Expected ABG in Uncorrected Anaphylactic Shock
In uncorrected anaphylactic shock, expect metabolic acidosis with elevated lactate, normal to low PaCO2 (from compensatory hyperventilation or respiratory distress), and variable PaO2 depending on respiratory involvement.
Primary ABG Pattern
The dominant finding in anaphylactic shock is metabolic acidosis resulting from tissue hypoperfusion and anaerobic metabolism 1, 2. This develops rapidly as vasodilation and increased capillary permeability cause up to 37% loss of circulating blood volume into the extravascular space within minutes 3.
Specific Expected Values:
- pH: Decreased (<7.35), reflecting metabolic acidosis
- Lactate: Elevated (>2 mmol/L), often significantly so in severe cases 1
- Base Excess: Negative (more negative with severity) 2
- Bicarbonate (HCO3-): Decreased due to metabolic acidosis
Respiratory Component
The respiratory pattern varies based on the dominant pathophysiology:
If bronchospasm/airway obstruction predominates:
- PaCO2: May be elevated (respiratory acidosis component)
- PaO2: Decreased due to ventilation-perfusion mismatch
- Mixed respiratory and metabolic acidosis develops
If cardiovascular collapse predominates without significant bronchospasm:
- PaCO2: Normal to decreased (from compensatory hyperventilation) 2
- Initial hyperventilation attempts to compensate for metabolic acidosis
- PaO2: May remain relatively preserved initially 1, 2
Critical Clinical Context
Metabolic acidosis develops even without obvious respiratory symptoms 1. The severity of acidosis is frequently underestimated during acute treatment 1. In experimental models, progressive metabolic acidosis indicating poor tissue oxygenation and perfusion develops rapidly, even when gas exchange remains relatively preserved 2.
Severity Correlation:
- Severe anaphylaxis leading to cardiac arrest: Metabolic acidosis is always present 1
- Less severe reactions with mildly impaired circulation: Metabolic acidosis is still frequently found 1
- The degree of lactate elevation and base deficit correlates with shock severity and tissue hypoperfusion 4, 5
Pitfalls to Avoid
Don't wait for ABG confirmation to treat: Anaphylaxis is a clinical diagnosis requiring immediate epinephrine administration 3, 6, 7
Don't assume normal PaO2 means adequate perfusion: Gas exchange may be preserved initially despite severe metabolic acidosis from circulatory collapse 2
Don't overlook mixed disorders: Patients may have combined metabolic acidosis from shock plus respiratory acidosis from bronchospasm, or respiratory alkalosis from compensatory hyperventilation 8
Serial measurements are essential: A single ABG provides limited information; trending values reveals adequacy of resuscitation 4, 5, 9
Practical Application
When you obtain an ABG in suspected anaphylactic shock, look for:
- Metabolic acidosis (low pH, low HCO3-, negative base excess)
- Elevated lactate as the most sensitive marker of tissue hypoperfusion
- Variable PaCO2 depending on respiratory involvement
- The combination of hypotension, metabolic acidosis, and elevated lactate confirms inadequate tissue perfusion requiring aggressive fluid resuscitation and vasopressor support 3, 9
The ABG helps quantify severity and guide resuscitation but should never delay immediate epinephrine administration, which remains the cornerstone of anaphylaxis treatment 3, 6, 7.