Chapter 14: Renal, Gynecologic, and Urinary Disease

ACID-BASE DISORDERS

Normal Acid-Base Balance

• pH 7.4

• PCO₂ 40

• O₂ sat 98–100%

• Bicarbonate 22–28

DIAGNOSIS

• Diagnose acid-base disorders by obtaining an arterial blood gas (ABG—pH and PCO₂) and an electrolyte panel (HCO₃⁻).

• Assess the acid-base disorder step by step:

  • Is the primary disorder an acidosis (pH <7.36) or alkalosis (pH >7.44)?

  • Is the disorder respiratory (pH and PCO₂ move in opposite directions)?

  • Is the disorder metabolic (pH and PCO₂ move in the same direction)?

  • Has there been respiratory/metabolic compensation? Is it a simple or mixed disorder?

ACIDOSIS

• Acidemia is an arterial pH <7.36 caused by metabolic (decreased serum HCO₃⁻) or respiratory (increased PCO₂) acidosis.

• An extracellular shift of potassium (exchange of hydrogen for potassium cation), and decreased binding of calcium to albumin, may lead to hyperkalemia and hypercalcemia.

• A lower affinity between O₂ and Hb results in more oxygen release to tissues, shifting the oxygen dissociation curve to the right.

Metabolic Acidosis

• A drop in pH occurs due to a decreased serum HCO₃⁻.

• Decreased serum HCO₃⁻ may result from (1) increased production of acids, (2) decreased excretion of acids, or (3) loss of alkaline fluids.

• The anion gap (AG) helps identify the cause of metabolic acidosis. It reflects unmeasured serum ions (organic acids, phosphates, etc.).

  • AG = [Na⁺] − ([Cl⁻] + [HCO₃⁻]).

  • In children, an AG above 14 is considered elevated.

• Normal AG metabolic acidosis (hyperchloremic metabolic acidosis) results when low serum HCO₃⁻ is balanced by increased renal Cl⁻ reabsorption. Etiologies include:

  • Diarrhea (most common), due to gastrointestinal HCO₃⁻ loss

  • Renal tubular acidosis, caused by ineffective HCO₃⁻ reabsorption proximally (type 2) or ineffective H⁺ excretion distally (type 1)

• AG metabolic acidosis can be caused by exogenous sources (drugs), endogenous acid production (lactic acidosis, ketoacidosis, etc.), or acid retention due to renal failure (uremia). See MUDPILES Ward Tip.

• Respiratory compensation (tachypnea) begins within 30 minutes. Severe metabolic acidosis (pH <7.2) results in compensatory hyperventilation (Kussmaul's breathing).

  • Appropriate change in PCO₂ can be predicted with the Winter's formula, i.e. expected PCO₂ = (1.5 × serum HCO₃⁻) + 8 ± 2.

• Treat ...