Acid-Base Imbalances

Basics of Acid-Base Balance

  • Definition of Acid:

    • An acid is a substance that can donate hydrogen ions (H+) to a base.

    • Examples of acids include:

    • Hydrochloric acid

    • Nitric acid

    • Ammonium ion

    • Lactic acid

    • Acetic acid

    • Carbonic acid (H2CO3)

  • Definition of Base:

    • A base is a substance that can accept or bind H+.

    • Examples of bases include:

    • Ammonia

    • Lactate

    • Acetate

    • Bicarbonate (HCO3-)

Acidity and Alkalinity

  • Determination of Acidity or Alkalinity:

    • The acidity or alkalinity of a solution depends on the concentration of hydrogen ions (H+).

    • An increase in hydrogen ions leads to acidity, while a decrease leads to alkalinity.

    • Hydrogen ion concentration is expressed in negative logarithm and symbolized as pH.

    • Relationship between pH and H+:

    • Lower pH → Higher H+

    • Higher pH → Lower H+

  • Acidic and Alkaline Solutions:

    • Acidic solution: pH < 7

    • Alkaline solution: pH > 7

    • Normal blood pH range: 7.35 – 7.45

    • Acidosis: pH < 7.35

    • Alkalosis: pH > 7.45

    • Life-threatening pH range: 6.8 to 7.8

Significance of pH Range

  • For normal enzyme and cell function, blood pH must stay in the narrow range of 7.35 to 7.45.

    • If blood is acidic, the force of cardiac contractions diminishes.

    • If blood is alkaline, neuromuscular function becomes impaired.

Mechanisms for Maintaining Acid-Base Balance

  • Bicarbonate to Acid Ratio:

    • Ratio of bicarbonate (HCO3-) to carbonic acid (H2CO3) = 20:1.

  • Three Mechanisms Regulating Acid-Base Balance:

    • Buffer Systems:

    • Fastest response; acts chemically to change strong acids to weak acids or bind acids to neutralize.

    • Major extracellular fluid (ECF) buffer: Bicarbonate (HCO3-) – Carbonic acid (H2CO3).

    • Less significant buffers in ECF: Inorganic phosphates and plasma proteins.

    • Respiratory System:

    • Lungs excrete CO2 and H2O, by-products of cellular metabolism.

    • CO2 + H2O → H2CO3; dissociates into H+ and HCO3-.

    • Free H+ buffered by hemoglobin.

    • Renal System:

    • Maintains acid-base balance by absorbing or excreting acids and bases.

    • Normal HCO3- level: 22 to 26 mEq/L.

    • Acidosis: Kidneys reabsorb HCO3- and excrete H+

    • Alkalosis: Kidneys excrete HCO3- and retain H+.

Acid-Base Imbalances

  • Acid-Base Imbalance Definition:

    • Occurs when the ratio of acid to base (1:20) changes.

    • Classified as respiratory or metabolic.

    • Respiratory Imbalance: Affects carbonic acid concentration.

    • Metabolic Imbalance: Affects bicarbonate concentration.

  • Types of Acidosis and Alkalosis:

    • Acidosis Causes:

    • Increased carbonic acid (Respiratory acidosis).

    • Decreased bicarbonate (Metabolic acidosis).

    • Alkalosis Causes:

    • Decreased carbonic acid (Respiratory alkalosis).

    • Increased bicarbonate (Metabolic alkalosis).

Compensatory Mechanisms

  • To regain balance:

    • Lungs may compensate for metabolic disorders.

    • Kidneys may compensate for respiratory disorders.

  • Partial compensation: pH remains abnormal.

  • Complete compensation: pH returns to normal.

  • Compensatory mechanisms won’t overcompensate.

Metabolic Acidosis (Bicarbonate Deficit)

  • Definition:

    • A common clinical disturbance characterized by low pH (<7.35) and low plasma bicarbonate concentration (<22 mEq/L).

    • Causes: Gain of H+ (acid) or loss of bicarbonate (base).

  • Types of Metabolic Acidosis:

    • High anion gap acidosis (hyperchloremic acidosis).

    • Normal anion gap acidosis.

    • Anion Gap Calculation:

    • Anion gap = Na+ - (Cl- + HCO3-)

    • Normal value: 8 to 12 mEq/L.

  • High Anion Gap Indicators:

    • Indicates an addition of endogenously or exogenously generated acids.

  • Normal Anion Gap Indicators:

    • Indicates a loss of bicarbonate.

Causes of Metabolic Acidosis

  • Ingestion of acidic substances (e.g., aspirin overdose).

  • Excess acid production (e.g., ketosis, lactic acidosis).

  • Inability of kidneys to excrete adequate acid.

  • Loss of base (e.g., severe diarrhea, gastrointestinal fistulas, shock).

Pathophysiology and Clinical Manifestations of Metabolic Acidosis

  • Metabolic acidosis occurs from:

    • Accumulation of acids other than carbonic acid.

    • Loss of bicarbonate due to processes like diabetic ketoacidosis and diarrhea.

  • Compensatory Response:

    • Increased CO2 excretion by the lungs, often manifested as Kussmaul breathing (deep, rapid breathing).

  • Clinical Manifestations:

    • Headache, confusion, drowsiness, increased respiratory rate, nausea, vomiting, diarrhea, peripheral vasodilation, decreased cardiac output, decreased blood pressure, cold clammy skin, dysrhythmias, shock, bradycardia, muscular twitching.

Assessment and Management of Metabolic Acidosis

  • Diagnostic Findings:

    • ABG measurement shows low bicarbonate (<22 mEq/L) and low pH (<7.35).

    • May cause hyperkalemia due to potassium shifts from cells.

    • ECG may reveal dysrhythmias from increased potassium.

    • Hyperventilation to decrease CO2 as compensatory action.

  • Medical Management:

    • Correct underlying metabolic defect.

    • Eliminate sources of excessive chloride.

    • Administer bicarbonate if pH < 7.1 and bicarbonate <10 mEq/L.

    • Monitor closely for hypo- and hyperkalemia.

Metabolic Alkalosis (Base Bicarbonate Excess)

  • Definition:

    • Clinical disturbance characterized by high pH (decreased H+ concentration) and high plasma bicarbonate concentration.

    • Causes: Gain of bicarbonate (base) or loss of H+ (acid).

  • Common Causes:

    • Vomiting or gastric suction (loss of H+ and Cl-).

    • Pyloric stenosis.

    • Diuretic therapy promoting potassium loss (e.g., thiazides, furosemide).

  • Pathophysiology:

    • Metabolic alkalosis occurs with loss of acid (such as prolonged vomiting or gastric suction) or gain of bicarbonate (e.g., ingestion of baking powder).

    • Compensatory mechanisms: Decreased respiratory rate to increase CO2, renal excretion of bicarbonate.

  • Clinical Manifestations:

    • Tingling fingers and toes, dizziness, decreased respiration (compensatory action), atrial tachycardia, decreased motility, paralytic ileus, headache, lethargy, tetany, seizures.

  • Assessment and Diagnostic Findings:

    • ABGs show pH > 7.45 and serum bicarbonate concentration > 26 mEq/L.

    • PaCO2 increases as lungs compensate by retaining CO2.

    • Hypokalemia may accompany metabolic alkalosis.

  • Management:

    • Supply sufficient chloride for kidneys to absorb sodium with chloride (facilitating bicarbonate excretion).

    • Administer sodium chloride fluids to restore normal volume.

    • Administer potassium as KCl to replace K+ and Cl- losses.

Respiratory Acidosis (Carbonic Acid Excess)

  • Definition:

    • Clinical condition with pH < 7.35 and PaCO2 > 45 mm Hg; can be acute or chronic.

  • Acute Respiratory Acidosis Causes:

    • Emergency situations such as acute pulmonary edema, foreign object aspiration, atelectasis, sedative overdose.

  • Pathophysiology:

    • Due to inadequate CO2 excretion resulting in elevated plasma CO2 levels and increased carbonic acid levels.

    • The kidneys conserve bicarbonate and increase urinary excretion of H+; compensation begins within 24 hours.

  • Clinical Manifestations:

    • Increased pulse and respiratory rate, increase in blood pressure, mental cloudiness, fullness in the head, cyanosis, tachypnea, ventricular fibrillation.

  • Assessment and Diagnostic Findings:

    • ABG shows pH < 7.35, PaCO2 > 45 mm Hg.

    • Variations in bicarbonate level depend on the duration of acidosis.

    • Chest X-ray and ECG may be indicated to identify pulmonary disease and cardiac involvement.

  • Management:

    • Improve ventilation; measures vary with the cause.

    • Bronchodilators for bronchial spasm, antibiotics for respiratory infections.

    • Thrombolytics or anticoagulants for pulmonary emboli.

    • Semi-Fowler’s position for improved chest wall expansion.

    • Adequate hydration (2-3 L/day) to keep mucous membranes moist for secretion removal.

Respiratory Alkalosis (Carbonic Acid Deficit)

  • Definition:

    • Clinical condition where arterial pH > 7.45 and PaCO2 < 35 mm Hg.

  • Common Causes:

    • Extreme anxiety, hypoxemia, salicylate intoxication, inappropriate ventilator settings.

  • Pathophysiology:

    • Due to hyperventilation, which causes excessive CO2 expulsion and decreases plasma carbonic acid concentration.

    • Compensatory mechanism: Renal excretion of bicarbonate.

  • Clinical Manifestations:

    • Lightheadedness, inability to concentrate, numbness, tingling, tinnitus, loss of consciousness, tachycardia, dysrhythmias, confusion, hyperventilation.

  • Assessment and Diagnostic Findings:

    • ABG confirms respiratory alkalosis with elevated pH, low PaCO2, and normal bicarbonate.

  • Management:

    • Correct the underlying cause, such as anxiety; instruct the patient to breathe more slowly.

    • In cases of severe anxiety, sedatives may be necessary.

ABG Analysis

  • Overview:

    • ABG analysis assesses ventilation effectiveness and acid-base balance.

    • Critical tests: pH, PaCO2, and HCO3-.

  • Normal Values for Adults:

    • pH: 7.35 to 7.45

    • PaCO2: 35 to 45 mm Hg

    • HCO3-: 22 to 26 mEq/L

  • Steps in ABG Analysis:

    • Step 1: List essential values: pH, PaCO2, HCO3-.

    • Step 2: Check against normal values:

    • Indicate abnormal results with:

      • "A" for excessive acid,

      • "B" for excessive base,

      • "N" for normal balance.

    • Step 3:

    • Circle results:

      • If pH and PaCO2 are circled, indicates respiratory disorder.

      • If pH and HCO3- are circled, indicates metabolic disorder.

      • If all three are circled, indicates combined disturbance.

    • Step 4: Check for compensation:

    • If uncircled value is abnormal in the opposite direction, compensation is occurring.

    • If it remains normal, no compensation.

  • Notes:

    • In respiratory disorders, pH and PaCO2 move in opposite directions; HCO3- remains normal.

    • In metabolic disorders, pH and HCO3- move in the same direction; PaCO2 remains normal.

    • Remember ROME: Respiratory Opposite, Metabolic Equal.

Case Studies

  • Case Study 1:

    • Patient: Mary Marfo, 34, presents with acute shortness of breath and pain.

    • ABG Results:

    • pH: 7.50

    • PaCO2: 29 mm Hg

    • PaO2: 64 mm Hg

    • HCO3-: 24 mEq/L

    • Interpretation:

    • Respiratory alkalosis without compensation.

    • Administer O2 therapy, encourage slow breathing; consider pulmonary embolism as probable cause.

  • Case Study 2:

    • ABG Results:

    • pH: 7.20

    • PaCO2: 35 mm Hg

    • HCO3-: 14 mEq/L

    • Interpretation:

    • Patient in metabolic acidosis due to low pH and low HCO3-.

Conclusion

  • Understanding and managing acid-base imbalances is essential in clinical practice to maintain homeostasis and ensure patient safety and health.