Week 2 Chapter 2 Basic Electrophysiology

*Phase 4—Return to Resting State* Heart is ? during this phase Ready for ? Cell will remain in this state until ?

"polarized" discharge reactivated by another stimulus

*PR segment* Part of the PR interval Horizontal line between end of P wave and beginning of QRS complex Normally isoelectric (flat)

*TP Segment* The TP segment is the portion of the ECG tracing between the end of the T wave and the beginning of the following P wave. When the heart rate is within normal limits, the TP segment is usually isoelectric. With rapid heart rates, the TP segment is often unrecognizable because the P wave encroaches on the preceding T wave.

*Horizontal Axis = Time* Width of each small box = Width of each large box (5 small boxes) =

0.04 second. 0.20 second

*Abnormal QRS Complexes* Duration of an abnormal QRS complex is greater than ?seconds

0.10

Duration of a QRS caused by an impulse originating in an ectopic pacemaker in the Purkinje network or ventricular myocardium is usually greater than ? seconds or greater

0.12 sec and often 0.16

ECG paper is graph paper made up of small and larger, heavy-lined squares Smallest squares are ? and ? ? between the heavier black lines ? within each large square

1 mm wide and 1 mm high 5 small squares 25 small squares

5 large boxes (each consisting of 5 small boxes) = 15 large boxes = 30 large boxes =

1 second. 3 seconds. 6 seconds.

*Q Wave* Normal (physiologic) Q waves Less than 0.04 sec Less than 1/3 the height of R wave in that lead Abnormal (pathologic) Q waves More than 0.04 sec More than ? of the following R wave in that lead

1/3 the height

Normal Characteristics of the P Wave Smooth and rounded No more than ? in height No more than ? in duration (width)

2.5 mm 0.11 sec

*Frontal Plane Leads* Six leads view the heart in the frontal plane

3 bipolar leads 3 unipolar leads

Normal T waves: Slightly asymmetric Usually ? in height in any limb lead Usually ? in height in any chest lead Usually ? in height in leads I and II

5 mm or less 10 mm or less 0.5 mm or more

*PR Interval (PRI)* The PR interval reflects depolarization of the right and left atria (P wave) and the spread of the impulse through the ?, ?, ?, and ?.

AV node, bundle of His, right and left bundle branches, and the Purkinje fibers (PR segment).

For a pacemaker cell to "fire" (produce an impulse), a flow of electrolytes across the cell membrane must exist. When a cell is stimulated, the cell membrane changes and becomes permeable to Na+ and K+. Permeability refers to the ability of a membrane channel to allow passage of electrolytes once it is open. Na+ rushes into the cell through Na+ channels. This causes the inside of the cell to become more positive. A spike (waveform) is then recorded on the ECG. When opposite charges come together, energy is released. When the movement of electrolytes changes the electrical charge of the inside of the cell from negative to positive, an impulse is generated. The impulse causes channels to open in the next cell membrane and then the next. The movement of charged particles across a cell membrane causing the inside of the cell to become positive is called depolarization. Depolarization must take place before the heart can mechanically contract and pump blood. Depolarization occurs because of the movement of Na+ into the cell. Depolarization proceeds from the innermost layer of the heart (endocardium) to the outermost layer (epicardium). An impulse normally begins in the pacemaker cells found in the sinoatrial (SA) node of the heart. A chain reaction occurs from cell to cell in the heart's electrical conduction system until all the cells have been stimulated and depolarized. This chain reaction is a wave of depolarization. The chain reaction is made possible because of gap junctions that exist between the cells. Eventually the impulse is spread from the pacemaker cells to the working myocardial cells. The working myocardial cells contract when they are stimulated. When the atria are stimulated, a P wave is recorded on the ECG. Thus the P wave represents atrial depolarization. When the ventricles are stimulated, a QRS complex is recorded on the ECG. Thus the QRS complex represents ventricular depolarization.

After the cell depolarizes, it quickly begins to recover and restore its electrical charges to normal. The movement of charged particles across a cell membrane in which the inside of the cell is restored to its negative charge is called repolarization. The cell stops the flow of Na+ into the cell and allows K+ to leave it. Negatively charged particles are left inside the cell. Thus, the cell is returned to its resting state. This causes contractile proteins in the working myocardial cells to separate (relax). The cell can be stimulated again if another electrical impulse arrives at the cell membrane. Repolarization proceeds from the epicardium to the endocardium. On the ECG, the ST segment and T wave represent ventricular repolarization.

Left bundle branch Divides into three fascicles ? ? and ?

Anterior fascicle Posterior fascicle Septal fascicle

The AV node is a group of cells located in the floor of the right atrium immediately behind the tricuspid valve and near the opening of the coronary sinus. The AV node is supplied by the right coronary artery in 85% to 90% of the population. In the remainder, the left circumflex artery provides the blood supply. The AV node is supplied by both sympathetic and parasympathetic nerve fibers.

As the impulse from the atria enters the AV node, there is a delay in conduction of the impulse to the ventricles. This delay occurs in part because the fibers in the AV junction are smaller than those of atrial muscle and have few gap junctions. If this delay did not occur, the atria and ventricles would contract at about the same time. The delay in conduction allows the atria to empty blood into the ventricles before the next ventricular contraction begins. This increases the amount of blood in the ventricles, increasing stroke volume.

AV node Divided into three functional regions according to their action potentials and responses to electrical and chemical stimulation ?, ?, and ?

Atrionodal (AN) Nodal (N) region Nodal-His (NH)

*Waveforms* ?: A straight line recorded when electrical activity is not detected ?: Movement away from the baseline in either a positive or negative direction ?: A line between waveforms; named by the waveform that precedes or follows it ?: A waveform and a segment ?: Several waveforms

Baseline (isoelectric line) Waveform Segment Interval Complex

Cardiac Action Potential Electrons Current Polarity Voltage Electrolytes Ions

Before we discuss of the cardiac action potential, think about how a battery releases energy. A battery has two terminals; one terminal is positive and the other is negative. Charged particles exert forces on each other, and opposite charges attract. Electrons (negatively charged particles) are produced by a chemical reaction inside the battery. If a wire is connected between the two terminals, the circuit is completed and the stored energy is released. This allows electrons to flow quickly from the negative terminal along the wire to the positive terminal. If no wire is connected between the terminals, the chemical reaction does not take place and no current flow occurs. Current is the flow of electrical charge from one point to another. Separated electrical charges of opposite polarity (positive versus negative) have potential energy. The measurement of this potential energy is called voltage. Voltage is measured between two points. In the battery example, the current flow is caused by the voltage, or potential difference, between the two terminals. Voltage is measured in units of volts or millivolts. In the normal heart, electrical activity occurs because of changes that occur in the body's cells. Human body fluids contain electrolytes, which are elements or compounds that break into charged particles (ions) when melted or dissolved in water or another solvent. The main electrolytes that affect the function of the heart are Na+, K+, Ca++, and chloride (Cl). Body fluids that contain electrolytes conduct an electric current in much the same way as the wire in the battery example. Electrolytes move about in body fluids and carry a charge, just as electrons moving along a wire conduct a current. The action potential is a five-phase cycle that reflects the difference in the concentration of these charged particles across the cell membrane at any given time.

*ECG Paper*

Fck YeAH!!$£

A record of electrical activity between two electrodes Allow viewing of the heart's electrical activity in two different planes ? , ? Each lead records the average current flow at a specific time in a portion of the heart

Frontal (coronal) Horizontal (transverse)

Lead aVR Views the heart from the right shoulder Does not view any wall of the heart

Great. Lead aVR views the heart from the right shoulder (the positive electrode) and views the base of the heart (primarily the atria and the great vessels). This lead does not view any wall of the heart.

*Standard Limb Leads* Leads ? Right arm electrode is always ? Left leg electrode is always ?

I, II, and III Negative positive

A waveform or deflection is movement away from the baseline in either a positive (upward) or negative (downward) direction. Each waveform produced is related to a specific electrical event in the heart. When electrical activity is not detected, a straight line is recorded called the baseline or isoelectric line. If the wave of depolarization (electrical impulse) moves toward the positive electrode, the waveform recorded on ECG graph paper will be upright (positive deflection).

If the wave of depolarization moves toward the negative electrode, the waveform recorded will be upside down (inverted)

The point at which the QRS complex and the ST segment meet = "?

J point" or junction

*Phase 3—Final Rapid Repolarization* ? flows quickly out of the cell Entry of ? and ? stops Cell becomes progressively more ? and more ? Corresponds with ? on the ECG

K+ Ca++ and Na+ more electrically negative and more sensitive to external stimuli T wave

Bipolar lead A lead that consists of a positive and negative electrode

Leads I, II, and III

*Augmented Limb Leads* Leads aVR, aVL, aVF A = augmented V = voltage R = right arm L = left arm F = foot (usually left leg)

Leads aVR, aVL, and aVF are augmented limb leads. The electrical potential produced by the augmented leads is normally relatively small. The ECG machine augments (magnifies) the amplitude of the electrical potentials detected at each extremity by about 50% over those recorded at the bipolar leads. The "a" in aVR, aVL, and aVF refers to augmented. The "V" refers to voltage. The "R" refers to right arm, the "L" to left arm, and the "F" to left foot (leg). The position of the positive electrode corresponds to the last letter in each of these leads. The positive electrode in aVR is located on the right arm, aVL has a positive electrode at the left arm, and aVF has a positive electrode positioned on the left leg. While leads aVR, aVL, and aVF have a distinct positive pole, they do not have a distinct negative pole. Since they have only one true pole, they are referred to as unipolar leads. In place of a single negative pole these leads have multiple negative poles, creating a negative field (central terminal), of which the heart is at the center. Theoretically, this makes the heart the negative electrode (Phalen & Aehlert, 2006).

Escape Beats or Rhythms ? produces electrical impulses Assumes ? "Protective" mechanisms Maintain ? Originate in the ? or the ?

Lower pacemaker site responsibility for pacing the heart cardiac output AV junction or the ventricles

*Phase 1—Early Repolarization* ? partially close Brief outward movement of ? Results in ? within the cell

Na+ channels K+ fewer positive electrical charges

*QRS Complex* A complex consists of several waveforms. The QRS complex consists of the Q wave, R wave, and S wave. It represents the spread of the electrical impulse through the ventricles (ventricular depolarization).

Normally, depolarization triggers contraction of ventricular tissue. Thus, shortly after the QRS complex begins, the ventricles contract. The QRS complex is significantly larger than the P wave because depolarization of the ventricles involves a considerably greater muscle mass than depolarization of the atria. A QRS complex normally follows each P wave. One or even two of the three waveforms that make up the QRS complex may not always be present.

*QRS Variations* Although the term QRS complex is used, not every QRS complex contains a Q wave, R wave, and S wave. To review, when the first deflection of the QRS complex is negative (below the baseline), the waveform is called a Q wave. The R wave is the first positive deflection (above the baseline) in the QRS complex. A negative deflection following the R wave is called an S wave. If the QRS complex consists entirely of a positive waveform, it is called an ? If the complex consists entirely of a negative waveform, it is called a ? wave. If there are two positive deflections in the same complex, the second is called R prime and is written R'.

R wave QS

Vertical Axis = Voltage/Amplitude

Size or amplitude of a waveform is measured in millivolts (voltage) or millimeters (amplitude).

*Frontal Plane Leads* View the heart from the front of the body as if it were flat Directions (4)

Superior Inferior Right Left

After passing through the AV node, the electrical impulse enters the bundle of His. It is located in the upper portion of the interventricular septum. The bundle of His has pacemaker cells capable of discharging at an intrinsic rate of 40 to 60 bpm. The bundle of His conducts the electrical impulse to the right and left bundle branches.

The bundle of His is normally the only electrical connection between the atria and the ventricles. It receives a dual blood supply from branches of the left anterior and posterior descending coronary arteries. Because of this dual blood supply, the bundle of His is less vulnerable to ischemic damage. The term His-Purkinje system or His-Purkinje network refers to the bundle of His, bundle branches, and Purkinje fibers.

Phase 0 represents depolarization and is called the rapid depolarization phase. Phase 0 also is called the upstroke, spike, or overshoot. Phase 0 begins when the cell receives an impulse. Na+ moves rapidly into the cell through the Na+ channels, K+ leaves the cell, and Ca++ moves slowly into the cell through Ca++ channels. The cell depolarizes and cardiac contraction begins. Phase 0 is represented on the ECG by the QRS complex.

The cells of the atria, ventricles, and the Purkinje fibers of the conduction system have many sodium channels. The SA and AV nodes of the heart have relatively few sodium channels. If the flow of sodium through the sodium channels is slowed or blocked, the heart rate slows, the cells become less excitable, and the speed of conduction decreases.

*P Wave* Represents atrial depolarization and spread of the impulse throughout right and left atria

The first wave in the cardiac cycle is the P wave. The first half of the P wave is recorded when the electrical impulse that originated in the SA node stimulates the right atrium and reaches the AV node. The downslope of the P wave reflects stimulation of the left atrium. Thus the P wave represents atrial depolarization and the spread of the electrical impulse throughout the right and left atria. The atria contract a fraction of a second after the P wave begins. A waveform representing atrial repolarization is usually not seen on the ECG because it is small and buried in the QRS complex.

*The Conduction System* Conduction system Specialized electrical (pacemaker) cells Arranged in a system of pathways Primary pacemaker Sinoatrial (SA) node

The specialized electrical (pacemaker) cells in the heart are arranged in a system of pathways called the conduction system. Pacemaker cells may also be referred to as conducting cells. In the normal heart, the cells of the conduction system are interconnected. The conduction system ensures that the chambers of the heart contract in a coordinated fashion. The normal heartbeat is the result of an electrical impulse that begins in the SA node. The intrinsic rate of the SA node is 60 to 100 bpm. The SA node is normally the primary pacemaker of the heart because it has the fastest firing rate of all of the heart's normal pacemaker sites.

A bipolar lead consists of two electrodes of opposite polarity (a positive and negative electrode). Each lead records the difference in electrical potential between two selected electrodes. Leads I, II, and III are called "standard limb leads" or "bipolar" leads.

Unipolar lead A lead that consists of a single positive electrode and a reference point Augmented limb leads Leads aVR, aVL, and aVF

Polarization In the body, ions spend a lot of time moving back and forth across cell membranes. As a result, a slight difference in the concentrations of charged particles across the membranes of cells is normal. Thus potential energy (voltage) exists because of the imbalance of charged particles. This imbalance makes the cells excitable. Cell membranes contain pores or channels through which specific electrolytes and other small, water-soluble molecules can cross the cell membrane from outside to inside.

When a cell is at rest, K+ leaks out of it. Large molecules such as proteins and phosphates remain inside the cell because they are too big to easily pass through the cell membrane. These large molecules carry a negative charge. This results in more negatively charged ions on the inside of the cell. When the inside of a cell is more negative than the outside it is said to be polarized. The difference in electrical charges across the cell membrane is called the membrane potential. Electrolytes are quickly moved from one side of the cell membrane to the other by means of "pumps." These pumps require energy (ATP). The energy expended by the cells to move electrolytes across the cell membrane creates a flow of current. This flow of current is expressed in volts. Voltage can be measured and appears on an electrocardiogram (ECG) as spikes or waveforms. Thus an ECG is actually a sophisticated voltmeter.

*Types of Cardiac Cells* Myocardial cells ? or ? cells Responsible for

Working or mechanical contraction

Enlargement of the right atrium produces an ?. The latter part of the P wave is prominent in ?

abnormally tall initial part of the P wave left atrial enlargement

ST segment elevation in the shape of a "smiley" face (upward concavity) is usually benign, particularly when it occurs in an otherwise healthy, asymptomatic patient. The appearance of coved ("frowny face") ST segment elevation is called an ?

acute injury pattern.

Enlargement of the right ventricle produces ?; left ventricular enlargement produces an ?

an abnormally tall R wave abnormally deep S wave

*Abnormal PR Interval* A long PR interval (greater than 0.20 second) indicates the impulse was delayed as it passed through the ? or ?

atria or AV node.

Bundle of His Connects AV node with ? Pacemaker cells have an intrinsic rate of ? Conducts impulse to ?

bundle branches 40 to 60 bpm right and left bundle branches

Purkinje fibers Receive impulse from ? Relay it to ? Pacemaker cells have an intrinsic rate of ?

bundle branches ventricular myocardium 20 to 40 bpm

Supernormal period Weaker than normal stimulus can cause ? Corresponds with ?

cardiac cells to depolarize end of T wave

Tall and pointed (peaked) or wide and notched P waves may be seen in conditions such as ?, ?, ? or ? and may be indicative of atrial enlargement.

chronic obstructive pulmonary disease (COPD), congestive heart failure (CHF), or valvular disease

Does not provide information about the mechanical (?) condition of the myocardium Evaluated by assessment of ?

contractile pulse and blood pressure

*Antiarrhythmics* Medications used to ? and slow down hearts that beat too fast Classified by their effects on the ?

correct irregular heartbeats cardiac action potential

Cardiac cells have four primary characteristics. The heart is unique because it has pacemaker cells that can generate an electrical impulse without being stimulated by a nerve. The ability of cardiac pacemaker cells to create an electrical impulse without being stimulated from another source is called automaticity. Automaticity is a property of all cells of the heart. However, the heart's normal pacemaker usually prevents other areas of the heart from assuming this function. Normal concentrations of K+, Na+, and Ca++ are important in maintaining automaticity. Increased blood concentrations of these electrolytes ? Decreased concentrations of K+ and Ca++ in the blood ?

decrease automaticity. increase automaticity.

*The Electrocardiogram (ECG)* The ECG is a voltmeter Records electrical voltages (potentials) generated by ?

depolarization of heart muscle

*Q Wave* First negative, or downward, deflection following the P wave Always a negative waveform Represents ?

depolarization of interventricular septum

Enhanced Automaticity Cardiac cells not normally associated with a pacing function begin to ? or Pacemaker sites other than the SA node ?

depolarize spontaneously increase their firing rate beyond that considered normal

Contractility refers to the ability of cardiac cells to shorten, causing cardiac muscle contraction in response to an electrical stimulus. Contractility can be enhanced with certain medications, such as ???

digitalis, dopamine, and epinephrine.

Right sided ECGs Step 7: Once these leads are printed, the correct lead must be handwritten onto the ECG to indicate the origin of the tracing. Clearly label the upper portion of the ECG tracing "right chest ECG." The computer-generated interpretation also must be ? in the event that the cables have been moved.

disregarded

A PR interval of less than 0.12 second may be seen when the impulse originates in an ? A shortened PR interval may also occur if the electrical impulse progresses from the atria to the ventricles through an abnormal conduction pathway that bypasses the AV node and ?

ectopic pacemaker in the atria close to the AV node or in the AV junction. depolarizes the ventricles earlier than usual.

Pacemaker cells Specialized cells of ? system Spontaneously generate and ?

electrical conduction conduct impulses

*Normal T Waves* The direction of the T wave is normally the same as the QRS complex that precedes it. This is because depolarization begins at the ? and spreads to the ? Repolarization begins at the ? and spreads to the ?

endocardial surface epicardium. epicardium endocardium.

The SA and AV nodes do not possess ? but ? and ?

fast Na+ channels have slow Ca++ and slow Na+ channels.

Atria Fibers of SA node connect directly with ? Impulse leaves SA node Spreads from cell to cell across ?

fibers of atria atrial muscle

Prolonged PR intervals may be seen in ? (3) among other conditions. The P wave associated with a prolonged PR interval may be normal or abnormal.

first-degree AV block, hypothyroidism, and digitalis toxicity,

Cerebral T waves The ECG of a patient with acute subarachnoid hemorrhage shows ?

giant T wave inversions.

Tall, pointed (peaked) T waves are commonly seen in ?

hyperkalemia

If the impulse originates in a bundle branch, the duration of the QRS may be only slightly greater than 0.10 seconds. For example, a QRS measuring 0.10 to 0.12 seconds is called an A QRS measuring ? seconds is called a complete bundle branch block.

incomplete bundle branch block. more than 0.12

*Properties of Cardiac Cells* Automaticity Ability of pacemaker cells to ? without being ?

initiate an electrical impulse stimulated from another source

A horizontal ST segment (forms a sharp angle with the T wave) is suggestive of ?. Digitalis causes a depression (scoop) of the ST segment sometimes referred to as a "dig dip."

ischemia

Slow-response action potentials normally occur in the SA and AV nodes but can occur abnormally anywhere in the heart, usually secondary to ??? The speed (velocity) of conduction is slower and conduction is more likely to be blocked in cardiac tissues that have slow-response action potentials.

ischemia, injury, or an electrolyte imbalance.

Lead I Records difference in electrical potential between ? (+) and ? (-) electrodes Views ? of left ventricle

left arm right arm lateral wall

Lead III Records difference in electrical potential between ? (+) and ? (-) electrodes Views ? of left ventricle

left leg left arm inferior surface

Lead II Records difference in electrical potential between ? (+) and ? (-) electrodes Views ? of left ventricle

left leg right arm inferior surface

Subarachnoid hemorrhage may cause deeply inverted T waves, usually with ?, simulating the pattern seen in myocardial infarction.

markedly prolonged QT intervals

*ST Segment Elevation* ST segment depression may reflect ? or ?. ST segment elevation may represent a ?, ?, ?, or ?.

myocardial ischemia or hypokalemia normal variant, myocardial injury, pericarditis, or ventricular aneurysm

*Abnormal T Waves* The T wave following an abnormal QRS complex is usually opposite in direction of the QRS. Negative T waves suggest ?

myocardial ischemia.

Absolute refractory period Cells cannot be stimulated to conduct an electrical impulse, ? Onset of ? to approximate peak of ?

no matter how strong the stimulus QRS complex T wave

Triggered Activity Abnormal electrical impulses occur during repolarization (after depolarizations), when cells are ? Requires a ? to initiate depolarization

normally quiet stimulus

Can provide information about: The ? of the heart in the chest ? disturbances The electrical effects of ? and ? The mass of ? The presence of ?

orientation Conduction medications and electrolytes cardiac muscle ischemic damage

A biphasic (partly positive, partly negative) waveform or a straight line is recorded when the wave of depolarization moves ? to ?

perpendicularly to the positive electrode

Reentry (Reactivation) An impulse returns to stimulate tissue that was

previously depolarized

Conduction Disturbances May occur because of: Trauma Drug toxicity Electrolyte disturbances Myocardial ischemia or infarction Conduction may be too ?

rapid or too slow

*R-R Intervals* The R-R (R wave-to-R wave) and P-P (P wave-to-P wave) intervals are used to determine the

rate and regularity of a cardiac rhythm.

Conductivity Ability of a cardiac cell to ? and conduct that impulse to ?

receive an electrical stimulus an adjacent cardiac cell

*T Wave* Ventricular repolarization is represented on the ECG by the T wave. The absolute refractory period is still present during the beginning of the T wave. At the peak of the T wave, the ? has begun. It is during the ? that a stronger than normal stimulus may produce ?

relative refractory period relative refractory period ventricular dysrhythmias.

*Phase 2—Plateau Phase* Repolarization continues ? Slow inward movement of ? Slow outward movement of ? Responsible for ? on ECG

relatively slowly Ca++ K+ ST segment

*U Waves* A U wave is a small waveform that, when seen, follows the T wave. The mechanism of the U wave is not definitely known. One theory suggests that it represents ? Normal U waves are small, round, and less than 1.5 mm in amplitude. The ECG shows normal U waves (seen in leads V2 through V4) in a 22-year-old male.

repolarization of the Purkinje fibers.

Excitability (irritability) Ability of cardiac muscle cells to ?

respond to an outside stimulus

*ST Segment* Portion of the ECG tracing between QRS complex and T wave Represents early part of repolarization of

right and left ventricles

AV node Supplied by ? in most people Delays conduction of impulse from atria to the ventricles Allows time for

right coronary artery atria to empty into ventricles

Contractility Ability of cardiac cells to ?, causing cardiac muscle contraction in response to ?

shorten an electrical stimulus

*S Wave* The S wave is the negative waveform following the R wave. The R and S waves represent ? Because of its greater muscle mass, the QRS complex generally represents the ?

simultaneous depolarization of the right and left ventricles. electrical activity occurring in the left ventricle.

The QRS complex continues as a large, upright, triangular waveform called

the R wave. An R wave is always positive.

*Phase 0—Depolarization* Begins when ? ? moves rapidly into cell ? leaves cell ? moves slowly into cell Cell ?; contraction begins Responsible for ?

the cell receives an impulse Sodium Potassium Calcium depolarizes QRS complex on the ECG

Lead aVL Views the heart from ? Oriented to the lateral wall of the left ventricle

the left shoulder

Relative refractory period Cardiac cells can be stimulated to depolarize if ? Corresponds with ?

the stimulus is strong enough downslope of T wave

*Refractory Periods* The period of recovery that cells need after being discharged before?

they are able to respond to a stimulus

*QT Interval* The QT interval represents ? The term "QT" interval is used regardless of whether the QRS complex begins with a Q or R wave. The duration of the QT interval varies according to age, gender, and particularly heart rate.

total ventricular activity —the time from ventricular depolarization (activation) to repolarization (recovery).

Waveform Deflections If the wave of depolarization moves toward the positive electrode, the waveform recorded will be ?

upright

*Artifact* Distortion of an ECG tracing by electrical activity that is noncardiac in origin Can mimic various cardiac dysrhythmias, including ? Patient evaluation essential before initiating any medical intervention

ventricular fibrillation

Interval A ? Important intervals PR interval QT interval

waveform and a segment