Critical Care Trauma Centre


Question of the Week:  September 10, 1999

Identify the following rhythm.



Sinus arrhythmia
(P - P irregular, R - R irregular [same pattern of irregularity], all PR intervals the same)

In any sinus rhythm, the QRS is produced as a result of a wave that originated in the sinus node.  Consequently, each P wave is followed by a QRS that occurs following the same PR interval.

In normal sinus rhythm, the sinus node fires regularly.  As a result, the P - P is regular.  Because each P produces a QRS with the same PR interval, the R - R will also be regular, and will have the exact same rate as the P - P.

Sinus arrhythmia is a normal rhythm variation.  The only difference from normal sinus rhythm is that the sinus node depolarizes irregularly, as a result of changing intrathoracic pressures during breathing.  Once the sinus node fires, the sinus wave will travel through the conduction pathway in the normal manner.  Thus, each sinus beat is followed by a QRS that appears after the same PR interval.

There are 2 quick checks that will confirm sinus arrhythmia:
1)      i)    mark all of the Ps in sequence on a piece of paper
         ii)    shift the paper to the right, to align the "P mark" with the QRS that corresponds to the P
        iii)    the pattern of irregularity in the P - P will match the pattern of irregularity in the R - Rs
2)    measure the PR interval after each P wave; in sinus arrhythmia, the PR intervals will be the same