A 17 year old woman is admitted to the psychiatry service.  Prior to starting medications, a routine ECG is performed and shown below:

• Marked sinus bradycardia (P-waves noted with “P”)
• Junctional Escape Rhythm
• Intermittent Capture beats (3rd complex)

Comment: The p-waves are sinus and not retrograde because (1) they are occurring at a fixed interval and not related to the QRS complex, and (2) they are upright in the inferior leads and in lead I. All p-waves noted occur close enough to the QRS complex that the AV node is still refractory after firing of the junction, with the exception of the second p-wave which is conducted as a capture beat. Because the sinus node is firing slower than the junction, AV dissociation exists but is not called complete heart block as the AV node is clearly capable of conducting (capture beat present).  This rhythm is also known as isorhythmic dissociation.