Vectors and Cardiac Axis Flashcard

A. Rationale: Normal axis deviation has a frontal QRS axis of −30° to +90°.

Answer choice B – Left axis deviation has a frontal QRS axis of −30° to −150°.
Answer choice C – Right axis deviation has a frontal QRS axis of +90° to +120°.
Answer choice D – The extreme axis deviation has a frontal QRS axis of +120° to −150°.

A. Rationale: An isoelectric QRS vector is the most equiphasic ECG tracing. The ECG lead detects almost 0 mV because the impulses that travel away from and toward that lead are equal. Hence, the ECG tracing will represent an R wave amplitude equal to an S wave amplitude.

Answer choice B – Peaked T waves typically indicate elevated potassium levels.
Answer choice C – A prolonged PR interval represents a 1st-degree atrioventricular block.
Answer choice D – ST depression typically represents myocardial ischemia.

A. Rationale: The frontal QRS axis determination in ECG is an essential tool for ECG diagnosis. Left axis deviation is a QRS axis < −30°. Normal QRS axis is between −30° and 90°. Right axis deviation is a QRS axis > 90°. Extreme right axis deviation is a QRS axis between 180° and 270°.

C. Rationale: Einthoven’s triangle pertains to an imaginary inverted triangle that represents the frontal leads I, II, III, aVR, aVL, and aVF. The summation of all potentials in the Einthoven triangle is 0 mV. The leads representing the triangle can be used to determine the axis of the heart.

Answer choice A – The precordial leads are the chest leads V1–V6.
Answer choice B – Leads V1–V6 are the chest leads.
Answer choice D – Leads V8 and V9 are posterior leads used in 15-lead ECGs.

D. Rationale: In the normal heart, ventricular excitation originates in the middle part of the septum. The septal vector represents vector 1. It travels from the center of the septum toward the anterior aspect and to the right with a slightly downward projection.

Answer choice A – The vector traveling medially represents right ventricular depolarization.
Answer choice B – The vector that travels inferior and laterally represents the distal left ventricle depolarization that occurs right after the septal vector 1.
Answer choice C – The vector that travels superiorly represents the excitation of the remaining ventricular heart muscles that have not been excited.