Cardiac Anatomy, Physiology, and Common Conditions
About 18% of the exam tests the anatomy and physiology behind the tracing. This is the smallest domain but the one that makes every other domain make sense: chest lead placement follows the position of the heart in the chest, escape rhythms follow the inherent rates of the conduction system, and the ischemic changes on a 12-lead follow the coronary anatomy. This chapter covers the chambers and valves, the path of blood, the conduction system and its intrinsic rates, the mechanics of the cardiac cycle, and the common cardiac conditions a technician will see referenced on requisitions every day.
Chambers, Valves, and the Path of Blood
The heart is a four-chambered pump enclosed in the pericardial sac. Its wall has three layers: the endocardium lining the chambers, the myocardium that does the contracting, and the epicardium on the outer surface. The right side is a low-pressure pump that sends blood a short distance to the lungs; the left side is a high-pressure pump that supplies the entire body, which is why the left ventricle wall is roughly three times thicker even though both ventricles eject about the same volume each beat. Deoxygenated blood returns from the body through the superior and inferior venae cavae into the right atrium, crosses the tricuspid valve into the right ventricle, and exits through the pulmonic valve to the lungs. Oxygenated blood returns through the pulmonary veins into the left atrium, crosses the mitral valve into the left ventricle, and leaves through the aortic valve to the systemic circulation.
The Electrical Conduction System and Its Intrinsic Rates
The impulse begins in the sinoatrial node in the upper right atrium, spreads across both atria to produce the P wave, and arrives at the atrioventricular node. There it is deliberately delayed about a tenth of a second, giving the atria time to finish contracting and top off ventricular filling; that delay is the flat segment inside the PR interval. The impulse then travels through the bundle of His, divides into the right and left bundle branches, and finishes in the Purkinje fibers, which spread it rapidly through the ventricular muscle to produce the QRS. Each level of this system can pace the heart on its own, and each has a slower inherent rate than the one above it, which is exactly why an escape rhythm's rate tells you where it originated.
Depolarization, Repolarization, and the Cardiac Cycle
Depolarization is the electrical discharge across the cell membrane that stimulates contraction; repolarization is the recovery back to the resting state. The EKG records only these electrical events, which is why an organized rhythm can appear on the screen while the heart produces no pulse at all, a condition called pulseless electrical activity. That single fact is the reason a technician always confirms the patient rather than treating the monitor. Mechanically, the cycle alternates between systole, when the ventricles contract and eject blood, and diastole, when they relax and fill. Cardiac output is the product of heart rate and stroke volume, normally about 4 to 8 liters per minute in an adult, so both an extremely slow and an extremely fast rate can reduce output, the slow rate by too few beats and the fast rate by cutting filling time short.
Common Cardiac Conditions and Their EKG Signs
Technicians are not permitted to diagnose, but recognizing the tracings that require urgent attention is part of the job. Coronary artery disease narrows the vessels that feed the myocardium. When the narrowing produces temporary oxygen shortage during exertion, the result is angina, typically relieved within minutes by rest or nitroglycerin and often accompanied by transient ST depression or T wave inversion. When a vessel occludes completely and muscle begins to die, the result is a myocardial infarction, classically showing ST segment elevation in a group of anatomically related leads and later pathologic Q waves. Congestive heart failure is a pumping problem rather than a conduction problem: the weakened ventricle cannot move the volume delivered to it, so fluid backs up into the lungs on the left side and into the peripheral tissues on the right. Chronic hypertension forces the left ventricle to work against high resistance until the muscle thickens, producing the increased voltage of left ventricular hypertrophy.
Last updated: July 2026