Atrioventricular block (AVB) types explained

When a dog develops an AVB this sometimes means that the cardiac cells (cardiomyocytes) in the atrioventricular node (AV-Node) conduct the depolarization front too slowly in each and every cardiac cycle ("first-degree" AV-Block), such as in the following case:

HR = 100 bpm, P = 60 ms, 0.5 mV, PR = 135 ms, QRS = 65 ms, R = 1.6 mV, QT= 250 ms  | view the full report

Some other times they do not conduct it at all (“Complete”, or “third-degree” AV Block) in any of the cardiac cycles (where no P is followed by a resultant QRS and no QRS is preceded by a causatively related P). Alternatively, at times they do and then do not conduct the depolarization across, in an intermittent fashion (“second-degree AV-Block”).

HR = 24 bpm (P wave rate 126 bpm), QRS = 0.125 sec, P wave 0.06 sec, 0.2 mV, 180<MEA<210

The QRS complexes in this rhythm strip result from an Escape Rhythm, which is needed because of a complete (3rd Degree) AV-Block. The escape rhythm in this specific example happens to originate from a ventricle (based on the wide QRS complex appearance at a relatively slow rate). This ventricle happens to be the left one (based on the QRS negative dominance in Leads I & aVF). Had these escape beats originated from the AV-Node, instead (a "higher" escape focus both in terms of anatomy AND in terms of functional hierarchy) they would have been normal looking in terms of morphology, and their rate would have been a little higher than the current one).

Note that while the QRS-to-QRS interval is nearly fixed (and very long), and the P-to-P interval is nearly fixed (and rather short, likely due to the elevated sympathetic tone driving the SA-Node as a response to the dropped cardiac output resulting from the severe background bradycardia), the PR-interval is actually highly variable. These integrated findings reflect the absence of a causative relationship between the P-waves and the QRS complexes that follow them. This means a complete dissociation ("block") between their respective cycles is present.

The intermittent, or second-degree AV block, can sometimes manifest itself in a progressive manner, where the PR-interval (also known as the PQ-interval, and defined as the time needed from the onset of the P-wave to the onset of the QRS-complex) gradually lengthens over several consecutive cardiac cycles until it is infinite for just a single cycle. This results in a non-conducted P-wave, followed by no QRS complex. Just after such an event, a new series of cardiac cycles begin, where P-waves are conducted with a progressively longer PR-interval, culminating in another non-conducted P-wave. This cyclic phenomenon can repeat itself constantly or rather come and go, depending on changes in vagal (parasympathetic) tone. This specific type of second-degree AV-block is called "Mobitz Type-1" or "Wenckebach Phenomenon" (two synonyms), and can be found, for example, here:

HR = 115-143 bpm, PR = 95-160 ms, QRS = 45 ms, R = 1.2 mV, QT = 205 ms | view the full report

In some cases, such as this one:

HR = 100 bpm

The PR-interval can be fixed rather than progressively prolonged, in all cardiac cycles where a longer-than-normal PR-interval is identified. This is called a Mobitz Type-II, second-degree AV-Block.

The nomenclature related to second-degree AV-block includes, in addition to “degree” or “type”, the term ”grade”: when up to three non-conducted P-waves are inscribed between every two consecutive QRS-complexes, this is a “low grade”, second-degree AV-block. When there are four or more such non-conducted P-waves between consecutive QRS-complexes, the grade can be considered “high”.

A concomitant first AND second-degree AV-Block can also occur, where some P-waves are not conducted (second-degree block) while those that are conducted, have a longer than normal (e.g. 160-180 ms, instead of ≤120 ms, in dogs) PR-interval (first-degree block). An example of such coexisting phenomena is seen here:

HR = atrial rate is 120-130 bpm, and the ventricular rate is 80 bpm, P = 40 ms, 0.2 mv, PR = 160 ms, when conducted, QRS = 65 ms, R = 0.65 mv, QT = 210 ms | view full report