The correct interpretation includes sinus bradycardia with marked sinus arrhythmia and junctional escape beats, and an intraventricular conduction delay.
This type of ECG is not ideal for calculating the ventricular rate via the 30/150/100 method. An easier method is to multiply the number of QRS complexes in the rhythm strip by six (an ECG at standard paper speed takes 10 s; 6 × 10 s = 60 s). In the absence of a permanent pacemaker, variation of a few beats/min from the computer reading is acceptable. In this case, multiplying 9 x 6 yields a rate of 54 beats/min (close to the computer reading of 55) and reveals sinus bradycardia.
Looking at the lead I rhythm strip, notice that while the QRS complexes in the fourth and eighth beats look similar to the others, they are not preceded by P waves; the T waves of these two beats are also not similar to the others. These represent junctional escape beats, with a possible retrograde P wave in the T-wave complex. The long pauses and the absence of a P wave prior to the fourth and eighth beats make this a sinus arrhythmia.
The diagnosis of an intraventricular conduction delay can be made by the duration of the QRS complex (122 ms), which is above normal limits. This ECG does not meet the clear criteria for a right bundle branch block (QRS ≥ 120 ms, terminal broad S wave in lead I, RSR’ in V1) or left bundle branch block (QRS ≥ 120 ms, ST depressions and inverted T waves, particularly in I, aVL, V5, and V6).