When conduction delay in the AV node exists (PR greater than 0.20 seconds), but every P-wave conducts to the ventricles, first-degree AV block is said to be present. If none of the P-waves conduct and the ventricular rhythm is regular, arising from an escape focus firing slower than the atrial rate, then third-degree or complete AV block is said to be present (as seen above). Second-degree AV block implies that the sinus rhythm is regular but that some P-waves fail to conduct to the ventricles. The key in managing patients with second-degree AV block is identifying the anatomical level of the block. AV block may occur high up in the compact AV node or it may occur in the infranodal tissues, including the His bundle and the three hemifascicles. When second-degree AV block is intranodal (i.e. high), progression to complete AV block is usually associated with a junctional escape rhythm at rates between forty and fifty beats per minute. This is adequate to prevent malignant symptoms. Therefore, intranodal block is only treated with a pacemaker when symptoms are clearly due to the presence of bradycardia. On the other hand, AV block occurring at the level of the His bundle or below is much more ominous since the development of complete block at that level may be associated with an inadequate escape rhythm. Commonly, idioventricular foci at rates of 20-30/minute can serve as the escape rhythm, but asystole may result from complete infranodal AV block. This is the reason why infranodal AV block should be treated with a permanent pacemaker even in the absence of symptoms. Determining the level of block is most accurately done with a His bundle electrogram. However, clues may be present on the surface ECG to allow us to infer the level of the block. Calcium-dependent tissues such as the AV node exhibit “decremental” conduction properties. This means that atrial impulses that are too fast for the AV node to conduct in a 1:1 manner generally exhibit progressive conduction delay (prolongation in the PR interval) before the dropped beat. This can be identified by a Type I or Mobitz I (Wenckebach) behavior on the surface ECG. These patients usually do not exhibit a bundle branch block since the disease is predominately above the bundle branches. Therefore, with Type I second-degree AV block and a narrow QRS complex, one can infer that the level of the block is in the AV node itself. On the other hand, the infranodal tissues are sodium-dependent and exhibit an “all or none” conduction pattern. Infranodal block is usually associated with the presence of a bundle branch block or bifascicular / trifascicular block. In general, the PR interval does not increase prior to the blocked beat (Type II or Mobitz II behavior). In an asymptomatic patient, if the level of the block is unclear from the surface ECG (e.g. Wenckebach block in combination with a bundle branch block or Type II second-degree AV block with a normal PR interval and no bundle branch block), then a His bundle study is indicated to determine whether permanent pacing is appropriate. Symptomatic patients with second-degree AV block should receive a permanent pacemaker (providing that the symptoms were clearly a consequence of bradycardia). Patients with third-degree AV block should receive a permanent pacemaker regardless of symptoms, particularly if ventricular rates of less than forty beats per minute or pauses greater than three seconds are detected.