Arterial blood gas analysis

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 [[P_aO₂/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

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.

• 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