ABG interpretation

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Basics

pH

  • Measurement of acidity or alkalinity of solution . The normal range is 7.35 to 7.45
  • pH > 7.45 = alkalosis
  • pH< 7.35 = acidosis

PaO2

  • The partial pressure of oxygen that is dissolved in the arterial serum that is not carried by hemoglobin. The normal range is 80 to 100 mm Hg.
    • Used to assess alveolar-arterial (A-a) gradient, or [[PaO2/FiO2 ratio]].

SaO2

The arterial oxygen saturation. The normal range is 95% to 100%.

pCO2

  • The amount of carbon dioxide dissolved in arterial blood. The normal range is 35 to 45 mm Hg.
  • pCO2 >45 = primary respiratory acidosis
  • pCO2 <35 = primary respiratory alkalosis

HCO3

The calculated value of the amount of bicarbonate in the bloodstream. The normal range is 22 to 26 mEq/liter HCO3 > 26 = alkalosis HCO3 < 22 = acidosis

B.E.

  • The base excess indicates the amount of excess or insufficient level of bicarbonate in the system.
  • The normal range is -2 to +2 mEq/liter.
  • A negative base excess indicates a base deficit in the blood.


ABG in Hypothermia

  • Blood gas analyzers typically warm blood to 37°C before making calculations however physicians should simply interpret the values as given by the lab
  • At any temperature, a pH of 7.4 and a PCO2 of 40 mm Hg represent normal acid-base balance

The best (simplest) approach is to use uncorrected ABG values compared with the normal values at 37°C[1]

TEMPERATURE CORRECTION OF PO2 and PCO2

Oxygen and carbon dioxide increase in solubility as water temperature lowers, so warmed ABGs from hypothermic patients with show a higher PaO2, higher PaCO2 and a lower pH than what is actually present in the patient’s blood although the clinical implications of this change is not significant.[2]

  • PO2 is 5 mmHg lower for each degree below 37C°
  • PCO2 is 2mmHg lower for each degree below 37C°

ABG after Cardiac Arrest

pH of blood dependent on:

  • cause of arrest
  • duration of arrest when ABG is taken
  • the quality of the CPR and ventilation the patient is receiving
  • temperature of the patient when ABG taken
  • drugs administered during arrest (ie. Adrenaline and NaHCO3)

Elevated pCO2

  • often due to inadequate ventilation during resuscitation

Low PaO2

  • insufficient oxygenation during resuscitation
  • aspiration
  • pulmonary edema
  • PE
  • primary respiratory pathology

Hyperkalemia Causes

  • cell death
  • loss of cell membrane integrity and acidosis
  • other sources such as missed dialysis

References

  1. Burnett RW, Noonan DC. Calculations and correction factors used in determination of blood pH and blood gases. Clin Chem. 1974 Dec;20(12):1499-506. Review.
  2. Ashwood ER, Kost G, Kenny M. Temperature correction of blood-gas and pH measurements. Clin Chem. 1983 Nov;29(11):1877-85. Review. PubMed PMID: 6354511.