ECG/EKG Interpretation: Heart Electrical Activity and Arrhythmias

ECG/EKG

The electrical energy of the heart, recorded as time (horizontal) and amplitude (vertical).

3-lead pads placement

White: right side below clavicle ("white to right"); Black: left side below clavicle; Red: below chest muscle ("smoke over fire").

Green lead in 4-lead ECG

Ground lead, placed below the right.

12-lead ECG

A 360° view of the heart.

Einthoven's Triangle

Orientation of frontal plane leads: Lead I (right to left), Leads II & III (superior to inferior).

Normal electrical axis range

-30° to +90° (normal conduction travels upper right to lower left).

P wave

Atrial depolarization.

QRS complex

Ventricular depolarization.

ST-T wave

Ventricular repolarization.

Electrolytes driving ECG signals

Na+, K+, Ca++, Cl-.

One small ECG box

0.04 seconds.

One large ECG box

0.20 seconds (5 small boxes).

Calculate HR from a strip

Count R peaks in a 6-second strip and multiply ×10.

Inherent rates

SA: 60-100 bpm; AV: 40-60 bpm; Ventricles: 20-40 bpm.

Normal PR interval range

0.12-0.20 seconds.

Normal QRS duration

0.04-0.12 seconds.

Steps to reading a rhythm

Rate (60-100), regularity, complex width, presence of P wave, complexes look the same.

Normal sinus rhythm

Rate ~60 bpm, regular, narrow QRS, P waves present, uniform complexes.

Sinus bradycardia

Regular rhythm, P wave present, rate <60 bpm.

Sinus tachycardia

Regular rhythm, P wave present, rate >100 bpm.

Arrhythmia/dysrhythmia

Rhythm not regular, does not "march out."

Atrial fibrillation hallmark

No P wave, fibrillating baseline, irregular QRS.

Atrial flutter hallmark

"Sawtooth" baseline, no atrial kick, irregular P wave activity.

Hyperkalemia ECG change

Peaked ("tented") T waves.

Ventricular fibrillation hallmark

No organized QRS or P waves, chaotic activity.

Asystole hallmark

Flatline, no rhythm or electrical activity.

Multifocal PVCs hallmark

QRS complexes look different (arising from different foci).

Unifocal PVCs hallmark

QRS complexes look the same (from one focus).

First-degree heart block

Slowed conduction, prolonged PR interval.

Second-degree block (Mobitz I)

PR interval lengthens until a beat drops (Wenckebach).

Second-degree block (Mobitz II)

Dropped QRS without PR lengthening.

Third-degree block

Complete block, atria and ventricles beat independently.