Surgery: pH stat is superior to alpha-stat in infants

Perioperative effects of alpha-stat versus pH-stat strategies for deep hypothermic cardiopulmonary bypass in infants. Adre J. du Plessis et al. JTVCS 1997;114:991-1001.

Randomized, single-center trial in infants, n=182 (90 alpha-stat & 92 pH-stat)

Early death : 4 in alpha-stat, 0 in pH-stat

EEG seizures: 5/57 in alpha-stat, 1/59 in pH-stat

Clinical seizures: 4 in alpha-state, 2 in pH-stat

First EEG activity occurred sooner in alpha-stat gp.

Higher CI, lower inotrope requirement, less frequent acidosis & hypotension and shorter ventilation time.

Conclusion: pH stat is superior to alpha-stat in infants.

Method:

Alpha-stat - During induction of hypothermia, there is a natural tendency for blood to become more alkalotic. With no intervention, this constitutes alpha-stat strategy.

pH-stat: Patient's pH is read from blood gas analyzer at 37 deg C. pH is adjusted to 7.40 at patient's tympanic membrane temperature using a nomogram. This is accomplished by addition of CO2 to the ventilating gas in the oxygenator during cooling and low-flow bypass. During rewarming, alpha-stat management is instituted for all patients.

Blood flow during cooling and rewarming was 150 ml/kg/min; during low-flow bypass 50 ml/kg/min. Patients were rewarmed to a rectal temperature of 35 deg C.

Discussion:

pH-stat causes cerebral vasodilatation. Cerebral oxygen supply is enhanced with decreased use. Alternative explanation is extracellular acidosis noted in pH-stat strategy inhibits cerebral excitotoxicity. And, CO2 used in pH-stat strategy seems to have suppressive effect on cerebral metabolic rate - decreasing oxygen demand. In rodents, intraischemic CO2 is neuroprotective.