| Aberrancy | A waveform or complex that deviates from the normal or expected appearance indicating that its conduction pathway is abnormal.
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| Absolute refractory period | The phase during repolarization during which no stimulus, no matter how strong will depolarize the cell.
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| Acetylcholine | The chemical compound used to transmit impulses across the nerve synapse, the major neurotransmitter for the parasympathetic (cholinergic) nervous system.
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| Action potential | The measure over time of the change in electrical charge of the cell. The action potential reflects the ability of the cell to depolarize.
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| Actin | One of the two components of the contractile filaments involved in the muscular contraction.
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| Accessory pathway | A conduction pathway from the sinoatrial (SA) node to the ventricles that does not pass through the normal atrioventricular (AV) node pathway.
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| Acute | Sudden and recent onset of a condition, sign, or symptom.
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| Acute coronary syndrome | The condition of ischemic chest pain consisting of the following conditions: unstable angina, non-ST segment elevation myocardial infarction, and ST segment elevation myocardial infarction.
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| Adrenalin (epinephrine) | A natural catecholamine that causes bronchodilation, increased atrioventricular (AV) conduction, muscular contractility, heart rate, and vasoconstriction.
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| Adrenergic | Nerves that release epinephrine or epinephrine-like substances.
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| Afterload | The pressure against which the left ventricle must pump blood during contraction.
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| Alpha adrenergic receptors | Any of the adrenergic receptor tissues that respond to norepinephrine.
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| Amplitude | The height and depth, or waveform size, of an ECG complex.
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| Anemia | A deficiency in the oxygen-carrying hemoglobin molecule contained in blood.
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| Aneurysm | A pathological blood-filled dilation of a blood vessel.
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| Angina | See angina pectoris.
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| Angina pectoris | Chest pain resulting when the supply of oxygen and coronary perfusion is insufficient to that demanded by the heart muscle.
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| Antegrade | Occurring or performed in a direction of the normal or forward direction of conduction or flow.
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| Artery | A vessel that carries blood away from the heart.
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| Artifact | An abnormality of the ECG arising from electrical interference unrelated to the electrical activity of the heart.
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| Aorta | The major vessel that carries oxygenated blood from the heart to the body.
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| Aortic valve | The valve that separates the left ventricle from the aorta.
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| Apex | Inferior end of the heart that tapers to a blunt, rounded point.
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| Arrhythmia | Technically speaking means the absence of a rhythm but is used synonymously with the term dysrhythmia.
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| Asystole | Absence of cardiac electrical activity.
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| Atherosclerosis | An accumulation of fat-containing deposits within the arterial wall.
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| Atria | The chamber of the heart that collects blood returning from the rest of the body.
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| Atrial kick | The additional volume of blood pushed into the ventricle by the contraction of the atria.
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| Atrioventricular dissociation (AV dissociation) | Any rhythm characterized by the atria and ventricles beating independently such as that seen in 3rd-degree AV heart block.
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| Atrioventricular junction (AV junction) | The area of conductive tissue that includes the AV node, its atrial pathways, and the bundle of His.
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| Atrioventricular node (AV node) | Specialized tissue at the base of the wall between the two upper heart chambers (atria).
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| Augmented limb leads | The limb leads aVR, aVL, and aVF. The ECG waveforms produced by these leads is so small that the ECG machine enhances, or augments, them by 50% so their amplitude is comparable to other leads.
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| Automaticity | The ability of cardiac cells to initiate spontaneous electrical impulses.
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| Autonomic nervous system | The involuntary portion of the peripheral nervous system that regulates vital body functions. It is separated into the parasympathetic and sympathetic divisions. This system controls cardiac, smooth muscle and glandular activity.
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| Axis | The sum of all the vectors of electrical activity associated with conduction of the action potential through the heart.
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| Axis deviation | An abnormal shift of the normal axis of the cardiac muscle action potential often associated with ventricular hypertrophy and certain conduction defects.
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| Baroreceptors | Pressure sensitive nerve endings in the walls of the atria of the heart and in some larger blood vessels such as the carotid artery.
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| Beta receptors | Adrenergic receptors of the nervous system that respond to adrenaline. Activation causes relaxation of smooth muscles and increases in cardiac rate and contractility.
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| Bicuspid valve | See mitral valve.
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| Bigeminy | Cardiac electrical activity characterized by a pattern of one ectopic beat followed by one normal beat. These ectopic beats can be either atrial or ventricular in origin and indicate cardiac irritability.
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| Bipolar limb lead | An ECG lead with both a positive and negative electrode. Leads I, II, and III are bipolar limb leads.
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| Biphasic | Having two phases, one negative and one positive.
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| Bradycardia | A heart rate below 60 beats per minute, when related to atrial or sinus rhythms.
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| Bundle branch | Either of the parts of the bundle of His passing respectively to the right and left ventricles.
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| Bundle of His | Specialized muscle fibers in the intraventricular septum that carry the electric impulses to the ventricles.
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| Burst | See Salvo.
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| Calibration | As it relates to the ECG machine it involves the internal regulation of the device such that 1 mV electrical signal results in a 10 mm deflection.
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| Catecholamine | Sympathetic nervous system neurotransmitter such as epinephrine, norepinephrine, and dopamine.
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| Cardiac tamponade | See pericardial tamponade.
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| Cardiac output | The volume of blood pumped by the heart, usually measured in milliliters per minute. Calculated by multiplying heart rate and stroke volume.
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| Cardiomyopathy | Any structural or functional disease of heart muscle that is marked especially by hypertrophy of cardiac muscle, by enlargement of the heart, by rigidity and loss of flexibility of the heart walls, or by narrowing of the ventricles.
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| Chemoreceptor | A sensory nerve cell that is activated by chemicals such as the carbon dioxide detectors in the brainstem.
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| Cholinergic | Nerve fibers that release acetylcholine.
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| Chordae tendineae | Thin, strong strings of connective tissue that anchor the papillary muscles to the floor of the heart.
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| Chronotropic Affecting | The rate of the heartbeat. A positive chronotropic effect increases the heart rate while a negative chronotropic effect decreases the heart rate.
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| Circadian rhythm | The biological clock in humans based on a 24-hour cycle. At regular intervals each day, the body becomes active or tired. Some medications affect the body more at certain times during the day than others.
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| Compensatory pause | The pause following a premature complex that allows the original rhythm to begin again at its normal rate.
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| Contractility | Ability of the cardiac muscle to contract in response to electrical stimulation.
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| Conductivity | The ability of the cardiac muscle to transmit an electrical stimulus from cell to cell.
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| Coronary arteries | A pair of arteries that branch from the ascending aorta and supply oxygenated blood to the myocardium.
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| Coronary artery disease | The gradual narrowing and hardening of the coronary arteries. Usually the result of atherosclerosis.
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| Coronary veins | Vessels that transport deoxygenated blood from the capillaries of the heart to the right atrium.
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| Couplet | Two consecutive complexes.
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| Cyanosis | A condition in which a person's skin is discolored to a bluish hue because of inadequate oxygenation of the blood.
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| Delta wave | A slurred or widened upstroke at the beginning of the cardiac QRS complex causing prolongation of the complex. It indicates anomalous impulse conduction and is diagnostic of Wolff-Parkinson-White syndrome.
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| Depolarization | Loss of the difference in charge between the inside and outside of the plasma membrane of a muscle or nerve cell due to a change in permeability and migration of sodium ions into the cell.
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| Desmosomes | A specialized local thickening of the plasma membrane of myocytes that serves to anchor contiguous cells together and prevent them from pulling apart during contraction.
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| Diastole | The period of relaxation during the cardiac cycle, when the cardiac muscle fibers lengthen, causing the heart to dilate and fill with blood. Coronary perfusion occurs during this phase.
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| Dilation | Expansion or widening of an organ, opening, or vessel.
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| Dysrhythmia | Any abnormality in the otherwise normal rhythmic pattern of the heartbeat.
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| Dyspnea | Difficulty in breathing.
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| Ectopic | A cardiac complex originating from somewhere other than the SA node. It may be early (premature) or late (escape) in nature.
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| Einthovans Triangle | An electrical triangle formed by the patients' right arm, left arm, and left leg, used to position electrodes for ECG monitoring. This positioning is used to determine the direction of cardiac vectors to various leads.
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| Electrical axis | See axis.
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| Electrocardiogram | The tracing made by an electrocardiograph. Also known as ECG or EKG.
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| Electrocardiograph | Device used to record electrical variation within cardiac tissue.
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| Electrode | The electrical sensor applied to the body to record the ECG.
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| Electrolyte | Element or compound that, when melted or dissolved in water or other solvent, dissociates into ions (atoms able to carry an electric charge). Sodium, potassium, and calcium are the primary electrolytes involved in myocardial activity.
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| Electromechanical dissociation (EMD) | A condition characterized by seemingly normal electrical heart activity without discernible mechanical activity. This term is no longer used and has been replaced by "Pulseless Electrical Activity" (PEA).
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| Endocardium | The serous membrane that lines the inner aspect of the four chambers of the heart and its valves.
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| Enlargement | An increase in size.
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| Epinephrine | See adrenalin.
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| Epicardium | A thick serous membrane that constitutes the smooth outer surface of the heart.
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| Escape complex | A beat that occurs after the normal pacemaker fails to fire.
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| Excitability | The degree to which a myocardial cell is reactive to external stimuli.
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| Extrasystole | A premature complex.
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| f waves | Waves associated with atrial fibrillation. Occur at a rate of greater than 350 times per minute.
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| F waves | Saw-toothed waves associated with atrial flutter. Occur at a rate of 250 to 350 times per minute.
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| Fascicle | A small bundle of conducting fibers.
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| Fibrillation | Very rapid irregular contractions of the muscle fibers of the heart resulting in a lack of synchronism between the heartbeat and pulse.
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| Fibrillatory | A waveform associated with atrial fibrillation.
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| Flutter | Rapid depolarization of a single focus in atria at a rate of 250 to 350 beats per minute
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| Fusion beat | Occurs when two complexes originating from different pacemakers fuse to form one complex as seen with some types of PVCs.
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| Ganglia | A group of nerve cells forming a nerve center along the thoracic and sacral spine associated with the parasympathetic nervous system.
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| Gap junctions | An area of the conducting cell that improves the rate of electrical conduction.
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| Hemiblock | Failure of conduction of the muscular excitatory impulse in either of the two fascicles of the left branch of the bundle of His.
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| Hypertrophy | Increase in bulk (as by thickening of muscle fibers) without multiplication of parts.
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| Hypertension | A condition in which a person's blood pressure is abnormally high. For normal adults, the blood pressure should be less than 130 mm Hg systolic and less than 85 mm Hg diastolic. Pressures above 140/90 indicate a mild form of hypertension; above 180/110 is considered severe.
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| Idioventricular | Relating to, or arising in the ventricles of the heart independent of the atria.
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| Infarction | Irreversible cell death caused by prolonged obstruction of arterial blood supply to an area of the myocardium.
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| Inotropic | Something that increases the force of contraction of the heart.
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| Intercalated disks | The specialized regions of the cardiac muscle cells that comprise the longitudinal and end-to-end junctions between adjacent cells and that function to connect them mechanically and electrically.
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| Interpolated | A beat that occurs between normal heartbeats without disturbing the succeeding beat or the basic rhythm of the heart.
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| Intraatrial pathway | Electrical pathways that travel through the atria from the SA node to the AV node.
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| Intranodal pathway | The conduction pathways through the atrioventricular (AV) node.
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| Ion | An atom or group of atoms that carries a positive or negative electric charge as a result of having lost or gained one or more electrons.
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| Ischemia | Local tissue hypoxia caused by an obstruction to arterial blood supply to an area of the myocardium.
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| Isoelectric | A condition of the cell that is neither negatively nor positively charged in relation to its neighboring cells. This also refers to the baseline on the ECG that indicates neither a positive or negative deflection.
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| J point | The point at which the QRS complex meets the ST segment. Also called the junction.
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| Junctional | A cardiac rhythm resulting from impulses coming from a locus of tissue in the area of the atrioventricular (AV) node.
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| Left bundle branch | The portion of the heart's conduction system that supplies the left ventricle. Composed of two fascicles; the left anterior fascicle and the left posterior fascicle.
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| M mitrale | Left atrial enlargement associated with mitral valve stenosis.
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| Mediastinum | The space in the chest between the pleural sacs of the lungs that contains all the viscera of the chest except the lungs and includes the heart, aorta, and vena cava.
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| Mitral valve | A valve in the heart between the left atrium and the left ventricle. Prevents the blood in the ventricle from returning to the atrium during systole, and consists of two triangular flaps. Also called bicuspid valve or left atrioventricular valve.
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| Morphology | The configuration or shape of a wave or complex on the ECG.
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| Multifocal | Complexes that arise from different sites within the heart; usually refers to ventricular complexes.
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| Myocardial infarction | See infarction.
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| Myocardial injury | A degree of cellular damage beyond ischemia.
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| Myocardial ischemia | See ischemia.
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| Myocardium | The muscular layer of the heart.
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| Myocytes | The individual muscle cells of the heart.
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| Myosin | One of the two components of the contractile filaments involved in the muscular contraction.
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| Neurotransmitter | A substance, such as norepinephrine or acetylcholine, used to transmit a nerve impulse across the synapse between nerve cells or to the organ being stimulated (innervated).
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| Noncompensatory pause | The pause following a premature complex that resets the rate of the original rhythm. The R-R interval following a noncompensatory pause is less than two preceeding R-R intervals; as seen following a PAC.
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| Norepinephrine | The chemical compound used to transmit impulses by the sympathetic nervous system. A precursor to epinephrine and referred to as noradrenaline.
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| Osborn wave | A narrow, positive deflection wave at the junction of the QRS complex and the ST segment, associated with hypothermia.
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| Oscilloscope | An instrument that displays the fluctuating electrical quantity as a visible waveform on the fluorescent screen of a cathode-ray tube.
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| P wave | The electrical representation of atrial contraction. Precedes the QRS complex.
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| Papillary muscles | The thin muscles that connect to the cusps of the mitral and tricuspid valves and contract to facilitate their ability to resist prolapsing during systole.
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| Pericarditis | Inflammation of the pericardium.
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| P mitrale | Right atrial enlargement associated with severe lung disease such as COPD and pulmonary hypertension.
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| P pulmonale | Hypertrophy of the right ventricle associated with severe lung disease such as emphysema.
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| PR segment | The isoelectric period that represents the conduction of the electrical impulse through the AV node.
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| Parasympathetic | The part of the autonomic nervous system that contains chiefly cholinergic fibers. Tends to induce secretion, to increase the tone and contractility of smooth muscle, and to slow the heart rate.
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| Parietal pericardium | The tough thickened membranous outer layer of the pericardium that is attached to the central part of the diaphragm and the posterior part of the sternum.
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| Paroxysmal | An abrupt beginning and ending of an event such as a different cardiac rhythm.
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| PEA | See Pulseless Electrical Activity.
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| Pericardial tamponade | A condition characterized by a collection of fluid within the pericardial sac that constricts the heart and inhibits its ability to fill during diastole. Also called cardiac tamponade.
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| Pericardium | The conical sac of serous membranes that encloses the heart and the roots of the aorta, pulmonary artery, and vena cava composed of one layer that is closely adherent to the heart while the other lines the inner surface of the outer coat with the intervening space being filled with pericardial fluid.
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| Polarization | Refers to the cell returning to its resting state during which no electrical activity occurs and its intracellular fluid is negatively charged relative to the extracellular fluid.
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| PR segment | The flat isoelectric line that represents the electrical impulse traveling through the His-Purkinje system.
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| Precordial | Situated in front of the heart.
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| Preload | The stretched condition of the heart muscle at the end of diastole just before contraction.
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| Pulmonic valve | The heart valve that separates the right ventricle from the pulmonary artery. Also called the semilunar valve.
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| Pulseless Electrical Activity (PEA) | A condition characterized by seemingly normal electrical heart activity without discernible mechanical activity.
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| Purkinje fibers | Specialized heart cells capable of conducting electrical impulses and directly stimulating ventricular myocardial cells to contract.
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| Q wave | The first negative deflection of the QRS complex not following an R wave.
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| QRS complex | The waveform that represents electrical conduction through the ventricles. Associated with ventricular contraction.
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| QT interval | Period of time measured from the beginning of the Q wave to the end of the T wave representing the time required to depolarize and repolarize the ventricles.
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| Quadrigeminal | Cardiac electrical activity characterized by a pattern of one ectopic beat followed by three normal beats. These ectopic beats can be either atrial or ventricular in origin and indicate cardiac instability.
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| Reentry | A cardiac mechanism that is held to explain certain abnormal heart actions (as tachycardia) and that involves the transmission of a wave of depolarization along an alternate pathway when the original pathway is blocked with return of the impulse along the blocked pathway resulting in a reinitiation of the impulse.
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| Refractory period | The phase during repolarization during which a stimulus may or may not cause the cell to depolarize. Also see relative and absolute refractory period.
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| Relative refractory period | A later phase of repolarization during which a sufficiently strong stimulus will depolarize the cell.
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| Repolarization | Restoration of the difference in charge between the inside and outside of the plasma membrane of a muscle fiber or cell following depolarization.
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| Resting membrane potential (RMP) | The relative difference between the electrical charge of the inside as compared to the outside of the cell.
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| Retrograde | Occurring or performed in a direction opposite to the normal or forward direction of conduction or flow.
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| Right bundle branch | The portion of the heart's conduction system that supplies the right ventricle.
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| R-R interval | The interval of time between consecutive R waves.
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| R wave | The positive deflection in the QRS complex.
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| SA node | See sinoatrial node.
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| Salvo | A series of three or more consecutive and identical beats.
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| Sarcolemma | The plasma membrane surrounding the myocytes of the heart.
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| Sarcoplasmic reticulum | The specialized area of the myocyte for storage of calcium required to trigger contraction of the actin and myosin filaments.
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| Semilunar valve | See pulmonic valve.
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| Septum | The curved slanting wall that separates the right and left ventricles of the heart and is composed of a muscular lower part and a thinner more membranous upper part.
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| Sick sinus syndrome | A condition characterized by persistent sinus bradycardia, sinoatrial block manifested as sinoatrial Wenckebach, complete sinoatrial block, or sinus arrest.
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| Sinoatrial (SA) node | A small mass of tissue made up of nerve fibers,which is embedded in the musculature of the right atrium. Initiates the impulses stimulating the heartbeat and is the primary pacemaker of the heart. Also called SA node, sinus node.
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| Sodium-potassium pump | Refers to the energy-requiring process by which the cellular membrane actively moves sodium out of the cell and potassium into the cell.
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| ST segment | Represents the phase following ventricular depolarization before repolarization begins and is helpful in assessing for the presence of cardiac ischemia.
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| Stroke volume | The volume of blood pumped from a ventricle of the heart in one beat.
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| Supraventricular | Relating to or being a rhythmic abnormality of the heart caused by impulses originating above the ventricles.
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| Sympathetic | The part of the autonomic nervous system that contains chiefly adrenergic fibers. Tends to decrease glandular secretion and to speed the heart rate.
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| Syncytium | Refers to the network of anastomosis (connections) between adjoining myocytes.
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| Systole | The contraction of the ventricles during which blood is ejected from the ventricles into the aorta and pulmonary artery.
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| T wave | Represents ventricular repolarization.
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| Tachycardia | A heart rate greater than 100 when referring to sinus rhythms or a rate greater than is normally expected with any rhythm.
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| Thrombolytics | Medications that attack and accelerate the body's ability to break up or lyse a blood clot (thrombus).
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| Torsades de pointes | Ventricular tachycardia characterized by fluctuation of the QRS complexes around the electrocardiographic baseline. Typically caused by a long QT interval.
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| Tricuspid valve | Heart valve situated between the right atrium and the right ventricle. Resembles the mitral valve in structure but consists of three triangular membranous flaps. Also called the right atrioventricular valve.
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| Trigeminal | Cardiac electrical activity characterized by a pattern of one ectopic beat followed by two normal beats. These ectopic beats can be either atrial or ventricular in origin and indicate cardiac instability.
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| Unifocal | Complexes originating from the same location (focus).
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| U wave | A positive wave that may follow the T wave representing the final repolarization phase of the ventricles. It is rarely seen.
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| Vagus nerve | Either of the pair of tenth cranial nerves that arise from the medulla and supply chiefly the heart with autonomic sensory and motor fibers.
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| Vector | The geometric direction of travel of an electrical impulse in the heart.
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| Vein | A vessel that carries blood toward the heart.
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| Ventricles | The chambers of the heart that receive blood from a corresponding atrium and from which blood is forced into the arteries.
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| Ventricular standstill | A condition where the atria continue to beat but the ventricles have stopped.
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| Visceral pleura | The fibrous connective tissue adhering to the outer surface of an organ.
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