© Steve Allen/Brand X Pictures/Alamy Images
© Steve Allen/Brand X Pictures/Alamy Images
A
aberrancy – The abnormal conduction of the electrical impulse through the heart. This aberrant conduction gives rise to wide complexes that are morphologically different from those that have gone through the normal pathways.
absolute refractory period – That period in the cell firing cycle at which it is impossible to restimulate a cell to fire off another impulse.
absolute value – The amount of a number with no regard to its sign (positive or negative).
accessory pathway – A pathway, other than the AV node, for the transmission of the impulse from the atria to the ventricles.
acetylcholine – A chemical neurotransmitter used by the parasympathetic system.
acidosis – The acid state on the pH balance scale.
actin – A muscle protein that is part of the contractile element.
action potential – The electrical firing of the myocyte, which leads to contraction. Consists of four phases.
afferent nerves – Nerves that bring information into the CNS.
amplitude – The total height of a wave or complex.
antegrade conduction – Normal conduction of the electrical impulse through the AV node from the atria to the ventricles.
anterior – Before or toward the front.
anterior wall – The vertical wall that lies along the anatomic front of the heart, that is, closest to the anterior chest wall.
antidromic conduction – A circus movement of the electrical impulse found in patients with Wolff-Parkinson-White syndrome in which the impulse travels down the Kent bundle and then reenters the atria via the AV node. Leads to a wide-complex morphology of the QRS complexes.
approach – When related to the AV node, the approaches or tracts are the pathways that lead to the AV node. (also known as tract)
arrhythmia – An abnormal rhythm of the heart. (also known as dysrhythmia)
arrhythmogenic – Can potentiate arrhythmias.
arteries – Vessels of the circulatory system that carry blood away from the heart.
Ashman’s phenomenon – The propensity of a complex to be transmitted aberrantly if it arrives prematurely after a long pause. In other words, if there is a long pause and then a short pause (the premature complex), the QRS complex of the short pause is usually transmitted aberrantly.
asynchronous – The events do not occur at the same time.
atrial kick – A vernacular term used to describe the blood ejected into the ventricles during atrial systole which leads to ventricular overfilling and stretching of the ventricular myocytes and increases the force of contraction.
atrioventricular (AV) node – Part of the electrical conduction system. It is responsible for slowing down conduction from the atria to the ventricles just long enough for atrial contraction to occur. This slowing allows the atria to “overfill” the ventricles and helps maintain the output of the heart at a maximum level.
atrium – Small, thin-walled, chamber of the heart. Each atrium acts as a priming pump for the ventricles. There are two: the left and the right. (plural = atria)
augmented limb leads – The limb leads aVR, aVL, and aVF.
automaticity – The ability of a pacemaker to repolarize and automatically trigger another interval by itself.
autonomic nervous system – The part of the nervous system that is responsible for unconscious or involuntary body functions (such as heart rate).
AV dissociation – An incomplete block of the AV node leading to the independent firing of the atria and the ventricles. The atrial impulse exerts some minimal control over the ventricular rate; however, with this incomplete block, the atrial rate is the same as or close to the ventricular rate.
AV junction – The area immediately surrounding and encompassing the AV node.
AV node dependent rhythms – Rhythms that have an ectopic focus triggered at the level of the AV node, or rhythms that need the AV node to function as part of the circuit to maintain the arrhythmia (AV node dependent). Used to classify the SVTs.
AV node independent – Rhythms that do not involve the AV node itself. Used to classify the SVTs.
B
Bachman bundle – Part of the electrical conduction system that transmits the impulses through the interatrial septum.
bigeminy – A premature complex that occurs at every second beat. The complexes can be either supraventricular or ventricular.
biphasic – A term used to describe a wave with both negative and positive components. Usually used in conjunction with P and T waves.
Bix’s rule – A rule stating that any arrhythmia with a P wave halfway between two QRS complexes could contain a buried P wave hiding inside the QRS complex between them.
block – Obstruction to flow of the impulse or depolarization wave.
bradycardia – A slow rhythm (< 60 BPM).
Brugada’s sign – The presence of an abnormally long interval from the R wave to the bottom of the S wave > 0.10 seconds long. This is an abnormality that, if present, will help identify a widecomplex tachycardia as VTach in comparison to an aberrantly conducted supraventricular tachycardia.
Brugada’s syndrome – A channelopathy that is usually genetically transmitted in families and which leads to the formation of tachyarrhythmias that result in early adult sudden cardiac death.
bundle branch block (BBB) – A physiologic block of either the left or right bundle branch of the electrical conduction system.
bundle of His – Part of the electrical conduction system. It originates in the AV node and ends in the bundle branches.
C
calibration box – A box or step-like displacement of the ECG baseline at the end of the ECG that is used to ensure that the ECG conforms to a standard format. When a standard size, it is 10 mm high and 0.20 seconds wide. It can-also be set at half-standard or doublestandard when evaluating height, or 25-mm or 50-mm standard in regard to width.
calipers – A tool with two identical legs whose terminal points (pins) are used to evaluate distance. A measuring tool used in electrocardiography, architecture, and navigation.
cannon A waves – A physical examination finding that occurs intermittently, representing an abrupt and sudden elevation in the jugular venous system, caused by a transient increase in jugular venous pressure. The elevation of the jugular venous pressure occurs when the right atria contract prematurely against a closed tricuspid valve.
capillary refill – Obtained by pushing down on the nail until the nail bed turns white. The pressure is then released and the number of seconds it takes for the normal pink color to return is measured. It should be less than 2 seconds in the normal patient. It is a sign of a hypoperfused cardiovascular state.
capture beat – Occurs in AV dissociation when P wave falls during a period that enables it to innervate the ventricle, forming complexes that narrower (identical to or close to the appearance of the normal complexes) than the ectopic ventricular beats due to the transmission of the electrical impulse down the normal conduction pathway.
cardiac output – The amount of blood that the heart pumps per minute from either the left or right ventricles. It equals stroke volume × heart rate.
central nervous system (CNS) – The brain and spinal cord.
channelopathy – A disease occurring in the small ionic channels that exist in the cell membrane.
circus movement – A self-propagating, recurrent looping pattern of impulse transmission in which the depolarization wave continuously restimulates itself.
coarse atrial fibrillation – A morphologic type of atrial fibrillation in which the baseline shows grossly large fibrillatory (f) waves. The coarse f waves may sometimes be mistaken for atrial flutter.
collateral circulation – Shared arterial perfusion areas that occur between the arteries of the heart. In other words, one area is perfused by multiple different arteries or arterioles.
compact zone – The center or core of the AV node. This area is composed of cells very similar in histologic appearance and function to those cells found in the sinoatrial node.
compensatory pause – A pause immediately following a premature complex that is longer than the interval between two normally conducted beats, allowing the rhythm to proceed, without any alteration of cycle length, around the premature complex. In essence, the pause compensates for the short interval preceding the premature complex and allows the rate to proceed on schedule.
complete block – Blockage of all transmission through the AV node. The result is that the atria and the ventricles are functioning completely isolated from each other electrically.
concealed conduction pathway – Conduction through an accessory pathway that does not create a delta wave. This type of conduction is electrocardiographically silent and there is no way (short of electrophysiologic testing) to identify the presence of these silent conduction pathways. Note that the potential for tachyarrhythmias and abnormal transmission exists in these cases. This type of conduction is the most common presentation for patients with accessory pathways.
concordance – All going in the same direction, either positive or negative.
concordance of the QRS complexes in the precordial leads – The presence of monophasic, wide QRS complexes in the precordial leads (V1 to V6) that all have the same polarity or direction.
conduction – The process by which bioelectricity is transferred through the myocardial tissue. It refers to a successive stimulation of cells that, in essence, create an electrical current.
controlled atrial fibrillation – Atrial fibrillation at normal rates.
critical rate – The rate that distinguishes normal conduction from aberrant conduction. It is the cutoff rate where rate-related aberrancy will occur.
couplet – Two PVCs occurring sequentially.
cyanosis – A purplish discoloration of the skin caused by lack of oxygen delivery to the affected tissues.
D
decompensated atrial fibrillation – Atrial fibrillation with rates between 100 and 200 BPM. (also known as uncontrolled atrial fibrillation)
delta wave – A slurring of the upstroke of the first part of the QRS complex that occurs in Wolff-Parkinson-White syndrome.
depolarization – A state in which the cell becomes more positive, moving toward equilibrium with the extracellular fluid. Takes place during the latter part of resting state and is completed during activation by the action potential.
diagnostic trial – Performance of procedures or administration of pharmaceutical agents for the purpose of assisting in making a diagnosis. (also known as diagnostic maneuver)
diastole – The normal “resting” phase of the heart when the heart is not actively contracting.
digitalis effect – A scooped-out appearance of the ST segment and the T waves in patients taking digoxin or digitalis-like substances.
distal – Anatomically, describes the direction and relative distance away from midline. An object that is further away from the midline, as compared to one that is proximal.
dual response – A type of response from a pulse generator or pacemaker in which the device responds in a triggered manner for some preprogrammed cases and in an inhibitory manner for other preprogrammed events.
E
electrical alternans – A finding associated with a fluctuation of the electrical axis of the ventricles over two or more beats. It is usually associated with large pericardial effusions.
electrical axis – The summation of the individualized vectors for all of the ventricular myocytes during activation.
electrical conduction system – A specialized collection of cells that coordinate the bioelectrical activity of the heart. They are involved in initiating (pacemaking) and conducting the electrical impulse. They also coordinate the sequencing of the atrial and ventricular contractions to allow efficient pumping of blood.
electrical potential – The difference between the charges on the outside and the inside of the cell wall. The value for the resting myocyte is usually about −70 to −90 mV.
electrocardiogram (ECG) – A 12-lead electrocardiographic recording used to evaluate the heart and its rhythm.
electrode – The electrical sensors placed on the chest to record the bioelectrical activity of the heart.
electrophysiologic studies (EPS) – Invasive studies used to diagnose where an arrhythmia originates or to map the surface of the heart to evaluate for abnormalities or reentry circuits which could be arrhythmogenic.
electrolyte – A chemical agent or molecule that can conduct an electrical current.
emergent phase of patient management – A life-threatening state in which immediate attention to diagnosis, treatment, and stabilization should not be delayed. Severely hemodynamically unstable patients and unresponsive patients fit within this category.
end-diastolic PVCs – When a PVC falls after the next normally occurring sinus P wave. These occur during the late diastolic phase of the previous complex.
endocardial ischemia – Relative or absolute hypoxemia of the endocardial surface of the heart.
endocardium – The internal lining of the atrial and ventricular walls.
epinephrine – Chemical neurotransmitter used by the sympathetic system.
escape complex – A beat that occurs after a normal pacemaker fails to fire. The R-R interval is longer in these cases.
escape rhythm – A rhythm that occurs after a normal pacemaker fails to fire.
extreme right quadrant – The quadrant of the hexaxial system that is located between –90° to –180°.
F
fibrillatory (f) waves – The fine or coarse waves caused by the wavelet depolarizations in atrial fibrillation.
fine atrial fibrillation – A morphologic type of atrial fibrillation in which the baseline shows very small fibrillatory (f) waves. The coarse f waves may sometimes be mistaken for a complete absence of atrial activity.
flutter (F) waves – The sawtooth waves formed by the circus movements found in atrial flutter. They can be either in a positive or negative direction.
flutter-fibrillation pattern – A rhythm that switches between atrial flutter and atrial fibrillation often.
fusion beat – Occurs when two complexes with different inciting pacemakers fuse to form a complex unlike either the normal or the ectopic complex. Commonly seen in VTach.
fusion complex – Occurs when two complexes with different inciting pacemakers fuse to form a complex unlike either the normal or the ectopic complex. Commonly seen in premature complexes, VTach, idioventricular rhythms, and multifocal atrial tachycardias.
G
gap junction – The small area where a nerve ending and the muscle tissue come together. Communication occurs between the cells via neurotransmitters that are released into the extracellular space at the junction.
H
heart rate – The rate at which the heart contracts per minute.
hexaxial system – The system developed to describe the coronal plane that is created by the limb leads (I, II, III, aVR, aVL, and aVF).
high-yield learning – A type of learning that provides the maximum amount of information in the smallest amount of space, and that is as concise and compact as possible.
hyperkalemia – Elevated potassium level.
hypocalcemia – Low level of calcium.
hypomagnesemia – Low level of magnesium.
hypovolemia – Low fluid or volume of the blood or circulatory system.
hypoxemia – Low oxygen level in the blood.
I
incessant ventricular tachycardia – A hemodynamically stable form of ventricular tachycardia that occurs nonstop for periods longer than one hour. May progress to cardiomyopathy.
inferior wall – Anatomically, the inferior wall of the heart, which lies on the diaphragm.
inhibited response – The pacemaker responds to a sensed event by not firing a response for a certain interval of time (the VEI).
innervation – Activation of the myocardial cells by an electrical impulse.
internodal pathways – Three pathways of the electrical conduction system found in the atria that transmit the impulse from the SA node to the AV node.
interpolated PVC – A PVC that falls exactly between two sinus complexes and does not alter the cadence of the rhythm in any way.
intrinsicoid deflection – The amount of time it takes the electrical impulse to travel from the Purkinje system in the endocardium to the surface of the epicardium. It is measured from the beginning of the QRS complex to the beginning of the downslope of the R wave, and is usually measured in leads with no Q wave.
ion – An atom or molecule that carries an electrical charge.
isoelectric – When referring to a wave, the wave is neither positive nor negative. When referring to a lead, the lead in question is exactly 90° from the electrical axis. It is usually the lead with the smallest amplitude and the one that is the closest to being neither positive nor negative.
J
Josephson’s sign – A small notching near the low point of the S wave in ectopic ventricular complexes.
K
Kent bundle – An accessory pathway found in patients with Wolff-Parkinson-White syndrome.
L
lateral wall – The lateral wall of the heart, along its left side.
LBBB-like morphology – An arrhythmia that occurs in ventricular complexes and tachycardias, that has an appearance that is negative in lead V1 on a 12-lead ECG, greater than or equal to 0.12 sec, and/or which shows the presence of any Q wave in lead V6.
lead – (1) Any of the electrodes or conductors used to measure the biochemical activity of the heart. (2) The actual representation of the electrical activity of the heart based on the placement of the electrodes, analogous to camera angles.
lead placement – The exact position on the body of the ECG electrodes.
left anterior fascicle (LAF) – Part of the electrical conduction system. Responsible for innervating the anterior and superior areas of the left ventricle. It is a single-stranded cord terminating in the Purkinje cells.
left bundle branch (LBB) – The bundle branch that branches toward the left side. It is responsible for depolarizing the right side of the interventricular septum and the right ventricle.
left bundle branch block (LBBB) – A physiologic block of the left bundle branch causing the characteristic ECG pattern of a QRS complex > 0.12 seconds, monomorphic S wave in V1 and monomorphic R wave in leads I and V6.
left posterior fascicle (LPF) – Part of the electrical conduction system. Responsible for innervating the posterior and inferior areas of the left ventricle. It is a widely distributed, fan-like structure terminating in the Purkinje cells.
limb leads – The leads that form the hexaxial system, dividing the heart along a coronal plane into anterior and posterior segments. These leads include I, II, III, aVR, aVL, and aVF.
Lown-Ganong-Levine (LGL) syndrome – A syndrome characterized by a short PR interval and a normal QRS complex.
M
macroreentry circuit – A reentry circuit which occurs at the macroscopic level (typically encompassing larger expanses or distances). Examples of these rhythms include atrial flutter and AV reentry tachycardia.
microreentry circuit – A reentry circuit which occurs at the microscopic level. It usually requires electrophysiology studies to diagnose.
microcollateral circulation – A shared arterial perfusion area that occurs between the arteries of the heart. In other words, one area is perfused by multiple different arterioles.
mnemonic – Refers to the use of an object, thought, memory, imagery, or concept for the purposes of creating a memory tool or link to help recall the relevant information faster and more efficiently.
monomorphic – One morphologic appearance.
morphology – The physical appearance of a wave or complex.
multifocal PVCs – PVCs that are morphologically different because they either originate in separate ventricular ectopic foci or are transmitted through different depolarization routes.
myocardial infarction (MI) – A process, acute or chronic, that is characterized by the formation or presence of dead myocardial tissue.
myocardium – A specialized term to describe the muscle tissue of the heart.
myocyte – An individual heart muscle cell.
myofibrils – Another term for the individual muscle cells or myocytes.
myosin – A muscle protein that is part of the contractile element.
N
narrow-complex supraventricular tachycardia – A supraventricular tachycardia where the QRS complexes are less than 0.12 seconds wide.
noncompensatory pause – A pause, immediately following a premature complex, that alters the rhythm, causing a resetting of the pacemaker and an alteration of cycle length, after the premature complex. In essence, this pause does not compensate for the short interval preceding the premature complex, and the rate is completely reset after the event.
nonsustained ventricular tachycardia – Ventricular tachycardia that lasts less than 30 seconds.
nonurgent phase of patient management – A category of patients who do not require immediate attention and are hemodynamically stable, allowing the patient to be evaluated in a timely (but still as rapid as possible) manner prior to instituting a course of action.
norepinephrine – A chemical neurotransmitter used by the sympathetic system.
normal quadrant – The quadrant of the hexaxial system represented by 0° to 90°.
O
orthodromic conduction – A circus movement of the electrical impulse found in patients with Wolff-Parkinson-White syndrome, in which the impulse travels normally down the AV node and then reenters the atria via the Kent bundle.
P
P wave – The deflection used to identify atrial depolarization. It is the first wave of a complex or beat.
P wave axis – The calculated electrical axis for the P waves.
pacemaker – The location that initiates cardiac depolarization and dictates the rate at which the heart will cycle. Its function is usually performed by the cells of the electrical conduction system, although any myocardial cell can perform this function. It can be an intrinsic part of the heart or an external unit.
pacemaker syndrome – The clinical syndrome that occurs because of a lack of atrial contribution to ventricular filling and the inability of the heart rate to compensate for any form of exertion or exercise, giving rise to various clinical symptoms related to decreased cardiac output and hypotension. These include fatigue, light-headedness, syncope, dyspnea, exercise intolerance, congestive heart failure, and anginal symptoms due to myocardial ischemia.
parasympathetic nervous system – The part of the nervous system that causes a slowing down or calming effect. Its main chemical messenger is acetylcholine.
pericardium – The external lining or surface of the heart.
peripheral nerves – The nerves that are outside of the central nervous system and are responsible for bringing information to and from the brain and spinal cord.
physiologic block – The normal delay period in the conduction of the electrical impulse caused by the AV node in order to allow the atria and the ventricles to contract synchronously.
posterior wall – Anatomically in the heart, the vertical wall that lies closest to the posterior wall of the chest or thorax.
P-P interval – The interval represented by the space between the P waves of two consecutive complexes.
PR interval – That interval of time that occupies the space between the beginning of the P wave and the beginning of the QRS complex.
PR segment – The segment of the complex that occupies the space between the end of the P wave and the beginning of the QRS complex.
precordial system – Another term used to describe the chest leads. They are labeled from V1 through V6. They divide the heart along a sagittal plane. (also known as precordial leads)
preexcitation syndrome – A morphologic term used to apply to the cardiac complex that includes a short PR interval and the presence of a delta wave. Signifies the presence of an underlying accessory pathway.
premature complex – A complex that arrives earlier than expected for the cadence of the rhythm.
proximal – Anatomically, describes the direction and relative distance away from midline. An object that is closer to the midline, as compared to one that is distal.
pseudo-R wave – See pseudo-R’ wave.
pseudo-R’ wave – The fusion of the P wave with the QRS complex of another complex to give the appearance of the presence of an R wave.
Purkinje system – Specialized cells that act as the final pathway of the electrical conduction system of the heart. They directly innervate the ventricular myocytes.
Q
Q wave – The first negative wave of the QRS complex.
QRS complex – The wave complex represented by ventricular depolarization. It may consist of individual or multiple waves in succession, which may appear in any combination: the Q wave, R wave, and S wave.
QRS interval – The interval of time occupied by the QRS complex.
QT interval – The interval of time represented by the space from the beginning of the QRS complex to the end of the T wave; may vary with heart rate.
QTc interval – The QT interval, corrected mathematically for the heart rate.
quadrants – The hexaxial system can be broken down into four quadrants: the normal, left, right, and extreme right quadrants. Each represents 90° of the hexaxial system.
quadrigeminy – A premature complex every fourth beat. The complexes can be either supraventricular or ventricular.
R
R wave – The first positive wave of the QRS complex.
R’ wave – A second positive wave inside a QRS complex.
rabbit ears – A slang term for the RSR’ pattern traditionally found in V1 in an RBBB.
rapid filling phase – Part of the cardiac filling cycle that occurs during early diastole. It starts when the AV valves open and blood rushes in to fill the ventricles. The phase ends when the atria begin to contract. Most of the blood enters the ventricles during this period.
rate – The number of beats per minute.
RBBB-like morphology – An arrhythmia that occurs in ventricular complexes and tachycardias, that has an appearance that is positive in lead V1 on a 12-lead ECG and is greater than or equal to 0.12 sec.
reentry – A self-propagating, recurrent looping pattern of impulse transmission in which the depolarization wave continuously restimulates itself. (see circus movement)
refractory state – A short period of time, immediately after depolarization, in which the myocytes are not yet repolarized and are unable to fire or conduct an impulse
relative hypotension – The discrepancy that can occur for any given patient between the clinical signs and symptoms of hypoperfusion while being relatively normotensive.
relative refractory period – That period in the cell firing cycle at which it is possible but difficult to restimulate the cell to fire another impulse. The pattern taken by the depolarization wave is usually abnormal in these cases.
reperfusion arrhythmia – An arrhythmia that develops in a patient after a thrombolytic or an external device has reopened a clotted artery in a patient undergoing an acute myocardial infarction.
repolarization – A state in which the cell becomes more negative, moving away from equilibrium with the extracellular fluid. This is an active process.
retrograde conduction – The conduction of the electrical impulse backward through the AV node, from the ventricles or AV node to the atria.
review of systems (ROS) – A systematic series of questions that are typically addressed as part of the patient’s history in order to unearth various signs and symptoms of the patient’s condition.
rhythm strip – A single-lead electrocardiographic recording used to evaluate the patient’s rhythm.
right bundle branch (RBB) – The bundle branch that branches toward the right side. It is responsible for depolarizing the right side of the interventricular septum and the right ventricle.
right bundle branch block (RBBB) – A physiologic block of the right bundle branch causing the characteristic ECG pattern of a QRS complex > 0.12 seconds, slurred S wave in leads I and V6 and an RSR’ pattern in V1.
R-on-T phenomenon – When a premature ventricular contraction falls on the T wave of the previous complex.
RP interval – The distance from the beginning of the QRS complex to the onset of the next P wave.
R-R interval – The interval represented by the space between the R waves of two consecutive complexes.
S
S wave – The second negative wave of the QRS complex. (The first one would be the Q wave.)
S’ wave – The third negative wave inside a QRS complex. (The first one would be the Q wave, next would be the S wave.)
salvo – When three or more PVCs occur sequentially. Commonly used when the presence of ventricular tachycardia is suspected.
second marks – Small marks at the bottom of the ECG to represent an interval of time. They are usually found every 3 or 6 seconds depending on the system. At a 25-mm standard, five large boxes represent 1 second.
sensitivity – A statistical term used to describe the ability of a test or finding to correctly identify those patients who have the disease in question.
septum – The muscular or fibrous wall that separates the atria or the ventricles in the heart. When the word is used by itself it usually refers to the ventricular septum.
short QT syndrome (SQTS) – An inherited cardiac channelopathy which can lead to an ECG finding characterized by an abnormally short QT and/or QTc interval. Associated with an increased risk for atrial and ventricular arrhythmias which can lead to sudden cardiac death. The diagnosis can be made if the QTc interval is less than or equal to 330 ms, or if the QTc is less than or equal to 360 ms and is associated with one or more of the following: (1) History of cardiac arrest or syncope; (2) Family history of sudden cardiac death at 40 years of age or younger; (3) A family history of this syndrome.
sick sinus syndrome – A diseased state of the sinoatrial node that leads to multiple abnormal rhythm states. In this diseased state, the rhythms can quickly alternate between tachycardia and bradycardia, and atrial flutter and fibrillation are common.
sinoatrial (SA) node – The main pacemaker of the heart. Located anatomically in the right atrium.
slurred S wave – A slow upstroke of the S wave noted in RBBB in leads I and V6. These can have various morphologies.
somatic nervous system – The part of the nervous system that is responsible for conscious and controllable muscle movement and functions.
specificity – A statistical term used to describe the ability of a test or finding to correctly identify those without the disease in question.
ST segment – The section of the complex from the end of the QRS complex to the beginning of the T wave. Electrically, it represents the period of inactivity between ventricular depolarization and repolarization. Mechanically, it represents the time that the myocardium is maintaining contraction.
stroke volume – The amount of blood ejected from the ventricle during a single ventricular contraction.
supraventricular – Refers to an impulse or rhythm that originated above the ventricles.
supraventricular tachycardia (SVT) – A rhythm with a ventricular rate greater than or equal to 100 BPM where the rhythm starts or is maintained by tissue located in either the sinus node, atria, AV node, bundle of His, or some combination of these sites.
sustained ventricular tachycardia – Ventricular tachycardia that lasts greater than 30 seconds or is shorter than 30 seconds but required either electrical or pharmaceutical intervention to terminate.
sympathetic nervous system – The part of the nervous system that is responsible for the fight-or-flight response. Uses epinephrine and norepinephrine as the chemical messengers.
sympatholytic – Interfering with or inhibiting the effect of the impulses from the sympathetic nervous system.
sympathomimetic – Effects resembling those caused by stimulation of the sympathetic nervous system, such as the effects seen following the injection of epinephrine into a patient.
synchronized – All events occur at the same time.
T
T wave – The wave that represents ventricular repolarization.
tachycardia – A rapid rhythm (>100 BPM).
therapeutic trial – Use of an agent or procedure to cure a problem. (also known as therapeutic maneuver)
threshold potential – The electrical value at which an action potential is triggered.
TP segment – The area of baseline between the end of the T wave and the beginning of the next P wave. A line drawn from one of these to the one of a consecutive complex is the true baseline of the ECG.
Tp wave – A wave representing the repolarization of the atria. It usually presents as PR depression or the ST segment depression seen in very fast tachycardias.
tract – When related to the AV node, the pathways that lead to the AV node. (also known as approach)
transitional cell zone – A small area of tissue that surrounds the tip of the AV node. This zone is full of autonomic fibers and is very arrhythmogenic.
trigeminy – A premature complex every third beat. The complexes can be either supraventricular or ventricular.
triggered response – The pacemaker activates a depolarization wave or “fires” to a sensed event.
triplet – Three PVCs occurring sequentially.
T-wave alternans – Alternating polarity of the T wave between positive and negative that occurs on an ECG or rhythm strip.
U
U wave – A small, flat wave sometimes seen after the T wave and before the next P wave. It could represent ventricular after depolarization and endocardial repolarization.
umbrella term – A term that captures multiple rhythms or findings under one generalized category.
uncontrolled atrial fibrillation – Atrial fibrillation with rates between 100 and 200 BPM. (also known as decompensated atrial fibrillation)
unifocal PVCs – Singly occurring PVCs or PVCs that share the same common morphologic appearance because they originate in the same ectopic focus.
urgent phase of patient management – A category of patients who present with life-threatening or limb- threatening clinical states where the need for attention and treatment is necessary as quickly as possible, but a small amount of “wiggle-room” is acceptable to analyze the situation in greater depth before deciding on the clinical course. Patients with initial presentations showing a functional but decreased or decreasing level of hemodynamic instability fall under this category.
V
vagus nerve – Main nerve or pathway of the parasympathetic nervous system.
vector – A diagrammatic term used to show the strength and direction of an electrical impulse.
ventricle – Large, thick-muscled chambers of the heart, the primary pumping chambers. There are two: the left and the right.
ventricular activation velocity ratio (Vi/Vt) – The amount of vertical distance travelled by the tracing during the first 0.04 seconds or one small block (Vi) over the distance traveled during the last 0.04 seconds (Vt) of the QRS complex.
ventricular escape interval (VEI) – The interval of time during which a pacemaker is dormant before triggering the next response.
ventricular overfilling – The mechanism in the heart that increases the force of contractility of the ventricular musculature, causing more blood to be ejected during ventricular systole.
W
wave – A deflection from the baseline in either a positive or negative direction, representing an electrical event of the cardiac cycle.
wavelet – A small depolarization wave that is found in cases of atrial fibrillation.
wide-complex tachycardia – A tachycardia in which the QRS complexes are morphologically wider than 0.12 seconds; not to be confused with a wide-complex supraventricular tachycardia.
wide-complex supraventricular tachycardia – A supraventricular tachycardia where the QRS complexes are greater than or equal to 0.12 seconds wide.
Wolf-Parkinson-White (WPW) pattern – An electrocardiographic pattern characterized by short PR intervals, delta waves, and nonspecific ST-T wave changes caused by the presence of an accessory pathway.
Wolf-Parkinson-White (WPW) syndrome – A syndrome characterized by short PR intervals, delta waves, nonspecific ST-T wave changes, and paroxysmal episodes of tachycardia caused by the presence of an accessory pathway.