Supine hypotension syndrome during caesarean section.

The supine hypotension syndrome was described by Hansen in 1942. Its clinical relevance became apparent in 1957 when it was postulated by Holmes as the cause of death in healthy young women during caeserean section under spinal analgesia. At first it was thought that the cause was simple mechanical compression of the inferior vena cava. But as this occurs in the majority of women lying supine in late pregnancy, a more complex process is involved. It was shown by Kerr et al. (1964) that on occlusion of the inferior vena cava collateral venous channels open through the paravertebral and azygous system of veins. These may or may not be adequate to maintain the venous return to the heart. When caval occlusion occurs one of two effects results. (1) The collateral system is not quite adequate to maintain the normal venous return, with a resulting moderate drop in cardiac output compensated for by increased arterial vasoconstriction. Blood pressure and pulse remain unchanged. (2) The collateral system fails to maintain an adequate venous return. There is a drop in cardiac output, increased vasoconstriction, and an increase in pulse rate. This is followed by bradycardia, which further reduces the cardiac output. The supine hypotension syndrome results.