Blood gases. Alfa-stat and pH-stat strategies during CPB.

Maintain arterial oxygen level not higher than 250 mmHg and less than 85 mmHg.

Some clinical perfusions for cardiac surgery are performed

at normothermia (≈37°C) and others at various levels of

hypothermia: mild (30°C-35°C), moderate (25°C-30°C),

or deep ( less than 25).Therefore, it is necessary to consider the
strategy for controlling PaCO2 and, indirectly, pH.

    The alpha-stat strategy is based on using the pH measured at 37°C

and uncorrected for the temperature of the patient’s blood, and

 maintaining this level at pH 7.4. That is, the ventilation

of the oxygenator is maintained at the level appropriate for

a body temperature of 37°C, no matter how low the

temperature. This hyperventilation during hypothermia

results in a decrease in PaCO2 and an increase in pH when

the values for these are corrected for the temperature of

the patient’s blood. Swan and Reeves and Rahn and colleagues

have all emphasized that at low temperatures, neutrality

exists at a higher pH than at normothermia, because

of the change of the dissociation constant of water with

temperature. The alpha-stat strategy results in

optimal function of a number of important enzyme systems,

including lactate dehydrogenase, phosphofructokinase, and

sodium-potassium ATPase.

    In contrast, the pH-stat strategy strives for the same values

of pH and PaCO2, corrected to the temperature of the patient’s

blood, during hypothermia as at normothermia. This represents

a state of respiratory acidosis and hypercarbia. Cerebral

blood flow usually increases under these circumstances. This

may be considered advantageous in some situations, but

so-called luxury perfusion may expose the brain to a larger

number of microemboli than would otherwise be the case,

and therefore could be disadvantageous.

   At a cellular enzyme level, the alpha-stat strategy may be

preferable, but which is preferable in clinical cardiac surgery

in neonates, children, and adults is the subject of continued

investigation and debate. The alpha-stat strategy results

in a lower PaCO2, which may adversely affect cerebral blood

flow. This may be of particular importance for patients

with cyanotic congenital heart disease (e.g., tetralogy of

Fallot with pulmonary atresia) for whom low PaCO2 may

result in pulmonary vasodilatation in addition to cerebral

vasoconstriction. Thus, there can be a steal of blood from

the cerebral to the pulmonary vascular bed. Several

studies in infants suggest that pH-stat management results in

superior neurologic outcome during deep hypothermic CPB and hypothermic circulatory arrest. 

   The pH-stat technique may depress cardiac function. However, at least

in dogs, regional distribution of blood flow during normothermic

and hypothermic full-flow CPB is similar with the

alpha-stat and pH-stat strategies.

   A recent review of 16 best-evidence published papers concluded

that better results were achieved with the alpha-stat

technique in adult patients and with the pH-stat technique

in pediatric patients.