Sinus Arrest - ECG

Sinus arrest is a condition when the formation of electric impulse in sinoatrial (SA) node is temporary disrupted. This manifests in ECG as an abnormally long isoelectric line that is located behind a QRS complex and its T wave.

Two options are possible when a sinus arrest occurs:

a) The SA node returns back to work or it is replaced by another area of heart atria. In that case the isoelectric line ends with a P wave followed by a normally looking thin QRS complex. When the electric signal comes from the SA node, the P wave has the same shape as any other P waves before the isoelectric line occurrence. When the signal occurred from another atrial location, the P wave has a slightly different shape that the preceding P waves.

 

Sinus Arrest - ECG
In this case, the isoelectric line is followed by a normal-shaped P wave and normal QRS complex. This suggests that the electric impulse occurs again in the temporary disrupted SA node.
 

b) The SA node and the atrial conduction system does not respond and the initiative is taken by the myocardium of AV node (junction rhythm) or ventricular myocardium (ventricular rhythm). In junction rhythm we find narrow QRS complexes without the P waves (as the source of the signal is the AV node and not the atria). In ventricular rhythm the P wave is also missing and the QRS complex is wide (over 120ms) and has a more bizarre appearance. As the AV node and ventricular myocardium are unable to emit the signals in high frequency, the outcome is alwyas bradycardia of various degrees.

Sinus Arrest - ECG
In this case, the sinus arrest is followed by a slim QRS complex without P waves (blue). It is a junction rhythm coming from the area of the AV node.

 

Conclusion: Sinus arrest can be caused by ischemia of the SA node and sometimes it occurs in digoxin overdose.