TY - JOUR ID - 84024 TI - WHAT IS THE MECHANISM OF GROUP BEATING? JO - Iranian Heart Journal JA - IHA LA - en SN - AU - SHOJAEIFARD, M. AU - ARIA, ARASH. AU - HAGHJOU, M. AU - SADR AMELI, M. AD - AD - DEPARTMENT OF PACEMAKER AND ELECTROPHYSIOLOGY, RAJAIE CARDIOVASCULAR MEDICAL AND RESEARCH CENTER, MELLAT PARK, VALI-ASR AVENUE, TEHRAN 1996911151, IRAN Y1 - 2006 PY - 2006 VL - 7 IS - 2 SP - 42 EP - 43 DO - N2 - 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.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.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 (4th 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.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. UR - http://journal.iha.org.ir/article_84024.html L1 - http://journal.iha.org.ir/article_84024_7eceeec0081d6d283bfb66e0447cbdb4.pdf ER -