Subarachnoid blockade is defined as a temporary interruption of nerve impulse transmission produced by injecting of a local anesthetic into the subarachnoid space. The only reason for the clinical effects of subarachnoid anaesthesia on the cardiovascular system is a preganglionic sympathetic block, which affects all the haemodynamic parameters determining the blood pressure. The decrease in blood pressure following subarachnoid blockade is caused by a decrease in both cardiac output and in peripheral resistance. The decrease in cardiac output is due to a decrease in preload secondary to an increase in venous capacity leading to a lower filling pressure to the heart. During high subarachnoid block the reduction in cardiac output is also caused by a decrease in heart rate and perhaps by a decrease in the contractility of the heart.