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Vagal influences on respiratory mechanics, pressures, and control in rats
Respiration Physiology (1988) 73, 43-54 Elsevier
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RSP 01414
Vagal influences on respiratory mechanics, pressures, and control in rats Marina P.R. Caldeira, Paulo H . N . Saldiva and Walter A. Zin Laboratdrio de PoluigSo Atmosf~rica Experimental and lnstituto do Cora~8o - Faculdade de Medicina da Universidade de SSo Paulo, SSo Paulo, Brazil and Instituto de Biofisica Carlos Chagas Filho - Universidade Federal do Rio de Janeiro, gab de Janeiro, Brazil (Accepted for publication 2 February 1988) Abstract. In eight spontaneously breathing anesthetized rats airflow, volume, and tracheal pressure were measured. The passive and active mechanical properties of the respiratory system, the shape of the tracheal occlusion pressure wave (P°tr), the decay ofinspiratory muscle pressure during expiration, and parameters related to the control of breathing were computed both before and after bilateral cervical vagotomy. Preand post-vagotomy values of passive elastance, resistance, and time constant were similar. Active mechanics disclosed an increase of elastance and a decrease in resistance and in the time constant after vagotomy. The time course of P°tr showed a downward concavity and was not modified by vagotomy in the range of control inspiratory times, whereas the shape of inspiratory muscle pressure decay during expiration was changed. The present data help to explain why after vagotomy the load-compensatory mechanisms are less effective.
Control of breathing; Driving pressure; Respiratory mechanics; Respiratory muscle; Vagus nerve
Although the effects of vagotomy on the behaviour of the respiratory system have been extensively studied, the recent introduction of new concepts about its mechanical and controlling aspects strongly suggests the need of further investigation on vagotomized animals. The aim of this work is to study the consequences of bilateral cervical vagotomy on: (a) the active mechanical properties of the respiratory system; (b)the relationships between active and passive mechanical parameters; (c) the shape of the driving pressure of the respiratory system; (d)the decay of inspiratory muscle pressure during expiration; and (e) some parameters related to the timing of breathing. In addition, based on the present results some of the phenomena involved in the weaker loadcompensatory response of vagotomized animals were demonstrated.
Correspondence address: Dr. Paulo H.N. Saldiva, Faculdade de Medicina da USP, Laborat6rio de Polui~o Atmosf6rica Experimental, Av. Dr. Arnaldo, 455, 01246 - Sgto Paulo - SP, Brazil. 0034-5687/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
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M.P.R. CALDEIRA et al.
Material and methods
The experiments were performed on eight spontaneously breathing 3-month-old male Wistar albino non-Specific Pathogen Free rats (170-189 g, mean weight: 180.5 g), anesthetized with pentobarbital sodium (30 mg/kg i.p.). This dose was sufficient to suppress the corneal reflex throughout the experiment, which lasted about 1 h. With the animals resting in the supine position a snugly fitting tracheal cannula (I.D. = 1.5 mm) was inserted into the trachea. It was connected to a pneumotachograph for the measurement of airflow (~') and, by electronic integration, of changes in lung volume (V) (on the pen-recorder, see below). The flow resistance of the equipment, constant up to flow rates of 26 ml. sec- 1, was 0.109 cm H 2 0 . m l - 1. sec. Because abrupt changes in diameter were not present in our circuit, errors of measurement of flow resistance were avoided. The equipment dead space (tracheal cannula included) amounted to 0.32 ml.Tracheal (airway opening) pressure (Ptr) was measured with a Sanborn 270 differential pressure transducer. For controlling purposes esophageal pressure (Pes) was measured with a 30-cm-long water-filled catheter (PE-240) with side holes at the tip, connected to a Beckman 4-327-0129 pressure transducer. The catheter was passed into the esophagus. Its proper positioning was assessed using the 'occlusion test' (Baydur et al., 1982). The vagus nerves were isolated in the mid-cervical region and warmed mineral oil was poured around them. All the manoeuvres described below were first performed with intact vagi and repeated 20 min after bilateral vagotomy. All variables were registered on a Beckman 511A pen-recorder. Flow, Ptr, and Pes signals were also fed through an 8-bit analog-to-digital converter into a microcomputer. The sampling rate was 62.5 Hz. Volume was obtained by digital integration of the V signal. The accuracy of the digital calculations was tested by comparing the computer results with those obtained from the recorder tracings in two rats. As the results were similar, the data presented herein were provided by digital processing. All rats exhibited an end-expiratory pause and hence their lung volume at end expiration presumably represented the elastic equilibrium volume of the respiratory system (Vr). Passive mechanical properties of the respiratory system. These were measured according to the 'single-breath method' described by Zin et al. (1982a). Briefly, the airways were occluded at the end of tidal inspirations (V-r) and shortly afterwards the animals relaxed their respiratory muscles. This was demonstrated by the presence of a plateau on the tracheal pressure tracing. The passive elastance of the total respiratory system (Ers) was computed by dividing the tracheal pressure obtained after the relaxation of the respiratory muscles (representing the elastic recoil pressure of the respiratory system, Pel,rs) by VT. The occlusions were then released, and the rats expired freely (relaxed expiration). In all instances, after the first moments of the relaxed expirations, a linear relationship between V and "~ was obtained over most of the expired volume, allowing determination
MECHANICS AND CONTROL OF BREATHING IN RATS
45
of the time constant of the passive respiratory system ('ors). The corresponding passive respiratory system resistance (Rrs) was then calculated by multiplying Ers by zrs. The linear equipment flow resistance was taken into account during the calculations, and the data will be presented as intrinsic values of Rrs and zrs.
Active mechanical properties of the respiratory system. These were determined according to Zin et aL (1982b). During spontaneous breathing the airways were occluded at end-expiratory lung volume (Vr) for one breath to obtain the tracheal occlusion pressure (P°tr), which represents the driving pressure of the system. The active elastance and active resistance of the respiratory system (E'rs and R'rs, respectively) were computed according to the equation: -P°tr(t)/V(t)--- R'rs + E ' r s . V(t)/~'(t)
(1)
This is a linear function (Y = a + bX), where the intercept on the Y-axis is R'rs and the slope represents E'rs. As for the passive manoeuvre the equipment flow resistance was subtracted in order to obtain intrinsic R'rs. z'rs was computed by dividing R'rs by E'rs. P°tr(t) was measured during the occluded inspiratory effort and "v'(t) and V(t) were obtained on the immediately preceding unoccluded inspiration.
Shape of the tracheal occlusion pressure wave. According to previous studies on different species (Marsh et al., 1981; Zin et al., 1982b, 1983, 1986), the P°tr(t) wave can be described as a power function of time: - P°tr(t) = a. t b
(2)
where t is time in sec from the onset of inspiration, b is a dimensionless index of the shape of the curve, and a is the extrapolated pressure at 1 sec after the start of the breath, an index of the intensity of the neuromuscular drive. The same approach was used in the present study.
Decay of inspiratory muscle pressure during expiration. The antagonistic pressure exerted by the inspiratory muscles during spontaneous expiration (Pmusi) is given by (Zin et al., 1982a): Pmusi(t ) = Ers. V(t) - Rrs. 9 ( 0
(3)
By using eq. (3) the time course of Pmus~ was computed with V and V values obtained at 0.016 sec intervals after the onset of spontaneous expirations and Ers and Rrs values pertaining to each rat obtained both before and after vagotomy.
Breathing pattern. In addition to the variables mentioned above, VT, respiratory frequency (f), minute ventilation ('¢E), and inspiratory (TI), expiratory (TE), and total
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M.P.R. CALDEIRA et al.
respiratory cycle duration (TT) were determined from the V signal. From these, the duty cycle (TI/TT) and the mean inspiratory flow (VT/TI) were computed. The time from the onset till the peak of P°tr (T°I) was also measured.
Statistical analysis.
All comparisons were performed using Student's paired t-test with a significance level of 5 ~o-
TABLE 1 Respiratory parameters before and after vagotomy. Values are means ( -+ SD) of eight rats (6 determinations in each animal). Ers, Rrs, and zrs, respiratory system passive elastance, resistance, and time constant, respectively; E'rs, R'rs, and z'rs, respiratory system active elastance, resistance, and time constant, respectively; r, correlation coefficient; Pmusl, inspiratory muscle pressure during expiration; Prnusl,0, inspiratory muscle pressure at start of expiration; Ts0 and Tz, times required for Prnus t to decay to 50 and 0 yo of Pmus x,0, respectively; VT, tidal volume; TI, TE, TT, inspiratory, expiratory, and total respiratory cycle durations, respectively; f, respiratory frequency; '¢E, minute ventilation; VT/Tt, mean inspiratory flow; TI/TT, duty cycle; T°I, inspiratory duration of occluded effort. * Significantly different from pre-vagotomy value (P < 0.05). Variable
Before vagotomy
After vagotomy
~°/o Change
Passive
E r s ( c m H z O . m l 1) Rrs (cm H 2 0 . m 1 - 1 . s e c ) zrs (sec) r
4.51 _+ 0.229_+ 0.051 _+ 0.974 -
0.90 0.086 0.018 1
4.95 0.231 0.048 0.980
-+ _+ + -
1.03 0.113 0.024 1
5.44 -+ 0.306 -+ 0.057 -+ 0.930-
0.72 0.073 0.014 0.999
6.99 + 1.98" 0.248 + 0.090* 0.036 + 0.010" 0,985- 1
+9.8 +0.9 -7.7
Active
E'rs(cmH20.m1-1) R'rs (cm H20- m l - 1. sec) r'rs (sec) r
+28.5 18.9 - 36.8
P m us 1
Pmusl,0 (cm H20 ) Tso(sec ) Tzsec)
7.57 _+ 1.14 0.03 +_ 0.01 0.13 _+ 0.03
10.71 _+ 1.68" 0.07 + 0.02* 0.18 + 0.03*
+41.5 +133.3 +38.5
Ventilation
Vy(ml) Tt (sec) TE(sec) TT(sec) f (min- l) •~1E (ml. mi n i) VT/TI ( m l . s e c - 1 ) Tt/T'r T°I (sec)
1.56 0.24 0.56 0.80 78.8 121.1 6.9 0.30 0.34
_+ 0.17 -+ 0.05 _+ 0.16 + 0.19 + 16.2 _+24.1 _+ 1.6 _+ 0.05 _+ 0.09
2.10 0.41 0.83 1.24 52.3 112.4 5.5 0.34 0.43
-+ 0.43* -+ 0.09* -+ 0.31 -+ 0.35* + 13.7" _+43.1 _+ 2.2 _+ 0.08 _+ 0.07*
+34.6 +70.8 +48.2 +55.0 33.6 -7.2 -20.3 +13.3 +26.5
MECHANICS AND CONTROL OF BREATHING IN RATS
47
Results
Passive and active mechanical properties of the respiratory system.
Table 1 shows the values of Ers, Rrs, vrs, and the range o f correlation coefficients (r) obtained in the estimation of zrs both before and after vagotomy. Post-vagotomy Ers, Rrs and vrs presented non significant variations in relation to the corresponding pre-vagotomy values. The relationships between - P ° t r / ' ¢ and V/V were linear in all instances, the correlation coefficient (r) ranging between 0.930 and 1, as shown in table 1. Table 1 also provides the mean values ( + SD) of E ' r s and R ' r s , and the corresponding time constants (z'rs = R ' r s / E ' r s ) . Post-vagotomy E ' r s , R ' r s , and z'rs varied significantly in relation to the corresponding pre-vagotomy values.
Shape of tracheal occlusion pressure wave. Over the range of unoccluded TI's, the experimental data o f our rats closely fitted eq. (2). In the intact animals P°tr data points up to unoccluded TI were used, whereas after vagotomy the curves were fitted up to (a) the average unoccluded control TI pertaining to the same rats, and (b) the corresponding post-vagotomy TI. The average values ( + SD) of a and b, and the range of r in the eight rats are given in table 2. Average values of a obtained in vagotomized rats with pre-vagotomy and postvagotomy TI values did not vary in relation to the mean pre-vagotomy a. On the other hand, average post-vagotomy b values obtained with pre-vagotomy and post-vagotomy unoccluded TI values varied - 6 . 1 ~ o and - 2 2 . 4 ~ o , respectively, in relation to prevagotomy mean b, the latter difference being statistically significant.
Decay of inspiratory muscle pressure during expiration.
The time course of Pmusi was computed in the eight rats by using eq. (3). Figure 1 shows the relationships between Pmus~ and expiratory time in the eight rats both before (@) and after ( O ) vagotomy. Each point is the average of six determinations and the bars represent 1 SD. While the pre-vagotomy curves in general present a monotonic decay, the post-vagotomy ones show a sigmoidal profile. TABLE 2
Constants a and b in eq. (2). Values are means ( + SD) of eight rats. a, pressure developed 1 sec after onset of inspiration; b, dimensionless index of the shape of the curve; r, range of correlation coefficient. * Significantly different from pre-vagotomy value (P < 0.05). Before vagotomy
a (cm H20.sec -l) b r
39.81 _+ 13.59 0.791 _+ 0.124 0.977 - 1
After vagotomy Pre-vagotomy TI
Post-vagotomy TI
49.23 _+20.22 0.743 + 0.113 0.931 - 0.999
32.07 + 8.56 0.614 + 0.065* 0.912 - 0.990
48
M.P.R. CALDEIRA et al.
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TE (ms) Fig. 1. Inspiratory muscle pressure during spontaneous expiration (Pmusl) plotted against expiratory time (TE) before ( 0 ) and after vagotomy (O) in eight rats. Mean values + SD (bars) of six determinations in each rat.
By taking each curve individually, Pmus~,0 (Pmusx at TE = 0), Tso, and Tz (times required for Pmus~ to decay to 50, and 0Yo of Pmusx,0, respectively) were determined. The average values ( + SD) of six measurements in each case are depicted in table 1. The mean post-vagotomy values of Pmus~,0, Tso, and Tz were significantly higher than those obtained in intact rats. Breathing pattern. In all rats the breathing pattern was regular and varied little during the experiments. Table 1 lists the average values ( + SD) of the ventilatory variables in the eight rats studied. Among the variables only TE, "~E,VT/TI and TI/TT did not show a statistically significant difference between intact and vagotomized conditions. The average increase of TE was not statistically significant because one rat showed a reduction in its expiratory duration after vagotomy (as opposed to the remaining 7 animals).
Discussion Whether vagotomy induces changes in functional residual capacity is a disputed issue, with conflicting results reporting both increase (Lim et al., 1958) or no alterations (Colebatch and Halmagyi, 1963; D'Angelo and Agostoni, 1975; Marsh et al., 1981). Our results support the latter hypothesis, since vagotomy did not result in any regular change in end-expiratory Pes. It is therefore unlikely to have produced any physiologically significant change in lung volume.
MECHANICS AND CONTROL OF BREATHING IN RATS
49
The present mean values of Ers, Rrs, and zrs in intact rats were very similar to those found in the literature (for references see Leiter et al., 1986 and Saldiva et aL, 1987). Lung and respiratory system elastances, as well as pulmonary and airway resistances, were found to either increase or decrease, or remain unchanged after vagotomy in some mammalian species (Lim et al., 1958; Colebatch and Halmagyi, 1963; Karczewski and Widdicombe, 1969; Inoue etal., 1983; Mortola etal., 1984; Clement etaL, 1986). Unfortunately, no rats were studied under these circumstances. In the present report the mean values of Ers, Rrs, and zrs did not change significantly after vagotomy (table 1). During active inspiration R ' r s . V represents the pressure developed in overcoming Rrs and the additional resistive impedance (R"rs. ~') resulting from the force-velocity properties of the contracting inspiratory muscles. Similarly, E'rs represents the pressure used to overcome Ers and the additional elastic impedance (E"rs .V) owing to force-length properties of active muscles and to distortion of the respiratory system from its relaxed configuration. These additional pressure losses (R"rs. ~') = R ' r s . ~' - Rrs. ~' and E " r s . V = E ' r s . V - Ers. V) represent potentially available respiratory muscle force dissipated during the process of active muscular contraction unable to be used for generating external mechanical work. It follows that E'rs and R'rs are bound to be somewhat greater than the corresponding passive values. The reported percentage increase of E'rs compared to Ers measured in different species (no rats had been studied) varied from 28 to 90 ~o and for R'rs in relation to Rrs ranged from 17 to 45~o (for references see Baydur et aL, 1986; Zin et aL, 1986). In our rats, there were significant increases of 20.6 and 33.6~o for E'rs and R'rs, respectively, in relation to Ers and Rrs. As E'rs and R'rs presented changes of the same magnitude before vagotomy, it follows that z'rs did not differ appreciably from zrs. The active mechanical behaviour of our rats presented marked alterations after vagotomy. Post-vagotomy E'rs showed a significant increase over pre-vagotomy E'rs. The higher VT achieved after vagotomy would cause more distortion of the respiratory system from its resting configuration and would result from greater shortening of the active inspiratory muscles. Both factors would tend to increase E" rs. V. Post-vagotomy R'rs was significantly smaller than pre-vagotomy R'rs. It is interesting to recall that preand post-vagotomy Rrs (measured during the relaxed expirations) were virtually identical. Colebatch and Halmagyi (1963) also found in sheep that vagotomy reduced inspiratory resistance to airflow, the percentage decrease amounting to - 22 ~o, a value not very different from the present percentage change in post-vagotomy R'rs ( - 18.9~o). In this context, some authors have reported a loss in bronchial muscle tone after vagotomy (Colebatch and Halmagyi, 1963; Karczewski and Widdicombe, 1969; Hahn et al., 1976). Thus, during active inspiration the radial pull on the bronchial wall increases bronchial lumen. In vagotomized animals an augmented tidal volume increases elastic forces and tends to dilate airways to a greater extent. The postvagotomy percentage increases of E'rs compared to Ers and R'rs in relation to Rrs were not of the same magnitude (41.2 and 7.4~o, respectively), so that z'rs decreased after vagotomy (table 1). The shorter post-vagotomy active time constant results in a smaller
50
M.P.R. CALDEIRA et aL
phase shift between driving pressure and volume, thus reducing the dead volume (Milic-Emili and Zin, 1982) in vagotomized rats. Since after vagotomy the elastic component of impedance (E'rs) increased whereas the resistive one (R' rs) fell, the same approach of Marsh et al. (1981) was used to calculate an index of active impedance of the respiratory system (Z'rs). The results disclosed a significant decrease of Z ' r s (mean + SD values: 6.53 + 2.06 and 4.40 + 1.22 cm H 2 0 . m l - ~. sec, pre- and postvagotomy, respectively). The time course of P°tr over the range of unoccluded was studied in different anesthetized intact animals and found to closely fit eq. (2) (table 2), and the P°tr vs time wave presented a concavity towards the X-axis. In order to evaluate whether the time course of P°tr is influenced by vagotomy, the post-vagotomy curves were fitted up to (a) the average unoccluded control TI pertaining to the same rats, and (b)the corresponding post-vagotomy TI. In the former case a and b did not show statistically significant differences in relation to the corresponding pre-vagotomy values. It can be concluded that in the range of control TI the time course of P°tr is barely affected by vagal influences. In the latter case, post-vagotomy a was not significantly different from its control value. However, vagotomy caused a significant decrease of b ( - 22.4~o). Since the terminal segment of the P°tr wave is less steep than the initial part, the fitting over the whole curve is bound to produce smaller b values. In line with these results, D'Angelo (1982) studied the mean rate of rise of integrated diaphragmatic and intercostal electrical activity and obtained similar results. In addition, D'Angelo and Schieppati (1982) demonstrated that peak integrated electrical activities of the diaphragm and intercostal muscles of occluded breaths were not significantly different between intact and vagotomized animals. The decay of inspiratory muscle pressure during spontaneous expiration was calculated according to eq. (3). A major advantage of this approach is that it allows the quantification of Pmus~ from the onset of expiration up to the moment when the inspiratory muscles become completely relaxed. It can be seen in fig. 1 that in general pre-vagotomy Pmus~ vs TE curves ( 0 ) decayed monotonically, as previously described in cats (Zin et al., 1982b) and pigs (Clement et al., 1986), whereas after vagotomy ((3) they presented a sigmoidal shape. The same finding was observed in pigs by Clement et al. (1986). Since post-vagotomy VT is greater than its pre-vagotomy value, it follows that Pmus~,0 found in vagotomized rats exceeded pre-vagotomy Pmus~,0. Since Pmus~,0 was VT-dependent, the relative decay of the time course of Pmus I was studied, and two indexes were measured: Tso, and Tz. Both proved to be significantly larger in vagotomized rats. In addition, the Tz/TE and Tso/TE ratios were compared before and after vagotomy. No statistically significant difference was found for the former, whereas post-vagotomy Tso/TE was significantly greater than before vagotomy. Hence, it seems that Tz/TE is fixed and not sensitive to the afferent vagal information from the lung. The modification of the shape of the decay of the inspiratory muscle pressure during expiration may result from a different pattern of neuronal firing to the respiratory muscles consequent either to the lack of vagal activity or to the initial higher lung volume. In this respect, it was discussed above that the shape of the P°tr wave
MECHANICS AND CONTROLOF BREATHINGIN RATS
51
changes towards end inspiration in vagotomized rats. In addition, Clement et al. (1986) suggested that the increased chest wall distortion (due to augmented VT) would be the mechanical factor responsible for the change in the decay of Pmusi. Based only on the available data, it is not possible to exclude any of these possibilities and further investigation is needed. In all rats the pattern of breathing was regular both before and after vagotomy. The inspiratory duration is set by the bulbopontine pacemaker under the influence of volume-dependent vagal feedback mechanisms, comprising both phasic (Clark and von Euler, 1972) and tonic (Bartoli et al., 1973) afferent impulses. Phasic vagal inspiratory-inhibitory activity depends on phasic lung inflation and helps to end inspiration by increasing inspiratory-inhibitory activity generated within the respiratory pacemaker itself (Clark and von Euler, 1972). On the other hand, tonic vagal feedback is independent of acute changes in lung volume, but such afferent information also leads to termination of inspiration. The determinants of TE are less clear. In pentobarbitalanesthetized cats, TE depends on the preceding TI (Clark and von Euler, 1972). In anesthetized dogs given chloralose after halothane-nitrous oxide and thiopental (Bartoli et al., 1975) TE is affected by tonic vagal afferents but is independent of the preceding Tl. The present results indicate that after vagotomy there was a significant increase in TI and a non-significant rise in TE. In the present study one rat behaved differentlyfrom the remaining animals, decreasing TE after vagotomy. When this rat was excluded from the calculations of TI and TE, the ratio TE/TI averaged 2.3 before and 2.1 after vagotomy, values which are not statistically different. Thus, the present results support the notion that the bulbopontine pacemaker sets the relationship between TE and TI independently of vagal afferent information. In accordance with Marsh et al. (1981), we also observed that T°I was very close to TI after vagotomy. However, there is a controversy concerning T°I. Marsh et al. (1981) in dogs anesthetized with halothane or enflurane found that T°I tended to increase after bilateral vagotomy in both groups of animals, suggesting that tonic vagal afferents inhibited inspiration. This suggestion agrees with D'Angelo and Agostoni (1975), who reported a significant increase in T°I in vagotomized rabbits, cats, and dogs anesthetized with a combination of urethane and pentobarbital sodium. These are in contrast with the findings of Bartoli e t a L (1975) in dogs anesthetized with a combination of chloralose, halothane-nitrous oxide, and thiopental in sequence, and those of D'Angelo (1978) in rabbits anesthetized with the same mixture used by D'Angelo and Agostoni (1975). In the present investigation vagotomy increased the mean T°I. These discrepant findings may reflect differences in species, in anesthetic agents, or in the depth of anesthesia achieved, or in combination of these, the actual reason remaining to be elucidated. Finally, it has been shown that vagotomized animals present a weaker load-compensatory response to imposed mechanical loads (Zechman et al., 1976). Based on the present results, some of the phenomena involved in this finding (Milic-Emili and Zin, 1986) can be elucidated. Firstly, the smaller Z'rs. Secondly, the increased TI improves the response to added resistive loads (AR) but by contrast, the compensation to added
52
M.P.R. CALDEIRA et al.
elastic l o a d s ( A E ) is i m p a i r e d . T h i r d l y , t h e fall in b (eq. (2)) w o u l d i m p r o v e t h e r e s p o n s e t o A R b u t d e c r e a s e s t h e ability o f t h e s y s t e m t o r e a c t a g a i n s t AE. F o u r t h l y , t h e d e c r e a s e in a (eq. (2)) w o u l d c o m p r o m i s e t h e b e h a v i o u r o f t h e s y s t e m in t h e face o f b o t h k i n d s of load.
Acknowledgements.We are grateful to Prof. GyOrgy Mikl6s B0hm for his constant stimulation throughout this work. The authors are indebted to Andrea B0ddener and D6bora Fernandes Calheiros for their skillful help in data analysis, and to Ruberval da Silva for technical assistance. This work was supported by the following Brazilian agencies: FAPESP (grant no. 85/0340-4), Brazilian Council for Scientific and Technological Development (CNPq), and Financing for Studies and Projects (FINEP).
References Bartoli, A., B.A. Cross, A. Guz, A. Huszczuk and R. Jefferies (1975). The effect of varying tidal volume on the associated phrenic motoneurone output: studies of vagal and chemical feedback. Respir. Physiol. 25: 135-155. Baydur, A., P.K. Behrakis, W.A. Zin, M. Jaeger and J. Milic-Emili (1982). A simple method for assessing the validity of the esophageal balloon technique. Am. Rev. Respir. Dis. 126: 788-791. Baydur, A., C. S. H. Sasson and C.M. Stiles (1986). Active inspiratory impedance and load compensation: effects of duration of anesthesia. Anesth. Analg. 65: 1-8. Clark, F.J. and C. von Euler (1972). On the regulation of depth and rate of breathing. J. Physiol. (London) 222: 267-295. Clement, M.G., J.P. Mortola, M. Albertini and G. Aguggini (1986). Effects of vagotomy on respiratory mechanics in newborn and adult pigs. J. Appl. Physiol. 60: 1992-1999. Colebatch, H.J.H. and D.F. Halmagyi (1963). Effect of vagotomy and vagal stimulation on lung mechanics and circulation. J. AppL Physiol. 18: 881-887. D'Angelo, E. and E. Agostoni (1975). Tonic vagal influences on inspiratory duration. Respir. Physiol. 24: 287-302. D'Angelo, E. (1978). Central and direct vagal dependent control of expiratory duration in anaesthetized rabbits. Respir. Physiol. 34:103-119. D'Angelo, E. (1982). Inspiratory muscle activity during rebreathing in intact and vagotornized rabbits. Respir. Physiol. 47: 193-218. D'Angelo, E. and M. Schieppati (1982). Effects of thoracic dorsal rhizotomy or vagotomy on inspiratory muscle activity at various levels of chemical drive. Respir. Physiol. 50: 221-238. Hahn, H. L., P. D. Graf and J. A. Nadel (1976). Effect of vagal tone on airway diameters and on lung volume in anesthetized dogs. J. Appl. Physiol. 41: 581-589. Inoue, H., M. Ishii, T. Fuyuki, C. Inoue, N. Matsumoto, H. Sasaki and T. Takishima (1983). Sympathetic and parasympathetic nervous control of airway resistance in dog lungs. J. Appl. Physiol. 54: 1496-1504. Karczewski, W. and J.G. Widdicombe (1969). The effect of vagotomy, vagal cooling and efferent vagal stimulation on breathing and lung mechanics of rabbits. J. Physiol. (London) 201: 259-270. Leiter, J.C., J.P. Mortola and S.M. Tenney (1986). A comparative analysis of contractile characteristics of the diaphragm and of respiratory system mechanics. Respir. Physiol. 64: 267-276. Lim, T. P. K., V. C. Luft and F. S. Grodins (1958). Effects of cervical vagotomy on pulmonary ventilation and mechanics. J. Appl. Physiol. 13: 317-324. Marsh, H.M., K. Rehder and R.E. Hyatt (1981). Respiratory timing and depth of breathing in dogs anesthetized with halothane or enflurane. J. Appl. Physiol. 51: 19-25. Milic-Emili, J. and W.A. Zin (1982). Mechanical aspects of ventilatory control. Bull. Eur. Physiopathol. Respir. 18 (suppl. 4): 97-102.
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53
Milic-Emili, J. and W.A. Zin (1986). Breathing responses to imposed mechanical loads. In: Handbook of Physiology. Section 3. The Respiratory System. Vol. II. Control of Breathing, edited by N. S. Cherniack and J.G. Widdicombe. Washington, DC, American Physiological Society, pp. 751-769. Mortola, J.P., J.T. Fisher and G. Sant'Ambrogio (1984). Vagal control of the breathing pattern and respiratory mechanics in the adult and newborn rabbit. Pfliigers Arch. 401: 281-286. Saldiva, P.H.N., W.V. Cardoso, M.P.R. Caldeira and W.A. Zin (1987). Mechanics in rats by the end-inflation occlusion and single-breath methods. J. Appl. Physiol. 63:1711-1718. Zechman, F.W., D.T. Frazier and D.A. Lally (1976). Respiratory volume-time relationships during resistive loading in the cat. J. Appl. Physiol. 40: 177-183. Zin, W. A., L. D. Pengelly and J. Milic-Emili (1982a). Single-breath method for measurement of respiratory mechanics in anesthetized animals. J. Appl. Physiol. 52: 1266-1271. Zin, W. A., L. D. Pengelly and J. Milic-Emili (1982b). Active impedance of respiratory system in anesthetized cats. J. Appl. Physiol. 53: 149-157. Zin, W. A., D. Marlot, M. Bonora, N. M. Siafakas, H. Gautier and J. Milic-Emili (1983). Active and passive respiratory mechanics and control of breathing in kittens. J. Appl. Physiol. 55: 183-190. Zin, W.A., A. Boddener, P.R.M. Silva, T.M.P. Pinto and J. Milic-Emili (1986). Active and passive respiratory mechanics in anesthetized dogs. J. Appl. Physiol. 61: 1647-1655.
43
RSP 01414
Vagal influences on respiratory mechanics, pressures, and control in rats Marina P.R. Caldeira, Paulo H . N . Saldiva and Walter A. Zin Laboratdrio de PoluigSo Atmosf~rica Experimental and lnstituto do Cora~8o - Faculdade de Medicina da Universidade de SSo Paulo, SSo Paulo, Brazil and Instituto de Biofisica Carlos Chagas Filho - Universidade Federal do Rio de Janeiro, gab de Janeiro, Brazil (Accepted for publication 2 February 1988) Abstract. In eight spontaneously breathing anesthetized rats airflow, volume, and tracheal pressure were measured. The passive and active mechanical properties of the respiratory system, the shape of the tracheal occlusion pressure wave (P°tr), the decay ofinspiratory muscle pressure during expiration, and parameters related to the control of breathing were computed both before and after bilateral cervical vagotomy. Preand post-vagotomy values of passive elastance, resistance, and time constant were similar. Active mechanics disclosed an increase of elastance and a decrease in resistance and in the time constant after vagotomy. The time course of P°tr showed a downward concavity and was not modified by vagotomy in the range of control inspiratory times, whereas the shape of inspiratory muscle pressure decay during expiration was changed. The present data help to explain why after vagotomy the load-compensatory mechanisms are less effective.
Control of breathing; Driving pressure; Respiratory mechanics; Respiratory muscle; Vagus nerve
Although the effects of vagotomy on the behaviour of the respiratory system have been extensively studied, the recent introduction of new concepts about its mechanical and controlling aspects strongly suggests the need of further investigation on vagotomized animals. The aim of this work is to study the consequences of bilateral cervical vagotomy on: (a) the active mechanical properties of the respiratory system; (b)the relationships between active and passive mechanical parameters; (c) the shape of the driving pressure of the respiratory system; (d)the decay of inspiratory muscle pressure during expiration; and (e) some parameters related to the timing of breathing. In addition, based on the present results some of the phenomena involved in the weaker loadcompensatory response of vagotomized animals were demonstrated.
Correspondence address: Dr. Paulo H.N. Saldiva, Faculdade de Medicina da USP, Laborat6rio de Polui~o Atmosf6rica Experimental, Av. Dr. Arnaldo, 455, 01246 - Sgto Paulo - SP, Brazil. 0034-5687/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
44
M.P.R. CALDEIRA et al.
Material and methods
The experiments were performed on eight spontaneously breathing 3-month-old male Wistar albino non-Specific Pathogen Free rats (170-189 g, mean weight: 180.5 g), anesthetized with pentobarbital sodium (30 mg/kg i.p.). This dose was sufficient to suppress the corneal reflex throughout the experiment, which lasted about 1 h. With the animals resting in the supine position a snugly fitting tracheal cannula (I.D. = 1.5 mm) was inserted into the trachea. It was connected to a pneumotachograph for the measurement of airflow (~') and, by electronic integration, of changes in lung volume (V) (on the pen-recorder, see below). The flow resistance of the equipment, constant up to flow rates of 26 ml. sec- 1, was 0.109 cm H 2 0 . m l - 1. sec. Because abrupt changes in diameter were not present in our circuit, errors of measurement of flow resistance were avoided. The equipment dead space (tracheal cannula included) amounted to 0.32 ml.Tracheal (airway opening) pressure (Ptr) was measured with a Sanborn 270 differential pressure transducer. For controlling purposes esophageal pressure (Pes) was measured with a 30-cm-long water-filled catheter (PE-240) with side holes at the tip, connected to a Beckman 4-327-0129 pressure transducer. The catheter was passed into the esophagus. Its proper positioning was assessed using the 'occlusion test' (Baydur et al., 1982). The vagus nerves were isolated in the mid-cervical region and warmed mineral oil was poured around them. All the manoeuvres described below were first performed with intact vagi and repeated 20 min after bilateral vagotomy. All variables were registered on a Beckman 511A pen-recorder. Flow, Ptr, and Pes signals were also fed through an 8-bit analog-to-digital converter into a microcomputer. The sampling rate was 62.5 Hz. Volume was obtained by digital integration of the V signal. The accuracy of the digital calculations was tested by comparing the computer results with those obtained from the recorder tracings in two rats. As the results were similar, the data presented herein were provided by digital processing. All rats exhibited an end-expiratory pause and hence their lung volume at end expiration presumably represented the elastic equilibrium volume of the respiratory system (Vr). Passive mechanical properties of the respiratory system. These were measured according to the 'single-breath method' described by Zin et al. (1982a). Briefly, the airways were occluded at the end of tidal inspirations (V-r) and shortly afterwards the animals relaxed their respiratory muscles. This was demonstrated by the presence of a plateau on the tracheal pressure tracing. The passive elastance of the total respiratory system (Ers) was computed by dividing the tracheal pressure obtained after the relaxation of the respiratory muscles (representing the elastic recoil pressure of the respiratory system, Pel,rs) by VT. The occlusions were then released, and the rats expired freely (relaxed expiration). In all instances, after the first moments of the relaxed expirations, a linear relationship between V and "~ was obtained over most of the expired volume, allowing determination
MECHANICS AND CONTROL OF BREATHING IN RATS
45
of the time constant of the passive respiratory system ('ors). The corresponding passive respiratory system resistance (Rrs) was then calculated by multiplying Ers by zrs. The linear equipment flow resistance was taken into account during the calculations, and the data will be presented as intrinsic values of Rrs and zrs.
Active mechanical properties of the respiratory system. These were determined according to Zin et aL (1982b). During spontaneous breathing the airways were occluded at end-expiratory lung volume (Vr) for one breath to obtain the tracheal occlusion pressure (P°tr), which represents the driving pressure of the system. The active elastance and active resistance of the respiratory system (E'rs and R'rs, respectively) were computed according to the equation: -P°tr(t)/V(t)--- R'rs + E ' r s . V(t)/~'(t)
(1)
This is a linear function (Y = a + bX), where the intercept on the Y-axis is R'rs and the slope represents E'rs. As for the passive manoeuvre the equipment flow resistance was subtracted in order to obtain intrinsic R'rs. z'rs was computed by dividing R'rs by E'rs. P°tr(t) was measured during the occluded inspiratory effort and "v'(t) and V(t) were obtained on the immediately preceding unoccluded inspiration.
Shape of the tracheal occlusion pressure wave. According to previous studies on different species (Marsh et al., 1981; Zin et al., 1982b, 1983, 1986), the P°tr(t) wave can be described as a power function of time: - P°tr(t) = a. t b
(2)
where t is time in sec from the onset of inspiration, b is a dimensionless index of the shape of the curve, and a is the extrapolated pressure at 1 sec after the start of the breath, an index of the intensity of the neuromuscular drive. The same approach was used in the present study.
Decay of inspiratory muscle pressure during expiration. The antagonistic pressure exerted by the inspiratory muscles during spontaneous expiration (Pmusi) is given by (Zin et al., 1982a): Pmusi(t ) = Ers. V(t) - Rrs. 9 ( 0
(3)
By using eq. (3) the time course of Pmus~ was computed with V and V values obtained at 0.016 sec intervals after the onset of spontaneous expirations and Ers and Rrs values pertaining to each rat obtained both before and after vagotomy.
Breathing pattern. In addition to the variables mentioned above, VT, respiratory frequency (f), minute ventilation ('¢E), and inspiratory (TI), expiratory (TE), and total
46
M.P.R. CALDEIRA et al.
respiratory cycle duration (TT) were determined from the V signal. From these, the duty cycle (TI/TT) and the mean inspiratory flow (VT/TI) were computed. The time from the onset till the peak of P°tr (T°I) was also measured.
Statistical analysis.
All comparisons were performed using Student's paired t-test with a significance level of 5 ~o-
TABLE 1 Respiratory parameters before and after vagotomy. Values are means ( -+ SD) of eight rats (6 determinations in each animal). Ers, Rrs, and zrs, respiratory system passive elastance, resistance, and time constant, respectively; E'rs, R'rs, and z'rs, respiratory system active elastance, resistance, and time constant, respectively; r, correlation coefficient; Pmusl, inspiratory muscle pressure during expiration; Prnusl,0, inspiratory muscle pressure at start of expiration; Ts0 and Tz, times required for Prnus t to decay to 50 and 0 yo of Pmus x,0, respectively; VT, tidal volume; TI, TE, TT, inspiratory, expiratory, and total respiratory cycle durations, respectively; f, respiratory frequency; '¢E, minute ventilation; VT/Tt, mean inspiratory flow; TI/TT, duty cycle; T°I, inspiratory duration of occluded effort. * Significantly different from pre-vagotomy value (P < 0.05). Variable
Before vagotomy
After vagotomy
~°/o Change
Passive
E r s ( c m H z O . m l 1) Rrs (cm H 2 0 . m 1 - 1 . s e c ) zrs (sec) r
4.51 _+ 0.229_+ 0.051 _+ 0.974 -
0.90 0.086 0.018 1
4.95 0.231 0.048 0.980
-+ _+ + -
1.03 0.113 0.024 1
5.44 -+ 0.306 -+ 0.057 -+ 0.930-
0.72 0.073 0.014 0.999
6.99 + 1.98" 0.248 + 0.090* 0.036 + 0.010" 0,985- 1
+9.8 +0.9 -7.7
Active
E'rs(cmH20.m1-1) R'rs (cm H20- m l - 1. sec) r'rs (sec) r
+28.5 18.9 - 36.8
P m us 1
Pmusl,0 (cm H20 ) Tso(sec ) Tzsec)
7.57 _+ 1.14 0.03 +_ 0.01 0.13 _+ 0.03
10.71 _+ 1.68" 0.07 + 0.02* 0.18 + 0.03*
+41.5 +133.3 +38.5
Ventilation
Vy(ml) Tt (sec) TE(sec) TT(sec) f (min- l) •~1E (ml. mi n i) VT/TI ( m l . s e c - 1 ) Tt/T'r T°I (sec)
1.56 0.24 0.56 0.80 78.8 121.1 6.9 0.30 0.34
_+ 0.17 -+ 0.05 _+ 0.16 + 0.19 + 16.2 _+24.1 _+ 1.6 _+ 0.05 _+ 0.09
2.10 0.41 0.83 1.24 52.3 112.4 5.5 0.34 0.43
-+ 0.43* -+ 0.09* -+ 0.31 -+ 0.35* + 13.7" _+43.1 _+ 2.2 _+ 0.08 _+ 0.07*
+34.6 +70.8 +48.2 +55.0 33.6 -7.2 -20.3 +13.3 +26.5
MECHANICS AND CONTROL OF BREATHING IN RATS
47
Results
Passive and active mechanical properties of the respiratory system.
Table 1 shows the values of Ers, Rrs, vrs, and the range o f correlation coefficients (r) obtained in the estimation of zrs both before and after vagotomy. Post-vagotomy Ers, Rrs and vrs presented non significant variations in relation to the corresponding pre-vagotomy values. The relationships between - P ° t r / ' ¢ and V/V were linear in all instances, the correlation coefficient (r) ranging between 0.930 and 1, as shown in table 1. Table 1 also provides the mean values ( + SD) of E ' r s and R ' r s , and the corresponding time constants (z'rs = R ' r s / E ' r s ) . Post-vagotomy E ' r s , R ' r s , and z'rs varied significantly in relation to the corresponding pre-vagotomy values.
Shape of tracheal occlusion pressure wave. Over the range of unoccluded TI's, the experimental data o f our rats closely fitted eq. (2). In the intact animals P°tr data points up to unoccluded TI were used, whereas after vagotomy the curves were fitted up to (a) the average unoccluded control TI pertaining to the same rats, and (b) the corresponding post-vagotomy TI. The average values ( + SD) of a and b, and the range of r in the eight rats are given in table 2. Average values of a obtained in vagotomized rats with pre-vagotomy and postvagotomy TI values did not vary in relation to the mean pre-vagotomy a. On the other hand, average post-vagotomy b values obtained with pre-vagotomy and post-vagotomy unoccluded TI values varied - 6 . 1 ~ o and - 2 2 . 4 ~ o , respectively, in relation to prevagotomy mean b, the latter difference being statistically significant.
Decay of inspiratory muscle pressure during expiration.
The time course of Pmusi was computed in the eight rats by using eq. (3). Figure 1 shows the relationships between Pmus~ and expiratory time in the eight rats both before (@) and after ( O ) vagotomy. Each point is the average of six determinations and the bars represent 1 SD. While the pre-vagotomy curves in general present a monotonic decay, the post-vagotomy ones show a sigmoidal profile. TABLE 2
Constants a and b in eq. (2). Values are means ( + SD) of eight rats. a, pressure developed 1 sec after onset of inspiration; b, dimensionless index of the shape of the curve; r, range of correlation coefficient. * Significantly different from pre-vagotomy value (P < 0.05). Before vagotomy
a (cm H20.sec -l) b r
39.81 _+ 13.59 0.791 _+ 0.124 0.977 - 1
After vagotomy Pre-vagotomy TI
Post-vagotomy TI
49.23 _+20.22 0.743 + 0.113 0.931 - 0.999
32.07 + 8.56 0.614 + 0.065* 0.912 - 0.990
48
M.P.R. CALDEIRA et al.
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I
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T
I
T
I
I
I
"
;
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TE (ms) Fig. 1. Inspiratory muscle pressure during spontaneous expiration (Pmusl) plotted against expiratory time (TE) before ( 0 ) and after vagotomy (O) in eight rats. Mean values + SD (bars) of six determinations in each rat.
By taking each curve individually, Pmus~,0 (Pmusx at TE = 0), Tso, and Tz (times required for Pmus~ to decay to 50, and 0Yo of Pmusx,0, respectively) were determined. The average values ( + SD) of six measurements in each case are depicted in table 1. The mean post-vagotomy values of Pmus~,0, Tso, and Tz were significantly higher than those obtained in intact rats. Breathing pattern. In all rats the breathing pattern was regular and varied little during the experiments. Table 1 lists the average values ( + SD) of the ventilatory variables in the eight rats studied. Among the variables only TE, "~E,VT/TI and TI/TT did not show a statistically significant difference between intact and vagotomized conditions. The average increase of TE was not statistically significant because one rat showed a reduction in its expiratory duration after vagotomy (as opposed to the remaining 7 animals).
Discussion Whether vagotomy induces changes in functional residual capacity is a disputed issue, with conflicting results reporting both increase (Lim et al., 1958) or no alterations (Colebatch and Halmagyi, 1963; D'Angelo and Agostoni, 1975; Marsh et al., 1981). Our results support the latter hypothesis, since vagotomy did not result in any regular change in end-expiratory Pes. It is therefore unlikely to have produced any physiologically significant change in lung volume.
MECHANICS AND CONTROL OF BREATHING IN RATS
49
The present mean values of Ers, Rrs, and zrs in intact rats were very similar to those found in the literature (for references see Leiter et al., 1986 and Saldiva et aL, 1987). Lung and respiratory system elastances, as well as pulmonary and airway resistances, were found to either increase or decrease, or remain unchanged after vagotomy in some mammalian species (Lim et al., 1958; Colebatch and Halmagyi, 1963; Karczewski and Widdicombe, 1969; Inoue etal., 1983; Mortola etal., 1984; Clement etaL, 1986). Unfortunately, no rats were studied under these circumstances. In the present report the mean values of Ers, Rrs, and zrs did not change significantly after vagotomy (table 1). During active inspiration R ' r s . V represents the pressure developed in overcoming Rrs and the additional resistive impedance (R"rs. ~') resulting from the force-velocity properties of the contracting inspiratory muscles. Similarly, E'rs represents the pressure used to overcome Ers and the additional elastic impedance (E"rs .V) owing to force-length properties of active muscles and to distortion of the respiratory system from its relaxed configuration. These additional pressure losses (R"rs. ~') = R ' r s . ~' - Rrs. ~' and E " r s . V = E ' r s . V - Ers. V) represent potentially available respiratory muscle force dissipated during the process of active muscular contraction unable to be used for generating external mechanical work. It follows that E'rs and R'rs are bound to be somewhat greater than the corresponding passive values. The reported percentage increase of E'rs compared to Ers measured in different species (no rats had been studied) varied from 28 to 90 ~o and for R'rs in relation to Rrs ranged from 17 to 45~o (for references see Baydur et aL, 1986; Zin et aL, 1986). In our rats, there were significant increases of 20.6 and 33.6~o for E'rs and R'rs, respectively, in relation to Ers and Rrs. As E'rs and R'rs presented changes of the same magnitude before vagotomy, it follows that z'rs did not differ appreciably from zrs. The active mechanical behaviour of our rats presented marked alterations after vagotomy. Post-vagotomy E'rs showed a significant increase over pre-vagotomy E'rs. The higher VT achieved after vagotomy would cause more distortion of the respiratory system from its resting configuration and would result from greater shortening of the active inspiratory muscles. Both factors would tend to increase E" rs. V. Post-vagotomy R'rs was significantly smaller than pre-vagotomy R'rs. It is interesting to recall that preand post-vagotomy Rrs (measured during the relaxed expirations) were virtually identical. Colebatch and Halmagyi (1963) also found in sheep that vagotomy reduced inspiratory resistance to airflow, the percentage decrease amounting to - 22 ~o, a value not very different from the present percentage change in post-vagotomy R'rs ( - 18.9~o). In this context, some authors have reported a loss in bronchial muscle tone after vagotomy (Colebatch and Halmagyi, 1963; Karczewski and Widdicombe, 1969; Hahn et al., 1976). Thus, during active inspiration the radial pull on the bronchial wall increases bronchial lumen. In vagotomized animals an augmented tidal volume increases elastic forces and tends to dilate airways to a greater extent. The postvagotomy percentage increases of E'rs compared to Ers and R'rs in relation to Rrs were not of the same magnitude (41.2 and 7.4~o, respectively), so that z'rs decreased after vagotomy (table 1). The shorter post-vagotomy active time constant results in a smaller
50
M.P.R. CALDEIRA et aL
phase shift between driving pressure and volume, thus reducing the dead volume (Milic-Emili and Zin, 1982) in vagotomized rats. Since after vagotomy the elastic component of impedance (E'rs) increased whereas the resistive one (R' rs) fell, the same approach of Marsh et al. (1981) was used to calculate an index of active impedance of the respiratory system (Z'rs). The results disclosed a significant decrease of Z ' r s (mean + SD values: 6.53 + 2.06 and 4.40 + 1.22 cm H 2 0 . m l - ~. sec, pre- and postvagotomy, respectively). The time course of P°tr over the range of unoccluded was studied in different anesthetized intact animals and found to closely fit eq. (2) (table 2), and the P°tr vs time wave presented a concavity towards the X-axis. In order to evaluate whether the time course of P°tr is influenced by vagotomy, the post-vagotomy curves were fitted up to (a) the average unoccluded control TI pertaining to the same rats, and (b)the corresponding post-vagotomy TI. In the former case a and b did not show statistically significant differences in relation to the corresponding pre-vagotomy values. It can be concluded that in the range of control TI the time course of P°tr is barely affected by vagal influences. In the latter case, post-vagotomy a was not significantly different from its control value. However, vagotomy caused a significant decrease of b ( - 22.4~o). Since the terminal segment of the P°tr wave is less steep than the initial part, the fitting over the whole curve is bound to produce smaller b values. In line with these results, D'Angelo (1982) studied the mean rate of rise of integrated diaphragmatic and intercostal electrical activity and obtained similar results. In addition, D'Angelo and Schieppati (1982) demonstrated that peak integrated electrical activities of the diaphragm and intercostal muscles of occluded breaths were not significantly different between intact and vagotomized animals. The decay of inspiratory muscle pressure during spontaneous expiration was calculated according to eq. (3). A major advantage of this approach is that it allows the quantification of Pmus~ from the onset of expiration up to the moment when the inspiratory muscles become completely relaxed. It can be seen in fig. 1 that in general pre-vagotomy Pmus~ vs TE curves ( 0 ) decayed monotonically, as previously described in cats (Zin et al., 1982b) and pigs (Clement et al., 1986), whereas after vagotomy ((3) they presented a sigmoidal shape. The same finding was observed in pigs by Clement et al. (1986). Since post-vagotomy VT is greater than its pre-vagotomy value, it follows that Pmus~,0 found in vagotomized rats exceeded pre-vagotomy Pmus~,0. Since Pmus~,0 was VT-dependent, the relative decay of the time course of Pmus I was studied, and two indexes were measured: Tso, and Tz. Both proved to be significantly larger in vagotomized rats. In addition, the Tz/TE and Tso/TE ratios were compared before and after vagotomy. No statistically significant difference was found for the former, whereas post-vagotomy Tso/TE was significantly greater than before vagotomy. Hence, it seems that Tz/TE is fixed and not sensitive to the afferent vagal information from the lung. The modification of the shape of the decay of the inspiratory muscle pressure during expiration may result from a different pattern of neuronal firing to the respiratory muscles consequent either to the lack of vagal activity or to the initial higher lung volume. In this respect, it was discussed above that the shape of the P°tr wave
MECHANICS AND CONTROLOF BREATHINGIN RATS
51
changes towards end inspiration in vagotomized rats. In addition, Clement et al. (1986) suggested that the increased chest wall distortion (due to augmented VT) would be the mechanical factor responsible for the change in the decay of Pmusi. Based only on the available data, it is not possible to exclude any of these possibilities and further investigation is needed. In all rats the pattern of breathing was regular both before and after vagotomy. The inspiratory duration is set by the bulbopontine pacemaker under the influence of volume-dependent vagal feedback mechanisms, comprising both phasic (Clark and von Euler, 1972) and tonic (Bartoli et al., 1973) afferent impulses. Phasic vagal inspiratory-inhibitory activity depends on phasic lung inflation and helps to end inspiration by increasing inspiratory-inhibitory activity generated within the respiratory pacemaker itself (Clark and von Euler, 1972). On the other hand, tonic vagal feedback is independent of acute changes in lung volume, but such afferent information also leads to termination of inspiration. The determinants of TE are less clear. In pentobarbitalanesthetized cats, TE depends on the preceding TI (Clark and von Euler, 1972). In anesthetized dogs given chloralose after halothane-nitrous oxide and thiopental (Bartoli et al., 1975) TE is affected by tonic vagal afferents but is independent of the preceding Tl. The present results indicate that after vagotomy there was a significant increase in TI and a non-significant rise in TE. In the present study one rat behaved differentlyfrom the remaining animals, decreasing TE after vagotomy. When this rat was excluded from the calculations of TI and TE, the ratio TE/TI averaged 2.3 before and 2.1 after vagotomy, values which are not statistically different. Thus, the present results support the notion that the bulbopontine pacemaker sets the relationship between TE and TI independently of vagal afferent information. In accordance with Marsh et al. (1981), we also observed that T°I was very close to TI after vagotomy. However, there is a controversy concerning T°I. Marsh et al. (1981) in dogs anesthetized with halothane or enflurane found that T°I tended to increase after bilateral vagotomy in both groups of animals, suggesting that tonic vagal afferents inhibited inspiration. This suggestion agrees with D'Angelo and Agostoni (1975), who reported a significant increase in T°I in vagotomized rabbits, cats, and dogs anesthetized with a combination of urethane and pentobarbital sodium. These are in contrast with the findings of Bartoli e t a L (1975) in dogs anesthetized with a combination of chloralose, halothane-nitrous oxide, and thiopental in sequence, and those of D'Angelo (1978) in rabbits anesthetized with the same mixture used by D'Angelo and Agostoni (1975). In the present investigation vagotomy increased the mean T°I. These discrepant findings may reflect differences in species, in anesthetic agents, or in the depth of anesthesia achieved, or in combination of these, the actual reason remaining to be elucidated. Finally, it has been shown that vagotomized animals present a weaker load-compensatory response to imposed mechanical loads (Zechman et al., 1976). Based on the present results, some of the phenomena involved in this finding (Milic-Emili and Zin, 1986) can be elucidated. Firstly, the smaller Z'rs. Secondly, the increased TI improves the response to added resistive loads (AR) but by contrast, the compensation to added
52
M.P.R. CALDEIRA et al.
elastic l o a d s ( A E ) is i m p a i r e d . T h i r d l y , t h e fall in b (eq. (2)) w o u l d i m p r o v e t h e r e s p o n s e t o A R b u t d e c r e a s e s t h e ability o f t h e s y s t e m t o r e a c t a g a i n s t AE. F o u r t h l y , t h e d e c r e a s e in a (eq. (2)) w o u l d c o m p r o m i s e t h e b e h a v i o u r o f t h e s y s t e m in t h e face o f b o t h k i n d s of load.
Acknowledgements.We are grateful to Prof. GyOrgy Mikl6s B0hm for his constant stimulation throughout this work. The authors are indebted to Andrea B0ddener and D6bora Fernandes Calheiros for their skillful help in data analysis, and to Ruberval da Silva for technical assistance. This work was supported by the following Brazilian agencies: FAPESP (grant no. 85/0340-4), Brazilian Council for Scientific and Technological Development (CNPq), and Financing for Studies and Projects (FINEP).
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