Different cases of heart conduction and ECG abnormalities and how to assess them
Dr Toni Hazell is a GP in North London
You are reviewing an ECG you arranged for a 56-year-old man with newly diagnosed hypertension. The trace is normal but the computerised report notes ‘first-degree heart block’. Looking at the patient’s record you see he has no other obvious past medical history and is on no medication.
1. What is first-degree heart block – both in terms of what it means within the heart and how that translates into ECG findings – and how common is it?
The electrical activity in the heart starts in the sinoatrial (SA) node and spreads through the atrial muscle to the atrioventricular (AV) node, then downwards through the bundle of His to the ventricles. The time taken for the depolarisation wave to spread from the SA node down to the ventricular muscle is indicated by the PR interval on the ECG, that is from the start of the P wave to the start of the QRS complex. A PR interval of more than 0.2s shows slowed conduction (which may represent delay at any point along the conduction pathway) and is known as first-degree heart block. It is estimated that around 0.5-2% of the population will have first-degree heart block. It is often asymptomatic.1,2
2. How accurate are the computer algorithms that pick up these abnormalities, and how, if the abnormality is confirmed, should it be explained to the patient?
ECG reading is difficult, and it is often tempting to rely on the computer printout. One study of junior doctors showed that the presence of the computer interpretation made no significant difference to the accuracy of the junior doctor’s reading of the ECG, which was significantly less good than the reading of senior clinicians who were used as a control.3 The computer made two major errors out of 50 ECGs. Other papers have suggested that the computer is around 95% accurate when it says that the ECG is normal, but that this accuracy drops to just over 50% for abnormal rhythms.4
It is therefore important that all ECGs are reviewed by a person with the appropriate experience. If you feel you don’t have that experience in your practice, possible options are to address this at a CPD event, or to discuss with your CCG what support they can offer, for example via a local cardiology GPSI or the advice and guidance service.
The patient should be told that there is a block in the electrical conduction of the heart and that this is a common issue, found in up to one person in 50. Assuming that other cardiac disease is not suspected, they can be told that no treatment is needed, other than avoiding certain medications. Pathological causes can include coronary heart disease, potassium imbalance and acute illness such as endocarditis.
3. What are the implications for the patient in terms of what might develop from this – and therefore the symptoms it might cause? And what medications should be avoided?
If the patient is asymptomatic and there are no signs of underlying disease (such as coronary artery disease) then no action need be taken, but the finding should be coded in the notes. First-degree heart block is associated with increased future risks of atrial fibrillation, which is also often asymptomatic and with a small increased risk in all-cause mortality, so as with any other condition the patient should declare it when applying for any type of health-related insurance. Caution should be used when considering the use of drugs that prolong transmission across the AV node, such as ß-blockers, diltiazem and digoxin.
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