End-of-Chapter Case Study Answers

b is correct. The circulation time will be shortened because some blood passes through the shunt (short circuit).

c is correct. When the extracellular K ⁺ concentration increases, the Nernst equation indicates that the transmembrane potential will become less negative.

d is correct. When the extracellular K ⁺ concentration increases and depolarizes the membrane, the rate of rise of the action potential is diminished because some Na ⁺ channels are inactivated by the persistent depolarization.

d is correct. The Purkinje fibers are automatic fibers, and they generate action potentials at a low frequency whenever they are not depolarized by action potentials that originate in higher-frequency pacemaker sites.

a is correct. When ventricular pacing at 75 beats per minute was discontinued, spontaneous pacemaker activity in the ventricles was suppressed for several seconds because the preceding period of artificial pacing had hyperpolarized the ventricular pacemaker cells (Purkinje fibers).

c is correct because the PR interval, which indicates the time for conduction from atria through the AV node and ventricular conducting system to the ventricle, is normal.

e is correct because the QT interval measures the duration of the average ventricular action potential.

a is correct because heart rate can be taken from either the R-R or the P-P interval when conduction is normal.

d is correct because there is greater activation of repolarizing K ⁺ channels when heart rate increases.

d is correct. The murmur is characteristic of mitral stenosis.

c is correct. The pulse is totally irregular.

b is correct. A “loop” diuretic like furosemide would relieve the excessive preload and allow the cardiac output to improve and the edema and ascites to subside.

b is correct. The elevated left atrial pressure would be transmitted back to the wedged catheter (wedge pressure).

c is correct. 300 mL O ₂ /min /[(18 − 8) mL O ₂ /100 mL blood] = 3000 mL/min = 3 L/min.

d is correct. A decrease in arterial pressure would reflexly increase sympathetic activity and thereby increase the neuronal release of norepinephrine.

d is correct. The ACh released from vagal fibers acts on muscarinic receptors on SA node automatic cells, and these receptors interact very quickly with specific K ⁺ channels because no second messenger intervenes.

a is correct. The cardiac responses to vagal stimulation develop and decay rapidly, but the responses to sympathetic stimulation develop and decay very slowly. Hence respiratory dysrhythmia is mediated almost entirely by the vagus nerves, and this dysrhythmia would be abolished by a potent muscarinic antagonist.

a is correct. Heart rate increases during inspiration in this dysrhythmia, and this increase is mediated mainly by a reduction in vagal activity.

e is correct. Acetylcholinesterase is abundant in atrial tissues, and especially in the SA and AV nodes.