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SECTION V. CARDIOVASCULAR PHYSIOLOGY

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Which part of the ECG (eg, Figure 29–5) corresponds to ventricular repolarization?

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FIGURE 29–5: Waves of the ECG. Standard names for individual waves and segments that make up the ECG are shown. Electrical activity that contributes the observed deflections are discussed in the text and in Table 29–2.

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Table Graphic Jump Location
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TABLE 29–2: ECG intervals.
Normal Durations
Intervals Average Range Events in the Heart during Interval
PR intervala 0.18b 0.12–0.20 Atrioventricular conduction
QRS duration 0.08 to 0.10 Ventricular depolarization
QT interval 0.40c to 0.43 Ventricular action potential
ST interval (QT minus QRS) 0.32 Plateau portion of the ventricular action potential

aMeasured from the beginning of the P wave to the beginning of the QRS complex.

bShortens as heart rate increases from average of 0.18 s at a rate of 70 beats/min to 0.14 s at a rate of 130 beats/min.

cCan be lower (0.35) depending on the heart rate.

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A. The P wave

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B. The QRS duration

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C. The T wave

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D. The U wave

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E. The PR interval

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Which of the following normally has a slowly depolarizing “prepotential”?

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A. Sinoatrial node

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B. Atrial muscle cells

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C. Bundle of His

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D. Purkinje fibers

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E. Ventricular muscle cells

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In second-degree heart block

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A. the ventricular rate is lower than the atrial rate.

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B. the ventricular ECG complexes are distorted.

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C. there is a high incidence of ventricular tachycardia.

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D. stroke volume is decreased.

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E. cardiac output is increased.

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Currents caused by opening of which of the following channels contribute to the repolarization phase of the action potential of ventricular muscle fibers?

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A. Na+ channels

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B. Cl channels

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C. Ca2+ channels

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D. K+ channels

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E. HCO3 channels

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