Atrioventricular (AV) block is defined as a delay or interruption in the transmission of an impulse, either transient or permanent, from the atria to the ventricles due to an anatomic or functional impairment in the conduction system. The conduction can be delayed, intermittent, or absent. The commonly used terminology includes first degree AV block (slowed conduction without missed beats), second degree AV block (missed beats, often in a regular pattern, eg, 2:1, 3:2, or higher degrees of block), and third degree or complete AV block.
The PR interval includes atrial depolarization (the P wave) and subsequent conduction through the AV node, His bundle, bundle branches and fascicles, and terminal Purkinje fibers (figure 1). The normal PR interval is between 120 to 200 msec (0.12 to 0.20 sec) and tends to shorten with increases in heart rate due in part to rate-related shortening of action potentials. Children under age 14 tend to have a PR interval of about 140 msec. (See "ECG tutorial: Basic principles of ECG analysis", section on 'PR interval'.)
Atrioventricular impulse transmission is delayed in first degree AV block, resulting in a PR interval longer than 200 msec (>210 msec at slow heart rates). On occasion, the PR interval may be prolonged in the absence of apparent heart disease [1-5]. As an example, first degree AV block, with PR intervals as long as 280 msec has been reported in 1.6 percent of healthy aviators . (See "ECG tutorial: Atrioventricular block", section on 'First degree atrioventricular block'.)
The remainder of this topic will discuss the manifestations of AV block due to conduction delay at different sites in the conducting system, as this distinction is important clinically in terms of prognosis and the indications for implantation of a pacemaker. The etiologies of AV block, as well as second and third degree AV block, are discussed separately. (See "Etiology of atrioventricular block" and "Second degree atrioventricular block: Mobitz type I (Wenckebach block)" and "Second degree atrioventricular block: Mobitz type II" and "Third degree (complete) atrioventricular block".)
SITES OF FIRST DEGREE AV BLOCK WITH A NORMAL QRS DURATION
While first degree AV block with a normal QRS duration (waveform 1) can result from conduction delay in the atrium, the AV node, the bundle of His, or in the specialized infra-Hisian conduction system, the majority of cases with a normal QRS results from atrial or AV nodal delay. However, involvement of more than one of these sites has been reported in 20 to 80 percent of cases [6-8].