A, ECG 1A. There is a regular rhythm at a rate of 42 bpm. There is a P wave before each QRS complex (+) with a stable PR interval (0.46 second). This is a first-degree atrioventricular (AV) block or prolonged AV conduction. The P waves are broad and prominently notched in leads II, III, and aVF, characteristic of left atrial hypertrophy or abnormality. A second P wave is seen after each QRS complex (▾). It is obvious in lead V1 but is also noted as a notching of the upstroke of the T wave in most of the leads. There is a stable PP interval at a rate of 84 bpm. Therefore, this is a normal sinus rhythm, and there is a first- and a second-degree AV block (defined as an occasional nonconducted P wave) with a pattern of 2:1 AV conduction (or block). A 2:1 AV block may be either Mobitz type I or II. The type can be established by seeing a change in the pattern of conduction. The QRS complex duration is increased (0.16 second) and has a morphology of a typical right bundle-branch block with an RSR′ in lead V1 (→) and a terminal broad S wave in leads I, V5, and V6 (←). The axis is extremely leftward between −30° and −90° (positive QRS complex in lead I and negative complex in leads II and aVF). Causes of an extreme leftward axis include an inferior wall myocardial infarction in which there is a deep Q wave or a left anterior fascicular block in which there is an rS complex in leads II and aVF. The left axis is to the result of a left anterior fascicular block. The presence of a right bundle-branch block and a left anterior fascicular block is called bifascicular disease. Because there is also a first- and second-degree AV block, this pattern is often referred to as trifascicular disease. However, this cannot be definitively established because the first- and second-degree AV block may be a result of a conduction abnormality of the AV node or His-Purkinje system. B, ECG 1B. The rhythm is irregular, but there is a pattern with long intervals (└┘), all of which are the same, and short intervals (┌┐), which are all the same. Similar to the QRS complexes in ECG 1A, the QRS complexes are wide (0.16 seconds), and there is a right bundle-branch block morphology. There are P waves seen before each QRS complex (+) with a stable PR interval (0.44 second); as in ECG 1A, this is a first-degree AV block or prolonged AV conduction. The P waves are positive in leads I, II, aVF, and V4 through V6; these are sinus P waves. The P waves are broad and notched in leads II, III, and aVF; this is left atrial hypertrophy or abnormality. There are additional P waves after the QRS complex that are nonconducted (▾); hence, there is a stable PP interval (↔) and a sinus rate of 88 bpm. An occasional nonconducted on-time P wave is characteristic of a second-degree AV block. The QRS complexes 3, 6, and 9 (*) are early. All the QRS complexes have the same right bundle-branch block morphology. These early QRS complexes are preceded by an on-time sinus P wave, and they have the same PR interval (^) as the other QRS complexes. Hence, these are conducted complexes. Because there are 2 conducted sinus complexes with the same PR interval, the second-degree AV block can be established as Mobitz type II. Therefore, the 2:1 AV block in ECG 1A is the result of a Mobitz type II. Although all of the QRS complexes have a right bundle-branch block, the early QRS complexes have a different axis. The QRS complexes associated with 2:1 AV block (complexes 1, 2, 4, 5, 7, and 8; ▴) have an extreme left axis between −30° and −90° (positive QRS complex in lead I and negative complex in leads II and aVF). This is a left anterior fascicular block. The early QRS complexes (ie, complexes 3, 6, and 9; *) have a different axis. These QRS complexes are negative in lead I and positive in lead II. Although not seen in lead aVF, the axis is now rightward. Causes of a rightward axis include right ventricular hypertrophy (which cannot be established in the presence of a right bundle-branch block), a lateral wall myocardial infarction (in which there is a deep Q wave in leads I and aVL), Wolff-Parkinson-White pattern (short PR interval and prolonged QRS complex duration caused by a delta wave), right-left arm lead switch (in which there are negative P and T waves in leads I and aVL and positive P, QRS, and T waves in lead aVR), dextrocardia (which resembles right-left arm lead switch and is associated with reverse R-wave progression in leads V1–V6), and a left posterior fascicular block (diagnosed if there is no other cause of a right axis). In this case, the rightward axis is attributable to a left posterior fascicular block. There is an underlying right bundle-branch block with alternating left anterior and left posterior fascicular blocks; this is indicative of trifascicular disease. Therefore, this finding and the presence of bifascicular disease associated with a Mobitz type II confirm the presence of trifascicular disease.