A patient with a VVI pacemaker experiences intraoperative bradycardia after hyperventilation. What is the most likely cause?

A patient with a VVI pacemaker experiences intraoperative bradycardia after hyperventilation. What is the most likely cause?

• A. Hypocarbia
• B. Vancomycin allergy
• C. Acute increase in intracranial pressure
• D. A side effect of thiopental

Answer: (A)

Hyperventilation lowers arterial CO2 (hypocapnia), which triggers respiratory alkalosis and cerebral vasoconstriction. The fall in cerebral blood flow and the associated shift in autonomic balance can increase vagal tone, leading to bradycardia. In a patient with a VVI pacemaker, this vagal-bradycardic response can manifest intraoperatively as bradycardia even as the device attempts to maintain rate, because the underlying cause is the autonomic effect of hypocapnia rather than a problem with the pacing system itself. Vancomycin allergy would produce allergic signs and hemodynamic changes not specifically tied to the ventilation state. An acute rise in intracranial pressure would more classically present with hypertension and a different breathing pattern (Cushing response), not bradycardia driven by hyperventilation. Thiopental can cause bradycardia as a drug effect, but the scenario links the event to hyperventilation and consequent hypocapnia, making hypocarbia the most likely explanation.

Hyperventilation lowers arterial CO2 (hypocapnia), which triggers respiratory alkalosis and cerebral vasoconstriction. The fall in cerebral blood flow and the associated shift in autonomic balance can increase vagal tone, leading to bradycardia. In a patient with a VVI pacemaker, this vagal-bradycardic response can manifest intraoperatively as bradycardia even as the device attempts to maintain rate, because the underlying cause is the autonomic effect of hypocapnia rather than a problem with the pacing system itself.

Vancomycin allergy would produce allergic signs and hemodynamic changes not specifically tied to the ventilation state. An acute rise in intracranial pressure would more classically present with hypertension and a different breathing pattern (Cushing response), not bradycardia driven by hyperventilation. Thiopental can cause bradycardia as a drug effect, but the scenario links the event to hyperventilation and consequent hypocapnia, making hypocarbia the most likely explanation.