McKenna's Pharmacology for Nursing, 2e

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P A R T 9  Drugs acting on the renal system

molecule. Because the tubule is not able to reabsorb all of the sugar pulled into it, large amounts of fluid are lost in the urine. The effects of these osmotic drugs are not limited to the kidneys because the injected substance pulls fluid into the vascular system from extravascular spaces, including the aqueous humour. Therefore, these drugs are often used in acute situations when it is necessary to decrease intraocular pressure before eye surgery or during acute attacks of glaucoma. They also are the diuretics of choice in cases of increased cranial pressure or acute renal failure due to shock, drug overdose or trauma. See Table 51.2 for usual indications for each of these agents. Pharmacokinetics Mannitol is only available for intravenous use. This drug is freely filtered at the renal glomerulus, poorly reab- sorbed by the renal tubule, not secreted by the tubule and resistant to metabolism. Its action depends on the concentration of the osmotic activity in the solution. It is not known whether it can cause fetal harm. In addition, the effects of mannitol during breastfeeding are not well understood. Contraindications and cautions These drugs are contraindicated in individuals with renal disease and anuria from severe renal disease, pul- monary congestion, intracranial bleeding, dehydration Prototype summary: Mannitol Indications: Prevention and treatment of the oliguric phase of renal failure; reduction of intracranial pressure and treatment of cerebral oedema; reduction of elevated intraocular pressure; promotion of urinary excretion of toxic substances; diagnostic use for measurement of glomerular filtration rate; also available as an irrigant in transurethral prostatic resection and other transurethral procedures. Actions: Elevates the osmolarity of the glomerular filtrate, leading to a loss of water, sodium and chloride; creates an osmotic gradient in the eye, reducing intraocular pressure; creates an osmotic effect that decreases swelling after transurethral surgery. Pharmacokinetics: Route Onset Peak Duration IV 30–60 mins 1 hour 6–8 hours Irrigant Rapid Rapid Short T 1/2 : 15–100 minutes; excreted unchanged in urine. Adverse effects: Dizziness, headache, hypotension, rash, nausea, anorexia, dry mouth, thirst, diuresis, fluid and electrolyte imbalances.

■■ BOX 51.2  Potassium-rich foods

avocados bananas broccoli

lima beans navy beans nuts

prunes rhubarb spinach sunflower seeds tomatoes watermelon

cantaloupe dried fruits grapefruit

oranges peaches potatoes

gynaecomastia, deepening of the voice and irregular menses. Clinically important drug–drug interactions The diuretic effect decreases if potassium-sparing diu- retics are combined with salicylates. Dose adjustment may be necessary to achieve therapeutic effects. Prototype summary: Spironolactone Indications: Primary hyperaldosteronism, adjunctive therapy in the treatment of oedema associated with HF, nephrotic syndrome, hepatic cirrhosis; hypokalaemia occurs; essential hypertension. Actions: Competitively blocks the effects of aldosterone in the renal tubule, causing loss of sodium and water and retention of potassium. Pharmacokinetics: Route Onset Peak Duration Oral 24–48 hours 48–72 hours 48–72 hours T 1/2 : 20 hours; metabolised in the liver and excreted in urine. Adverse effects: Dizziness, headache, drowsiness, rash, cramping, diarrhoea, hyperkalaemia, hirsutism, gynaecomastia, deepening of the voice, irregular menses. treatment of hypokalaemia or prevention of hypokalaemia in people at high risk if O smotic diuretics Osmotic diuretics pull water into the renal tubule without sodium loss. The osmotic diuretics include glucose and mannitol ( Osmitrol ). Therapeutic actions and indications Some non-electrolytes are used intravenously to increase the volume of fluid produced by the kidneys. Mannitol, for example, is a sugar that is not well reabsorbed by the tubules; it acts to pull large amounts of fluid into the urine due to the osmotic pull exerted by the large sugar