3.3 Effects of anesthesia on cardiovascular system 

3.3.1 General anesthesia 

•  Induction and maintenance of anesthesia is usually accompanied by a fall in the patient's blood pressure; cardiac output falls due to decreased preload and myocardial contractility and the reduced afterload decreases systemic vascular resistance. The situation may be worsened by the fact that some drugs also depress the baroreceptor reflex to a varying degree and so there may not be a compensatory tachycardia. 
• The anesthetics also depress the medulla reducing the sympathetic response to the fall in blood pressure. Fortunately, most healthy adults will tolerate a fall in blood pressure of approximately 20% and once surgery starts this increases sympathetic activity and blood pressure is restored. If the blood pressure remains low, then treatment may be required. From the information above we can now formulate a logical method to restore the blood pressure and ensure an adequate blood flow to the various organs. 

3.3.2 Spinal anesthesia 

This technique results in vasodilatation (reduced SVR) proportional to the height of the block. As the local anaesthetic drug spreads cranially, there is an increasing block of the sympathetic nerves leaving the spinal cord (the sympathetic chain) that supply the vascular beds. If cardiac output is maintained there will be a slight fall in blood pressure (remember, BP=CO x SVR). If the cardiac output also falls due to reduced preload (failure to maintain an adequate venous return) the blood pressure will fall significantly. This may be compounded further if the block extends to reach above the level of T5 (anesthesia above the nipple) as the sympathetic supply to the heart will be reduced and result in a bradycardia (due to unopposed parasympathetic activity) and profound hypotension.