Chapter 14. Acid-Base Disorders

(N Engl J Med 1998;339:819)

• Buffering systems:
  • Extracellular buffering systems: proteins, phosphates, carbonic acid–bicarbonate buffer system
  • Bicarbonate–carbon dioxide buffer system: dissolved CO2 + H2O ↔ H2CO3 ↔ HCO3− + H+
• Henderson-Hasselbach equation: pH = 6.10 + log (HCO3−)/(0.03 × pCO2)
  • Concentration of H2CO3 (carbonic acid) is normally so low that its role can be ignored
• Regulation of acid-base status:
  • Respiratory: medullary chemoreceptors in brainstem sense changes in pCO2
  • Renal: excretion or reabsorption of HCO3−, excretion of acid (ex: H+, NH4+), generation of new HCO3−

• Arterial blood gas (ABG) interpretation: reported as pH/pCO2/pO2/SaO2 or pH/pCO2/pO2/FiO2
  • Normal values: pH 7.40 (range 7.35–7.45), pCO2 35–45mmHg, pO2 80–100mmHg, HCO3− 22–28mEq/L (from serum chemistry, not blood gas), SaO2 95–100% (% hemoglobin fully saturated with O2)

Signs & Symptoms

(N Engl J Med 1998;338:26; N Engl J Med 1998;338:107)

• Acidemia (↓ pH) → hyperventilation, CNS depression, ↓ cardiac contractility, hyperkalemia, hypotension, & hypothermia; catecholamines not as effective in an acidic environment
• Alkalemia (↑ pH) → hypokalemia, muscle cramping, paresthesias, confusion, neuromuscular excitation, arrhythmias, respiratory depression

Compensation

(Prim Care Clin Office Pract 2008;35:195)

• Lungs: regulate acute changes (occurs within seconds); regulates pCO2 through respiration rate & depth changes
• Renal: regulates chronic changes (takes hours to days to compensate); regulates HCO3− & H+ through excretion/reabsorption
• Primary metabolic disorder will have respiratory compensation (metabolic acidosis → respiratory alkalosis; metabolic alkalosis → respiratory acidosis)
• Primary respiratory disorder will have metabolic compensation (respiratory acidosis → metabolic alkalosis; respiratory alkalosis → metabolic acidosis)

Steps to Evaluate Blood Gases

1. Begin by looking at the pH (pH <7.4 = acidemia; pH >7.4 = alkalemia)

2. Determine if primary respiratory or metabolic problem

   1. For primary respiratory disorders, pH & CO2 → opposite direction

   2. For primary metabolic disorders, pH & HCO3− → same direction

3. Calculate the anion gap (AG = Na+ − Cl− − HCO3−); AG = unmeasured anions, ex: albumin, plasma proteins, phosphates, sulfate; must correct for low albumin (↑AG by 2.5 for every 1g/dL↓ in albumin)

4. Is the compensatory response appropriate? ...