Interatrial Blocks May be of First, Second, and Third Degree

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Interatrial Blocks May be of First, Second, and Third Degree

The interatrial blocks, by analogy with other types of block, (sinoatrial, atrioventricular, and/or bundle-branch block at the ventricular level), may be of first (partial), second (transient interatrial block is part of atrial aberrancy), or third degree (advanced).

Electrocardiographic Pattern of First-degree (Partial) Interatrial Block (see Figures 1-B, 4)

The P wave has a normal electrical axis. The electrical impulse is conducted from the right to the LA through the normal propagation route but with a delay. The endocardial recording confirms that the coronary sinus activation time is delayed.

The ECG shows that (a) a P wave of 120 milliseconds or more, usually bimodal, is especially visible in leads I, II, or III, and (b) the P wave morphology in V1 in the absence of left atrial enlargement (LAE) presents a P wave negative mode that is less evident than in cases of LAE, because in the absence of LAE, the P loop is directed in a less backward direction (see Figures 2 and 4).

However, in the instance of P wave duration of 120 milliseconds or more the presence of LAE is common. In fact, it has been considered that the wide and bimodal P wave of LAE is better explained in terms of underlying interatrial block rather than the longer distance that the impulse has to travel⁷.

LAE is often found in the elderly⁸, in patients with heart failure⁹ and those with other advanced heart diseases. We also demonstrated that it is a risk marker of atrial fibrillation/atrial flutter (AF/Afl)¹⁰.

Electrocardiographic Pattern of Second-degree Interatrial Block (see Figure 5)

Similar to the conduction block at the level of AV junction, SA junction, or the ventricles, the interatrial block may occur transiently on a beatby- beat basis or may be recorded momentarily.

In a typical case, the morphology of the P wave may change in the same recording from interatrial block pattern (first or third degree) to normal pattern, usually transiently in relation to the preceding premature beats, before then again presenting the pattern of interatrial block. These changes may be considered as atrial aberrancy, similar to ventricular aberrancy^2,4.

Atrial aberrancy may also present a transient bizarre P wave without the morphology of interatrial block. In these cases, the location of the block is usually the right atrium.

Electrocardiographic Pattern of Third-degree (advanced) Interatrial Block(Figures 6–8)

The electrical impulse is blocked especially in the upper and middle part of the interatrial septum, in the Bachmann’s bundle zone, and/or in the upper part of LA so that retrograde left atrial activation occurs mainly via muscular connections in the vicinity of coronary sinus^2,3,11,12. In rare occasions, the right atrium and LA can demonstrate dissociated electrical activity.

The ECG shows that (a) P wave duration of 120 milliseconds or more and (b) the morphology of P wave are usually bimodal in lead I and biphasic [±] in leads II, III, and VF because of caudocranial activation of the LA, and also often in V1 to V3–V5(see Figure 6).

The electrophysiological mechanism underlying this ECG pattern has been explained using deductive ECG-VCG data correlated with recordings obtained from the high and low right atrium, coronary sinus, right pulmonary artery, and oesophagus and also with electroanatomic mapping^11,12(see Figures 7 and 8).

It has been also demonstrated that the block may be located in the LA not affecting conduction through the proximal Bachmann’s bundle. However, this does not modify the terminal inferosuperior activation of the LA that explains the typical morphology ± in II, III, and VF. In fact, in dogs, the same ± morphology appears after cutting the Bachmann’s bundle at the right and left sides of the septum⁵(see Figure 3).

Bayés de Luna et al¹³ defined the ECG-VCG criteria and demonstrated that this type of block is a very specific (90 %) but insensitive marker of LAE. The duration of P wave is 120 milliseconds or more (120–170 milliseconds in this sample of 83 cases). However, it is the morphology of the P wave (± in inferior leads) that pinpoints that there is a retrograde activation of the LA. The positive mode of P waves in leads II, III, and VF is at times not well-observed, probably because of fibrosis, and the diagnosis of junctional rhythm due to an apparently negative P wave in II, III, and VF may be made. It was demonstrated that this type of block is very frequently accompanied by paroxysmal atrial arrhythmia, especially atypical atrial flutter in patients with valvular heart diseases and cardiomyopathies. As a result, this association is considered an ECG clinical syndrome^10,14.

During long-term follow-up, especially in patients with mitral valve disease or cardiomyopathy, the successive appearance of first-degree and later third-degree interatrial block may be seen(see Figure 9).