A 63-year-old man was admitted to the hospital 1 hour after experiencing substernal chest pain.He was weak and diaphoretic,and his heart was beating rapidly at 90 beats/minute as a consequence of a coronary artery occlusion that impaired blood flow to the left ventricle.From an ECG,it was determined that the tachycardia originated in the sinoatrial (SA)node.Before therapy could be started,the man became much weaker; his arterial pulse rate was about 45 beats/minute,and yet the ECG revealed an atrial rate of 90 beats/minute.A cardiac pacemaker was inserted because the atrioventricular (AV)node had been damaged by ischemia.With the implanted pacemaker set at 75 beats/minute,he felt somewhat better,and drug therapy was initiated. Soon after coronary artery occlusion,the interstitial fluid [K⁺] rose substantially in the flow-deprived region.In this region,the high extracellular [K⁺]: A)Increased the propagation velocity of the myocardial action potentials. B)Decreased the postrepolarization refractoriness of the myocardial cells. C)Depolarized the resting transmembrane potential to a less negative value. D)Diminished the automaticity of the myocardial cells. E)Decreased the likelihood of reentry dysrhythmias.

Question

Quizplus · Accepted Answer

The high extracellular [K⁺] in the flow-deprived region would cause depolarization of the resting transmembrane potential to a less negative value. This can lead to ectopic pacemaker activity and contribute to the development of arrhythmias.

A 63-Year-Old Man Was Admitted to the Hospital 1 Hour