16.3 Conduction of Electrical Impulses

Electrical Conduction System of the Heart

The heart’s rate and rhythm are controlled by a conduction system that uses electrical impulses to cause the heart to pump blood throughout the body.

The conduction system of the heart generates and carries the impulses that initiate and regulate the heart rate. The heart rate can be measured by palpation of the pulse on an artery such as the radial artery (known as a peripheral pulse) or by auscultation on the chest at the apex of the heart (known as an apical pulse). Each beat correlates with ventricular contraction. The pacemaker of the heart is the sinoatrial (SA) node in the atrium, which possesses automaticity, or the ability to spontaneously generate an electrical impulse that initiates the heartbeat. The impulse from the sinoatrial node is conducted through a specialized pathway down through the ventricles and eventually reaches the cardiac muscle cells, or cardiac myocytes, to trigger coordinated contraction of the heart chambers at their respective times in the cardiac cycle.

The origination of the impulse from the sinoatrial node in the atrium and the pathway of conduction is what leads to a normal heart rate (60–100 beats per minute) and rhythm, which is called normal sinus rhythm.

Impulse generation and conduction work through depolarization of the cells in the conduction system. Depolarization is the cell membrane potential increasing or becoming more positive as compared to its surroundings. The cells of the cardiac conduction system maintain a negative resting voltage (i.e., ionic charge), also known as membrane potential. An action potential describes rapid depolarization of the cell, followed by repolarization (the cell membrane potential decreasing back to the resting voltage).

The mechanisms for depolarization and repolarization vary, but in general, they rely on electrolytes or electrically charged ions. Positively charged ions such as sodium (Na⁺), potassium (K⁺), or calcium (Ca²⁺) entering the cell make it less negative and mediate repolarization. Ions or substances entering the cell is referred to as influx. Positively charged ions such as Na⁺, K⁺, or Ca²⁺ leaving the cell make it more negative and mediate repolarization. Ions or substances leaving the cell is referred to as efflux. Given how central electrolytes are to cardiac function and the electrical conduction system of the heart, it is important to monitor clients’ blood levels to ensure they are sufficient. Hypokalemia, or a low potassium blood level, is a risk factor for dysrhythmias and should be avoided.

Conductivity

The cells of the cardiac conduction system receive the spontaneous impulses from the sinoatrial node and conduct, or carry, the signal through the atrioventricular node and then the bundle of His, which then splits into the left and right bundle branches. Each bundle branch gives rise to Purkinje fibers, which conduct the impulse into the cardiac muscle to facilitate contraction. Depolarization spreads from cell to cell via gap junctions, which are membranous connections between the cells. Figure 16.5 depicts the structures of the cardiac conduction system.

Conductivity is mediated by action potentials. The resting membrane potential is negative. During conduction, rapid depolarization is mediated by sodium influx. The cell remains depolarized for a time period while calcium influx (through L-type calcium channels) and potassium efflux are relatively balanced, and then repolarization occurs via potassium efflux. A refractory period follows depolarization when the sodium channels that usually mediate depolarization are inactive. During the refractory period, the cell cannot be depolarized until the refractory (or inactive) period is over.

Figure 16.5 This image of the heart shows the conduction tissue and pathway. Specialized conducting components of the heart include the sinoatrial node, the internodal pathways, the atrioventricular node, the atrioventricular bundle, the right and left bundle branches, and the Purkinje fibers. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Electrocardiography

Electrocardiography (EKG or ECG) is a common diagnostic tool that allows health care professionals to monitor various aspects of the client’s heart including rate, rhythm, and the presence of ischemia. It is also used for monitoring medications. In a typical ECG, 12 leads are placed on the client’s chest and limbs in a specific orientation that allows them to detect the electrical activity of the cardiac conduction system. The client’s heart rate and rhythm are recorded visually through waves and intervals, which represent various aspects of conduction through the heart. Figure 16.6 shows an example of an electrocardiogram and its components.

Waveforms

When the cardiac electrical impulse is conducted in the direction of a lead, it creates an upward deflection; when traveling away, it causes a downward deflection. These are called waveforms. A segment on an ECG is the space between two waves and does not include a waveform. An interval, on the other hand, includes the space between two waves and a waveform. The final ECG represents the summation of the impulses from all leads. This creates a pattern of waves for each cardiac cycle (heartbeat) that repeats for the duration of monitoring.

In a healthy heart, each cardiac cycle consists of a P wave, followed by a QRS complex (a combination of a Q wave, R wave, and S wave), and lastly a T wave. The P wave represents atrial depolarization, which is when the electrical impulse travels from the sinoatrial node through the atria to cause atrial contraction. After the P wave, the tracing comes back to baseline as the signal is transmitted through the AV node. The QRS complex represents ventricular depolarization, which is the electrical conduction signal that causes ventricular contraction or ventricular systole. Finally, the T wave represents ventricular repolarization. (Atrial repolarization is not visualized.) The interval from the beginning of the QRS complex to the end of the T wave is called the QT interval. Segments are the regions between two waves.

Figure 16.6 A normal electrocardiogram tracing shows the P wave, QRS complex, and T wave. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)