the hypoxemia observed in broilers developing pulmonary hypertension, based on observations that tidal volume and total ventilation were not correlated to blood oxygen saturations ranging from 35 to 88%, whereas respiratory frequency was negatively correlated to blood oxygen saturation (Reeves et al., 1991). Furthermore, the hypoxemia, acidosis, and hypercapnia induced by acute unilateral pulmonary artery occlusion were accompanied by a visibly increased respiratory effort, indicating the respiratory control systems of broilers detected the changes in blood gas composition and initiated an appropriate hyperventilatory response (Wideman and Kirby, 1995a). Nevertheless, the possibility remained that ongoing air flow through both lungs during unilateral pulmonary artery occlusion created an effective ventilatory mismatch that amplified the perfusion-dependent hypoxemia (Wideman and Kirby, 1995a). The results of the present study demonstrate conclusively that simultaneously forcing the entire respiratory minute volume toward the lung receiving the entire CO does not attenuate the hypoxemia, hypercapnia, and acidosis caused by independent tightening of the pulmonary artery snare alone. Therefore, healthy male broilers must be considered highly susceptible to a perfusion-dependent V / Q mismatch that can be initiated by relatively moderate (two times or less) increases in CO. This perfusion-dependent V / Q
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F
G
Sample Intervals FIGURE 8. Mean systemic arterial pressure (mean ± SEM, n = 10) at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples ) and K). "Peak" values were measured over a 20- 40-s interval bracketing the maximum (Peak) increase in pulmonary artery pressure during the first 5 min after tightening the pulmonary artery snare, or within the first 2 min after tightening the bronchial snare. A) systemic arterial pressure measured as a mean over several respiratory cycles; B) systemic arterial pressure measured as systolic and diastolic pressures at the peak of inspiration or at the trough of expiration. Asterisks designate significant differences (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare(s).
mismatch apparently cannot be counteracted by a matching increase in ventilation, presumably because blood residence time at the gas exchange surfaces is too brief for effective diffusive gas equilibration. In order to suitably accelerate O2 diffusion by amplifying the driving gradient under such circumstances, the P02
levels in the air capillaries may have to increase well above the 100 mm Hg level normally needed to fully saturate avian hemoglobin. It is in this context that the very similar alterations in blood gas partial pressures observed during independent and simultaneous pulmonary artery and bronchial occlusion would be consistent
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Sample Intervals
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WIDEMAN JR. ET AL. TABLE 1. Body weight and heart values for normal broilers (CONTROL), sham-operated broilers (SHAM), broilers in which one extrapulmonary primary bronchus was clamped (BRONCHUS CLAMP), and broilers in which one pulmonary artery was clamped (PA-CLAMP)1 Treatment group
Variable
CONTROL (n = 19)
SHAM (n = 12)
BRONCHUS CLAMP (n = 14)
Body weight, g Right ventricle (RV) weight, g Left ventricle + septum (LV+S) weight, g Total ventricle (TV) weight, g RV:TV2 Percentage ascites
1,394 + 14" 1.47 ± 0.05b 4.97 ± 0.10b 6.44 ± 0.13" 0.228 ± 0.006= 0 (0/19)b
1,455 ± 19" 1.60 ± 0.10b 5.56 ± 0.11" 7.17 ± 0.14" 0.223 ± 0.012= 0 (0/12) b
1,224 ± 19b 2.02 ± 0.15" 4.36 ± 0.16= 6.39 ± 0.23" 0.314 ± 0.019b 7 (l/14) b
PA-Clamp (n = 17) 968
± 47= 1.97 ± 0.12" 2.81 ± 0.23d 4.78 ± 0.28b 0.417 ± 0.017" 52 (9/17)"
a_d
Means within a variable with no common superscript differ significantly (P < 0.05). Data are means ± SEM. 2 RV:TV = right ventricle to total ventricle weight ratio. J
left ventricular CO and thus systemic arterial hypotension. Confirmation of these interpretations await measurements of CO during unilateral pulmonary artery occlusion. Acute tightening of the bronchial snare triggered a modest but significant pulmonary hypertension in anesthetized male broilers, whereas tightening the pulmonary artery snare caused a significantly greater degree of pulmonary hypertension in the same birds (Figure 8A). Experiment 2 was conducted to determine whether these differences in the acute pulmonary hypertensive responses would be maintained throughout a more prolonged period of unilateral bronchus or pulmonary artery obstruction. Indeed, clamping one pulmonary artery for 10 to 12 d caused a large increase in the RV:TV ratio, a high incidence of ascites, and a profound reduction in growth, all of which are symptomatic of the extreme challenge presented to these broilers by the pulmonary artery clamp model (Wideman and Kirby, 1995b, 1996). In contrast, clamping one extra-pulmonary primary bronchus for the same period of time led to more moderate reductions in growth rate and increases in RV:TV ratios, reflecting a lower increment in PAP triggered by the chronic bronchus clamp model than for the pulmonary artery clamp model. These results demonstrate that the differences observed during acute physiological studies are relevant to the performance of growing broilers, and further confirm the direct correlation between pulmonary arterial pressures and RV:TV ratios as reported elsewhere (Burton and Smith, 1967; Burton et al, 1968). It is curious that acutely tightening a bronchial snare for more than 4 min caused severe arterial acidosis followed by death during pilot studies preceding Experiment 1, whereas mortality was nonexistent when one bronchus was chronically clamped in Experiment 2. The physiological explanation for this key observation remains to be determined. The phenomenon may reflect the comparatively greater degree of surgical insult necessary to conduct Experiment 1 in contrast to the very brief period during which the thoracic inlet was
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with simple parabronchial hypoventilation, reflecting an inability of broilers to sufficiently elevate the P02 and depress the Pco2 0 l parabronchial gas to the extent necessary to prevent systemic arterial hypoxemia and hypercapnia. Normobaric and hypobaric hypoxia consistently trigger systemic arterial hypotension that coincides with the onset of hypoxemia and pulmonary arterial hypertension in chickens (Burton et al, 1968; Besch and Kadono, 1978; Peacock et al, 1989; Owen et al, 1995). The systemic arterial hypotension has been attributed to a reduction in total peripheral resistance (peripheral vasodilation) caused by the direct dilatory effect of hypoxemia on systemic vascular smooth muscle. Systemic hypotension also may develop if hypoxemia reduces cardiac muscle contractility and thus CO (Butler, 1967; Besch and Kadono, 1978; Peacock et al, 1989; Wideman and Bottje, 1993; Owen et al, 1995). Alternatively, the pulmonary vascular constriction caused by hypoxia and hypoxemia may be sufficient to significantly impede venous return to the left ventricle, thereby reducing CO. In the present study, independently tightening the bronchial snare triggered a significantly greater systemic arterial hypoxemic response than did independently tightening the pulmonary artery snare (Figure 1); however, tightening the pulmonary artery snare caused a significantly greater increase in pulmonary arterial pressure and a greater decrease in systemic arterial pressure than did tightening the bronchial snare (Figures 7A, 8A). This dissociation of the vascular responses from the hypoxemic response suggests that the systemic hypotension during pulmonary hypertension cannot be attributed primarily to the direct effects of hypoxemia on cardiac or peripheral vascular muscle. Instead, these data support the hypothesis that the maximal "acute" increment in PAP attainable by the right ventricle is inadequate to propel the requisite CO through an effectively constricted pulmonary vasculature. The resulting congestive right-sided pooling of blood would, in turn, impede venous return to the left ventricle, causing a reduction in
OCCLUSION OF THE PULMONARY ARTERY AND PRIMARY BRONCHUS o p e n in E x p e r i m e n t 2. Alternatively, the age or b o d y m a s s of the broilers at the time of unilateral b r o n c h u s obstruction m a y d e t e r m i n e their ability to function w i t h o n e ventilated l u n g . T h e chronic b r o n c h u s c l a m p s w e r e placed in 16- to 18-d-old chicks, w h e r e a s the b r o n c h u s s n a r e t e c h n i q u e w a s c o n d u c t e d u s i n g 6- to 7-wk-old broilers w e i g h i n g 2.2 kg. The extreme d e g r e e of respiratory distress n o t e d d u r i n g acute b r o n c h i a l occlusion in the older broilers m a y reflect a m o r e t e n u o u s relationship b e t w e e n functional p u l m o n a r y capacity a n d metabolic d e m a n d as b o d y m a s s increases.
ACKNOWLEDGMENTS This research w a s s u p p o r t e d b y the A r k a n s a s P o u l t r y Federation E n d o w m e n t a n d b y grants from H u b b a r d F a r m s , Walpole, N H 03608.
Besch, E. L., and H. Kadono, 1978. Cardiopulmonary responses to acute hypoxia in domestic fowl. Pages 71-78 in: Respiratory Function in Birds Adult and Embryonic. J. Piiper, ed. Springer-Verlag, New York, NY. Burton, R. R., E. L. Besch, and A. H. Smith, 1968. Effect of chronic hypoxia on the pulmonary arterial blood pressure of the chicken. Am. J. Physiol. 214:1438-1442. Burton, R. R., and A. H. Smith, 1967. The effect of polycythemia and chronic hypoxia on heart mass of the chicken. J. Appl. Physiol. 22:782-785. Butler, P. J., 1967. The effect of progressive hypoxia on the respiratory and cardiovascular systems of the chicken. J. Physiol. 191:309-324. Fedde, M. R., 1986. Respiration. Pages 191-220 in: Avian Physiology. 4th ed. P. J. Sturkie, ed. Springer-Verlag, New York, NY. Julian, R. J., and S. M. Mirsalimi, 1992. Blood oxygen concentration of fast-growing and slow-growing broiler chickens, and chickens with ascites from right ventricular failure. Avian Dis. 36:730-732. Lutz, P. L., I. S. Longmuir, and K. Schmidt-Nielsen, 1974. Oxygen affinity of bird blood. Respir. Physiol. 20:325-330. National Research Council, 1984. Nutrient Requirements of Poultry. 8th rev. ed. National Academy Press, Washington, DC.
Owen, R. L., R. F. Wideman, and B. S. Cowen, 1995. Changes in pulmonary arterial and femoral arterial blood pressure upon acute exposure to hypobaric hypoxia in broiler chickens. Poultry Sci. 74:708-715. Peacock, A. J., C. Pickett, K. Morris, and J. T. Reeves, 1989. The relationship between rapid growth and pulmonary hemodynamics in the fast-growing broiler chicken. Am. Rev. Respir. Dis. 139:1524-1530. Peacock, A. J., C. Pickett, K. Morris, and J. T. Reeves, 1990. Spontaneous hypoxaemia and right ventricular hypertrophy in fast growing broiler chickens reared at sea level. Comp. Biochem. Physiol. 97A:537-541. Powell, F. L., R. H. Hastings, and R. W. Mazzone, 1985. Pulmonary vascular resistance during unilateral pulmonary artery occlusion in ducks. Am. J. Physiol. 249: R39-R43. Reeves, J. T., G. Ballam, S. Hofmeister, C. Pickett, K. Morris, and A. Peacock, 1991. Improved arterial oxygenation with feed restriction in rapidly growing broiler chickens. Comp. Biochem. Physiol. 99A:481^85. SAS Institute, 1982. SAS® User's Guide: Statistics. SAS Institute Inc., Cary, NC. Sturkie, P. D., 1986. Heart and circulation: anatomy, hemodynamics, blood pressure, blood flow. Pages 130-166 in: Avian Physiology. 4th ed. P. J. Sturkie, ed. SpringerVerlag, New York, NY. Wideman, R. F., and W. G. Bottje, 1993. Current understanding of the ascites syndrome and future research directions. Pages 1-20 in: Nutrition and Technical Symposium Proceedings. Novus International, Inc., St. Louis, MO. Wideman, R. F., and E. G. Buss, 1985. Arterial blood gas, pH, and bicarbonate values in laying hens selected for thick or thin eggshell production. Poultry Sci. 64:1015-1019. Wideman, R. F., and Y. K. Kirby, 1995a. Evidence of a ventilation-perfusion mismatch during acute unilateral pulmonary artery occlusion in broilers. Poultry Sci. 74: 1209-1217. Wideman, R. F., and Y. K. Kirby, 1995b. A pulmonary artery clamp model for inducing pulmonary hypertension syndrome (ascites) in broilers. Poultry Sci. 74:805-812. Wideman, R. F., Jr., and Y. K. Kirby, 1996. Electrocardiographic evaluation of broilers during the onset of pulmonary hypertension initiated by unilateral pulmonary artery occlusion. Poultry Sci 75:407-416.
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REFERENCES
1427
(Key words: ascites, pulmonary hypertension, respiration, blood pressure, acidosis, ventilation-perfusion inequality) 1996 Poultry Science 75:1417-1427
INTRODUCTION The reduced oxygen content of systemic arterial blood (hypoxemia) during the onset of pulmonary hypertension syndrome (PHS, ascites) in broilers may be symptomatic of an imbalance between ventilation (V) of the lungs with air and perfusion (Q) of the lungs with blood (Peacock et al, 1989, 1990; Reeves et al, 1991; Wideman and Kirby, 1995b). A ventilation-perfusion
Received for publication April 1, 1996. Accepted for publication July 26, 1996. JPublished as A r k a n s a s Agricultural Experiment Station manuscript Number 96032 with the approval of the Experiment Station Director. 2 To whom correspondence should be addressed: Department of Poultry Science, O-402 Poultry Science Center, University of Arkansas, Fayetteville, AR 72701.
(V/Q) mismatch theoretically would trigger hypoxemia in susceptible broilers if a high cardiac output (CO) caused blood to flow too rapidly through the lungs to permit adequate oxygen uptake, or if an excessively high blood flow rate was necessary to convey a normal CO through a pulmonary vascular bed having an anatomically inadequate capacity (Powell et al., 1985; Wideman and Bottje, 1993; Wideman and Kirby, 1995a,b). Unilateral pulmonary artery obstruction has been used as an experimental model for effectively doubling the volume of blood flow through the unobstructed lung in birds (Burton et ah, 1968; Powell et ah, 1985). Evidence obtained using chronic and acute adaptations of this model supports the hypothesis that a proportionally high CO coupled with a low capacity, noncompliant pulmonary vasculature can lead to a V / Q mismatch, hypoxemia, and pulmonary hypertension in
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ABSTRACT Acutely tightening a snare around one pulmonary artery previously was shown to trigger a reversible ventilation-perfusion (V/Q) mismatch in broilers, as reflected by decreases in the partial pressure of oxygen in arterial blood (hypoxemia), accompanied by increases in the hydrogen ion concentration (acidosis) and partial pressure of carbon dioxide (hypercapnia). In the present study, snares were loosely implanted around the right pulmonary artery and the right extrapulmonary primary bronchus in anesthetized male broilers. These snares were tightened and released independently and then simultaneously to evaluate the possibility that directing the entire respiratory minute volume toward the left lung might attenuate the V / Q mismatch caused by forcing the entire cardiac output (CO) through the left lung. Fully reversible arterial blood hypoxemia, acidosis, and hypercapnia occurred when either snare was tightened independently. Presumably, tightening the bronchial snare restricted ventilation but not blood flow to the right lung, thereby permitting blood to perfuse poorly ventilated gas exchange surfaces. Simultaneously tightening both
1418
WIDEMAN JR. ET AL.
MATERIALS AND METHODS Experiment 1 Male by-product chicks of the Hubbard3 breeder pullet line were reared on fresh wood shavings litter in an environmental chamber (8 m 2 floor space). They were brooded at 32 and 30 C during Weeks 1 and 2, respectively, and thereafter the temperature was maintained at 24 C. They were fed corn-soybean meal-based broiler rations formulated to meet or exceed the minimum NRC (1984) standards for all ingredients. Feed and water were provided for ad libitum consumption. The surgical protocol was described previously (Wideman and Kirby, 1995b). When the birds were 6 to 7 wk of age, 10 of the largest clinically healthy individuals were anesthetized to a surgical plane by injecting allobarbital (5,5-diallyl-barbituric acid; 4 25 mg/mL, 50 m g / k g body mass) into the Pectoralis muscle. They were fastened in dorsal recumbency on a heated surgical board that was thermostatically regulated to maintain a surface temperature of 35 C. The end of the surgical board was elevated to a 20 C head-up angle. A midline incision was made through the skin of the thoracic inlet, and the connective tissues surrounding the right pulmonary artery were separated to allow a snare, consisting of a loop of braided 2-0 silk suture thread 5 passed through a 6-cm length of PE290 polyethylene tubing, 6 to be placed loosely around
3Hubbard Farms, Walpole, N H 03608. 4 Sigma Chemical Co., St. Louis, MO 63178-9916. 5 Arista Surgical Supply Co., Inc., New York, NY 10010-1898. 6 Intramedic Co., Parsippany, NJ 07054. 7 Miltex-Instruments Co., Lake Success, NY 11042. 8 Dow Corning Corp., Midland, MI 48686-0994. 9 World Precision Instruments, Sarasota, FL 34230. 10 Biopac Systems, Inc., Goleta, CA 93117. n Criticare Systems, Inc., Milwaukee, WI 53226. 12 Pilot studies demonstrated that more prolonged tightening of the bronchus snare caused severe arterial blood acidosis (pH < 7.15) and death (R. F. Wideman, personal observations).
the pulmonary artery. A similar snare was placed loosely around the right extra-pulmonary primary bronchus. The skin of the thoracic inlet was sealed with surgical wound clips. 7 The left cutaneous ulnar vein was cannulated with Silastic® Laboratory Tubing (0.012 internal diameter x 0.025 outside diameter) 8 filled with 0.8% sodium chloride containing 200 IU heparin/mL. 4 The distal end of the cannula tubing was attached to a BLPR blood pressure transducer 9 interfaced through a Transbridge™ preamplifier9 to a Biopac MP 100 data acquisition system using AcqKnowledge software. 10 The cannula tubing was advanced slowly through the right atrium and ventricle into a pulmonary artery using continuous monitoring of characteristic pulse pressures to identify the location of the tip of the cannula, as described by Owen et al. (1995). If the cannula was in the right pulmonary artery, as assessed by loss of pulse pressure when the snare was briefly tightened, then the cannula was withdrawn into the right ventricle and repositioned until it entered the left pulmonary artery. The left brachial artery and left anterior tibial vein were cannulated with PE50 polyethylene tubing 6 filled with heparinized saline. The arterial cannula was advanced to a position near the descending aorta and, except when used for arterial blood sampling, was attached to a blood pressure transducer for continuous monitoring of systemic arterial pressure. A solution of 25 g mannitol 4 /L of water was infused at a constant rate of 0.05 m L / m i n through the venous cannula to hydrate the birds throughout the remainder of the experiment. The probe of a pulse oximeterii was positioned on the right wing to illuminate the tissue between the radius and ulna for noninvasive measurements of percentage saturation of hemoglobin with oxygen (Peacock et al, 1990; Julian and Mirsalimi, 1992; Wideman and Kirby, 1995a,b). When surgical preparations were complete and a stabilization period of 20 to 30 min had elapsed, control data were collected for 15 min, and two arterial blood samples were withdrawn 5 and 10 min into the control period (Samples A and B). The pulmonary artery snare was tightened to fully occlude the right pulmonary artery for 10 min, and an arterial blood sample was withdrawn 5 min after tightening the snare (Sample C). The pulmonary artery snare was released, and arterial blood samples were collected 5 and 10 min into the 15-min recovery period (Samples D and E). The bronchial snare was gently tightened to fully occlude the right extra-pulmonary primary bronchus for 4 min, and an arterial blood sample was collected 2 min after tightening the bronchial snare (Sample F). 12 The bronchial snare was released, and arterial blood samples were collected 5 and 10 min into the 15-min recovery period (Samples G and H). Both the pulmonary artery and bronchial snares were tightened simultaneously for 4 min, and an arterial blood sample was collected 2 min after both snares were tightened (Sample I). Both snares were released, and arterial Samples "J" and "K" were collected 5 and 10 min into the subsequent 15-min recovery period.
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fast-growing broilers (Wideman and Kirby, 1995a,b, 1996). However, these physiological interpretations are confounded by the likelihood that the pulmonary air flow remained fairly evenly distributed between both lungs during unilateral pulmonary artery occlusion (Wideman and Kirby, 1995a). This potential ventilatory mismatch raises the possibility that effective hypoventilation contributes significantly to the hypoxemic response observed during acute unilateral pulmonary artery occlusion. In the present study, snares implanted around the right pulmonary artery and the right extrapulmonary primary bronchus in male broilers were tightened and released independently and then simultaneously to evaluate the possibility that forcing the entire respiratory minute volume toward the left lung may attenuate the V / Q mismatch caused by forcing the entire CO through the left lung.
OCCLUSION OF THE PULMONARY ARTERY AND PRIMARY BRONCHUS
13
Radiometer America Inc., Westlake, OH 44145.
the expiratory wave excursions for three consecutive respiratory cycles preceding and three respiratory cycles following each blood sampling interval.
Experiment
2
Male by-product chicks were reared as described in Experiment 1. When the chicks reached 16 to 18 d of age, they were anesthetized to a surgical plane, the thoracic inlet was opened, and a silver clip was positioned to fully obstruct the left pulmonary artery (PA-Clamp Group; 17 chicks) or the left extra-pulmonary primary bronchus (Bronchus Clamp Group; 14 chicks). A detailed description of the surgical procedures has been provided previously (Wideman and Kirby, 1995b). Sham-operated chicks were anesthetized and the thoracic inlet was opened; however, no clamps were inserted on the pulmonary artery or bronchus (Sham Group; 12 chicks). Chicks in the Control Group (19 chicks) were not anesthetized and did not undergo surgical procedures. Following recovery from surgery, the chicks were reared together in the same environmental chamber and were fed the same diets until they reached 28 d of age (10 to 12 d postsurgery). Then they were killed with an overdose of anesthetic, and the heart was removed, dissected, and weighed for calculation of the RV:TV ratio.
Statistical
Analysis
Because pH is a logarithmic function, all blood p H values were converted to hydrogen ion concentrations ([H+])for statistical analysis. In Experiment 1, data were analyzed using the General Linear Models procedure of SAS® (SAS Institute, 1982) using repeated measures ANOVA for comparisons over time. Differences between two sample intervals were confirmed by a paired t test. Responses obtained when a snare was tightened were considered significant when they differed (P < 0.05) from both sample intervals immediately preceding tightening of the snare. In Experiment 2, means were analyzed using ANOVA followed by t tests for pairwise comparisons; a chi-square test was used to compare the proportions of ascites among the four treatment groups.
RESULTS The 10 male broilers used in Experiment 1 weighed 2,246 ± 92 g (mean + SEM), had a RV:TV ratio of 0.257 ± 0.009, and their lung weights averaged 7.20 ± 0.41 g (left lung) and 6.87 + 0.32 g (right lung). Hematocrits averaged 31 ± 1% during Sample Intervals A to F, decreased to 30 + 1 % during Sample Intervals G to I, and decreased to 29 + 1% during Sample Intervals J to K. As shown in Figure 1, the P02 of arterial blood averaged between 97 and 100 mm Hg during all periods when the pulmonary artery and bronchial snares were loose (Sample Intervals A and B, D and E, G and H, and J and K), and decreased by 20 mm Hg or more when the
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All arterial blood samples were 1 mL each. No saline or blood replacement was administered. All samples were withdrawn anaerobically with a syringe, as described previously (Wideman and Kirby, 1995a), and were immediately injected into a Radiometer ABL330 AcidBase Laboratory. 13 Appropriate function of the blood gas analyzer was assessed by periodically injecting Blood Gas Qualicheck® reference standards, as described previously (Wideman and Buss, 1985). The primary arterial blood values for p H and partial pressures of oxygen (P02) a n d carbon dioxide (Pco2) were generated by the ABL330 operating at a sample chamber temperature of 37 C, and were recalculated by the ABL330 for a temperature of 41C to match the normal body temperature of domestic fowl (Lutz et al, 1974; Fedde, 1986). A portion of the arterial blood was used for duplicate hematocrit (HCT) determinations using heparinized capillary tubes and a microhematocrit centrifuge. Birds were killed with an overdose of anesthetic at the end of the experiment; the heart was removed, dissected, and weighed for calculation of the righfctotal ventricular weight ratio (RV:TV ratio), which serves as a reliable index of pulmonary hypertension (Burton and Smith, 1967; Burton et ah, 1968); and the lungs were removed and weighed. The Biopac MP 100 data acquisition system was used to continuously collect data from three channels throughout each experiment, including systemic arterial pressure (millimeters of Hg), pulmonary arterial pressure (PAP, millimeters of Hg), and percentage saturation of hemoglobin with oxygen (%HbC>2). Heart rate (HR, beats per minute) was obtained by counting systolic peaks over time in the PAP recording. Respiratory rate (RR, breaths per minute) was obtained by counting the wave cycles associated with respiratory movement that comprise an integral part of the pulmonary and systemic arterial pressure recordings (Sturkie, 1986). Average values for these parameters were measured electronically during the 30-s time intervals immediately preceding and following withdrawal of each arterial blood sample, then the separate pre- and postsampling measurements were averaged to yield a single value representative of the data collected throughout the blood sampling interval. Values for these parameters also were averaged over an approximately 20- to 40-s interval coincident with the maximum ("Peak") increase in pulmonary artery pressure attained before collecting arterial blood Samples C, F, and I. The measurements thus obtained for systemic arterial pressure and PAP incorporated multiple respiratory wave cycles in the overall estimation of mean arterial pressure (MAP) and mean PAP. To evaluate possible influences of the respiratory excursions on blood pressure measurements, data from each bird were analyzed separately to obtain discrete values for systolic and diastolic systemic arterial pressure and PAP during the maximum of the inspiratory wave excursions and during the minimum of
1419
1420
WIDEMAN JR. ET AL. P. Arterial Snare P Tightened ~|
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pulmonary artery and bronchial snares were tightened independently (Sample Intervals C and F) or simultaneously (Sample Interval I). The hypoxemic P02 values during Sample Intervals F and I did not differ from each other but were lower than those of Sample Interval G. Pulse oximetry measurements indicated that the percentage saturation of hemoglobin with oxygen decreased from average values of 85% when the pulmonary artery
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FIGURE 1. Partial pressure of oxygen (mean ± SEM, n = 10) in arterial blood at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). Blood samples were not obtained for blood gas analysis during the "Peak" intervals. Asterisks designate significant differences (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare(s).
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FIGURE 3. Partial pressure of carbon dioxide (mean ± SEM, n = 10) in arterial blood at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). Blood samples were not obtained for blood gas analysis during the "Peak" intervals. Asterisks designate significant differences (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare(s).
The Pco2 values during Sample Intervals C, F, and I did not differ from one another. Significant acidosis also developed whenever the pulmonary artery or bronchial snares were tightened independently or simultaneously (Figure 4). The p H and [H+] values during Sample Intervals C, F, and I did not differ from one another. The respiratory rate increased and the heart rate decreased when the pulmonary artery snare was
tightened alone, but not when the bronchial snare was tightened alone or in combination with the pulmonary artery snare (Figures 5 and 6). Tightening the pulmonary artery and bronchial snares independently or simultaneously caused PAP to increase regardless of whether PAP was measured as a mean over several respiratory cycles (Figure 7A), or was evaluated as discrete systolic and diastolic pressures at the peak of
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Sample Intervals
WIDEMAN JR. ET AL.
1422 P. Arterial Snare
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FIGURE 5. Respiratory rate (mean ± SEM, n = 10) at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). "Peak" values were measured over a 20- to 40-s interval bracketing the maximum (Peak) increase in pulmonary artery pressure during the first 5 min after tightening the pulmonary artery snare, or within the first 2 min after tightening the bronchial snare. Asterisks designate significant difference (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare.
inspiration (Figure 7B) and at the trough of expiration (Figure 7C). The increase in the mean PAP was greater when the pulmonary artery snare was tightened alone or in combination with the bronchial snare than when the bronchial snare was tightened alone. Due to the gradual onset of pulmonary hypertension when the bronchial snare was tightened, the "Peak" PAP values measured during the 2-min interval immediately after tightening the snare tended to be lower than the PAP values bracketing the collection interval for blood Sample F. When the systemic arterial pressure was
Bronchus Snare I - Tightened ~~|
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evaluated as a mean over several respiratory cycles (MAP; Figure 8A) or as discrete systolic and diastolic pressures at the peak of inspiration and the trough of expiration (Figure 8B), systemic hypotension developed after the pulmonary artery snare was tightened independently or simultaneously with the bronchial snare, but not when the bronchial snare was tightened independently. The results of Experiment 2 are summarized in Table 1. When compared with the Control and Sham groups, chronically clamping one bronchus (Bronchus Clamp
E
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Sample Intervals FIGURE 6. Heart rate (mean ± SEM, n = 10) at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). "Peak" values were measured over a 20- to 40-s interval bracketing the maximum (Peak) increase in pulmonary artery pressure during the first 5 min after tightening the pulmonary artery snare, or within the first 2 min after tightening the bronchial snare. Asterisks designate significant difference (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare.
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Sample Intervals FIGURE 7. Pulmonary arterial pressure (PAP; mean + SEM, n = 10) at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples J and K). "Peak" values were measured over a 20- to 40-s interval bracketing the maximum (Peak) increase in pulmonary artery pressure during the first 5 min after tightening the pulmonary artery snare, or within the first 2 min after tightening the bronchial snare. A) PAP measured as a mean over several respiratory cycles; B) PAP measured as systolic and diastolic pressures at the peak of inspiration; C) PAP measured as systolic and diastolic pressures at the trough of expiration. Asterisks designate significant differences (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare(s).
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1424
WIDEMAN JR. ET AL.
group) led to a reduced Day 28 body weight, an increased right ventricle weight, a decreased left ventricle plus septum weight, and an increased RV:TV ratio. Chronically clamping one pulmonary artery (PA-Clamp group) triggered larger reductions in the body weight and left ventricle plus septum weight, and a larger increase in . the RV:TV ratio when compared with chronically clamping one bronchus. The incidence of ascites by Day 28 was higher in the PA-Clamp group than in the Control, Sham, and Bronchus Clamp groups (Table 1).
DISCUSSION
Independently tightening the bronchial snare also initiated a fully reversible V / Q mismatch accompanied by a modest pulmonary hypertensive response. In this case, tightening the snare around the right extrapulmonary primary bronchus directed the respiratory minute volume toward the left lung, whereas pulmonary arterial blood presumably continued to perfuse both lungs. Consequently, the blood exiting the poorly ventilated right lung would essentially remain "venous" in its gas composition, leading to the observed rapid onset of systemic arterial hypoxemia, hypercapnia, and acidosis. Unexpectedly, these alterations in systemic arterial blood composition failed to stimulate an increased respiratory rate when the bronchial snare was tightened independently or simultaneously along with the pulmonary artery snare. Perhaps the increased resistance to air flow through the conducting airways, caused by obstructing an extra-pulmonary primary
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As demonstrated previously (Wideman and Kirby, 1995a), acutely tightening a snare around one pulmonary artery in healthy male broilers triggered an immediate pulmonary hypertension accompanied by systemic arterial hypoxemia, hypercapnia, and acidosis. These symptoms of a V / Q mismatch developed in spite of an increased respiratory rate throughout the period when the pulmonary artery snare was independently tightened. In the absence of direct measurements of CO and pulmonary vascular resistance (PVR), these results indicate that halving the pulmonary vascular capacity by tightening the pulmonary artery snare effectively doubled PVR and forced the right ventricle to substantially increase PAP in order to propel the entire CO through the unobstructed lung. Pressure and flow characteristics of this nature are to be expected if the pulmonary vasculature of broilers has a low compliance and little reserve capacity for capillary recruitment (Burton et al, 1968; Powell et ah, 1985; Wideman and Bottje, 1993; Wideman and Kirby, 1995a,b). Within these limitations, even moderate increases in CO must dictate correspondingly proportional increases in PAP and in the rate at which blood flows past the gas exchange surfaces, making broilers highly susceptible to pulmonary hypertension and a perfusion-induced V / Q mismatch.
bronchus, forced any increase in pulmonary ventilation to be accomplished through amplification of the tidal volume rather than by accelerating the respiratory rate. Tidal volume was not measured in the present study; however, the pulmonary and systemic arterial pressure wave excursions associated with intra-thoracic and intra-abdominal pressure changes during the respiratory cycle did not appreciably widen when the bronchial snare was tightened, as would have been expected if tidal volume increased substantially while a constant respiratory rate was maintained (Figures 7 A to C and 8 A to B). Alternatively, hypoxia coupled with tightening of the bronchial snare may have introduced contradictory input from various stretch and chemoreceptors that regulate avian respiration (Butler, 1967; Fedde, 1986). In either case, the unilateral bronchial snare technique provides a model of avian V / Q mismatch induced by effective hypopnea. The modest pulmonary hypertension triggered by independently tightening the bronchial snare probably reflects the increased right ventricular work required to overcome pulmonary vasoconstriction (increased PVR) triggered by local hypoxia and hypoxemia within the right lung, or systemic hypoxemia affecting the vasculature of both lungs (Burton et al., 1968; Besch and Kadono, 1978; Peacock et al, 1989; Owen et al., 1995). The unilateral bronchus clamp technique should provide an experimental model for mimicking field outbreaks of PHS thought to be induced by respiratory damage associated with disease, dust, and poor air quality. Hypoventilation previously was discounted as a cause for the hypoxemia observed in broilers developing pulmonary hypertension, based on observations that tidal volume and total ventilation were not correlated to blood oxygen saturations ranging from 35 to 88%, whereas respiratory frequency was negatively correlated to blood oxygen saturation (Reeves et al., 1991). Furthermore, the hypoxemia, acidosis, and hypercapnia induced by acute unilateral pulmonary artery occlusion were accompanied by a visibly increased respiratory effort, indicating the respiratory control systems of broilers detected the changes in blood gas composition and initiated an appropriate hyperventilatory response (Wideman and Kirby, 1995a). Nevertheless, the possibility remained that ongoing air flow through both lungs during unilateral pulmonary artery occlusion created an effective ventilatory mismatch that amplified the perfusion-dependent hypoxemia (Wideman and Kirby, 1995a). The results of the present study demonstrate conclusively that simultaneously forcing the entire respiratory minute volume toward the lung receiving the entire CO does not attenuate the hypoxemia, hypercapnia, and acidosis caused by independent tightening of the pulmonary artery snare alone. Therefore, healthy male broilers must be considered highly susceptible to a perfusion-dependent V / Q mismatch that can be initiated by relatively moderate (two times or less) increases in CO. This perfusion-dependent V / Q
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Sample Intervals FIGURE 8. Mean systemic arterial pressure (mean ± SEM, n = 10) at 5-min intervals before tightening the pulmonary artery snare (Samples A and B), 5 min after the snare was tightened (Sample C), 5 and 10 min after the pulmonary artery snare was released (Samples D and E), 2 min after tightening a bronchial snare (Sample F), 5 and 10 min after the bronchial snare was released (Samples G and H), 2 min after tightening both snares (Sample I), and 5 and 10 min after both snares were released (Samples ) and K). "Peak" values were measured over a 20- 40-s interval bracketing the maximum (Peak) increase in pulmonary artery pressure during the first 5 min after tightening the pulmonary artery snare, or within the first 2 min after tightening the bronchial snare. A) systemic arterial pressure measured as a mean over several respiratory cycles; B) systemic arterial pressure measured as systolic and diastolic pressures at the peak of inspiration or at the trough of expiration. Asterisks designate significant differences (P < 0.05) compared with both of the samples collected immediately prior to tightening the snare(s).
mismatch apparently cannot be counteracted by a matching increase in ventilation, presumably because blood residence time at the gas exchange surfaces is too brief for effective diffusive gas equilibration. In order to suitably accelerate O2 diffusion by amplifying the driving gradient under such circumstances, the P02
levels in the air capillaries may have to increase well above the 100 mm Hg level normally needed to fully saturate avian hemoglobin. It is in this context that the very similar alterations in blood gas partial pressures observed during independent and simultaneous pulmonary artery and bronchial occlusion would be consistent
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Sample Intervals
1426
WIDEMAN JR. ET AL. TABLE 1. Body weight and heart values for normal broilers (CONTROL), sham-operated broilers (SHAM), broilers in which one extrapulmonary primary bronchus was clamped (BRONCHUS CLAMP), and broilers in which one pulmonary artery was clamped (PA-CLAMP)1 Treatment group
Variable
CONTROL (n = 19)
SHAM (n = 12)
BRONCHUS CLAMP (n = 14)
Body weight, g Right ventricle (RV) weight, g Left ventricle + septum (LV+S) weight, g Total ventricle (TV) weight, g RV:TV2 Percentage ascites
1,394 + 14" 1.47 ± 0.05b 4.97 ± 0.10b 6.44 ± 0.13" 0.228 ± 0.006= 0 (0/19)b
1,455 ± 19" 1.60 ± 0.10b 5.56 ± 0.11" 7.17 ± 0.14" 0.223 ± 0.012= 0 (0/12) b
1,224 ± 19b 2.02 ± 0.15" 4.36 ± 0.16= 6.39 ± 0.23" 0.314 ± 0.019b 7 (l/14) b
PA-Clamp (n = 17) 968
± 47= 1.97 ± 0.12" 2.81 ± 0.23d 4.78 ± 0.28b 0.417 ± 0.017" 52 (9/17)"
a_d
Means within a variable with no common superscript differ significantly (P < 0.05). Data are means ± SEM. 2 RV:TV = right ventricle to total ventricle weight ratio. J
left ventricular CO and thus systemic arterial hypotension. Confirmation of these interpretations await measurements of CO during unilateral pulmonary artery occlusion. Acute tightening of the bronchial snare triggered a modest but significant pulmonary hypertension in anesthetized male broilers, whereas tightening the pulmonary artery snare caused a significantly greater degree of pulmonary hypertension in the same birds (Figure 8A). Experiment 2 was conducted to determine whether these differences in the acute pulmonary hypertensive responses would be maintained throughout a more prolonged period of unilateral bronchus or pulmonary artery obstruction. Indeed, clamping one pulmonary artery for 10 to 12 d caused a large increase in the RV:TV ratio, a high incidence of ascites, and a profound reduction in growth, all of which are symptomatic of the extreme challenge presented to these broilers by the pulmonary artery clamp model (Wideman and Kirby, 1995b, 1996). In contrast, clamping one extra-pulmonary primary bronchus for the same period of time led to more moderate reductions in growth rate and increases in RV:TV ratios, reflecting a lower increment in PAP triggered by the chronic bronchus clamp model than for the pulmonary artery clamp model. These results demonstrate that the differences observed during acute physiological studies are relevant to the performance of growing broilers, and further confirm the direct correlation between pulmonary arterial pressures and RV:TV ratios as reported elsewhere (Burton and Smith, 1967; Burton et al, 1968). It is curious that acutely tightening a bronchial snare for more than 4 min caused severe arterial acidosis followed by death during pilot studies preceding Experiment 1, whereas mortality was nonexistent when one bronchus was chronically clamped in Experiment 2. The physiological explanation for this key observation remains to be determined. The phenomenon may reflect the comparatively greater degree of surgical insult necessary to conduct Experiment 1 in contrast to the very brief period during which the thoracic inlet was
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with simple parabronchial hypoventilation, reflecting an inability of broilers to sufficiently elevate the P02 and depress the Pco2 0 l parabronchial gas to the extent necessary to prevent systemic arterial hypoxemia and hypercapnia. Normobaric and hypobaric hypoxia consistently trigger systemic arterial hypotension that coincides with the onset of hypoxemia and pulmonary arterial hypertension in chickens (Burton et al, 1968; Besch and Kadono, 1978; Peacock et al, 1989; Owen et al, 1995). The systemic arterial hypotension has been attributed to a reduction in total peripheral resistance (peripheral vasodilation) caused by the direct dilatory effect of hypoxemia on systemic vascular smooth muscle. Systemic hypotension also may develop if hypoxemia reduces cardiac muscle contractility and thus CO (Butler, 1967; Besch and Kadono, 1978; Peacock et al, 1989; Wideman and Bottje, 1993; Owen et al, 1995). Alternatively, the pulmonary vascular constriction caused by hypoxia and hypoxemia may be sufficient to significantly impede venous return to the left ventricle, thereby reducing CO. In the present study, independently tightening the bronchial snare triggered a significantly greater systemic arterial hypoxemic response than did independently tightening the pulmonary artery snare (Figure 1); however, tightening the pulmonary artery snare caused a significantly greater increase in pulmonary arterial pressure and a greater decrease in systemic arterial pressure than did tightening the bronchial snare (Figures 7A, 8A). This dissociation of the vascular responses from the hypoxemic response suggests that the systemic hypotension during pulmonary hypertension cannot be attributed primarily to the direct effects of hypoxemia on cardiac or peripheral vascular muscle. Instead, these data support the hypothesis that the maximal "acute" increment in PAP attainable by the right ventricle is inadequate to propel the requisite CO through an effectively constricted pulmonary vasculature. The resulting congestive right-sided pooling of blood would, in turn, impede venous return to the left ventricle, causing a reduction in
OCCLUSION OF THE PULMONARY ARTERY AND PRIMARY BRONCHUS o p e n in E x p e r i m e n t 2. Alternatively, the age or b o d y m a s s of the broilers at the time of unilateral b r o n c h u s obstruction m a y d e t e r m i n e their ability to function w i t h o n e ventilated l u n g . T h e chronic b r o n c h u s c l a m p s w e r e placed in 16- to 18-d-old chicks, w h e r e a s the b r o n c h u s s n a r e t e c h n i q u e w a s c o n d u c t e d u s i n g 6- to 7-wk-old broilers w e i g h i n g 2.2 kg. The extreme d e g r e e of respiratory distress n o t e d d u r i n g acute b r o n c h i a l occlusion in the older broilers m a y reflect a m o r e t e n u o u s relationship b e t w e e n functional p u l m o n a r y capacity a n d metabolic d e m a n d as b o d y m a s s increases.
ACKNOWLEDGMENTS This research w a s s u p p o r t e d b y the A r k a n s a s P o u l t r y Federation E n d o w m e n t a n d b y grants from H u b b a r d F a r m s , Walpole, N H 03608.
Besch, E. L., and H. Kadono, 1978. Cardiopulmonary responses to acute hypoxia in domestic fowl. Pages 71-78 in: Respiratory Function in Birds Adult and Embryonic. J. Piiper, ed. Springer-Verlag, New York, NY. Burton, R. R., E. L. Besch, and A. H. Smith, 1968. Effect of chronic hypoxia on the pulmonary arterial blood pressure of the chicken. Am. J. Physiol. 214:1438-1442. Burton, R. R., and A. H. Smith, 1967. The effect of polycythemia and chronic hypoxia on heart mass of the chicken. J. Appl. Physiol. 22:782-785. Butler, P. J., 1967. The effect of progressive hypoxia on the respiratory and cardiovascular systems of the chicken. J. Physiol. 191:309-324. Fedde, M. R., 1986. Respiration. Pages 191-220 in: Avian Physiology. 4th ed. P. J. Sturkie, ed. Springer-Verlag, New York, NY. Julian, R. J., and S. M. Mirsalimi, 1992. Blood oxygen concentration of fast-growing and slow-growing broiler chickens, and chickens with ascites from right ventricular failure. Avian Dis. 36:730-732. Lutz, P. L., I. S. Longmuir, and K. Schmidt-Nielsen, 1974. Oxygen affinity of bird blood. Respir. Physiol. 20:325-330. National Research Council, 1984. Nutrient Requirements of Poultry. 8th rev. ed. National Academy Press, Washington, DC.
Owen, R. L., R. F. Wideman, and B. S. Cowen, 1995. Changes in pulmonary arterial and femoral arterial blood pressure upon acute exposure to hypobaric hypoxia in broiler chickens. Poultry Sci. 74:708-715. Peacock, A. J., C. Pickett, K. Morris, and J. T. Reeves, 1989. The relationship between rapid growth and pulmonary hemodynamics in the fast-growing broiler chicken. Am. Rev. Respir. Dis. 139:1524-1530. Peacock, A. J., C. Pickett, K. Morris, and J. T. Reeves, 1990. Spontaneous hypoxaemia and right ventricular hypertrophy in fast growing broiler chickens reared at sea level. Comp. Biochem. Physiol. 97A:537-541. Powell, F. L., R. H. Hastings, and R. W. Mazzone, 1985. Pulmonary vascular resistance during unilateral pulmonary artery occlusion in ducks. Am. J. Physiol. 249: R39-R43. Reeves, J. T., G. Ballam, S. Hofmeister, C. Pickett, K. Morris, and A. Peacock, 1991. Improved arterial oxygenation with feed restriction in rapidly growing broiler chickens. Comp. Biochem. Physiol. 99A:481^85. SAS Institute, 1982. SAS® User's Guide: Statistics. SAS Institute Inc., Cary, NC. Sturkie, P. D., 1986. Heart and circulation: anatomy, hemodynamics, blood pressure, blood flow. Pages 130-166 in: Avian Physiology. 4th ed. P. J. Sturkie, ed. SpringerVerlag, New York, NY. Wideman, R. F., and W. G. Bottje, 1993. Current understanding of the ascites syndrome and future research directions. Pages 1-20 in: Nutrition and Technical Symposium Proceedings. Novus International, Inc., St. Louis, MO. Wideman, R. F., and E. G. Buss, 1985. Arterial blood gas, pH, and bicarbonate values in laying hens selected for thick or thin eggshell production. Poultry Sci. 64:1015-1019. Wideman, R. F., and Y. K. Kirby, 1995a. Evidence of a ventilation-perfusion mismatch during acute unilateral pulmonary artery occlusion in broilers. Poultry Sci. 74: 1209-1217. Wideman, R. F., and Y. K. Kirby, 1995b. A pulmonary artery clamp model for inducing pulmonary hypertension syndrome (ascites) in broilers. Poultry Sci. 74:805-812. Wideman, R. F., Jr., and Y. K. Kirby, 1996. Electrocardiographic evaluation of broilers during the onset of pulmonary hypertension initiated by unilateral pulmonary artery occlusion. Poultry Sci 75:407-416.
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REFERENCES
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