Passage
The heart is a muscular organ comprising four chambers, two atria and two ventricles, that function synergistically to pump blood through a vast, closed network of blood vessels. The chambers are separated by membranous muscular barriers known as septa. Oxygenated blood returning from the lungs via the pulmonary veins fills the left atrium and enters the left ventricle, which then pumps the blood into the systemic arteries to supply other organs in the body. By contrast, deoxygenated blood returning from systemic veins first enters the right atrium, then the right ventricle, and is ultimately siphoned into the pulmonary arteries leading to the lungs. The necessary oxygen, nutrient, and waste exchange occurs between the thinnest blood vessels (capillaries) and neighboring tissues of the systemic and pulmonary circuits.Blood flow throughout the circulatory system is dictated by blood (hydrostatic) pressure, vascular resistance (force opposing blood flow through a vessel), and cardiac output (blood volume expelled from the ventricles per unit time). When blood traverses a vessel, it exerts hydrostatic pressure on the vessel walls, which results in the forced movement of fluid out of the vessel and into the interstitial space. The circulating plasma proteins cause the osmotic pressure within the vessel to be higher than that of the interstitial fluid. In turn, osmotic pressure causes fluid to flow from the interstitial space into the blood vessel, opposing hydrostatic pressure.Cardiac output depends in part on heart rate, which is tightly regulated by the sinoatrial (SA) and atrioventricular (AV) nodes, specialized groups of self-depolarizing cells located in the upper right atrial wall and lower interatrial septum, respectively. Action potentials (APs) generated by SA nodal cells stimulate atrial contraction as they travel to the AV node. The AV node delays AP transmission to ventricular cells, ensuring ventricular filling is complete prior to heart contraction.A 63-year-old male patient collapsed while exercising and was hospitalized. The patient presented with an elevated heart rate, low systemic blood pressure, and abnormally low blood oxygen levels. In addition, x-rays revealed excess fluid in his lungs.
-The attending physician orders an electrocardiogram (ECG) to measure the electrical activity of the patient's heart. The results of the ECG of a healthy individual at rest (A) and the ECG of the patient at rest (B) are shown below. Which of the following conclusions regarding the patient's cardiac function is best supported by these results?

Question

Passage
The heart is a muscular organ comprising four chambers, two atria and two ventricles, that function synergistically to pump blood through a vast, closed network of blood vessels.  The chambers are separated by membranous muscular barriers known as septa.  Oxygenated blood returning from the lungs via the pulmonary veins fills the left atrium and enters the left ventricle, which then pumps the blood into the systemic arteries to supply other organs in the body.  By contrast, deoxygenated blood returning from systemic veins first enters the right atrium, then the right ventricle, and is ultimately siphoned into the pulmonary arteries leading to the lungs.  The necessary oxygen, nutrient, and waste exchange occurs between the thinnest blood vessels (capillaries) and neighboring tissues of the systemic and pulmonary circuits.Blood flow throughout the circulatory system is dictated by blood (hydrostatic) pressure, vascular resistance (force opposing blood flow through a vessel), and cardiac output (blood volume expelled from the ventricles per unit time).  When blood traverses a vessel, it exerts hydrostatic pressure on the vessel walls, which results in the forced movement of fluid out of the vessel and into the interstitial space.  The circulating plasma proteins cause the osmotic pressure within the vessel to be higher than that of the interstitial fluid.  In turn, osmotic pressure causes fluid to flow from the interstitial space into the blood vessel, opposing hydrostatic pressure.Cardiac output depends in part on heart rate, which is tightly regulated by the sinoatrial (SA) and atrioventricular (AV) nodes, specialized groups of self-depolarizing cells located in the upper right atrial wall and lower interatrial septum, respectively.  Action potentials (APs) generated by SA nodal cells stimulate atrial contraction as they travel to the AV node.  The AV node delays AP transmission to ventricular cells, ensuring ventricular filling is complete prior to heart contraction.A 63-year-old male patient collapsed while exercising and was hospitalized.  The patient presented with an elevated heart rate, low systemic blood pressure, and abnormally low blood oxygen levels.  In addition, x-rays revealed excess fluid in his lungs.
-The attending physician orders an electrocardiogram (ECG) to measure the electrical activity of the patient's heart.  The results of the ECG of a healthy individual at rest (A) and the ECG of the patient at rest (B) are shown below.  Which of the following conclusions regarding the patient's cardiac function is best supported by these results?

Quizplus · Accepted Answer

11ecba2d_1037_591c_a3bf_d9487d254d60_MD0008_00 As the passage states, heart rate is tightly regulated by the SA and AV nodes, specialized clusters of self-depolarizing cells found in the upper atrial wall and lower interatrial septum, respectively. Normally, depolarization occurs via the influx of positive ions (ie, Na⁺, Ca²⁺) into SA and AV nodal cells, increasing cell membrane potentials and generating action potentials (APs). APs fired from the SA node travel through cardiac muscle cells of the atria, inducing atrial contraction and filling of the ventricles. At the AV node, relay of APs to ventricular cells is delayed, ensuring that ventricular filling is complete prior to ventricular contraction. Next, APs travel through both ventricles, causing them to contract and expel blood into the arteries. The electrical activity of the heart is often measured using an ECG, a device that records APs generated by SA and AV nodal cells as well as cardiac muscle cells. In this scenario, the physician compared an ECG of the patient to that of a healthy individual. Compared to the healthy individual's ECG, the patient's ECG shows more waves of atrial and ventricular depolarization over the same time period. This indicates an increased number of APs fired from the patient's SA and AV nodal cells, leading to more frequent atrial and ventricular contraction (Choices A and B). Because depolarization and AP generation depend on the influx of positive ions into cells, a more rapid influx of positively charged ions into SA nodal cells may lead to increased firing of APs from the SA node and more frequent atrial contraction, as observed in the patient. (Choice C) Although ventricular depolarization and contraction are occurring more frequently in the patient, ECG results show that the duration of each ventricular depolarization event is the same in both the patient and the healthy individual. Educational objective: Heart rate is regulated by the activity of specialized clusters of self-depolarizing cells known as the SA and AV nodes. APs are initiated in the SA node and travel through the atria, stimulating atrial contraction. These APs then reach the AV node and, after a brief delay, are relayed to ventricular cells, stimulating ventricular contraction.