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)

2000px-Acid-base_nomogram.svg

 

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