The Electrocardiogram

Flashcards about the Electrocardiogram and Cardiac Conduction System

What percentage of cardiac muscle cells are pacemaker cells?

Approximately 1%

What is the main function of pacemaker cells?

To rhythmically and spontaneously depolarize and have action potentials, triggering contractile cells.

Name the three groups of pacemaker cells in the heart.

Sinoatrial node (SA node), atrioventricular node (AV node), and Purkinje fiber system.

What is the location and typical depolarization rate of the SA node?

Superior portion of the right atrium; about 60 times per minute.

Where is the AV node located and what is its depolarization rate?

Posterior and medial to the tricuspid valve; about 40 times per minute.

Describe the pathway of the Purkinje fiber system.

Begins with the AV bundle, splits into right and left bundle branches, then into terminal branches that contact ventricular contractile cells.

What is the cardiac conduction system?

The 'hardwiring' of the heart that allows rapid spread of depolarizations from pacemaker cells to other pacemaker and contractile cells.

What causes AV node delay?

Slowing of conduction at the AV node, allowing the atria to depolarize and contract before the ventricles.

What happens if the AV node is blocked?

The atria may be paced by the SA node and the ventricles by the AV node or Purkinje fiber system, leading to different heart rates and rhythms.

What does the ECG record?

Changes in the electrical activity of cardiac muscle cells over time.

What does a flat line on the ECG indicate?

No net change in electrical activity, although cells may still be in some phase of an action potential.

Why is pacemaker cell activity not usually detectable on a standard ECG?

There are too few pacemaker cells in the heart.

What does the P wave represent?

Depolarization of the cells of the right and left atria.

What does the QRS complex represent?

Depolarization of the right and left ventricles.

What does the T wave represent?

Repolarization of the right and left ventricles.

What is the P-P interval?

The period of time between two P waves.

What is the R-R interval?

The period of time between two R waves, representing the duration of the spread of an action potential through the heart.

What is the P-R interval, and what does it include?

Period from the beginning of the P wave to the beginning of the QRS complex; includes the AV node delay.

What is the Q-T interval?

Time from the beginning of the Q wave to the end of the T wave, during which ventricular cells are depolarizing and repolarizing.

What is the S-T segment, and what occurs during this time?

The segment between the end of the S wave and the beginning of the T wave; corresponds to the ventricles' plateau phase.

What is a normal sinus rhythm?

When the SA node is pacing the heart at 60-100 beats per minute.

What is a dysrhythmia (or arrhythmia)?

Any deviation from the normal sinus rhythm.

At what speed does an electrocardiograph record the tracing?

25 mm/second

How is heart rate calculated based on an ECG tracing?

Count the number of large boxes between R waves, and divide 300 by this number.