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Chapter 14

Assessment and Care of Patients with Acid-Base

Imbalances

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Normal Blood pH

Keeping pH within the normal range involves balancing acids and bases in body fluids.

Normal pH for arterial blood—7.35 to 7.45. Normal pH for venous blood—7.31 to 7.41.

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Normal Blood pH (Cont’d)

Changes from normal blood pH interfere with many normal functions by: Changing the shape of hormones and

enzymes. Changing the distribution of other electrolytes,

causing fluid and electrolyte imbalances. Changing of excitable membranes. Decreasing the effectiveness of many

hormones and drugs.

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Introduction to Acid-Base Chemistry

Acids Bases Buffers Body fluid chemistry:

Bicarbonate ions Relationship between carbon dioxide and

hydrogen ions Calculation of free hydrogen ion level

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Sources of Acids

Incomplete breakdown of glucose Destruction of cells Bicarbonate

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Respiratory Acid-Base Control Mechanisms

When chemical buffers alone cannot prevent changes in blood pH, the respiratory system is the second line of defense against changes: Hyperventilation Hypoventilation

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Respiratory Acid-Base Control Mechanisms (Cont’d)

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Renal Acid-Base Control Mechanisms

The kidneys are the third line of defense against wide changes in body fluid pH.

Stronger for regulating acid-base balance but take longer than chemical and respiratory mechanisms to completely respond.

Kidney movement of bicarbonate. Formation of acids. Formation of ammonium.

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Compensation

The body attempts to correct changes in blood pH.

pH below 6.9 or higher than 7.8 is usually fatal. Respiratory system is more sensitive to acid-base

changes; can begin compensation efforts within seconds to minutes.

Renal compensatory mechanisms are much more powerful and result in rapid changes in ECF composition not fully triggered unless imbalance continues for several hours to days.

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Respiratory Compensation

Lungs compensate for acid-base imbalances of a metabolic origin.

Example: Prolonged running causes buildup of lactic acid, hydrogen ion levels in the ECF increase, pH drops; breathing is triggered in response to the increased carbon dioxide levels to bring the pH level back to normal.

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Renal Compensation

A healthy kidney can correct or compensate for changes in blood pH when the respiratory system either is overwhelmed or is not healthy.

Example: Person has chronic obstructive pulmonary disease, retains carbon dioxide in the blood, blood pH level falls (becomes more acidic); kidney excretes more hydrogen ions and increases the reabsorption of bicarbonate back into the blood.

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Acid-Base Imbalances

Metabolic acidosis Respiratory acidosis Combined metabolic and respiratory

acidosis Metabolic alkalosis Respiratory alkalosis

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Metabolic Acidosis

Overproduction of hydrogen ions Under-elimination of hydrogen ions Underproduction of bicarbonate ions Over-elimination of bicarbonate ions

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Respiratory Acidosis

Retention of CO: Respiratory depression Inadequate chest expansion Airway obstruction Reduced alveolar-capillary diffusion

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Combined Metabolic and Respiratory Acidosis

Uncorrected respiratory acidosis always leads to poor oxygenation and lactic acidosis.

Combined acidosis is more severe than metabolic or respiratory acidosis alone.

Cardiac arrest is an example of a problem leading to combined metabolic and respiratory acidosis.

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Collaborative Care

History CNS changes Neuromuscular changes Cardiovascular changes Respiratory changes:

Kussmaul respiration Skin changes Psychosocial assessment

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Laboratory Assessment

Metabolic acidosis: pH <7.35 Bicarbonate <21 mEq/L PaO2 normal PaCO2 normal or slightly decreased Serum potassium high

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Laboratory Assessment (Cont’d)

Respiratory acidosis: pH <7.35 PaO2 low PaCO2 high Serum bicarbonate variable Serum potassium levels elevated if acidosis is

acute Serum potassium levels normal or low if renal

compensation is present

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Interventions—Metabolic Acidosis

Hydration Drugs:

Insulin to treat DKA Antidiarrheal drugs Bicarbonate only if serum bicarbonate levels

are low

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Interventions—Respiratory Acidosis

Maintain a patent airway, and enhance gas exchange

Drug therapy Oxygen therapy Pulmonary hygiene Ventilation support Prevent complications

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Alkalosis

Metabolic alkalosis: Base excesses—excessive intake

bicarbonates, carbonates, acetates, and citrates

Acid deficit—prolonged vomiting, excess cortisol, hyperaldosteronism, thiazide diuretics, prolonged NG suction

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Alkalosis (Cont’d)

Respiratory alkalosis: Hyperventilation—anxiety, fear, improper vent

settings, stimulation of central respiratory center due to fever, DNS lesion, and salicylates

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Collaborative Care

Assessment CNS changes—positive Chvostek’s and

Trousseau’s signs Neuromuscular changes—tetany Cardiovascular changes Respiratory changes

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Interventions

Prevent further losses of hydrogen, potassium, calcium, and chloride ions.

Restore fluid balance. Monitor changes.