Question 1

How long does it normally take for the ventricles to depolarize?

Accepted Answer

The QRS complex normally lasts approximately 0.08 to 0.10 second.

Question 2

Which of the following are features of the PR interval?
I.It is measured from the beginning of the P wave to the first ventricular deflection.
II.It is measured from the beginning of the P wave to the S wave.
III.The normal PR interval is between 0.12 and 0.20 second.
IV.The PR interval can be shorter for fast heart rates than for slow heart rates.

Accepted Answer

The PR interval is the time required for the sinus node's impulse to reach the ventricles. It is measured from the beginning of the P wave to the first ventricular deflection, whether it is a Q or an R wave. The normal PR interval is between 0.12 and 0.20 second; a PR interval greater than 0.20 second indicates abnormally slowed impulse conduction. The PR interval is normally shorter for fast heart rates than for slow heart rates.

Question 3

Why is a prolonged QT interval associated with life-threatening heart rhythm disturbances?

Accepted Answer

A prolonged QT interval is associated with life-threatening heart rhythm disturbances because it is associated with a longer vulnerable period.

Question 4

What is the normal MCV angle?

Accepted Answer

Figure 19-3 shows that the normal mean cardiac vector (MCV) points toward +60 degrees.

Question 5

Which of the following limb leads are unipolar? I.Leads aVR, aVL, aVF II.Leads V₁, V₂, V₃ III.Leads V₄, V₅, V₆ IV.Leads I, II, III

Accepted Answer

The symbols for the three unipolar leads are aVR, aVL, and aVF.

Question 6

Atrial depolarization is represented by which of the following waves in the ECG?

Accepted Answer

The P wave (see Figure 19-2, A) is produced as a result of atrial depolarization.

Question 7

Which of the following conditions are detected by the ECG?
I.Abnormal heart rhythms
II.Conduction problems
III.Location of damaged heart muscle
IV.Force of contraction

Accepted Answer

From the ECG it is possible to detect abnormal heart rhythms, conduction problems, and the location of damaged heart muscle.

Question 8

When the heart rate is 150 beats/min, the interval between two QRS complexes in the typical 25 mm/sec ECG is?

Accepted Answer

When the heart rate is 150 beats/min, the interval between two QRS complexes is two large squares.

Question 9

The normal MCV is parallel to which of the following leads?

Accepted Answer

Figure 19-3 shows that the normal MCV points toward +60 degrees. This direction is parallel to standard limb lead II's axis and points toward lead II's positive electrode.

Question 10

In which direction does ventricular depolarization normally occur?
I.Leftward
II.Downward
III.Anterior
IV.Posterior

Accepted Answer

The ventricles depolarize in a leftward, downward, and posterior direction.

Question 11

Which of the following are characteristics of the normal ST segment?
I.The ST segment is normally flat, lying on the baseline.
II.This segment may be elevated as much as 2 mm above the baseline.
III.This segment may be depressed as much as 2 mm below the baseline.
IV.An ST segment that becomes depressed more than 0.5 mm is abnormal.

Accepted Answer

The ST segment is normally flat, lying on the baseline. However, this segment may be elevated as much as 2 mm above the baseline or depressed as much as 0.5 mm below the baseline and still be considered normal. An ST segment that becomes depressed more than 0.5 mm during a stress exercise test signals myocardial ischemia.

Question 12

A 60-year-old man with COPD is chronically hypoxemic and hypercapnic.A standard electrocardiogram (ECG) reveals a strongly negative QRS complex in lead I and a more positive than negative QRS complex in aV_F.What explains this finding?

Accepted Answer

These QRS recordings reveal a right axis deviation. In other words, the mean direction of current flow in the ventricles is toward the +90-degree to +180-degree quadrant. A strongly negative QRS in lead I indicates that the heart's current is flowing almost 180 degrees away from lead I's positive electrode. This finding is consistent with the patient's history.

Question 13

Ventricular depolarization is represented by which of the following waves in the ECG?

Accepted Answer

The QRS complex is produced by ventricular depolarization and is composed of three separate waves: the Q, R, and S waves.

Question 14

Ventricular repolarization is represented by which of the following waves in the ECG?

Accepted Answer

The T wave is produced by ventricular repolarization and is associated with ventricular relaxation (diastole).

Question 15

Which of the following findings are consistent with a left axis deviation? I.A positive QRS in lead I II.A positive QRS in lead II III.A negative QRS in lead aV_F IV.A positive QRS in lead aV_R

Accepted Answer

A positive QRS in lead I and a negative QRS in lead aVF meAnswer the cardiac vector points to the left upper quadrant (0 degrees to -90 degrees), which indicates a left axis deviation.

Question 16

Which of the following is the only parameter that the ECG measures?

Accepted Answer

The electrocardiogram (ECG) measures only electrical events.

Question 17

An abnormally elevated ST segment is indicative of which of the following?

Accepted Answer

An elevated ST segment generally indicates myocardial tissue injury.

Question 18

Which of the following leads allow the cardiac vector to be located within the normal 90-degree quadrant? Which two leads are best utilized to determine the direction of the cardiac vector? I.I II.II III.aV_L IV.aV_F

Accepted Answer

Comparing QRSs from lead I and lead aV_F allows the cardiac vector to be located within a 90-degree quadrant (Figure 19-15). A positive QRS complex recorded by both leads meAnswer the cardiac vector is located in the left lower quadrant, or the normal range (-0 degrees to +90 degrees).

Question 19

What is the normal variation between the shortest and longest R-R interval in a regular sinus rhythm?

Accepted Answer

If the variation between the shortest and longest R-R interval is less than 0.16 second, the ECG rhythm is regular.

Question 20

In cases of extreme obesity, where may the MCV be located?

Accepted Answer

Extreme obesity pushes the diaphragm up, displacing the heart to a more horizontal position, causing a left axis deviation.

Quiz 19: The Electrocardiogram and Cardiac Arrhythmias In Adults