Physiology

Maintaining the proper balance of acids and bases in the body is crucial for sustaining life. Disruptions to this balance can lead to various life-threatening conditions, including acidosis and alkalosis. Understanding the causes and effects of acid-base imbalances is crucial for proper diagnosis and treatment. Effective management of acid-base disorders requires addressing the underlying cause and restoring normal pH, oxygenation, and ventilation. Arterial blood gas analysis provides essential information for assessing acid-base status and guiding therapy

Normal States

Normal Blood pH: 7.35-7.45
- Maintaining blood pH within this narrow range is essential for optimal enzyme function, protein structure, and cellular metabolism
Normal Arterial Blood Gas Values
- Pa\(O_2\): 80-100 mmHg
  - Partial pressure of oxygen in arterial blood
  - Indicates the amount of oxygen available to tissues
- Pa\(CO_2\): 35-45 mmHg
  - Partial pressure of carbon dioxide in arterial blood
  - Reflects the effectiveness of ventilation
- \(HCO_3^-\): 22-26 mEq/L
  - Bicarbonate concentration in arterial blood
  - Represents the metabolic component of acid-base balance
- Base Excess (BE): -2 to +2 mEq/L
  - Represents the amount of acid or base needed to restore normal pH
  - Positive BE: Indicates excess base (alkalosis)
  - Negative BE: Indicates excess acid (acidosis)
- Sa\(O_2\): 95-100%
  - Oxygen saturation
  - Indicates the percentage of hemoglobin saturated with oxygen

Abnormal States

Acidosis: A condition characterized by an excess of acid in the body, resulting in a blood pH below 7.35
- Respiratory Acidosis
- Metabolic Acidosis
Alkalosis: A condition characterized by an excess of base in the body, resulting in a blood pH above 7.45
- Respiratory Alkalosis
- Metabolic Alkalosis
Hypoxemia: A condition characterized by low oxygen levels in the blood
Hypercapnia: A condition characterized by elevated carbon dioxide levels in the blood
Mixed Acid-Base Disorders: The presence of more than one primary acid-base disorder

Acid-Base Disorders

Respiratory Acidosis

Definition: A condition characterized by a decrease in blood pH due to increased carbon dioxide levels (hypercapnia)
Causes
- Hypoventilation: Impaired breathing due to lung disease (e.g., COPD, pneumonia), central nervous system depression (e.g., drug overdose), or neuromuscular disorders (e.g., muscular dystrophy)
- Increased Carbon Dioxide Production: In rare cases, can be caused by increased carbon dioxide production (e.g., malignant hyperthermia)
Laboratory Findings
- pH: < 7.35
- Pa\(CO_2\): > 45 mmHg
- \(HCO_3^-\): Normal in acute respiratory acidosis; elevated in chronic respiratory acidosis (due to renal compensation)
Compensation
- Renal Compensation: Kidneys increase bicarbonate reabsorption and acid excretion over several days
Symptoms
- Confusion, lethargy, headache, shortness of breath

Metabolic Acidosis

Definition: A condition characterized by a decrease in blood pH due to decreased bicarbonate levels or increased acid production
Causes
- Increased Acid Production:
  - Diabetic Ketoacidosis (DKA): Accumulation of ketoacids due to insulin deficiency
  - Lactic Acidosis: Accumulation of lactic acid due to anaerobic metabolism
  - Starvation: Breakdown of fats and proteins leads to increased ketoacid production
  - Toxins: Ingestion of methanol, ethylene glycol, or salicylates
- Loss of Bicarbonate:
  - Diarrhea: Loss of bicarbonate-rich intestinal secretions
  - Renal Tubular Acidosis (RTA): Impaired reabsorption of bicarbonate by the kidneys
- Impaired Acid Excretion:
  - Kidney Failure: Reduced ability to excrete acids and regenerate bicarbonate
Laboratory Findings
- pH: < 7.35
- Pa\(CO_2\): Normal or decreased (due to respiratory compensation)
- \(HCO_3^-\): < 22 mEq/L
- Base Excess (BE): More negative than -2 mEq/L
Anion Gap
- Definition: The difference between measured cations (Na+, \(K^+\)) and anions (Cl-, \(HCO_3^-\)) in serum or plasma
- Calculation: Anion Gap = [\(Na^+\)] + [\(K^+\)] - [\(Cl^-\)] - [\(HCO_3^-\)]
- Normal Anion Gap: 8-16 mEq/L (with \(K^+\)), 10-20 mEq/L (without \(K^+\))
High Anion Gap Metabolic Acidosis
- Increased production of organic acids (e.g., ketoacids, lactic acid)
- Common causes: DKA, lactic acidosis, toxins (methanol, ethylene glycol, salicylates), kidney failure
Normal Anion Gap (Hyperchloremic) Metabolic Acidosis
- Loss of bicarbonate or impaired acid excretion
- Common causes: Diarrhea, renal tubular acidosis (RTA), administration of chloride-containing solutions
Compensation
- Respiratory Compensation: Increased ventilation to blow off \(CO_2\) and raise pH
- Renal Compensation: Kidneys increase acid excretion and bicarbonate reabsorption
Symptoms
- Rapid breathing, fatigue, confusion, nausea, vomiting

Respiratory Alkalosis

Definition: A condition characterized by an increase in blood pH due to decreased carbon dioxide levels (hypocapnia)
Causes
- Hyperventilation: Increased breathing rate due to anxiety, pain, hypoxia, or certain medical conditions (e.g., pulmonary embolism, asthma)
- Mechanical Ventilation: Excessive ventilation by a mechanical ventilator
- Central Nervous System Disorders: Conditions that stimulate the respiratory center in the brain
Laboratory Findings
- pH: > 7.45
- Pa\(CO_2\): < 35 mmHg
- \(HCO_3^-\): Normal in acute respiratory alkalosis; decreased in chronic respiratory alkalosis (due to renal compensation)
Compensation
- Renal Compensation: Kidneys decrease bicarbonate reabsorption and acid excretion over several days
Symptoms
- Lightheadedness, dizziness, tingling, numbness, muscle cramps, rapid breathing

Metabolic Alkalosis

Definition: A condition characterized by an increase in blood pH due to increased bicarbonate levels or decreased acid levels
Causes
- Loss of Acid:
  - Vomiting: Loss of gastric acid (hydrochloric acid, HCl)
  - Nasogastric Suctioning: Removal of gastric acid
- Increased Bicarbonate:
  - Excessive Bicarbonate Intake: Ingestion of antacids
  - Administration of Bicarbonate: During resuscitation or treatment of acidosis
  - Contraction Alkalosis: Due to volume depletion, leading to increased bicarbonate concentration
- Mineralocorticoid Excess:
  - Hyperaldosteronism: Increased aldosterone leads to increased sodium reabsorption and increased hydrogen ion excretion
Laboratory Findings
- pH: > 7.45
- Pa\(CO_2\): Normal or increased (due to respiratory compensation)
- \(HCO_3^-\): > 26 mEq/L
- Base Excess (BE): More positive than +2 mEq/L
Compensation
- Respiratory Compensation: Decreased ventilation to retain \(CO_2\) and lower pH
- Renal Compensation: Kidneys decrease bicarbonate reabsorption and increase acid excretion
Symptoms * Confusion, lightheadedness, muscle cramps, nausea, vomiting

Other Abnormal States

Hypoxemia

Definition: A condition characterized by low oxygen levels in the blood
Causes
- Reduced Inspired Oxygen: High altitude, breathing air with low oxygen content
- Hypoventilation: Decreased breathing rate or depth
- Diffusion Impairment: Thickening or damage to the alveolar membrane
- Ventilation-Perfusion Mismatch: Imbalance between ventilation and blood flow in the lungs
- Shunt: Blood bypasses the lungs without being oxygenated
Laboratory Findings
- Decreased Pa\(O_2\): < 80 mmHg
- Decreased Sa\(O_2\): < 95%
Symptoms * Shortness of breath, rapid breathing, confusion, cyanosis (bluish discoloration of the skin)

Hypercapnia

Definition: A condition characterized by elevated carbon dioxide levels in the blood
Causes
- Hypoventilation: Decreased breathing rate or depth
- Increased Dead Space Ventilation: Ventilation of areas of the lung that are not perfused with blood
- Increased Carbon Dioxide Production: In rare cases, can be caused by increased carbon dioxide production (e.g., malignant hyperthermia)
Laboratory Findings
- Increased Pa\(CO_2\): > 45 mmHg
Symptoms
- Headache, confusion, lethargy, shortness of breath

Mixed Acid-Base Disorders

Definition: The presence of more than one primary acid-base disorder occurring simultaneously
Examples
- Metabolic Acidosis and Respiratory Acidosis: Occurs in patients with COPD and kidney failure
- Metabolic Alkalosis and Respiratory Alkalosis: Occurs in patients with anxiety and vomiting
Diagnosis
- Requires careful analysis of arterial blood gas results and clinical context
- May involve calculating the expected compensation and comparing it to the actual compensation

Key Terms

Acidosis: A condition in which the blood pH is below the normal range
Alkalosis: A condition in which the blood pH is above the normal range
Hypoxemia: Low oxygen levels in the blood
Hypercapnia: Elevated carbon dioxide levels in the blood
Arterial Blood Gas (ABG): A test that measures the levels of oxygen and carbon dioxide in arterial blood
Partial Pressure of Oxygen (Pa\(O_2\)): A measure of the oxygen level in arterial blood
Partial Pressure of Carbon Dioxide (Pa\(CO_2\)): A measure of the carbon dioxide level in arterial blood
Bicarbonate (\(HCO_3^-\)): A major buffer in the blood
Base Excess (BE): A measure of the amount of acid or base needed to restore normal pH
Oxygen Saturation (Sa\(O_2\)): A measure of the percentage of hemoglobin saturated with oxygen
Respiratory Acidosis: Acidosis caused by increased carbon dioxide levels
Metabolic Acidosis: Acidosis caused by decreased bicarbonate levels or increased acid production
Respiratory Alkalosis: Alkalosis caused by decreased carbon dioxide levels
Metabolic Alkalosis: Alkalosis caused by increased bicarbonate levels or decreased acid levels
Anion Gap: The difference between measured cations and anions in serum or plasma
Hyperventilation: Increased rate and depth of breathing
Hypoventilation: Decreased rate and depth of breathing
COPD: Chronic obstructive pulmonary disease

CO2 & O2 Transport

Procedures