When interpreting an EKG rhythm strip, the nurse knows that one large square on the paper equals how many seconds?

Choice A rationale

Gastroesophageal Reflux Disease (GERD) pain is often described as burning and is typically exacerbated by meals, bending over, or lying down, which increases intra-abdominal pressure and allows gastric acid reflux. The patient's pain, which occurs when sitting and is not worse with activity, is less typical of GERD-related chest pain.

Choice B rationale

Prinzmetal's angina, or variant angina, is caused by transient, spontaneous spasm of a coronary artery, often occurring at rest or during the night/early morning, rather than with exertion. Its resolution is typically spontaneous or with nitrates, aligning well with the described pattern of intermittent pain when "just sitting around.”.

Choice C rationale

A first-degree heart block is an electrical conduction delay through the AV node, characterized by a prolonged PR interval (normal range: 0.12–0.20 seconds) on an ECG. It is usually asymptomatic and does not directly cause chest pain, making it an unlikely cause of the described intermittent symptoms.

Choice D rationale

Chronic stable angina is classically and predictably brought on by exertion or emotional stress due to fixed atherosclerotic plaque limiting blood flow (supply-demand mismatch). The pain is relieved by rest or nitroglycerin; the patient's pain not worsening with activity makes this diagnosis less likely. —.

Choice A rationale

The Bundle of His is not the natural pacemaker but a component of the cardiac conduction pathway transmitting impulses from the AV node to the ventricles. It provides electrical continuity through the interventricular septum, ensuring synchronous depolarization. However, its intrinsic automaticity rate (20–40 beats/min) is insufficient for maintaining effective cardiac output, functioning only as a secondary pacemaker during atrioventricular nodal or sinoatrial failure.

Choice B rationale

The sinoatrial (SA) node is the natural pacemaker of the heart, located at the junction of the superior vena cava and right atrium. It generates spontaneous depolarizations through automaticity, firing at 60–100 impulses/min under normal physiologic conditions. SA nodal cells possess leaky sodium channels and calcium-dependent action potentials that determine intrinsic rhythm, initiating electrical conduction through atrial myocardium and establishing the baseline sinus rhythm that drives cardiac output.

Choice C rationale

Purkinje fibers function as specialized conductive fibers located in the subendocardial layer of ventricles, transmitting impulses rapidly to facilitate synchronized ventricular contraction. Although capable of spontaneous depolarization at 20–40 beats/min, they are tertiary pacemakers. They assume pacemaking only when higher pacemakers fail. Their primary function is propagation rather than initiation of impulses, ensuring efficient myocardial contraction and ejection fraction maintenance.

Choice D rationale

The atrioventricular (AV) node serves as a relay station slowing conduction from atria to ventricles, allowing ventricular filling during diastole. It has intrinsic automaticity at 40–60 beats/min, functioning as a secondary pacemaker only during SA nodal failure. The delay at the AV node ensures atrial contraction precedes ventricular systole, optimizing stroke volume. However, its primary role is conduction delay, not initiation of the intrinsic cardiac rhythm.