In these patients (the validation group) an appropriately sized and calibrated electromagnetic flow probe (Cineflow II; Carolina Medical; King, NC) was placed around the ascending aorta immediately above the coronary artery. Estimations of apneic cardiac output, which were achieved by averaging three thermodilution curves by using 10 mL 5% dextrose solution at 4°C, were used for initial calibration of the electromagnetic flow signal. Integration of the aortic flow signal per beat was used to derive LV stroke volume. Stroke volume changes during ventilation were compared to SA changes to see if TEE-ABD signals accurately tracked LV volume changes. generic doxycycline
The protocol sequence consisted of observing the effects of a brief apneic interval (15 to 20 s), followed by performing standard positive-pressure ventilation (tidal volume [Vt], 8 to 10 mL/kg; frequency, 15 breaths/min; fraction of inspired oxygen, 100%) on the dependent measured variables. This apnea-ventilation sequence was repeated three times at each step within the surgical procedure. Data were recorded before and after bypass and during both open and closed chest conditions, thereby yielding four sequential, separate steps. We assumed that the differences between the responses that occurred during closed conditions and the responses that occurred during open chest conditions would reflect the differences in ITP swings during ventilation, whereas the differences between responses that occurred before bypass and responses that occurred after bypass would reflect changes in the contractile state because a bypass induces a transient decrease in contractility. The validation group studies were only performed during open chest conditions. The pericardium was kept closed during the study before and after bypass.