📈 Cardiovascular System Lesson

P-QRS-T: reading the ECG waveform

The ECG is a direct readout of the conduction pathway you just learned — each wave corresponds to a specific electrical event you can now name.

Atrial depol.

AV delay

QRS

Vent. depol.

Plateau

Vent. repol.

📖 Full Breakdown

Matching each ECG segment to the electrical event producing it

Depolarization triggers contraction; repolarization resets the cell for the next beat.

P wave

Atrial depolarization

Produced as the SA node fires and the signal spreads through the atria, causing them to contract.

PR interval

AV node delay

The flat segment representing the AV node's built-in pause. Normal duration is 0.12–0.20 seconds; a prolonged PR interval signals a heart block.

QRS complex

Ventricular depolarization

Represents the ventricles contracting. Normal duration is under 0.12 seconds; a widened QRS suggests a bundle branch block, since the signal is taking a slower, less efficient route through the ventricles.

ST segment

Between depolarization and repolarization

A flat baseline segment. Elevation here is the hallmark of a STEMI (ST-elevation myocardial infarction); depression suggests ischemia or an NSTEMI.

T wave

Ventricular repolarization

Represents the ventricles electrically resetting after contraction. Peaked T waves suggest hyperkalemia; inverted T waves suggest ischemia.

🩺 Clinical / Exam Application

A patient arrives with crushing chest pain and the ECG shows ST elevation in the anterior leads. Because you know ST elevation specifically indicates a full-thickness myocardial infarction (STEMI), and you know from the coronary arteries lesson that anterior territory is supplied by the LAD, you can predict which artery is likely occluded before any imaging is done — this is exactly how emergency teams triage chest pain patients toward the cath lab within minutes.

⚠️ Exam Alert

ST elevation vs. ST depression is one of the most consequential single findings in all of cardiology — elevation usually means a complete blockage requiring emergency intervention, while depression suggests a partial blockage or ischemia without complete occlusion. Do not treat them as minor variations of the same finding.

🚧 Common Trap

Don't confuse the QRS complex (ventricular depolarization/contraction) with the T wave (ventricular repolarization/resetting) just because both involve "ventricular" in the name — one triggers the contraction, the other resets the cells afterward, and they represent opposite electrical processes.

✅ Quick Check

A patient's ECG shows a peaked T wave. What electrolyte abnormality does this suggest, and which wave represents ventricular contraction itself?

📝 Exam Prep

Common Exam Questions

❓ What does an elevated ST segment on ECG indicate?

✅ ST elevation indicates a STEMI (ST-elevation myocardial infarction) — a full-thickness heart attack requiring immediate intervention.

❓ What is the difference between what the QRS complex and the T wave represent?

✅ The QRS complex represents ventricular depolarization (contraction), while the T wave represents ventricular repolarization (the cells electrically resetting afterward) — they are opposite phases of the same electrical cycle.