Oral anti-hypertensive drugs and anaesthesia

Anti-hypertensive drugs and anaesthesia, general as well as neuro-axial anaesthesia, interfere with the normal regulation of the cardiovascular system. It is therefore not surprising that there are numerous clinically important interactions between the blood pressure-lowering drugs and anaesthetic agents. A thorough knowledge of the mechanisms of action of the individual compounds and a good understanding of the possible interactions is a prerequisite for a safe conduct of anaesthesia in patients treated for hypertension.

Beta-adrenoceptor antagonists seldom show untoward interactions with anaesthesia and anaesthetic agents. In contrast, they have a beneficial effect on the heart and the haemodynamics in hypertensive patients. By preventing an excessive increase in heart rate and myocardial contractility, they decrease the myocardial oxygen demand and protect the myocardium from ischaemia. They should not be withdrawn prior to anaesthesia and surgery, and continuation of the beta-adrenoceptor blockade throughout the peri-operative period is warranted. The interaction between calcium antagonists and volatile and local anaesthetics can lead to adverse effects in cardiac impulse generation and conductance and can increase drug toxicity. Close attention must be paid to the choice and dosage of the individual agents. The renin-angiotensin system plays a pivotal role in blood pressure regulation during the peri-operative period. The inhibition of the angiotensin-converting enzyme or blockade of the angiotensin receptors deranges the neurohumoral defence against the effects of anaesthesia and blood loss, and may induce severe hypotension in patients treated with these drugs. They should therefore be withdrawn before surgery. Centrally acting alpha-2 agonists exert beneficial effects on the haemodynamics and anaesthetic requirements, and they are becoming useful adjuvants of anaesthesia, particularly in patients with arterial hypertension. Considering the large number of treated hypertensive patients undergoing surgery, any newly developed anti-hypertensive agents should be tested for their compatibility and interactions with anaesthesia and anaesthetic agents.