Wagner_Marriot's Practical Electrocardiography, 12e

ATRIOVENTRICULAR NODAL BLOCK

V 1

A AV

75

75

75

75

76

75

21

31

35

23

V

140

85

79

139

F I G U R E 2 2 . 1 2 . A lead V1 rhythm strip accompanied by a ladder diagram with atrial ( A ), AV nodal ( AV ), and ventricular ( V ) levels. The various intervals are indicated in hundredths of a second.

The classic form of AV nodal block is reflected by the Wenckebach sequence , in which the PR interval may begin within normal limits but is usually somewhat prolonged. With each successive beat, the PR interval gradually lengthens until there is failure to conduct an impulse to the ventricles. Examples are presented in Figures 22.3, 22.4A, and 22.5A. Following the nonconducted P wave, the PR interval reverts to normal (or near nor- mal) and the sequence is repeated. At times, the PR interval may increase to surprising lengths. Progressive lengthening of the PR interval occurs in the Wenckebach sequence because each successive atrial impulse arrives progressively earlier in the relative refractory pe- riod of the AV node and therefore takes progressively longer to penetrate the node and reach the ventricles. This is a physiologic mechanism during atrial flutter/fibrillation, but its occurrence at normal heart rates implies impairment of AV conduction. 16 The progres- sive lengthening of the PR interval usually follows a predictable pattern: The maximal increase in the PR interval occurs between the first and second cardiac cycles, and the increase between subsequent cycles then becomes progressively smaller. Three charac- teristic features of the cardiac cycle, which can be figuratively referred to as the footprints of the Wenckebach sequence , occur with AV nodal block: (a) the beats tend to cluster in small groups, particularly in pairs, because 3:2 P-to-QRS ratios are more common than 4:3 ratios, which are more common than 5:4 ratios, and so forth; (b) in each group of ventricular beats, the first cycle is longer than the second cycle, and there is a tendency for progressive shortening to occur in successive cycles; and (c) the longest cycle (the one containing the dropped ventricular beat) is less than twice the length of the shortest cycle (Fig. 22.12). This phenomenon influences the rhythm of the ventricles. After the pause produced by complete failure of AV conduction, the RR intervals in the ECG tend to decrease progres- sively, and the long cycle (the one containing the nonconducted beat) is of shorter duration than two of the shorter cycles because it contains the shortest PR interval. This pattern of progressively decreasing RR intervals preceding a pause in AV conduction that lasts for less than twice the duration of the shortest RR interval is of only academic interest in the presence of AV nodal block, but a similar pattern of PP intervals may provide the only clue to the presence of sinus nodal exit block (see Chapter 21).

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SECTION III: Abnormal Rhythms