This 65-year-old woman is brought to the Emergency Department following an episode of syncope lasting 30 seconds. In the previous 2 weeks she reports feeling weak and lightheaded:

• Sinus rhythm with high grade AV block
• Sinus rate 76/min, ventricular rate 37/min
• Junctional escape complexes
• Right bundle branch block
• Non-specific ST abnormality

Notice that the PP intervals containing a QRS complex are shorter (“ventriculophasic sinus arrhythmia”). These variations in the sinus cycle may be reflex-mediated, as the result of changes in the intracardiac volume and pressure and stroke volume. Variations in the perfusion of the sinus node may also play a role. Ventriculophasic sinus arrhythmia is frequently seen in complete heart block.