Lecture #26: Physiological Basis/Basic EKG Leads, Vectors, Intervals

What is an electrocardiogram (ECG/EKG)?

A graphic representation of the electrical activity of the heart that detects changes in electrical potential produced in successive areas of the myocardium during the cardiac cycle.

What is the normal cardiac conduction pathway?

SA node → atria → AV node → bundle of His → right and left bundle branches → Purkinje fibers → ventricles.

What are the intrinsic pacemaker rates of the SA node, AV node, and His-Purkinje/ventricular system?

SA node: 60–100 beats/min; AV node: 40–60 beats/min; bundle of His/Purkinje fibers/ventricles: 25–40 beats/min.

Which part of the cardiac conduction system conducts impulses fastest?

The Purkinje system has the fastest conduction velocity, followed by atrial muscle, ventricular muscle, and then the AV node.

Which part of the cardiac conduction system conducts impulses slowest?

The AV node has the slowest conduction velocity.

What does the P wave represent?

The P wave represents atrial depolarization as the electrical impulse originating from the SA node spreads through both atria, followed by atrial contraction.

What happens to atrial repolarization on the ECG?

Atrial repolarization occurs but is masked by the much larger QRS complex.

What does the PR interval represent?

The PR interval represents the time from the beginning of atrial depolarization to the beginning of ventricular depolarization and includes conduction of the impulse from the atria through the AV node toward the ventricles.

What is the normal PR interval?

120–200 ms.

What does prolongation of the PR interval suggest physiologically?

PR prolongation reflects delayed conduction through the AV node.

What does the QRS complex represent?

The QRS complex represents ventricular depolarization.

What is the normal duration of the QRS complex?

Normally less than 120 ms.

Why are QRS amplitude and duration clinically important?

Changes in the amplitude or duration of the QRS complex are important in many ECG diagnoses because they reflect abnormalities in ventricular electrical activity.

What does the ST segment represent?

The ST segment is normally isoelectric and represents the period when the ventricles are depolarized and undergoing mechanical contraction.

What is the significance of ST-segment elevation?

The lecture associates ST-segment elevation with transmural injury.

What is the significance of ST-segment depression?

The lecture associates ST-segment depression with subendocardial injury.

What is the true isoelectric point of the ECG?

The TP segment is the true isoelectric point and can be used to assess ST-segment changes.

What does the T wave represent?

The T wave represents ventricular repolarization.

What electrolyte is especially associated with changes in T-wave amplitude?

Potassium. Changes in T-wave amplitude can suggest hyperkalemia or hypokalemia; the lecture mnemonic is that potassium "lives" under the T wave.

Why is the T wave normally a positive deflection despite representing repolarization?

Its positive inflection results from the direction of ventricular repolarization and the resting potential of the cells.

What does the QT interval represent?

The QT interval encompasses ventricular depolarization, mechanical ventricular contraction, and ventricular repolarization; it therefore corresponds to ventricular systole.

What is the general normal relationship between the QT interval and R-R interval?

The QT interval is generally less than one-half of the R-R interval.

Why is a prolonged QT interval clinically important?

A prolonged QT interval increases the risk of ventricular arrhythmias.

What major factors can cause QT prolongation?

Congenital long-QT syndrome, electrolyte imbalances, and certain medications can prolong the QT interval.

What drug classes associated with QT prolongation are emphasized in the lecture?

Antiarrhythmics classes IA and III, macrolide antibiotics, antipsychotics such as haloperidol, tricyclic antidepressants, antiemetics such as ondansetron, and azole antifungals.

What mnemonic is used for drugs that prolong the QT interval?

"Anti-ABCDE": Anti-Arrhythmics, Anti-Biotics, Anti-"C"ychotics, Anti-Depressants, Anti-Emetics, and Anti-Fungals.

Why would imipramine use raise concern about the QT interval?

Imipramine is a tricyclic antidepressant, and TCAs are included among the drug classes associated with QT prolongation.

What are the two major groups of leads on a standard 12-lead ECG?

The standard ECG contains six limb leads—I, II, III, aVR, aVL, and aVF—and six precordial/chest leads—V1 through V6.

What are the six precordial leads?

V1, V2, V3, V4, V5, and V6.

What are the six limb leads displayed on a 12-lead ECG?

I, II, III, aVR, aVL, and aVF.

Where is V1 placed?

V1 is placed in the 4th intercostal space at the right sternal border.

Where is V2 placed?

V2 is placed in the 4th intercostal space at the left sternal border.

Where is V3 placed?

V3 is placed between V2 and V4.

Where is V4 placed?

V4 is placed in the 5th intercostal space at the midclavicular line.

Where is V5 placed?

V5 is placed at the anterior axillary line on the same transverse plane as V4.

Where is V6 placed?

V6 is placed at the midaxillary line on the same transverse plane as V4.

What limb electrodes are used to obtain a standard ECG?

Right arm (RA), left arm (LA), right leg (RL), and left leg (LL) electrodes are placed on the four limbs.

What mnemonic does the lecture give for limb-electrode placement?

"Green and White on the right, Christmas trees below the knees."

What is the difference between a 12-lead ECG and a rhythm strip?

A standard 12-lead ECG displays 12 leads, whereas a rhythm strip displays only one lead.

Where is the SA node located?

The SA node is located at the junction of the right atrium and superior vena cava.

Where is the AV node located?

The AV node is located in the posteroinferior portion of the interventricular septum.

What is an electrical axis?

The axis is the major direction of the heart's electrical activity.

What is the purpose of an electrical vector in ECG interpretation?

A vector describes the amplitude and direction of the heart's electrical potential and forms the basis for understanding how the limb leads view cardiac electrical activity.

What does Einthoven's triangle represent?

Einthoven's triangle represents the relationship among the three standard bipolar limb leads—I, II, and III—formed using the right arm, left arm, and left leg.

What electrical potential does Lead I measure?

Lead I measures the potential between the right arm and left arm: RA → LA.

What electrical potential does Lead II measure?

Lead II measures the potential between the right arm and left leg: RA → LL.

What electrical potential does Lead III measure?

Lead III measures the potential between the left arm and left leg: LA → LL.

What mnemonic helps remember Leads I, II, and III?

I = "1 L" = RA → LA; II = "2 L" = RA → LL; III = "3 L" = LA → LL.

What are the directions of aVR, aVL, and aVF?

aVR points toward the right arm, aVL points toward the left arm, and aVF points toward the feet.

How are aVR, aVL, and aVF oriented relative to the standard limb leads?

aVR is perpendicular to Lead III, aVL is perpendicular to Lead II, and aVF is perpendicular to Lead I.

What mnemonic helps remember the augmented limb leads?

"RaLF 3,2,1": aVR points Right and is perpendicular to III; aVL points Left and is perpendicular to II; aVF points toward the Feet and is perpendicular to I.

Which pair of leads is emphasized for determining cardiac axis?

Lead I and aVF are used together to assess the overall electrical axis.

What are the highest-yield ECG component associations from this lecture?

P wave = atrial depolarization; PR interval = atrial-to-ventricular conduction; QRS = ventricular depolarization; ST segment = ventricles fully depolarized; T wave = ventricular repolarization; QT interval = ventricular depolarization through ventricular repolarization.