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C HAP TE R 7 / Fluid and Electrolyte and Acid–Base Balance and Imbalance 157

the oral route; other routes of electrolyte intake include the in-
Table 7-5 ■ CAUSES OF HYPERNATREMIA travenous and rectal routes, and also through tubes into various
body cavities. Electrolytes that are taken into the gastrointestinal
Category Clinical Examples
tract must be absorbed into the blood. Although some elec-
Loss of water relative to salt Endocrine: Lack of ADH (diabetes insipidus) trolytes (e.g., potassium) are absorbed readily by mechanisms
Renal: Osmotic diuresis; renal concentrating based on gradients, the absorption of other electrolytes (e.g., cal-
disorders cium and magnesium) is more complex and can be impaired by
Other: Inadequate water replacement after
diarrhea or excessive diaphoresis many factors.
Gain of salt relative to water Decreased intake of water: Inability to Electrolytes are distributed into all body fluids, but their con-
respond to thirst (coma, aphasia, paralysis, centrations in the different body fluid compartments vary greatly.
confusion); lack of access to water; Substantial amounts of most electrolytes are located in pools out-
difficulty swallowing fluids (advanced side the extracellular fluid. For example, the major pool of potas-
Parkinsonism); prolonged nausea
Increased intake of salt: Excessive hypertonic sium is inside cells; the major pool of calcium is in the bones.
NaCl or NaHCO 3 ; near-drowning in salt Electrolyte excretion occurs through the normal routes of
water; tube feedings without adequate urine, feces, and sweat. Any removal of electrolytes through other
water intake routes can be considered loss of electrolytes through an abnormal
route. Examples of these abnormal routes are emesis, nasogastric
suction, fistula drainage, and hemorrhage.
To maintain normal balance of any specific electrolyte, elec-
3
malaise, confusion, lethargy, seizures, and coma. Thirst (except in trolyte intake and absorption must equal electrolyte excretion and
some older adults) and oliguria (except in hypernatremia caused electrolyte loss through abnormal routes, and the electrolyte must
by decreased ADH) may also occur. As with hyponatremia, the be distributed properly within the body. Alterations in any of
extent of these manifestations depends on the speed with which these processes can cause an electrolyte imbalance. 1
hypernatremia develops as well as its severity. Hypernatremia is
much less common than hyponatremia in cardiac patients who do
not have other pathophysiologies, although it is common in crit- ELECTROLYTE IMBALANCES
ically ill patients. 18 Hypernatremia also does not have significant
clinical effects on cardiac electrophysiology or function.
Plasma electrolyte imbalances can have profound effects on car-
diovascular function. Because cardiac function depends on ion
Mixed ECV and Osmolality currents across myocardial cell membranes, action potential gen-
Imbalances eration, impulse conduction, and myocardial contraction are all
vulnerable to alterations in electrolyte status. In addition to their
ECV and osmolality imbalances may occur at the same time in effects on the myocardium itself, some electrolyte imbalances have
the same person. For example, in a person who has severe gas- vascular effects.
troenteritis without proper fluid replacement, concurrent ECV
deficit and hypernatremia (clinical dehydration) will develop. The Potassium Balance
fluid lost in the emesis and diarrhea, plus the usual daily fluid ex-
cretion (urine, feces, respiratory, insensible through skin), is hy- Potassium balance is the net result of potassium intake and ab-
potonic sodium-containing fluid (analogous to isotonic saline that sorption, distribution, excretion, and abnormal losses. These
has extra water added). People who have chronic heart failure fre- components are summarized in Table 7-6. Although the plasma
quently develop concurrent ECV excess and hyponatremia, some- potassium concentration describes the status of potassium in the
times called a hypervolemic hyponatremia. 12 extracellular fluid, it does not necessarily reflect the amount of
The signs and symptoms of such mixed fluid imbalances are a potassium inside the cells. The plasma potassium concentration
combination of the clinical manifestations of the two separate im- has a circadian rhythm, rising during the hours a person is usu-
balances. In the example of clinical dehydration, the individual ally active and reaching its trough when a person is usually asleep.
has the sudden weight loss, manifestations of decreased vascular A classic study demonstrated that the kidneys handle an intra-
volume, and signs of decreased interstitial volume that result from venous potassium load much less efficiently during the hours a
ECV deficit plus the thirst and nonspecific signs of cerebral dys- person is customarily asleep, which has implications for potas-
3
function that result from hypernatremia. In heart failure, the sium administration to ICU patients. 19
clinical manifestations include the weight gain, distended neck The potassium concentration of the extracellular fluid has a
veins, and edema of ECV excess plus the nonspecific signs of cere- major influence on the function of the myocardium. Specifically,
bral dysfunction of hyponatremia. the resting membrane potential of cardiac cells is proportional to
the ratio of potassium concentrations in the extracellular and in-
tracellular fluids. The potassium concentration within cardiac
PRINCIPLES OF ELECTROLYTE cells is approximately 140 mEq/L; the normal potassium concen-
BALANCE tration of the extracellular fluid is 3.5 to 5 mEq/L. A small change
in the extracellular concentration of potassium has a large effect
Electrolyte balance is the net result of several concurrent dy- on the extracellular-to-intracellular concentration ratio because
namic processes. These processes are electrolyte intake, absorp- the initial extracellular value is relatively small. A similar change
tion, distribution, excretion, and loss through abnormal routes 1 in the intracellular potassium concentration has a lesser effect be-
(Table 7-6). Electrolyte intake in healthy people is primarily by cause the initial intracellular value is so large.