![]() The impulse is propagated from the right atrium to the left atrium via Bachmann’s bundle.Ĭonduction to the AV node occurs in specialised tissue called intermodal tracts. Normally, electrical activity is spontaneously generated by the SA node (the ‘physiological pacemaker’) Signals arising from the SA node stimulate the atria to contract and are conducted to the AV node (the P wave on the ECG). The conduction pathways of the heart consist of: A final section describes drug therapy and pacing in more detail.įigure 1: The conduction pathways of the heart. The causes, clinical presentations and management for each type of bradycardia are discussed in more detail in the sections below. May be asymptomatic worsening of angina pectoris or heart failure. Other symptoms may relate to the underlying cause. Organophosphates (including nerve agents)īradycardia may present as an incidental finding or with symptoms related to hypotension: Various drugs can also affect the electrophysiology of the heart myocytes and their nerve supply (Table 1). Electrolyte disturbance: Hyperkalaemia and hypokalaemia.Normal physiological variants as seen, for example, in athletes.The causes of bradycardia can be broadly categorised as: Bradycardia is defined as a heart rate of less than 60 beats per minute. Non-paroxysmal junctional tachycardia with type I exit block. I am grateful to everyone who has contributed to the discussion and to Su Baxter for sending me the ECG. Please feel free to continue to comment on the ECG and my explanation. The failure to conduct in these circumstances is physiological rather than pathological. The reason that none of the sinus impulses conduct is simply because they always arrive at the AV node when it is in its refractory state due to the rapid junctional discharges. None of the P waves in this ECG conduct to the ventricles, so is there any heart block? No, none at all. We often see the opposite of this in complete AV block, where there is a reflex sinus tachycardia in the presence of a slow escape rhythm. As Scott ingeniously suggests, this may be an automatic reaction to the junctional tachycardia, a circulatory system reflex that aims to correct the abnormally high cardiac output. ![]() This phenomenon has been reported previously, although it seems that it is usually caused by digitalis toxicity (2).Īn interesting side-issue is the sinus bradycardia in a 10-year-old child. A possible explanation for this is that there is Wenckebach exit block from the junctional focus. I don’t think this is an AVNRT because of the relatively slow ventricular rate and the presence of AV dissociation: in AVNRT there is a common upper pathway that makes retrograde block, and therefore AV dissociation, very unlikely (1).Īlthough the tachycardia is almost perfectly regular, there is a pause in the ventricular rhythm that is just under two normal RR intervals in duration. I think this is because there is a junctional ectopic, or non-reentrant junctional, tachycardia. The ventricular rate is, for the most part, just above 100 bpm and the QRS is narrow. ![]() But they are all P waves, and this brings us to one of key abnormalities of the rhythm: there is AV dissociation. Some of these P waves fall immediately before QRS complexes, perhaps creating the impression of delta waves some occur immediately after, resembling J waves. There is therefore an underlying sinus bradycardia/arrhythmia. Here we have P waves that are positive in leads I and II and that occur at a rate of about 50/minute but are slightly irregular. It’s often useful when trying to solve an arrhythmic puzzle to start with identifying all the atrial activity. I have tried to illustrate what I think is happening in the below laddergram that accompanies the rhythm strip (RR intervals shown in milliseconds). Well, this one elicited quite a few responses, with several different explanations proposed. The only information I have about this case is that the ECG was recorded from a 10-year-old child, but my question is a simple one: what is the rhythm? Please describe exactly what you think is happening.
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