Arrhythmias

Medicine — Cardiology, NMC MBBS licence examination syllabus (Nepal Medical Council).

Arrhythmias

Two measurements sort almost every abnormal rhythm — and one question, asked before either of them, decides whether you have time to take the measurements at all.

The ECG chapter covers reading a trace. This one covers what to do when the rhythm on it is wrong: how to classify it quickly, which ones are emergencies, and why the commonest management error in cardiology is treating two separate decisions as one.

Stability comes before classification

Before naming the rhythm, decide whether it is perfusing the patient. Shock, syncope, ischaemic chest pain or heart failure in the presence of an arrhythmia mean the rhythm is the emergency — and an unstable patient is treated immediately rather than after the diagnosis is refined. Getting this order wrong is how a patient deteriorates while the ECG is being interpreted.

Stability is a clinical judgement rather than a number. A rate one patient tolerates comfortably will decompensate another whose ventricle is already poor, which is why no threshold defines it.

Two measurements sort the fast rhythms

Narrow or broad? A narrow QRS means the impulse used the normal conducting system, so it started above the ventricles. A broad QRS means it did not — and a broad-complex tachycardia is treated as ventricular tachycardia until proven otherwise.

Regular or irregular? Irregularly irregular with narrow complexes is atrial fibrillation. Regular and narrow points to a supraventricular tachycardia. Regular and broad points to VT.

🩺 Why assume the dangerous diagnosis: a broad-complex tachycardia can be VT, or a supraventricular rhythm conducted through a diseased bundle. Treating VT as if it were supraventricular can cause it to degenerate; treating a supraventricular rhythm as VT is usually recoverable. The asymmetry in the consequences, not the probabilities, is what justifies the rule — and examiners test whether you can state that reasoning rather than just the rule.

Atrial fibrillation: two decisions, not one

AF is the commonest sustained arrhythmia, and it generates two entirely separate management questions that are constantly collapsed into one.

Rate or rhythm control addresses the arrhythmia itself — either slow the ventricular response or attempt to restore sinus rhythm. This decision is driven by symptoms and by how well the patient functions.

Anticoagulation addresses stroke, and it is decided by the patient's risk profile — not by their heart rate, not by their symptoms, and not by whether sinus rhythm has been restored.

The commonest error in this topic: controlling the rate does nothing to the stroke risk. Fibrillating atria fail to empty properly and clot regardless of how fast the ventricles are responding, so a comfortable, rate-controlled patient carries the same risk as a symptomatic one. The two decisions are made independently, from different information.

The stroke risk itself comes from stasis: the atria quiver instead of contracting, blood pools, and a clot that forms there travels to the brain. That mechanism is why AF causes strokes rather than merely accompanying them.

Bradycardia and heart block

The degrees of block describe how far the impulse gets.

First degree — every impulse arrives, just late. A long PR interval with a QRS after every P. It usually needs nothing.

Second degree — some impulses are dropped. In one pattern the PR interval lengthens progressively until a beat fails; in the other, beats drop suddenly with the PR interval unchanged. The second is the more concerning, because it reflects disease lower in the conducting system and is more likely to progress.

Third degree (complete) — nothing crosses. The atria fire at their own rate, the ventricles at a slower independent one, and there is no relationship between them.

💡 Why complete block is urgent even in a patient who looks well: the ventricles are being driven by an escape rhythm — a backup pacemaker that has taken over because nothing else is arriving. Escape rhythms are slow and unreliable, and they can stop without warning. The patient is stable because of a mechanism that may not last, which is not the same as being safe.

Look for a cause before treating the trace

Many arrhythmias are a symptom rather than a disease, and treating the rhythm while ignoring what provoked it means it returns.

Electrolytes, potassium above all — both high and low disturb the membrane excitability that every heartbeat depends on. Drugs, including the antiarrhythmics themselves, digoxin toxicity, stimulants and excess thyroid hormone. And ischaemia, sepsis, hypoxia, pain or bleeding — a sinus tachycardia in a septic patient is the heart responding correctly, and slowing it removes a compensation the patient needs.

Clinical reasoning: four presentations

An irregularly irregular pulse with narrow complexes in a comfortable outpatient. Atrial fibrillation. Two decisions follow: how to manage the rate or rhythm, and — separately, on risk — whether to anticoagulate.

A regular broad-complex tachycardia in a patient who is pale with a low blood pressure. Unstable, so treatment does not wait for the diagnosis to be refined. It is VT until proven otherwise, and this patient needs immediate senior help.

P waves at a steady rate, QRS complexes at a slower steady rate, no relationship between them. Complete heart block. Even if the patient is talking comfortably, they depend on an escape rhythm and need monitoring and urgent senior involvement.

A sinus tachycardia in a patient with a fever and low blood pressure. The rhythm is a response, not the problem. Treating the sepsis addresses it; slowing the heart removes a compensation.

Commonly confused

Rate control and stroke prevention. Independent decisions in AF, made from different information. This is the single most-tested distinction in the topic.

Broad complex meaning ventricular. Broad means conduction was abnormal. It is treated as ventricular because of the consequences of being wrong, not because it always is.

Stable meaning safe. Complete heart block in a comfortable patient is stable and not safe.

Treating the rhythm and treating the patient. A sinus tachycardia usually has a cause worth finding.

Rapid revision

Stability first — shock, syncope, ischaemic pain, heart failure.

Narrow or broad, regular or irregular. Broad-complex tachycardia is VT until proven otherwise.

Irregularly irregular and narrow = atrial fibrillation.

AF: rate/rhythm and anticoagulation are separate decisions. Rate control does not reduce stroke risk.

Complete heart block runs on an escape rhythm that can fail without warning.

Check potassium, drugs, and whether the rhythm is a response to something else.

Syllabus points

Assessing stability before classifying: shock, syncope, ischaemic chest pain, heart failure
Why stability is a clinical judgement rather than a rate threshold
QRS width: narrow means supraventricular origin, broad means it was not
Regularity: irregularly irregular with narrow complexes as atrial fibrillation
Broad-complex tachycardia treated as ventricular tachycardia, and why the asymmetry justifies it
Atrial fibrillation: rate or rhythm control as one decision
Anticoagulation as a separate decision made on risk, not on symptoms or rate
Why rate control does not reduce stroke risk
Stasis in the fibrillating atrium as the mechanism of stroke
First degree block: delayed but every impulse conducted
Second degree block: progressive PR lengthening, or sudden dropped beats
Third degree block: atria and ventricles independent
The escape rhythm in complete heart block, and why stable is not safe
Electrolytes, especially potassium, as a reversible cause
Drugs, digoxin toxicity, stimulants and thyroid excess as causes
Sinus tachycardia as an appropriate response to sepsis, hypoxia, pain or bleeding

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