A sinus pause is caused by a delay in the formation of a sinus impulse in the sinus node itself. The delay is represented on the rhythm strip as a longer pause between the complexes (Figure 12-4). In other words, a sinus pause is represented electrocardiographically as a longer P-P interval.
Figure 12-4 A sinus pause refers to a delay or failure of the sinus node to create a depolarization wave. In this figure, the expected firing time for the sinus node is represented by the pink star. Note the delay in the timing of the actual firing. The net result is that the normal P, QRS, and T wave complex is not formed on time. The sinus pause is not a direct multiple of the P-P interval.
© Jones & Bartlett Learning.
Sinus pauses do not occur as exact multiples of the normal P-P interval because the delay usually occurs in the sinus node itself. When the sinus node returns after the pause, it may be set at the same rate as before the pause or it may be a different rate.
A sinus arrest is caused by a longer delay in the formation of an atrial impulse (Figure 12-5). A surface ECG is not accurate enough to adequately interpret if sinus blocks can also be found within the pause of a sinus arrest. One thing is certain: Sinus arrests can be long, but they are never an exact multiple of the normal P-P interval.
Figure 12-5 A sinus arrest refers to a delay or failure of the sinus node to create a depolarization wave, but at an interval much longer than that seen during a sinus pause. The net result is that the normal P, QRS, and T wave complex is not formed on time. The sinus arrest is not a direct multiple of the P-P interval.
© Jones & Bartlett Learning.
Where the cut-off point between a sinus pause and a sinus arrest exactly falls is not clear. There is no clear consensus as to where a sinus pause ends and a sinus arrest begins. Basically, we use this rule of thumb: If the pause is shorter than three times the normal P-P interval (at least two nonconducted complexes), we say that it is a sinus pause. If the pause is greater than three times the normal P-P interval, we say it is a sinus arrest. Is this system correct? Well, no one can say this is right or wrong because no one knows the answer. This system is completely arbitrary, but does work for our purposes.
Additional Information
Sick Sinus Syndrome
It is difficult to cover a topic as complicated as sick sinus syndrome (SSS) at this point in the book because we have not covered many of the rhythms involved. For that reason, we are placing this SSS section as an Additional Information box. You can skip over this section and return to it later after you have covered some of the sections, or to quickly refresh your memory on those rhythm abnormalities, you can turn to Chapter 7, How to Interpret a Rhythm Strip.
SSS refers to a series of arrhythmias that are frequently seen in patients with diseased SA nodes. The diseased node can cause severe bradycardias, marked sinus arrhythmias, and sinus blocks, pauses, and arrests to develop. The diseased node can also frequently alternate between very slow bradycardias and very fast tachycardias (the so-called tachy-brady syndrome, which is a variant of SSS).
In addition to sinus node dysfunction, the same processes that cause the sinus node to start to malfunction will also affect the AV node and the rest of the conduction system. This can lead to the formation of AV blocks, junctional escape rhythms, and chronic atrial fibrillation.
The arrhythmias can develop in any combination and not every patient will exhibit all of them. Typically, the syndrome is suspected when the patient develops symptoms of fatigue, exercise intolerance, or congestive heart failure and has an ECG or rhythm strip with any of the rhythms mentioned earlier. Verification of the syndrome and its manifestations is usually made by obtaining a 24-hour ambulatory monitoring study.
The most common treatment for SSS is to give the patient an antiarrhythmic to prevent the formation or reduce the severity of the tachycardias (e.g., procainamide, digoxin) and to place an artificial pacemaker to control the bradycardias and blocks.
ARRHYTHMIA RECOGNITION
Sinus Blocks, Pauses, and Arrests
| Rate: | These are either single or multiple events. |
| Regularity: | Regular with events |
| P wave:
Morphology: Upright in II, III, and aVF: |
Nonconducted or delayed
Not applicable Not applicable |
| P:QRS ratio: | Not applicable |
| PR interval: | Not applicable |
| QRS width: | Not applicable |
| Grouping: | None |
| Dropped beats: | Yes |
DIFFERENTIAL DIAGNOSIS
Sinus Blocks, Pauses, and Arrests
This list is not inclusive but reflects the most common causes of the rhythm disturbance.