Electrocardiographic changes associated with the nasopharyngeal reflex in conscious rabbits: vago-sympathetic co-activation

Electrocardiographic responses were assessed in conscious rabbits when the nasopharyngeal reflex was elicited by inhalation of formaldehyde vapour. There was a profound fall in heart rate (224±5 to 64±4 beats per min (bpm)) associated with abnormal or absent P-waves. There were no changes in the QRS complex. The R–T interval (control value 118±4 ms) was initially shortened to 107±3 ms and then prolonged to 130±4 ms. Heart rate and P-wave changes were prevented by muscarinic cholinergic blockade with methylscopolamine. The R–T shortening was reduced by 79±4% by β-adrenergic blockade with propranolol. Methylscopolamine also unmasked small tachycardic responses (5–25 bpm) in 5/7 animals. This tachycardia was prevented by propranolol. Thus both parasympathetic vagal cardiac nerves and sympathetic cardiac nerves are activated during the nasopharyngeal reflex, with increased vagal effects in the sino-atrial node, and increased sympathetic effects in the ventricular myocardium.