ECG Basic Principles Flashcard 1

A. Rationale: Measuring an irregular heart rate is possible using an electrocardiogram. Take a 6-second strip and count the number of QRS complexes within the strip. Multiply that number by 10 to calculate the number of beats per minute. In the case of bradycardia, a 12-second strip may be taken to improve accuracy. Count the number of QRS complexes within the 12-second strip, and then multiply that number by 5 to get the heart rate per minute.

D. Rationale: All myocardial cells are polarized, with the intracellular environment being more negative compared to the extracellular environment. Even if potassium is positively charged, the extracellular environment, which is already rich in sodium, is even more so. The intracellular environment is relatively more negative than the extracellular environment. If we consider the myocyte, its resting potential is –90mV, meaning the cell is more negative than the outside environment by 90mV.

C. Rationale: In the normal heart, the SA node is the pacemaker that generates the impulse. It is located high in the right atrium, where the superior vena cava enters the heart. Lead V1 is a right-sided precordial lead placed near the right ventricle in the 4th intercostal space, immediately to the right of the sternum. Therefore, in relation to the SA node, lead V1 is below and to the right. An impulse generated from the SA node will travel away from lead V1. The ECG tracing should register small R waves with deep S waves. Since the ECG tracing showed tall R waves, it can be deduced that lead V1 was not placed on the 4th intercostal space to the right of the sternum but instead somewhere over the left ventricle. This is a common error, and healthcare workers should always be mindful of lead placement.

A. Rationale: Sometimes, certain foci within the heart exceed their inherent rates and take over the pacemaker cells. This is known as irritability. Escape is a failsafe mechanism of the heart that allows other areas to take over when higher pacemakers such as the SA node fail.
Torsade’s de pointes is an arrhythmia described as a polymorphic ventricular tachycardia.
Wenckebach phenomenon refers to an atrioventricular block.”

B. Rationale: The PR interval of the normal sinus rhythm is 120 milliseconds to 200 milliseconds. Any interval faster than 120 milliseconds or slower than 200 milliseconds is not considered to be a normal sinus rhythm. Furthermore, a PR interval should be consistent throughout the ECG strip.

A. Rationale: The normal interval of the QRS complex is 0.06 to 0.10 seconds (60–100 milliseconds). This is approximately 1-½ to 2-½ small squares in the ECG rhythm strip.

C. Rationale: Heart rate is established by the autonomic nervous system and the inherent rates of the conduction system. The autonomic nervous system contains two branches: the sympathetic and the parasympathetic. These branches oppose each other to regulate the heart. The sympathetic branch influences the atria and ventricles to increase the rate, increase the conduction velocity through the AV node, and increase irritability. Alpha receptors on the heart regulate vasodilation and vasoconstriction of the vessels. Beta receptors located on the heart increase the strength of cardiac contraction when stimulated by the sympathetic nervous system. The parasympathetic branch counteracts the sympathetic branch by reducing the rate, irritability, and the conduction velocity through the AV node.

D. Rationale: The electrical charges of cardiac cells come from the sodium and potassium concentrations inside and outside of the cells. This concentration is regulated by the sodium-potassium channels within the myocyte cell walls. These channels give the electrical cells automaticity, and they can discharge electrical activity without a stimulus by changing the conformation of the sodium-potassium channels.

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

C. Rationale: The normal QRS interval is 0.06–0.10 seconds. In a standard ECG, one small square is equivalent to 0.04 seconds. Therefore, 4 small squares is equivalent to 0.16 seconds, meaning the QRS complex (ventricular depolarization) in this instance is too slow. There is insufficient information to conclude that this is an irregular rhythm.