An ectopic beat is one that arises from a focus outside the sinoatrial node (SAN). This focus can be in the atria, the atrioventricular node or atrioventricular junctional areas (the His bundle), or distal to the bifurcation of the His Bundle (the Purkinje system or the ventricles).
An ectopic beat can be described according to its presumed origin (atrial, nodal, junctional or ventricular), its shape, or its effects onthe sequence of cardiac activity. The latter approach involves discussion of intervals and pauses
An interval is the space between two objects or parts or the lapse of time between two events. We are familiar with the PR interval and the Q-T interval, but there are other intervals that are useful when describing ectopic beats or arrhythmias
The P-P interval is the time interval between consecutive P waves.
The R-P interval is the time interval between a QRS complex (either from the major deflection or the end of the QRS complex) and the start of the next P wave. This term is useful when discussing tachycardias where P waves are seen between the QRS complexes.
If the RP interval is shorter than the PR interval (RP < PR) the P wave is close to the end of the QRS complex, suggesting it is a retrograde P wave. When the RP interval is the same as the PR interval (RP = PR) the P wave is located midway between two consecutive QRS complexes. A RP interval that is longer than the PR interval (RP > PR) is seen in sinus rhythm, but also occurs in some types of tachycardia.
The R-R interval is the time interval between consecutive QRS complexes. The P-P interval can be used to measure the regularity of the atrial rhythm, and the R-R interval the regularity of the ventricular rhythm. If the PR interval is constant the P-P interval and the R-R interval are the same, and the R-R intervalis used to assess the regularity of the rhythm.
The RR interval in sinus rhythm is usually regular, with the intervals being exactly the same or only showing slight variation (with the difference between any two RR intervals being 15 percent or less)
Causes of Variations in the R-R interval
1. Sinus arrhythmia: The R-R interval (or the P-P interval) periodically decreases and then increases. This may be related to breathing, with the R-R interval shortening during inspiration.
2. Premature beat: The rhythm may be interrupted infrequently or frequently by a beat that occurs sooner than expected i.e. the R-R interval between the preceding normal beat and the ectopic beat is less than the R-R interval between normal beats. The premature beat may be followed by a pause. The main causes of premature beats are extrasystoles (atrial, junctional or ventricular); other causes are capture beat(s) or a period of better conduction interrupting poor conduction.
3. A pause: This isa sudden prolongation of the R-R interval.
The causes of a pause include:
• Failure of the sinus node pacemaker to produce a action potential, or failure to conduct the sinus node action potential to the atrial muscle
• Non conducted atrial extrasystole.
• Atrio-ventricular node (AVN) block
• Tachycardia-bradycardia syndrome (sick sinus syndrome).
4. Group beating: This occurs when a group of beats is followed by a pause.
The main causes of group beatingare:
• Sinus rhythm with two or more consecutive extrasystoles
• Recurrent bursts of tachycardia (ventricular or supraventricular) that lasts only a short time and ceases spontaneously (non-sustained tachycardia)
• Wenckebach phenomenon
4. Totally irregular rhythm
The causes of an irregularly irregular rhythm include:
• Atrial fibrillation.
• Atrial flutter with varying AV conduction.
• Multifocal ventricular extrasystoles.
• Shifting (wandering) pacemaker
• Multifocal atrial tachycardia.
Measuring R-R Intervals
The above rhythm strip has 10 complexes that are labelled 1 to 10. The R-R interval (in msec) between each complex is shown at the top of the strip. The R-R interval between complexes 1 - 3 and between complexes 5 - 6 is (essentially) the same at 840 to 880 msec. The R-R interval between complexes 3 and 6 is (about thirteen percent) less at 740 to 760 msec. The R-R interval between complexes 6 - 9 is also slightly variable, but has increased to between 1020 and 1120 msec.
The P waves in complexes 1 - 3 and and complexes 7 - 10 are narrow (1 mm width) and have a low amplitude (1 mm). The P waves in complexes 4 - 6 are wider (2 mm width), taller (amplitude is about 2 mm) and are notched. These differences suggest that there are two different locations within the atrium that trigger an action potential that is conducted to the ventricle.
What is thediagnosis? The rhythm is irregular (varying R-R interval) and the ventricular rate is about 80 beats per minute. The presence of an upright P wave before each QRS complex, and the marked variation in the R-R intervals, suggest sinus arrhythmia. The differences in P wave morphology suggest the presence of a wandering atrial pacemaker.
The final diagnosis is sinus arrhythmia and wandering atrial pacemaker.
Ectopic Beats & Intervals & Pauses
Figure 2A shows normal sinus rhythm (each ventricular complex is marked S; P waves are not shown). The R-R interval is constant at 0.8 seconds (800 msec), and is labelled DC for "dominant cycle"
Figure 2B shows sinus rhythm with a dominant cycle interval of 0.8 seconds (800 msec). The third beatis a atrial ectopic beat (APB). The APB is inscribed earlier than normal, so the interval between the preceding sinus beat and the APB is 0.44 seconds (440 msec). Another term for the interval between an ectopic beat and the normal complex that immediately precedes the ectopic beat is "coupling interval" (indicated as CI in the diagram). The coupling interval is less than the dominant (normal) R-R interval of 0.8 seconds (800 msec).
The interval between the APB and the next sinus beat has different names, including post ectopic interval or return cycle. The term return cycle (indicated as RC) is used in the diagram. The RC after the APB is 0.92 seconds (920 msec), which is longer than the dominant cycle.
The term compensatory pause (CP) is sometimes used as a synonym for the RC but it has a more specific meaning. It is defined as the sum of the coupling interval and the return cycle interval i.e. CP = CI + RC. The CP interval is compared to the sum of two dominant cycles i.e. to 2 x DC. In this case the CP interval is (440 + 920) msec i.e. 1360 msec, and the 2 x DC interval is (2 x 800) msec i.e. 1600 msec.
When the CP interval is less than the 2 x DC interval (as in this case) the compensatory pauseis (said to be) not fully compensated.
Figure 2C shows sinus rhythm with a dominant cycle interval of 0.8 seconds (800 msec). The secondbeat is a ventricular ectopic beat (VPB).
The coupling interval between the VPB and the preceding sinus beat (S) is 0.36 seconds (360 msec).
The return cycle interval after the VPB is 1.24 second (1240 msec).
The compensatory pause interval is (360 + 1240) msec i.e. 1600 msec.
The sum of two dominant cycles in this case is (2 x 800) msec i.e. 1600 msec.
When the CP interval is equal to the 2 x DC interval (as in this case) the compensatory pause is (said to be) fully compensated.
The calculation of the compensatory pause can help when trying to determine the origin of a ectopic beat.
- A compensatory pause that is not fully compensated is usually associated with atrial ectopic beats.
- A compensatory pause that is fully compensated is usually associated with ventricular ectopic beats.