The QRS complexes are numbered and the R-R intervals (in msec) are shown at the top of the rhythm strip.
Sinus bradycardia is present in complexes 1 - 5, with a slight irregular rhythm (R-R intervals are 1040 - 1200 msec) and a rate of 50 - 58 beats per minute.
The heart rate decreases after the 5th complex. The R-R intervals between the 5th and 8th complexes lengthen to between 2240 and 2600 msec, forming three pauses (labelled A - C). The 6th and 7th complexes are sinus beats and the 8th complex is a junctional beat. Pause A has a R-R interval of 2240 msec, which is exactly double the R-R interval between complexes 1 and 2. Pause C has a R-R interval of 2400 msec, which is exactly double the R-R interval between complexes 2 and 3.
This suggests that during pauses A and C the sinoatrial node has discharged but the impulse has not been conducted into the atrium. This is called sino-atrial (exit) block. The R-R interval between complexes 6 and 7 is 2600 msec, and is probably due to failure of discharge of the sinoatrial node. This is called sinoatrial arrest.
The complexes between 8 and 15 have the same QRS morphology that is seen in sinus rhythm but there are no preceding P wave. The R-R interval is slightly irregular, with an interval between 1280 msec and 1640 msec (which corresponds to a ventricular rate of 37 - 47 beats per minute). This is a junctional escape rhythm.
Sinus rhythm returns in complexes 16 to 21 with one pause of 2160 msec (labelled D) and a R-R interval of 1040 msec to 1320 msec (corresponding to a ventricular rate of 45 to 58 beats per minute).
- Sinus bradycardia
- Sinoatrial block and sinoatrial arrest
- Junctional escape rhythm