First-degree atrioventricular block and very long PR

ECG: Sinus rhythm (positive P waves in leads I, II, V5, V6, negative in aVR); very prolonged (520 ms) and fixed PR interval: first-degree AV block; narrow QRS with no abnormality;

Comments: A significant prolongation of the PR interval due to a relatively fast heart rate can lead to a superposition of the P wave on the preceding T wave which renders interpretation difficult, with a risk of misinterpretation with a junctional rhythm. Variations in heart rate generally allow rectifying the diagnosis by helping in the visualization of the P waves. Generally, prolongation does not exceed 400 ms even though PR interval values exceeding 800 ms have been observed. In extreme cases, the duration of the PR can exceed that of the PP and the P wave responsible for the QRS complex is not the one preceding it on the tracing. The location of the slowing is most often nodal (especially when the QRS is narrow) and is therefore of relatively good prognosis. Electrophysiological study revealed a prolongation of the AH interval indicative of a conduction delay in the atrioventricular node.

The presence of a very long PR may be associated with the occurrence of symptoms, if the response to exertion is not physiological. Indeed, as in this patient, an exercise test can document an absence of reduction of the PR interval in parallel with the increase in heart rate. If the RR intervals become shorter and the PR interval remains fixed and prolonged, the atrial systole following atrial depolarization occurs prematurely in diastole or may even occur at the end of the ventricular systole leading to a loss of active atrial contribution to cardiac ejection, a shortening of left ventricular filling time, diastolic mitral regurgitation and, in the more severe cases, atrial contraction during clodes mitral valves. This can be accompanied by a more or less marked symptomatology (exertion dyspnea, impression of retrograde flow in the jugular veins, palpitations, malaise) similar to that observed in patients paced in VVI mode with retrograde conduction (pacemaker syndrome). A number of uncontrolled studies suggest that pacemaker implantation reduces symptoms and improves functional status.

In the latest European guidelines, there is a class IIA indication for implantation of a pacemaker for this type of presentation: “implantation of a pacemaker should be considered in patients with first-degree AV block (PR> 300 ms) and symptoms consistent with pacemaker syndrome.”

While this patient did not present any symptoms such as malaise, fatigue or syncope, he was still implanted with a pacemaker allowing to reduce the symptoms of exertion intolerance by resynchronizing atrial with ventricular activity both at rest and during exercise.

Take-home message: First-degree AV block most often reflects a delay in atrioventricular node conduction; the longer the PR interval, the greater the likelihood of an intra-nodal slowing; when the PR interval is extremely prolonged, the physiological activation/contraction sequence between atria and ventricles is no longer respected, leading to a contraction of the atria while the mitral valves are closed. A pacemaker may be indicated in this setting if the patient is symptomatic during exercise.