A heartbeat is an electrical wave that sweeps through the heart in a fixed order. The ECG is simply a recording of that wave from the body surface.

Every heartbeat starts as an electrical impulse, and the heart has its own built-in wiring to carry it. The signal follows the same path in the same order every time: it begins in the sinoatrial (SA) node in the right atrium and spreads across the atria, then pauses briefly at the AV node so the atria can finish emptying into the ventricles. From there it travels down the bundle of His, through the right and left bundle branches, and into the Purkinje fibers, which fire both ventricles almost at once. Because that sequence is so orderly, each step leaves its own fingerprint on the ECG, a distinct wave that appears in the same order on every beat.

The SA node fires fastest, so it normally sets the pace, a rhythm we call "sinus rhythm." If it ever fails, slower backup pacemakers further down the pathway can step in to keep the heart beating.

1. SA node: the pacemakerSpontaneously fires ~60–100×/min in the right atrium. Sets the heart rate.
2. Atria depolarize → P waveThe impulse spreads across both atria, producing the P wave.
3. AV node: the gatekeeperDeliberately delays the impulse (~0.1 s) so the atria can finish emptying. This pause, with the impulse's passage through the bundle of His, makes up most of the flat PR segment.
4. His bundle & bundle branchesThe signal speeds into the ventricular septum via the right and left bundle branches.
5. Purkinje fibers → QRSFast fibers fire both ventricles almost simultaneously, the sharp QRS complex.

Electrodes are the sensors on the skin. A lead is a point of view: the voltage measured from a negative pole to a positive pole. Ten electrodes give the twelve leads of a standard ECG, and one rule explains which way each wave of depolarization points.1

Now watch the leads at work. Einthoven's triangle puts the right arm, left arm and left leg at the corners of a triangle with the heart in the middle. Each side is one lead, running from its negative electrode to its positive one. Watch a whole heartbeat pass through it, then drag the arrow to turn the heart's main direction and see what each lead draws. Switch to One wave to follow a single wave of depolarization on its own.

The augmented leads: aVR, aVL and aVF

The three sides of the triangle look at the heart from three directions, and that leaves gaps. The augmented leads fill them using the same three limb electrodes, with no extra stickers. Each one takes a single limb electrode as its positive end and compares it with the average of the other two. Averaging two electrodes makes a point halfway between them, so each augmented lead looks along a line from the middle of one side of the triangle, through the heart, to the opposite corner.

The names spell it out: a for augmented, V for voltage (or vector), and R, L or F for the right arm, left arm or foot. "Augmented" because wiring them this way draws the wave half as tall again (1.5 times) as the older method, which compared the limb with the average of all three limb electrodes.1

The six limb leads look at the heart in the frontal plane, as if you were facing the patient; the angles above are their directions in that plane, measured from lead I, with positive angles turning clockwise, toward the feet. The six chest leads look at it in the horizontal plane, like a slice across the chest. The same rule applies to every one of them: depolarization moving toward the positive end is up. Repolarization works the other way round, which is why a normal T wave usually points the same way as its QRS.1

Each part of the beat, what it is, and exactly where it sits on a real Lead II tracing. Colours match Waveform Studio; normal values follow the AHA/ACCF/HRS standardization series.34567

ECG paper uses the same grid everywhere: 1 mm small boxes inside 5 mm large boxes. What each box means depends on the paper speed and gain, which are almost always 25 mm/s and 10 mm/mV, so check the calibration pulse before you measure. Time runs left to right; voltage runs up and down.1

Now read it yourself. Click two points on the strip to drop a caliper; it reads the span in seconds across and millivolts up. Count the boxes between your points to see where the numbers come from.

In a regular rhythm, the gap between two R waves gives the rate: count the large boxes between them and divide into 300 (for more precision, count the small boxes and divide into 1500). If the rhythm is irregular, count the QRS complexes in a 6-second strip (30 large boxes) and multiply by 10.1 Tap a rate below to see exactly what it looks like on the grid.

The same checklist works on every strip. Run it in order and the diagnosis usually names itself.

1. Rate  Fast, slow, or normal?
   60–100 bpm is normal. < 60 = bradycardia, > 100 = tachycardia.
2. Regularity  Are the R–R intervals even?
   Regular, regularly-irregular, or irregularly-irregular (think AFib).
3. P waves  What is the atrial activity?
   An upright, uniform P in lead II before every QRS = sinus origin. Sawtooth flutter waves = atrial flutter. No P waves on a wavy, fibrillating baseline = AFib. No visible P, or an inverted P just before or just after each QRS = junctional.
4. PR interval  Constant and 120–200 ms?
   Applies only when P waves are present. A long or lengthening PR points to AV block; a short PR (under 120 ms) suggests pre-excitation or a P wave arising near the AV junction.
5. QRS width  Narrow or wide?
   Under 0.12 s (3 small boxes) is narrow: the impulse came from above the ventricles. 0.12 s or wider is wide: a bundle branch block or a ventricular origin.
6. P : QRS relationship  Does every P conduct?
   Dropped or dissociated beats reveal the type of block.

Now try it yourself. Work each strip through the checklist under the monitor; pick the finding that fits to move on, and the rhythm is named only at the end.

The basics in action.2789 Each card maps the rhythm back to the checklist above. See them live in the ecg_heartTrue Sim.