The following ECG results are obtained
from a patient: The QRS is equally
biphasic in lead II (no net deflection),
and the QRS has a net positive voltage in
lead aVL. What is the approximate mean
electrical axis?
a. -30°
b. 0°
c. +60°
d. +120°
10. An ECG rhythm strip shows a complete
dissociation between P waves and QRS
complexes. The atrial rate is 95 beats/min
and regular, and the ventricular rate is
about 60 beats/min and regular. The QRS
complexes are of normal shape and dura-
tion. This ECG represents
a. First-degree AV nodal block.
b. Second-degree
V nodal block.
c. Third-degree AV nodal block.
d. Premature ventricular complexes.
The correct answer is “d” because
the sarcolemmal Na+/K+-ATPase is
an electrogenic pump that generates
hyperpolarizing currents; inhibition
of this pump results in depolarization.
Furthermore, inhibition of the pump
leads to an increase in intracellular sodi-
um and a decrease in intracellular potas-
sium, both of which cause depolariza-
tion. Choices “a” and “b” are incorrect
because decreased calcium and sodium
conductance reduces the inward move-
ment of positive charges that normally
depolarize the membrane. Choice “c” is
incorrect because increased potassium
conductance hyperpolarizes the mem-
brane (see Equation 2-4).
The correct answer is “c” because
slow depolarization leads to closure of
the h-gates, which inactivates the fast
sodium channels. Choice “a” is incor-
rect because the m-gates open at the
onset of phase 0, which activates the
fast sodium channels. Choice “b” is
incorrect because it is the closure of
the h-gates that inactivates the channel.
Choice “d” is incorrect because L-type
(long-lasting) calcium channels have
a prolonged phase of activation before
they become inactivated.
The correct answer is “d” because the
membrane potential during phase 4 is
primarily determined by the high potas-
sium conductance. Choices “a,” “b,”
and “c” are incorrect because the overall
potassium conductance is reduced dur-
ing phases 0 through 2, and it begins to
recover only during early phase 3.
The correct answer is “a” because one
effect of (3-adrenoceptor activation is to
increase If, which enhances the rate of
spontaneous depolarization. Choice “b”
is incorrect because vagal stimulation
reduces pacemaker firing rate, in part,
by decreasing the slope of phase 4.
Choice “c” is incorrect because fast
sodium channels do not play a role
in SA nodal action potentials; inward
calcium currents are responsible for
phase 0. Choice “d” is incorrect because
increasing potassium conductance dur-
ing phase 4 hyperpolarizes the cell so it
takes longer to reach threshold.
The correct sequence of activation and
conduction within the heart is choice “a”.
The correct answer is “b” because ace-
tylcholine released by the vagus nerve
binds to M2 receptors, which decreases
AV nodal conduction velocity and
increases the PR interval. Removal of
vagal tone through the use of a musca-
rinic receptor antagonist (e.g., atropine)
leads to an increase in conduction
velocity. Choice “a” is incorrect because
blocking p-adrenoceptors would
decrease the influence of sympathetic
nerves on the AV node and lead to
a decrease in conduction velocity.
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