Fig. 55.2 Long PR interval. It is not easy to see the P wave in those leads usually best suited to examine the P wave (leads II and V1). However, in lead V1 a prominent ‘hump’, easily confused with a component of the T wave (arrowed) is actually the P wave. This can be confirmed by examining lead V2, where a deflection on the downslope of the T wave is obviously seen to be the P wave. The PR interval is nine little squares, i.e. 360 ms, clearly pathologically prolonged. The ECG otherwise shows small QRS complexes generally, and poor anterior R wave progression. This patient had obesity, normal left ventricular function and isolated conducting tissue disease, which intermittently resulted in high-grade heart block, so requiring a pacemaker.

The PR interval reflects the time it takes for the depolarizing wave front to travel from the sinus node to the main body of the ventricle. It is made up of a number of constituent parts (Fig. 55.1): a long PR interval (also known as first degree heart block) can be due to disturbances in function in one (or more) of several sites. It is rather difficult non-invasively to diagnose where the problem is sited. Though knowledge of the site of the problem is often not of clinical significance, occasionally it is! For example, long PR interval due to disease of the atrioventricular (AV) node progresses only rarely to higher forms of AV block, whereas a long PR interval due to infra-Hissian disease not infrequently progresses to higher-grade heart block.

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