The effect of pH on the hypothermic ventricular fibrillation threshold

Comparison of Early Outcomes for Normothermic and Hypothermic Cardiopulmonary Bypass in Children Undergoing Congenital Heart Surgery

Objective: Comparison of early outcomes of normothermic cardiopulmonary bypass (N-CPB, ≥35°C) with hypothermic cardiopulmonary bypass (H-CPB, 28-34°C) for congenital heart defects. Methods: Data from 99 patients <2 years operated with N-CPB (n = 48) or H-CPB (n = 51) were retrospectively reviewed: aortic X-clamping and CPB duration, vasoactive inotropic score (VIS), arterial lactate, pH and base excess, urine output, extubation, PICU stay, transfusion requirements, chest drain losses, costs of transfusions, and costs of PICU stay. Results: The two groups were homogeneous for diagnosis, risk factors, surgery and demographic variables: N-CPB age 7.7 ± 6.1 months, weight 6.2 ± 2.4 kg, and H-CPB age 6.6 ± 6.5 months, weight 6.1 ± 2.4 kg. There were no hospital deaths in either group. VIS in N-CPB was lower than H-CPB on PICU arrival (9.7 ± 5.9 vs. 13.4 ± 7.9, P < 0.005), after 4 h (7.0 ± 5.2 vs. 11.1 ± 7.3, P < 0.001) and 24 h (2.8 ± 3.6 vs. 5.6 ± 5.6, P < 0.003); arterial pH was better at PICU arrival (7.33 ± 0.09 vs. 7.30 ± 0.09, P = 0.046) after 4 h (7.35 ± 0.07 vs. 7.32 ± 0.07, P = 0.022) and after 24 h (7.37 ± 0.05 vs. 7.35 ± 0.05, P = 0.01). Extubation was earlier in N-CPB than in H-CPB (22 ± 27 vs. 48 ± 57 h, P = 0.003) as PICU discharge (61 ± 46 h vs. 87 ± 69 h, P = 0.021). Transfusion requirements in operating room were lower in N-CPB vs. H-CPB for RBC, FFP, cryoprecipitate, and platelets, while during the first 24 h in PICU were lower only for cryoprecipitate and platelets. Chest drain losses (mL/kg) on PICU arrival, after 4 and 24 h were lower with N-CPB vs. H-CPB (respectively 1.5 ± 1.4 vs. 2.5 ± 2.7, P = 0.013, 7.8 ± 6.0 vs. 10.9 ± 8.7, P = 0.025, and 23.0 ± 12.0 vs. 27.9 ± 15.2, P = 0.043). Tranexamic acid infusion was required in 7/48 (14.6%) patients with N-CPB vs. 18/51(= 35.3%) in H-CPB (P = 0.009). The average total costs/patient of blood and blood products (RBC, FFP, cryoprecipitate, platelets) were lower in N-CPB vs. H-CPB for both the first 24 h after surgery (£204 ± 169 vs. £306 ± 254, P = 0.011) as well as during the total duration of PICU stay (£239 ± 193 vs. £427 ± 337, P = 0.001). The average cost/patient/day of stay in PICU was lower in N-CPB than in H-CPB (£4,067 ± 3,067 vs. £5,800 ± 4,600, P = 0.021). Conclusions: N-CPB may reduce inotropic and respiratory support, shorten PICU stay, and decrease peri-operative transfusion requirements, with subsequent costs reduction, compared to H-CPB. Future studies are needed to validate and support wider use of N-CPB.

Haemocompatibility of membrane and bubble oxygenators

During clinical hypothermic cardiopulmonary bypass (CPB), the haemocompatibility of six groups of membrane oxygenators (Cobe CML2, Shiley M2000, Maxima, Bard HF4000, Bard HF5000, Capiox E has been studied in 60 patients having open-heart surgery. A standardized anaesthetic and perfusion protocol was used, during which the abilityof the perfusionist to achieve target blood gas values (PaO2 20kPa and PaCO2 5.3kPa: alpha-stat) using inline electrodes was assessed. Haemocompatibility was evaluated by measurement of platelet numbers and function, betathromboglobulin (BTG), plasma haemoglobin, complement (C3a des Arg) and white blood cell (WBC) count pre- and post-CPB. Platelet and WBC numbers were also measured every five minutes throughout CPB.

All oxygenators allowed the perfusionist to control blood gases adequately to prescribed levels. There were only minor differences in the degree and pattern of platelet depletion, reduction in platelet aggregation, elevation of BTG and C3a des Arg observed between oxygenator groups, which did not appear to be influenced by membrane type (flat plate versus hollow fibre). The membrane oxygenator haematological data was amalgamated with that obtained in previous clinical studies using membrane and bubble oxygenators (Cobe CML, Polystan Venotherm, Harvey H 1700, Bentley BIO-10, Bentley 1 0B, Bentley 1 OPlus, Gambro 10 and Shiley S100A HED) in which a similar evaluation protocol was employed.

Comparison of the percentage change in platelet count when the pre- and post-CPB values were compared, demonstrated statistically significantly less platelet depletion (p <0.001 ) in the membrane oxygenator groups (-0.2 ± 8.3%) when compared to the bubble oxygenator groups (-21.7 ± 8.7%). A significantly lower percentage rise in BTG was also observed in the membrane oxygenator group when compared to the bubble oxygenator groups (p <0.001 ). All oxygenator groups showed elevation of both WBC count and plasma haemoglobin with a nonspecific fall in platelet aggregation over the period of bypass but no significant differences could be found between the two types of oxygenator.

Membrane oxygenators, when compared to bubble oxygenators, exhibit lower GME production and improved haemocompatibility and allow superior blood gas control. Membrane oxygenators manifestly must be the oxygenator type of choice for clinical CPB.

A clinical evaluation of the Bentley 10B and Bentley 10Plus bubble oxygenators

During clinical hypothermic cardiopulmonary bypass (CPB) in 20 adult patients, the ability of the perfusionist, using an alphastat acid-base blood gas management technique, to control blood gas values has been evaluated in the Bentley 10B and Bentley 10Plus bubble oxygenators. Superior flexibility in control of the blood gas values to within a defined target range (PaO2 20 ± 3.3 kPa, PaCO2 5.3 ± 0.6 kPa) with significantly improved control of PaO2 was demonstrated in the Bentley 10Plus when compared with the Bentley 1 0B. The percentage of values when both PaCO2 and PaO2 were within the target range was higher in the Bentley 10Plus (14%) than in the Bentley 10B (11 %). The incorporation of an Integral gas proportioning valve in the Bentley 10Plus oxygenator offers a degree of independence of control of PaO 2 and PaCO2 which is unique in a bubble oxygenator. When the two oxygenator groups were compared, the rise in plasma haemoglobin was significantly less (p<0.005) in the Bentley 10Plus but no significant differences could be demonstrated between the two groups with respect to alteration in other formed blood elements (platelets or white blood cells) or in gaseous microemboli production.

Review article : Acid-base status, hypothermia and cardiac surgery

The optimal method of pH management during hypothermia is controversial. A review of current experimental and laboratory investigations comparing different pH schemes is presented. The biochemical and metabolic consequences of keeping pH constant (pH-stat) as compared to allowing the pH to increase with hypothermia (alpha-stat) are discussed. It is concluded that, on the basis of experimental and clinical studies, the alpha-stat pH management scheme may be preferable to that of keeping pH constant throughout a wide temperature range in humans.

Warm pediatric cardiac surgery: European experience

Cold pediatric cardiac surgery has been a dogma for 50 years. However, the beneficial effects of cold perfusion are counterbalanced by the drawbacks of hypothermia. Thus, we propose a major paradigm shift from hypothermic surgery to warm perfusion and intermittent warm blood cardioplegia. This approach gives satisfactory results even with prolonged aortic crossclamp times. The major advantages are reduced time to extubation and shorter intensive care unit stay. Warm pediatric surgery is an anecdotal phenomenon no more; over 10,000 procedures have been carried out in Europe. All types of cardiopathy have been treated, including arterial switch, total pulmonary anomalous venous return, interruption of the aortic arch, and hypoplastic left heart syndrome. Once surgeons decide to shift from hypothermia to normothermia, they never decide to shift back to hypothermia. This fact is evidence of the satisfactory clinical outcome obtained with this technique. The technique and the composition of microplegia is identical in all European centers, the only variable factor being the interval between microplegia injections, which varies from 10 to 25 min. We hope that the increasing interest in pediatric warm surgery will hearten new candidates.

Adrenalectomy and Caval Thrombectomy in a Cat With Primary Hyperaldosteronism

A 13-year-old, castrated male, domestic longhaired cat was diagnosed with primary hyperaldosteronism from an adrenal gland tumor and a thrombus in the caudal vena cava. Clinical signs included cervical ventriflexion, lethargy, weakness, inappetence, and diarrhea. Laboratory tests revealed hypokalemia, normonatremia, hyperglycemia, hypophosphatemia, and elevated creatine kinase activity. Hypokalemia worsened despite oral potassium supplementation. An adrenalectomy and caval thrombectomy were successfully performed utilizing deliberate hypothermia followed by progressive rewarming.

Arterial blood gas management during cardiopulmonary bypass

This study sought to investigate the effects of α-stat and pH stat regimens on cardiac outcome during moderate hypothermic cardiopulmonary bypass. 100 patients undergoing elective coronary artery bypass grafting (CABG) were randomly assigned with respect to the target value for PaCO(2) during cardiopulmonary bypass (CPB) into 2 groups. In 50 patients the target PaCO(2) was 40 mmHg, measured at a standard electrode temperature of 37°C while in the other 50 patients the target PaCO(2) was 40 mmHg, corrected to the patients nasopharyngeal temperature (lowest value reached: 32±0.5°C). There were no significant differences between groups with regards to cardiac outcome such as appearance of new 'Q' waves on the electrocardiogram, postoperative creatinine kinase-MB fraction, systemic vascular resistance (SVR), cardiac index (Cl), need for inotropic or intra-aortic balloon pump support and the length of postoperative ventilation or intensive care unit stay. These findings support the hypothesis that CO(2) management during CPB at moderate hypothermia has no clinically significant effect on cardiac outcome.

Comparison of pH-Stat Versus Alpha-Stat During Hypothermic Cardiopulmonary Bypass in the Prevention and Control of Acidosis in Cardiac Surgery

OBJECTIVES

To compare the effects of blood-gas management using either alpha-stat (temperature-uncorrected blood-gas management) or pH-stat (temperature-corrected blood-gas management) strategies, 30 patients undergoing coronary artery bypass surgery allocated randomly to either one of the approaches were studied. Acid-base balance, tissue oxygenation, and biochemical parameters were measured at distinct times: before bypass, after 15 min of hypothermia at 32 degrees C, after 45 min of hypothermia at 32 degrees C, after 15 min of rewarming at 37 degrees C, and 45 min after the end of bypass in normothermic conditions.

RESULTS

The groups were similar with regard to physical characteristics, physiological parameters, and bypass time. In the pH-stat group, CO2 administered with the aim of correcting pH for the patients hypothermic temperature caused a significant increase in temperature-uncorrected PaCO2 and a decrease in arterial temperature-uncorrected pH at 45 min. During the rewarming period and following bypass, the pH was lower and PaCO2 higher in the pH-stat group (P < 0.001).

CONCLUSION

It was found that during the rewarming period and following bypass, the resulting acidosis caused by the procedure was less in the alpha-stat group. It was found that there were no difference between the two groups, with regard to tissue perfusion, as is seen by the tissue oxygenation parameters and lactic acid concentration.

Acid-base management during cardiopulmonary bypass. Current trends in the United Kingdom

Controversy exists over which method of acid-base management should be used during hypothermic cardiopulmonary bypass: the alpha-stat method or the pH-stat method. We surveyed 40 centres in the United Kingdom to assess current practice; whether practice has changed recently; and if so, why. Ten years ago, 31 centres (77.5%) used pH-stat and seven (17.5%) used alpha-stat. Currently nine centres (22.5%) are using pH-stat, 25 (62.5%) are using alpha-stat and four (10%) are using both. One centre (2.5%) has always used a modified pH-stat technique, and one continues to use alpha-stat in children and pH-stat in adults. Twenty-one of the 22 centres which have changed their practice did so because of increasing reports incriminating pH-stat in postoperative morbidity, particularly cerebral dysfunction. Our results show a marked trend towards using alpha-stat methods rather than the more traditional pH-stat approach.

A randomized study of the influence of perfusion technique and pH management strategy in 316 patients undergoing coronary artery bypass surgery. I. Mortality and cardiovascular morbidity

The impact of perfusion technique and mode of pH management during cardiopulmonary bypass has not been well characterized with respect to postoperative cardiovascular outcome.

METHODS

This double-blind, randomized study comparing outcomes after alpha-stat or pH-stat management and pulsatile or nonpulsatile perfusion during moderate hypothermic cardiopulmonary bypass was undertaken in 316 patients undergoing coronary artery bypass operations.

RESULTS

Cardiovascular morbidity and mortality were not affected by pH management, and the incidence of stroke (2.5%) did not differ between groups. Overall in-hospital mortality was 2.8%, eight of the nine deaths occurring in the nonpulsatile group (5.1% versus 0.6%; p = 0.018). The incidence of myocardial infarction was 5.7% in the nonpulsatile group and 0.6% in the pulsatile group (p = 0.010), and use of intraaortic balloon pulsation was significantly more common in the nonpulsatile group (7.0% versus 1.9%; p = 0.029). The overall percentage of patients having major complications was also significantly higher in the nonpulsatile group (15.2% versus 5.7%; p = 0.006). Duration of cardiopulmonary bypass, age, and use of nonpulsatile perfusion all correlated significantly with adverse outcome.

CONCLUSIONS

Use of pulsatile perfusion during cardiopulmonary bypass was associated with decreased incidences of myocardial infarction, death, and major complications.

The effects of normothermic and hypothermic cardiopulmonary bypass on defibrillation energy requirements and transmyocardial impedance. Implications for implantable cardioverter-defibrillator implantation

The influence of normothermic and hypothermic cardiopulmonary bypass on defibrillation energy requirements and transcardiac impedance is not well characterized. However, this relationship is of clinical importance during automatic defibrillator implantation done with concomitant cardiac surgery, and there is anecdotal information that criteria for successful implantation are harder to achieve after such operations. We studied the effect of controlled hypothermia on defibrillation energy requirements and transcardiac impedance in a canine model of cardiopulmonary bypass in which 26 animals underwent right atrial and femoral arterial cannulation, as well as continuous hemodynamic and intramyocardial temperature monitoring. The defibrillation energy requirements were evaluated at 60-minute intervals with an epicardial patch system, and transcardiac impedance was measured before and after the multiple inductions and terminations of ventricular fibrillation. In group 1 (n = 10) defibrillation energy requirements were evaluated immediately after initiation of cardiopulmonary bypass at 37 degrees C (T0), after gradual cooling to 28 degrees C (T1), and after rewarming to 37 degrees C (T2). Group 2 (n = 16) comprised time controls that were identically instrumented and studied, but maintained at 37 degrees C throughout. Percent successful defibrillation was plotted against delivered energy, and the raw data fit by logistic regression. The energy at which 50% of shocks were successful (E50) was 3.23 +/- 0.89 joules at T0, 5.12 +/- 1.85 joules at T1, and 4.42 +/- 1.22 joules at T2 in group 1; this was not significantly different from the corresponding group 2 E50 values, which were 3.11 +/- 1.39 joules, 4.95 +/- 2.47 joules, and 5.59 +/- 3.18 joules, respectively. Both groups demonstrated a significant increase in E50 during the first hour of cardiopulmonary bypass (mean increase from T0 to T1 was 1.89 joules in group 1 and 1.84 joules in group 2, p < 0.05). Transmyocardial impedance fell progressively during the group 2 experiments from 73.6 +/- 12.9 omega at the beginning of the T0 shock series to 61.4 +/- 8.9 omega at the end of the T2 shock series. A similar reduction in transmyocardial impedance was observed during the course of all the group 1 experiments; however, at the beginning of the T1 shock series impedance was significantly elevated to 77.4 +/- 12.3 omega (p < 0.05 compared with group 2 and with end T0 in group 1). There was no relationship between defibrillation energy requirements and transcardiac impedance; there was also no correlation between either of these parameters and intramyocardial extracellular pH or left ventricular end-diastolic pressure.(ABSTRACT TRUNCATED AT 400 WORDS)

Hemodynamics and metabolism during surface-induced hypothermia in the dog: a comparison of pH management strategies

The management of blood pH during hypothermia remains controversial. The present study was designed to determine whether hemodynamics and oxygen consumption during hypothermia are different between the alpha-stat and pH-stat strategies. Theoretical considerations of enzyme kinetics suggest that the alpha-stat strategy would result in a higher oxygen consumption during hypothermia. Because hypothermia is used to decrease oxygen consumption for protection during ischemia, a pH scheme that results in a greater oxygen demand for any level of ischemia would be detrimental. The core temperature of 22 dogs was lowered to 26 degrees C by combined surface cooling and gastric irrigation. Either the alpha-stat (N = 9) or the pH-stat (N = 13) pH strategy was used. The arterial pressure was different between the two groups at 26 degrees C (65 +/- 6 vs 85 +/- 6 mm Hg, alpha-stat vs pH-stat, respectively, P less than 0.05). Neither systemic oxygen consumption nor the Q10 was different between groups. There were no differences in any other hemodynamic parameters. In summary, during moderate hypothermia alpha-stat pH management results in an arterial pressure lower than that of pH-stat management, possibly resulting in improved peripheral perfusion. Despite theoretical predictions, the alpha-stat pH scheme does not result in an oxygen consumption higher than that of the pH-stat scheme.

Autoperfusing ectothermic heart-lung preservation system

A portable heart-lung preservation system was developed to enable distant organ procurement. In 8 dogs, a functioning heart-lung system was isolated, cannulated in situ, removed en bloc, and placed into a Plasmalyte-filled, temperature-controlled (15-38 degrees C) chamber. The perfusion circuit consisted of an adjustable-height, autologous-blood reservoir. The heart ejected through the aortic cannula with venous return (VR) into the superior vena cava. Respiration was maintained with a positive pressure ventilator. Intramyocardial tissue pressure (IMP), tissue pH, right atrial (RAP), aortic pressure (AOP), and cardiac output (CO) were monitored. Autoperfusion at normothermia and CO of 50 mL/min/kg resulted in early (3 h) deterioration of pulmonary function with progressive interstitial lung edema. Pulmonary dysfunction always preceded changes in regional myocardial contractile viability (peak IMP and d(IMP)/dt) and global function (CO, AOP). When tissue pH was regulated ectothermically (as in cold-blooded animals) (delta pH/degrees C = -0.015) the heart maintained a stable pumping mode (greater than 6 h) at myocardial temperatures of 17-28 degrees C, pH = 7.70-7.55, and heart rate of 25-50 bpm, respectively. The results indicate that a viable (greater than 6 h) autoperfused, working heart-lung system can be achieved by reducing the circulating blood flow to 30-50% of normal CO. More significantly, ectothermic alpha-stat modulation of perfusate pH and pCO2 allows a substantial reduction in organ temperature and metabolic demand without endangering induction of fibrillation and ultimate allograft failure.

The effects of antishock trouser inflation during hypothermic cardiovascular depression in the canine model

The purpose of this study was to investigate the value and to identify any deleterious effects of antishock trouser use during hypothermic cardiovascular depression. Thirteen mongrels were made hypotensive by cooling to a core temperature of approximately 27 degrees C. Eight dogs had antishock trousers inflated for one hour and five dogs served as controls. Metabolic and hemodynamic variables were measured at regular intervals during cooling, during trouser inflation, and after trouser deflation. No study animal experienced ventricular fibrillation. Neither central temperature, pH, or serum potassium nor mean arterial BP or systemic vascular resistance were significantly affected by trouser inflation or deflation. Antishock trouser use during the early phase of hypothermia before rewarming does not appear to result in a central bolus of cold, acidotic, hyperkalemic blood or the precipitation of ventricular fibrillation. There appears to be no significant hemodynamic benefit of antishock trouser use early in the management of hypotension caused by moderate hypothermia.

Multicenter hypothermia survey

A multicenter survey evaluated the clinical presentation, treatment, and outcome of accidental hypothermia. Data were collected from 13 emergency departments, with 401 of the 428 cases presenting during a two-year study period. Core temperatures ranged from 35 C to 15.6 C (mean, 30.57 C +/- 3.53) with 272 cases (63.6%) less than or equal to 32.2 C. There were no significant differences by age in presenting temperature, rewarming strategies, or mortality. The first hour rewarming rate was significantly (P less than .05) faster in the population less than or equal to 59 years (1.08 +/- 1.39 C/hr) than in those greater than or equal to 60 years (0.75 +/- 1.16 C/hr). Male core temperatures averaged 30.27 +/- 3.44 C versus female temperatures of 31.1 +/- 3.61 C. There were no clinically significant differences in male (N = 296) versus female (N = 132) profiles. High ethanol levels (315 to 800 mg%) did not affect outcome. Nine of 27 (33%) patients who received CPR initiated in the field survived, versus six of 14 (43%) with CPR begun in the ED. The profile of the CPR versus non-CPR population differed significantly (P less than .05) in location (outdoors), initial temperature (24.8 +/- 3.77 C vs 30.94 +/- 3.12 C), third-hour rewarming rate (2.28 +/- 1.53 C vs 1.17 +/- 1.18 C/hr), and numerous laboratory parameters. Tracheal intubation was performed without incident in 117 cases, of which 97 were less than or equal to 32.2 C. There were 73 fatalities (17.1%). Of these, 84.9% (N = 62) were less than or equal to 32.2 C. Predisposing conditions in this group included "serious" illness (30), systemic infection (28), trauma (15), immersion (ten), frostbite (seven), and overdose (two). The initial pulse, hemoglobin, and first-hour rewarming rate was lower in the deceased population, while the potassium, urea nitrogen, creatinine, and phosphorus were elevated. Excluding treatment combinations, outcome with exclusive use of a single rewarming strategy was passive external rewarming, 14 deaths below 32.2 C, 13 above; active external rewarming, six deaths below 32.2 C, two above; active core rewarming, 38 deaths below 32.2 C, none above. Refinements of the American Heart Association's CPR standards in hypothermia and a Hypothermia Survival Index are proposed.

Hypothermic cardiopulmonary bypass: a comparison between alpha and pH stat regulation in the dog

The accepted normal pH of 7.40 may not be optimal at lower temperatures. This study evaluated the effect of maintaining pH in the accepted normal range at hypothermia (group 1, pH stat) or at normothermia (group 2, alpha stat) on organ blood flow and hemodynamics in dogs. The desired pH was achieved at all temperatures by adjusting pCO2. Hypothermia to 20 degrees C was induced by high flow bypass in both groups followed by 45 min of reduced flow before rewarming. In group 1 (n = 10), pH was 7.45 +/- 0.02 at 20 degrees C and in group 2 (n = 11) it was 7.64 +/- 0.01. A greater base excess developed by the end of the low flow period in pH stat animals (-9.4 +/- 1.1 vs -2.8 +/- 0.8, P less than .001) and resulted in continued acidosis after rewarming in the pH stat group compared to those in the alpha stat group (7.32 +/- 0.03 vs 7.38 +/- 0.01, P less than .02). Expressed as a percentage of baseline, regional blood flows after rewarming for group 1 were: brain 112 +/- 8%, renal cortex 48 +/- 8% (P less than 0.005, rewarm vs baseline), renal medulla 36 +/- 9% (P less than 0.005, rewarm vs baseline), epicardium 198 +/- 40% (P less than 0.05, rewarm vs baseline), endocardium 151 +/- 25%.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial preservation

The principles used to develop techniques for myocardial preservation in cardiac surgery have been successfully applied to the protection of the donor heart in transplant surgery. This article reviews the latest advances in myocardial preservation during cardiac surgery and shows how they have been adopted in current cardiac transplant techniques.

Recent advances in cardiopulmonary bypass and the clinical application of myocardial protection

Basic scientific research has provided the impetus to develop cardioplegic solutions that offer excellent myocardial preservation. Future research will continue to develop methods for better delivery of cardioplegia to all myocardial regions. In addition, earlier detection of evolving ischemic damage during aortic cross-clamping might provide a basis for earlier intervention to reverse developing myocardial injury. At the present time, the cardiac surgeon has many cardioplegic solutions and delivery systems from which to choose. Only by understanding the principles involved in myocardial preservation will the surgeon be able to develop a system that will work best in his or her clinical practice.

Blood acid-base status of an awake heterothermic rodent, Spermophilus tereticaudus

The desert ground squirrel Spermophilus tereticaudus is shown to show both reptilian style alphastat regulation and the pH-stat regulation typical of mammalian hibernation, depending upon the range of body temperature and the state of vigilance. Temperature corrected arterial pH and PCO2 of torpid squirrels (Tb 11-28 degrees C) were independent of Tb and about equal to euthermic values at 37 degrees C. Torpid squirrels show a progressive respiratory acidosis as Tb is lowered. In awake heterothermic squirrels (Tb 30-42.5 degrees C), blood acid-base status is like that of many ectothermic vertebrates: from 30 to 40 degrees C delta pHa/delta Tb was -0.0121, delta PaCO2/delta Tb was 1.057 and [HCO3-] remained about constant. Arterial blood from awake heterothermic squirrels measured at standard temperature (37 degrees C) showed no significant change with Tb, similar to blood undergoing anaerobic temperature changes in vitro. In vitro, the delta pH/delta T of blood of constant CCO2 was -0.014. Constant blood pH with change in Tb is thus not a general feature of mammalian acid-base regulation but appears in this species to be a feature of the respiratory and metabolic poise of torpor.