AÂ recent caseÂ over at Burned-Out Medic has reminded me how much Iâ€™ve benefited from the study of implantable medical devices.
As I stated in my interview with EP Lab Digest:
â€œAs for the nuts and bolts of cardiac rhythm analysis, it’s the realization that the heart has two rhythms happening at the same time, one atrial and the other ventricular (hopefully in a 1:1 relationship with a functioning AV node). This is probably why studying implantable medical devices has deepened my understanding of heart rhythms. After all, pacemakers function by offsetting defects in the heart’s electrical conduction system.â€
We are often faced with patients who have implantable medical devices, and the indications for these devices are continually expanding. (See also my series on Ineffective or Inappropriate ICD Shocks.)
There are a lot of myths out there about paced rhythms. The worst of all is this one.
â€œItâ€™s impossible to identify acute STEMI in the presence of a paced rhythm.â€
This myth is so pervasive that Iâ€™ve witnessed situations where paramedics didnâ€™t place the patient on a monitor because the patient had a pacemaker!
In the first place, you can use Sgarbossaâ€™s criteria to identify acute STEMI in the presence of a simple paced rhythm (where the pacing lead in the apex of the right ventricle which is typical). Iâ€™m adding that qualifier because modern bi-ventricular pacemakers are an entirely different animal.
Secondly, you should always capture a baseline 12-lead ECG, regardless of what kind of rhythm is present! How else can you look for changes on serially obtained ECGs? Changing ST-segments and T-waves suggest the dynamic supply vs. demand characteristics of ACS!
Even when I worked on the Critical Care Stepdown unit as a cardiac monitoring technician, it was unusual for anyone to bother selecting whatever lead (and gain) was required to identify the underlying atrial rhythm of a paced rhythm, which is the key to understanding a pacemakerâ€™s behavior. By behavior I mean the pacemakerâ€™s programming and timing cycles.
The best interpretation you would generally get is â€œsingle chambered paced rhythmâ€ or â€œdual chambered paced rhythmâ€.
In the field, we are often blinded to the type of pacemaker and how it is programmed (although thereâ€™s a good chance the patient carries an I.D. card â€” you can always ask to see it). However, if you are observant, you can make some pretty good assumptions about whether or not the patient has a pacemaker and whether or not itâ€™s functioning properly.
Most modern pacemakers are DDD pacemakers according to the NBG pacemaker code.
Even though the chart shows five columns, in clinical practice the first three columns are the ones most often mentioned. So a DDD pacemaker may actually be a DDDRO pacemaker. Weâ€™re just discussing the basics here.
So that means that a DDD pacemaker is capable of pacing and sensing both the atria and ventricles. For our purposes that means that a DDD pacemaker is capable of atrial tracking. In other words, if the patientâ€™s sinus node is firing within acceptable parameters, the atrial pacing lead will not pace. Rather, the pacemaker will track each P-wave and ensure it is followed by a QRS-complex. If not, the pacemaker will provide one.
Letâ€™s go back to the case on Burned-Out Medicâ€™s blog. The first ECG appears to show 3AVB with wide complexes. The wide complexes, it turns out, are paced. But there is no relationship between P-waves and paced QRS-complexes. So we know this pacemaker is not functioning effectively in DDD mode, because itâ€™s not â€œtrackingâ€ P-waves and providing QRS-complexes after the P-waves to take advantage of the â€œatrial kickâ€.
However, we can say that the ventricular pacing lead is functioning and has â€œcaptureâ€ which is a good thing for this patient, because there is no apparent intrinsic ventricular activity!
The Basics of Paced Rhythms at ECG Medical Training