Enhanced Vagal Tone

Patients usually experience cardiac asystole during open-heart surgery or other types of operations performed with cardiac bypass. Iatrogenically induced vagal tone causes cardiac asystole, which disappears spontaneously as the patient recovers from surgery. Patients may require a temporary pacemaker to bridge this period.

“Laboratory” Sinus Node Dysfunction

Some asymptomatic patients have ECG findings of prolonged sinus node recovery time and sinoatrial conduction time that is not related to sinus node dysfunction. These patients should not be treated with a pacemaker. Instead, the underlying cause needs to be determined and treated. Left untreated, these patients often progress to sinus node dysfunction within the next few months to years.

Atrioventricular-Blocked Premature Atrial Contractions

Sometimes a PAC can be blocked in the AV node and produce a measurable ventricular pause that extends up to twice the usual heart cycle. This is especially likely to be the case if the P wave is hidden inside the previous T wave. This ECG finding can mimic sinus nodal dysfunction.

Hypersensitive Carotid Sinus Syndrome

Hypersensitive carotid sinus syndrome is caused by abnormal vagal function, baroreflex hypersensitivity, and hyperresponsiveness to acetylcholine. It is found in older patients with chronic heart disease. 

The cardioinhibitory type causes ventricular arrest of at least 3 seconds after carotid massage. The ventricular pause is attributed to sinoatrial block and its presence is determined by the absence of a QRS complex. Longer ventricular pauses cause patients to experience presyncope or syncope. Patients are treated with a pacemaker.

Another type of hypersensitive carotid sinus syndrome is called the vasodepressor type. Patients exhibit a drop in systolic blood pressure of 30–50 mm Hg after carotid massage without any rhythm disturbance noted on ECG. Patients can be treated with sodium-retaining drugs. Some cases may require radiation ablation or surgical denervation of the carotid sinus.

The cardioinhibitory and vasodepressive types of hypersensitive carotid sinus syndrome may coexist.