[Blood volume changes caused by position under general anesthesia]

The cardiovascular changes in response to general anaesthesia are related to several interacting mechanisms. In addition to the intrinsic effects of anaesthetic agents on the heart and vessels which depress the physiological mechanisms of adaptation, the additive independent effects of posture and mechanical ventilation on intravascular blood volume have to be considered. Physiopathologic studies show that the ultimate related mechanism is the decrease of venous return to the heart. The major effect of posture is the change in the distribution of the blood volume, as the posture modifies the influence of gravity in addition to direct vascular compression and stretching. The deterioration of the cardiac venous return results in dramatic or insidious clinical consequences which lead finally to a low cardiac output. As cardiac function is not a limiting factor of output, any decrease of blood pressure in a patient with a healthy heart must be considered as an hypovolaemic state due to an abnormal contents to container ratio, and must be managed as such.