Blood Gases and Acid-Base Disorders




DEFINITIONS



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  • pH = power of Hydrogen defined by the Henderson-Hasselbalch equation:


    pH=pK+log ([HCO3−]/0.03 PCO2)




  • Acidosis = decrease in arterial pH




    • Respiratory acidosis – caused by CO2 retention. This increases the denominator in the Henderson-Hasselbalch equation and depresses the pH.




      • Hypoventilation and ventilatory failure can cause respiratory acidosis.





    • Metabolic acidosis – reduced pH not explained by increased PCO2. Caused by a primary fall in the numerator [HCO3−] of the Henderson-Hasselbalch equation.




      • It is usually associated with an increased anion gap (see later).





  • Alkalosis = increase in arterial pH




    • Respiratory alkalosis – the decreased PCO2 explains the increased pH.




      • Seen in alveolar hyperventilation.





    • Metabolic alkalosis – raised pH out of proportion to changes in PCO2.




      • It is associated with hypokalemia, exogenous alkali administration, or volume contraction (e.g., severe prolonged vomiting) when the plasma bicarbonate concentration rises.





  • Anion gap: helps differentiate between acid gain and HCO3−loss.


    Anion gap=[Na+]−([Cl−]+[HCO3−])Normal gap 8−12 mEq/L





  • Anion gap acidosis: gap >12 mEq/L




    • Caused by decrease in HCO3−balanced by an increase in unmeasured acid ions, not by an increase in chloride.



    • Causes include salicylates, methanol, paraldehyde, ethylene glycol, lactic acidosis, ketoacidosis (from diabetes or starvation), and uremia.




  • Non–anion gap acidosis: gap 8 to 12 mEq/L




    • Caused by a decrease inHCO3− balanced by an increase in chloride.



    • Causes include renal tubular acidosis, carbonic anhydrase inhibitor, and diarrhea.




  • Table 22-1 gives normal values for infants and children.



  • Fencl-Stewart approach to understanding acid-base balance: looks for unmeasured anions, which could contribute to a pH disturbance.


    BEexplained=H2O+Cl−+albumin




  • H2O contribution = 0.3 (Na + 140)



  • Cl contribution = 102 − (Cl × 140/Na)



  • Albumin contribution = 3.4 (4.5-albumin)



  • BE: base excess




BEunexplained=*BEmeasured−BEexplained



*Get BEmeasured from blood gas


Jan 14, 2019 | Posted by in PEDIATRICS | Comments Off on Blood Gases and Acid-Base Disorders
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