Page 3, USMLE, Dr. D. Penney



USMLE-Type Examination Questions - Electrocardiography:


125.  The sinoatrial node is the pacemaker for the heart because it
	(A)	is the most richly innervated structure in the heart
	(B)	is the only structure in the heart capable of generating action potentials
	(C)	has the highest rate of automatic discharge
	(D)	has the most stable transmembrane potential
	(E)	is the cardiac cell least sensitive to catecholamines


127.  The last part of the ventricles to be activated after atrial activation is
	(A)	the base of the right ventricle
	(B)	the base of the left ventricle
	(C)	the endocardium of the right ventricle
	(D)	the epicardium of the apex
	(E)	the endocardium of the apex


129.  In standard electrocardiogram lead III, the right leg electrode is grounded through the recorder and the potential difference is recorded between
	(A)	the right arm and left leg
	(B)	the right arm and left arm
	(C)	the left arm and left leg
	(D)	the right arm and a central terminal using resistors and the left arm and left leg
	(E)	the left arm and a central terminal using resistors and the right arm and left leg


132.  The depolarization of the atrium consists of numerous waves of depolarization (ie, waves of negativity) moving away from the sinoatrial node.
These waves attract positive charges.  It proves useful to characterize all the electrical changes in the heart at any one instant in terms of a single, two-dimensional dipole.
This dipole has a positive head and a negative tail.  The negativity represents the wave of depolarization.
On that basis, one would expect the atrial dipole on the frontal plane to be moving
	(A)	cephalad (ie, -90o)
	(B)	caudad (ie, +90o)
	(C)	toward the right arm (180o)
	(D)	toward +60o
	(E)	toward -60o


135.	A patient was admitted to the hospital and ECG lead III was recorded.
It was found to contain no S wave.  The P, R, and T waves appeared normal.  What conclusions can you draw?
	(A)	activation of parts of the base of the heart are abnormal
	(B)	activation of parts of the apex of the heart are abnormal
	(C)	there has been cardiac depression
	(D)	there is left bundle branch block
	(E)	there are no indications of cardiac abnormalities


137.  A patient had an ECG with a prolonged PR segment.  What might have caused this?
	(A)	increased sympathetic tone to the atrioventricular (AV) node
	(B)	increased parasympathetic tone to the AV node
	(C)	increased sympathetic tone to the sinoatrial (SA) node
	(D)	increased parasympathetic tone to the SA node
	(E)	decreased carotid sinus pressure


143.  An independence of the P wave and of the QRS complex of the ECG indicates
	(A)	an early repolarization of the ventricular fibers
	(B)	a failure of the AV node to conduct
	(C)	depression of the SA node
	(D)	slowing of conduction at the AV node
	(E)	a conduction block in the left bundle branch



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