Adult who came in due to nausea and vomiting.
Figure 1 (lead2)
The long lead II shows a narrow QRS rhythm at ~88 bpm (?regular).
Figure 2 (full disclosure)
Search for the P waves
The full disclosure will only reveal the positive nibs which are most likely atrial activities in leads II, III and aVF and a little in V1.
Figure 3 3(P waves marked)
Red arrows are the obvious P. Green arrows - not so obvious P and blue arrows are most likely the hidden P's. I have to use a caliper to march these missing P's. The atrial rate is about 187 bpm (8 small squares).
If you use a caliper and measure the RR interval in R9R10, it is longer compared to the rest. It also shows a distinct P wave. This means that the hidden P wave after QRS 9 is the dropped beat of the Wenckebach cycle.
It is now known that atrial flutter rate can be slower than the usual 250-350 range. It can be due to antiarrhythmics or atrial myopahty. With slowing of the atrial rate, isoelectric interval can be seen. Atrial tachycardia on the other hand can be faster than the usual rate of about 240 bpm. An electrophysiologic study is needed to differentiate the two.
Another differential diagnosis is AV dissociation with capture beat on QRS # 10. According to Dr. HJ Marriott:
"The way to recognize ventricular capture is not by finding an appropriate PR interval but rather detecting a shortening of the RR interval."
There is lengthening of the RR interval in R9R10. So, this is not the capture beat we see in AV dissociation.
Interpretation: Atrial tachycardia with a block vs. AFL with Wenckebach
HJ Marriot. 1998. Pearls and Pitfalls in Electrocardiography (2ed). MA Williams and Wilkins