The effect of maternal hypothermic cardiopulmonary bypass on fetal lamb temperature, hemodynamics, oxygenation, and acid-base baance

Objective: To evaluate fetal-maternal temperature relationship and fetal cardiovascular and metabolic response during maternal hypothermic cardiopulmonary bypass in pregnant ewes. Methods: Cardiopulmonary bypass was instituted in 9 pregnant ewes, reaching 2 different levels of maternal hypothermia: 24°C to 20°C (deep hypothermia) in group A (5 cases) and less than 20°C (very deep hypothermia) in group B (4 cases). Hypothermic levels were maintained for 20 minutes, then the rewarming phase was started. Fetal and maternal temperature, blood pressure, heart rate, electrocardiogram, blood gases, and acid-base balance were evaluated at different levels of hypothermia and during recovery. Results: Fetal survival was related to maternal hypothermia: all group A fetuses survived, while 2 of 4 fetuses of group B in which maternal temperature was lowered below 18°C died in a very deep acidotic and hypoxic status. Maternal temperature was always lower than fetal temperature during cooling; during rewarming the gradient was inverted. The start of cardiopulmonary bypass and cooling was associated with transient fetal tachycardia and hypertension; then, both fetal heart rate and blood pressure progressively decreased. The reduction of fetal heart rate was of 7 beats per minute for each degree of fetal cooling. Deep maternal hypothermia was associated with fetal alkalosis and reduction of PO ₂. Very deep hypothermia, in particular below 18°C, caused irreversible fetal acidosis and hypoxia. Conclusions: Deep maternal hypothermic cardiopulmonary bypass was associated with reversible modifications in fetal cardiovascular parameters, blood gases, and acid-base balance and therefore with fetal survival. On the contrary, fetuses did not survive to a very deep hypothermia below 18°C.