Electrical Activity of the Heart

Cardiac cycle

The pattern of physiological events during each beat of the heart.

Systole

The contraction of the atria or ventricles.

Diastole

The relaxation of the atria or ventricles.

Action potentials

Electrical events where the polarity across the cell membrane changes.

Electrocardiography

The technique used to measure the electrical activity of the heart.

Electrocardiograph

The recording voltmeter that produces the ECG.

ECG (electrocardiogram)

The waveform graph produced by a recording voltmeter.

P wave

This wave represents depolarization of the atrial walls, which precedes atrial systole.

QRS complex

This set of waves results from the more or less concurrent ventricular depolarization and atrial repolarization

T wave

This wave represents ventricular repolarization, which precedes ventricular diastole.

Sinoatrial (SA) node

• Specialized section of cardiac muscle in the upper lateral wall of the right atrium

• Generates its own action potentials faster than surrounding muscle

• Acts as the pacemaker of the heart

Atrioventricular (AV) node

• Pacemaker in the lower right atrium, subordinate to the SA node

• Generates action potentials in response to signals from the SA node through the atrial walls

AV bundle (of His)

• Bundle of cardiac muscles in the interventricular septum specialized to conduct action potentials rapidly

• Branches into subendocardial fibers (Purkinje fibers) through the lateral ventricular walls

PR/PQ interval

The time required for a signal to pass from the SA node, through the atrial myocardium (depolarizing it), to the AV node, and through its branches.

QT interval

Represents the time needed for complete depolarization and recovery of the ventricular myocardium.

Einthoven

Proposed three basic arrangements, or leads, of the ECG electrodes.

Lead I

One electrode on the left arm, one on the right arm.

Lead 2

One electrode on the right arm, one on the left leg.

Lead 3

One electrode on the left arm, one on the left leg.

Einthoven triangle

Formed by the appendicular leads.

Tachycardia

Condition of a heart rate greater than 100 beats/min; may be caused by excitement (stress), high body temperature, or toxicity.

Bradycardia

Condition of a heart rate less than 60 beats/min; normal in conditioned athletes or may result from excessive vagal stimulation.

First-degree heart block

PR interval greater than 0.2 seconds; may result from inflammation of the AV bundle, slowing conduction to the ventricles.

Second-degree heart block

PR interval 0.25–0.45 seconds with some P waves not followed by QRS complexes; may result from AV node damage or excessive vagal stimulation.

Complete heart block

P wave independent of QRS complex, P rate ~100 per minute, QRS <40 per minute; indicates full dissociation between atrial and ventricular activity.

Augmented leads

Leads produced by combining two negative electrodes and measuring them against one positive electrode; not bipolar leads or separate electrode placements.

aVr

The LA and II electrodes are combined to form the negative electrode; RA is the positive electrode.

aVl

The RA and II electrodes form the negative electrode; LA is the positive electrode.

aVf

The RA and LA electrodes form the negative pole; LL is the positive pole

Chest (precordial) leads

Use RA, LA, and LL combined to form the negative pole and each of six chest electrodes to form the positive poles; named V1 through V6.