The computer's got it wrong again. I think the atria are under the control of the sinus node at a rate of 103bpm but there is a faster AV junctional tachycardia controlling the ventricles at a rate of 115bpm, producing AV dissociation. The AV dissociation isn't complete, however, and occasional sinus P waves fall in such a place that they can conduct to the ventricles and produce a capture beat. Theses are the 'premature' beats 5 and 15.

GREAT tracing by Jason! I agree completely with Dave R's interpretation. This is AV dissociation by usurpation (ie, as per Dave - the slightly faster junctional rhythm takes over = "usurps" the pacemaking function despite underlying sinus tach). KEYS to recognizing this require the use of calipers (this being a perfect example of a rhythm you just can't interpret without using calipers! ).

• Look first for 2 definite P waves in a row. I took the P waves seen before beats #6 and #7. Using calipers you can now march out a perfectly regular P-P interval throughout the entire rhythm strip!
• Look next for the "break" in the rhythm. As per Dave - beats #5 and #15 occur early. A perfectly-on-time P wave with constant PR interval is seen before each of these beats. This proves they are capture beats! That fact that P waves can conduct when given opportunity to do so (and conduct with a reasonable PR interval) indicates that there definitely is not complete AV block. In fact - there may not be any AV block at all (we just don't know - the rate is too fast and the rhythm strip too short to tell ... ).

Taking a final look at the tracing (to see if we might find a cause of this rhythm disturbance) - we see suggestion of prior inferior infaraction (large inferior Q waves) - but really no acute changes. So we have Sinus tach plus junctional tach resulting in AV dissociation by usurpation - the combination of which suggest Dig toxicity as the first clinical entity to rule out.

I'm sure Jason will elucidate further with a gorgeous laddergram! THANKS for posting this.