Rabbits are a commonly used laboratory animal species and an increasingly popular household pet. As such, rabbits are being anesthetized for invasive surgical procedures, yet anesthesia in rabbits remains challenging to practitioners and researchers. In a large-scale study1 in small animal veterinary practices, the anesthesia-associated mortality rate in rabbits was 6 and 8 times the rates in cats and dogs, respectively. Inhalation anesthetic agents are commonly used for induction and maintenance of anesthesia in rabbits. The considerable hypotension that develops in rabbits during inhalation anesthesia may be 1 contributing factor to their high anesthetic-related mortality rate.2
Although rabbits have been used extensively in cardiovascular research, it remains difficult to ascertain the complete dose-dependent hemodynamic effects of inhalation anesthetic agents. Such data are necessary to enable appropriate management of cardiovascular depression or to develop strategies to combat cardiovascular depression in rabbits undergoing inhalation anesthesia and ultimately reduce the anesthetic-related mortality rate in this species.
The purpose of the study reported here was to determine the cardiopulmonary effects of 3 concentrations of isoflurane with and without controlled mechanical ventilation in healthy adult New Zealand white rabbits. In addition, to mimic clinical situations, hemodynamic measurements were repeated during application of a supramaximal noxious stimulus. We hypothesized that cardiopulmonary depression would be greater at higher anesthetic doses and with intermittent positive pressure ventilation and would be offset by mechanical stimulation.
Materials and Methods
Animals—Six healthy adult female New Zealand white rabbits were used in the study. The mean ± SEM weight of the rabbits was 3.83 ± 0.25 kg. The study was approved by the Institutional Animal Care and Use Committee.
Anesthesia and instrumentation—For each rabbit, anesthesia was induced with isoflurane in oxygen delivered into an induction chamber and then via face mask. Each rabbit's trachea was then intubated with a cuffed endotracheal tube, and anesthesia was maintained with isoflurane in oxygen delivered via a Bain circuit with a fresh gas flow rate of 500 mL/kg/min.
A 22-gauge, 2.5-cm cathetera was placed in a lateral saphenous vein for delivery of lactated Ringer's solutionb at a rate of 3 mL/kg/h. A 14-gauge, 5-cm cathetera was placed into a jugular vein via surgical cutdown and connected to a hemostasis valve.c A 4F, 75-cm thermodilution catheterd was inserted through the introducer into the pulmonary artery under fluoroscopic guidance. This catheter was used to measure cardiac output, mean pulmonary artery pressure, pulmonary artery occlusion pressure, central venous pressure, and core body temperature and for collection of mixed venous blood samples. A 22-gauge, 4.4-cm cathetere was placed into an external carotid artery via surgical cutdown to enable measurement of arterial blood pressures and collection of arterial blood samples. A calibrated temperature probe was placed into the esophagus to the level of the heart for constant temperature monitoring. Core body temperature was maintained between 38° and 39°C by use of circulating warm water blankets and a forced air warming unitf as needed.
Each rabbit was positioned in right-lateral recumbency. Heart rate, SAP, DAP, MAP, mean pulmonary artery pressure, central venous pressure, and core body temperature were monitored continuously and recorded by use of a physiographg and acquisition software.h All pressure transducers were calibrated against a mercury manometer with the zero point set at the level of the sternum.
A catheter was positioned down the lumen of the endotracheal tube so that its tip lay close to the distal end of the tube. This catheter was used to collect samples of end-expired gases. Inspired and expired oxygen, carbon dioxide, and isoflurane were monitored continuously by use of a spectrometer.i Expired gas samples were collected manually (10-mL volume collected during 7 to 10 respiratory cycles) and analyzed in triplicate with a spectrometer.j This analyzer was calibrated daily with 30 mL each of 4 known concentrations of isoflurane.
Arterial and mixed venous blood pH, Pco2, Po2, and lactate concentration were measured and bicarbonate concentration was calculated by use of a blood gas analyzer.k Arterial and mixed venous blood hemoglobin concentration and oxygen saturation were measured by use of a hemoximeter.l In arterial blood samples, PCV was measured by a microcentrifugation technique and total protein concentration was measured via refractometry.
Cardiac output was determined in triplicate by use of thermodilution technique and cardiac output computer.m Three milliliters of iced 5% dextrose was injected through the proximal port of the thermodilution catheter for each determination. The mean of the 3 measurements was then calculated.
Cardiac index, SI, RPP, SVRI, pulmonary vascular resistance index, LVSWI, RVSWI, arterial and mixed venous oxygen content, oxygen delivery, oxygen consumption, oxygen extraction ratio, Pao2 - Pao2, and shunt fraction were calculated with standard equations.3–5 Barometric pressure was obtained from the University of California climate station, and body surface area was based on data from a recently reported study6 in rabbits.
For the controlled mechanical ventilation experiments, intermittent positive pressure ventilation was commenced after instrumentation and continued for the remainder of the anesthetic episode by use of a pressure cycled ventilator.n Peak inspiratory pressure was set at 12 cm H2O, and the respiratory rate was adjusted to maintain normocapnia for each rabbit.
Experimental protocol—Each rabbit underwent anesthesia with spontaneous or controlled mechanical ventilation in separate experiments conducted at least 2 weeks apart. At each of the 3 isoflurane concentrations with spontaneous or mechanical ventilation, variables of interest were assessed before and during noxious stimulation; hence, data were obtained from each rabbit in 12 experimental conditions. The order of delivery of anesthetic dose was randomized, and for the first experiment for each animal, the selection of spontaneous or mechanical ventilation was random.
Following instrumentation, isoflurane concentration was randomly set at either 1.0, 1.5, or 2.0 times the published MAC of isoflurane for rabbits (2.07%).2,7 Following each change in anesthetic concentration, a 30-minute period was allowed to elapse for equilibration. Samples of end-expired gas were then collected manually for measurement of end-expired isoflurane concentration. Heart rate and blood pressure measurements were recorded, and a sample (0.3 mL) each of arterial and mixed-venous blood were collected and placed into two 125-μL glass capillary tubeso containing heparin; tubes were capped and stored in ice water until analysis, which occurred within 30 minutes after collection. Cardiac output was measured as described. A supramaximal electrical stimulusp (50 Hz; 40 V) was then delivered to the rabbit via needle electrodes placed on either side of the tail base.8 After 2 minutes, application of the stimulus continued while all measurements and blood sample collections were repeated; the stimulus was then terminated.
The concentration of isoflurane was changed, and another equilibration period was allowed to elapse. Measurements and blood samples were obtained as described. The experimental protocol was repeated again, so that all 3 anesthetic doses were evaluated.
At the end of the experiment, isoflurane was adjusted to approximately 2.5% and buprenorphineq (0.05 mg/kg) was administered IV. The thermodilution, jugular, and carotid catheters were removed, and hemostasis was achieved by application of pressure to the neck for 15 minutes followed by placement of skin sutures. Each rabbit was then allowed to recover from anesthesia. Meloxicamr (0.5 mg/kg, SC) was administered following extubation and the lateral saphenous catheter was removed when the rabbit was actively moving.
Statistical analysis—Data are reported as mean ± SEM unless otherwise stated. Data were analyzed for the effects of isoflurane concentration and mode of ventilation by repeated-measures 2-way ANOVA.s The effects of isoflurane concentration on variables measured before noxious stimulation during either ventilation mode were analyzed by repeated-measures ANOVA. When significant effects were detected, pairwise comparisons were made with a sequentially rejective Bonferroni technique to correct for multiple comparisons.9 Paired t tests were used to evaluate the effect of noxious stimulation on variables at a given isoflurane concentration during a given ventilation mode. Significance was set at a value of P < 0.05.
Results
Mean time for anesthetic induction and instrumentation was 67 minutes, regardless of whether rabbits were undergoing spontaneous or controlled mechanical ventilation. There was no significant difference in the actual concentration of isoflurane at a given dose between the spontaneous or mechanical ventilation experiments. Overall, the isoflurane concentrations for the experimental doses at 1.0×, 1.5×, and 2.0× MAC were 2.11 ± 0.04%, 3.14 ± 0.07%, and 4.15 ± 0.06%, respectively.
Effects of ventilation mode—Mode of ventilation had significant effect on cardiac output (P = 0.046) and ventilatory variables (Paco2 [P < 0.001], arterial blood pH [P = 0.019], mixed venous partial Pco2 [P = 0.009], and mixed venous blood pH [P = 0.013]). Additionally, for all of those variables, there was an interaction between isoflurane concentration and ventilation status. At an isoflurane concentration of 4.15%, cardiac output (P = 0.010), SAP (P = 0.009), MAP (P = 0.012), SI (P = 0.036), RPP (P = 0.023), LVSWI (P = 0.044), oxygen delivery (P = 0.006), Paco2 (P < 0.001) and mixed venous Paco2 (P = 0.002) were significantly higher and arterial (P = 0.002) and mixed venous blood pH (P = 0.006) significantly lower during spontaneous ventilation, compared with findings for rabbits receiving mechanical ventilation.
Effects of isoflurane concentration in rabbits undergoing spontaneous ventilation without noxious stimulation—Cardiopulmonary data were obtained from the rabbits during anesthesia at each of the 3 isoflurane concentrations with spontaneous ventilation and no noxious stimulation (Tables 1 and 2). Increases in anesthetic dose were associated with progressive ventilatory depression. Compared with findings at an isoflurane concentration of 2.11%, Paco2 (P = 0.041) and mixed venous Pco2 (P = 0.04) were increased and arterial pH (P = 0.077) and mixed venous pH (P = 0.137) were decreased at an isoflurane concentration of 3.14%. Compared with findings at isoflurane concentrations of 3.14% and 2.11%, Paco2 (P = 0.005 and P < 0.001, respectively) and mixed venous Pco2 (P = 0.009 and P < 0.001, respectively) were increased and arterial pH (P < 0.001 and P < 0.001, respectively) and mixed venous pH (P = 0.002 and P < 0.001, respectively) were decreased at an isoflurane concentration of 4.15%.
Mean ± SEM cardiovascular variables before and during application of a supramaximal noxious stimulus in 6 New Zealand white rabbits anesthetized with 3 concentrations of isoflurane under conditions of spontaneous ventilation.
| Concentration of isoflurane | ||||||
|---|---|---|---|---|---|---|
| 2.11% | 3.14% | 4.15% | ||||
| Variable | Before stimulation | During stimulation | Before stimulation | During stimulation | Before stimulation | During stimulation |
| Heart rate (beats/min) | 220 ± 8 | 225 ± 5 | 225 ± 6 | 227 ± 5 | 220 ± 14 | 233 ± 9 |
| SAP (mm Hg) | 74 ± 1 | 77 ± 2 | 69 ± 3 | 70 ± 5 | 63 ± 3* | 65 ± 2 |
| MAP (mm Hg) | 47 ± 3 | 53 ± 2 | 40 ± 3* | 41 ± 3 | 34 ± 2* | 35 ± 1 |
| DAP (mm Hg) | 39 ± 3 | 43 ± 3 | 30 ± 3* | 30 ± 3 | 23 ± 1* | 23 ± 1 |
| CVP (mm Hg) | 6 ± 1 | 7 ± 1 | 6 ± 1 | 7 ± 1 | 7 ± 1 | 8 ± 1 |
| MPAP (mm Hg) | 13 ± 2 | 13 ± 2 | 12 ± 2 | 13 ± 2 | 13 ± 3 | 14 ± 3 |
| PAOP (mm Hg) | 5 ± 1 | 6 ± 1 | 4 ± 2 | 6 ± 2 | 5 ± 1 | 5 ± 2 |
| Cardiac output (L/min) | 0.57 ± 0.05 | 0.60 ± 0.06 | 0.54 ± 0.05 | 0.57 ± 0.06 | 0.51 ± 0.04 | 0.55 ± 0.04 |
| SI ([mL/beat]/m2) | 10.7 ± 0.6 | 10.9 ± 0.7 | 10.0 ± 0.7 | 10.3 ± 0.7 | 9.9 ± 1.1 | 9.8 ± 0.6 |
| SVRI ([{dynes•s}/cm5]/m2) | 1,462 ± 199 | 1,560 ± 201 | 1,240 ± 168 | 1,186 ± 146 | 1,052 ± 128 | 983 ± 92 |
| PVRI ([{dynes•s}/cm5]/m2) | 280 ± 51 | 239 ± 51† | 287 ± 58 | 256 ± 48 | 271 ± 63 | 318 ± 75 |
| RPP (beats/min•mm Hg) | 10,420 ± 822 | 11,822 ± 547† | 9,008 ± 862 | 9,247 ± 763 | 7,490 ± 793* | 8,088 ± 518 |
| LVSWI ([g•m]/m2) | 7.2 ± 0.4 | 8.2 ± 0.3† | 5.7 ± 0.5* | 6.0 ± 0.5 | 4.7 ± 0.4*‡ | 4.9 ± 0.3 |
| RVSWI ([g•m]/m2) | 2.0 ± 0.4 | 2.1 ± 0.4 | 1.8 ± 0.4 | 2.0 ± 0.4† | 1.8 ± 0.4 | 2.1 ± 0.5 |
Each rabbit was administered isoflurane in oxygen at each of the 3 anesthetic doses (1.0, 1.5, or 2.0 times the published MAC of 2.07%); the mean ± SEM isoflurane concentrations used were 2.11 ± 0.04%, 3.14 ± 0.07%, and 4.15 ± 0.06%. For each rabbit at each isoflurane concentration, data were collected before and during application of a supramaximal electrical stimulus.
Within a row, value is significantly (P < 0.05) different from the value obtained at an isoflurane concentration of 2.11%.
Within an anesthetic dose, value is significantly (P < 0.05) different from that obtained before noxious stimulation.
Within a row, value is significantly (P < 0.05) different from the value obtained at an isoflurane concentration of 3.14%.
CVP = Central venous pressure. MPAP = Mean pulmonary artery pressure. PAOP = Pulmonary artery occlusion pressure. PVRI = Pulmonary vascular resistance index.
Mean ± SEM blood gas and derived cardiorespiratory variables before and during application of a supramaximal noxious stimulus in 6 New Zealand white rabbits anesthetized with 3 concentrations of isoflurane under conditions of spontaneous ventilation.
| Concentration of isoflurane | ||||||
|---|---|---|---|---|---|---|
| 2.11% | 3.14% | 4.15% | ||||
| Variable | Before stimulation | During stimulation | Before stimulation | During stimulation | Before stimulation | During stimulation |
| Arterial blood pH | 7.45 ± 0.02 | 7.46 ± 0.02 | 7.40 ± 0.03 | 7.40 ± 0.03 | 7.26 ± 0.02*‡ | 7.25 ± 0.02 |
| Paco2 (mm Hg) | 35 ± 1 | 34 ± 1 | 42 ± 4* | 42 ± 5 | 59 ± 4*‡ | 60 ± 4 |
| Pao2 (mm Hg) | 410 ± 33 | 421 ± 31 | 382 ± 28 | 390 ± 40 | 373 ± 38 | 370 ± 40 |
| Arterial hemoglobin (g/dL) | 9.9 ± 0.3 | 9.6 ± 3† | 10.2 ± 0.5 | 9.5 ± 0.4† | 10.0 ± 0.4 | 9.3 ± 0.5† |
| Mixed venous blood pH | 7.40 ± 0.02 | 7.41 ± 0.02 | 7.37 ± 0.03 | 7.37 ± 0.03 | 7.24 ± 0.02*‡ | 7.23 ± 0.02 |
P co2 (mm Hg) | 43 ± 1 | 41 ± 1 | 50 ± 4* | 50 ± 5 | 66 ± 4*† | 69 ± 4 |
| Pv2 (mm Hg) | 67 ± 3 | 67 ± 3 | 71 ± 6 | 67 ± 4 | 71 ± 4 | 70 ± 3 |
| Mix2ed venous hemoglobin (g/dL) | 10.2 ± 0.3 | 9.9 ± 0.2† | 10.2 ± 0.4 | 10.0 ± 0.4 | 9.9 ± 0.4 | 9.5 ± 0.4† |
| Lactate (mmol/L) | 2.0 ± 0.2 | 2.1 ± 0.2 | 1.5 ± 0.1 | 1.6 ± 0.1 | 1.6 ± 0.2 | 1.7 ± 0.2 |
| Cao2 (mL/dL) | 14.6 ± 0.5 | 14.3 ± 0.4 | 15.0 ± 0.8 | 14.1 ± 0.7† | 14.6 ± 0.6 | 13.7 ± 0.7† |
| Cvo22 (mL/dL) | 12.6 ± 0.3 | 12.3 ± 0.2 | 12.5 ± 0.4 | 12.3 ± 0.5 | 11.7 ± 0.3* | 11.4 ± 0.3 |
| Oxygen delivery (mL/min) | 81.7 ± 5.0 | 84.2 ± 6.2 | 79.7 ± 5.8 | 78.8 ± 7.3 | 74.1 ± 5.1*‡ | 73.5 ± 5.1 |
| Oxygen consumption (mL/min) | 10.7 ± 1.5 | 11 ± 1.0 | 12.5 ± 1.9 | 8.7 ± 1.3 | 14.1 ± 2.0* | 11.4 ± 2.1 |
| Oxygen extraction ratio | 0.14 ± 0.03 | 0.14 ± 0.02 | 0.16 ± 0.03 | 0.12 ± 0.03 | 0.20 ± 0.03* | 0.16 ± 0.03 |
| Pao2 - Pao2 (mm Hg) | 243 ± 34 | 234 ± 33 | 254 ± 31 | 246 ± 43 | 235 ± 39 | 237 ± 41 |
| Qs/Qt | 0.32 ± 0.05 | 0.28 ± 0.05 | 0.27 ± 0.06 | 0.34 ± 0.07 | 0.23 ± 0.06 | 0.30 ± 0.08 |
Pco2 = Mixed venous partial pressure of carbon dioxide. Po2 = Mixed venous partial pressure of oxygen. Cao2 = Arterial oxygen content. Co2 = Mixed venous oxygen content. Qs/Qt = Shunt fraction (or venous admixture).
See Table 1 for remainder of key.
Oxygen delivery was significantly lower at an isoflurane concentration of 4.15% than at an isoflurane concentration of 3.14% (P = 0.002) or 2.11% (P = 0.024). At an isoflurane concentration of 4.15%, values of oxygen consumption (P = 0.012) and oxygen extraction ratio (P = 0.003) were significantly higher and mixed venous oxygen content (P = 0.013) was significantly lower, compared with findings at an isoflurane concentration of 3.14%.
Effects of isoflurane concentration in rabbits undergoing controlled mechanical ventilation without noxious stimulation—In anesthetized rabbits undergoing controlled mechanical ventilation without noxious stimulation, there were incremental reductions in SAP, MAP, DAP, and cardiac output with increases in isoflurane dose (Table 3). Compared with findings at an isoflurane concentration of 2.11%, SAP (P = 0.038), MAP (P = 0.029), DAP (P = 0.025), and cardiac output (P = 0.003) were decreased at an isoflurane concentration of 3.14%. Compared with findings at isoflurane concentrations of 3.14% and 2.11%, SAP (P = 0.01 and P < 0.01, respectively), MAP (P = 0.006 and P = 0.001, respectively), DAP (P = 0.015 and P = 0.001, respectively), and cardiac output (both P = 0.004) were decreased at an isoflurane concentration of 4.15%. Values for SVRI (P = 0.002), RPP (P = 0.001), LVSWI (P = 0.001), and RVSWI (P = 0.011) were all significantly lower at the highest anesthetic dose, compared with findings at the lowest anesthetic dose. At an isoflurane concentration of 4.15%, RPP (P = 0.008) and LVSWI (P = 0.011) were also significantly lower than those at an isoflurane concentration of 3.14%.
Mean ± SEM cardiovascular variables before and during application of a supramaximal noxious stimulus in 6 New Zealand white rabbits anesthetized with 3 concentrations of isoflurane under conditions of controlled mechanical ventilation.
| Concentration of isoflurane | ||||||
|---|---|---|---|---|---|---|
| 2.11% | 3.14% | 4.15% | ||||
| Variable | Before stimulation | During stimulation | Before stimulation | During stimulation | Before stimulation | During stimulation |
| Heart rate (beats/min) | 225 ± 9 | 231 ± 6 | 221 ± 12 | 218 ± 11 | 201 ± 11 | 200 ± 10 |
| SAP (mm Hg) | 75 ± 4 | 83 ± 4† | 66 ± 5* | 69 ± 4† | 51 ± 3*‡ | 51 ± 4 |
| MAP (mm Hg) | 51 ± 5 | 54 ± 3 | 38 ± 4* | 40 ± 4 | 27 ± 3*‡ | 27 ± 3 |
| DAP (mm Hg) | 42 ± 5 | 44 ± 3 | 30 ± 4* | 30 ± 4 | 20 ± 2*‡ | 19 ± 2 |
| CVP (mm Hg) | 7 ± 1 | 8 ± 1 | 7 ± 1 | 8 ± 1 | 8 ± 1 | 9 ± 1 |
| MPAP (mm Hg) | 16 ± 1 | 16 ± 1 | 15 ± 2 | 17 ± 1 | 15 ± 2 | 17 ± 3 |
| PAOP (mm Hg) | 7 ± 1 | 7 ± 1 | 6 ± 1 | 6 ± 1 | 9 ± 1 | 8 ± 2 |
| Cardiac output (L/min) | 0.50 ± 0.05 | 0.56 ± 0.05† | 0.46 ± 0.03* | 0.49 ± 0.04† | 0.34 ± 0.03*‡ | 0.36 ± 0.04 |
| SI ([mL/beat]/m2) | 9.1 ± 0.7 | 9.9 ± 0.5† | 8.7 ± 0.6 | 9.4 ± 0.7† | 6.9 ± 0.5 | 7.4 ± 0.5 |
| SVRI ([{dynes•s}/cm5]/m2) | 1,715 ± 103 | 1,650 ± 166 | 1,306 ± 206 | 1,292 ± 169 | 1,074 ± 83* | 926 ± 38 |
| PVRI ([{dynes•s}/cm5]/m2) | 315 ± 49 | 318 ± 32 | 385 ± 61 | 431 ± 72 | 373 ± 85 | 441 ± 84 |
| RPP (beats/min•mm Hg) | 11,619 ± 1,348 | 12,502 ± 639 | 8,587 ± 1,323* | 8,833 ± 1,203 | 5,450 ± 784*‡ | 5,414 ± 779 |
| LVSWI ([g•m]/m2) | 7.7 ± 0.9 | 8.1 ± 0.5 | 5.2 ± 0.7* | 5.7 ± 0.5 | 3.2 ± 0.7*‡ | 3.2 ± 0.4 |
| RVSWI ([g•m]/m2) | 2.1 ± 0.2 | 2.3 ± 0.2 | 1.9 ± 0.2 | 2.3 ± 0.3† | 1.5 ± 0.2* | 1.7 ± 0.3 |
See Table 1 for key.
Assessment of the blood gas and associated calculated variables obtained from rabbits during mechanical ventilation without noxious stimulation revealed a significant reduction in oxygen delivery at an isoflurane concentration of 4.15%, compared with values at isoflurane concentrations of 3.14% (P = 0.001) and 2.11% (P = 0.002). Also the oxygen extraction ratio at an isoflurane concentration of 4.15% was significantly increased, compared with findings at an isoflurane concentration of 3.14% (Table 4).
Mean ± SEM blood gas and derived cardiorespiratory variables before and during application of a supramaximal noxious stimulus in 6 New Zealand white rabbits anesthetized with 3 concentrations of isoflurane under conditions of controlled mechanical ventilation.
| Concentration of isoflurane | ||||||
|---|---|---|---|---|---|---|
| 2.11% | 3.14% | 4.15% | ||||
| Variable | Before stimulation | During stimulation | Before stimulation | During stimulation | Before stimulation | During stimulation |
| Arterial blood pH | 7.46 ± 0.02 | 7.46 ± 0.03 | 7.46 ± 0.02 | 7.46 ± 0.02 | 7.45 ± 0.04 | 7.42 ± 0.02 |
| Paco2 (mm Hg) | 31 ± 0 | 31 ± 1 | 32 ± 1 | 30 ± 1 | 30 ± 2 | 32 ± 1 |
| Pao2 (mm Hg) | 428 ± 18 | 449 ± 16 | 364 ± 33 | 326 ± 25 | 390 ± 32 | 380 ± 36 |
| Arterial hemoglobin (g/dL) | 9.8 ± 0.4 | 9.6 ± 0.4 | 10.0 ± 0.6 | 9.1 ± 0.6† | 9.6 ± 0.5 | 9.2 ± 0.4† |
| Mixed venous blood pH | 7.41 ± 0.02 | 7.42 ± 0.03 | 7.43 ± 0.01 | 7.42 ± 0.02 | 7.41 ± 0.03 | 7.39 ± 0.03 |
| Pco2 (mm Hg) | 39 ± 1 | 39 ± 2 | 40 ± 2 | 39 ± 2 | 38 ± 2 | 40 ± 2 |
| Po2 (mm Hg) | 57 ± 3 | 61 ± 3t | 55 ± 2 | 57 ± 2 | 52 ± 5 | 54 ± 6 |
| Mixed venous hemoglobin (g/dL) | 10.1 ± 0.4 | 9.9 ± 0.4 | 10.3 ± 0.7 | 9.5 ± 0.7† | 9.6 ± 0.5 | 9.4 ± 0.5 |
| Lactate (mmol/L) | 2.4 ± 0.5 | 2.7 ± 0.3 | 1.7 ± 0.2 | 1.8 ± 0.2 | 2.5 ± 0.3 | 2.6 ± 0.4 |
| Cao2 (mL/dL) | 14.6 ± 0.6 | 14.4 ± 0.6 | 14.6 ± 0.8 | 13.4 ± 0.8† | 14.2 ± 0.7 | 13.6 ± 0.6† |
| Co2 (mL/dL) | 12.2 ± 0.3 | 12.3 ± 0.4 | 12.3 ± 0.6 | 11.5 ± 0.5† | 10.6 ± 0.8 | 10.6 ± 0.9 |
| Oxygen delivery (mL/min) | 72.1 ± 6.8 | 79.3 ± 6.2 | 66.0 ± 4.4 | 64.9 ± 5.7 | 48.4 ± 5.8*‡ | 50.2 ± 7.0 |
| Oxygen consumption (mL/min) | 11.4 ± 2.0 | 11.1 ± 1.8 | 9.1 ± 1.5 | 8.8 ± 2.1 | 10.5 ± 1.5 | 9.3 ± 0.7 |
| Oxygen extraction ratio | 0.16 ± 0.02 | 0.14 ± 0.02 | 0.14 ± 0.03 | 0.13 ± 0.03 | 0.24 ± 0.03* | 0.21 ± 0.04 |
| Pao2 - Pao2 (mm Hg) | 230 ± 19 | 209 ± 16 | 285 ± 33 | 325 ± 24 | 255 ± 31 | 261 ± 35 |
| Qs/Qt | 0.25 ± 0.05 | 0.27 ± 0.05 | 0.32 ± 0.06 | 0.39 ± 0.07 | 0.21 ± 0.03 | 0.23 ± 0.03 |
See Tables 1 and 2 for key.
Effects of noxious stimulation—In rabbits anesthetized with isoflurane and undergoing spontaneous ventilation, effects of noxious stimulation on cardiovascular variables were minimal (Table 1). Noxious stimulation of the rabbits anesthetized with isoflurane and undergoing controlled mechanical ventilation resulted in significant increases in SAP (P = 0.033), cardiac output (P = 0.026), and SI (P = 0.047) at an isoflurane concentration of 2.11% and in SAP (P = 0.041), cardiac output (P = 0.028), SI (P = 0.040), and RVSWI (P = 0.018) at an isoflurane concentration of 3.14% (Table 2). With either mode of ventilation, there were significant reductions in hemoglobin concentration and oxygen content in arterial and mixed venous blood during noxious stimulation of rabbits at some anesthetic doses (Tables 3 and 4).
Discussion
In the present study, increases in the concentration of isoflurane delivered to rabbits undergoing spontaneous ventilation were associated with progressively worsening hypoventilation and associated acidemia as well as reductions in arterial blood pressures (at an isoflurane concentration of 4.15%), but no significant reduction in cardiac output. When controlled mechanical ventilation was applied, the rabbits’ Paco2 remained unchanged, but increasing isoflurane concentration resulted in progressively lower arterial blood pressures and cardiac output.
The effect of increasing anesthetic dose on cardiovascular variables was more severe when the study rabbits were provided with controlled mechanical ventilation than when they were allowed to spontaneously ventilate. These findings are consistent with those for isoflurane-anesthetized cats10; however, to our knowledge, the present study is the first to describe in detail the cardiovascular effects for a range of isoflurane concentrations in rabbits.
The cardiac outputs determined for the rabbits in the present study were similar to values (300 to 600 mL/min) determined with a variety of measurement techniques in other studies11,12 of anesthetized rabbits. The cardiac outputs for the rabbits in the present study were lower than the value of 225 mL/kg/min measured by thermodilution in awake rabbits,13 which would equate to 855 mL/min for rabbits with the mean body weight of those in the present study.
Development of severe cardiovascular depression in isoflurane-anesthetized rabbits receiving mechanical ventilation in the present study was consistent with our hypothesis and can be explained by several potential contributing factors. Intermittent positive-pressure ventilation can impair cardiovascular performance by increasing intrathoracic pressure, thereby reducing venous return. Alternatively, or in addition, the negative effect of intermittent positive-pressure ventilation may be secondary to removal of the cardiovascular stimulating effects of high Paco2. In a study14 of sevoflurane-anesthetized cats, no significant effect of mechanical ventilation on cardiovascular or respiratory variables was detected at anesthetic doses up to 1.75× MAC. When data obtained from cats undergoing spontaneous and mechanical ventilation were pooled and reanalyzed in that study,14 there was a significant increase in Paco2 at 1.75× MAC of sevoflurane, compared with findings at 1.25× MAC. However, the mean Paco2 at the highest anesthetic dose in that study14 was 36 mm Hg, compared with 59 mm Hg in the rabbits undergoing spontaneous ventilation at the highest anesthetic dose in the present study. In another study,2 the mean Paco2 in rabbits anesthetized at 2.0× MAC of isoflurane was even higher (80 mm Hg).
Increases in Paco2 activate chemoreceptors, which subsequently increases CNS sympathetic output.15 In awake people, an increase in end-expired Pco2 by 10 mm Hg results in increases in arterial blood pressure, heart rate, and cardiac output and reduced total peripheral resistance.16 In the anesthetized rabbits undergoing spontaneous ventilation in the present study, there was no reduction or lesser magnitude of reduction in those variables as Paco2 increased.
In the present study, application of noxious stimulation of mechanically ventilated rabbits anesthetized at an isoflurane concentration of 2.11% or 3.14% resulted in significant increases in SAP, cardiac output, and SI; these changes were not evident at an isoflurane concentration of 4.15%. The high dose of isoflurane may have been sufficient to block autonomic responses to noxious stimulation. For isoflurane, the partial pressure that blocks autonomic response to a noxious stimulus (ie, the MACBAR), has been reported to be 2.6× MAC for goats17; when isoflurane is delivered in 60% nitrous oxide to humans, the MACBAR is 1.3× MAC.18 The MACBAR of isoflurane has not been investigated in rabbits, to our knowledge, but may be close to 2.0× MAC.
Interestingly, noxious stimulation had minimal effect on any measured variable in the isoflurane-anesthetized rabbits undergoing spontaneous ventilation in the present study. It is possible that sympathetic stimulation or the animals’ ability to respond to increases in sympathetic stimulation was already maximal in those rabbits because of their high Paco2. Additional studies would be needed to investigate this hypothesis.
Hypotension in rabbits undergoing inhalation anesthesia is not a new finding. In a previous study,2 mean MAPs in rabbits anesthetized with isoflurane at 1.0× and 1.5× MAC were 47 and 36 mm Hg, respectively. During anesthesia at isoflurane concentrations equivalent to 1.0×, 1.5×, and 2.0× MAC in the rabbits in the present study, mean MAP was 51, 38, and 27 mm Hg, respectively, with controlled mechanical ventilation and 47, 40, and 34 mm Hg, respectively, with spontaneous ventilation. Mean MAP in awake rabbits has been reported to be 80 mm Hg.13 This dose-dependent reduction in MAP associated with inhalation anesthesia in normocapnic animals is consistently reported across anesthetic agents and species.19
In the present study, MAP reduction associated with controlled mechanical ventilation was attributable to decreases in both cardiac output and SVRI at an isoflurane concentration of 4.15%. At an isoflurane concentration of 3.14%, although MAP was significantly reduced, the small reduction in cardiac output (8%) and larger reduction in SVRI (24%) were not significant. At clinically relevant concentrations of isoflurane (1.0× to 1.5× MAC), reduction in MAP is reported to be a result of a decrease in SVRI with minor change in cardiac output.19 In the present study, measurements of SVRI were much lower than values reported for cats14 and dogs20 undergoing inhalation anesthesia. Although not directly assessed in the present study, lower systemic vascular resistance may be the reason for the substantially lower arterial blood pressures in rabbits undergoing inhalation anesthesia, compared with findings in other species.
Because we did not measure tissue blood flow, it is only possible to make generalizations from the variables that were actually measured. Decreased SVR may facilitate blood flow to some organs, but decreases in MAP may reduce the necessary pressure gradient to produce organ flow. Ultimately, oxygen delivery is what is critical to each organ. In the present study, globally, oxygen delivery was reduced at an isoflurane concentration of 4.15% in the absence of significant alteration in arterial oxygen content. Oxygen extraction ratio increased accordingly as a compensatory change.
An interesting finding in the present study was the relatively high degree of venous admixture in all of the anesthetized rabbits, regardless of isoflurane concentration or mode of ventilation. Blood gas values with large Pao2 - Pao2 in rabbits have been reported previously. In awake sedated rabbits breathing room air, mean Pao2 - Pao2 between 15 and 25 mm Hg have been reported.21 Extrapolation from publications by other authors indicates a Pao2 - Pao2 of 250 to 400 mm Hg for rabbits anesthetized via injectable22 and inhalant-based23 techniques while breathing 100% O2, or close to it.
With regard to the present study, there may be some debate over whether the delivered anesthetic dose should have been based on multiples of an individual rabbit's MAC rather than multiples of a previously reported group MAC. The concept of MAC has allowed dose comparisons to be made across agents and across species for many years. However, the determination of MAC is based on the presence or absence of movement in response to a supramaximal stimulus and not the cardiovascular response or any other effect of the inhaled agent. Whether the dose-response relationships between different effects of inhalation anesthetics have the same slope has not been extensively studied. It has been suggested that the slope of the MAC curve differs from that of the MACBAR curve.24 In a small group of dogs, it has been demonstrated that cardiopulmonary variables do not differ significantly whether the inhalation anesthetic agent is administered at a multiple of an individual dog's MAC or at the same specific multiple of the group MAC.25
The concentrations of isoflurane evaluated in the present study were selected to provide a broad range over which to evaluate the cardiorespiratory effects of isoflurane in an experimental setting. For clinical use, the 2.0× MAC equivalent dose (ie, isoflurane concentration, 4.15%) is excessive and, as highlighted in the present study, associated with extreme cardiovascular and respiratory depression. Nevertheless, we chose to evaluate that high anesthetic dose to more fully characterize the cardiopulmonary effects of isoflurane in rabbits. What our findings do emphasize is that monitoring and adjustment of delivered anesthetic concentration to the minimum adequate level are critical for reducing the negative impact of isoflurane on the cardiovascular and respiratory systems in this species.
ABBREVIATIONS
| DAP | Diastolic arterial blood pressure |
| LVSWI | Left ventricular stroke work index |
| MAC | Minimum alveolar concentration |
| MACBAR | Minimum alveolar concentration to block autonomic reflexes |
| MAP | Mean arterial blood pressure |
| Pao2 - Pao2 | Alveolar-arterial difference in partial pressure of oxygen |
| RPP | Rate pressure product |
| RVSWI | Right ventricular stroke work index |
| SAP | Systolic arterial blood pressure |
| SI | Stroke index |
| SVRI | Systemic vascular resistance index |
Insyte catheter, Becton-Dickinson, Sandy, Utah.
Baxter Healthcare, Deerfield, Ill.
Check-Flo, Cook Inc, Bloomington, Ind.
Thermodilution balloon catheter, Arrow International, Reading, Pa.
Intracan B Braun, Melsungen, Germany.
Bair Hugger, Arizant Healthcare Inc, Eden Praire, Minn.
Physiograph, Gould Instrument Systems, Valley View, Ohio.
Ponemah, version 3.0, Gould Instrument Systems, Valley View, Ohio.
Rascal II, Ohmeda, Salt Lake City, Utah.
Beckman Medical gas analyzer LB1, Beckman Instruments, Schiller Park, Ill.
ABL 705, Radiometer, Copenhagen, Denmark.
OSM3, Radiometer, Copenhagen, Denmark.
COM-1, American Edwards Laboratories, Irvine, Calif.
Bird Mark 4, Bird Products Corp, Palm Springs, Calif.
Clinitube, Radiometer Medical, Copenhagen, Denmark.
Grass Instruments, Quincy, Mass.
Buprenex injectable, Reckitt Benckiser Healthcare, Richmond, Va.
Metacam, Boehringer Ingelheim, Germany.
GraphPad Prism, version 5, GraphPad Software Inc, La Jolla, Calif.
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