Heart Anatomy, Blood Flow and Electrical Conduction

Scope and Responsibilities of the EKG Technician

In healthcare settings, the EKG technician performs a vital clinical role centered on technical accuracy and patient safety. An EKG technician is responsible for patient preparation, accurate lead placement, and acquiring diagnostic-quality tracings. Preparing the patient properly and positioning leads with exactness ensure that the resulting record accurately captures the heart's electrical events.

During data collection, electrical interference or patient movement can distort the recording. An EKG technician must identify artifact and take appropriate corrective measures to ensure tracing clarity. The technician should identify and minimize artifact when feasible, document persistent artifact as appropriate, and never delay reporting a suspected life-threatening rhythm to obtain a cleaner tracing.

Clinical safety also requires vigilance and rapid communication. An EKG technician must immediately recognize and alert the healthcare provider to life-threatening cardiac rhythms. While the technician identifies these critical rhythms and flags emergencies, definitive medical interpretation and clinical diagnosis of an EKG tracing are solely the responsibility of the licensed provider.

Heart Wall Layers, Chambers, and Unidirectional Blood Flow

The heart is an efficient muscular organ surrounded by protective structures. The heart wall comprises three distinct tissue layers: the epicardium, myocardium, and endocardium. The endocardium is the smooth inner lining of the heart chambers and valves, consisting of endothelium and underlying connective tissue. The myocardium is the middle, muscular layer of the heart wall responsible for mechanical contraction, acting much like an engine cylinder generating driving force. The epicardium is the outer visceral layer of the serous pericardium that lies on the heart surface, while the enclosing pericardial sac contains serous fluid that reduces friction between tissue layers during cardiac motion.

Internally, the human heart consists of four chambers: the right atrium, left atrium, right ventricle, and left ventricle. Heart valves prevent the backflow of blood, maintaining forward flow throughout the cardiac cycle. The tricuspid valve regulates one-way blood flow between the right atrium and the right ventricle, whereas the mitral (bicuspid) valve regulates one-way blood flow between the left atrium and the left ventricle. Chordae tendineae and papillary muscles anchor the atrioventricular valves to prevent regurgitation during systole.

The right side of the heart manages low-oxygen blood arriving from the body. The right atrium receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus. This blood passes across the tricuspid valve into the right ventricle. The right ventricle pumps deoxygenated blood through the pulmonary valve into the pulmonary trunk, which divides into the right and left pulmonary arteries. The pulmonary artery carries deoxygenated blood to the lungs for gas exchange.

Once refreshed with oxygen, four pulmonary veins return oxygen-rich blood from the lungs to the left atrium. This blood moves through the mitral (bicuspid) valve into the left ventricle. The left ventricle pumps oxygenated blood through the aortic semilunar valve into the aorta for systemic delivery. Because it must push blood throughout the entire body against significant resistance, the myocardium of the left ventricle is thicker than that of the right ventricle due to higher systemic vascular resistance.

Coronary Circulation and Perfusion Timing

The working heart muscle requires its own dedicated blood supply to support continuous mechanical work. The right coronary artery supplies much of the right atrium and right ventricle. In individuals with right-dominant coronary anatomy, the right coronary artery also supplies the inferior wall of the left ventricle.

The left side of the coronary tree originates from a single stem before branching out. The left main coronary artery divides into the left anterior descending (LAD) and the left circumflex (LCx) arteries. The left anterior descending artery supplies the anterior wall of the left ventricle and the anterior portion of the interventricular septum. The left circumflex artery supplies the lateral wall of the left ventricle and may supply its posterior wall, depending on coronary dominance.

The timing of coronary blood flow depends on the mechanical pressure generated by the contracting chambers. Left ventricular coronary perfusion occurs primarily during ventricular diastole, when the thick ventricular myocardium relaxes and allows blood to enter the coronary vessels. In contrast, right ventricular perfusion occurs during both systole and diastole because the lower pressure in the right ventricle does not compress its vessels completely.

The Cardiac Conduction System and Pacemaker Hierarchy

The cardiac conduction system consists of specialized cells that generate and propagate electrical impulses. The sinoatrial (SA) node is located in the upper right atrium near the opening of the superior vena cava. The SA node serves as the primary, dominant pacemaker of the healthy heart. From the SA node, electrical impulses travel through the right atrial myocardium to the atrioventricular (AV) node. Simultaneously, Bachmann's bundle conducts electrical impulses from the right atrium across the atrial roof to the left atrium.

The AV node is situated in the lower right atrium near the tricuspid valve and the interatrial septum. The AV node briefly delays impulse transmission, allowing time for atrial contraction to contribute to ventricular filling. The bundle of His then delivers the electrical impulse from the AV node across the fibrous skeleton toward the ventricles.

Within the interventricular septum, the bundle of His bifurcates into the right bundle branch and the left bundle branch. The left bundle branch subdivides into an anterior fascicle and a posterior fascicle. Purkinje fibers distribute electrical impulses rapidly throughout the inner ventricular myocardium. Through this network, ventricular myocardial depolarization spreads from the endocardium outward toward the epicardium.

The conduction system operates under a strict hierarchy in which the pacemaker site with the fastest rate normally suppresses and overrides lower, slower pacemakers. The intrinsic firing rate of the sinoatrial (SA) node is 60 to 100 beats per minute. If impulses from the SA node do not reach the AV junction, the AV junction can act as a secondary escape pacemaker with an intrinsic firing rate of 40 to 60 beats per minute. If impulses from higher pacemakers fail to reach the ventricles or are too slow, an idioventricular escape rhythm can arise from the Purkinje network and ventricular pacemakers at their intrinsic firing rate of 20 to 40 beats per minute.

Cellular Electrophysiology and Surface EKG Deflections

Cardiac electrical events originate from shifts in ionic charges across cell membranes. Polarization is the resting state of a cardiac cell in which the intracellular charge is negative relative to the outside. The resting membrane potential of a ventricular myocyte is approximately negative 90 millivolts. Depolarization is an electrical event triggered by the rapid influx of positive ions that precedes mechanical contraction. Specifically, rapid sodium influx produces the fast upstroke of depolarization in atrial and ventricular contractile myocytes.

Following the initial upstroke, calcium influx during the plateau phase of the action potential drives excitation-contraction coupling in myocytes, linking electrical activity directly to muscular contraction. Repolarization is the electrical recovery process during which cardiac cells restore their resting baseline charge. Potassium ion efflux out of the cell is the predominant mechanism responsible for cardiac repolarization.

During these electrical phases, cells show varying responsiveness to new stimuli. During the absolute refractory period, cardiac cells cannot respond to any incoming electrical stimulus. During the relative refractory period, a sufficiently strong electrical impulse can cause premature depolarization.

These cellular shifts generate the waveforms recorded on a surface EKG tracing. On a surface EKG tracing, the P wave represents atrial depolarization. The PR interval extends from the start of the P wave to the start of the QRS complex and reflects conduction from the atria through the AV node to the ventricles. The QRS complex represents ventricular depolarization on the surface EKG. Atrial repolarization occurs simultaneously with ventricular depolarization and is typically obscured by the QRS complex. The ST segment corresponds mainly to the plateau phase of the ventricular action potential, when the ventricles are largely depolarized. Finally, the T wave represents the rapid phase of ventricular repolarization on the EKG.