While many ECGs present subtle, benign patterns, others wave bright red flags. Signals that demand immediate attention. Recognising these is critical to safeguarding your patient.

ST elevation in two or more contiguous leads is a hallmark of ST-Elevation Myocardial Infarction (STEMI). These aren’t just squiggly lines, they’re a heart muscle crying out for oxygen. If you see it, don’t wait for a troponin! Activate the PPCI pathway. Remember: time is myocardium.

Note the ST-Elevation in Leads II, III and aVF (Inferior Leads)

A broad complex tachycardia is ventricular tachycardia until proven otherwise. It may look organised, but it’s dangerous, especially if the patient is hypotensive or showing signs of poor perfusion. If unsure, treat as VT. Better to err on the side of caution than miss a potentially fatal rhythm.

A new left bundle branch block (LBBB) with chest pain is a STEMI equivalent. The LBBB may mask ST elevation and that’s the danger.
If unsure, remember WiLLiaM MoRRoW:

In Right BBB, you’ll see an M in V1 and a W in V6 – MoRRoW.
In Left BBB, look in V1 and V6: you’ll see a W shape in V1 and an M shape in V6 – WiLLiaM.

Bradycardia under 40 bpm with syncope, dizziness or hypotension may be a sign of high-grade heart block. Whether due to complete AV dissociation or sinus node dysfunction, the heart rate is too slow to sustain cerebral or coronary perfusion. Action is needed fast.

Remember to divide 300 by the number of big squares between the R-R to calculate the rate. What is the rate in the ECG below?

A complete (3rd degree) heart block? The atria and ventricles are working on entirely different schedules. There’s no coordination, and output can drop dangerously. These patients often need pacing, temporary or permanent.

And don’t forget the QTc interval. Anything over 500 ms significantly raises the risk of torsades de pointes – a twisting, polymorphic VT that can deteriorate into cardiac arrest. Drugs, hypokalaemia, hypomagnesaemia… know your culprits and correct them.

Red flags aren’t subtle. See one? Act. Reassure later.

Conclusion: Making Sense of the Rhythm of Life

Interpreting ECGs isn’t about memorising patterns, it’s about understanding what the heart is trying to tell you. Every P wave, every QRS, every subtle deviation is a clue. As clinicians, our role is to translate those electrical signals into safe, timely, and effective care.

You’ve explored the fundamentals, from electrode placement to axis, waveform to rhythm strip. You’ve learned to spot red flags that can’t wait, and how to unravel complex rhythms using structure, analogies, and confidence. Whether it’s a jazz-band atrial fibrillation, a soloing ventricle in VT, or a dysfunctional couple in heart block, ECGs are never just lines, they’re stories waiting to be read.

Mastery comes not from rushing to diagnose, but from developing rhythm in your own approach. Be methodical. Be curious. And never ignore what doesn’t quite fit.

So whether you’re on the wards, in primary care, working under pressure in resus or in a patient’s living room at 3º’clock in the morning, keep the basics strong, the red flags sharp, and the patient at the centre.

Your stethoscope listens to the beat. Your ECG tells the full story.

You’re ready to read it, so Read away, and Lead the way… You’ve got this!