Cardiac Conduction System

SA node

the primary pacemaker of the heart, initiates electrical impulses at an inherent rate of 60 to 100 beats per minute (bpm). Note, this is the same as a normal pulse rate because the SA node is pacing the heart. These impulses move across the atria via internodal pathways causing the atria to contract and then move onto the AV node that is located low in the atrial septum.

AV node

a slight delay occurs, which synchronizes atrial and ventricular activity and permits adequate time for the ventricles to fill. If the SA node fails in its pacemaker role, the AV node is capable of assuming this function by discharging electrical impulses at an inherent rate of 40 to 60 bpm. If this occurs, the patient’s heart rate is considered bradycardic (less than 60 bpm).

Purkinje fibers

the point at which the ventricles are stimulated to contract (ventricular systole). The conduction tissue in the ventricles has an inherent rate of 30 to 40 bpm, which will assume the pacemaker role should the SA and AV nodes fail to provide electrical impulses.

vagus nerve

slow the heart rate

beta₁ receptor sites

increase heart rate

Holter monitor

Ambulatory monitoring may also be used to monitor patients who have hypertrophic cardiomyopathy, or in patients with variant angina. The patient wears a portable recording device that is connected to the chest by electrodes and lead wires. The patient keeps a diary while wearing this device, noting the type and time of symptoms or performance of unusual activities.

loop monitor or an implantable loop recorder

categorized as intermittent recorders and selected when patients have infrequent symptoms. The monitoring period may be as short as 24 to 48 hours or as long as several months.

small block

equals 0.04

P wave

represents the electrical impulse starting in the sinus node and spreading through the atria (atrial depolarization leading to atrial contraction).

PR interval

measured from the beginning of the P wave to the beginning of the QRS complex and represents the time needed for sinus node stimulation, atrial depolarization, and conduction through the AV node before ventricular depolarization. In adults, the normal range for the PR is 0.12 to 0.20 second.

QRS complex

represents ventricular depolarization.

normally less than 0.10 second in duration

T wave

represents ventricular repolarization or electrical recovery.

ST segment

represents early ventricular repolarization

analyzed to identify whether it is above or below the isoelectric line, which may be, among other signs and symptoms, a sign of cardiac ischemia.