accelerated idioventricular rhythm: an enhanced ectopic ventricular rhythm, usually transient, that comprises at least three consecutive ventricular beats; the rate is > 40 bpm, the inherent rate of the ventricles

accelerated junctional rhythm: an enhanced rhythm of 60–100 bpm; a backup pacemaker for the heart

agonal rhythm: an idioventricular rhythm, usually < 20 bpm; signifies a dying heart

aorta: the main “trunk” of the systemic arterial system. The aorta arises from the base of the left ventricle and ends at the left side of the fourth lumbar vertebra, where it divides and forms the right and left iliac arteries.

aortic valve: the valve that lies between the left ventricle and the ascending aorta; comprises three leaflets in the anterior, right posterior, and left posterior positions

artifact: interference on an ECG that prevents an accurate reading of the heart’s impulses; may be caused by patient movement, loose leads, or other noncardiac causes

asystole: the absence of contractions of the heart; when all electrical activity ceases

atrial ectopic: an extra heartbeat caused by a signal to the atria from an abnormal atrial focus; also called an atrial premature beat or a premature atrial contraction

atrial fibrillation: fibrillation of the heart in which the normal rhythmic contractions of the atria are replaced by rapid and irregular twitching (fibrillation) of the muscular wall. When the rate is > 100 bpm, the ventricular response is said to be uncontrolled.

atrial flutter: rapid, regular atrial contractions, usually 250–330 bpm, triggered by an irritable atrial focus; often produces characteristic saw-toothed wave patterns on an ECG

atrial foci: the natural pacemakers of the heart; located in the atria

atrial tachycardia: an abnormally rapid heart rhythm originating in the atria

atrioventricular junctional rhythm: the cardiac rhythm that occurs when the heart is controlled by the AV junction

atrioventricular node: a small node of modified cardiac muscle fibers located in the AV junction

atrioventricular valves: valves that separate the atria from the ventricles, allow blood to flow from the atria into the ventricles, and prevent backward flow in the opposite direction (regurgitation)

automaticity: the ability of certain cells within the cardiac conduction pathway to spontaneously depolarize

biphasic wave: two wave components that develop on alternate sides of the baseline

bicuspid valve: the valve that lies between the left atrium and left ventricle; also called the mitral valve

bigeminy: the occurrence of heartbeats in pairs; ventricular bigeminy occurs when there are premature ventricular contractions occurring every other beat, interspersed with normal sinus beats

bradycardia: a slow heart rate; defined as a heart rate < 60 bpm

bundle branches: part of the electrical pathway of the heart that delivers electrical impulses to the ventricles, divides into a right bundle and a left bundle.

bundle of His: a slender bundle of modified cardiac muscle connecting the atria with the ventricles; maintains the normal sequence of the heartbeat

concordant: in agreement; together

couplet: two successive atrial or ventricular premature complexes, one after the other

depolarization: a relative reduction in the magnitude of polarization

early diastole: the early part of the diastole cycle, occurring when the heart cavities begin to fill with blood

ectopic beat: a cardiac beat that originates somewhere other than the sinoatrial node

electrical vector projections: a description of the electrical generator of the heart on an ECG

electrocardiogram (ECG): a graphical representation of the heart’s electrical activity that allows one to measure specific activities occurring during the cardiac cycle

endocardium: the innermost lining of the chambers of the heart; comprises endothelial and subendothelial connective tissue

epicardium: the inner serous layer of the pericardium; also referred to as the visceral pericardium

escape beat: an automatic beat, usually arising from the AV junction or ventricle, which occurs after the next expected normal beat has defaulted; a late beat

escape mechanism: protects against total cardiac standstill in the event of sinus node failure or complete heart block

f wave: The waves of atrial flutter; usually seen in ECG leads V2, V3, and aVF

His-Purkinje system: carries the electrical signals throughout the ventricles to make them contract; includes the His bundle, the right bundle branch, the left bundle branch, and the Purkinje fibers

iatrogenic: an illness caused by medical treatment

idiopathic progressive cardiac conduction disease: degeneration of the conduction system of the heart; eventually results in complete heart block

idioventricular rhythms: a transient type of arrhythmia; usually occurs in patients with acute myocardial infarction or digitalis toxicity, and usually benign

impulse conduction: the pathway through which the heart creates its own electrical impulses and controls the route those impulses take

impulses: electrical energy that causes the heart to contract and relax; electrical stimulus generated by the heart

inferior vena cava: the large vein that receives blood from the lower part of the body; it empties into the right atrium of the heart

intermodal pathway: transfers the impulse from the SA node to AV node, located in the floor of the right atrium

interpolated premature ventricular complex: a premature ventricular complex that does not produce a compensatory pause

intra-atrial pathways: pathways through which impulses travel from the sinus node to the AV node

irritability: the ability to respond to stimuli; abnormal sensitivity to stimuli of an organ or body part

isoelectric line: the baseline of an electrocardiogram

isovolumetric contraction: occurs when blood stops moving into or out of the ventricles due to closure of the AV and semilunar valves

isovolumetric relaxation: the part of the cardiac cycle during which the ventricular muscle decreases its tension without lengthening so the ventricular volume remains unchanged

j point: the point marking the end of the QRS complex and the beginning of the S or T wave

junctional escape rhythm: a sequence of three or more junctional escapes beats; rate is 40–60 bpm; occurs when the primary pacemaker defaults or the AV node blocks the atrial impulse

junctional tachycardia: a supraventricular tachycardia arising from the AV junction; junctional escape rhythm that has a rate > 60 bpm

left atrium: the upper left chamber of the heart

left ventricle: the lower left chamber of the heart

mitral valve: the valve that lies between the left atrium and the left ventricle; also called the bicuspid valve

Mobitz type I: a type of second-degree AV block in which the PR interval progressively lengthens with each beat until the atrial impulse fails to conduct and a QRS complex is dropped

Mobitz type II: a type of second-degree AV block in which beats are intermittently non-conducted and QRS complexes are dropped, usually in a repeating cycle of every third P wave

monitoring lead: the single lead on an ECG machine selected to represent all the basic waveforms; lead II is the most used monitoring lead because all complexes are upright in this lea

myocardium: the middle layer of the heart, consisting of cardiac muscle

myocytes: a muscle cell

normal sinus rhythm: the normal cardiac rhythm arising from the sinoatrial node; its rate is 60–100 bpm in healthy adults

P cells: cells found in the sinoatrial and atrioventricular nodes; often a source of electrical impulses

P wave: represents the electrical activity of the upper chambers of the heart; occurs before the QRS complex in normal sinus rhythm and has a normal duration of 120–200 milliseconds (0.12–0.2 seconds)

pacemaker: a regulator of heart rate activity

parasympathetic branch: one of two branches of the autonomic nervous system; while both branches control energy expenditure, the parasympathetic branch serves a restorative function and works in direct opposition to the sympathetic nervous system to maintain homeostasis

parasympathetic nerve: one of the nerves of the parasympathetic nervous system

pericardial sac: a conical sac of fibrous tissue surrounding the heart and the beginning of the great blood vessels; also referred to as the pericardium

polarized: the state in which cardiac cells are at rest, meaning no electrical activity takes place

PR interval: the time between the onset of atrial depolarization and the onset of ventricular depolarization, normally 120–200 milliseconds (0.12–0.20 seconds)

premature atrial contraction: early atrial heart beats that are not produced by the sinus node

premature junctional complex: premature depolarizations from the AV node or the proximal portion of the His-Purkinje system

premature ventricular complex: the premature occurrence of a QRS complex that is unusually shaped and is generally > 120 milliseconds in duration; may have an opposite deflection compared to the underlying rhythm

pulmonary arteries: arterial trunks that carry oxygen-deficient blood to the lungs

pulmonary vein: any one of four veins (two from each lung) that return oxygenated blood from the lungs to the superior part of the left atrium

pulmonic valve: the valve at the entrance to the pulmonary trunk from the right ventricle; blood leaves the heart through the pulmonic valve, moving into the pulmonary artery and the lungs

Purkinje fibers: ventricular “highways” that spread widely across the ventricles

QRS complex: the electrical activity of the lower ventricles; recorded as a complex of three waves

QT interval: the distance from the beginning of the QRS complex to the end of the T wave, representing the time it takes for the heart muscle to contract and then recover

quadrigeminy: a cardiac arrhythmia in which heartbeats are grouped in fours, each group composed of one sinus beat followed by three extra systoles

R-on-T phenomenon: a cardiac event in which a ventricular stimulus causes the premature depolarization of cells that have not completely repolarized; may result in ventricular tachycardia or ventricular fibrillation

R-R interval: the time interval from the beginning of one QRS complex to the beginning of the next, as measured on an ECG

repolarization: a change in membrane potential, making it more negative; the return of cell membrane potential to its resting potential following depolarization

retrograde conduction: transmission of a cardiac impulse backwards, particularly conduction from the atrioventricular node into the atria; implicated in certain types of tachyarrhythmias

right atrium: the right upper chamber of the heart

right ventricle: the right lower chamber of the heart

S wave: the negative (downward) deflection of the QRS complex following the R wave; represents depolarization in the Purkinje fibers

semilunar valves: either of two valves, one of which is situated at the opening between the heart and the aorta and the other at the opening between the heart and the pulmonary artery

sinoatrial node: a cluster of specialized cells located in the right atrium; produces the electrical impulses that set the heart’s rate and rhythm

sinus arrest: the cessation of sinus activity; the ventricles may continue to beat under ectopic atrial, AV junctional, or idioventricular control

sodium-potassium channels: generate the cardiac cells’ electrical activity, both inside and outside of the cells; potassium channels are responsible for terminating action potentials and contractions while initiating repolarization, while sodium channels initiate action potential and trigger contractions of the cardiac muscle fibers

ST segment: the brief period between the end of the QRS complex (J point) and the beginning of the T wave; has a duration of 80–120 milliseconds (0.080–0.120 seconds)

superior vena cava: returns blood from the upper body to the right atrium

sympathetic branch: one of two branches of the autonomic nervous system. While both branches control energy expenditure, the sympathetic branch mediates the expenditure; acts in direct opposition to the parasympathetic nervous system to maintain homeostasis.

sympathetic nerves: any nerve of the sympathetic nervous system

T cells: assist in distributing sinus impulses to the rest of the atrial myocardium

T wave: the recovery phase of the ventricles

tachycardia: a resting heart rate > 100 bpm

tricuspid valve: the valve that lies between the right atrium and the right ventricle

trigeminy: a cardiac arrhythmia in which the heartbeats are grouped in trios, usually a sinus beat followed by two extrasystoles

U wave: appears commonly in patients with hypokalemia, hypomagnesemia, or ischemia, although sometimes present in healthy patients as well; best seen in leads V2 and V3.

ventricular diastole: the period of relaxation and repolarization of the ventricles

ventricular fibrillation: rapid, quivering fibrillatory movements of the ventricles; replaces normal contraction; a non-perfusing rhythm that rapidly results in patient death if not treated

ventricular systole: contraction of the ventricles

ventricular tachycardia: a regular, faster-than-normal heart rate that arises from the lower chambers of the heart; can be life-threatening as it is often a prelude to ventricular fibrillation

Wolff-Parkinson-White syndrome: an electrical abnormality in the heart; associated with supraventricular tachycardia

zero net charge: the point at which total positive and negative charges are equal