Vignette: A 50 yo who is scheduled for surgery.
Figure 1 - Long lead V1,II and V5
a. Sinus rhythm with AV dissociation and junctional escape
b. Sinus rhythm with complete heart block
c. Ventriculophasic sinus arrhythmia, second degree advanced heart block
d. Sinus rhythm, second degree AV block type II (Mobitz II)
Answer: c. Ventriculophasic sinus arrhythmia, second degree advanced heart block
Figure 2 - ECG case labeled
This is an irregular (irregular R to R interval), narrow QRS rhythm with a ventricular rate of about 35-50 bpm. The P to P interval is also irregular with a rate of about 50's-60's.
This is sinus rhythm because the P waves are upright in lead II. However, it is irregular. The PP interval encompassing a QRS complex is shorter than the PP interval without an intervening QRS complex (P2P3 of 1080 ms vs P3P4 of 1160 ms). This is called ventriculophasic sinus arrhythmia which is one of the subtypes of sinus arrhythmia and seen in the presence of AV block.
Some of the P waves are not followed by a QRS. Only P1, P4 and P8 are followed by a QRS. The QRS following those P waves have similar morphology (R2, R4 and R7) as can be seen in V1, II and V5 (red arrows). This means that P1,P4 and P8 are conducted sinus P waves. P2,P3,P5,P6 and P7 are nonconducted or blocked P waves.
R3, R5 and R6 have a different QRS morphology compared to R2,R4 and R7. The RR interval of R5R6 are the same with R2R3 and R4R5. This means that R3,R5 and R6 are junctional escape complexes.
This is basically NOT complete heart block (CHB) because some of the sinus P waves are conducted. A simple rule to remember for complete heart block is that the RR interval MUST be regular. If the RR interval is not regular then some of the sinus P waves a conducted.
An easy way to understand such a complex rhythm is by using a ladder diagram or a laddergram. A laddergram will show the behavior of an impulse in the A (atria) tier, AV tier and V (ventricle) tier. The dots represent the source or origin of the impulse. Thus, in the ladder diagram the sinus impulse is conducted in a 3:1 and 4:1 pattern or for every 3 P waves or 4 P waves only 1 is conducted. The rhythm is maintained by a subsidiary pacemaker which is coming from the junction. This is how second degree advanced or high-grade AV block looks like on the surface ECG and on a ladder diagram.