Iranian Heart Association Iranian Heart Journal 7 2 2006 03 01 WHAT IS THE MECHANISM OF GROUP BEATING? 42 43 84024 EN M. SHOJAEIFARD ARASH. ARIA DEPARTMENT OF PACEMAKER AND ELECTROPHYSIOLOGY, RAJAIE CARDIOVASCULAR MEDICAL AND RESEARCH CENTER, MELLAT PARK, VALI-ASR AVENUE, TEHRAN 1996911151, IRAN M. HAGHJOU M. SADR AMELI Journal Article 2019 03 07 An eighty-year-old male was referred to our center because of dizziness and palpitation for the previous two weeks. He had no history of syncope and the drug history was unremarkable. Figure 1 shows the resting ECG, interpreted as complete heart block by the referring physician. A transthoracic echocardiography was done and showed a left ventricular ejection fraction of 35% and a severely calcified mitral valve with an estimated mitral valve orifice area of 1.4 cm².<br />Group beating was observed on the ECG, and a closer look at the ECG suggested the diagnosis of sino-atrial node Wenckebach (SANW) exit block. Premature atrial beat trigeminy could have also explained the group beating of the P-waves. However, the similarity of the P-wave morphologies and the confirmation of the diagnosis of sick sinus syndrome by electrophysiology study made this diagnosis unlikely and suggested the SANW as the cause of P-wave group beating.<br />Apparently there seems to be a conduction defect at the atrio-ventricular nodal (AVN) level. The PR interval of the first conducted P-wave is longer (first black arrow) than the subsequent PR interval, which is then followed by a non-conducted P-wave. Nevertheless, a close look at the ECG reveals the real underlying mechanism of apparent AV block. Due to SANW, the PP intervals gradually shorten and the third P-wave is blocked as it encounters the physiologic refractory period of the AVN. After the third P-wave, there is a pause due to SA exit block (4^th P-wave, blocked at the SAN level) followed by a conducted P-wave with a long PR interval. This phenomenon could be best explained by phase-four (bradycardia-dependent) conduction blocks.<br />Electrocardiographic changes in this patient could also be explained by the effect of cyclelength duration on infra-Hissian conduction. Thus, PR interval prolongation can be due to His-Purkinje refractoriness in the longer cycle length. The shorter PR interval in the next beat may be due to a decrease in the HV interval in the shorter cycle length. In this scenario, the last P-wave would be blocked in the AV node. https://journal.iha.org.ir/article_84024_27290b81e2340fc4fe3690469860d728.pdf