Na⁺/K⁺-ATPase activity in the erythrocytes of infants after normothermic and deep hypothermic low-flow cardiopulmonary bypass

Objectives

Previous studies reported impairments of membrane-bound ionic pumps as a result of membrane damage and changes in brain tissue enzyme activity correlated with the progression of cerebral morphological changes and increased permeability of brain microvessels. The aim of the present study was to assess whether an alteration of Na⁺/K⁺-ATPase activity after normothermic and deep hypothermic low-flow cardiopulmonary bypass (CPB) is also detectable on paediatric erythrocytes (RBC) membranes in cerebral venous blood samples.

Method

After IRB approval and informed consent, 20 infants and children were grouped as follows: 1, deep hypothermic low-flow cardiopulmonary bypass (DHCPB; n = 10, rectal temperature 16.7 ± 3.1 °C), and 2, normothermic CPB (rectal temperature 35.2 ± 0.8 °C). Age and weight were 3.2 ± 2.9 months and 3.8 ± 0.5 kg for group 1, and 6.9 ± 1.4 months and 5.8 ± 1.9 kg for group 2. Two hundred μ l heparinized blood samples were obtained from an jugular bulb catheter before the induction of CPB, at 1 h (immediately after CPB termination), and subsequently at 2, 3 and 4 h postoperatively. The protein content of haemoglobin-free erythrocyte membranes (ghosts) was determined and the linear rate of NADH-oxidation was measured for 10 min at 340 nm. The enzyme activity was calculated from the difference of NADH-oxidation in the absence and in the presence of 1 mmol/l ouabain. One unit Na⁺/K⁺-ATPase represents 1 nmol ATP degradation/mg protein per h.

Results

The Na⁺/K⁺-ATPase activity pre-CPB was higher in the DHCPB-group (419 ± 15.2 versus 329 ± 20.5). In the early postoperative phase the Na⁺/K⁺-ATPase activity was restored to prebypass levels in group 2, but remained decreased in group 1 (P < 0.05) compared with prebypass. Plasma monovalent cationic levels did not alter significantly during the observation period.

Conclusion

The decrease in Na⁺/K⁺-ATPase activity of RBC membranes in infants during the early postoperative period after deep hypothermic CPB is detectable and indicate an alteration of cell membrane proteins/structures (possibly due to hypoxic stress or lipid peroxidation). The RBC membrane Na⁺/K⁺-ATPase measurements appears to be an sensitive marker of subtle cerebral deterioration.