CPPS 302 Cardiovascualr System (ECG)

What is an electrocardiogram (ECG/EKG)

A graphic recording of the electrical potentials (biopotentias) generated by myocardium during the cardiac cycle

Why is an ECG considered a vector?

Because the electrical acitivyt recorded by an ECG has both MAGNITUDE (strength) and DIRECTION

What electrical activities does an ECG reflect

The rhythmic depolarization and repolarization of the atria and ventricles, which initiate contraction and relaxation respectively

What happens on an ECG when a wave of depolarization or repolarization travels towards the positive (+) recording electrode?

Depolarization → Upward (+) deflection

Repolarization → Downward (-) defelction

How many leads are present in a standard ECG, and how are they calssified

A srandard ECG contains 12 leads

• 6 limb leads: I, II, III, aVR, aVL, aVF

• 6 chest leads: V1-V6

Which ECG leads are bipolar and which are unipolar

• Bipolar: I, II, III

• Unipolar: aVR, aVL, aVF, and V1-V6

What are the positive (+) and negative (-) electrode positions for bipolar limb leads I, II, III

• Lead I: (-) Right arm → (+) left arm

• Lead II: (-) Right arm → (+) left leg

• Lead III: (-) Left arm → (+) left leg

Where is the ground electrode placed in a standard ECG?

On the right leg (RL)

What are the three augmented unipolar limb leads, and where are their positive (+) and negative (-) electrodes

aVR: (+) Right arm; (-) left arm and left leg

aVL: (+) Left arm; (-) right arm and left leg

aVF: (+) left leg; (-) right arm and left arm

What are the standard positions of the uniipolar chest (precordial) leads V1-V3

• V1 = 4th intercostal space, right sternum

• V2 = 4th intercostal space, left of sternum

• V3 = between V2 and V4

What are the standard positions of the unipolar chest (precordial) leads V4-V6

V4 = 5th intercostal space, midclavicular line

V5 = same horzontal level as V4, left anterior axillary line

V6 = same horizontal level as V4 and V5, midaxillary line

What is the difference between ECG electrode placement and the regions reflected by ECG leads

Electrode placement refers to where the electrodes are attached to the skin, while ECG leads reflect electrical activity from different regions of the heart.

Which ECG leads reflect the different regions of the heart?

• Anterior: V1-V6

• Anteroseptal: V1-V2

• Anteroapical: V3-V4

• Anterolateral: V5-V6, I, aVL

• Inferior: II, III, aVF

• Posterior: Tall R waves in V1-V2

Why does the left side of the ventricular septum depolarize first?

The left bundle branch has a larger diamter and lower cystolic resistance, allowing impulses to reach and activate the left side of the septum first

What is the normal direction of ventricular septal depolarization, and what ECG wave does it produce in aVL

Left → Right (producing the initial Q wave in aVL)

What happens when the bundle branches and Purkinje fibres reach the apex

They initiate depolarization of the apical region, producing the beginning fo the R wave

Which bundle branch activates each ventricle

Left bundle branch → left ventricle myocardium

Right bundle branch → right ventricular myocardium

In what directions does ventircular depolaization spread after reaching the apex

From the apex towards the base and from the endocardium toward the epicardium

What happens to the ECG during ventricular wall depolarization (Stage D)

The depolarization wave continues spreading through the ventricular myocardium, contributing to the continuation of the R wave in aVL

Why do chest leads V1-V6 procude different QRS complexes

Because each lead views vnetricular depolarization from a different direction

How does the QRS complex normally change from V1 to V6

The R wave becomes progressively larger and the S wave becomes smaller, changing from a predominantly negative to a predominantly positive QRS complex

Why are V1-V2 typically predominantly negative, whole V5-V6 are predominantly positive

The main ventricular depolarization wave moves away form the V1-V2 and toward the V5-V6

What does the P wave reprsent, and what mechanism event follows

Atrial depolarization, followed by atrial contraciton

What does the QRS complex represent, and what mechanism event follows

Ventricualr depolarization, followed by ventricular contraction

What does the T wave represent

Ventricular repolarization, associated with ventricular relaxation

What does the P-Q (PR) interval represent?

The time from the beginning of atrial depolarization to the beginning of ventricular depolarization

What do the ST segment and QT interval represent

• ST segment: time between the end of ventricular depolarization and beginnig of ventricular repolarization

• QT interval: total duration of ventricular depolarization and repolarization

In a normal cardaic cycle, how dos the duration of ventricular systole compare to ventricular diastole?

Ventricular systole is substantially shorter than ventricular diastole

Which heart sounds mark the beginning of ventricular systole and diastole

S1: Beginning of ventricular systole (AV valves close)

S2: Beginning of ventricular diastole (semilunar valves close)

What does the U wave represent on an ECG

Its exact source is unknown. The lecture suggests possible repolarization of the papillary muscles or delayed repolarization of Purkinje Fibers

Which ECG component represents atrial repolarization

Atrial repolarization occurs during the QRS complex but is normall hidden by the much larger ventricular depolarization signal

What happens during a third-degree heart block

A third degree heart block (complete AV block)

Electrical impulses form the atria cannot reach the ventircles, causing the atria and vnetricles to depolarize independently

What is the characterisitc ECG finding in 3rd degree heart block

P waves and QRS complexes occur independently, with no consistent relationship between them

Why can atrial repolarization sometimes be observed on an ECG during a 3rd degree heart block

Because atrial and ventricular activity are independent, atrial repolarization may occur separately form the QRS complex instead of being hidden by ventricular depolarization

What does the PR interval represent on an ECG

The time from the beginning of atrial depolarization (start of P wave) to the beginning of ventricular depolarization (start of QRS complex)

What does the PR segment represent on an ECG

The time from the end of atrial depolarization (end of P wave) to the beginning of ventricular depolarization (start of QRS complex).

It mainly reflects the conduction delay through the AV node

What is the main difference between the PR interval vs the PR segment?

PR interval: includes the P wave and PR segemnt

PR segment: includes only the portion between the end of the P wave and beginning of the QRS complex

What does the QRS duration indicate on an ECG

• The time required for ventricular depolarization.

• It reflects the functional integrity of the ventricular conduction system

  • (basically how well the ventircualr conduction system works)

    • (Bundle of His, bundle branches, purkinje fibres) and contractile myocardium

What does the ST segment represent on an ECG

Time between end of ventricular depolarization and the beginning of ventricular repolarization. It is normally isoelectirc (flat) because theres is no significant net electrical activity recorded

What is the QT interval, and what does it represent?

The time from the beginning of ventricular depolarization (start of QRS complex) to the end of ventricular repolarization (End of T wave).

It represents the total duration of ventricular electrical activity

Which ECG interval or segment can be used to roughly estimate the duration of ventricular aciton potentials

QT interval

Which ECG interval or segment is correlated with ehart rate

QT interval.

• It generally shortens as heart rate increases and lengthens as heart rate decreases (inverse relationship)

What are the normal P wave values in Lead II

• Height: < 2.5 mm

• Width: < 0.11 seconds

What is the normal PR interval duration

Between 0.12 - 0.20 seconds

What would a shortened vs a prolonged PR interval indicate

Shortened PR interval: Wolff-Parkinsons-White (WPW) syndrome

• Pre-excitation of the ventricle

Prolonged PR interval: Heart block

• Disruption of conduction

What is the normal duration of the QRS complex?

Less than 0.12 seconds

What could a widened vs tall QRS complex indiacte

Wide QRS:

• Bundle branch block

• ventricular rhythm

• Hyperkalemia (High extracellular K)

Tall QRS:

• Ventricular hypertrophy (increased ventricular muscle mass).

How does hyperkalemia affect the QRS complex and why

Causes wide QRS complex because increased extracellular K partially depolarizes the resting membrane potential, slowing ventricular conduction

• normally Na channels open to depolarize

• Increased extracellular K caused Na channels to remain more inactive

• Fewer Na channels are unable to open during next aciton potential

• Less inward Na → slower phase 0 depolarization

• Prolongs QRS → widens it

What are the two gates of fast voltage-gated Na channels, and what is their state at rest

Activation gate: Closed at rest

Inactivation gate: Open at rest

What is partial depolarization of the membrane potential, and what causes it

Partial depolarization occurs when the resting membrane potential become less negative than normal

In hyperkalemia, increased extracellular K reduces the K concentration gradient, causing the membrane potential to remain less negative and preventing full recovery to its normal resting state

What happens to the inactivation gates of fast Na channels during depolarization and repolarization

• During depolarization, the inactivation gates close.

• During repolarization, as the membrane returns towards its resting potential, they REOPEN adn reset the Na channel

What happens to the activation and inactivation gates of fast Na channels when the membrane returns to its resting state

• Activation gates close

• Inactivation gates reopen

• Na channels are ready for another action potential

How does hyperkalemia cause a widened QRS complex

Hyperkalemia causes partial membrane depolarization, preventing some fast Na channles from recovering form inactivqtion. This redcues Na channel availability, slows ventricular conduciton, and widens the QRS complex

Why do we need to calculate the corrected QT inerval (QTc), and how is it calculated

The QT interval is closley related to heart rate (HR), so QTc is calculated to correct for HR

• Formula: QTc = QT / √RR

• RR = interval between two consecutive R waves (in seconds).

• Normal QTc (lecture): 0.30–0.44 seconds

What is the ST segement also known as, and what can deviations from it indiacte? Include the difference between ST elevation and depression

• AKA the isoelectric line (normally at baseline

• Deviation can indicate acute myocardial ischemia

• ST elevation: epicardial ischemia

• ST depression: endocardial ischemia

What is the normal orientation of the T wave, and what can an inverted T wave indicate

• T wave is usually upright

• An inverted T wave may indicate:

  • Myocardial ischemia

  • Intraventricualr conduction delay

  • Anxiety attack