The Systematic Read

Step 1, rate. Count it before anything else, because the rate reframes everything after it. At 25 mm/s, a large box is 0.2 s, so dividing 300 by the number of large boxes between two QRS complexes gives the ventricular rate at a glance. A rate of 38 and a rate of 190 send us looking for entirely different problems, so we fix the number first. Chapter 2 built the counting methods and the sinus range.

Step 2, rhythm. Ask where each beat comes from. Is there one P wave before every QRS, upright in II and inverted in aVR, marching at a regular interval? That is sinus. Anything else means an ectopic pacemaker, a conduction block, or a reentry circuit has taken over, and the P-to-QRS relationship tells us which. Chapter 3 walked the rhythm decision tree.

Step 3, axis. The mean QRS axis is the direction the ventricular depolarization vector points in the frontal plane. Lead I and aVF give the quadrant: both upright is normal, and a leftward or rightward shift flags hypertrophy, a hemiblock, or a shifted heart. Chapter 1 built the vector and the hexaxial reference that makes axis a lookup rather than a guess.

Step 4, intervals. Measure the PR, the QRS, and the QT. The PR times AV conduction, the QRS width times how the ventricle depolarizes, and the QT times repolarization. A PR over 0.20 s, a QRS at or above 0.12 s, or a corrected QT past 0.44 s each names a specific mechanism. Chapter 4 built the intervals and their normal bounds.

Step 5, morphology. Now, and only now, look at the shape of each wave. The first four steps have loaded the context; the fifth reads the detail into it. Morphology is where the P wave, the QRS, the ST segment, and the T wave each get inspected on their own terms, and it is worth the rest of this chapter.

The P wave is the sum of two atrial depolarization waves laid end to end: the right atrium fires first, the left atrium a moment later. In a normal atrium the two overlap so tightly that the P wave in lead II is a single smooth hump under 0.12 s wide and under 2.5 mm tall.

Enlarge one atrium and you separate the two humps in time or in size. A dilated right atrium fires with more voltage, so its early component grows: the P wave in II becomes tall and peaked, over 2.5 mm, the pattern called P pulmonale. A dilated left atrium takes longer to activate, so its late component drags out: the P wave in II widens past 0.12 s and often notches into two bumps, and V1 shows a deep, wide terminal negative deflection. That is P mitrale. The mechanism is the same in both: the shape of the P wave is a direct readout of how long and how strongly each atrium depolarizes.

The QRS carries the most information because it records the whole ventricular wavefront. Three questions sit inside it: are there pathologic Q waves, is the voltage too tall, and is the complex too wide.

A small Q wave in the lateral leads is normal septal activation. A pathologic Q wave is deeper than a quarter of the following R or wider than 0.04 s, and it marks a wall that no longer depolarizes toward the electrode: dead tissue from an old infarct is electrically silent, so the vector points away from it and the lead over that territory writes an initial downward stroke. The Q wave is a window onto muscle that is gone.

Tall voltage points the other way, toward too much muscle. When the left ventricular wall thickens, its depolarization vector grows, and the lateral leads write taller R waves while the right precordial leads write deeper S waves. The Sokolow-Lyon rule, an S in V1 plus an R in V5 or V6 summing past 35 mm, is one way to flag left ventricular hypertrophy from voltage alone.

A wide QRS, at or above 0.12 s, means the ventricles did not depolarize through the fast His-Purkinje system all the way. In bundle branch block, one bundle is out, so that ventricle is activated late and slowly by muscle-to-muscle spread from the other side. A right bundle branch block leaves the classic rSR' in V1, an M-shaped complex, because the right ventricle finishes last and its late vector points back toward V1. A left bundle branch block gives a broad, notched, all-positive R in V5 and V6 with a deep, broad S in V1, because activation now crosses from right to left. Wide QRS complexes that are not fixed block are the signature of aberrancy or a ventricular origin, which is where the rhythm question and the morphology question meet.

The ST segment sits between the end of depolarization and the start of repolarization, when the whole ventricle is depolarized together and no net current flows, so it normally sits flat on the baseline. Break that uniformity and the segment lifts or drops. Acute transmural ischemia injures a wall so that it cannot hold a normal resting potential; the injured region reads at a different voltage from its neighbors during the ST window, and the lead over it records ST elevation, the electrical fingerprint of injury. Subendocardial ischemia produces the reverse offset and gives ST depression. The same event seen from the opposite side of the heart shows up as reciprocal depression, which raises confidence that an elevation is a true injury and not a normal variant.

The T wave records repolarization, and its shape tracks how evenly the ventricle recovers. Ischemia makes recovery uneven and can flatten the T wave or flip it into symmetric inversion. Electrolytes rewrite it directly: hyperkalemia speeds the terminal phase of the action potential and pulls the T wave up into a narrow peak, while hypokalemia flattens the T wave and raises a U wave behind it. Each T-wave shape is a report on the timing and uniformity of recovery, which is why it is the last wave we read but never the one we skip.

The discipline of the fixed order is what turns squiggles into a sentence. We state the rate, then the rhythm, then the axis, then the intervals, then a wave-by-wave morphology, and by the end the reading reads like a diagnosis because it was built like one. "Rate 45, sinus with dropped beats, normal axis, PR that lengthens then a beat drops, otherwise normal morphology" is already a diagnosis rather than a description we then have to interpret. It names a Mobitz I block on its own.

The fixed order matters most when two rhythms look alike at a glance. The two strips below are both second-degree AV block with the same slow escape appearance. They separate only when we do step four and measure the PR interval beat to beat, exactly the situation the sequence is built to catch.

In the first strip the PR interval stretches from beat to beat until a QRS drops out, then the cycle resets: Mobitz I, or Wenckebach, from a decrementing AV node. In the second strip the PR interval is constant on every conducted beat, and then one drops with no warning: Mobitz II, from an all-or-none block below the node. Skip the interval step and the two look identical. Measure it and they are two different diseases with two different prognoses. That is the entire argument for reading the same order every time.

Work through a full annotated example, step by step and lead by lead, in the interactive ECG Lab, which walks an unknown tracing through all five steps to a named diagnosis.