The ECG trace is a dynamic map of how electricity flows through the heart’s muscle tissue. Whether a waveform points up (positive) or down (negative) depends on whether the electrical activity is moving towards or away from the lead in question. Think of it like standing at the end of a tunnel: if a train (current) comes toward you, the headlights shine bright (positive deflection); if it moves away, it fades into the distance (negative deflection).

Let’s walk through the cardiac cycle:

P Wave – Atrial Depolarisation

The P wave represents the electrical spark from the sinoatrial (SA) node; the heart’s natural pacemaker, travelling across both atria. The impulse moves from top to bottom and right to left, which is toward leads such as II, causing a small positive deflection.

This is like flicking on the hallway lights before entering a room. You’re preparing the chambers for action.

PR Interval – A Pause for Ventricular Prep

After the atria contract, the impulse slows briefly at the atrioventricular (AV) node, which gives the ventricles time to fill with blood. This delay forms the flat PR segment, sitting on the baseline. No muscle depolarisation happens here, it’s an electrical pause.

Meanwhile, the current races down the Bundle of His, splits into the right and left bundle branches, and travels rapidly through the Purkinje fibres like wiring across a scaffold, reaching the far corners of the ventricles in milliseconds.

QRS Complex – Ventricular Depolarisation

Now comes the action.

Q wave: A small initial downward deflection, the septum depolarises left to right, away from lead II.

R wave: The main upward spike. The large left ventricle depolarises toward the recording lead, causing a big positive deflection.

S wave: A small downward deflection as the final parts of the ventricle depolarise away from the lead.
Altogether, this is the ventricles saying “Go!”, firing together like two pistons to generate contraction.

ST Segment – The Silent Phase

After contraction, there’s a pause while the ventricles remain fully depolarised. This is the ST segment, again resting on the baseline. No net electrical movement means no deflection. It’s electrically silent, like holding your breath between two waves.

T Wave – Ventricular Repolarisation

Finally, the ventricles repolarise, reset and recharge. Strangely, the current flows opposite to depolarisation, but because it moves away from the lead, and it’s a reverse charge, the T wave ends up positive in most leads. It’s like running backwards up a hill and still arriving at the top.

Together, the P-QRS-T complex shows the heart’s electrical choreography. If the pattern or polarity shifts, something’s disrupted, from electrolyte imbalance to infarction.