In a normal ECG wave form, which wave has the maximum amplitude ?
R-wave
The R wave is the tall spike of the QRS complex and dominates the trace — option 2.
| Wave | Typical amplitude | What the heart is doing |
|---|---|---|
| P | 0.1 – 0.25 mV | Atrial depolarisation |
| Q | Small, negative | Septal depolarisation |
| R | 1 – 2 mV | Ventricular depolarisation |
| S | Small, negative | Late ventricular depolarisation |
| T | 0.1 – 0.5 mV | Ventricular repolarisation |
Why the R wave is largest. Amplitude on the surface reflects how much muscle depolarises at once. The ventricles are the thick-walled pumping chambers, with far more muscle mass than the atria, and their conduction system spreads the impulse through them almost simultaneously. A large mass depolarising in a short interval produces a large, brief current dipole — hence a tall, narrow deflection. The atria are thin-walled and small, so the P wave is correspondingly modest, and repolarisation is a slower and less synchronised process, which spreads the T wave out and lowers its peak.
What this means for the instrument. The ECG amplifier must resolve a P wave of a tenth of a millivolt while accommodating an R wave twenty times larger without clipping — a demanding dynamic range for a signal of this size. Two further difficulties follow from the same smallness:
Common-mode interference. Mains hum appears on both electrodes at perhaps a volt — thousands of times the signal — so a differential amplifier with a CMRR above 100 dB is essential, together with a driven right-leg circuit.
Electrode offset. The half-cell potential at the skin-electrode interface can reach hundreds of millivolts of DC, far larger than the ECG itself, so the amplifier must be AC coupled below about 0.05 Hz — low enough not to distort the slow ST segment.
The R wave's prominence has a practical use : it is the feature that heart-rate monitors and pacemaker sensing circuits detect, precisely because its height and sharpness make it easy to pick out of noise by threshold or derivative detection.
Hence, the wave of maximum amplitude is the R-wave.
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