diuretics
If a single drug is not effective, other anti-hypertensive drugs may be added, usually at intervals of at least four weeks, until good control of BP is achieved. Where hypertension is relatively mild (systolic BP less than 160mmHg, and diastolic less than 100mmHg), drugs may be substituted rather than used together.
Thiazide Diuretics
Thiazide diuretics (e.g. bendrofluazide, hydrochlorothiazide) are generally used as a first-line treatment in the elderly, or are combined with other anti-hypertensive drugs (e.g. a beta-blocker or ACE inhibitor) to boost their action in younger patients.
They lower blood pressure by increasing loss of salts through the kidneys into the urine. This tends to draw fluid out of the circulation, causes mild dilation of small arteries and lowers arteriolar resistance. The diuretics act within an hour or two of being given and are usually taken in the morning so you do not have to get up at night to pass water. When you first start taking the tablets, you may notice that you have to pass water more frequently than usual for the first few days;, then this effect tends to disappear as dilation of the arterioles occurs. Only low doses of hiazide diuretic are needed to bring your diastolic BP down by around 5 mmHg – higher doses have no further effect on BP and are more likely to cause side effects such as salt imbalances.
They should not be used by people with diabetes or with sodium, potassium or calcium imbalances, severe kidney or liver problems, active gout or Addison’s disease.
Beta-blockers
The way beta-blockers lower blood pressure is not fully understood but is thought to result from a combination of actions in which they:
In general, beta-blockers are used as a first-line treatment in young people with hypertension and in people who have coronary heart disease. Because they also affect receptors in the lungs, they should not be used in people with asthma as they may trigger an asthma attack. Beta-blockers have been shown to significantly reduce the risk of having a second heart attack and may prolong life in high-risk individuals.
Beta-blockers should not be withdrawn suddenly, but must be tailed off slowly so that rebound high blood pressure (or angina) does not occur.
Alpha-blockers
Alpha-blockers (e.g. doxazosin, indoramin, prazosin, terazosin) lower blood pressure by dilating both arteries and veins. They are particularly helpful for older males who have both high blood pressure and problems associated with benign enlargement of the prostate gland. They sometimes cause a rapid fall in blood pressure after the first dose so treatment should be started with caution – usually at night so that if low blood pressure does occur, this is after you have retired to bed.
If taking Indoramin, you should avoid alcohol as it boosts alcohol absorption.
Calcium Channel Blockers
Calcium channel blockers (e.g. diltiazem, felodipine, isradipine, lacidipine, nicardipine, nifedipine) work by:
Treatment must not be stopped suddenly, but should be tailed off slowly to prevent rebound angina. Verapamil is slightly different from the others in the way it works, and should not be used together with a beta-blocker.
ACE Inhibitors
ACE inhibitor drugs are so-named because they block formation of Angiotensin Converting Enzyme (ACE). This in turn prevents formation of a substance called angiotensin II – a powerful constrictor of blood vessels – leading to dilation of both small arteries and veins. This reduces total peripheral resistance and arterial blood pressure. ACE inhibitors also increase blood flow to the kidneys, so more fluid and sodium is lost as urine. They are usually considered for treating hypertension when thiazides diuretics or beta-blockers are contraindicated, not tolerated, or fail to control high blood pressure.
They can cause a sudden fall in BP on giving the first dose, especially in people who are taking diuretics or who are dehydrated. Where possible, diuretic treatment is therefore usually stopped a few days before ACE inhibitor treatment is started. For some, the first dose is best taken at night on retiring to bed. Kidney function and salt balance should be checked before treatment is started. ACE inhibitors may be less effective in people of Afro-Caribbean descent unless combined with a thiazide diuretic.
