Atrial fibrillation (AF)

This is a supraventricular tachycardia originating from multiple foci in the atrial myocardium of either one or both atria. Unlike the sinoatrial node (SA-Node) or the atrioventricular node (AV-Node), none of these foci is a physiological pacemaker. Rather, they are all pathological micro-re-entry “rotors” that generate very rapid (~400-600 times per minute) and unsynchronized electrical activity in the atrial myocardium, resulting in a chaotic electrophysiological activity reminiscent of seizure activity in other excitable tissues such as the brain. This does not lead to an organized or orchestrated mechanical atrial contraction, but rather to coarse or fine quivering of the atrial walls.

Therefore, there is no longer any atrial contribution to the diastolic ventricular filling process (a contribution that, depending on the heart rate, is normally responsible for at least 10-15% and at the most 30% of the end-diastolic ventricular volume). On the other hand, despite the lack of any organized or synchronized atrial contraction, the highly frequent electrical activity passes through the “bottleneck” known as the atrioventricular (AV) node towards the ventricular myocardium, constantly “bombarding” *it* with depolarization fronts.

Although the AV-node is designed to serve as a highly efficient filter of such activity, it is unable to filter it all but rather filters only about a third of it, and is nearly overwhelmed by its rapidly repetitive activation. Consequently, the ventricular response rate is very fast (~160-240/min) AND highly irregular. The cardiac output, therefore, is now compromised by three different mechanisms, including the absence of atrial contribution to ventricular diastolic filling, the rapid ventricular response rate that shortens the ventricular diastolic duration, and the irregularly irregular ventricular rhythm, which all too often shortens diastolic filling duration even further. When the process is chronic, both anatomic and electrophysiological remodeling of atrial (and possibly ventricular) myocardium develops, along with processes that lead to (often right-sided) congestive heart failure, as well as other secondary complications such as sarcopenia, and ultimately even death.

Electrocardiographically, there are three hallmarks of atrial fibrillation: 1) A typically rapid ventricular depolarization (depicted as a highly frequent QRS complex). Please note that in some dogs this activity is not as rapid as expected, which does not rule out the diagnosis, as in the following case:

HR = 150 bpm, QRS = 54 ms, R = 0.17 mV, QT = 203 ms  | view the full report

2) The ventricular rhythm is irregularly irregular, which can sometimes be hard to ascertain if the heart rate is very high, necessitating visualization of a long segment of a rhythm strip before irregularity is confirmed beyond doubt.

3) There are no discernible P-waves. In many but not all cases, coarse or fine, seemingly “artefactual” undulations on the ECG baseline are inscribed called f-waves, one can see such f-waves here or in the next viewer below

HR = 186 bpm, QRS = 80 ms, R = 2.125 mV, QT = 205 ms, 90>MEA>0 (normal)  | view the full report

Some of these occasionally appear to “pretend” to be P-waves but these are typically not consistent in morphology or timing, and, unlike a true P-wave, are not a reflection of a true, organized or synchronized bi-atrial depolarization. Such f-waves are not always present and therefore are not a condition or a reliable diagnostic criterion that has to be present when making the diagnosis of atrial fibrillation. Here's an example of such a case:

HR = 240 bpm, QRS = 60 ms, R = 0.87 mV, QT = 180 ms | view the full report

The presence of atrial fibrillation does not mean that other, co-existing arrhythmia or intra-ventricular conduction anomalies cannot be present as well. When a bundle branch block coexists with atrial fibrillation, both a wider-than-normal QRS complex AND an absence of P-waves coexist. This combination results in a so-called “wide-complex” tachycardia, which can highly resemble ventricular tachycardia and the distinction between these two entities can sometimes be challenging.

HR = 186 bpm, QRS = 80 ms, R = 2.125 mV, QT = 205 ms, 90>MEA>0 (normal)  | view the full report