With enormous growth in our understanding of arrhythmias and exciting new technologies, it is easy to forget that electrocardiography remains the fundamental cornerstone of clinical electrophysiology and the key port of patient entry to our care. It is often the first screening test that guides our management, and intelligent use of the ECG allows focus on the next step while avoiding further intervention when it is not required.
The ECG and intracardiac electrograms are indivisible. Both recording the same phenomena but from complementary vantage points. The surface ECG provides a “far-field” view of the landscape via wide bipolar recording (limb leads) and unipolar leads (precordial leads). The electrograms provide a “near-field” close-up view in specific strategic intracardiac locations. The ECG allows the recording of spontaneous naturally occurring events, while the presence of the intracardiac recordings allows manipulation of the rhythm through pacing at specific sites in addition to passive recording. One can simply think of the ECG as the view from the air, while the intra-cardiac recordings provide the view on the ground.
The following examples will try to demonstrate this complementary relationship. Of course, analysis requires applying many physiologic principles that simply need to be learned. It is surprising what can be learned from the ECG using these principles before an intracardiac catheter is placed! The ECG is of course the more difficult art form; it requires recognition of low-voltage atrial activity and interpretation of QRS patterns in the universe of patients with highly variable body habitus and degrees of cardiac disease. One can’t understand what is going on without this vision.
Irregular SVT in a 66-yr-old woman with palpitations.
This is an irregular tachycardia not infrequently mistaken for atrial fibrillation in the community, although marching through the tracing with calipers makes it clear that it is not “irregularly” irregular. We are fortunate to see the onset and offset of the tachycardia with a “clean” noise-free tracing.
At the onset, we see a peaked P, which facilitates diagnosis. This peaked P is easily recognized as we continue through the tracing (blue arrows). The first tachycardia beat is followed by a second beat with NO preceding P Wave (very smooth and uninterrupted ST segment and T wave). This second beat can only result from one of the following:
Junctional extrasystole with aberration
A “2 for 1” phenomenon where the first of the pair conducts over a fast AV node pathway, and the second conducts over a slow AV node pathway.
Continuing through the tracing, this pattern repeats itself, sometimes with a wide QRS (red asterisk) or a normal ...