1. P Wave
1. Produced by atrial depolarization.
2. Occurs just before atrial contraction.
2. QRS Complex
1. Represents ventricular depolarization.
2. Typically includes Q, R, and S waves, but sometimes fewer or more
apparent waves.
3. Occurs just before ventricular contraction.
3. T Wave
1. Represents ventricular repolarization.
2. Occurs 0.25–0.35 seconds after depolarization.
• Depolarization Wave Movement of electrical potential from the resting
negative state toward a positive state inside the cells.
• P wave (atria) and QRS complex (ventricles).
• Repolarization Wave: Movement back toward the resting negative state.
• T wave: Ventricular muscle recovery

• Depolarization (Red in Figure 11-2)
• Inner membrane potential changes
from negative to slightly positive.
• Outside of the membrane
becomes negative.
• Repolarization
• Membrane potential returns from positive
back to negative inside.
• Outside of the membrane returns
to positive.

1. Halfway Depolarization (Figure 11-2A)
§ Left half of the fiber: Depolarized (inside positive, outside
negative).
§ Right half: Still polarized (inside negative, outside
positive).
§ Electrode Reading: A large positive deflection because
the left electrode is in an area of negativity (extracellular)
and the right electrode is in positivity.
2. Complete Depolarization (Figure 11-2B)
§ Entire fiber is depolarized.
§ Both electrodes now detect equal negativity outside the
fiber.
§ Meter Reading: Returns to zero baseline (no potential
difference).
3. Halfway Repolarization (Figure 11-2C)
§ Positivity begins returning to the left of the fiber’s outside
surface.
§ Left electrode: Area of positivity; Right electrode: Area of
negativity.
§ Polarity Reversal: Opposite to the depolarization phase
→ Negative deflection on the meter.
4. Complete Repolarization (Figure 11-2D)
§ Entire fiber repolarized (outside surface positive again).
§ Both electrodes in areas of equal positivity → Potential
difference is zero.
§ Meter reading returns to baseline.

• Duration: ~0.25–0.35 seconds in ventricular muscle.
• Depolarization: Rapid upswing of the action potential.
• Repolarization: Return to baseline.
§ Recording Method:
• Microelectrode inserted inside a single ventricular muscle fiber.
1. QRS Complex and Depolarization
§ QRS appears at the start of the monophasic action potential.
§ Indicates ventricular depolarization.
2. T Wave and Repolarization
§ T wave appears near the end of the action potential.
§ Reflects ventricular repolarization.
3. No ECG Deflection When Fully Polarized or Depolarized
§ ECG waves occur only when parts of the ventricle have different
membrane potentials.
§ Complete polarization or complete depolarization = No voltage
difference → No deflection on the ECG.

• Depolarization → Contraction:: Muscle contraction begins only
after depolarization spreads through the muscle.
§ Atrial Events
§ P Wave and Atrial Contraction
§ P wave initiates atrial depolarization → Atrial contraction soon
follows.
§ Atrial Repolarization (Atrial T Wave)
§ Occurs ~0.15–0.20 sec after the P wave ends.
§ Usually masked by the large QRS complex.
§ Rarely visible in a normal ECG.
§ Ventricular Events
§ QRS Complex and Ventricular Contraction
§ Depolarization wave (QRS) starts ventricular contraction.
§ Ventricles remain contracted until after the end of the T wave.
§ Ventricular Repolarization (T Wave)
§ Begins about 0.20 sec after the start of QRS; can extend up to 0.35
sec in some fibers.
§ Prolonged repolarization (~0.15 sec) → Broader, lower-voltage T
wave compared to QRS.

• Voltage Calibration
• Typically, 10 small line divisions (vertical) = 1 mV.
• Positive deflection = upward; Negative deflection =
downward.
• Time Calibration
• Standard ECG speed: 25 mm/sec.
• 1 second = 25 mm horizontally.
• Each 5 mm = 0.20 second (darker vertical lines).
• Each small division (thin lines) within the 5 mm = 0.04 second.
• Amplitude Reading
• Count vertical divisions to assess voltage in millivolts.
• Timing Analysis
• Each large box (5 mm) corresponds to 0.20 second.
• Each small box (1 mm) corresponds to 0.04 second.
• Clinical Use
• Accurately measure wave amplitudes (e.g., QRS height).
• Calculate heart rate and interval durations (PR, QRS, QT).

§ Influencing Factors
• Electrode placement (distance from the heart, body surface location).
• Proximity of electrodes to the ventricles influences voltage amplitude.
§ Typical Ranges
• QRS Complex:
• 1.0–1.5 mV (arm-to-arm or arm-to-leg leads).
• Can reach 3–4 mV if one electrode is directly over ventricles.
• P Wave: ~0.1–0.3 mV.
• T Wave: ~0.2–0.3 mV.
§ Comparison to Monophasic Action Potential
• Direct Cardiac Recording: Monophasic action potential can be ~110
mV at the muscle fiber membrane.
• Surface ECG: Significantly lower voltages due to tissue
attenuation and distance from the heart.

• Time from start of the P wave (atrial depolarization) to the start of the
QRS (ventricular depolarization).
• Reflects onset of atrial excitation to onset of ventricular excitation.
• Normal duration: ~0.16 seconds.
• If the Q wave is absent, it’s commonly referred to as the P-R interval.
• Atrioventricular (A-V) Node Conduction
• Major contributor to this interval.
• Ensures atria contract and fill ventricles before ventricular contraction
begins.
• Heart Rate Influence
• Faster heart rates: P-R interval shortens.
• Due to increased sympathetic or decreased
parasympathetic activity.
• Speeds up A-V nodal conduction.
• Slower heart rates: P-R interval lengthens.
• Caused by increased parasympathetic tone or reduced
sympathetic activity.
• Slows A-V nodal conduction.

• Q-T: Extends from the start of the Q (or R) wave to the end
of the T wave.
• Significance
• Ventricular contraction lasts almost the entire Q-T
interval.
• Normal duration: ~0.35 seconds.
§ Determining Heart Rate from the ECG
§ R-R Interval
§ Measure the time (seconds) between two successive R
waves.
§ Heart rate (beats/min) = 60 / (R-R interval in seconds).
§ Example
§ If R-R interval = 0.83 seconds
§ Heart rate = 60 / 0.83 ≈ 72 beats/min.

• Conductive Surroundings:
• Lungs, fluids, and tissues around the heart conduct electricity to a notable degree.
• Heart is effectively “suspended” in a conductive environment.
• Electrical Circuits:
• When part of the ventricles is depolarized (electronegative), current flows
from depolarized to polarized regions.
• Current paths form curving or elliptical lines around the heart.
§ Sequence of Depolarization (Figure 11-5)
§ Initial Ventricular Depolarization
§ Begins in the ventricular septum (Purkinje system).
§ Spreads from endocardium to epicardium (inside outward).
§ Electronegative Interior Surfaces
§ Depolarized (negative) internal walls contrasted with still positive external walls.
§ Current Flow: Negative base → Positive apex (the red arrows or negative signs in the figure).
§ Late Depolarization Reversal
§ At the end of depolarization (~0.01 sec), current briefly reverses direction (apex → base).
§ Occurs because the last areas to depolarize are the outer walls near the base of the
ventricles.

• Net Vector:
• For most of the depolarization process, current flows
from the base (negative) toward the apex (positive).
• Surface Electrodes:
• An electrode placed near the base is relatively negative.
• An electrode near the apex is relatively positive.
• ECG Reading: Produces a positive deflection when
measured from base to apex.
§ Key Takeaways
• Main Depolarization Direction: Base → Apex, yielding
a positive reading on standard leads.
• End of Depolarization: Brief reversal (apex → base)
before full depolarization completes.

• Definition:
• Bipolar means the ECG is recorded between two electrodes on different sides of the
heart.
• Each lead uses a pair of limb electrodes (e.g., right arm, left arm, left leg).
• The ECG measures voltage differences between these two points.
• Complete Circuit: Each bipolar limb lead forms a two-electrode circuit with the ECG
machine.
• Voltage Direction: ECG deflections are determined by the direction of current
flow relative to each pair of electrodes.
• Clinical Relevance:
• The three standard limb leads provide different views of the heart’s electrical activity.
• Often used together to diagnose conduction pathways and arrhythmias.

§ Lead I
• Electrode Connections:
• Negative terminal: Right arm (RA)
• Positive terminal: Left arm (LA)
• Positive Deflection on ECG
• Occurs when the right arm is more negative relative to the left arm.
• Visually recorded above the baseline.
§ Lead II
• Electrode Connections:
• Negative terminal: Right arm (RA)
• Positive terminal: Left leg (LL)
• Positive Deflection on ECG
• Occurs when the right arm is more negative relative to the left leg.
• Most commonly used lead for standard ECG rhythm monitoring.
§ Lead III
• Electrode Connections:
• Negative terminal: Left arm (LA)
• Positive terminal: Left leg (LL)
• Positive Deflection on ECG
• Occurs when the left arm is more negative relative to the left leg.

§ A conceptual triangle formed by the electrical connections at the
two arms and the left leg, enclosing the heart in the center.
§ Apex Points: RA, LA, LL
§ Electrical Significance
§ Depicts how limb leads I, II, and III form a geometric
relationship around the heart.
§ Each vertex (limb) connects electrically with the fluids
around the heart, providing different vantage points of
cardiac activity.
§ Einthoven’s Law: Lead I + Lead III = Lead II
§ Meaning
§ If ECGs are recorded simultaneously with the three limb
leads, the potential in Lead II equals the sum of the potentials
in Lead I and Lead III.

§ Similar Waveforms
§ Leads I, II, and III all show:
§ Positive P wave (atrial depolarization).
§ Positive T wave (ventricular repolarization).
§ Main portion of QRS is also positive in each lead.
§ Einthoven’s Law Demonstrated
§ At any given instant: Lead I + Lead III = Lead II
§ Confirms the algebraic relationship among the three bipolar limb leads.
§ Arrhythmia Diagnosis
§ Time intervals between waves (e.g., P-R interval) are key.
§ Any of the three limb leads generally suffices to evaluate arrhythmic patterns
because all show similar timing relationships of the waves.
§ Localizing Myocardial Damage or Conduction Abnormalities
§ Different leads can highlight abnormal patterns in specific regions of the
heart.
§ Some leads may show marked changes while others appear near-normal.
§ Example: Identifying ventricular muscle injury or Purkinje system
issues requires careful lead selection.

• Definition & Setup:
• ECGs can be recorded with an electrode placed on the anterior
chest directly over the heart.
• This electrode is connected to the positive terminal of the
electrocardiograph.
• Indifferent Electrode (Wilson Central Terminal):
• The negative electrode is created by connecting the right arm,
left arm, and left leg through equal resistances.
• Standard Precordial Leads:
• Six chest (precordial) leads: V1, V2, V3, V4, V5, and V6.
• The chest electrode is placed sequentially at six standard
positions (as shown in Figure 11-8).
• Each chest lead mainly records the electrical activity of the cardiac
musculature directly beneath the electrode.
• Minute abnormalities, especially in the anterior ventricular wall, can
produce marked changes in these recordings.

• Leads V1 and V2:
• QRS complexes are predominantly negative.
• Reason: Electrodes in V1 and V2 are positioned
closer to the base of the heart, which is the
direction of electronegativity during ventricular
depolarization.
• Leads V4, V5, and V6:
• QRS complexes are predominantly positive.
• Reason: Electrodes in these leads are located
closer to the apex of the heart, which is the
direction of electropositivity during ventricular
depolarization.
• Clinical Implication:
• The spatial orientation of the precordial leads helps
in identifying regional abnormalities in ventricular
depolarization.

• Definition:
• A variation of limb lead recordings that
uses three electrodes placed on the limbs.
• Recording Setup:
• Two limbs are connected through equal
resistances to the negative terminal of the
electrocardiograph.
• The third limb is connected to
the positive terminal.
• Naming Conventions:
• aVR: Positive electrode on the Right Arm.
• aVL: Positive electrode on the Left Arm.
• aVF: Positive electrode on the Left Leg.

• Normal ECG Recordings:
• Augmented limb leads produce waveforms
similar to standard limb leads.
• Recorded patterns help provide additional
perspectives of the heart’s electrical activity.
• Unique Feature:
• The aVR lead is
characteristically inverted (negative
deflections).
• Reason:
• The polarity connections are such that the positive
terminal (right arm) in aVR views the heart's
electrical activity from an opposite perspective
compared to aVL and aVF.
• Study the electrode configuration and polarity of the
augmented system to understand why this
inversion occurs.

• Three Lead Groupings:
• Standard Bipolar Limb
Leads: I, II, III
• Augmented Limb
Leads: aVR, aVL, aVF
• Precordial Leads: V1–V6
• Recording Method:
• Each lead is derived from two
electrodes forming a complete
circuit.
• Digital Advances:
• Modern ECGs are displayed
digitally rather than on paper.
• Clinical Utility:
• The grouping helps provide
different “views” of the heart’s
electrical activity.

• Electrical Currents in the Chest: When a cardiac impulse occurs, current flows from depolarized (electronegative) to still-polarized
(electropositive) areas through the conductive medium of the chest.
• Depolarization & Repolarization: These processes create transient voltage differences that are recorded by electrodes on the
body surface.
• Standard Bipolar Limb Leads: Leads I, II, and III are formed from electrodes on the right arm, left arm, and left leg; they surround
the heart as illustrated by Einthoven’s Triangle.
• Einthoven’s Law: The potential in Lead II equals the sum of the potentials in Leads I and III when properly measured with attention
to polarity.
• ECG Waveforms: Normal recordings from the limb leads show positive P waves, predominantly positive QRS complexes, and
positive T waves.
• Precordial Leads: Six chest leads (V1–V6) record localized electrical activity, with QRS polarity varying by electrode position
relative to the heart’s base and apex.
• Augmented Limb Leads: With modified electrode connections (aVR, aVL, aVF), these leads offer additional diagnostic views; aVR
is typically inverted.
• ECG Display Organization: Modern ECG displays group leads into standard bipolar limb, augmented limb, and precordial
categories, each with calibrated voltage (mV) and time (mm/sec) scales.
• Ambulatory ECG Monitoring: Used to capture transient or infrequent arrhythmias by extending monitoring over hours to years (via
Holter monitors, intermittent recorders, or implantable loop recorders).
• Digital Advances: Current systems allow for continuous or intermittent digital transmission of ECG data with real-time computerized
analysis.
• Clinical Relevance: Each lead system contributes complementary perspectives for diagnosing arrhythmias, myocardial damage,

KEY
TAKEAWAY
S
• Cardiac electrical activity spreads through a conductive medium in the
chest, enabling non-invasive ECG recordings.
• Depolarization and repolarization waves create transient potential
differences that are detected by surface electrodes.
• The standard bipolar limb leads (I, II, III) form Einthoven’s Triangle,
establishing a fundamental geometric basis for ECG interpretation.
• Einthoven’s Law ensures that if two lead potentials are known, the third
can be accurately calculated, reinforcing measurement consistency.
• Precordial leads (V1–V6) capture detailed regional electrical activity of
the heart, with QRS polarity varying by proximity to the heart’s base or
apex.
• Augmented limb leads (aVR, aVL, aVF) provide additional views; the
aVR lead is characteristically inverted due to its electrode configuration.
• Accurate ECG display requires proper calibration of voltage and time
scales, with 1 mV corresponding to 10 small vertical divisions and 25
mm/sec representing 1 second.
• Ambulatory electrocardiography extends monitoring beyond a resting
ECG, capturing transient arrhythmic events during daily activities.
• Technological advancements now permit digital, real-time ECG data
transmission and automated analysis, enhancing diagnostic capabilities.
• Each lead grouping (limb, augmented, precordial) supplies unique
information that is vital for comprehensive cardiac assessment.
• Understanding the relationships among these leads is critical for
diagnosing both rhythm disturbances and regional myocardial
abnormalities.

QUESTIONS