ECG: lectures 1-14

Define electrical polarization in cardiac cells

Cardiac muscle cells at rest are considered polarized. This means the electrical charges are evenly distributed such that the outside of the cell is relatively positive and the inside of the cell is relatively negative. In this state, there is no net electrical vector, and a voltmeter will record no deflection, remaining at the isoelectric baseline

how does blood flow from the body through the heart and back into the body

systemic veins → inferior and superior venae cavae → right atrium → tricuspid valve → right ventrical → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → gas exchange in the lungs → pulmonary veins → left atrium → bicuspid valve → left ventricle → aortic semilunar valve → aorta → gas exchange in peripheral tissues

Identify the ions responsible for maintaining the resting membrane potential in cardiac cells.

• Responsible Ions: The primary ions involved in the electrical state of the cell are Sodium (Na+), Potassium (K+), and Calcium (Ca2+).

• Resting Potential: For a non-pacemaker cardiomyocyte, the stable resting membrane potential is approximately -90 mV

ligand gated ion channels

open or close based on presence of a chemical (ex: hormones, neurotranmitters, second-messengers)

voltage gated ion channels

open or close based on electrical potenital across membrane

how can ions move

move passively based on concentration gradients or/and voltage gradient (membrane potential)

Identify the role of membrane pumps and ion channels in maintaining the resting electrical state of cardiac cells

• Na+ -K+ ATP Pump: This membrane pump is active in maintaining the potential.

• Ion Channels: These are transmembrane proteins that allow ions to pass based on their charge and size. They can be voltage-gated (opening/closing based on electrical potential) or ligand-gated (responding to chemicals like hormones or neurotransmitters).

• Leaky K+ Channels: These channels are specifically identified as active during Phase 4, the resting potential phase

Recognize depolarization as the fundamental electrical event of the heart.

• In pacemaker cells is occurs spontaneously and in others it is initiated by the arrival of an electrical impulse that causes positively charged ions to cross the cell membrane 

• Is propagated from cell to cell, producing a wave of depolarization that can be transmitted across the entire heart 

• Wave of depolarization represents a flow of electricity, an electrical current, that can be detected by electrodes placed on the surface of the body

• Initiation: In non-pacemaker cells, it is initiated by an external stimulus that opens voltage-gated Na+ channels, causing a rapid influx of positively charged sodium ions.

Differentiate between spontaneous depolarization in pacemaker cells and depolarization initiated by an external electrical impulse.

• Pacemaker cells possess automaticity, meaning they are "self-firing" and do not require an external impulse. Their resting potential is unstable and slowly drifts upward until it fires.

• Non-pacemaker cells lack automaticity and require an external electrical impulse to begin depolarization.

• Propagation: The wave of depolarization is propagated from cell to cell via gap junctions.

• Detection: This moving wave represents a flow of electricity (an electrical current) that creates a charge difference between depolarized and resting tissue. This current is detected by electrodes on the body surface as an electrical vector with a specific direction and magnitude

how does depolarization spread between cells?

via gap junctions which allows for the rapid depolarization across the cardiac tissue

what distinguishes cardiac muscle cells from skeletal muscle cells?

cardiac musclse cells have prolonged depolarization during phase 2

Differentiate the physiology of a pacemaker action potential from a cardiomyocyte action potential

• pacemaker cells (found in the SA and AV nodes) and non-pacemaker cardiomyocytes (found in the atria and ventricles) have several distinct physiological differences

• Primary Role: Pacemaker cells are responsible for setting the heart rate, while cardiomyocytes are designed to generate force.

• Automaticity: Pacemaker cells possess automaticity, meaning they are self-firing. Non-pacemaker cells lack this property and require an external electrical impulse to initiate depolarization.

• Resting Membrane Potential: The resting potential of pacemaker cells is unstable and slowly drifts upward toward a threshold. Cardiomyocytes maintain a stable resting potential of approximately -90 mV.

• Phase 0 (Depolarization): In pacemaker cells, Phase 0 is slow and Ca2+ -mediated. In cardiomyocytes, it is fast and mediated by Na+ influx through voltage-gated channels.

• Action Potential Shape: Cardiomyocytes feature a prolonged plateau phase (Phase 2) caused by Ca2+ influx, which prevents tetany and distinguishes them from skeletal muscle. Pacemaker cells have no true plateau phase.

what are the stages of cardiac depolarization and repolarization

phase 0: slow and Ca2+ -mediated. In cardiomyocytes, it is fast and mediated by Na+ influx through voltage-gated channels.

phase 1: sodium channels close

phase 2: prolonged plateau phase caused by Ca2+ influx, which prevents tetany and distinguishes them from skeletal muscle. Pacemaker cells have no true plateau phase.

phase 3: rapid repolarization where potassium leaks out and calcium channels close

phase 4: repolarization where potassium continues to leak out to return cell to resting membrane potential

Describe how depolarization propagates through cardiac tissue and how it produces an electrical current detectable on the body surface.

• Depolarization is the fundamental electrical event of the heart and propagates through the following mechanism:

  • Cell-to-Cell Propagation: The wave of depolarization spreads throughout the myocardium via gap junctions.

  • Current Generation: When a region of a cell depolarizes, it becomes less negative inside and relatively negative outside compared to the resting tissue. This creates a charge difference between the depolarized and resting areas, causing a current of depolarization to flow.

  • Surface Detection: This flow of electricity represents an electrical vector with a specific direction and magnitude. As this wave moves toward a positive electrode placed on the body, the voltmeter senses a voltage difference, causing the ECG tracing to deflect upward.

  • Magnitude and Return to Baseline: As more myocardium depolarizes, the voltmeter senses a larger voltage difference, increasing the magnitude of the deflection. Once the tissue is fully depolarized, the charges are again evenly distributed, the charge gradient disappears, and the tracing returns to the isoelectric baseline

what occurs in action potentials for cardiac myocytes

• rapid, transient change in membrane potential in cardiac mucsle cells

• initiated by voltage-gates sodium channels opening causing depolarization

• characterized by a prolonged plateau phase due to L-type calcium influx

• followed by repolarization mediated by potassium effux

• results in coordinated myocardial contraction and long refractory period that prevents tetany

Define repolarization and identify the mechanism by which cardiac cells return to their resting electrical state.

• Definition: Repolarization is defined as the return to a negative membrane potential, specifically back to -90 mV.

• Primary Mechanism: This process is primarily mediated by K+ efflux, which is the movement of potassium ions out of the cell.

• Phases of Return: In cardiomyocytes, following the Ca2+ -driven plateau, Phase 3 (Rapid Repolarization) occurs as Ca2+ channels close and K+ continues to exit the cell. The cell then enters Phase 4, the resting potential, where leaky K+ channels and the Na+-K+ ATP pump help maintain the resting electrical state

Recognize that both depolarization and repolarization generate measurable electrical signals

The ECG is a technical recording that summarizes the electrical activity of the heart. It measures both depolarization and repolarization events. Because the heart contains 2 to 4 billion cardiomyocytes, several million cells are experiencing these electrical events at any given instant; the ECG provides a summation of these signals as they occur over time.

Associate ECG waveforms with the processes of depolarization and repolarization

• The waveforms recorded on an ECG correspond directly to the electrical state of the cardiac tissue:

  • Depolarization Waveform: A wave of depolarization moving toward a positive electrode is recorded as an upward deflection.

  • Repolarization Waveform: A wave of repolarization detected by a positive electrode is recorded as a downward deflection.

  • Baseline (Isoelectric Line): The signal remains at or returns to the baseline when the heart is either resting (fully polarized) or fully depolarized, because in these states, charges are evenly distributed and no charge gradient exists for the voltmeter to detect.

measuring repolarization

• means returning to negative membrane potential (-90mV)

• wave of repolarization detected on (+) electrode as downward deflection

• when fully repolarized, signal returns back to baseline

what is/purpose of an electrocardiogram (ECG)?

• measures the depolarization and repolarization events occurring in the heart

• measurements are recorded as upward and downward delfection from a baseline over time depending in depolarization or repolarization events

• some number to consider:

  • there are 2-4 billion cardiomyocytes in the adult human heart

  • in any instant, several million will be experiencing a depolarization or repolarization event

  • the ECG is technically recording all of these at once but the recording is a summation of all events

nonpacemaker cells vs. pacemaker cells

nonpacemaker cells;

• atria/ventricles

• primary role: generate force

• resting membrane potential: stable at about -90mV

• automatcity: absent

• phase 0 depolarization: sodium meditated, fast

• action potential shape: plateau phase present

pacemaker cells

• AV/SA nodes

• primary role: set heart rate

• resting membrane potential: unstable (slowly drifts upward)

• automatcity: absent

• phase 0 depolarization: sodium meditated, fast

• action potential shape: plateau phase present

what is the vertical axis on a ECG represent

voltage

what does the horizontal axis on ECG represent

time

Describe the electrical events of the cardiac cycle that produce the P wave, QRS complex, and T wave on the EKG.

• P wave: atrial depolarization 

• QRS complex: ventricular depolarization 

• T wave: ventricular repolarization 

Identify the three defining characteristics of EKG waveforms and explain how they are measured.

• Waveforms are defined by three characteristics: 

  • Amplitude (energy), 

  • Duration (time)

  • Morphology (shape/appearance).

• Measurement: Amplitude is measured on the vertical axis (voltage/height), while duration is measured on the horizontal axis (time)

Interpret time intervals and voltage amplitudes on standard EKG paper using small and large squares.

• Horizontal (Time): One small square represents 0.04 seconds. One large square (5 small squares) represents 0.2 seconds. Five large squares equal 1 second.

• Vertical (Voltage): One small square represents 0.1 mV (1 mm). One large square represents 0.5 mV (5 mm)

what is the print speed for an ECG

25 mm/sec

P waves (location, amplitude, duration, and morpholgy)

• location: one P wave preceded each QRS complex

• amplitude: 0.5-2.5mm

• duration: 0.06-0.10 seconds (1.5-2.5 small squares)

• morphology: rounded and upright

P waves in Limb leads I, II, III

• lead I: upright

• Lead II: upright

• Lead III: upright, biphasic or inverted

P waves in aVR, aVL, and aVF

• lead aVR: inverted

• lead aVL: upright, biphasic, or inverted

• lead aVF: upright

P waves in the chest leads

• V1: biphasic

• V2: small positive or biphasic

• V3: positive

• V4: positive

• V5: positive

• V6: positive but often small

PR interval (location, duration, morphology)

• location: starting at the beginning of the P wave and ends at the beginning of the Q wave (or R wave if Q is absent)

• duration: 0.12-0.20

• morphology: P wave followed by a flat line

QRS complex (location, amplitude, duration, morphology)

• location: follows the PR interval

• amplitude: 5-30 mm

• duration: 0.06 to 0.12 seconds

• morphology: varies significantly by lead

Explain the sequence of atrial and ventricular depolarization and how it is reflected in EKG wave morphology

• The heart depolarizes in a specific sequence reflected on the EKG: atrial depolarization (P wave) occurs first, followed by ventricular depolarization (QRS complex).

• P wave morphology: Usually rounded and upright.

• QRS morphology: Varies significantly by lead but follows a specific naming convention based on deflections.

  • Q wave: first negative deflection from the baseline following the p wave

  • R wave: first positive deflection in QRS

  • S wave: first negative deflection that extends below the baseline after the R wave 

• J point: This represents the exact point where ventricular depolarization ends and repolarization begins.

• T wave: positive deflection after each QRS complex

• U wave: small deflection immediately following the T wave, usually in the same direction as the T wave → cause unknown 

QRS complex in limb leads:

• lead I: upright

• leads II: upright

• lead III: small but mostly upright or biphasic

QRS complex in aVF, aVL, aVR:

• lead aVF: upright

• lead aVL upright or biphasic

• lead aVR: negative

QRS complex in chest leads

• V1: predominately negative

• V2: predominately negative or biphasic

• V3: biphasic

• V4: upright or biphasic

• V5: upright

• V6: upright

measuring QRS width

• use the most obvious tracing

• start with the first deflection

• measure to the beginning of the ST segment

the J point

• junction between the termination of the QRS complex and the beginning of the ST segment

• the height of the j point is where we measure ST elevation

• represents the end of ventricular depolarization and the beginning of repolarization

the ST segment (location, rep, what causes abnormalitity)

• the flat, isoelectric line between the end of the J point and the beginning of the T wave

• represents the interval between ventrivcular depolarization and repolariztion

• most important cause of ST segment abnormality is myocardial ischemia or infaraction

the t wave (location, reps, where is it upright/inverted, amplitude)

• is the positive deflection after each QRS complex

• repersents ventricular repolarization

• upright in all leads expect aVR and V1

• amplitude:

  • less then 5mm in limb leads

  • less then 10mm in precordial leads

    • 10mm in males

    • 8mm in females

QT interval (location, what does it rep, range)

• beginning of QRS complex to end of T wave

• represents ventricular depolarization and repolariation

• normal range:

  • 0.36 -0.44 but is dependent in heart rate

the U wave

• small (0.5mm) deflection immediately following the T wave

• usually in the same direction as the T wave

• best seen in leads V2 and V3

• cause unknown

Describe the physiologic role of the AV node and predict how autonomic influences affect AV conduction.

• Physiologic Role of the AV Node: 

  • Impulse Delay: It slows the cardiac impulse, allowing time for the atria to contract fully and fill the ventricles.

  • Atrial-Ventricular Separation: Acts as the only normal pathway between atria and ventricles, preventing premature electrical activation of the ventricles.

  • Protection (Rate Limiting): Limits the number of impulses conducted to the ventricles during atrial tachycardia or fibrillation.

  • Backup Pacemaker: Generates electrical signals at a lower intrinsic rate 

    • 40–60 bpm if the Sinoatrial (SA) node fails. 

• Autonomic Influence on AV Conduction: 

  • Sympathetic Stimulation (Fight or Flight): Increases conduction velocity (positive dromotropy) and reduces the nodal delay, allowing for faster heart rates.

  • Parasympathetic Stimulation (Rest and Digest): Via the vagus nerve, this decreases conduction velocity (negative dromotropy), increases the nodal delay, and increases the refractory period. This can lead to increased AV block, slowing the ventricular rate. 

Identify the components of the ventricular conduction system and relate their activation to the QRS complex

Components of the Ventricular Conduction System

• Atrioventricular (AV) Bundle/Bundle of His: Conducts the impulse from the AV node through the fibrous skeleton of the heart into the interventricular septum.

• Bundle Branches (Left and Right): The Bundle of His splits into the left and right branches, which carry the impulse down the interventricular septum.

• Purkinje Fibers: A fast-conducting network that spreads the impulse rapidly throughout the ventricular myocardium, starting from the endocardium and moving to the epicardium.

Relationship to the QRS Complex

• Q Wave: Initial depolarization of the interventricular septum.

• R Wave: Rapid, major depolarization of the left and right ventricular free walls by the Purkinje network.

• S Wave: Final depolarization of the base of the ventricles.

• Activation Sequence: The impulse travels from the Bundle of His to the branches, then to the Purkinje fibers, causing the swift, near-simultaneous activation of the ventricles. A wider QRS indicates a delay in this, such as a bundle branch block.

Apply standard naming conventions to identify and label the components of the QRS complex.

• Q wave: The first negative deflection following the P wave.

• R wave: The first positive deflection in the complex.

• S wave: The first negative deflection that extends below the baseline after the R wave.

  • Small letters (q, r, s) are used for small deflections, and capital letters (Q, R, S) are used for large deflections

Compare ventricular depolarization and repolarization with respect to timing, waveform shape, and amplitude.

• Depolarization 

  • Time: fast QRS lasting 0.06 to 0.12 seconds 

  • Waveform: a sharp, narrow, often multi-phasic complex (QRS) and it represents a fast, synchronous activation of the ventricles

  • Amplitude: high

• Repolarization 

  1. Time: occurs over a longer internal following the ST segment, usually lasting 0.10 to 0.25 seconds 

  2. Waveform: broader, lower-amplitude wave that is usually asymmetrical and rounded  

  3. Amplitude: low 

Differentiate EKG segments from intervals and determine what electrical events each represents.

• Intervals: Include at least one total wave plus a connecting straight line (e.g., PR Interval includes the P wave; QT Interval includes the QRS and T wave).

• Segments: Refer only to the flat, isoelectric line between waves (e.g., ST Segment is the line between the J point and the T wave)

ECG Lead polarity

• bipolar has negative and positive electrodes (leads I, II, and III)

• unipolar has positive electrodes only; the negative electrode is substituted by a reference point calculated by the ECG machine (leads aVR, aVL, aVF, and V1-V6)

what are the limb leads

I, II, III, aVR, aVF, aVL

bipolar standard limb leads

I,II,III

unipolar leads

• augmented right (aVR), left (aVL), foot (aVF)

• precordial chest leads: V1-V6

where do the limb leads look at in the body

frontal plane

where do the precordial leads look at in the body

cross section or horizontal plane

chest lead placement

• V1: 4th intercostal space, just to the right of the sternum

• V2: 4th intercostal space, just to the left of the sternum

• V3: midway between V2 and V4

• V4: 5th intercostal space, mid-clavicular line

• V5: anterior axillary line, between V4 and V6

• V6: mid-axillary line, horizontal with V4

Predict EKG waveform polarity based on the direction of depolarization or repolarization relative to a recording electrode.

• Upward deflection: Produced when an electrical impulse travels toward a positive electrode.

• Downward deflection: Produced when an impulse travels away from a positive electrode (or toward a negative one).

• Biphasic waveform: Produced when an impulse travels perpendicular to the positive electrode

Explain why multiple leads are required to interpret cardiac electrical activity and define what constitutes an EKG lead.

• A lead is a "point of view." The 12-lead EKG uses 12 different perspectives to interpret the heart's electrical activity.

• Lead Types:

  1. Bipolar: Uses both a negative and a positive electrode (Leads I, II, III).

  2. Unipolar: Uses only a positive electrode, with a calculated reference point acting as the negative (aVR, aVL, aVF, and V1–V6).

Identify the orientation, anatomic perspective, and regional associations of the six limb leads.

• The six limb leads (I, II, III, aVR, aVL, aVF) view the heart in the frontal plane.

  1. Standard Limb Leads (Bipolar): 

     • Lead I (0°)

     • Lead II (+60°)

     • Lead III (+120°).

  2. Augmented Leads (Unipolar): 

     • aVL (-30°)

     • aVR (-150°)

     • aVF (+90°)

Identify standard precordial lead placement and relate each lead to the cardiac region it best visualizes.

• The precordial leads (V1–V6) view the heart in the horizontal plane.

  • V1 & V2: 4th intercostal space (R/L of sternum); visualizes the Anteroseptal region.

  • V3 & V4: V4 is 5th intercostal space (mid-clavicular); visualizes the Anteroapical region.

  • V5 & V6: V6 is mid-axillary line; visualizes the Anterolateral region

Recognize how patient positioning and electrode misplacement can alter EKG appearance and lead to misinterpretation.

Electrode misplacement, such as Lead Reversal, significantly alters the EKG. For example, if the Right Arm (RA) and Left Arm (LA) electrodes are swapped, the appearance of Lead I will be inverted, and the augmented leads (aVR and aVL) will be swapped, potentially leading to misinterpretation of the heart's rhythm or axis

Wave Polarity Logic:

An electrical impulse traveling toward a positive electrode is recorded as an upward (positive) deflection, while an impulse traveling away from a positive electrode is recorded as a downward (negative) deflection

Direction of Atrial Depolarization:

Atrial depolarization (represented by the P wave) starts at the SA node, travels down the right atrium, and then moves from the right atrium to the left atrium

Expected Morphology in p wave:

The P wave is normally upright in most leads, inverted in lead aVR, and biphasic in lead V1.

P-Wave Limits:

Normal P-wave amplitude is less than 0.25 mV (equivalent to < 2.5 mm or 2.5 small boxes). Its duration should be less than 0.10 seconds.

what do pathological Q waves mean?

prior myocardial infarction

Identify the normal direction of atrial depolarization and predict expected P-wave morphology in limb and precordial leads.

• Normal Direction: Atrial depolarization (the P wave) starts at the SA node, travels down the right atrium, and then moves from the right atrium to the left atrium.

• Expected Morphology: Because of this direction, the P wave is normally upright in most leads

  • leads I, II, III, aVF, aVL, V2-V6 → upright

  • Lead aVR → inverted

  • lead V1 → biphasic

Recognize which ECG leads normally display positive, negative, or biphasic P waves.

• Positive: Upright in most leads.

• Negative: Normally inverted in lead aVR.

• Biphasic: Normally biphasic in lead V1

Identify normal P-wave amplitude limits and the leads in which the P wave is typically most positive and most negative.

• Amplitude Limit: The normal P-wave amplitude is less than 0.25 mV (equivalent to < 2.5 mm or 2.5 small boxes).

• Duration Limit: The duration should be less than 0.10 seconds.

• Leads: It is typically most positive in lead II (implied by previous history) and is inverted (most negative) in lead aVR

Identify normal variation in ECG wave appearance due to differences in cardiac orientation and recognize normal vector ranges.

1. The appearance of ECG waves varies based on the heart's anatomic position:

2. Horizontal Heart: Often associated with obesity or pregnancy, leading to Left Axis Deviation.

3. Vertical Heart: Often associated with emphysema, leading to Right Axis Deviation

Define the PR interval and PR segment and identify their normal duration and appearance on an ECG

1. PR Interval: Represents atrial depolarization plus the physiologic pause in the AV node. This pause allows the atria to contract and fully empty blood into the ventricles.

2. Normal Duration: 0.12 to 0.20 seconds (3–5 mm or 3–5 small boxes).

3. PR Segment: This is the isoelectric line used as the baseline to evaluate for ST-segment elevation or depression

Identify the normal sequence of ventricular depolarization, including septal depolarization and depolarization of the ventricular myocardium

1. Septal Depolarization: Depolarization travels down the septal fascicle of the left bundle into the left side of the septum, followed by left-to-right movement through myocytes into the right side of the septum.

2. Ventricular Myocardium: Following the septum, the electrical signal continues through the rest of the ventricular myocardium, creating the R and S waves

Recognize normal septal Q waves and distinguish them from abnormal Q waves based on size and lead location.

• Normal Septal Q Wave: Small in size; amplitude is approximately 0.1 mV (1 mm) and duration is approximately 0.04 seconds (1 small box). It must be less than 1/3rd the height of the corresponding R wave.

  • Lateral Leads (I, aVL, V5, V6): Small, narrow q-waves (septal q-waves) representing normal left-to-right depolarization of the septum.

  • Inferior Leads (II, III, aVF): Frequently seen in individuals with a vertical heart axis.

  • Lead aVR: Usually shows an initial negative deflection (QS complex).

  • Lead III: An isolated, often large Q-wave is a common normal variant (respiratory q-wave).

• Abnormal (Pathologic) Q Waves: These are "Big" (greater than the limits above) and represent a prior myocardial infarction

  • Anterior Wall MI: Leads V1–V4 (often Q waves in V1-V2 are considered significant if present).

  • Inferior Wall MI: Leads II, III, and aVF. A Q wave in lead III alone is less specific and can be normal, but is significant in conjunction with II and aVF.

  • Lateral Wall MI: Leads I, aVL, V5–V6.

  • Posterior MI: Often indicated by reciprocal changes (tall R wave) in V1–V2, but can present with pathological Q waves in lateral leads.

  • Septal: V1–V3.

Identify normal QRS morphology across limb and precordial leads, including normal R-wave progression and the transition zone.

• Normal Septal Q Wave: Small in size; amplitude is approximately 0.1 mV (1 mm) and duration is approximately 0.04 seconds (1 small box). It must be less than 1/3rd the height of the corresponding R wave.

• Abnormal (Pathologic) Q Waves: These are "Big" (greater than the limits above) and represent a prior myocardial infarction

Identify the normal durations and relationships of the QRS complex, ST segment, T wave, and QT interval.

1. QRS Complex: Represents ventricular depolarization and hides atrial repolarization.

2. ST Segment: Represents the transition from ventricular depolarization to repolarization. It starts at the J point and should sit on the isoelectric line.

3. T Wave: Represents ventricular repolarization. Its duration varies with heart rate (fast rate = shorter duration).

4. QT Interval: Represents the time for full ventricular depolarization and repolarization. It is often corrected for heart rate (QTc)

Recognize key limitations of computerized ECG interpretation and identify situations in which clinical context alters ECG interpretation.

frequently struggles with arrhythmia detection, pacemaker rhythms, and subtle myocardial infarctions, often producing false-positive results. Key limitations include poor accuracy in non-sinus rhythms, failure to interpret, and inability to integrate clinical context

Identify which ECG waveforms are evaluated to assess atrial enlargement and ventricular hypertrophy (P wave vs QRS complex).

1. Atrial Enlargement: Evaluated by examining the P wave.

2. Ventricular Hypertrophy: Evaluated by examining the QRS complex.

Recognize the three fundamental ECG changes that may occur with chamber hypertrophy or enlargement:

• The three fundamental changes are

  • Increased wave duration.

  • Increased amplitude (voltage).

  • Axis deviation

Explain why increased ECG wave amplitude is the most clinically useful criterion for diagnosing hypertrophy or enlargement and recognize nonpathologic causes of increased voltage.

• it directly reflects the increased electrical potential generated by a larger, thicker muscle mass, or by a chamber closer to the electrodes.

• Although its sensitivity is low (many patients with hypertrophy do not show increased voltage), its high specificity makes it a crucial, rapid, and cost-effective marker of structural heart changes that indicate significant cardiovascular risk.

Define the electrical axis as the direction of the mean electrical vector and identify the plane in which the axis is determined.

The electrical axis is the direction of the mean instantaneous electrical vector generated during a specific phase of the cardiac cycle (atrial depolarization, ventricular depolarization, or ventricular repolarization) and is determined in the frontal plane by using the limb leads

Identify the normal QRS axis range and recognize normal versus abnormal axis orientation using leads I and aVF.

• normal axis is determined using Leads I and aVF simultaneously

  • Lead I: Must show a positive deflection.

  • Lead aVF: Must show a positive deflection.

• Left atrial devation

  • Lead 1: positive

  • Lead aVF: negative

• right atrial devation

  • lead 1: negative

  • Lead aVF: positive

• extreme atrial devation

  • lead 1: negative

  • lead aVF: negative

Left Axis Deviation (LAD) leads

• Lead I is positive

• Lead aVF is negative

• Causes include a

  • horizontal heart

  • obesity

  • pregnancy

  • LVH

Right Axis Deviation (RAD) leads

• Lead I is negative

• Lead aVF is positive

• Causes include a

  • vertical heart

  • emphysema

  • RVH

Extreme Axis Deviation leads

• Both Lead I and Lead aVF are negative

• causes include:

  • same ad LAD and RAD but more marked

Estimate the precise electrical axis by identifying the most nearly biphasic limb lead and applying perpendicular lead logic.

• Find the lead that in which the QRS complex is equally biphasic and the axis must the be oriented approximately perpendicular to this lead 

  • QRS complex in lead III (orientation, +120°) is biphasic, then the axis must be oriented at right angles (90°) to this lead, at either +30° or −150°. And, if we already know that the axis is normal—that is, if the QRS complex is positive in leads I and aVF—then the axis cannot be −150° but must be +30°.

Recognize normal axis ranges for the P wave and T wave and identify the expected relationship between the QRS axis and T-wave axis.

• P Axis: approx between 0-70 degrees

• T Axis: is variable but should approximate the QRS axis lying within 50-60 degrees 

•  Relationship: The T wave should generally be in the same orientation as the corresponding R wave (and thus follow the QRS axis), except in lead aVR

Explain how chamber hypertrophy alters electrical vectors, including its effects on QRS axis, wave amplitude, and wave duration.

• Hypertrophy, defined as an increase in muscle mass typically seen in the ventricles, alters the electrical activity of the heart in three primary ways:

  1. Increased Amplitude: There is an increase in the voltage of the waveform (e.g., a P wave taller than 2.5 mm).

  2. Increased Duration: The waveform takes longer to complete (e.g., a P wave longer than 0.10 seconds).

  3. Axis Deviation: The mean electrical vector shifts away from the normal range (e.g., left axis deviation). Atrial changes are reflected in the P wave, while ventricular changes are reflected in the QRS complex

Differentiate left versus right axis deviation and relate each to underlying ventricular hypertrophy.

• Left Axis Deviation (LAD): Defined as an axis ≤ -30°, this is a key ECG criterion for Left Ventricular Hypertrophy (LVH).

• Right Axis Deviation (RAD): Defined as an axis between +90° and -180° (clinically relying on ≥ +100°), this is a key criterion for Right Ventricular Hypertrophy (RVH)

Identify the ECG features of atrial enlargement using leads II and V1, including how right and left atrial components contribute to P-wave morphology.

• Atrial enlargement is assessed using Leads II and V1:

  1. Lead II: Parallel to the mean P-wave vector.

  2. Lead V1: Positioned directly over the right atrium.

  3. P-Wave Components: The beginning of the P wave represents right atrial (RA) depolarization, and the tail end represents left atrial (LA) depolarization

Describe the effect of right atrial enlargement may have on leads III and aVF.

• Tall, Peaked Waves (P Pulmonale): The most specific sign is a high-amplitude, pointed wave, which is often best seen in leads II, III, and aVF.

  • Amplitude Criterion: wave amplitude > 2.5 mm in the inferior leads is indicative of RAE.

  • Narrow Duration: Unlike left atrial enlargement, RAE usually does not significantly broaden the wave duration; it remains < 0.12 seconds

  • Axis Shift: The wave axis may shift to the right, becoming more vertical, which accentuates the height in leads III and aVF.

Distinguish right atrial enlargement from left atrial enlargement based on P-wave amplitude, duration, and terminal negativity in V1.

• right atrial enlargement:

  • P-wave amplitude: taller than 2.5 mm

  • P wave duration: normal

  • terminal negativity in V1: biphasic; initial phase taller then terminal

• left atrial enlargement:

  • P-wave amplitude: normal or increased

  • P wave duration: abnormally wide and often notched

  • terminal negativity in V1: biphasic; terminal phase is much larger

Compare and contrast the terms P pulmonale and P mitrale.

• P pulmonale:

  • otherwise known as right atrial enlargement

  • tall. peaked P waves, and is associated with pulmonary disease

  • ECG findings:

    • amplitude: greater then 2.5 mm in inferior leads (II, III, aVF)

    • shape: peaked or pointed

    • V1: prominent intital positive component greater than 1.5 mm

    • duration: normal

• P mitrale:

  • left atrial enlargement

  • wide, notched P waves associated with mitral valve disease

  • ECG findings:

    • duration: wide

    • shape: notches or M shapes in lead II

    • V1: deep/wide terminal negative component

Recognize the limb-lead and precordial criteria for right ventricular hypertrophy, including right axis deviation and abnormal R-wave progression.

1. Limb-Lead Criteria: Presence of Right Axis Deviation (+90° to -180°).

2. Precordial Criteria: A change in the normal progression of the R wave across chest leads, specifically where R > S in Lead V1 and S > R in Lead V6

Describe how the amplitudes of the R waves and S waves in leads V1 and V6 can help determine the presence of right ventricular hypertrophy.

1. In RVH, the normal electrical dominance of the left ventricle is overcome, resulting in:

   • Lead V1: The R wave becomes taller than the S wave (R > S).

   • Lead V6: The S wave becomes deeper than the R wave (S > R)

Recognize the most common causes of right ventricular hypertrophy.

• pulmonary diseases (e.g., primary pulmonary hypertension)

• congenital heart diseases (e.g., pulmonic stenosis)

Apply standard voltage criteria to diagnose left ventricular hypertrophy in both precordial and limb leads.

1. Limb Leads:

   • R > 11 mm in aVL

   • R > 13 mm in I

   • R > 20 mm in aVF

   • R in I + S in III > 25 mm.

2. Precordial Leads:

   • R in V5 or V6 + S in V1 or V2 > 35 mm

   • R > 20 mm in V6

   • R > 26 mm in V5.

Explain how younger age and low body mass can produce increased ECG voltages and lead to false-positive left ventricular hypertrophy criteria.

bringing the heart closer to the chest electrodes and having less tissue (fat/muscle) to dampen the electrical signal, causing high voltage readings. These physiological, non-pathological factors often exceed standard ECG criteria for Left Ventricular Hypertrophy (LVH), creating false-positive results.

List three rules for identifying LVH in the precordial leads, four rules for the limb leads and one rule that combines limb and precordial leads (i.e., aVL and V3).

• 3 Precordial Rules: 

  1. R in V5/V6 + S in V1/V2 > 35 mm; 

  2. R > 20 mm in V6; 

  3. R > 26 mm in V5.

• 4 Limb Lead Rules: 

  1. R > 11 mm in aVL; 

  2. R > 13 mm in I; 

  3. R > 20 mm in aVF; 

  4. R in I + S in III > 25 mm.

• 1 Combined Rule:

  1. R in aVL + S in V3 > 20 mm in women or > 28 mm in men.

What are the most common systemic causes of left ventricular hypertrophy?

The most common causes are systemic hypertension and valvular heart disease (e.g., aortic stenosis).

Describe how combined ventricular hypertrophy may appear on ECG and explain why left ventricular findings often dominate.

occurs when both the left and right ventricles are thickened, commonly due to chronic pressure or volume overload affecting both chambers. On an electrocardiogram (ECG), this condition often presents as a complex mix of features, but left ventricular hypertrophy (LVH) findings frequently dominate because the left ventricle is naturally larger and generates much stronger electrical forces.

Identify secondary repolarization abnormalities associated with ventricular hypertrophy, including asymmetric ST depression and T-wave inversion, and distinguish these from ischemic changes.

Hypertrophy often causes altered repolarization in thickened myocardium, appearing as downsloping ST-segment depression and T-wave inversion.

• hypertrophy

  • t wave: asymmetric. shallow/slow

  • ST segment: sloping, often depressed

  • QRS: high voltage

  • time course: chronic, stable

• ischemic

  • T wave: symmetric, deep/arrowhead

  • ST segment: horizontal or downsloping

  • QRS: often normal or low voltage

  • time course: acute, dynamic