CHAPTER 2 • ELECTRICAL ACTIVITY OF THE HEART
39
Choice “c” is incorrect because L-type
calcium channel blockers reduce con-
duction velocity by decreasing the rate of
calcium entry into the cells during depo-
larization, which decreases the slope
of phase 0 in AV nodal cells, thereby
decreasing conduction velocity. Choice
“d” is incorrect because enhancing vagal
activity decreases AV nodal conduction
velocity and increases the PR interval.
7.
The correct answer is “c” because the
T wave represents repolarization of the
ventricular muscle. Choice “a” is incor-
rect because the normal PR interval is
between 0.12 and 0.20 seconds. Choice
“b” is incorrect because the duration
of the ventricular action potential is
most closely associated with the QT
interval. Choice “d” is incorrect because
the duration of the QRS complex is
normally <0.1 seconds.
8.
The correct answer is “a” because the T
wave is normally positive when the last
cells that depolarize are the first to repo-
larize. When the direction of repolariza-
tion is reversed, the T wave becomes
inverted. Choice “b” is incorrect because
accidental reversal of the electrode polar-
ity would lead to an inverted QRS and
inverted T wave. Choice “c” is incorrect
because when depolarization and repo-
larization occur in opposite directions
(which is normal), both the QRS and T
wave are upright. Choice “d” is incorrect
because the QRS, which represents ven-
tricular depolarization, is normal.
9.
The correct answer is “a” because when
lead II is biphasic, the mean electrical
axis must be perpendicular to that lead,
and therefore it is either -30° or +150°.
Because aVL
is positive, the mean electri-
cal axis must be -30° because that is the
axis for aVL. All the other choices are
therefore incorrect.
10. The correct answer is “c” because
a complete dissociation between P
waves and QRS complexes indicates a
complete (third-degree) AV nodal block.
Furthermore, the rate of ventricular
depolarizations and the normal shape
and duration of the QRS complexes
suggest that the pacemaker driving
ventricular depolarization lies within
the AV node or bundle of His so that
conduction follows normal ventricu-
lar pathways. Choice “a” is incorrect
because a first-degree AV nodal block
increases only the PR interval. Choice
“b” is incorrect because all of the QRS
complexes would still be preceded by a
P wave in a second-degree block. Choice
“d” is incorrect because premature
ventricular complexes normally have
an irregular discharge rhythm and the
QRS is abnormally shaped and has a
longer-than-normal duration.
ANSWERS TO PROBLEMS AND CASES
PROBLEM 2-1
Using Equation 2-1, the membrane potential
(actually, the equilibrium potential for potas-
sium) with 4 mM external potassium would
be -96 mV Solving the equation for 40 mM
external potassium results in a membrane
potential of —35 mV. This is the membrane
potential predicted by the Nemst equation
assuming that no other ions contribute to the
membrane potential (see Equation 2-3). This
calculation also neglects any contribution
of electrogenic
pumps
to
the membrane
potential. Nevertheless, a high concentration
of external potassium causes a large depolari-
zation, as predicted by the Nemst equation.
PROBLEM 2-2
Because phase 0 of myocyte action poten-
tials is generated by activation of fast sodium
channels, partial inactivation of these chan-
nels would decrease the upstroke velocity of
phase 0 (decrease the slope of phase 0). Partial
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