Acid Base Disorders

Links:

Understanding Acid-Base Disorders

---

Normal pH

• Important to keep metabolic pathways in their ionized state
• Optimal environment for protein function

Notes on assessment:

• Before obtaining ABG, FiO₂ should be stable for 15 min to allow PaO₂ and PaCO₂ to equilibrate
• Check temperature: measured PaO₂ and PaCO₂ increase as blood is warmed (O₂ dissociation curve shifts right, and solubility of gases decrease)
  • Sample should be analyzed immediately or placed on ice
  • PaCO₂ may increase if sample left at room temp, or if generated by metabolically active leukocytes, red cells
  • PaO₂ may decrease if consumption in vitro in setting of marked leukocytosis or thrombocytosis
• O₂ may diffuse through plastic tube (glass is preferred)
• Normal VBG pH 7.30 – 7.35, Normal PaO₂ 40 – 44, Normal PaCO₂ 45
• Expect exaggerated discordance in arterial to venous pH in heart failure
• Acidemia
  • Decreased sarcoplasmic Ca²⁺ release, decreased mitochondrial respiration, decreased enzyme activity
    • CNS depression
    • Myocardial depression with reduced responsiveness to catecholamines
    • Sympathetic overdrive
    • Peripheral vasodilitation, Pulmonary vasoconstriction
  • Alkalemia
    • Decreased ionized Ca²⁺
      • Paresthesias, tetany, seizures, arrhythmias

 

Base excess = the amount of strong acid that must be added to 1L fully oxygenated blood to return the pH to 7.40, at pCO₂ 40 and temp 37

= HCO₃^– + 10(pH – 7.40) – 24

Bicarb is the predominant base. Negative base excess (base deficit) means that bicarb is depleted

Calculation cannot tell you whether pathologic or compensatory

Bicarbonate: 8.4% (1 mEq/mL) NaHCO₃ 50 mL = 1 amp (so 3 amps in 1L of D5W gets you 150mEq of NaHCO₃)

Image care of https://commons.wikimedia.org