Effect of hypercapnia on the arrhythmogenic dose of epinephrine in horses anesthetized with guaifenesin, thiamylal sodium, and halothane.

The research article investigates the impact of an elevated level of carbon dioxide in the blood (hypercapnia) on the susceptibility of horses, anesthetized with a certain combination of drugs, to heart rhythm complications triggered by epinephrine. Results suggest that a high carbon dioxide level in the blood heightens the risk of developing ventricular arrhythmias in horses under these anesthetic conditions.

Research Methodology

• The study involved 14 horses where the arrhythmogenic dose of epinephrine (ADE) – the minimum amount of epinephrine required to induce abnormal heart rhythms – was determined under different conditions of blood gas levels.
• Anesthesia was initiated using guaifenesin and thiamylal sodium, maintained with a constant range of halothane concentration.
• Cardiac parameters such as ECG, cardiac output, and blood pressure were continuously monitored and recorded.
• ADE was determined at a normal level of carbon dioxide (normocapnia), a high level (hypercapnia), and after returning to normocapnia.
• The process involved the infusion of epinephrine at progressively increasing rates, with the ADE being designated as the lowest rate at which four premature ventricular complexes (a type of arrhythmia) happened within a 15-second period.

Findings

• The ADE during hypercapnia was significantly lower than during normocapnia, meaning less epinephrine was required to trigger arrhythmias when carbon dioxide levels were high in the blood of the anesthetized horses.
• The ADE after returning to normocapnia was not significantly different from the ADEs recorded during both high and normal carbon dioxide levels.
• Systolic and diastolic arterial pressures and cardiac output decreased after returning to normocapnia.
• No significant changes were observed in the arterial partial pressure of O2 or the base excess during the experiment.
• The experiment was risky as two horses developed a serious variation of arrhythmia, called ventricular fibrillation, and died during normocapnic determinations of ADE.

Implications

• The study concluded that a state of hypercapnia is associated with a heightened risk of developing ventricular arrhythmias in anesthetized horses, implying caution during veterinary anesthesia procedures in situations where elevated carbon dioxide levels in blood could occur.