Sinus arrest and sinus exit block: Identifying ECG features

Rhythm:

Basic rhythm usually regular; there is a sudden pause in the basic rhythm (causing irregularity) with one or more missing beats; heart rate may slow down for several beats after pause (temporary rate suppression) but returns to basic rate

Rate:

That of underlying rhythm, usually sinus

P waves:

Sinus P waves with basic rhythm; absent during pause

PR interval:

Normal (0.12 to 0.20 second) with basic rhythm; absent during pause

QRS complex:

Normal (0.10 second or less) with basic rhythm; absent during pause

Differentiating features

Sinus block:

Basic rhythm (R-R regularity) resumes on time after pause

Sinus arrest:

Basic rhythm (R-R regularity) doesn't resume on time after pause

Both sinus arrest and sinus exit block originate in the sinus node and are characterized by a sudden pause in the sinus rhythm in which one or more beats (cardiac cycles)

Figure 6-6.

Rhythm:

Rate:

P waves:

PR interval: QRS complex: Comment:


Normal sinus rhythm with sinus block.

Basic rhythm regular; irregular during pause

Basic rhythm 84 beats/minute

Normal in basic rhythm; absent during pause

0.16 to 0.18 second in basic rhythm; absent during pause

0.08 to 0.10 second in basic rhythm; absent during pause

ST-segment depression is present.

R-R regularity

S-R-R regularity interrupted; rhythm does not resume on time after pause

Figure 6-7. Normal sinus rhythm with sinus arrest.

Rhythm:  Basic rhythm regular, irregular during pause

Rate:  Basic rhythm 94 beats/minute

P waves:  Normal in basic rhythm; absent during pause

PR interval:  0.16 to 0.18 second in basic rhythm; absent during pause

QRS complex: 0.06 to 0.08 second in basic rhythm; absent during pause.


Figure 6-8. Normal sinus rhythm with sinus arrest; rate suppression is present following pause.

Rhythm:  Basic rhythm regular; irregular during pause

Rate:  Basic rhythm rate 84 beats/minute; rate slows to 56 beats/minute following pause (temporary rate suppression may occur

Following a pause in the basic rhythm)

P waves:  Sinus in basic rhythm; absent during pause

PR interval:  0.16 to 0.18 second in basic rhythm; absent during pause

QRS complex: 0.08 to 0.10 second in basic rhythm; absent during pause.


Are missing. The P waves in the underlying rhythm will be normal in size, shape, and direction; positive in lead II (a positive lead), with one P wave preceding each QRS complex. The duration of the PR interval and the QRS complex in the underlying rhythm is within normal limits. The distinguishing feature of both rhythms is the abrupt pause in the underlying sinus rhythm in which one or more beats are missing, followed by a resumption of the basic rhythm after the pause.

Sinus arrest is caused by a failure of the SA node to initiate an impulse and is therefore a disorder of automatic-ity. This failure in the automaticity of the SA node upsets the timing of the sinus node discharge, and the underlying rhythm won’t resume on time after the pause.

With sinus exit block, an electrical impulse is initiated by the SA node, but is blocked as it exits the sinus node, preventing conduction of the impulse to the atria. Thus, SA exit block is a disorder of conductivity. Because the regularity of the sinus node discharge isn’t interrupted (just blocked), the underlying rhythm will resume on time after the pause. Once the rhythm resumes after the pause (in both sinus arrest and sinus exit block) it’s common for the rate to be slower for several cycles (ratesuppression). Rate

Figure 6-9. Sinus arrhythmia with sinus pause.

Rhythm:  Basic rhythm irregular

Rate:  60 beats/minute

P waves:  Normal in basic rhythm; absent during pause

PR interval:  0.14 to 0.16 second in basic rhythm; absent during pause

QRS complex:  0.06 to 0.08 second in basic rhythm; absent during pause

Comment:  Because of the irregularity of the basic rhythm, sinus arrest can't be differentiated from sinus block, and the rhythm is inter

Preted using the broad term sinus pause, indicating that either rhythm could be present.

Suppression is temporary and will cause a brief irregularity in the underlying rhythm, but after several cycles the basic rate and rhythm will return. An example of rate suppression is shown in Figure 6-8.

Differentiating between the two rhythms involves comparing the length of the pause with the underlying P-P or R-R interval to determine if the underlying rhythm resumes on time after the pause. This can be determined only if the underlying rhythm is regular. If the underlying rhythm is irregular, as in sinus arrhythmia (Figure 6-9), it’s impossible to distinguish sinus arrest from sinus exit block. In this case, the rhythm would best be interpreted using the broad term sinus pause, indicating that either rhythm could be present. From a clinical viewpoint, distinguishing between sinus arrest and sinus exit block usually isn’t essential.

Sinus arrest or sinus exit block can be caused by numerous factors, including:

¦  increase in vagal (parasympathetic) tone on the SA node

¦  myocardial ischemia or infarction

¦  use of certain drugs such as digoxin, beta blockers, or calcium channel blockers.

The patient may become symptomatic if the pauses associated with sinus arrest or sinus exit block are frequent or prolonged. Another danger is that the SA node may lose pacemaker control. When the sinus node slows down below its minimum firing rate of 60 beats per minute because of bradycardia or a pause in the underlying rhythm, an opportunity is provided for pacemaker cells in other areas of the conduction system to usurp control from the sinus node and become the dominant pacemaker of the heart. The term ectopic is commonly applied to rhythms that originate from any site other than the SA node. Ectopic sites in the atria, AV node, or ventricles may assume pacemaker control for one beat, several beats, or continuously.

If symptomatic, the rhythm is treated the same as in symptomatic sinus bradycardia. In addition, all medications that depress sinus node discharge or conduction should be stopped.

A summary of the identifying ECG features of sinus arrhythmias can be found in Table 6-1.

Table 6-1.

Sinus arrhythmias: Summary of identifying ECG features

Rhythm

Rate (beats/ minute)

P waves (lead II)

PR interval

QRS complex

Normal sinus rhythm

Regular

60 to 100

Positive in lead II; normal in size, shape, and direction; one P wave precedes each QRS complex

Normal (0.12 to 0.20 second)

Normal (0.10 second or less)

Sinus

Bradycardia

Regular

40 to 60

Positive in lead II; normal in size, shape, and direction; one P wave precedes each QRS complex

Normal (0.12 to 0.20 second)

Normal (0.10 second or less)

Sinus

Tachycardia

Regular

100 to 160

Positive in lead II; normal in size, shape, and direction; one P wave precedes each QRS complex

Normal (0.12 to 0.20 second)

Normal (0.10 second or less)

Sinus

Arrhythmia

Irregular

60 to 100 (normal) or < 60 (slow)

Positive in lead II; normal in size, shape, and direction; one P wave precedes each QRS complex

Normal (0.12 to 0.20 second)

Normal (0.10 second or less)

Sinus block and sinus arrest

Basic rhythm usually regular; there is a sudden pause in the basic rhythm (causing irregularity) with one or more missing beats; temporary rate suppression common following pause

That of underlying rhythm, usually sinus

Sinus P waves with basic rhythm; absent during pause

Normal (0.12 to 0.20 second) with basic rhythm; absent during pause

Normal (0.10 second or less) with basic rhythm; absent during pause

Differentiating features Sinus block:

Basic rhythm resumes on time after pause

Sinus arrest:

Basic rhythm does not resume on time after pause

Note: If the basic rhythm is irregular (sinus arrhythmia), sinus arrest can't be differentiated from sinus block, and the rhythm is interpreted as sinus arrhythmia with sinus pause.

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