Post-Induction Low Cardiac Output Syndrome in Coronary Artery Bypass Graft Surgery

Hemodynamic change after induction of general anesthesia is usually judged from mean arterial pressure. Pressure, however, is a surrogate for flow, and the two can separate: cardiac output may fall while arterial pressure remains apparently adequate. The interval that follows induction of anesthesia, before surgical stimulation begins and while hemodynamic conditions are determined by anesthetic management alone, has not been characterized in terms of flow. This study is designed to define post-induction low cardiac output (PiLCOs) in patients undergoing coronary artery bypass grafting. This is a prospective, observational, two-centre cohort study. Consecutive adults undergoing isolated coronary artery bypass grafting with cardiopulmonary bypass at two tertiary cardiovascular surgery centre will be enrolled. No aspect of anesthetic or surgical management will be altered for the purposes of the study. An arterial catheter is sited before induction while the patient is awake, and cardiac index is recorded continuously at 30-second intervals by arterial waveform analysis. Mean arterial pressure, heart rate, stroke volume index and cardiac index are recorded at the same resolution. The analysis window is the 30 minutes that follow induction of anesthesia. The period of cardiopulmonary bypass is excluded from all analyses, because flow during bypass is generated by the pump rather than by the heart. The primary outcome has two components, both defined over that window and both referenced to a cardiac index threshold of 2.0 L/min/m2. The first is the total time spent below threshold, in minutes. The second is the cumulative low-output burden, defined as the integral over time of the difference between the threshold and the measured cardiac index, taken only where that difference is positive. Dimensional analysis reduces the unit of burden to L/m2, a body-surface-indexed volume of undelivered flow, which multiplied by body surface area yields an absolute flow deficit in