The effect of arterial pH on whole body oxygen uptake during hypothermic cardiopulmonary bypass in man

Acid-base regulation, alpha-stat, and the emperor's new clothes

Considerable time and effort have been expended to determine the most appropriate technique for management of a patient's acid-base status during hypothermic cardiopulmonary bypass. A critical question is whether to maintain plasma pH at 7.4 regardless of temperature (pH-stat) or to permit a relative alkalosis as the patient is cooled (alpha-stat). Until recently, there has been a remarkable lack of evidence in the literature for a consistent physiological benefit provided by one protocol over the other. The alpha-stat versus pH-stat controversy has taken on the characteristics of the emperor's new clothes, with alpha-stat winning by default because of theoretical arguments and because it was technically easier to perform. Part of the explanation is the realization that cellular mechanisms are capable of maintaining intracellular pH despite fluctuations in extracellular conditions. The prevailing plasma pH does have strong influence over cerebral blood flow, even to the point of overriding normal autoregulatory mechanisms. Recent evidence suggests that cerebral blood flow variations between alpha-stat and pH-stat conditions have important implications for patient outcomes.

The relationship between systemic oxygen uptake and delivery during moderate hypothermic cardiopulmonary bypass: critical values and effects of vasodilation by hydralazine

The relationship between oxygen delivery (DO2) and uptake (VO2) has been studied during moderately hypothermic cardiopulmonary bypass (CPB) in 15 patients undergoing myocardial revascularization. As soon as nasopharyngeal temperature was lowered to 32 degrees C, blood flow was decreased from 2.4 to 2.0 l/min/m2. Arterial and mixed venous blood gas analyses were performed five and eight minutes later and DO2 and VO2 were calculated; VO2 stabilized in five minutes after changing blood flow and neither DO2 nor VO2 values changed three minutes later (DO2: 217 +/- 19 versus 215 +/- 17 ml/min/m2; VO2: 63 +/- 12 versus 66 +/- 14 ml/min/m2). Blood flow then was increased to 2.3 l/min/m2 and DO2 and VO2 were determined again, five minutes later; they both increased significantly, to 243 +/- 20 and 74 +/- 13 ml/min/m2, respectively. However, a further blood flow increase to 2.6 l/min/m2 which caused DO2 to increase to 277 +/- 24 ml/min/m2, did not affect VO2 which was unchanged five minutes later (76 +/- 13 ml/min/m2); VO2 dependence on DO2 values higher than 243 +/- 20 ml/min/m2 was consequently ruled out. Ten patients, having a mean arterial pressure higher than 80 mmHg, were eventually vasodilated with hydralazine, 0.1 mg/kg intravenously, and DO2 and VO2 were determined after five and ten minutes. As blood flow did not change, DO2 was unaffected while a slight increase in VO2 mean values was observed which was not statistically significant (prior to hydralazine: 78 +/- 15 ml/min/m2; five minutes later: 82 +/- 17 ml/min/m2; 10 minutes later: 76 +/- 18 ml/min/m2). In conclusion, during hypothermic CPB at 32 degrees C, VO2 plateau ranges between 48 and 102 ml/min/m2 (mean +/- 2 SD) in 95% of patients, corresponding to 66 and 141 ml/min/m2 at 37 degrees C; this finding closely matches other literature reports. Consequently, lower VO2 values suggest inadequate oxygen supply to tissues. Critical DO2 at 32 degrees C is lower than 283 ml/min/m2 in 97.5% of patients. Finally, arterial vasoconstriction does not seem to play a significant role in tissue hypoperfusion.

Hemodynamics and oxygen metabolism in the pig during long-term hypothermia: comparison of 2 pH strategies

The aim of this study was to investigate the influence of acid-base management (pH stat or alpha stat) on hemodynamics and oxygen metabolism during long-term hypothermia in the pig. Seventeen female pigs were anesthetized, slowly cooled to 28 degrees C with cooling mats and kept at this temperature for 36 h. Thereafter, slow rewarming was performed with inhalation of a 40 degrees C warm air/oxygen mixture and insulation. Eight of the 17 pigs were ventilated according to the pH stat strategy and 9 according to the alpha stat strategy. Both groups were compared 4 times each for hemodynamics and metabolism during cooling, hypothermia and rewarming. The 2 strategies showed no significant difference in cardiac output, heart rate or mean arterial pressure. The only difference in hemodynamics was observed in mean pulmonary artery pressure and pulmonary artery resistance during hypothermia, showing higher values in pH stat animals. As for oxygen metabolism, oxygen consumption during hypothermia was significantly higher in alpha stat animals. Long-term hypothermia with spontaneous circulation revealed significant differences in oxygen metabolism and pulmonary artery pressure as well as resistance between alpha and pH stat acid-base management. These differences may be of importance when using moderate hypothermia in a clinical or experimental setting.

Cardiopulmonary bypass for adult patients: a survey of equipment and techniques

OBJECTIVES

The techniques and equipment used for cardiopulmonary bypass for adult cardiac surgery vary among institutions and may change over time. This study sought to document the changing patterns of practice.

DESIGN

Voluntary survey of meeting participants.

SETTING

13th Annual San Diego Cardiothoracic Surgery Symposium (February 1993).

PARTICIPANTS

There were 331 responses from perfusionists (79.5%), cardiac surgeons (11.2%), and anesthesiologists (6.3%). The majority of these participants were from institutions where more than 1,000 cardiac operations were performed annually.

MEASUREMENTS AND MAIN RESULTS

It was found that 91.5% of the respondents used membrane oxygenators currently, compared with 5% in 1982 (as reported in a previous survey). Over 80% of the institutions surveyed used some type of perioperative cell-salvaging technique. Arterial line filters were used by 92% of the respondents compared with 64% in 1982. Also, 80% of the respondents were aware of the availability of leukocyte-depleting filters.

CONCLUSIONS

The probable reasons for the increased utilization of membrane oxygenators and arterial line filters include less damage to the formed elements of blood, fewer gaseous microemboli, and better control of carbon dioxide elimination and oxygenation. The authors anticipate that future surveys will document increased use of leukocyte-depleting filters because of the literature implicating neutrophils as mediators of tissue destruction in various disease processes, including myocardial reperfusion injury.