It is often difficult to differentiate on a surface electrocardiogram 1) a depression in automaticity and 2) an abnormality in impulse conduction (sinoatrial block). Only the actual depolarization of the atria (P wave) is recorded while the sinus activity is not. The underlying reasoning for determining the mechanism involved is indirect and based on the respective position of the P waves.
The pattern of this tracing is compatible with the diagnosis of second-degree sinoatrial block, with 1 out of 2 impulses being blocked (2:1 sinoatrial block). Indeed, the drop in heart rate is abrupt with a PP interval approximately doubled that of the preceding interval.
Theoretically, it is possible to differentiate (although difficult or impossible in practice and subject to controversy) three degrees of sinoatrial block (by analogy with the atrioventricular block):
- the first-degree sinoatrial block which is undetectable on a surface electrocardiogram, the P waves appearing normal and at regular intervals; the transmission of the sinus impulse is only slowed between the sinus and the atria;
- the second-degree sinoatrial block which corresponds to an intermittent interruption of sinoatrial transmission; the electrocardiogram is characterized by isolated absences of atrial activities; one can distinguish the second-degree sinoatrial block type 1 (Wenckebach), where the PP interval shortens before the missing atrial beat, from the second-degree sinoatrial block type II (Mobitz 2), where the PP interval remains constant until the loss of one or more atrial activities; the resulting pause corresponds to the double or a multiple of the normal interval; this second pattern (Mobitz 2) is the most commonly observed sinoatrial block and the only type that is easily identifiable;
- the third-degree sinoatrial block which corresponds to the permanent interruption of impulse transmission between the sinus and the right atrium; the electrocardiogram is characterized by the absence of a P wave and the frequent presence of a junctional escape rhythm; the two previous tracings (junctional escape with retrograde conduction) may correspond to this mechanism.