Deck 16: Elements of Cardiac Function

Question 1

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 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 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. While the heart was being paced,the cardiologist discontinued ventricular pacing periodically to test the patient's cardiac status.The cardiologist found that the ventricles did not begin beating spontaneously until about 5 to 10 seconds after cessation of pacing because the preceding period of pacing led to:

A)Overdrive suppression of the automatic cells in the ventricles.
B)Release of norepinephrine from the cardiac sympathetic nerves.
C)Release of neuropeptide Y from the cardiac sympathetic nerves.
D)Fatigue of the ventricular myocytes.
E)Release of acetylcholine from the cardiac parasympathetic nerves.

Answer 1

A

Question 2

A patient presents for her annual physical examination in seemingly good health.However,she is concerned about her heart because of family history,and you proceed to work up an examination of cardiovascular function.She has a blood pressure of 128/83 mm Hg,and echocardiography indicates that end-diastolic volume is 140 mL and end-systolic volume is 80 mL.A routine ECG is obtained and is shown as follows: According to the echocardiographic data,the patient's stroke volume amounts to:

A)80 mL
B)70 mL
C)60 mL
D)50 mL
E)45 mL

Answer 2

C

Question 3

A patient presents for her annual physical examination in seemingly good health.However,she is concerned about her heart because of family history,and you proceed to work up an examination of cardiovascular function.She has a blood pressure of 128/83 mm Hg,and echocardiography indicates that end-diastolic volume is 140 mL and end-systolic volume is 80 mL.A routine electrocardiogram (ECG)is obtained and is shown as follows: The ECG reveals the patient is in normal sinus rhythm with an R-R interval of 0.76 seconds.This corresponds to a heart rate of:

A)70 beats/minute
B)74 beats/minute
C)79 beats/minute
D)84 beats/minute
E)88 beats/minute

Answer 3

C

Question 4

A patient presents for her annual physical examination in seemingly good health.However,she is concerned about her heart because of family history,and you proceed to work up an examination of cardiovascular function.She has a blood pressure of 128/83 mm Hg,and echocardiography indicates that end-diastolic volume is 140 mL and end-systolic volume is 80 mL.A routine ECG is obtained and is shown as follows: Inspection of the ECG indicates a PR interval of:

A)0.08 seconds
B)0.10 seconds
C)0.12 seconds
D)0.16 seconds
E)0.20 seconds

Question 5

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 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 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. The most likely mechanism responsible for the patient's arterial pulse rate of about 45 beats/minute after impulse conduction through the AV junction was blocked is:

A)Excitation of the ventricles via an AV bypass tract.
B)Conversion of ventricular myocardial fibers to automatic cells.
C)Firing of ventricular ectopic cells that have the same electrophysiological characteristics as SA node cells.
D)Firing of automatic cells (Purkinje fibers)in the specialized conduction system of the ventricles.
E)Excitation of ventricular cells by the rhythmic activity in the autonomic neurons that innervate the heart.

Question 6

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 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 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. The mechanism by which the SA node generated impulses at a rapid rate during the early stages of the coronary artery occlusion involves an increase in:

A)The slope of the action potential upstroke (phase 0)of the automatic cells.
B)The slope of the slow diastolic depolarization of the automatic cells.
C)Firing threshold (less negative potential)of the automatic cells.
D)Negativity (hyperpolarization)of the initial portion of the slow diastolic depolarization.
E)Action potential amplitude of the automatic cells.

Question 7

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 8

A patient presents for her annual physical examination in seemingly good health.However,she is concerned about her heart because of family history,and you proceed to work up an examination of cardiovascular function.She has a blood pressure of 128/83 mm Hg,and echocardiography indicates that end-diastolic volume is 140 mL and end-systolic volume is 80 mL.A routine ECG is obtained and is shown as follows: You estimate this patient's cardiac output as:

A)4.4 L/minute
B)4.7 L/minute
C)5.2 L/minute
D)5.5 L/minute
E)6.3 L/minute