Acid-Base Balance and ABG Interpretation

pH Regulation and Buffer Systems

The body tightly regulates pH to maintain normal function. When pH deviates from the normal range, the body employs various mechanisms to restore balance. If the body senses an acidic condition (low pH), it increases bases; if it senses a basic condition (high pH), it increases acids.

Carbonic Anhydrase and the Bicarbonate Buffering System

• Enzymes: Carbonic anhydrase is a crucial enzyme that interconverts carbon dioxide and water into carbonic acid, which then dissociates into bicarbonate and hydrogen ions.
• Reaction: CO2 + H2O \rightleftharpoons H2CO3 \rightleftharpoons HCO_3^- + H^+
• Lungs: Regulate carbon dioxide levels by expelling CO_2. The lungs can quickly adjust ventilation to alter CO_2 levels: Hyperventilation decreases CO_2, while hypoventilation increases CO_2.
• Kidneys: Regulate bicarbonate (HCO_3^-) and hydrogen ion (H^+) levels. The kidneys can excrete or reabsorb HCO_3^- and H^+ to maintain pH balance. This process is slower than the respiratory response but provides long-term control.

pH Scale and Key Players

• pH and Acidity: Lower pH indicates higher acidity, while higher pH indicates higher alkalinity (basicity).
• Acids: Carbon dioxide (CO_2) and hydrogen ions (H^+) are acids.
• Base: Bicarbonate (HCO_3^-) is a weak base.

Body's Buffer Systems

1. Proteins and Phosphates: Act as buffer systems. Proteins, such as albumin and hemoglobin, can bind to H^+ ions, while phosphates buffer pH in intracellular fluids and urine.
2. Carbonic Acid-Bicarbonate Buffering System: Primarily involves the lungs and kidneys.

Lungs and Kidneys: A Dynamic Duo

The lungs and kidneys work in concert to maintain pH balance.

• Lungs (The Rock): React quickly to imbalances by regulating carbon dioxide (CO_2) levels through respiration.
• Low pH (Acidic): Lungs increase respiration to expel CO_2. The increased respiratory rate helps to reduce the amount of acid in the blood.
• Normal Respiration Rate: 12 to 20 breaths per minute.
• High pH (Basic): Lungs decrease respiration to retain CO_2. The decreased respiratory rate helps to increase the amount of acid in the blood.
• Kidneys (Kevin Hart): Respond more slowly, regulating bicarbonate (HCO_3^-) and hydrogen ion (H^+) levels.
• Acidic: Kidneys retain bicarbonate (base) and excrete hydrogen ions (acid). This process increases the amount of base in the blood to balance the acid.
• Basic: Kidneys retain hydrogen ions (acid) and excrete bicarbonate (base). This process increases the amount of acid in the blood to balance the base.

Central Chemoreceptors

Located in the brain, these receptors monitor pH levels and signal the lungs to adjust respiration accordingly. These chemoreceptors are highly sensitive to changes in CO_2 and pH, allowing for rapid adjustments in ventilation.

Acid-Base Disorders

• Respiratory: Originates in the lungs (excess CO_2). Respiratory disorders affect the lungs’ ability to expel CO_2, leading to imbalances.
• Metabolic: Originates outside the lungs. Metabolic disorders involve imbalances in the production or excretion of acids or bases by the kidneys or other bodily systems.

Arterial Blood Gas (ABG)

An ABG test is essential for diagnosing acid-base disorders. Key values to memorize:

• pH: 7.35 to 7.45
• PaCO2: 35 to 45 mmHg (Partial pressure of carbon dioxide)
• HCO3: 22 to 26 mEq/L (Bicarbonate level)

Allen's Test

Required before ABG draw to ensure adequate circulation in the artery. This test helps to prevent ischemia in the hand if the radial artery is damaged during the ABG draw.

Anion Gap

Determined from blood work to further evaluate acid-base disturbances. The anion gap helps differentiate between different causes of metabolic acidosis.

Potassium and Hydrogen Relationship

Changes in hydrogen ion concentration affect potassium levels and vice versa.

• Acidemia (Excess Hydrogen Ions): Hydrogen ions enter cells, displacing potassium, leading to hyperkalemia (high potassium). This is because the body tries to maintain electrical neutrality by shifting potassium out of cells when hydrogen ions enter.
• Alkalemia (Low Hydrogen Ions): Hydrogen ions exit cells, and potassium enters, leading to hypokalemia (low potassium). This is because the body tries to maintain electrical neutrality by shifting potassium into cells when hydrogen ions exit.
• Hyperkalemia (Excess Potassium): Potassium ions enter cells pushing hydrogen ions out of the cells into the bloodstream.
• Hypokalemia (Potassium deficiency): Hydrogen moves out from the bloodstream into the cells from Potassium leaving to a more basic state, so your alkalosis.

Calcium and Hydrogen Ions

Albumin binds to both calcium and hydrogen ions. In acidemia (excess hydrogen), more hydrogen ions bind to albumin, displacing calcium. This leads to an increase in free (unbound) calcium in the blood, which can affect nerve and muscle function.

Respiratory Alkalosis

Condition where the blood is too basic due to excessive loss of carbon dioxide from the lungs.

• Causes: Anxiety, salicylate intoxication (aspirin overdose), changes in altitude. Hyperventilation due to anxiety or high altitude can lead to excessive CO_2 loss.
• Symptoms: Tingling extremities, tetany, dizziness, palpitations, dysrhythmia, confusion, anxiety. These symptoms are often related to the decreased levels of ionized calcium.
• Treatment: Breathing into a brown paper bag to rebreathe carbon dioxide. This helps to increase the CO_2 levels in the blood.

Metabolic Acidosis

Condition where the blood is too acidic due to metabolic causes.

• pH: Less than 7.35
• Causes: Diabetic ketoacidosis, lactic acidosis (sepsis), alcohol. These conditions result in the accumulation of acids in the body.
• Respirations: Kussmaul respirations (deep, labored breathing). This is the body's attempt to compensate by blowing off excess CO_2.
• Treatment: Correct underlying cause

Metabolic Alkalosis

Condition where the blood is too basic due to metabolic causes.

• pH: Greater than 7.45
• Causes: Excessive loss of acid (kidney dysfunction), NG tube suctioning. These conditions result in the loss of acids or the accumulation of bases in the body.
• Treatment: Correct underlying cause.

Interpreting Arterial Blood Gases (ABGs)

Steps

1. Assess pH (acidic, basic, or normal).
2. Assess PaCO2 (high, low, or normal).
3. Assess HCO3 (high, low, or normal). If all three are abnormal, consider compensation.

The Arrow Method

Use arrows to indicate whether each value is high (↑), low (↓), or normal (↔).

• Opposite Arrows (pH and PaCO2): Indicate a respiratory disturbance.
• Same Direction Arrows (pH, and HCO3): Indicate a metabolic disturbance.

Example 1: Respiratory Acidosis (Uncompensated)

• pH: 7.21 (↓)
• PaCO2: 65 (↑)
• HCO3: Normal (↔)

Example 2: Metabolic Acidosis (Uncompensated)

• pH: 7.14 (↓)
• PaCO2: Normal (↔)
• HCO3: Low (↓)

Compensation

• **Partial