EKG Waveforms – PQRST Complex, Intervals, and 5-Lead Placement

The electrocardiogram (ECG or EKG) is one of the most important diagnostic tools in cardiology. It records the electrical activity of the heart, giving clinicians insights into heart rhythm, conduction abnormalities, and cardiac diseases.

To interpret ECGs effectively, students and healthcare professionals must understand the PQRST waveforms, intervals and segments, and lead placements. This article breaks down these essentials in a clear, exam-friendly, and clinically relevant way.

The PQRST Complex

The PQRST complex represents the electrical events of a single cardiac cycle.

P Wave

Represents atrial depolarization (atrial contraction).

Normal duration: ≤ 0.12 seconds (3 small ECG squares).

Abnormal findings:

• Tall P wave → Right atrial enlargement (P pulmonale).
• Wide/Notched P wave → Left atrial enlargement (P mitrale).

QRS Complex

Represents ventricular depolarization (ventricular contraction).

Normal duration: 0.06 – 0.10 seconds.

Abnormal findings:

• Wide QRS (>0.12s) → Bundle branch block or ventricular rhythm.
• Tall QRS → Left ventricular hypertrophy.

T Wave

Represents ventricular repolarization (ventricular relaxation and filling).

Normally upright in most leads.

Abnormal findings:

• Inverted T wave → Ischemia.
• Tall peaked T wave → Hyperkalemia.

Key Intervals and Segments

PR Interval

Represents time taken for electrical activity to travel from atria to ventricles.

Normal: 0.12 – 0.20 seconds.

Abnormal:

• Prolonged (>0.20s) → First-degree AV block.
• Short PR (<0.12s) → Wolff-Parkinson-White (WPW) syndrome.

ST Segment

Time between ventricular depolarization and repolarization (plateau phase).

Should be isoelectric (flat).

Abnormal:

• ST elevation → Myocardial infarction (STEMI).
• ST depression → Ischemia or digoxin effect.

QT Interval

Represents total time for ventricular depolarization and repolarization.

Normal: 0.36 – 0.44 seconds (rate dependent).

Abnormal:

• Prolonged QT → Risk of torsades de pointes, sudden cardiac death.
• Short QT → Hypercalcemia.

Clinical Importance of PQRST and Intervals

Understanding these waveforms helps in diagnosing:

• Arrhythmias (atrial fibrillation, ventricular tachycardia).
• Myocardial infarction (ST elevation/depression, Q waves).
• Electrolyte abnormalities (K⁺, Ca²⁺, Mg²⁺ imbalances).
• Conduction blocks (AV block, bundle branch block).

5-Lead ECG Placement

In clinical practice, a 5-lead ECG is often used for continuous cardiac monitoring in ICUs and emergency settings.

Mnemonic for placement:

White on Right → RA (Right Arm) electrode.

Smoke over Fire → LA (Left Arm) = Black (smoke), LL (Left Leg) = Red (fire).

Chocolate in my Heart → V (Brown) chest lead, placed near sternum (V1 or modified V5 position).

Green goes Last → RL (Right Leg) electrode, grounding lead.

Placement Breakdown

1. RA (White) – Right upper chest below clavicle.

2. LA (Black) – Left upper chest below clavicle.

3. RL (Green) – Right lower abdomen.

4. LL (Red) – Left lower abdomen.

5. V (Brown) – Precordial position (commonly V1 or modified).

Practical Applications of 5-Lead ECG

• Used in ICU monitoring, ambulances, and post-operative cardiac care.
• Detects arrhythmias, ischemic changes, and conduction abnormalities.
• Easier and faster than a full 12-lead ECG, but less comprehensive.

Table: PQRST Interpretation Summary

Wave/Interval	Represents	Normal Value	Abnormal Findings
P Wave	Atrial depolarization	≤ 0.12s	Tall P = RAE, Notched P = LAE
PR Interval	Atrial to ventricular conduction	0.12–0.20s	Prolonged = AV block, Short = WPW
QRS Complex	Ventricular depolarization	0.06–0.10s	Wide = BBB, Tall = LVH
ST Segment	Ventricular plateau	Isoelectric	Elevation = MI, Depression = Ischemia
T Wave	Ventricular repolarization	Upright	Inverted = Ischemia, Peaked = Hyperkalemia
QT Interval	Total ventricular activity	0.36–0.44s	Long = Torsades, Short = Hypercalcemia

Frequently Asked Questions (FAQ)

Q1. What does the P wave represent in ECG?

The P wave corresponds to atrial depolarization (contraction).

Q2. What is a normal PR interval?

A normal PR interval ranges from 0.12 to 0.20 seconds.

Q3. What is the difference between QRS and T waves?

The QRS represents ventricular depolarization (contraction), while the T wave represents ventricular repolarization (relaxation).

Q4. What does ST elevation mean?

It usually indicates an acute myocardial infarction (STEMI).

Q5. What is prolonged QT syndrome?

It is an abnormal condition where the QT interval is lengthened, predisposing to dangerous arrhythmias like torsades de pointes.

Q6. What is the role of the 5th lead in a 5-lead ECG?

It provides additional monitoring of precordial activity (usually V1 or V5), improving arrhythmia and ischemia detection.