4. Anti-Hypertensive Drugs and Anaesthesia

: patients should continue taking these drugs and in some cases may be prescribed them de novo.

Thiazides, such as bendrofluazide, which act on the distal tubule; and loop diuretics typically furosemide, which act on the loop of Henle These drugs decrease the active reabsorption of sodium and chloride by binding to the chloride site of the electroneutral Na+/Cl− co-transport system to inhibit its action. Anaesthetic implications: potassium loss can be significant, particularly in the elderly. Electrolytes should be checked prior to anaesthesia, and consideration should be given to withholding the drugs on the day of surgery.

L-type calcium channels and are of three main classes: phenylalkylamines (verapamil), dihydropyridines (nifedipine, amlodipine) benzothiazepines (diltiazem All three groups bind to the α1-subunit of the calcium channel and inhibit the slow inward calcium current in cardiac and smooth muscle cells Verapamil has primarily cardiac effects acts as a negative inotrope and chronotrope. Nifedipine and related drugs are more selective for vascular smooth muscle and so are usually used to treat hypertension. channel blockers are all negative inotropes, but because they offload the myocardium by vasodilatation, cardiac output is usually maintained.

there may be some synergistic action with volatile anaesthetic agents, which also affect slow Ca2+ channels in the myocardium and elsewhere. Nifedipine and verapamil may also potentiate the actions of nondepolarizing muscle relaxants.

the renin– angiotensin system Angiotensin acts on receptors to mediate vasoconstriction (its pressor activity is 40 times as powerful as that of noradrenaline), it also stimulates noradrenaline release from sympathetic nerve terminals, sodium reabsorption from proximal tubules aldosterone secretion from the adrenal cortex. ACE inhibitors include ramipril, captopril, enalapril, lisinopril and perindopril significant fall in BP in hypertensive subjects and reduce cardiac load by affecting both capacitance and resistance vessels no influence on cardiac contractility, although they do act preferentially on angiotensin-sensitive vascular beds in the myocardium, brain and kidney. Cough is a common side effect of their use, due to bradykinin accumulation

significant intraoperative hypotension may follow the concomitant administration of a general or regional anaesthetic in patients who are continuing to take an ACE inhibitor. If the ACE inhibitor has not been omitted, then volume loading and vasopressors may be needed to maintain normal arterial blood pressure,

pure antagonists of the angiotensin II receptor (examples include losartan, valsartan, irbesartan, candesartan) should in theory have a similar spectrum of benefit as ACE inhibitors. They have a better side effect profile and do not cause persistent cough, although they are less effective in the treatment of heart failure. Anaesthetic implications: these are broadly similar to those that apply to ACE inhibitors, although, if not discontinued prior to surgery, they are even more likely to cause profound and refractory intraoperative hypotension.