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ACID-BASE LEARNING OBJECTIVES

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  • Understand acid-base pathophysiology implications for nursing assessment

  • Explain how aging changes the body’s ability to maintain acid-base balance

  • Apply knowledge of anatomy, physiology, and pathophysiology to define processes by which the body maintains balance:

    • Acidosis

    • Alkalosis

CONCEPTS OF ACID-BASE BALANCE

The Power of Hydrogen

  • The water molecule separates into:

    • A hydrogen ion (H+)

    • A hydroxide ion (OH-)

  • pH is based on how water dissociates, or comes apart, in a solution.

  • What is pH? A measure of how many hydrogen ions or hydroxide ions are present in a solution.

  • What is arterial pH? A measure of the free hydrogen ion level in the blood and other body fluids.

  • The body regulates the presence and concentration of these ions in extracellular fluid (ECF) to maintain homeostasis.

  • Acid-Base Balance: pH maintenance between 7.35 and 7.45.

Definitions of Acid, Base, and Buffer

  • ACID:

    • Substances that release hydrogen ions when dissolved in H2O.

    • Increases the amount of free H+ ions.

    • Example: H2CO3 - Carbonic acid.

  • BASE:

    • Substances that bind free hydrogen ions.

    • Decreases the amount of free H+ ions.

    • Example: HCO3 - Bicarbonate.

  • BUFFER:

    • Substances that, when dissolved in a fluid, can react as an acid or a base.

    • Help keep body fluid pH in the normal range.

  • What is acid-base balance? It is the maintenance of arterial blood pH between 7.35 and 7.45 through the control of hydrogen ion production and elimination.

  • Maintaining pH within the normal range is part of homeostasis and is critical for optimal body function.

Conditions of Acid-Base Balance

  • What is acidosis?

    • Body fluids with a pH of less than 7.35.

  • What is alkalosis?

    • Body fluids with a pH of greater than 7.45.

  • Example Interpretation: On recent lab work, a client’s pH is 7.38. How would the nurse interpret this finding?

    • a. Acidosis

    • b. Alkalosis

    • c. Compensation

    • d. Acid-base balance

BODY FLUID CHEMISTRY: MAINTAINING BALANCE

  • The most common acid is Carbonic Acid (H2CO3).

  • The most common base in the human body is Bicarbonate (HCO3).

  • During homeostasis, the ratio of acid:base is 1:20.

    • H2CO3 is regulated through the respiratory system.

    • HCO3 is regulated through the renal system.

    • This constant ratio is related to balancing the production and elimination of CO2 and H+.

Carbon Dioxide and the Carbonic Anhydrase Equation

  • Free hydrogen ion levels and carbon dioxide levels are directly related; an increase in one causes an equal increase in the other.

  • The more CO2 you have, the more H+ you have and vice versa.

    • CO2 is regulated by the respiratory system and eliminated during exhalation.

    • When any condition causes the blood’s free H+ ion concentration to increase, extra CO2 is produced in the same proportion.

Bicarbonate Ions (HCO3-)

  • Bicarbonate is the most common base in human physiology.

  • Controlled by the kidneys.

  • It is critical in preventing major changes in body fluid pH.

  • **Effects of Bicarbonate:

    • An increase in HCO3- causes a decrease in H+ and an increase in pH.

    • A decrease in HCO3- causes an increase in H+ and a decrease in pH.**

Bicarbonate Regulation

  • Alkalosis: Increase HCO3- = Decreased H+ = High pH.

  • Acidosis: Decrease HCO3- = Increase H+ = Low pH.

  • Production of HCO3:

    • Bicarbonate is primarily produced in the pancreas, kidneys, and inside red blood cells.

pH AND HOMEOSTASIS: REVIEW OF KEY CONCEPTS

  1. pH is the Power of Hydrogen.

  2. Homeostasis is maintained via an arterial pH of 7.35-7.45.

  3. H+ ions are acidic.

  4. When the body produces CO2, equal parts of hydrogen ions are produced and vice versa.

  5. The lungs maintain acid-base balance by exhaling CO2 which decreases free H+ ions.

  6. The kidneys maintain acid-base balance by reabsorbing HCO3 and eliminating H+.

COMPENSATION IN ACID-BASE BALANCE

Two Disorders of Acid-Base Balance

  • Acidosis:

    • An excess of free H+ ions in body fluids.

    • pH below 7.35.

    • Condition caused by any disorder or pathology process associated with metabolic or respiratory issues.

    • Reduces excitability of muscles and neurons.

  • Alkalosis:

    • Excess base (especially HCO3-) and decrease H+.

    • pH greater than 7.45.

    • Condition caused by any disorder or pathology process associated with metabolic or respiratory issues.

    • Results in increased stimulation of the nervous, neuromuscular & cardiac systems.

Cause of Acid-Base Disorders

  • Metabolic Cause: Related to kidneys; the lungs will compensate.

  • Respiratory Cause: Related to lungs; the kidneys will compensate.

What is Compensation?

  1. The body senses a change in blood pH (either acidosis or alkalosis).

  2. The body adapts in an attempt to correct changes and maintain acid-base balance:

    • Respiratory compensation: The lungs correct for acid-base imbalances resulting from metabolic problems.

    • Kidney compensation: Healthy kidneys correct acid-base imbalance when the respiratory system is either overwhelmed or unhealthy.

NORMAL ARTERIAL BLOOD GAS (ABG) VALUES

  • pH: 7.35 – 7.45

  • PaO2: 80 – 100 mm Hg

  • PaCO2: 35 – 45 mm Hg

  • HCO3-: 23 – 30 mEq/L

ACID-BASE IMBALANCES

Respiratory Acidosis

  • Excess CO2 retention correlating with under-elimination of hydrogen ions.

  • Arterial Blood Gas Findings:

    • pH < 7.35

    • PaO2 < 90 mmHg

    • HCO3- 21-28 mmHg

    • PaCO2 > 50 mm Hg

  • Compensatory Mechanism:

    • Kidneys work to eliminate extra H+ ions/retain bicarbonate.

  • Possible Causes:

    • Respiratory depression/hypoventilation (drugs, electrolyte imbalance).

    • Inadequate chest expansion.

    • Airway obstruction.

    • Reduced alveolar-capillary diffusion.

Metabolic Acidosis

  • HCO3- loss, acid retention.

  • Arterial Blood Gas Findings:

    • pH < 7.35

    • PaO2 80-100 mmHg

    • HCO3- 15-20 mEq/L

    • PaCO2 35-40 mmHg

  • Compensatory Mechanism:

    • Lungs: Hyperventilation (Kussmaul’s respirations).

  • Possible Causes:

    • Over-elimination of HCO3- from diarrhea.

    • Under-elimination of H+ or underproduction of HCO3- = renal disease.

    • Underproduction of HCO3- = pancreatitis, liver failure.

    • Overproduction of H+ ions = Diabetic Ketoacidosis, Salicylate intoxication.

Respiratory Alkalosis

  • CO2 loss due to excess CO2 excretion.

  • Arterial Blood Gas Findings:

    • pH > 7.45

    • HCO3- < 22 mEq/L (if compensating)

    • PaCO2 < 35 mm Hg

  • Compensatory Mechanism:

    • Kidneys retain H+ ions.

  • Possible Causes:

    • Hyperventilation caused by anxiety, pain, or improper ventilator settings.

    • Respiratory stimulation due to drugs, disease, hypoxia, or fever.

    • Head injury or brain tumor.

    • ASA overdose.

Metabolic Alkalosis

  • Increase in serum HCO3- due to HCO3- retention or acid loss.

  • Arterial Blood Gas Findings:

    • pH > 7.45

    • HCO3- > 26 mEq/L

    • PaCO2 > 45 mm Hg (if compensating)

  • Compensatory Mechanism:

    • Lungs (slow, shallow respirations).

  • Possible Causes:

    • Oral ingestion of antacids.

    • Total Parenteral Nutrition (TPN).

    • Loss of hydrochloric acid from the stomach due to excessive vomiting.

    • Nasogastric (NG) suctioning.

CLINICAL MANIFESTATIONS

Acidosis (Box 14.1)

  • Caused by increase in free H+ ions and imbalance with K+ which disrupts function of all excitable tissues:

    • Nerves

    • Cardiac muscle

    • Skeletal muscle

Alkalosis (Box 14.4)

  • Caused by electrolyte imbalances that occur in response to decreased blood cation (positive charge ion), resulting in increased stimulation of:

    • Nervous system

    • Neuromuscular system

    • Cardiac System

Summary of Conditions

  • Metabolic Acidosis:

    • pH is low

    • Bicarb is low

  • Metabolic Alkalosis:

    • pH is high

    • Bicarb is high

  • Respiratory Acidosis:

    • pH is low

    • CO2 is high

  • Respiratory Alkalosis:

    • pH is high

    • CO2 is low

ANALYZING ABGS: ROME

  • ROME:

    • Respiratory: Opposite

    • Metabolic: Equal

    • Interpretations:

    • Respiratory cause = If CO2 is abnormal and goes in the opposite direction of pH.

    • Metabolic cause = if HCO3 is abnormal and moves in the same direction as pH.

Sample ABG Findings

  • ABG Values:

    • pH (7.35-7.45)

    • PaCO2 (35-45)

    • HCO3- (23-30)

  • Acid-Base Imbalance Findings:

    • Respiratory Acidosis: WNL (or high)

    • Respiratory Alkalosis: WNL (or low)

    • Metabolic Acidosis: WNL (or low)

    • Metabolic Alkalosis: WNL (or high)

CLINICAL APPLICATION QUESTIONS

  1. Question: A nurse is caring for a patient with chronic obstructive pulmonary disease (COPD) who is experiencing respiratory acidosis. Which of the following arterial blood gas (ABG) results would the nurse expect?

    • A. pH 7.48, PaCO₂ 32 mm Hg, HCO₃⁻ 22 mEq/L

    • B. pH 7.35, PaCO₂ 40 mm Hg, HCO₃⁻ 26 mEq/L

    • C. pH 7.30, PaCO₂ 50 mm Hg, HCO₃⁻ 24 mEq/L

    • D. pH 7.50, PaCO₂ 45 mm Hg, HCO₃⁻ 28 mEq/L

  2. Question: A nurse is assessing a patient who is experiencing acute respiratory alkalosis due to hyperventilation. Which of the following arterial blood gas (ABG) results would the nurse expect to find?

    • A. pH 7.50, PaCO₂ 30 mm Hg, HCO₃⁻ 24 mEq/L

    • B. pH 7.30, PaCO₂ 50 mm Hg, HCO₃⁻ 28 mEq/L

    • C. pH 7.48, PaCO₂ 45 mm Hg, HCO₃⁻ 22 mEq/L

    • D. pH 7.35, PaCO₂ 40 mm Hg, HCO₃⁻ 26 mEq/L

  3. Question: A nurse is reviewing the arterial blood gas (ABG) results of a patient who has been vomiting excessively and is suspected of having metabolic alkalosis. Which of the following ABG results would confirm this diagnosis?

    • A. pH 7.30, PaCO₂ 50 mm Hg, HCO₃⁻ 24 mEq/L

    • B. pH 7.48, PaCO₂ 40 mm Hg, HCO₃⁻ 32 mEq/L

    • C. pH 7.25, PaCO₂ 60 mm Hg, HCO₃⁻ 20 mEq/L

    • D. pH 7.52, PaCO₂ 28 mm Hg, HCO₃⁻ 18 mEq/L

  4. Question: A nurse is caring for a patient with diabetic ketoacidosis (DKA). The nurse suspects the patient is developing metabolic acidosis. Which arterial blood gas (ABG) result would confirm this diagnosis?

    • A. pH 7.50, PaCO₂ 40 mm Hg, HCO₃⁻ 30 mEq/L

    • B. pH 7.30, PaCO₂ 50 mm Hg, HCO₃⁻ 24 mEq/L

    • C. pH 7.28, PaCO₂ 35 mm Hg, HCO₃⁻ 18 mEq/L

    • D. pH 7.45, PaCO₂ 38 mm Hg, HCO₃⁻ 22 mEq/L