Cardiorespiratory effects of the sitting position in neurosurgery.

The hemodynamic and respiratory effects of raising patients from the supine into the sitting position for neurosurgical procedures were investigated in fifteen patients under fentanyl-nitrous oxide anesthesia. The change of position caused a significant decrease in the cardiac output and in the mean arterial blood pressure. An increase in the systemic vascular resistance and in the pulmonary vascular resistance occurred. Half an hour after positioning the patients the mean arterial blood pressure decreased so far that in one third of the patients the cerebral perfusion was threatened. In the sitting position the central venous pressure, measured at right atrium level, became subatmospheric in three patients. The central venous pressure and the capillary wedge pressure decreased, but the pressure gradient between the right and the left atrium reversed in most patients, bearing the risk of paradoxical air embolism. The alveolar-arterial difference in oxygen tension and the intrapulmonary shunt fraction were decreased after raising the patients; the dead space ventilation showed a significant increase.