McKenna's Pharmacology for Nursing, 2e

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P A R T 9  Drugs acting on the renal system

Thiazides, thiazide-like diuretics

Proximal convoluted tubule

Distal convoluted tubule

Spironolactone

Collecting duct

Osmotic diuretics

Carbonic anhydrase inhibitors

Descending loop of Henle

Ascending loop of Henle

Loop diuretics

FIGURE 51.1  Sites of action of diuretics in the nephron.

these segments of the tubule are impermeable to water, there is little increase in the volume of urine produced, but it will be sodium rich, a saluretic effect . Thiazides are considered to be mild diuretics compared with the more potent loop diuretics. These drugs are the first-line drugs used to manage essential hypertension when drug therapy is needed. See Table 51.2 for usual indications for these agents. Pharmacokinetics These drugs are well absorbed from the GI tract after oral administration, with onset of action ranging from 1 to 3 hours. They have peak effects within 4 to 6 hours and duration of effects of 6 to 12 hours. They are metabolised in the liver and excreted in the urine. These diuretics cross the placenta and enter breast milk. Hydrochlorothiazide is the most frequently used of the thiazide diuretics and the prototype of this class. Contraindications and cautions Thiazide and thiazide-like diuretics are contraindi- cated with allergy to thiazides or sulfonamides to prevent hypersensitivity reactions ; fluid and electrolyte imbalances, which can be potentiated by the fluid and

electrolyte changes caused by these diuretics ; and severe renal disease, which may prevent the diuretic from working or precipitate a crisis stage due to the blood flow changes brought about by the diuretic. Caution should be used with the following con- ditions: systemic lupus erythematosus (SLE), which frequently causes glomerular changes and renal dysfunc- tion that could precipitate renal failure in some cases ; glucose tolerance abnormalities or diabetes mellitus, which is worsened by the glucose elevating effects of many diuretics ; gout, which reflects an abnormality in normal tubule reabsorption and secretion ; liver disease, which could interfere with the normal metabolism of the drugs, leading to an accumulation of the drug or toxicity ; hyperparathyroidism, which could be exacer- bated by the renal effects of these drugs ; and bipolar disorder, which could be exacerbated by the changes in calcium levels that occur with these drugs . Routine use during pregnancy is not appropriate; these drugs should be reserved for situations in which the mother has pathological reasons for use, not pregnancy mani­ festations or complications, and only if the benefit to the mother clearly outweighs the risk to the fetus. If one of these drugs is needed during breastfeeding, another method of feeding the baby should be used because of