Answer to the last #DiagnosticDilemma
What does his ECG reveal?

Final Dx: Sinus tachycardia, right ventricular pacemaker, P-wave synchronous ventricular pacing (A-sensed V-paced), pseudo second-degree AV block due to failure of atrial sensing.

Learning Points:-
• There is a regular rhythm at a rate of 130 beats/min. The QRS complex has an increased duration (0.16 sec). Each QRS complex is preceded by a pacemaker stimulus (^). Because the QRS complex has a LBBB morphology with a broad R wave in lead I (←) and a deep QS complex in lead V1 (→), this is a right ventricular pacemaker.

• The QT/QTc intervals are prolonged (320/470 msec) but are normal when the prolonged QRS complex duration is considered (260/380 msec). A P wave occurs before each QRS complex (+); therefore, this is A sensing V pacing or P-wave synchronous ventricular pacing. The P waves are positive in leads I, II, aVF, and V4-V6, This is a sinus tachycardia.

• The pauses in the rhythm are the result of a nonconducted P wave (↓) that does not result in a pacemaker stimulus or paced QRS complex. This happens because the P wave is not sensed; it is falling within the post-ventricular atrial refractory period (PVARP). This is the "blanking" period—the time after the ventricular stimulus is delivered during which a P wave is not sensed.

• The PVARP determines the upper pacing rate of the pacemaker. The underlying sinus rate is close to the upper rate limit and most of the P waves are sensed, resulting in a ventricular stimulus. However, if there is a slight increase in the sinus rate, the pacemaker fails to sense the P wave, resulting in a pause (termed a pseudo second-degree AV block).

• The complex following each pause is AV sequentially paced, and two pacemaker stimuli are seen. The first is atrial (↑) and the second is ventricular (^). The lower rate limit of the pacemaker is 78 beats/min, as determined by the interval from the preceding ventricular stimulus to the atrial stimulus.

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