Differences in cerebral blood flow (CBF) between alpha-stat and pH-stat management depend on preserved responsiveness of the cerebral vasculature to changes in arterial carbon dioxide tension (Pa(CO2)). We tested the hypothesis that hypothermia-induced reductions in CBF would decrease the CBF response to changing Pa(CO2) (ΔCBF/ΔPa(CO2)). Anesthetized New Zealand white rabbits were randomly assigned to one of three temperature groups-group 1 (37° C, n = 9); group 2 (31° C, n = 10); or group 3 (25° C, n = 10)-and were cooled using cardiopulmonary bypass. After esophageal temperature equilibration (~ 40 min), oxygenator gas flows were serially varied to achieve Pa(CO2) values of 20, 40, and 60 mmHg (temperature-corrected). All animals were studied at all three Pa(CO2) levels in random order. At each level of Pa(CO2), CBF and masseter blood flow were determined using radiolabeled microspheres. There were no significant differences between groups with respect to mean arterial pressure (~ 80 mmHg), central venous pressure (~ 4 mmHg), or hematocrit (~ 22%). Prior normothermic studies have found ΔCBF/ΔPa(CO2) to be proportional to CBF. Nevertheless, in this study, with hypothermia-induced reductions in CBF, ΔCBF/ΔPa(CO2) was not significantly different between temperature groups. Thus, hypothermia either increased the sensitivity of the cerebral vasculature to carbon dioxide and/or increased the effective level of cerebrospinal fluid respiratory acidosis produced by each increment of temperature-corrected Pa(CO2). This latter possibility is consistent with 'alpha-stat' acid-base theory, wherein such increments of actual (temperature-corrected) Pa(CO2) would produce increasing degrees of 'respiratory acidosis' (measured at 37° C) with progressive hypothermia. When we compared ΔCBF/ΔPa(CO2) between temperature groups using Pa(CO2) values measured at 37° C, we found that ΔCBF/ΔPa(CO2) did decrease with hypothermia-induced reductions in CBF. Over the Pa(CO2) range of 40-60 mmHg (measured at 37° C), ΔCBF/ΔPa(CO2) equalled 0.97 ± 0.60 ml · 100 g-1 · mmHg-1 at 37° C versus 0.54 ± 0.23 ml · 100 g-1 · min-1 · mmHg-1 at 25° C, (P = 0.02). These findings suggest that the CBF response to changes in Pa(CO2) is sensitive to variations around electrochemical neutrality and not to absolute levels of Pa(CO2). Although hypothermia lead to marked reductions in CBF, extracranial blood flow (in the masseter muscle) was little affected. This alteration of intracranial/extracranial flow distribution could lead to underestimation of CBF with xenon clearance methods used during hypothermic cardiopulmonary bypass, if the detection systems used overlie extracranial muscle.
- Anesthesia: cardiovascular
- Brain: blood flow; carbon dioxide response
- Cardiopulmonary bypass: blood gas management
- Temperature: hypothermia