Acid-Base Balance Fundamentals and Nursing Care

Core Terminology and Physiology

  • Acid: Donor of hydrogen ion (H+H^+).

  • Base: Acceptor of hydrogen ions; Bicarbonate (HCO3−HCO_3^-) is the most abundant base in body fluids.

  • pH: Measurement of the power of hydrogen ion concentration; calculated as the negative logarithm of the H+H^+ concentration.

  • Buffer: A substance that reacts with an acid or base to prevent large changes in pH.

  • Physiological Significance: Normal pH is essential for electrolyte balance, enzyme activity, muscle contraction, and cellular function.

Normal Arterial Blood Values

  • pH: 7.35↔7.457.35 \leftrightarrow 7.45 (Values < 7.35 indicate acidemia/acidosis; > 7.45 indicate alkalemia/alkalosis).

  • PaCO2: 35↔4535 \leftrightarrow 45.

  • HCO3: 22↔2622 \leftrightarrow 26.

  • PaO2: 80↔10080 \leftrightarrow 100.

  • SaO2: 95↔100%95 \leftrightarrow 100\%.

  • Base Excess (BE): −2↔+2-2 \leftrightarrow +2.

  • Bicarbonate to Carbonic Acid Ratio: 20:120:1 (Ratio required to maintain normal pH range).

Regulatory Mechanisms for pH Balance

  • Chemical Buffers: Act immediately to protect tissues and cells. Includes Bicarbonate (blood/interstitial fluid), Phosphate (active in renal tubules), and Protein buffers (intracellular/extracellular).

  • Respiratory System: Responds within minutes by varying the rate and depth of breathing to regulate CO2CO_2 levels. Hypoventilation retains acid (CO2CO_2); hyperventilation excretes it.

  • Renal System: Provides long-term adjustment within hours or days by producing bicarbonate and excreting or retaining hydrogen ions (H+H^+).

Respiratory Imbalances

  • Respiratory Acidosis (pH < 7.35, PaCO_2 > 45):

    • Cause: Hypoventilation (COPD, pneumonia, atelectasis, sedatives, anesthesia, CNS injury).

    • Symptoms: Dyspnea, hypoxia, restlessness, lethargy, tachycardia, decreased BP.

  • Respiratory Alkalosis (pH > 7.45, PaCO_2 < 35):

    • Cause: Hyperventilation (anxiety, pain, fever, high altitudes, mechanical over-ventilation).

    • Symptoms: Light-headedness, tetany, tingling in extremities, arrhythmias.

Metabolic Imbalances

  • Metabolic Acidosis (pH < 7.35, HCO_3 < 22):

    • Cause: Excess acid production or base loss (DKA, renal failure, shock, severe diarrhea, starvation).

    • Key Sign: Kussmaul’s respirations (deep, rapid breathing to blow off CO2CO_2).

  • Metabolic Alkalosis (pH > 7.45, HCO_3 > 26):

    • Cause: Base excess or acid loss (severe vomiting, NG tube suctioning, antacid overuse, diuretic therapy).

    • Symptoms: Muscle cramps, hyperactive reflexes, tetany, confusion.

ABG Interpretation and Compensation

  • ROME Mnemonic:

    • Respiratory Opposite: If pH is low, PCO2PCO_2 is high (Acidosis); if pH is high, PCO2PCO_2 is low (Alkalosis).

    • Metabolic Equal: If pH is low, HCO3HCO_3 is low (Acidosis); if pH is high, HCO3HCO_3 is high (Alkalosis).

  • Compensation Processes:

    • Respiratory helps Metabolic: Lungs change breathing rate to compensate for bicarbonate imbalances.

    • Metabolic helps Respiratory: Kidneys alter bicarbonate and H+H^+ levels to compensate for CO2CO_2 imbalances.

Lifespan Considerations

  • Older Adults: Experience decreased kidney filtration and decreased total body fluid, making them more susceptible to imbalances.

  • Children: Total body fluid is 70−80%70 - 80\%, leading to quicker electrolyte changes; they are more prone to respiratory imbalances due to communicable diseases.