Chapter 42

Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc.

Chapter 42

Agents Affecting the Volume and Ion Content of Body Fluids

2Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc.

Drugs to Correct Disturbances Drugs used to correct disorders of fluid

volume and osmolality Drugs used to correct disturbances of

hydrogen ion concentration Drugs used to correct electrolyte imbalances


Agents Affecting the Volume and Ion Content of Body Fluids

Disorders of fluid volume and osmolality Acid-base disturbances Potassium imbalances Magnesium imbalances


Fluid Volume and Osmolality Good health requires that both the volume

and the osmolality of extracellular and intracellular fluids remain within a normal range

Maintenance of both is primarily the job of the kidneys

Volume contraction and volume expansion


Volume Contraction and Volume Expansion

Volume contraction Decrease in total body water Definition, causes, and treatment

• Isotonic, hypertonic, and hypotonic Volume expansion

Increase in total body water Definition, causes, and treatment

• Isotonic, hypertonic, and hypotonic


Isotonic Contraction Definition

Volume contraction in which sodium and water are lost in isotonic proportions

Decrease in total volume, but no change in osmolality Causes

Vomiting, diarrhea, kidney disease, and misuse of diuretics Treatment

Fluids that are isotonic to plasma 0.9% NS Replenish slowly to prevent pulmonary edema


Hypertonic Contraction Definition

Loss of water exceeds loss of sodium Reduced extracellular fluid volume and increase in

osmolality Causes

Excessive sweating, osmotic diuresis, concentrated food given to infants

Secondary to extensive burns or CNS disorders that interfere with thirst

Treatment Hypotonic fluids or fluids that contain no solutes at all (D5W) Initial therapy: drink water


Hypotonic Contraction Definition

Loss of sodium exceeds loss of water Both volume and osmolality of extracellular fluid are reduced

Causes Excessive loss of sodium through the kidney (diuretic

therapy, chronic renal insufficiency, lack of aldosterone) Treatment

Mild: infusing isotonic sodium chloride solution for injection Severe: hypertonic solution (3%) NaCl Watch for signs of fluid overload


Volume Expansion Definition

Increase in the total volume of body fluid May be isotonic, hypertonic, hypotonic

Causes Overdose with therapeutic fluids Disease states (congestive heart failure [CHF],

nephrotic syndrome, cirrhosis with ascites) Treatment

Diuretics Agents used for heart failure


Acid-Base Disturbances Acid-base balance is maintained by multiple

systems Bicarbonate–carbonic acid buffer system Respiratory system

• CO2 (increase lowers pH) Kidneys

• HCO3– (increase raises pH)


Acid-Base Disturbances Respiratory alkalosis Respiratory acidosis Metabolic alkalosis Metabolic acidosis


Respiratory Alkalosis Causes

Hyperventilation causes decrease in CO2

Treatment Mild: none needed More severe: rebreathe CO2-laden expired breath


Respiratory Acidosis Causes

Retention of CO2 secondary to hypoventilation• Depression of the medullary respiratory center• Pathologic changes in the lungs

Treatment Correction of respiratory impairment Infusion of sodium bicarbonate if severe


Metabolic Alkalosis Causes

Excessive loss of gastric acid Administration of alkalinizing salts

Treatment Solution of sodium chloride plus potassium

chloride


Metabolic Acidosis Causes

Chronic renal failure Loss of bicarbonate during severe diarrhea Metabolic disorders Poisoning by methanol and certain medications

Treatment Correction of the underlying cause of acidosis Alkalinizing salt if severe


Potassium Most abundant intracellular cation Extracellular concentrations are low Major role in:

Conducting nerve impulses Maintaining the electrical excitability of muscle Regulating acid-base balance


Regulation of Potassium Levels Primarily by the kidneys Renal excretion increased by aldosterone Excretion also increased by most diuretics Potassium-sparing diuretics are the exception Influenced by extracellular pH

Alkalosis potassium uptake enhanced Acidosis potassium exits cells

Insulin has a profound effect on potassium level


Hypokalemia Serum potassium levels less than 3.5 mEq/L Causes and consequences

Most common cause is treatment with a thiazide or loop diuretic• Less common: excessive insulin, alkalosis

Adverse effects on skeletal muscle, smooth muscle, blood pressure, and the heart

Hypokalemia increases the risk for hypertension and stroke


Hypokalemia Treatment Potassium salts preferred because chloride

deficiency frequently coexists with hypokalemia Oral potassium chloride: mild

• Sustained-release version has fewer GI effects• Abdominal discomfort, nausea and vomiting, diarrhea

IV potassium chloride: severe or cannot take PO• Must be diluted and infused slowly


Hypokalemia Treatment Contraindications to potassium use

Avoid in patients who are predisposed to hyperkalemia• Severe renal impairment, use of potassium-sparing

diuretics, hypoaldosteronism Principal complication of hypokalemia is hyperkalemia Assess renal function and changes in ECG

ECG = electrocardiogram.


Hyperkalemia Excessive elevation of serum potassium

Causes• Severe tissue trauma• Untreated Addison’s disease• Acute acidosis (draws K out of cells)• Misuse of potassium-sparing diuretics• Overdose with IV potassium


Hyperkalemia Consequences

Disruption of the electrical activity of the heart Earliest sign patient is in danger

• Mild elevation (5–7): T wave heightens; PR prolonged• Severe elevation (8–9): cardiac arrest can occur

Noncardiac signs• Confusion, anxiety, dyspnea, weakness or heaviness of

legs, numbness/tingling of hands/feet/lips


Hyperkalemia Treatment

Withhold foods that contain potassium Withhold medicines that promote potassium

accumulation Counteract potassium-induced cardiotoxicity Lower extracellular levels of potassium

• Calcium gluconate• Infusion of glucose and insulin• If acidotic: infusion of sodium bicarbonate


Magnesium Required for the activity of many enzymes Binding of messenger RNA to ribosomes


Magnesium Imbalances Hypomagnesemia

Causes• Diarrhea• Hemodialysis• Kidney disease• Prolonged intravenous feeding• Chronic alcoholism• Hypermagnesemia

Prevention and treatment• Magnesium gluconate and magnesium hydroxide• Magnesium sulfate


Magnesium Imbalances Hypermagnesemia

Most common in patients with renal insufficiency

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