Chapter 11 Electrocardiogram (ECG) Waveforms

• When a cardiac impulse travels through the heart, the electrical current spreads to adjacent tissues and to the body's surface
• Electrodes on the skin can record these electrical potentials in a process called electrocardiogram (ECG)

Waveforms Of The Normal Electrocardiogram

• A normal ECG consists of a P wave, a QRS complex, and a T wave
• The QRS complex sometimes consists of three separate waves: Q, R, and S
• The P wave represents atrial depolarization before atrial contraction
• The QRS complex represents ventricular depolarization before ventricular contraction
• Both the P wave and the QRS complex are depolarization waves
• The T wave represents ventricular repolarization
• Ventricular repolarization normally occurs 0.25 to 0.35 seconds after depolarization, the T wave is a repolarization wave
• The ECG consists of depolarization and repolarization waves

Cardiac Depolarization Waves Versus Repolarization Waves

• Figure 11-2 shows a single cardiac muscle fiber during depolarization and repolarization
• During depolarization, the inside of the fiber becomes positive and the outside becomes negative
• Figure 11-2A shows depolarization traveling left to right
• The left electrode is in an area of negativity, while the right electrode is in an area of positivity, causing a positive meter recording
• The recording reaches a maximum positive value when depolarization reaches the halfway mark
• Figure 11-2B shows complete depolarization, electrodes are in equal negativity, returning the recording to a zero baseline
• This completed wave is a depolarization wave because it results from the spread of depolarization along the muscle fiber membrane
• Figure 11-2C shows halfway repolarization with positivity returning to the outside of the fiber
• The left electrode is in an area of positivity, and the right electrode is in an area of negativity, reversing the polarity from Figure 11-2A, shown as a negative deflection
• Figure 11-2D shows complete repolarization, the electrodes are in areas of positivity so no potential difference is recorded and the potential returns to zero
• This completed negative wave is a repolarization wave

Relation Of The Monophasic Action Potential Of Ventricular Muscle To The QRS and T Waves in the Standard Electrocardiogram

• The monophasic action potential of ventricular muscle normally lasts between 0.25 and 0.35 second
• Figure 11-3 shows a monophasic action potential from a ventricular muscle fiber
• The upsweep of this action potential is caused by depolarization, the return to baseline is caused by repolarization
• The QRS waves appear at the beginning of the monophasic action potential, the T wave appears at the end
• No potential is recorded when the ventricular muscle is either completely polarized or completely depolarized
• Current flows from one part of the ventricles to another only when the muscle is partly polarized and partly depolarized, generating current to the body surface to produce the ECG

Relationship Of Atrial And Ventricular Contraction To The Waves Of The Electrocardiogram

• Depolarization must spread through the muscle to initiate chemical processes that lead to contraction
• The P wave occurs at the beginning of atrial contraction, QRS complex occurs at the beginning of ventricular contractions
• The ventricles remain contracted until after repolarization has occurred, after the end of the T wave
• The atria repolarize about 0.15 to 0.20 second after the P wave terminates, approximately during the QRS complex
• The atrial repolarization wave (atrial T wave) is obscured by the larger QRS complex, seldom observed on the ECG
• Ventricular repolarization wave is the T wave of the normal ECG
• Some ventricular muscle fibers begin to repolarize about 0.20 second after the beginning of the depolarization wave (QRS complex), while others take as long as 0.35 second
• Ventricular repolarization extends over a long period, about 0.15 second, resulting a prolonged T wave with lower voltage than the QRS complex

Electrocardiographic Calibration And Display

• ECG recordings are made with calibration lines on a display grid
• ECGs were historically printed onto paper, ECGs are now usually digitally displayed
• Horizontal calibration lines are arranged so that 10 small line divisions upward or downward represent 1 millivolt
• Positivity is in the upward direction, negativity in the downward direction
• Vertical lines on the ECG are time calibration lines
• A typical ECG is run at a speed of 25 millimeters per second
• Each 25-millimeter segment horizontally is 1 second, indicated by dark vertical lines for each 5-millimeter segment representing 0.20 second
• The 0.20-second intervals are broken into smaller intervals by thin lines, each representing 0.04 second

Normal Voltages In The Electrocardiogram

• Recorded voltages of the waves in a normal ECG depend on electrode placement and proximity to the heart
• Placing an electrode directly over the ventricles, with the second elsewhere on the body remote from the heart, the QRS complex may be as high as 3 to 4 millivolts
• Even 3 to 4 millivolts is small compared to the monophasic action potential of 110 millivolts recorded directly at the heart muscle membrane
• When ECGs are recorded from electrodes on the two arms or on one arm and one leg, the voltage of the QRS complex usually is 1.0 to 1.5 millivolts from the top of the R wave to the bottom of the S wave
• The voltage of the P wave is between 0.1 and 0.3 millivolts and the voltage of the T wave is between 0.2 and 0.3 millivolts

P-Q Or P-R Interval

• The time between the beginning of the P wave and the beginning of the QRS complex is the interval between the beginning of electrical excitation of the atria and the beginning of excitation of the ventricles (P-Q interval)
• The normal P-Q interval is about 0.16 second (often called the P-R interval because the Q wave is likely to be absent)
• The P-R interval shortens at faster heart rates because increased sympathetic or decreased parasympathetic activity, which increase atrioventricular node conduction speed
• Conversely, the P-R interval lengthens with slower heart rates due to slower atrioventricular nodal conduction caused by increased parasympathetic tone or withdrawal of sympathetic activity

Q-T Interval

• Ventricular Contraction lasts almost from the beginning of the Q wave (or R wave, if the Q wave is absent) to the end of the T wave which is about 0.35 second(Q-T interval)

Flow Of Current Around The Heart During The Cardiac Cycle

• Figure 11-4 shows a syncytial mass of cardiac muscle stimulated at its most central point
• The exteriors of the muscle cells are positive and the interiors are negative before stimulation
• As an area of cardiac syncytium depolarizes, negative charges leak to the outsides of the depolarized muscle fibers, making that part of the surface electronegative
• The surface of the heart which is still polarized remains positive
• A meter with its negative terminal on the area of depolarization and its positive terminal on one of the still polarized areas records positively
• Two other electrode placements and meter readings are demonstrations, understanding these placements helps understand how the placement affects readings
• Depolarization spreads in all directions through the heart, the potential differences persist for only a few thousandths of a second
• Voltage measurements is accomplished only with a high-speed recording apparatus

Flow Of Electrical Currents In The Chest Around The Heart

• Figure 11-5 shows the ventricular muscle in the chest
• The lungs conduct electricity to a degree, the fluids in the tissues conduct electricity more easily, so the heart is suspended in a conductive medium
• An electronegative portion of the ventricles relative to the remainder of the heart creates electrical current flow from the depolarized area to the polarized area in circuitous routes
• The cardiac impulse first arrives in the ventricles in the septum and spreads to the inside surfaces of the ventricles
• This process negativity on the insides of the ventricles and positivity on the outer walls, with electrical current flowing through the fluids surrounding the ventricles
• Average current flow is negativity toward the base of the heart and with positivity toward the apex
• Current continues flowing in the same direction and depolarization spreads
• Immediately before depolarization completes, the average direction of current flow reverses from the ventricular apex toward the base for roughly 0.01 second
• Current primarily flows from negative to positive in the direction from the base of the heart toward the apex during almost the entire cycle of depolarization, except at the very end
• An electrode connected near the base will be negative, the electrode near the apex will be positive, and the recording meter will show a positive recording

Electrocardiographic Leads

• Figure 11-6 shows electrical connections between the patient's limbs and the electrocardiograph

Three Standard Bipolar Limb Leads

• Bipolar means the ECG is recorded between electrodes on different sides of the heart
• A lead is not a single wore connecting from the body, but a combination of two wires and their electrodes making a complete circuit between the body and the electrocardiograph
• The diagrams present the electrocardiograph as an electrical meter, in reality they are high-speed computer electronic systems

Lead 1

• The negative terminal of the electrocardiograph connects to the right arm, the positive terminal connects to the left arm
• Electronegativity at the right arm connection relative to the left arm results in a positive recording
• The opposite polarity creates a negative recording

Lead 2

• The negative terminal connects to the right arm and the positive terminal connects to the left leg
• Right arm negativity versus the left leg positivity creates a recording

Lead 3

• The negative terminal connects to the left arm, and the positive terminal connects to the left leg
• Left arm negativity versus the left leg positivity creates a positive recording

Einthoven's Triangle

• Drawn around the area of the heart, and illustrates that the two arms and left leg form apices of a triangle surrounding the heart
• The two apices at the upper part of the triangle represent the points that which the two arms connect to the fluids surrounding the heart, and the lower apex is the point at which the left leg connects

Einthoven's Law

• If the ECGs are recorded simultaneously with the three limb leads, the sum of the potentials recorded in leads I and III will equal the potential in lead II; Lead I potential+Lead III potential = Lead II potential
• If the electrical potentials of any two of the three bipolar limb electrocardiographic leads are known at any given instant, the third can be determined by summing the first two (maintain proper positive and negatives when summing)
• For instance, assuming the right arm is –0.2 millivolts (negative), the left arm is +0.3 millivolts (positive), and the left leg is +1.0 millivolts (positive)
• Lead I records a positive potential of +0.5 millivolts (the difference between the -0.2 millivolts on the right arm and the +0.3 millivolts on the left arm), lead III records a positive potential of +0.7 millivolts, and lead II records a positive potential of +1.2 millivolts because these are the instantaneous potential differences between the respective pairs of limbs -The sum of the voltages in leads I and III equals the voltage in lead II, (0.5 + 0.7 = 1.2)

Normal Electrocardiograms Recorded from the Three Standard Bipolar Limb Leads

• The ECGs in leads I, II, and III are similar, recording positive P waves and positive T waves
• The major portion of the QRS complex is also positive in each ECG because the recordings are from all the bipolar limb leads
• Arrhythmias main diagnosis is on the time between cardiac cycles however a determination of ventricular or atrial muscle or in the Purkinje conducting system must have the particular leads recorded in mind as damage in cardiac muscle contraction or impulse conduction changes the patterns of ECGs significantly in some leads and in others very little

Precordial Leads

• ECGs are recorded with one electrode placed on the anterior chest surface directly over the heart
• The electrode connects to the positive terminal electrograph, the negative electrode(indifferent electrode or Wilson central terminal) connects through equal electrical resistances to the right arm, left arm, and left leg
• Six standard chest leads are recorded, one at a time, from the anterior chest wall and known as leads V1, V2, V3, V4, V5, and V6
• The ECGs from these six standard chest leads as recorded from the healthy heart record mainly the electrical potential of cardiac musculature because the surface is close the chest wall
• Minute abnormalities in the ventricles can cause marked changes in individual chest leads especially anterior wall

Augmented Limb Leads

• Two of the limbs are connected through electrical resistances to the negative terminal of the electrocardiograph, and the third limb is connected to the positive terminal
• A lead is identified as aVR when the connection is the right arm connecting the positive, aVL is the connection on the left arm, and aVF is the connection on the left leg
• Standard limb lead recordings identify normal recordings of the augmented limb leads, except from the aVR lead where the level is inverted

Electrocardiographic Display

• Leads are displayed into three groupings: the standard bipolar limb leads, then the augmented leads, and then the precordial leads

Ambulatory Electrocardiography

• Standard ECGs assess cardiac electrical events briefly, usually while the patient is resting
• Ambulatory electrocardiography extends the ECG to assess cardiac electrical events while the patient is ambulating during normal daily activities, evaluating changing cardiac electrical phenomena
• Ambulatory electrocardiographic monitoring is used when a patient demonstrates symptoms thought to be caused by transient arrhythmias or other transient cardiac abnormalities
• These symptoms may include chest pain, syncope (fainting) or near syncope, dizziness, and irregular heartbeats (palpitations)
• Key information is the ECG recording during the precise time the symptom is occurring
• Devices detect asymptomatic cardiac arrhythmias such as atrial fibrillation that heighten the risk of embolus formation
• There are continuous recorders (Holter monitors) for 24 to 48 hours to investigate the relationship of symptoms and electrocardiographics
• Intermittent recorders exist for longer periods (weeks to months) to provide brief intermittent recordings for detection of events that occur infrequently
• A small device called a loop recorder implantable just under the skin in the chest can monitor electrical activity continuously for as long as 2 to 3 years
• Solid-state digital technology are improving recorders that are equipped with microprocessors and that transmit digital electrocardiographic data over telephone lines, offering online computerized analysis of the data as they are acquired
• Emerging newer wearable devices continue to develop for home based heart rhythm monitoring