7.8.13 Accelerated idiojunctional and idioventricular rhythms
Idiojunctional and idioventricular rhythms occur when there is a slow sinoatrial rate but an abnormally high (accelerated) junctional or ventricular rate. The term idioventricular implies that the complexes are ventricular in origin, i.e. they have an abnormal configuration. A similar mechanism can result in an idionodal or idiojunctional rhythm when the complexes arise for the AV node or junctional tissue; this is defined as a supraventricular rhythm and the QRS complexes are normal in configuration. An idioventricular rhythm is a ventricular rhythm with a slow rate (usually of approximately 50 bpm). If the same pattern was seen with a higher rate (>~60 bpm) the rhythm would be defined as a ventricular tachyP waves occur less frequently than the QRS complexes, are unconducted and therefore do not bear any relationship to the QRS complex (Figure 7.13). The separation of atrial and ventricular rhythms is called AV dissociation. Occasionally, a P wave is conducted and a normal QRS complex follows at a normal P-R interval (a capture beat). Fusion beats can also be seen when sinus beats are conducted and coincide with depolarisation from the ectopic focus. These are abnormal configuration QRS complexes which occur with a short P-R interval.
Accelerated junctional or ventricular rhythms are most commonly seen in association with severe systemic disease, for example in horses after colic surwhere there is high vagal tone (and hence a low sinus rate, or AV block), but increased levels of catecholamines which raise the intrinsic junctional or venrate from their normal level of approximately 8-20 bpm. Occasionally, idioventricular or idiojunctional rhythms occur during anaesthesia. The rate of the arrhythmia is usually normal or just greater than normal (4~50 bpm), so it has relatively little direct effect on cardiac function. Once the systemic disease has resolved, the rhythm usually returns to normal sinus rhythm. There is therefore usually no requirement to treat these rhythms, but the ECG and carstatus should be closely monitored. However the systemic disease should be reversed if possible and this may require aggressive treatment.