AV dissociation is common clinical situation that can occur during both bradyarrhythmias and tachyarrhythmias .
Bradycardias
- Complete heart block
- During pacemaker rhythms
Tachycardias
- Accelerated junctional rhythm
- Idioventricular rhythm
- Ventricular tachycardia
AV dissociation is essentially an ECG diagnosis. But it is associated with some clinical signs ,which can be detected by an astute physician in the bedside. At rapid heart rates it may be really difficult at times to recognise theses findings, but a cardiology fellow should look for these whenever they encounter AV dissociation in ECG.
- Varying pulse volume
- Varying korotkoff sounds during BP measurement.
- Cannon a waves in JVP
- Varying intensity of first heart sound on auscultation
- Mitral regurtitant murmur may be heard
- Hypotension in compromised hearts
What is the mechanism of clinical signs of AV dissociation ?
During AV dissociation , the atrial and ventricular contractions occur out of phase and the sequential contraction is lost. So atrial contractions might occur with a closed AV valves . This result in reflux of blood into the neck resulting in cannon waves . It may be visible only in few beats as the retrograde conduction VA conduction , is highly variable.
Further , only some atrial beats contribute for ventricular filling some do not.This results in varying LV volumes and this could result in changing pulse volume.Occasionally the ventricular and atrial contraction occur simultaneously .When this happens , some amount of blood reguritates through the open tricuspid valve and mitral valve which result in MR or TR .
Clinical utility
This could be important , in differentiating the perennial issue of decoding the wide qrs VT from SVT with aberrancy .A rapid clinical assessment here could aid in the diagnosis of VT by identifying AV dissociation . An experienced cardiologists will realise even in a given ECG with VT identifying or ruling out AV dissociation is not always a pleasant excercise !
In this era of high tech gadget oriented cardiology is it not too much to call for clinical recognition of this entity ?
Definitely not , if we know Wencke bach recognised the classical type 1 2nd degree AV block in late 19th century even before the ECG machine was invented ,
Simply by looking at the neck , by carefully observing progressive prolongation of distance between a and c waves and subsequent dropping of c waves . Amazing isn’t it ?
Dr.S.Venkatesan MD 