![]() ![]() Circles on the top line represent impulses originating from the SA node. Solid circles represent the start of any impulse. The top horizontal line represents SA node, the 2nd horizontal line represents the beginning of AV conduction, and the 3rd horizontal line represents the end of AV conduction and the start of ventricular conduction. It is constructed of 3 tiers that represent the atria, AV junction, and ventricle. These P-waves look like the other sinus P-waves, which is expected if they are all initiated in the SA node.Ī ladder diagram uses lines with arrows and bars to illustrate the electrical activity within the cardiac conduction system that may not be evident on the surface EKG (ie, concealed conduction). These P-waves are sometimes entirely seen (after beat 4) and sometimes only partially seen (after beat 7) due to the close timing of the SA node in comparison to the junctional focus. The junctional focus occurs just before the SA node impulse, so these P-waves are non-conducting because the rest of the conduction system has already depolarized from the junctional focus. During this time a junctional focus competes with the SA node. This is referred to as an echo beat or reciprocal beat, and typically follows a QRS preceded by a prolonged PR interval.Īfter any beat with a retrograde P-wave, the atrial conduction system needs time to reset before it resumes pacing. Alternately, they conduct retrograde over a dual AV nodal pathway within the AV node. ![]() Most likely, they conduct via the “retrograde only” accessory pathway since the AV node will be refractory to retrograde conduction that quickly following the QRS. They reset the atrial conduction system, interrupting Mobitz I, so there isn’t the expected dropped QRS after a P-wave. These P-waves are inverted in the inferior leads, which is expected for retrograde conduction. Sinus beat followed by a retrograde P-wave( ↓) which resets the atrial conduction system.Atrial conduction system resets and resumes pacing = sinus beat.Sinus beat with longer PR (= Mobitz I) followed by a retrograde P-wave ( ↓) which resets the atrial conduction system (interrupting Mobitz I).Junctional beat (narrow QRS with no preceding P-wave) followed immediately by a non-conducting sinus P-wave (↓).Using the Lead II rhythm strip as a guide (see below), here’s the pattern: This ECG demonstrates a 2nd degree type I AV block (Mobitz I) with intermittent junctional beats, non-conducting P-waves, and retrograde P-waves. ![]()
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