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The cardiovascular dose–response effects of isoflurane alone and combined with butorphanol in the green iguana (Iguana iguana)
Veterinary Anaesthesia and Analgesia, 2004, 31, 64^72
The cardiovascular dose^response effects of isoflurane alone and combined with butorphanol in the green iguana (Iguana iguana) Craig AE Mosley
DVM, MSc, Diplomate ACVA,
Doris Dyson
DVM, DVSc, Diplomate ACVA
& Dale A Smithy,
DVM, DVSc
Departments of Clinical Studies and yPathobiology, University of Guelph, Guelph, Ontario, Canada
Correspondence: CAE Mosley, Veterinary Teaching Hospital, University of Guelph, Guelph, Ontario, Canada N1G 2W1. E-mail: cmosley@ uoguelph.ca
Abstract Objective To assess the cardiovascular e¡ects (arterial blood pressure, heart rate, and metabolic acid^ base status) of three doses (MAC multiples) of iso£urane alone and combined with butorphanol in the green iguana (Iguana iguana). Study design Prospective randomized double-blind, two-period cross-over trial. Animals Six mature healthy green iguanas (Iguana iguana). Methods The iguanas received each of two treatments, saline 0.1 mL kg 1 (SAL) and butorphanol 1.0 mg kg 1 (BUT) during iso£urane anesthesia. Treatments were separated by at least 1 week. The iguanas were exposed to each of the three minimum alveolar concentration (MAC) multiples (1.0, 1.5, and 2.0) in random order. Anesthesia was induced with iso£urane and maintained using controlled ventilation. Instrumentation included use of an ECG, airway gas monitor, cloacal thermometer, esophageal pulse oximeter, and the placement of a femoral arterial catheter. Body temperature was stabilized and maintained at 32 8C. The treatment was administered, and the animals were equilibrated for 20 minutes at each MAC multiple. At each concentration, the heart rate, blood pressure (systolic, mean, diastolic), endtidal CO 2, and SpO2 were measured. At 1.0 and 2.0 MAC, simultaneous blood samples were drawn from the tail vein/artery complex and femoral catheter for 64
blood gas analysis. Data were analyzed using a twoway analysis of variance for repeated measures looking for di¡erences between treatments and among MAC multiples. Results There were no signi¢cant di¡erences in any of the cardiovascular variables between the treatments. Signi¢cant di¡erences among iso£urane MAC multiples were observed for HR, mean, diastolic, and systolic blood pressures. Blood pressure and heart rate decreased with an increasing dose of anesthetic. There were no signi¢cant di¡erences between treatments or MAC multiples for any of the blood gas variables. The blood pH, PCO2, HCO3 , and hemoglobin saturation di¡ered signi¢cantly between sites. Pulse oximetry values measured from the carotid complex did not correlate with and were signi¢cantly di¡erent from the calculated hemoglobin saturation values determined using the gas analyzer. Conclusion and clinical relevance Cardiovascular depression associated with iso£urane anesthesia in the green iguana is dose dependent. The degree of cardiovascular depression was not signi¢cantly different when iso£urane was combined with butorphanol. This ¢nding suggests that the pre-emptive or intraoperative use of butorphanol is unlikely to be detrimental to cardiovascular function. Butorphanol may be a useful anesthetic adjunct to iso£urane anesthesia in the green iguana. Keywords anesthesia, butorphanol, green iguana, Iguana iguana, iso£urane, lizard.
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
Introduction Veterinarians are reliant upon the frequent and safe administration of anesthetics for even minor procedures in reptiles because of their size, disposition, and unique anatomy (e.g. lack of access in turtles and tortoises when they retract into their shells). However, there is a paucity of objective scienti¢c information regarding reptile anesthesia that is acknowledged by leading authors in the ¢eld of reptilian medicine and surgery (Bennett 1995; Schumacher1996; Heard 2001). Muchof what isknown about reptile anesthesia is based on the clinical impressions of, and case-series presented by, experienced reptile veterinarians. There is poor understanding of the e¡ects of analgesics in reptiles, in part, because of the relatively few studies investigating their use in these animals (Hinsch & Gandal 1969; Kanui & Hole 1992). Despite this, butorphanol is the most common opioid used for analgesia in reptiles, based on a survey of reptile veterinarians in attendance at the 1998 North American Veterinary Conference (personal data). Our previous work with iso£urane (Mosley et al. 2003) has shown the minimum alveolar concentration (MAC) to be 2.1% in the green iguana (Iguana iguana), and that the addition of an analgesic (butorphanol at 1.0 mg kg 1) does not result in a statistically signi¢cant reduction in this MAC value. Although there was no statistically signi¢cant di¡erence in the MAC of iso£urane with and without butorphanol, there was an overall 17% reduction in MAC between the saline-treated and the butorphanol-treated animals. It was thus concluded that butorphanol may have some MAC-reducing ability in some individuals and may be useful as an adjunct to iso£urane anesthesia. However, butorphanol may cause unfavorable cardiovascular e¡ects when combined with iso£urane during anesthesia. The cardiovascular e¡ects of this combination need to be compared to those with iso£urane alone before its use can be widely recommended. The objectives of this study were to assess some of the cardiovascular and metabolic acid^base e¡ects of iso£urane alone and combined with butorphanol in the green iguana.
Materials and methods Six mature iguanas (Iguana iguana), three males and three females weighing 2.19 0.35 kg (mean SEM), were used. General physical examination, complete blood counts, and biochemical pro¢les # Association of Veterinary Anaesthetists, 2004, 31, 64^72
were used to assess the health of the animals. The animals were admitted for an acclimatization period of at least 2 days before experiments. They were assigned a number for positive identi¢cations during the experimental period (clearly marked on the dorsal thorax of each animal). The study was carried out with approval of the Animal Care Committee of the University of Guelph and in compliance with the guidelines of the Canadian Council of Animal Care. The animals were group-housed at the University, in a large room where an appropriate temperature gradient (26^35 8C) was provided and a consistent photoperiod (16 hours light and 8 hours dark) was maintained. The animals had access to several UVemitting lights and infrared spotlights placed to provide basking areas in the room. The animals were fed a diet of 80% dark leafy green vegetables and 20% other vegetables and fruits with a light dusting of a calcium and phosphorus supplement. The iguanas were randomly assigned an order to receive each of the two treatments, saline (SAL) and butorphanol (BUT). The treatments were administered concurrently with iso£urane anesthesia. All animals were then subjected to a second randomization within treatment to determine the order of exposure to three MAC multiples (1.0, 1.5, and 2.0). The experiment was designed as a prospective randomized double-blind cross-over trial. All experiments were completed between 0830 and 1730 hours to reduce the impact of natural circadian rhythms. On the day of experimentation, the animals were transported from the housing facility to the anesthesia laboratory in a secure carrier that prevented the animals from seeing their surroundings and minimized any excitement as a result of the short transport (100 m). The animals were then placed on a circulating water heating blanket and mask induced with iso£urane (Aerrane, Janssen, Toronto, Ontario, Canada) at a dial setting of 5% and an oxygen £ow of 1 L minute 1 delivered using a Bain breathing system. Iso£urane was administered using an out-ofcircuit, agent-speci¢c, temperature- and pressurecompensated precision vaporizer (Fortec vaporizer, Fraser Harlake, Orchard Park, NY, USA). When the iguanas were su¤ciently immobilized, they were intubated using an uncu¡ed endotracheal tube (Sheridan Catheter Corp., Argyle, NY, USA) of the appropriate size (2.0^4.0 mm). Intubation was facilitated with the use of one drop (1^2 mg) of lidocaine (Xylocaine 2%, Astra Pharma Inc., Mississauga, Ontario, Canada) on the glottis.The tube was checked for correct ¢t by in£ating the lungs to a pressure of 65
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
15 mm Hg and listening for leaks, and then was fastened securely in place. All animals were ventilated mechanically using a pressure-limited ventilator (Bird Corporation, Palm Springs, CA, USA) with a frequency of 4 breaths minute 1 and a tidal volume of 30 mL kg 1. The minute ventilation and tidal volume were based on preliminary work on breathing patterns in the green iguana (Perry 1989a). The volume of each breath was determined using a respirometer (Wright's Respirometer, Lifetronics, Downsview, Ontario, Canada). The animals were then prepared for experimentation. A three-lead ECG (Criticare 1100 Patient Monitor, Criticare Systems Inc., Waukesha, WI, USA) was used to monitor heart rate and rhythm using Lead II. The ECG leads were attached to stainless steel wires placed subcutaneously in the right and left axilla and left groin region to ensure adequate contact. A cloacal thermometer (YSI Tele-thermometer, Yellow Springs Instrument Co. Inc., Yellow Springs, OH, USA) was used to ensure that the body temperature was maintained at 32 8C. Additional warmth was provided by a forced warm air blanket (Bair Hugger Model 505, Augustine Medical Inc., Eden Prairie, MN, USA) used, as needed, to maintain a stable body temperature. The gas analyzer (Criticare 1100 Patient Monitor, Criticare Systems Inc.,Waukesha,WI, USA) was connected between the Bain breathing system and the endotracheal tube using a standard gas sampling line connector. A 3-French extension line was fed down the endotracheal tube to a level near the bifurcation of the trachea for gas sampling. The extension line was used to ensure accurate monitoring of airway gases. A pulse oximeter (VetOx, Sensor Device Inc, Milwaukee, WI, USA) with a rectal probe was placed in the esophagus against the carotid complex to monitor functional hemoglobin saturation (SpO2). A sterile percutaneous cut-down was then performed on the ventromedial surface of the hind leg to facilitate the placement of a 24-SWG catheter into the femoral artery (Insyte W, Becton Dickinson, Sandy, UT, USA).The opposite leg was used for the second experiment. The catheter was sutured to the surrounding tissues and capped with an injection port (PRN Adapter, Becton Dickinson Infusion Therapy Systems Inc., Sandy, UT, USA). A disposable transducer system (Model DT-36, Ohmeda Medical Devices Division Inc., Madison,WI, USA) for direct blood pressure monitoring was connected to the Criticare1100 patient monitor. This monitor was calibrated using the manufacturer's recommended calibration gas 66
and checked for blood pressure accuracy with a mercury manometer before each experiment. During the instrumentation period, the animals were maintained between1.0 and1.5 MAC. Once the body temperature was stable between 30 and 32 8C, butorphanol tartrate (Torbugesic, Fort Dodge Laboratories, Inc., Fort Dodge, IA, USA) at 1.0 mg kg 1 or a saline (0.9% saline) placebo at 0.1 mL kg 1, providing equal volumes, was given IM in the forearm. All the treatments were given during anesthesia at least one and no more than 2 hours prior to the initiation of testing. The animals were then equilibrated for 20 minutes, after the per cent exhaled iso£urane appeared stable, at each of the randomly ordered MAC multiples. Once equilibration was complete, three readings of heart rate, blood pressure (systolic, mean, diastolic), end-tidal CO 2 (PE0 CO 2), and SpO2 were taken. Additionally, simultaneous blood samples were drawn from the tail vein/ artery complex and the femoral arterial catheter. The temperature, tidal volume, frequency of respiration, SpO2, and PE0 CO 2 were recorded at the time of blood gas sampling. Air was removed from the blood gas syringe, and the sample was immediately analyzed using an automated blood gas analyzer (Model ABL 3 Radiometer Corp., Copenhagen, Denmark), which corrected the reported values for body temperature to 32 8C. Sampling required approximately 10^15 minutes, after which the next randomly determined iso£urane level was set. The experiment continued until all three MAC levels had been tested. Upon completion of the measurements, all iguanas were given a dose of butorphanol (1 mg kg 1 IM) as an analgesic for recovery. The catheter was then removed, the artery was ligated, and the subcutaneous tissues and skin were closed. The iso£urane was discontinued, and the animals were ventilated until they could be stimulated to move. At this point, the animals were extubated whether or not spontaneous ventilation had occurred, and observed until respiration returned. Recovery was completed in an enclosed quiet area in the group housing room. Each response variable was analyzed by use of a repeated-measures analysis of variance. Statistical analysis was performed using the general linear model (GLM) and mixed (MIXED) procedures of SAS (SAS Institute Inc, SAS/STAT Users Guide, Version 6, 4th Edition, Cary, NC,1989). The cardiovascular data were analyzed ¢rst using a general linear model technique to identify the signi¢cant random e¡ects to be included in the ¢nal mixed model analysis. # Association of Veterinary Anaesthetists, 2004, 31, 64^72
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
Signi¢cance for inclusion in the mixed model was taken as p < 0.25. The mixed model was then used to determine di¡erences between treatments and among doses. A p < 0.05 was considered signi¢cant. If a signi¢cant treatment by dose interaction was present, the di¡erence of least square means was used to determine where di¡erences existed among the doses but within treatment. If no signi¢cant treatment by dose interaction was present, main e¡ects were tested and least square means was used to determine where di¡erences existed. Values of p < 0.05 were considered signi¢cant. A similar analysis was used for analyzing the blood gas variables between treatments (butorphanol or saline), sites (arterial or venous) and doses (1.0 or 2.0 MAC). The data for each variable were ¢rst analyzed using a GLM procedure to determine the random e¡ects to be used in the ¢nal MIXED model. Inclusion in the MIXED model was done if p < 0.25. The MIXED procedure was then used to determine where signi¢cant di¡erences existed (p < 0.05). While ¢tting and
testing all statistical models, assumptions of analysis of variance were assessed via residual analysis using the univariate (UNIVARIATE) and plot (PLOT) procedures of SAS. The need for transformations was also evaluated. Pulse oximetry and calculated hemoglobin saturation values from the blood gases were compared using a paired t-test. Correlation between the two was assessed using the Pearson's coe¤cient of correlation (r).
Results There were no signi¢cant di¡erences in any of the cardiovascular variables between the treatment groups (Table 1). Signi¢cant di¡erences between iso£urane doses were observed for HR, mean, diastolic, and systolic BP. Blood pressure decreased consistently with an increasing dose of iso£urane. Mean heart rate decreased with an increasing dose of anesthetic. At 2.0 and 1.5 MAC, heart rates were
Table 1 Cardiovascular parameters of green iguanas (Iguana iguana) compared between treatments, butorphanol 1 mg kg (BUT) and saline 0.1 mL kg 1 (SAL) and dose (1.0 MAC,1.5 MAC, and 2.0 MAC) of iso£urane (n 6)
1
Variable
Treatment
1.0 MAC
1.5 MAC
2.0 MAC
Heart rate (beats minute 1)
SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT
58 10 52 7 43 7 39 6 29 4 27 5 35 6 33 5 19 10 (2.5 1.3) 19 9 (2.5 1.2) 74 20 90 6 7.46 0.18 7.30 0.17 34.9 14.9 (4.6 2.0) 48.9 17.4 (6.5 2.3) 63.3 19.2 (8.4 2.6) 50.5 7.7 (6.7 1.0) 25.0 4.8 24.2 3.3 0.1 5.7 4.6 6.1 95.2 3.4 90.0 4.0
50 3y 51 13y 31 6 37 7 20 3y 23 5y 25 4 29 6 18 8 (2.4 1.1) 20 12 (2.7 1.6) 65 12 69 18 7.46 0.17 7.30 0.15 36.2 14.5 (4.8 1.9) 49.6 19.4 (6.6 2.6) 52.5 17.0 (7.0 2.3) 45.6 10.5 (6.1 1.4) 25.8 3.9 24.4 3.7 0.7 5.5 4.6 4.8 92.6 4.1 89.4 1.7
51 4y 48 8y 28 5 33 11z 19 3y 21 7y 23 4 25 9z 18 10 (2.4 1.3) 17 10 (2.3 1.3) 76 15 73 26 7.45 0.19 7.34 0.21 36.5 15.8 (4.9 2.1) 46.3 21.5 (6.2 2.9) 54.2 26.7 (7.2 3.6) 41.7 12.8 (5.6 1.7) 24.9 4.1 24.4 2.9 0.3 5.5 3.4 6.3 82.2 15.9 84.1 8.1
Systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg) Mean blood pressure (mm Hg) End-tidal CO2 (mm Hg) (kPa) Functional Hb saturation (%) pHa PaCO2 (mm Hg) (kPa) PaO2 (mm Hg) (kPa) Arterial HCO3 (mmol L 1) Arterial ABE (mmol L 1) Arterial Hb saturation (%)
Significantly different from1.0 MAC within each treatment (treatment by dose interaction, groups not combined for final analysis) at p < 0.05. ySignificantly different from1.0 MAC, combined treatments (no treatment by dose interaction) at p < 0.05. zSignificantly different from1.5 MAC within each treatment (treatment by dose interaction, groups not combined for final analysis) at p < 0.05.
# Association of Veterinary Anaesthetists, 2004, 31, 64^72
67
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
Heart Rate (beats minute-1)
70
SAL & BUT
60 50 a
a
40 30 20 10 0
a, Significantly different from 1.0 MAC (p<0.05)
1
1.5
2
MAC Multiple (MAC) Figure 1 Heart rate in green iguanas (Iguana iguana) at each MAC multiple for both treatments combined (saline 0.1 mL kg 1, SAL and butorphanol 1 mg kg 1, BUT). There was no signi¢cant ( p < 0.05) treatment by dose interaction (n 6). aSigni¢cantly di¡erent from1.0 MAC ( p < 0.05).
signi¢cantly less than those observed at 1.0 MAC (p 0.0322). Heart rates at 1.5 and 2.0 MAC were not signi¢cantly di¡erent from each other (Fig. 1). The analysis of BP revealed a treatment by dose interaction for mean and systolic BP (Figs 2 & 3), but not diastolic BP (Fig. 4). In general, it appeared that blood pressures in the saline treatment group declined more between 1.0 and 1.5 MAC than between 1.5 and 2.0 MAC where it declined at a slower rate. This rate di¡erence may have accounted for our treatment by dose interaction. The blood pressures in the butorphanol group appeared to decrease more uniformly between 1.0 and 1.5 MAC, and 1.5 and 2.0 MAC. The diastolic blood pressure at 1.0 MAC was signi¢cantly 50
di¡erent (p 0.0001) from the diastolic pressures at 1.5 and 2.0 MAC (Fig. 4).The diastolic blood pressures at1.5 and 2.0 MAC were not di¡erent from each other. There was a signi¢cant dose e¡ect for both systolic and mean arterial blood pressure (p 0.0001). As there was a treatment by dose interaction, these differences are described within treatments rather than combined, as was done for diastolic blood pressure. Within the saline treatment group, the mean and systolic blood pressures at 1.5 and 2.0 MAC were signi¢cantly di¡erent from those at 1.0 MAC, but not from each other.Within the butorphanol group, the mean and systolic blood pressures at 2.0 MAC were signi¢cantly di¡erent from those at 1.0 and 1.5 MAC. The mean and systolic blood pressures at1.0 and1.5 MAC were not signi¢cantly di¡erent from each other in either treatment. There were no signi¢cant di¡erences between treatments or doses for any of the blood gas variables. Site e¡ects were determined bycombining both doses and treatments (Table 2). The blood pH, PCO2, HCO3 , and hemoglobin saturation all di¡ered significantly between sites. There were no di¡erences between sites for PO2 and actual base excess. Pulse oximetry values recorded from the carotid artery did not correlate (a Pearson's coe¤cient of 0.33) with the values determined with the blood gas analyzer from femoral arterial samples (Fig. 5). Using the paired t-test, pulse oximetry (89 1.7%) and hemoglobin saturation values calculated from the blood gas (73 3.6%) values were signi¢cantly different (p 0.0005).
SAL
Systolic Blood Pressure (mm Hg)
BUT 40 a,b 30 a
a
20
10
a, Significantly different from 1.0 MAC (p<0.05) b, Significantly different from 1.5 MAC (p<0.05)
0 1
1.5 MAC Multiple (MAC)
68
2
Figure 2 Systolic arterial blood pressure in green iguanas (Iguana iguana) at each MAC multiple for each treatment (saline 0.1 mL kg 1, SAL and butorphanol1 mg kg 1, BUT). A significant ( p < 0.05) treatment by dose interaction was present (n 6). aSigni¢cantly di¡erent from 1.0 MAC ( p < 0.05); bSigni¢cantly di¡erent from1.5 MAC ( p < 0.05).
# Association of Veterinary Anaesthetists, 2004, 31, 64^72
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
50
SAL BUT
Figure 3 Mean arterial blood pressure in green iguanas (Iguana iguana) at each MAC multiple for each treatment (saline 0.1 mL kg 1, SAL and butorphanol 1 mg kg 1, BUT). A signi¢cant (p < 0.05) treatment by dose interaction was present (n 6). aSigni¢cantly di¡erent from 1.0 MAC ( p < 0.05), b Signi¢cantly di¡erent from 1.5 MAC ( p < 0.05).
Mean Blood Pressure (mm Hg)
40
30
a,b
a
20
10
a, Significantly different from 1.0 MAC (p<0.05) b, Significantly different from 1.5 MAC (p<0.05)
0 1
Diastolic Blood Pressure (mm Hg)
SAL BUT 40
30 a a 20 a
a
10 a, Significantly different from 1.0 MAC (p<0.05)
0 1.5
1.5
2
MAC Multiple (MAC)
50
1
a
2
MAC Multiple (MAC) Figure 4 Diastolic arterial blood pressure in green iguanas (Iguana iguana) at each MAC multiple for each treatment (saline 0.1 mL kg 1, SAL and butorphanol 1 mg kg 1, BUT). There was no signi¢cant ( p < 0.05) treatment by dose interaction (n 6). aSigni¢cantly di¡erent from1.0 MAC ( p < 0.05).
Discussion The cardiovascular e¡ects of iso£urane in the green iguana share some similarity with those reported in humans (Cromwell et al. 1971), dogs (Ste¡ey & Howland 1977), cats (Ste¡ey & Howland 1977), and birds (Ludders et al. 1989; Ludders et al. 1990). As end-tidal iso£urane concentrations are increased from1.0 to 2.0 MAC, blood pressure decreases. In our # Association of Veterinary Anaesthetists, 2004, 31, 64^72
study of iguanas, the heart rate was signi¢cantly higher at 1.0 MAC than at 1.5 and 2.0 MAC. This is unlike humans (Stevens et al.1971), and dogs (Ste¡ey & Howland 1977), where the heart rate tends to increase with increasing depth. However, in cats (Ste¡ey & Howland 1977) and birds (Ludders et al. 1989,1990), heart rates remain relatively unchanged despite an increasing dose of iso£urane. The heart rate changes observed in humans and dogs may be mediated through the baroreceptor response to iso£urane-induced hypotension. An increased heart rate tends to maintain cardiac output despite a reduced stroke volume (Ste¡ey & Howland 1977). Unfortunately, in our studies, we were unable to assess cardiac output.The lack of an increase in heart rate in the green iguana may be the result of lacking, weak or easily obtunded baroreceptor re£exes. In snakes, a very strong barore£ex response has been demonstrated (Berger1987), and in the green iguana, tachycardia has been demonstrated in response to graded hemorrhage, which suggests the presence of a baroreceptor response (Hohnke1975). The decreasing heart rate that was observed in the green iguana in response to an increasing dose of iso£urane was unexpected. This may represent a species-speci¢c response. In our experiments, it is also possible that the higher HR observed at 1.0 MAC was a result of sympathetic drive in some animals which were at subanesthetic levels, as MAC is de¢ned as the dose required to prevent purposeful movement in response to a supramaximal stimulus in 50% of subjects. Increasing the percentage of inhalant in the 69
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
Blood gas variable
Arterial
Venous
pH PCO2 (mm Hg) (kPa) PO2 (mm Hg) (kPa) HCO3 (mmol L 1) ABE Hemoglobin saturation (%)
7.39 0.04 41.1 3.7 (5.5 0.5) 53.0 4.1 (7.1 0.5) 24.7 0.8 1.9 1.2 90.2 1.4
7.37 0.04 44.0 3.5 (5.9 0.5) 49.1 6.1 (6.5 0.8) 25.2 0.8 2.1 1.2 78.4 5.0
Table 2 Blood gas results compared between sites (femoral artery, tail vein) in green iguanas (Iguana iguana)
No significant treatment ordose effects (n 24). Significantly different from arterial (p < 0.05).
iguanas would then bring them into a better anesthetic plane and thus eliminate the sympathetic drive. Other evidence indicating that some reptiles were excessively light (movement, ventilatory e¡orts) were not observed during the study. However, surgical stimulus was not present at the time of these measurements. The blood pressures we obtained in the green iguana were low compared with those documented in birds and mammals. However, there are substantial experimental data demonstrating that many reptiles have blood pressures markedly lower than those observed in mammals (Farrell 1991). Unfortunately, we were unable to locate any complete references documenting the `normal awake' values in the green iguana, which would have given us greater insight into whether iso£urane resulted in a decrease
Pulse oximetry (%)
100
80
60
40
20 20
40 60 80 100 Calculated hemoglobin saturation (%)
Figure 5 Pulse oximetry and calculated hemoglobin saturation values for each paired blood sample collected from green iguanas (Iguana iguana) anesthetized twice, once with iso£urane and once with iso£urane combined with butorphanol. Samples were taken simultaneously from the femoral artery and tail vein at 1.0 and 2.0 MAC in six green iguanas (n 29). 70
in blood pressure. We were only able to locate one report describing measurement of arterial blood pressure from the femoral artery of the green iguana (Hohnke 1975). Unfortunately, in this work, pressure measurements are not explicitly described; instead, one must read the blood pressure o¡ the scales of traces from single animals. In two animals, the awake mean blood pressures taken from the graphs and trace were approximately 75 and 60 mm Hg. Additionally, these animals were not `free roaming' but rather restrained on an elevated platform. It is unclear whether these values represent true mean pressures in a`normal awake'animal or those of relatively stressed iguanas. The response of blood pressure to an increasing dose of iso£urane within the two treatments was not the same as evidenced by the treatment by dose interaction. At 1.0 MAC, the butorphanol group had a lower blood pressure than the saline group, but as the dose of iso£urane was increased, the butorphanol values did not decrease as dramatically as those of the saline group. Based on our sample size and small number of doses tested, drawing speci¢c conclusions would not be warranted. However, our data suggest that butorphanol combined with iso£urane has greater depressive e¡ects on blood pressure than iso£urane alone at low doses, but as the dose of iso£urane is increased, the di¡erence becomes negligible. One might speculate that the level of anesthetic produced at 1.0 MAC was deeper with butorphanol as a result of its tendency to reduce MAC in some iguanas (Mosley et al. 2003). The individuals that responded to butorphanol with a reduction in MAC may have had decreased sympathetic drive, and consequently lower blood pressure. However, there is no clear evidence of this based on our data. It is not surprising that functional hemoglobin saturation measured using pulse oximetry was signi¢cantly di¡erent from that determined by blood gas analysis. Pulse oximetry measures the hemoglo# Association of Veterinary Anaesthetists, 2004, 31, 64^72
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
bin absorbance and uses the ratio of oxyhemoglobin to deoxyhemoglobin to determine the per cent saturation of hemoglobin (Alexander et al.1989). The absorbance ratio is then compared with an internal standard curve that plots the ratio of the absorbances of the two hemoglobins against the hemoglobin saturation at a de¢ned temperature. There is recent evidence that suggests that pulse oximetry is a valid tool for hemoglobin saturation determination in the green iguana (Diethelm et al. 1998). Unfortunately, it is not clear from the abstract how the hemoglobin saturation for the `gold standard' comparison was estimated. In our study, we used a human blood gas analyzer that calculates hemoglobin saturation (SO2) using the measured pH, PCO2, PO2, and temperature. These equations are derived from human blood, and it is likely that they di¡er from those needed for the hemoglobin saturation determination of iguana blood. Therefore, we suspect that the SO2 results from our blood gas analysis may be inaccurate in the green iguana and cannot, with certainty, draw conclusions regarding the use of pulse oximetry or the use of a human blood gas analyzer. Validation of these techniques for use in the green iguana is necessary. Very few conclusions can be drawn from our blood gas values. Only pH, PCO 2, and PO2 were measured directly; all other values were calculated based on complex formulae derived from human blood. In addition, all the blood gases were corrected to the animal's actual body temperature,32 8C, by the blood gas machine. This correction is based on human blood algorithms and may not accurately re£ect temperature-related changes in green iguana blood. It was expected that arterial and venous sampling sites would be signi¢cantly di¡erent from one another. Interestingly, PO2 was not signi¢cantly di¡erent: arterial 53.0 4.1 mm Hg (7.1 0.5 kPa) and venous 49.1 6.1 mm Hg (6.5 0.8 kPa). This result suggests that, in the green iguana anesthetized with iso£urane, with or without butorphanol, there is very little di¡erence in arterial and venous PO2. Additionally, PO2 values in arterial blood were much lower than would be expected in a normal mammal breathing 100% oxygen. Di¡erences in PO2 values in arterial and venous blood are created by tissue extraction of O2. It would thus appear that, in the anesthetized green iguana, there is very little tissue extraction of O2. It is unclear to us whether the lower than expected PO2 and O2 tissue extraction are a re£ection of normal reptilian physiology or whether they represent a physiologic adaptation to one or a combination # Association of Veterinary Anaesthetists, 2004, 31, 64^72
of the following factors: (i) high inspired oxygen; (ii) iso£urane; (iii) the anesthetized state irrespective of the drugs involved; or (iv) positive pressure ventilation. There are studies in other reptile species that have demonstrated a relatively low rate of oxygen extraction (Perry 1989b). A ¢nal possibility is that our venous sample drawn from the tail was not truly venous. The ventral surface of the tail contains a major artery, vein, and lymphatic system, and it is possible that our samples contained some combination of the three £uids. The lower than expected arterial oxygen tension cannot be termed a true hypoxemia, as hypoxemia has not been clearly de¢ned in reptiles. Pragmatically speaking, hypoxemia should describe the oxygen level below which metabolic requirements, at rest, are inadequate. The term relative hypoxemia may be an acceptable term as applied to the oxygen values we observed in the green iguana. Based on the predicted partial pressures of oxygen in the lung and complete equilibration of all the blood passing to the systemic circulation from the pulmonary circulation, we would expect the partial pressures of oxygen in the blood to be nearly equal to that of the lung (West 2000). This was never the case in any of our animals, although we were able to more completely saturate the iguana blood by exposing it to 100% oxygen in a syringe. The reason for the relative hypoxemia can most easily be explained by shunt £ow, in particular, a right-to-left shunt that bypasses the pulmonary circulation. This type of shunting has been documented in numerous studies involving several reptile species (Heisler et al. 1983; Ishimatsu et al. 1988; Comeau & Hicks 1994). Other causes may be an increase in the di¡usion barrier, hypoventilation (this is likely to be insigni¢cant while inspiring 100% O2), or ventilation^perfusion inequality (West 1995). Unfortunately, our study was not designed to elucidate or explain more clearly the blood gas changes we observed. In our study we demonstrated cardiovascular depression in response to an increasing dose of iso£urane. The amount of cardiovascular depression was not signi¢cantly di¡erent when iso£urane was combined with butorphanol. This ¢nding suggests that the pre-emptive or intraoperative use of butorphanol is unlikely to be detrimental to cardiovascular function, even when MAC reduction does not occur.We found that pulse oximetry using an esophageal probe did not re£ect values taken simultaneously from the femoral artery and analyzed on a human blood gas analyzer. It remains unclear at 71
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
this time whether either methodology is useful as a monitoring device for hemoglobin saturation. Additional validation and respiratory physiologic studies are needed to more clearly assess the utility of these techniques. The pH and PCO2 varied signi¢cantly between arterial and venous sites; however, the PO2 values did not di¡er signi¢cantly between the two sites. The signi¢cance and cause of this latter unexpected ¢nding remains unclear and warrants further investigation.
Acknowledgements This investigation was supported by an award from the Pet Trust Fund at the University of Guelph. Statistical consultation was provided by William C Sears, MS (Zoology), MSc (Statistics), Department of Population Medicine, University of Guelph, Guelph, Canada.
References Alexander CM,Teller LE, Gross JB (1989) Principles of pulse oximetry. Theoretical and practical considerations. Anesth Analg 68,368^376. Bennett RA (1995) Reptile anesthesia. In: Kirks' Veterinary Therapy XII. Small Animal Practice,12th edn. Bonagura JD, Kirk RW (eds).W.B. Saunders,Toronto, pp.1349^1353. Berger PJ (1987) The reptilian baroreceptor and its role in cardiovascular control. Am Zool 27,111^120. Comeau SG, Hicks JW (1994) Regulation of central vascular blood £ow in the turtle. Am J Physiol 267 (no. 2 Part 2), R569^R578. Cromwell TH, Eger EI II, Stevens WC et al. (1971) Forane uptake, excretion, and solubility in man. Anesthesiology 35, 401^408. Diethelm G, Mader DR, Grosenbaugh DA et al. (1998) Evaluating pulse oximetry in the green iguana (Iguana iguana). In: Proceedings of the 5th Annual Conference of the Association of Reptilian and Amphibian Veterinarians. Frahm MW, Boyer LC (eds). pp.11^12(Abstract). Farrell AP (1991) Introduction to cardiac scope in lower vertebrates. Can J Zool 69,1981^1984. Heard DJ (2001) Reptile anesthesia.Vet Clin North Am Exot Anim Pract 2001 (4),83^117. Heisler N, Neumann P, Maloiy GM (1983) The mechanism of intracardiac shunting in the lizard Varanus exanthematicus. J Exp Biol105,15^31.
72
Hinsch H, Gandal CP (1969) The e¡ects of etorphine (M-99), oxymorphone hydrochloride and meperidine hydrochloride in reptiles. Copeia, 404^405. Hohnke LA (1975) Regulation of arterial blood pressure in the common green iguana. Am J Physiol 228,386^391. Ishimatsu A, Hicks JW, Heisler N (1988) Analysis of intracardiac shunting in the lizard,Varanus niloticus: a new model based on blood oxygen levels and microsphere distribution. Respir Physiol 71,83^100. Kanui TI, Hole K (1992) Morphine and pethidine antinociception in the crocodile. J Vet Pharmacol Ther15,101^103. Ludders JW, Mitchell GS, Rode J (1990) Minimal anesthetic concentration and cardiopulmonary dose^response of iso£urane in ducks.Vet Surg19,304^307. Ludders JW, Rode J, Mitchell GS (1989) Iso£urane anesthesia in sandhill cranes (Grus canadensis): minimal anesthetic concentration and cardiopulmonary dose^response during spontaneous and controlled breathing. Anesth Analg 68,511^516. Mosley CA, Dyson D, Smith DA (2003) Minimum alveolar concentration of iso£urane in green iguanas and the e¡ect of butorphanol on minimum alveolar concentration. J Am Vet Med Assoc 222,1559^1564. Perry SF (1989a) Structure and function of the reptilian respiratory system. In: Lung Biology in Health and Disease. Comparative Pulmonary Physiology. Current Concepts, Vol. 39. Lenfant C, Wood SC (eds). Marcel Dekker, NewYork, pp. 210^211. Perry SF (1989b) Structure and function of the reptilian respiratory system. In: Lung Biology in Health and Disease. Comparative Pulmonary Physiology. Current Concepts,Vol.39. Lenfant C,Wood SC (eds). Marcel Dekker, NewYork, pp. 216^217. Schumacher J (1996) Reptiles and amphibians. In: Lumb and Jones' Veterinary Anesthesia, 3rd edn. Thurmon JC, Tranquilli WJ, Benson GJ, Lumb WV (eds). Lea & Febiger, Philadelphia, pp.670^685. Ste¡ey EP, Howland D, Jr. (1977) Iso£urane potency in the dog and cat. Am J Vet Res 38,1833^1836. Stevens W, Cromwell TH, Halsey M et al. (1971) The cardiovascular e¡ects of a new inhalation anesthetic, Forane, in human volunteers at constant arterial carbon dioxide tension. Anesthesiology 35,8^16. West JB (1995) Pulmonary Pathophysiology ^ the Essentials. Lippincott Williams & Wilkins, Philadelphia, USA, pp.17^34. West JB (2000) Respiratory Physiology ^ the Essentials. Lippincott Williams & Wilkins, Philadelphia, USA, pp. 45^77. Received17 June 2002; accepted 27 November 2002.
# Association of Veterinary Anaesthetists, 2004, 31, 64^72
The cardiovascular dose^response effects of isoflurane alone and combined with butorphanol in the green iguana (Iguana iguana) Craig AE Mosley
DVM, MSc, Diplomate ACVA,
Doris Dyson
DVM, DVSc, Diplomate ACVA
& Dale A Smithy,
DVM, DVSc
Departments of Clinical Studies and yPathobiology, University of Guelph, Guelph, Ontario, Canada
Correspondence: CAE Mosley, Veterinary Teaching Hospital, University of Guelph, Guelph, Ontario, Canada N1G 2W1. E-mail: cmosley@ uoguelph.ca
Abstract Objective To assess the cardiovascular e¡ects (arterial blood pressure, heart rate, and metabolic acid^ base status) of three doses (MAC multiples) of iso£urane alone and combined with butorphanol in the green iguana (Iguana iguana). Study design Prospective randomized double-blind, two-period cross-over trial. Animals Six mature healthy green iguanas (Iguana iguana). Methods The iguanas received each of two treatments, saline 0.1 mL kg 1 (SAL) and butorphanol 1.0 mg kg 1 (BUT) during iso£urane anesthesia. Treatments were separated by at least 1 week. The iguanas were exposed to each of the three minimum alveolar concentration (MAC) multiples (1.0, 1.5, and 2.0) in random order. Anesthesia was induced with iso£urane and maintained using controlled ventilation. Instrumentation included use of an ECG, airway gas monitor, cloacal thermometer, esophageal pulse oximeter, and the placement of a femoral arterial catheter. Body temperature was stabilized and maintained at 32 8C. The treatment was administered, and the animals were equilibrated for 20 minutes at each MAC multiple. At each concentration, the heart rate, blood pressure (systolic, mean, diastolic), endtidal CO 2, and SpO2 were measured. At 1.0 and 2.0 MAC, simultaneous blood samples were drawn from the tail vein/artery complex and femoral catheter for 64
blood gas analysis. Data were analyzed using a twoway analysis of variance for repeated measures looking for di¡erences between treatments and among MAC multiples. Results There were no signi¢cant di¡erences in any of the cardiovascular variables between the treatments. Signi¢cant di¡erences among iso£urane MAC multiples were observed for HR, mean, diastolic, and systolic blood pressures. Blood pressure and heart rate decreased with an increasing dose of anesthetic. There were no signi¢cant di¡erences between treatments or MAC multiples for any of the blood gas variables. The blood pH, PCO2, HCO3 , and hemoglobin saturation di¡ered signi¢cantly between sites. Pulse oximetry values measured from the carotid complex did not correlate with and were signi¢cantly di¡erent from the calculated hemoglobin saturation values determined using the gas analyzer. Conclusion and clinical relevance Cardiovascular depression associated with iso£urane anesthesia in the green iguana is dose dependent. The degree of cardiovascular depression was not signi¢cantly different when iso£urane was combined with butorphanol. This ¢nding suggests that the pre-emptive or intraoperative use of butorphanol is unlikely to be detrimental to cardiovascular function. Butorphanol may be a useful anesthetic adjunct to iso£urane anesthesia in the green iguana. Keywords anesthesia, butorphanol, green iguana, Iguana iguana, iso£urane, lizard.
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
Introduction Veterinarians are reliant upon the frequent and safe administration of anesthetics for even minor procedures in reptiles because of their size, disposition, and unique anatomy (e.g. lack of access in turtles and tortoises when they retract into their shells). However, there is a paucity of objective scienti¢c information regarding reptile anesthesia that is acknowledged by leading authors in the ¢eld of reptilian medicine and surgery (Bennett 1995; Schumacher1996; Heard 2001). Muchof what isknown about reptile anesthesia is based on the clinical impressions of, and case-series presented by, experienced reptile veterinarians. There is poor understanding of the e¡ects of analgesics in reptiles, in part, because of the relatively few studies investigating their use in these animals (Hinsch & Gandal 1969; Kanui & Hole 1992). Despite this, butorphanol is the most common opioid used for analgesia in reptiles, based on a survey of reptile veterinarians in attendance at the 1998 North American Veterinary Conference (personal data). Our previous work with iso£urane (Mosley et al. 2003) has shown the minimum alveolar concentration (MAC) to be 2.1% in the green iguana (Iguana iguana), and that the addition of an analgesic (butorphanol at 1.0 mg kg 1) does not result in a statistically signi¢cant reduction in this MAC value. Although there was no statistically signi¢cant di¡erence in the MAC of iso£urane with and without butorphanol, there was an overall 17% reduction in MAC between the saline-treated and the butorphanol-treated animals. It was thus concluded that butorphanol may have some MAC-reducing ability in some individuals and may be useful as an adjunct to iso£urane anesthesia. However, butorphanol may cause unfavorable cardiovascular e¡ects when combined with iso£urane during anesthesia. The cardiovascular e¡ects of this combination need to be compared to those with iso£urane alone before its use can be widely recommended. The objectives of this study were to assess some of the cardiovascular and metabolic acid^base e¡ects of iso£urane alone and combined with butorphanol in the green iguana.
Materials and methods Six mature iguanas (Iguana iguana), three males and three females weighing 2.19 0.35 kg (mean SEM), were used. General physical examination, complete blood counts, and biochemical pro¢les # Association of Veterinary Anaesthetists, 2004, 31, 64^72
were used to assess the health of the animals. The animals were admitted for an acclimatization period of at least 2 days before experiments. They were assigned a number for positive identi¢cations during the experimental period (clearly marked on the dorsal thorax of each animal). The study was carried out with approval of the Animal Care Committee of the University of Guelph and in compliance with the guidelines of the Canadian Council of Animal Care. The animals were group-housed at the University, in a large room where an appropriate temperature gradient (26^35 8C) was provided and a consistent photoperiod (16 hours light and 8 hours dark) was maintained. The animals had access to several UVemitting lights and infrared spotlights placed to provide basking areas in the room. The animals were fed a diet of 80% dark leafy green vegetables and 20% other vegetables and fruits with a light dusting of a calcium and phosphorus supplement. The iguanas were randomly assigned an order to receive each of the two treatments, saline (SAL) and butorphanol (BUT). The treatments were administered concurrently with iso£urane anesthesia. All animals were then subjected to a second randomization within treatment to determine the order of exposure to three MAC multiples (1.0, 1.5, and 2.0). The experiment was designed as a prospective randomized double-blind cross-over trial. All experiments were completed between 0830 and 1730 hours to reduce the impact of natural circadian rhythms. On the day of experimentation, the animals were transported from the housing facility to the anesthesia laboratory in a secure carrier that prevented the animals from seeing their surroundings and minimized any excitement as a result of the short transport (100 m). The animals were then placed on a circulating water heating blanket and mask induced with iso£urane (Aerrane, Janssen, Toronto, Ontario, Canada) at a dial setting of 5% and an oxygen £ow of 1 L minute 1 delivered using a Bain breathing system. Iso£urane was administered using an out-ofcircuit, agent-speci¢c, temperature- and pressurecompensated precision vaporizer (Fortec vaporizer, Fraser Harlake, Orchard Park, NY, USA). When the iguanas were su¤ciently immobilized, they were intubated using an uncu¡ed endotracheal tube (Sheridan Catheter Corp., Argyle, NY, USA) of the appropriate size (2.0^4.0 mm). Intubation was facilitated with the use of one drop (1^2 mg) of lidocaine (Xylocaine 2%, Astra Pharma Inc., Mississauga, Ontario, Canada) on the glottis.The tube was checked for correct ¢t by in£ating the lungs to a pressure of 65
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
15 mm Hg and listening for leaks, and then was fastened securely in place. All animals were ventilated mechanically using a pressure-limited ventilator (Bird Corporation, Palm Springs, CA, USA) with a frequency of 4 breaths minute 1 and a tidal volume of 30 mL kg 1. The minute ventilation and tidal volume were based on preliminary work on breathing patterns in the green iguana (Perry 1989a). The volume of each breath was determined using a respirometer (Wright's Respirometer, Lifetronics, Downsview, Ontario, Canada). The animals were then prepared for experimentation. A three-lead ECG (Criticare 1100 Patient Monitor, Criticare Systems Inc., Waukesha, WI, USA) was used to monitor heart rate and rhythm using Lead II. The ECG leads were attached to stainless steel wires placed subcutaneously in the right and left axilla and left groin region to ensure adequate contact. A cloacal thermometer (YSI Tele-thermometer, Yellow Springs Instrument Co. Inc., Yellow Springs, OH, USA) was used to ensure that the body temperature was maintained at 32 8C. Additional warmth was provided by a forced warm air blanket (Bair Hugger Model 505, Augustine Medical Inc., Eden Prairie, MN, USA) used, as needed, to maintain a stable body temperature. The gas analyzer (Criticare 1100 Patient Monitor, Criticare Systems Inc.,Waukesha,WI, USA) was connected between the Bain breathing system and the endotracheal tube using a standard gas sampling line connector. A 3-French extension line was fed down the endotracheal tube to a level near the bifurcation of the trachea for gas sampling. The extension line was used to ensure accurate monitoring of airway gases. A pulse oximeter (VetOx, Sensor Device Inc, Milwaukee, WI, USA) with a rectal probe was placed in the esophagus against the carotid complex to monitor functional hemoglobin saturation (SpO2). A sterile percutaneous cut-down was then performed on the ventromedial surface of the hind leg to facilitate the placement of a 24-SWG catheter into the femoral artery (Insyte W, Becton Dickinson, Sandy, UT, USA).The opposite leg was used for the second experiment. The catheter was sutured to the surrounding tissues and capped with an injection port (PRN Adapter, Becton Dickinson Infusion Therapy Systems Inc., Sandy, UT, USA). A disposable transducer system (Model DT-36, Ohmeda Medical Devices Division Inc., Madison,WI, USA) for direct blood pressure monitoring was connected to the Criticare1100 patient monitor. This monitor was calibrated using the manufacturer's recommended calibration gas 66
and checked for blood pressure accuracy with a mercury manometer before each experiment. During the instrumentation period, the animals were maintained between1.0 and1.5 MAC. Once the body temperature was stable between 30 and 32 8C, butorphanol tartrate (Torbugesic, Fort Dodge Laboratories, Inc., Fort Dodge, IA, USA) at 1.0 mg kg 1 or a saline (0.9% saline) placebo at 0.1 mL kg 1, providing equal volumes, was given IM in the forearm. All the treatments were given during anesthesia at least one and no more than 2 hours prior to the initiation of testing. The animals were then equilibrated for 20 minutes, after the per cent exhaled iso£urane appeared stable, at each of the randomly ordered MAC multiples. Once equilibration was complete, three readings of heart rate, blood pressure (systolic, mean, diastolic), end-tidal CO 2 (PE0 CO 2), and SpO2 were taken. Additionally, simultaneous blood samples were drawn from the tail vein/ artery complex and the femoral arterial catheter. The temperature, tidal volume, frequency of respiration, SpO2, and PE0 CO 2 were recorded at the time of blood gas sampling. Air was removed from the blood gas syringe, and the sample was immediately analyzed using an automated blood gas analyzer (Model ABL 3 Radiometer Corp., Copenhagen, Denmark), which corrected the reported values for body temperature to 32 8C. Sampling required approximately 10^15 minutes, after which the next randomly determined iso£urane level was set. The experiment continued until all three MAC levels had been tested. Upon completion of the measurements, all iguanas were given a dose of butorphanol (1 mg kg 1 IM) as an analgesic for recovery. The catheter was then removed, the artery was ligated, and the subcutaneous tissues and skin were closed. The iso£urane was discontinued, and the animals were ventilated until they could be stimulated to move. At this point, the animals were extubated whether or not spontaneous ventilation had occurred, and observed until respiration returned. Recovery was completed in an enclosed quiet area in the group housing room. Each response variable was analyzed by use of a repeated-measures analysis of variance. Statistical analysis was performed using the general linear model (GLM) and mixed (MIXED) procedures of SAS (SAS Institute Inc, SAS/STAT Users Guide, Version 6, 4th Edition, Cary, NC,1989). The cardiovascular data were analyzed ¢rst using a general linear model technique to identify the signi¢cant random e¡ects to be included in the ¢nal mixed model analysis. # Association of Veterinary Anaesthetists, 2004, 31, 64^72
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
Signi¢cance for inclusion in the mixed model was taken as p < 0.25. The mixed model was then used to determine di¡erences between treatments and among doses. A p < 0.05 was considered signi¢cant. If a signi¢cant treatment by dose interaction was present, the di¡erence of least square means was used to determine where di¡erences existed among the doses but within treatment. If no signi¢cant treatment by dose interaction was present, main e¡ects were tested and least square means was used to determine where di¡erences existed. Values of p < 0.05 were considered signi¢cant. A similar analysis was used for analyzing the blood gas variables between treatments (butorphanol or saline), sites (arterial or venous) and doses (1.0 or 2.0 MAC). The data for each variable were ¢rst analyzed using a GLM procedure to determine the random e¡ects to be used in the ¢nal MIXED model. Inclusion in the MIXED model was done if p < 0.25. The MIXED procedure was then used to determine where signi¢cant di¡erences existed (p < 0.05). While ¢tting and
testing all statistical models, assumptions of analysis of variance were assessed via residual analysis using the univariate (UNIVARIATE) and plot (PLOT) procedures of SAS. The need for transformations was also evaluated. Pulse oximetry and calculated hemoglobin saturation values from the blood gases were compared using a paired t-test. Correlation between the two was assessed using the Pearson's coe¤cient of correlation (r).
Results There were no signi¢cant di¡erences in any of the cardiovascular variables between the treatment groups (Table 1). Signi¢cant di¡erences between iso£urane doses were observed for HR, mean, diastolic, and systolic BP. Blood pressure decreased consistently with an increasing dose of iso£urane. Mean heart rate decreased with an increasing dose of anesthetic. At 2.0 and 1.5 MAC, heart rates were
Table 1 Cardiovascular parameters of green iguanas (Iguana iguana) compared between treatments, butorphanol 1 mg kg (BUT) and saline 0.1 mL kg 1 (SAL) and dose (1.0 MAC,1.5 MAC, and 2.0 MAC) of iso£urane (n 6)
1
Variable
Treatment
1.0 MAC
1.5 MAC
2.0 MAC
Heart rate (beats minute 1)
SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT SAL BUT
58 10 52 7 43 7 39 6 29 4 27 5 35 6 33 5 19 10 (2.5 1.3) 19 9 (2.5 1.2) 74 20 90 6 7.46 0.18 7.30 0.17 34.9 14.9 (4.6 2.0) 48.9 17.4 (6.5 2.3) 63.3 19.2 (8.4 2.6) 50.5 7.7 (6.7 1.0) 25.0 4.8 24.2 3.3 0.1 5.7 4.6 6.1 95.2 3.4 90.0 4.0
50 3y 51 13y 31 6 37 7 20 3y 23 5y 25 4 29 6 18 8 (2.4 1.1) 20 12 (2.7 1.6) 65 12 69 18 7.46 0.17 7.30 0.15 36.2 14.5 (4.8 1.9) 49.6 19.4 (6.6 2.6) 52.5 17.0 (7.0 2.3) 45.6 10.5 (6.1 1.4) 25.8 3.9 24.4 3.7 0.7 5.5 4.6 4.8 92.6 4.1 89.4 1.7
51 4y 48 8y 28 5 33 11z 19 3y 21 7y 23 4 25 9z 18 10 (2.4 1.3) 17 10 (2.3 1.3) 76 15 73 26 7.45 0.19 7.34 0.21 36.5 15.8 (4.9 2.1) 46.3 21.5 (6.2 2.9) 54.2 26.7 (7.2 3.6) 41.7 12.8 (5.6 1.7) 24.9 4.1 24.4 2.9 0.3 5.5 3.4 6.3 82.2 15.9 84.1 8.1
Systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg) Mean blood pressure (mm Hg) End-tidal CO2 (mm Hg) (kPa) Functional Hb saturation (%) pHa PaCO2 (mm Hg) (kPa) PaO2 (mm Hg) (kPa) Arterial HCO3 (mmol L 1) Arterial ABE (mmol L 1) Arterial Hb saturation (%)
Significantly different from1.0 MAC within each treatment (treatment by dose interaction, groups not combined for final analysis) at p < 0.05. ySignificantly different from1.0 MAC, combined treatments (no treatment by dose interaction) at p < 0.05. zSignificantly different from1.5 MAC within each treatment (treatment by dose interaction, groups not combined for final analysis) at p < 0.05.
# Association of Veterinary Anaesthetists, 2004, 31, 64^72
67
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
Heart Rate (beats minute-1)
70
SAL & BUT
60 50 a
a
40 30 20 10 0
a, Significantly different from 1.0 MAC (p<0.05)
1
1.5
2
MAC Multiple (MAC) Figure 1 Heart rate in green iguanas (Iguana iguana) at each MAC multiple for both treatments combined (saline 0.1 mL kg 1, SAL and butorphanol 1 mg kg 1, BUT). There was no signi¢cant ( p < 0.05) treatment by dose interaction (n 6). aSigni¢cantly di¡erent from1.0 MAC ( p < 0.05).
signi¢cantly less than those observed at 1.0 MAC (p 0.0322). Heart rates at 1.5 and 2.0 MAC were not signi¢cantly di¡erent from each other (Fig. 1). The analysis of BP revealed a treatment by dose interaction for mean and systolic BP (Figs 2 & 3), but not diastolic BP (Fig. 4). In general, it appeared that blood pressures in the saline treatment group declined more between 1.0 and 1.5 MAC than between 1.5 and 2.0 MAC where it declined at a slower rate. This rate di¡erence may have accounted for our treatment by dose interaction. The blood pressures in the butorphanol group appeared to decrease more uniformly between 1.0 and 1.5 MAC, and 1.5 and 2.0 MAC. The diastolic blood pressure at 1.0 MAC was signi¢cantly 50
di¡erent (p 0.0001) from the diastolic pressures at 1.5 and 2.0 MAC (Fig. 4).The diastolic blood pressures at1.5 and 2.0 MAC were not di¡erent from each other. There was a signi¢cant dose e¡ect for both systolic and mean arterial blood pressure (p 0.0001). As there was a treatment by dose interaction, these differences are described within treatments rather than combined, as was done for diastolic blood pressure. Within the saline treatment group, the mean and systolic blood pressures at 1.5 and 2.0 MAC were signi¢cantly di¡erent from those at 1.0 MAC, but not from each other.Within the butorphanol group, the mean and systolic blood pressures at 2.0 MAC were signi¢cantly di¡erent from those at 1.0 and 1.5 MAC. The mean and systolic blood pressures at1.0 and1.5 MAC were not signi¢cantly di¡erent from each other in either treatment. There were no signi¢cant di¡erences between treatments or doses for any of the blood gas variables. Site e¡ects were determined bycombining both doses and treatments (Table 2). The blood pH, PCO2, HCO3 , and hemoglobin saturation all di¡ered significantly between sites. There were no di¡erences between sites for PO2 and actual base excess. Pulse oximetry values recorded from the carotid artery did not correlate (a Pearson's coe¤cient of 0.33) with the values determined with the blood gas analyzer from femoral arterial samples (Fig. 5). Using the paired t-test, pulse oximetry (89 1.7%) and hemoglobin saturation values calculated from the blood gas (73 3.6%) values were signi¢cantly different (p 0.0005).
SAL
Systolic Blood Pressure (mm Hg)
BUT 40 a,b 30 a
a
20
10
a, Significantly different from 1.0 MAC (p<0.05) b, Significantly different from 1.5 MAC (p<0.05)
0 1
1.5 MAC Multiple (MAC)
68
2
Figure 2 Systolic arterial blood pressure in green iguanas (Iguana iguana) at each MAC multiple for each treatment (saline 0.1 mL kg 1, SAL and butorphanol1 mg kg 1, BUT). A significant ( p < 0.05) treatment by dose interaction was present (n 6). aSigni¢cantly di¡erent from 1.0 MAC ( p < 0.05); bSigni¢cantly di¡erent from1.5 MAC ( p < 0.05).
# Association of Veterinary Anaesthetists, 2004, 31, 64^72
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
50
SAL BUT
Figure 3 Mean arterial blood pressure in green iguanas (Iguana iguana) at each MAC multiple for each treatment (saline 0.1 mL kg 1, SAL and butorphanol 1 mg kg 1, BUT). A signi¢cant (p < 0.05) treatment by dose interaction was present (n 6). aSigni¢cantly di¡erent from 1.0 MAC ( p < 0.05), b Signi¢cantly di¡erent from 1.5 MAC ( p < 0.05).
Mean Blood Pressure (mm Hg)
40
30
a,b
a
20
10
a, Significantly different from 1.0 MAC (p<0.05) b, Significantly different from 1.5 MAC (p<0.05)
0 1
Diastolic Blood Pressure (mm Hg)
SAL BUT 40
30 a a 20 a
a
10 a, Significantly different from 1.0 MAC (p<0.05)
0 1.5
1.5
2
MAC Multiple (MAC)
50
1
a
2
MAC Multiple (MAC) Figure 4 Diastolic arterial blood pressure in green iguanas (Iguana iguana) at each MAC multiple for each treatment (saline 0.1 mL kg 1, SAL and butorphanol 1 mg kg 1, BUT). There was no signi¢cant ( p < 0.05) treatment by dose interaction (n 6). aSigni¢cantly di¡erent from1.0 MAC ( p < 0.05).
Discussion The cardiovascular e¡ects of iso£urane in the green iguana share some similarity with those reported in humans (Cromwell et al. 1971), dogs (Ste¡ey & Howland 1977), cats (Ste¡ey & Howland 1977), and birds (Ludders et al. 1989; Ludders et al. 1990). As end-tidal iso£urane concentrations are increased from1.0 to 2.0 MAC, blood pressure decreases. In our # Association of Veterinary Anaesthetists, 2004, 31, 64^72
study of iguanas, the heart rate was signi¢cantly higher at 1.0 MAC than at 1.5 and 2.0 MAC. This is unlike humans (Stevens et al.1971), and dogs (Ste¡ey & Howland 1977), where the heart rate tends to increase with increasing depth. However, in cats (Ste¡ey & Howland 1977) and birds (Ludders et al. 1989,1990), heart rates remain relatively unchanged despite an increasing dose of iso£urane. The heart rate changes observed in humans and dogs may be mediated through the baroreceptor response to iso£urane-induced hypotension. An increased heart rate tends to maintain cardiac output despite a reduced stroke volume (Ste¡ey & Howland 1977). Unfortunately, in our studies, we were unable to assess cardiac output.The lack of an increase in heart rate in the green iguana may be the result of lacking, weak or easily obtunded baroreceptor re£exes. In snakes, a very strong barore£ex response has been demonstrated (Berger1987), and in the green iguana, tachycardia has been demonstrated in response to graded hemorrhage, which suggests the presence of a baroreceptor response (Hohnke1975). The decreasing heart rate that was observed in the green iguana in response to an increasing dose of iso£urane was unexpected. This may represent a species-speci¢c response. In our experiments, it is also possible that the higher HR observed at 1.0 MAC was a result of sympathetic drive in some animals which were at subanesthetic levels, as MAC is de¢ned as the dose required to prevent purposeful movement in response to a supramaximal stimulus in 50% of subjects. Increasing the percentage of inhalant in the 69
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
Blood gas variable
Arterial
Venous
pH PCO2 (mm Hg) (kPa) PO2 (mm Hg) (kPa) HCO3 (mmol L 1) ABE Hemoglobin saturation (%)
7.39 0.04 41.1 3.7 (5.5 0.5) 53.0 4.1 (7.1 0.5) 24.7 0.8 1.9 1.2 90.2 1.4
7.37 0.04 44.0 3.5 (5.9 0.5) 49.1 6.1 (6.5 0.8) 25.2 0.8 2.1 1.2 78.4 5.0
Table 2 Blood gas results compared between sites (femoral artery, tail vein) in green iguanas (Iguana iguana)
No significant treatment ordose effects (n 24). Significantly different from arterial (p < 0.05).
iguanas would then bring them into a better anesthetic plane and thus eliminate the sympathetic drive. Other evidence indicating that some reptiles were excessively light (movement, ventilatory e¡orts) were not observed during the study. However, surgical stimulus was not present at the time of these measurements. The blood pressures we obtained in the green iguana were low compared with those documented in birds and mammals. However, there are substantial experimental data demonstrating that many reptiles have blood pressures markedly lower than those observed in mammals (Farrell 1991). Unfortunately, we were unable to locate any complete references documenting the `normal awake' values in the green iguana, which would have given us greater insight into whether iso£urane resulted in a decrease
Pulse oximetry (%)
100
80
60
40
20 20
40 60 80 100 Calculated hemoglobin saturation (%)
Figure 5 Pulse oximetry and calculated hemoglobin saturation values for each paired blood sample collected from green iguanas (Iguana iguana) anesthetized twice, once with iso£urane and once with iso£urane combined with butorphanol. Samples were taken simultaneously from the femoral artery and tail vein at 1.0 and 2.0 MAC in six green iguanas (n 29). 70
in blood pressure. We were only able to locate one report describing measurement of arterial blood pressure from the femoral artery of the green iguana (Hohnke 1975). Unfortunately, in this work, pressure measurements are not explicitly described; instead, one must read the blood pressure o¡ the scales of traces from single animals. In two animals, the awake mean blood pressures taken from the graphs and trace were approximately 75 and 60 mm Hg. Additionally, these animals were not `free roaming' but rather restrained on an elevated platform. It is unclear whether these values represent true mean pressures in a`normal awake'animal or those of relatively stressed iguanas. The response of blood pressure to an increasing dose of iso£urane within the two treatments was not the same as evidenced by the treatment by dose interaction. At 1.0 MAC, the butorphanol group had a lower blood pressure than the saline group, but as the dose of iso£urane was increased, the butorphanol values did not decrease as dramatically as those of the saline group. Based on our sample size and small number of doses tested, drawing speci¢c conclusions would not be warranted. However, our data suggest that butorphanol combined with iso£urane has greater depressive e¡ects on blood pressure than iso£urane alone at low doses, but as the dose of iso£urane is increased, the di¡erence becomes negligible. One might speculate that the level of anesthetic produced at 1.0 MAC was deeper with butorphanol as a result of its tendency to reduce MAC in some iguanas (Mosley et al. 2003). The individuals that responded to butorphanol with a reduction in MAC may have had decreased sympathetic drive, and consequently lower blood pressure. However, there is no clear evidence of this based on our data. It is not surprising that functional hemoglobin saturation measured using pulse oximetry was signi¢cantly di¡erent from that determined by blood gas analysis. Pulse oximetry measures the hemoglo# Association of Veterinary Anaesthetists, 2004, 31, 64^72
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
bin absorbance and uses the ratio of oxyhemoglobin to deoxyhemoglobin to determine the per cent saturation of hemoglobin (Alexander et al.1989). The absorbance ratio is then compared with an internal standard curve that plots the ratio of the absorbances of the two hemoglobins against the hemoglobin saturation at a de¢ned temperature. There is recent evidence that suggests that pulse oximetry is a valid tool for hemoglobin saturation determination in the green iguana (Diethelm et al. 1998). Unfortunately, it is not clear from the abstract how the hemoglobin saturation for the `gold standard' comparison was estimated. In our study, we used a human blood gas analyzer that calculates hemoglobin saturation (SO2) using the measured pH, PCO2, PO2, and temperature. These equations are derived from human blood, and it is likely that they di¡er from those needed for the hemoglobin saturation determination of iguana blood. Therefore, we suspect that the SO2 results from our blood gas analysis may be inaccurate in the green iguana and cannot, with certainty, draw conclusions regarding the use of pulse oximetry or the use of a human blood gas analyzer. Validation of these techniques for use in the green iguana is necessary. Very few conclusions can be drawn from our blood gas values. Only pH, PCO 2, and PO2 were measured directly; all other values were calculated based on complex formulae derived from human blood. In addition, all the blood gases were corrected to the animal's actual body temperature,32 8C, by the blood gas machine. This correction is based on human blood algorithms and may not accurately re£ect temperature-related changes in green iguana blood. It was expected that arterial and venous sampling sites would be signi¢cantly di¡erent from one another. Interestingly, PO2 was not signi¢cantly di¡erent: arterial 53.0 4.1 mm Hg (7.1 0.5 kPa) and venous 49.1 6.1 mm Hg (6.5 0.8 kPa). This result suggests that, in the green iguana anesthetized with iso£urane, with or without butorphanol, there is very little di¡erence in arterial and venous PO2. Additionally, PO2 values in arterial blood were much lower than would be expected in a normal mammal breathing 100% oxygen. Di¡erences in PO2 values in arterial and venous blood are created by tissue extraction of O2. It would thus appear that, in the anesthetized green iguana, there is very little tissue extraction of O2. It is unclear to us whether the lower than expected PO2 and O2 tissue extraction are a re£ection of normal reptilian physiology or whether they represent a physiologic adaptation to one or a combination # Association of Veterinary Anaesthetists, 2004, 31, 64^72
of the following factors: (i) high inspired oxygen; (ii) iso£urane; (iii) the anesthetized state irrespective of the drugs involved; or (iv) positive pressure ventilation. There are studies in other reptile species that have demonstrated a relatively low rate of oxygen extraction (Perry 1989b). A ¢nal possibility is that our venous sample drawn from the tail was not truly venous. The ventral surface of the tail contains a major artery, vein, and lymphatic system, and it is possible that our samples contained some combination of the three £uids. The lower than expected arterial oxygen tension cannot be termed a true hypoxemia, as hypoxemia has not been clearly de¢ned in reptiles. Pragmatically speaking, hypoxemia should describe the oxygen level below which metabolic requirements, at rest, are inadequate. The term relative hypoxemia may be an acceptable term as applied to the oxygen values we observed in the green iguana. Based on the predicted partial pressures of oxygen in the lung and complete equilibration of all the blood passing to the systemic circulation from the pulmonary circulation, we would expect the partial pressures of oxygen in the blood to be nearly equal to that of the lung (West 2000). This was never the case in any of our animals, although we were able to more completely saturate the iguana blood by exposing it to 100% oxygen in a syringe. The reason for the relative hypoxemia can most easily be explained by shunt £ow, in particular, a right-to-left shunt that bypasses the pulmonary circulation. This type of shunting has been documented in numerous studies involving several reptile species (Heisler et al. 1983; Ishimatsu et al. 1988; Comeau & Hicks 1994). Other causes may be an increase in the di¡usion barrier, hypoventilation (this is likely to be insigni¢cant while inspiring 100% O2), or ventilation^perfusion inequality (West 1995). Unfortunately, our study was not designed to elucidate or explain more clearly the blood gas changes we observed. In our study we demonstrated cardiovascular depression in response to an increasing dose of iso£urane. The amount of cardiovascular depression was not signi¢cantly di¡erent when iso£urane was combined with butorphanol. This ¢nding suggests that the pre-emptive or intraoperative use of butorphanol is unlikely to be detrimental to cardiovascular function, even when MAC reduction does not occur.We found that pulse oximetry using an esophageal probe did not re£ect values taken simultaneously from the femoral artery and analyzed on a human blood gas analyzer. It remains unclear at 71
Cardiovascular dose^response e¡ects of iso£urane in iguanas CAE Mosley et al.
this time whether either methodology is useful as a monitoring device for hemoglobin saturation. Additional validation and respiratory physiologic studies are needed to more clearly assess the utility of these techniques. The pH and PCO2 varied signi¢cantly between arterial and venous sites; however, the PO2 values did not di¡er signi¢cantly between the two sites. The signi¢cance and cause of this latter unexpected ¢nding remains unclear and warrants further investigation.
Acknowledgements This investigation was supported by an award from the Pet Trust Fund at the University of Guelph. Statistical consultation was provided by William C Sears, MS (Zoology), MSc (Statistics), Department of Population Medicine, University of Guelph, Guelph, Canada.
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Hinsch H, Gandal CP (1969) The e¡ects of etorphine (M-99), oxymorphone hydrochloride and meperidine hydrochloride in reptiles. Copeia, 404^405. Hohnke LA (1975) Regulation of arterial blood pressure in the common green iguana. Am J Physiol 228,386^391. Ishimatsu A, Hicks JW, Heisler N (1988) Analysis of intracardiac shunting in the lizard,Varanus niloticus: a new model based on blood oxygen levels and microsphere distribution. Respir Physiol 71,83^100. Kanui TI, Hole K (1992) Morphine and pethidine antinociception in the crocodile. J Vet Pharmacol Ther15,101^103. Ludders JW, Mitchell GS, Rode J (1990) Minimal anesthetic concentration and cardiopulmonary dose^response of iso£urane in ducks.Vet Surg19,304^307. Ludders JW, Rode J, Mitchell GS (1989) Iso£urane anesthesia in sandhill cranes (Grus canadensis): minimal anesthetic concentration and cardiopulmonary dose^response during spontaneous and controlled breathing. Anesth Analg 68,511^516. Mosley CA, Dyson D, Smith DA (2003) Minimum alveolar concentration of iso£urane in green iguanas and the e¡ect of butorphanol on minimum alveolar concentration. J Am Vet Med Assoc 222,1559^1564. Perry SF (1989a) Structure and function of the reptilian respiratory system. In: Lung Biology in Health and Disease. Comparative Pulmonary Physiology. Current Concepts, Vol. 39. Lenfant C, Wood SC (eds). Marcel Dekker, NewYork, pp. 210^211. Perry SF (1989b) Structure and function of the reptilian respiratory system. In: Lung Biology in Health and Disease. Comparative Pulmonary Physiology. Current Concepts,Vol.39. Lenfant C,Wood SC (eds). Marcel Dekker, NewYork, pp. 216^217. Schumacher J (1996) Reptiles and amphibians. In: Lumb and Jones' Veterinary Anesthesia, 3rd edn. Thurmon JC, Tranquilli WJ, Benson GJ, Lumb WV (eds). Lea & Febiger, Philadelphia, pp.670^685. Ste¡ey EP, Howland D, Jr. (1977) Iso£urane potency in the dog and cat. Am J Vet Res 38,1833^1836. Stevens W, Cromwell TH, Halsey M et al. (1971) The cardiovascular e¡ects of a new inhalation anesthetic, Forane, in human volunteers at constant arterial carbon dioxide tension. Anesthesiology 35,8^16. West JB (1995) Pulmonary Pathophysiology ^ the Essentials. Lippincott Williams & Wilkins, Philadelphia, USA, pp.17^34. West JB (2000) Respiratory Physiology ^ the Essentials. Lippincott Williams & Wilkins, Philadelphia, USA, pp. 45^77. Received17 June 2002; accepted 27 November 2002.
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