ECG waves P, QRS, T: why isn't atrial repolarisation visible? And heart attack vs cardiac arrest vs heart failure
Lakshmi TripathiDropper Sample
14 Oct 2026 · 1 min read
I usually don't post questions, but this chapter has defeated my reading-only method.
ECG: P wave, QRS complex, T wave. I know the NCERT lines: the P wave represents the electrical excitation (depolarisation) of the atria, which leads to contraction of both the atria. The QRS complex represents the depolarisation of the ventricles, which initiates ventricular contraction. The T wave represents the return of the ventricles from excited to normal state (repolarisation), and the end of the T wave marks the end of systole.
The thing I can't work out is: if depolarisation of atria gives a P wave, where is the wave for repolarisation of the atria? I have never seen it on the diagram. Is it missing?
Also the disorders. I use "heart attack", "cardiac arrest" and "heart failure" as though they are the same thing and I think they are not (puriyala, I'm confused). Plus hypertension values, angina, and coronary artery disease. Which are the key differences I can write in two lines each?
Last, how is heart rate read off an ECG?
6 answers & replies
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[+][–]Zoya Khan12thSample· 5d ago
The atrial repolarisation does happen, it's simply hidden. It occurs at about the same time as the ventricular depolarisation, and the QRS complex is much larger (ventricles have far more muscle mass), so the small atrial repolarisation signal is lost inside the QRS. So it's not missing, it is masked.
Heart rate: count the number of QRS complexes that appear in a given time (say one minute) and you have the heart rate. In the cardiac cycle of about 0.8 seconds that's about 72 beats per minute.
[+][–]Diya Menon11thSample· 5d ago
ahh the T wave is the atrial repolarisation! That's why there's no separate wave after P, the T wave is it. P = atria depolarise, T = atria repolarise, QRS = ventricles. Super neat.
[+][–]Nisha Verma12thSample· 5d ago
Diya, T is the ventricular repolarisation, not atrial. Atria's repolarisation is hidden in the QRS (as Zoya said). The three waves aren't atria-then-ventricle-then-atria. They go: P (atria depolarise) then QRS (ventricles depolarise) then T (ventricles repolarise). And the end of the T wave marks the end of ventricular systole.
[+][–]Lakshmi TripathiDropperSample· 5d agoAuthor
Now I see: three waves, only ventricles get both the big waves, and the atrial recovery is hidden. Heart attack vs cardiac arrest vs failure also sorted, thank you everyone. Nandri 🙏
[+][–]Pooja Iyer12thSample· 5d ago
Disorders!!
- Hypertension: blood pressure higher than normal. Normal is 120/80 (systolic/diastolic), and 140/90 or higher is hypertension.
- Coronary artery disease: the vessels supplying blood to the heart muscle get affected by deposits of calcium, fat, cholesterol and fibrous tissue (atherosclerosis), narrowing the lumen.
- Angina pectoris: acute chest pain when not enough oxygen reaches the heart muscle.
- Heart failure: the heart isn't pumping blood effectively enough to meet the body's needs (sometimes called congestive heart failure because lungs get congested).
- Cardiac arrest: the heart stops beating.
- Heart attack: heart muscle is suddenly damaged by an inadequate blood supply.
[+][–]Kavya NaikDropperSample· 5d ago
A bonus for the regulation part since it's in the same topic: the heart is myogenic (the SAN generates the impulse, so it's the pacemaker), but a neural centre in the medulla can change the rate through the autonomic nervous system. Sympathetic nerves increase the rate of heartbeat and the strength of ventricular contraction (so more cardiac output). Parasympathetic nerves decrease the rate and the speed of conduction. Adrenal medullary hormones also increase cardiac output. Cardiac output = stroke volume x heart rate, about 70 mL x 72 = roughly 5000 mL per minute.
