Physiologic process for ECG

[FOUNDATION] What does an electrocardiogram (ECG) measure?

A graphic representation of the average electrical activity of the heart, including depolarization and repolarization, over time

[FOUNDATION] Does an ECG directly measure cardiac contraction?

No. It records electrical activity, which precedes mechanical contraction

What two major cellular electrical processes produce ECG recordings?

Depolarization and repolarization

What does depolarization represent in cardiac cells?

A change in membrane potential toward a more positive value that contributes to electrical excitation

What does repolarization represent in cardiac cells?

The return of membrane potential toward its resting or maximum diastolic value after depolarization

Why does electrical activity precede cardiac contraction?

Depolarization triggers excitation–contraction coupling, which subsequently produces mechanical force

What does an ECG electrode detect?

Changes in extracellular electrical potential produced by cardiac electrical activity

Does an ECG record the action potential of one individual cardiomyocyte?

No. It records the combined electrical activity of many cardiac cells as detected from the body surface

Why does the ECG have different waveforms during the cardiac cycle?

Different regions of the heart depolarize and repolarize at different times and in different directions

What is the normal sequence of cardiac electrical activation?

SA node → atrial myocardium → AV node → bundle of His → bundle branches → Purkinje fibers → ventricular myocardium

What is the normal pacemaker of the heart?

The sinoatrial node

Why does the SA node normally determine the heart rate?

It has the fastest intrinsic rate of spontaneous depolarization to threshold

What is the role of the AV node in normal cardiac conduction?

It delays transmission of electrical excitation from the atria to the ventricles

Why is the AV nodal delay physiologically important?

It allows atrial contraction and ventricular filling before ventricular depolarization and contraction

What is the role of the His-Purkinje system?

Rapidly distribute electrical excitation throughout ventricular myocardium

Why does the ventricular conduction system propagate impulses rapidly?

Rapid propagation permits coordinated ventricular depolarization and effective contraction

[DRAW/MAP] Map the relationship between electrical activation and mechanical events.

SA node fires → atrial depolarization → atrial contraction → AV nodal delay → ventricular depolarization → ventricular contraction → ventricular repolarization → ventricular relaxation

What are the three principal ECG waveforms?

P wave, QRS complex, and T wave

What electrical event produces the P wave?

Atrial depolarization

What electrical event produces the QRS complex?

Ventricular depolarization

What electrical event produces the T wave?

Ventricular repolarization

What happens mechanically after the P wave?

Atrial contraction

What happens mechanically after the QRS complex?

Ventricular contraction

What happens mechanically after the T wave?

Ventricular relaxation

Why is atrial repolarization usually not visible as a separate ECG wave?

It occurs during ventricular depolarization and is obscured by the much larger QRS complex

Why is the QRS complex generally larger than the P wave?

Ventricular myocardium has substantially more muscle mass than atrial myocardium

What is the relationship between electrical activity and myocardial mass on the ECG?

Greater electrically active myocardial mass generally produces a larger net electrical signal

What is the first negative deflection of the QRS complex called?

Q wave

What is the first positive deflection of the QRS complex called?

R wave

What is a negative deflection occurring after an R wave called?

S wave

Must every normal ECG lead show distinct Q, R, and S waves?

No. The appearance of the QRS complex varies depending on the recording lead and direction of ventricular depolarization

What is a segment on the ECG?

The portion of the tracing between designated waves, such as the PR segment or ST segment

What is an interval on the ECG?

A measurement of time that includes at least one wave and may include adjacent segments

What is the PR interval?

The time from the beginning of the P wave to the beginning of the QRS complex

What electrical events are included in the PR interval?

Atrial depolarization and conduction through the AV node and conduction system until ventricular depolarization begins

What is the normal PR interval?

Approximately 0.12–0.20 seconds

What is the physiological significance of the PR interval?

It reflects the time required for electrical excitation to travel from the atria toward the ventricles

What is the PR segment?

The portion between the end of the P wave and the beginning of the QRS complex

What major physiological event occurs during the PR segment?

AV nodal conduction delay

What can a prolonged PR interval indicate?

Delayed atrioventricular conduction, as in first-degree AV block

What is the characteristic PR interval in first-degree AV block?

Greater than 0.20 seconds

What can an abnormally short PR interval suggest?

Accelerated atrioventricular conduction or ventricular pre-excitation through an accessory pathway

What is the QRS duration?

The time from the beginning to the end of ventricular depolarization

What is the normal QRS duration emphasized in introductory ECG interpretation?

Generally less than 0.12 seconds

What does a widened QRS complex suggest?

Delayed or abnormal ventricular depolarization

Why can ventricular conduction abnormalities widen the QRS complex?

Ventricular excitation takes longer to spread through myocardium

What is the ST segment?

The portion between the end of the QRS complex and the beginning of the T wave

What cellular action potential phase is closely associated with the ST segment?

The ventricular action potential plateau, Phase 2

Why is the ST segment normally near the isoelectric baseline?

Most ventricular cells are depolarized at approximately the same time, producing little net voltage difference

What is the QT interval?

The time from the beginning of the QRS complex to the end of the T wave

What electrical processes are represented by the QT interval?

Ventricular depolarization and ventricular repolarization

Why is the QT interval dependent on heart rate?

The duration of ventricular electrical activity generally decreases as heart rate increases

What is the relationship between heart rate and QT interval duration?

As heart rate increases, the QT interval generally shortens

What is the relationship between decreasing heart rate and QT interval duration?

The QT interval generally lengthens

What is QTc?

The QT interval corrected for heart rate

Why is a corrected QT interval useful?

It permits interpretation of QT duration while accounting for differences in heart rate

Why can prolonged ventricular repolarization be clinically significant?

It can increase susceptibility to certain ventricular arrhythmias

What is the RR interval?

The time between successive R waves

What does the RR interval allow you to calculate?

Heart rate

What does the PP interval represent?

The time between successive P waves

How can the PP interval help evaluate rhythm?

It reflects the timing and regularity of atrial depolarization

[DRAW/MAP] Map the principal ECG waves and intervals.

P wave = atrial depolarization → PR segment = AV nodal delay → QRS = ventricular depolarization → ST segment = ventricular plateau → T wave = ventricular repolarization. PR interval = P onset to QRS onset. QT interval = QRS onset to T end

[HIGH-YIELD] Which ECG waveform corresponds to atrial depolarization?

P wave

[HIGH-YIELD] Which ECG waveform corresponds to ventricular depolarization?

QRS complex

[HIGH-YIELD] Which ECG waveform corresponds to ventricular repolarization?

T wave

[HIGH-YIELD] Which ECG interval includes ventricular depolarization and repolarization?

QT interval

[HIGH-YIELD] Which ECG interval is used to assess atrioventricular conduction time?

PR interval

What is an electrical vector?

A quantity that has both magnitude and direction

Why is vector analysis useful for understanding ECG recordings?

Cardiac electrical activity has a direction and magnitude that determine how it appears in different leads

What is the mean electrical vector of the heart?

The average direction and magnitude of the heart's net electrical activity during a specified event

What is the mean QRS vector?

The average direction of ventricular depolarization

What does the mean electrical axis usually refer to in introductory ECG interpretation?

The mean QRS axis in the frontal plane

Why is the normal mean QRS vector directed generally toward the left?

The left ventricle contains more electrically active myocardial mass than the right ventricle

Why is the normal mean QRS vector directed generally inferiorly?

The orientation of the heart and sequence of ventricular depolarization direct the net electrical activity toward the apex

What is the normal general direction of ventricular depolarization?

Inferiorly and toward the left

What is the approximate normal mean QRS axis commonly emphasized in the lecture?

Approximately +60 degrees

What is the normal mean QRS axis quadrant used in the lecture?

Approximately 0 to +90 degrees

Why is the normal QRS axis not identical in every individual?

Cardiac anatomy, ventricular mass, body position, and conduction patterns vary among individuals

What is an ECG lead?

A particular electrical viewpoint of the heart established by a defined arrangement of recording electrodes

What is the difference between an electrode and a lead?

An electrode is a physical recording contact, while a lead is an electrical viewpoint produced by electrode configuration

What determines whether a lead records a positive or negative deflection?

The direction of the net electrical vector relative to the lead's positive recording direction

What happens when a depolarization vector moves toward a positive recording electrode?

The lead records a positive deflection

What happens when a depolarization vector moves away from a positive recording electrode?

The lead records a negative deflection

What happens when a depolarization vector is perpendicular to a recording lead?

The lead records an approximately isoelectric or biphasic deflection

What is an isoelectric QRS complex?

A QRS complex with approximately equal positive and negative deflections

What is a biphasic QRS complex?

A QRS complex containing both positive and negative components

Why is the angle between an electrical vector and a lead important?

It determines how much of the vector is projected onto the lead's recording axis

When does a lead record its largest positive deflection?

When the electrical vector is parallel to and directed toward the lead's positive axis

When does a lead record its largest negative deflection?

When the electrical vector is parallel to but directed away from the lead's positive axis

When does a lead record the smallest net deflection?

When the electrical vector is perpendicular to the lead axis

[DRAW/MAP] Map vector direction to ECG deflection.

Vector toward positive electrode → positive wave. Vector away from positive electrode → negative wave. Vector perpendicular to lead → isoelectric or biphasic wave

What is the conventional angular orientation of zero degrees in the frontal plane?

Horizontally toward the patient's left

What is the conventional angular orientation of +90 degrees?

Inferiorly

What is the conventional angular orientation of +180 degrees?

Horizontally toward the patient's right

What is the conventional angular orientation of -90 degrees?

Superiorly

[DRAW/MAP] Map the frontal-plane angular reference system.

0° = patient's left → +90° = inferior → +180° = patient's right → -90° = superior

Why can ventricular repolarization produce a positive T wave in a lead that records a positive QRS complex?

The direction of repolarization and the polarity of the electrical process are opposite, allowing a positive recorded deflection

What is the general relationship between ventricular depolarization and repolarization directions?

They generally occur in opposite spatial directions

Why can opposite directions of depolarization and repolarization produce deflections with the same polarity?

Repolarization reverses the underlying electrical polarity, so a repolarization wave moving opposite to depolarization can produce a similarly directed ECG deflection

Why is the T wave normally upright in many leads with an upright QRS complex?

Ventricular repolarization proceeds in a direction that, combined with its reversed electrical polarity, produces a positive deflection