Understanding ECGs: tachycardias
BMJ 2001; 323 doi: https://doi.org/10.1136/sbmj.0111410 (Published 01 November 2001) Cite this as: BMJ 2001;323:0111410
- Dominic Cox, specialist registrar in cardiology1,
- Hamish Dougall, general practitioner and research fellow in general practice2
- 1Newcastle upon Tyne
- 2University of Dundee
Tachycardias are the most exciting part of learning about ECGs. It is of course important to understand the mechanism causing tachycardias, but it is equally important to remember to look at the patient first. “Benign” arrhythmias can compromise a person, and “serious” arrhythmias can be asymptomatic. Look at the patient and act with commonsense, rather than treating the ECG.
We are not going to try to explain the emergency treatment of arrhythmias, but this should be something on all doctors' minds (and hearts). All the emergency treatment protocols are available from the European Resuscitation Council (website www.erc.edu/).
Tachyarrhythmias can be divided into two broad categories: supraventricular tachycardias (SVT) and ventricular tachycardias (VT). It is useful to think of these as narrow complex (QRS complex <120ms) tachycardias (SVT) and broad complex (QRS >120 ms) tachycardias (VT).
Why is the QRS complex narrow?
As described in the previous articles,12 normally the atrium passes the sinoatrial (SA) node's signal to the atrioventricular (AV) node. This then passes on through the ventricular specialised conducting tissue, which gives rapid, synchronous activation of the whole of the ventricle. Thus to get a narrow complex you must have an electrical signal that passes forward through the AV node, and you must have specialised conducting tissue, that works. A broad QRS complex means that either your conducting system is not working (bundle branch block) or the electrical circuit is not involving the AV node correctly.
So what causes arrhythmias?
Only two things cause abnormal heart rhythms. The most common mechanism involves an abnormal electrical circuit which allows the heart beat to cycle around it. This is called re-entry. The other is where a focal area of the heart starts to “spark off” and send out a shower …
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