The effect of posture on gas exchange following cardiac surgery

Abstract

THE CHANC~ from the supine to the sitting position is associated with changes in factors which have conflicting effects on gas exchange. Cardiac output is decreased, j tending to increase the alveolar to arterial gradient for oxygen (AaDO2) ? Functional residual capacity (FRC) increases, however, 1 promoting better distribution of ventilation in the lung. In certain subjects, this enables more airways to remain open during normal breathing, and decreases AaDOe 1.Patients in the postoperative period after cardiac surgery feel more comfortable in the semi-sitting position. It was the aim of our present study to see whether this is associated with any significant alteration in gas exchange, and whether one of the conflicting factors discussed above has a predominant effect. METHODNine male and three female patients were studied on the day following their operation. Mean age was 47 years, with a range of 23 to 57 years. The surgery was performed using extra-corporeal circulation and involved valve replacement, aortocoronary graft or ciosure of ventricular septal defects.Cardiac output was measured in triplicate by a dye-dilution method using indoeyanine green. The dye was injected through a central vein and sampled from a radial artery. A Waters system (XC 302 cuvette; D 400 densitometer) was used, with a 10-inch recorder (Honeywell Electronik 194). Curves were analyzed using the Stewart-Hamilton semi-logarithmic replot technique.Expired gas was collected over three minutes in a neoprene bag with a nonrebreathing valve of 35 ml dead space. Arterial blood was collected simultaneously from an in-dwelling catheter in the radial artery into a heparinized syringe. All blood and gas analyses were by conventional electrodes ( Instrumentation Laboratories 113 System). Alveolar oxygen partial pressure (Pn%) was calculated from the following form of the.alveolar air equationa: PAoz = Plo~ --Pacoz (PI% --PEo~ )/PEcov From the De]~artment