ECG Basic Principles Flashcard 1

D. Rationale: The standard complete ECG study makes use of 12 leads. These leads represent the following leads: I, II, III, aVR, aVL, aVF, and V1 to V6.

D. Rationale: The normal PR interval is about 0.12–0.2 seconds if the impulse is generated in the SA node. But since junctional rhythms are in close proximity to the ventricles, the PR interval is expected to be shorter than 0.12–0.2 seconds.

C. Rationale: Electrical signals that flow away from the positive electrode of the ECG device cause a downward deflection in the tracing generated by the ECG device.

C. Rationale: The sinoatrial node is the anatomical structure where normal impulse generation originates.

Answer choice A – The Bundle of His transmits impulses from the atrioventricular (AV) node.
Answer choice B – The Purkinje fibers are specialized cardiac muscle fibers that form a network in the ventricular walls that conducts the impulses responsible for ventricular contractions.
Answer choice D – The intraventricular septum separates the left and right ventricles and does not generate electrical impulses.

A. Rationale: Calipers can measure equidistant R-R intervals throughout the ECG printout, representing R-R wave regularity that demonstrates the overall regularity of the patient’s ECG.

C. Rationale: This action potential pattern refers to the electrical measurements in the working heart cell.

Answer choices A & B – The conduction cell and sinus node cell have similar patterns distinct from working heart cells. The only difference between them is that the action potential within a single conduction cell has a slower and more gradual depolarizing phase.
Answer choice D – Myocytes do not conduct electrical activity.

D. Rationale: One small square measured horizontally is 0.04 seconds, and thus 5 small squares measured horizontally is 0.2 seconds. One small square measured vertically is 0.1 millivolt, and 5 small squares measured vertically is 0.5 millivolts.

D. Rationale: The QRS complex represents ventricular depolarization. When impulses are generated from the ventricles, they will show a widened QRS complex on an ECG tracing (i.e., longer than 0.12 seconds). Sometimes, a supraventricular rhythm may also present with widened QRS complexes when there is a delay in impulse conduction from the supraventricular focus to the ventricles.

C. Rationale: The heart is a muscle working as a pump system that ejects blood into the systemic circulation by the action of myocytes. When stimulated by electrical impulses, the myocytes contract. The ECG measures the function of the heart’s conduction system as it generates impulses causing the myocardium to contract.

B. Rationale: The normal PR interval is 120–200 milliseconds.

Answer choice A – The normal QRS interval is 70–100 milliseconds.
Answer choice D – The normal QT interval is 360–440 milliseconds.