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Ann Thorac Surg 1986;41:525-530
© 1986 The Society of Thoracic Surgeons

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Articles

Cerebral Carbon Dioxide Reactivity during Nonpulsatile Cardiopulmonary Bypass

Departments of Neurosurgery, Cardiothoracic Surgery and Anesthesiology, Rikshospitalet, Oslo, Norway

Accepted for publication August 10, 1985.

^* Address reprint requests to Dr. Lundar, Department of Neurosurgery, Rikshospitalet, 0027 Oslo 1, Norway

Five patients undergoing extensive cerebral monitoring during cardiopulmonary bypass (CPB) procedures were subjected to studies on cerebral CO₂ reactivity during nonpulsatile CPB. The cerebral monitoring included recording of arterial blood pressure (BP), central venous pressure (CVP), epidural intracranial pressure (EDP), cerebral electrical activity by a cerebral function monitor (CFM), and middle cerebral artery (MCA) flow velocity by transcranial Doppler technique. The cerebral perfusion pressure (CPP) was thus continuously recorded (CPP = BP - EDP).

During steady-state CPB with constant hematocrit, temperature, and arterial carbon dioxide tension (PaCO₂), MCA flow velocity varied with changing CPP in a pressure-passive manner, indicating that the cerebral autoregulation was not operative.

During moderately hypothermic (28 to 32°C), nonpulsatile CPB, with steady-state hematocrit, temperature, and pump flow, we deliberately and rapidly changed PaCO₂ for periods of 1 or 2 minutes by increasing gas flow to the membrane oxygenator, thereby testing the cerebral CO₂ reactivity. Nineteen CO₂ reactivity tests, performed at CPP levels ranging from 17 to 75 mm Hg, disclosed that the cerebral CO₂ reactivity decreased with CPP, especially with CPP levels below 35 mm Hg. In these patients, concomitant changes in CPP during the CO₂ reactivity test could be compensated for by adjusting the observed change in MCA flow velocity. The corrected CO₂ reactivity values obtained in this way ranged from below 1.0 (observed at CPP levels below 20 mm Hg) to a 3.0 to 4.5% · mm Hg^–1 change in PaCO₂ (observed at CPP levels above 35 mm Hg).

Four CO₂ reactivity tests performed after rewarming was completed revealed corrected CO₂ reactivity values in the range from 1.3 to 1.9% · mm Hg^–1 change in PaCO₂, observed at CPP in the range of 41 to 50 mm Hg.

Cerebral CO₂ reactivity seems to be partially retained down to CPP levels of 20 mm Hg during moderately hypothermic, nonpulsatile CPB.

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