Page 10 - Medicine and Surgery

P1: JYS
BLUK007-01 BLUK007-Kendall May 12, 2005 17:17 Char Count= 0

6 Chapter 1: Principles and practice of medicine and surgery

Plasma osmolality is low in most cases of hypona- has a mortality of up to 12%, but asymptomatic hypona-

traemia. If it is normal or high, this is pseudohypona- traemia has a good outcome.
traemia, which may be due to abnormally high lipid
levelsorthepresenceofotherosmoticallyactiveagents
Potassium balance
inthebloodsuchasglucose(hyperglycaemia).Inthese
cases treatment is aimed at the underlying cause. Almost all of the body’s potassium stores are intracellu-
Urine osmolality helps to differentiate the causes of lar, with a high concentration of potassium maintained

hyponatraemia with a low plasma osmolality. If there in the intracellular fluid by the Na -K -ATPase pump
+
+
is raised ADH secretion (as in most cases) the urine re- exchanging it for sodium. This is important in maintain-
mains concentrated at ≥300 mosmol/kg. If the urine ingcellularmembranepotentialandsmallchangesinthe
is dilute, this suggests psychogenic polydipsia or ex- extracellular potassium level affect the normal function
cessiveinappropriateintravenousdextroseordextros- ofcells,particularlyofmusclecells,e.g.myocardiumand
aline. skeletal muscle.
Urine sodium concentration is low (≤20 mmol/L) in Various factors can act to change total body stores of

hypovolaemia (although it is falsely raised by diuretic potassium:
therapy or if there is renal salt-wasting). Fluid reple- Intake can be increased by a potassium-rich diet or by
tion should lead to the production of dilute urine (low oral or intravenous supplements.
osmolality)withhighersodiumconcentrations.How- Potassium is found in high levels in gastric juice and
ever, in SIADH, the urine remains concentrated de- most of this is reabsorbed in the small intestine. A
spite a low plasma osmolality. small amount of potassium is lost in the stool. Vom-
In addition, thyroid function tests and cortisol should iting or diarrhoea can reduce total body potassium.
be checked as there are often multifactorial causes in The kidneys are the main route of excretion of potas-
hyponatraemia and leaving these conditions undetected sium,excreting90%oftheintake.Potassiumexcretion
anduntreatedispotentiallylife-threatening.AshortSyn- by the kidneys is controlled by aldosterone, which acts
acthen test (see page 441) may also be indicated. on the distal tubules and collecting ducts to increase
sodium reabsorption and potassium excretion. Dis-
Management turbances of the renin–angiotensin–aldosterone sys-
In all cases, treating the underlying cause successfully tem can therefore cause alterations in the potassium
will lead to a return to normal values. level. In severe renal failure, when 90% of the renal
Fluid depletion is treated with saline or colloid re- function is lost, the kidneys become unable to excrete

placement. sufficient potassium.
Wateroverload is best treated by fluid restriction to The normal intracellular to extracellular ratio of potas-

as little as <1 L/day but in severe cases diuretics with sium is affected by acid–base status, insulin, cate-
hypertonic saline may be given. Mannitol can be used cholamines, aldosterone and drugs.
to reduce cerebral oedema. Anticonvulsants may be In most tissues, including the kidney, potassium and
necessary to treat fits. hydrogen ions compete with each other at the cell mem-
In salt and water overload, continued diuretics with brane to be exchanged for sodium. If the hydrogen

water restriction are used. Intravenous saline should concentration is high (acidotic conditions), the kidney
be avoided and patients must adhere to a low-sodium excretes hydrogen ions in preference to potassium; in
diet. In severe nephrotic syndrome with oedema, in- the tissues, hydrogen ions compete with potassium to
travenous albumin may be required together with di- be taken up by the cells, so extracellular potassium con-
uretics. centration rises (hyperkalaemia). As the acidosis is cor-
rected, potassium is taken up by the cells and may cause
Prognosis hypokalaemia. Conversely, in metabolic alkalosis potas-
Acute severe symptomatic hyponatraemia has a mortal- sium is excreted in exchange for hydrogen ions, leading
ityashighas50%.Chronicsymptomatichyponatraemia to hypokalaemia.