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(N Engl J Med 1998;339:819)

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  • 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

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  • 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)

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Table Graphic Jump Location
Table 14-1 Primary Acid-Base Disorders
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Signs & Symptoms

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

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  • 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

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Compensation

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

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  • 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)

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Steps to Evaluate Blood Gases

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  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? Determine the corrected HCO3...

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