Electrocardiogram by John H. Dirckx, M.D.
Answers to Questions by Susan Dooley, CMT, AHDI-F
1.B
Feedback: Electrocardiography was developed by the Dutch physician and physiologist Willem Einthoven around 1900.
2.D
Feedback: At the beginning of the 20th century, available methods of gathering diagnostic information about the heart included feeling the pulse, listening with a stethoscope, percussing the thorax, and using the newly developed x-ray. Though experiments with cardiac catheterization had been made as early as the 1700s on animals, the first successful human heart catheterization was in the 1930s.
3.A and B
Feedback: Before Einthoven’s experimentation with electrocardiography, earlier investigators had shown that the action of the heart generates small, transitory electrical waves that can be detected by applying an appropriate instrument to the outer surface of the body.
4.D
Feedback: Einthoven devised a highly responsive voltage detector called a string galvanometer to measure the electrical activity of the heart. Galvanometers measure any steady direct current, particularly those created by chemical processes.
5.C
Feedback: To make a permanent record of the rapidly shifting electrical potentials, Einthoven had the movements of the galvanometer needle deflect a beam of light as it fell on a moving strip of photographic film.
6.D
Feedback: Since Einthoven was Dutch, he spelled the new word, derived from the Greek kardia for heart, as it would be spelled in Dutch and German: Elektrokardiogramm. That is the source of the K in EKG¸ still widely used today as an alternative to ECG.
7.A
Feedback: In modern parlance, an electrocardiograph is a machine, and an electrocardiogram is the tracing it makes.
8. D
Feedback: Nerve and muscle tissue, including heart muscle and the highly specialized conducting system of the heart, consists of bundles or sheets of microscopic fibers, each of which is a living cell.
9.B and C
Feedback: A constant chemical pumping action of the cell membrane tends to keep it polarized—that is, the outside of the cell is electrically positive with respect to the inside.
10.C
Feedback: The travel of an impulse along the course of a nerve or muscle fiber is a wave of depolarization, followed a fraction of a second later by a return to normal polarity (repolarization). It is important to be aware that the flow of electricity through a nerve or muscle fiber is entirely different from the flow of electrical current through a wire.
11. A
Feedback: It is impossible to measure the passage of an electrical impulse through a single fiber of heart muscle from outside the body. What the electrocardiograph actually detects, with electrodes placed at strategic sites on the body surface, is net effect of all the electrical activity going on in the heart from one instant to the next, as damped and diffused by the body tissues intervening between the heart and the electrodes. Like testing a battery’s voltage by applying the meter to both terminals, the electrocardiograph is always testing the difference of potential between two points.
12.D
Feedback: To our knowledge, Einthoven stuck to studying the electricity of the heart.
13.C
Feedback: Lead I is for the right arm and left arm; lead II for the right arm and left leg, and lead III is for the left arm and left leg. A good mnemonic for this is to relate the roman numeral I to the number of Ls.
14.D
Feedback: A single lead can provide only limited data about the size, shape, and position of the heart, and may not show any sign of disease affecting parts of the heart other than those facing the recording electrode.
15.C
Feedback: Einthoven chose the three electrode positions because they formed a roughly equilateral triangle with the heart at the center.
16.A
Feedback: The three unipolar limb leads are aVR, aVL, and aVF. V6 is a precordial, or chest, lead.
17.D
Feedback: In the nomenclature for unipolar leads, a stands for augmented and V for vector. The third letter in each abbreviation refers to the site of the recording electrode, i.e.: aVR, Right Arm; aVL, Left Arm; aVF, Left Leg (Foot).
18. E
Feedback: Precordial or chest lead electrodes are placed on the chest in front of the heart. The standard electrode positions for the six chest leads occur at approximately equal intervals around the anterior chest starting from the right sternal border and ending in the left midaxillary line.
19.E
Feedback: A standard 12-lead electrocardiogram consists of the standard limb leads I, II, and III; the augmented bipolar limb leads, aVR, aVL, and aVF; and precordial leads V1 through V6, in that order.
20.C
Feedback: The bipolar and unipolar limb leads together give a composite view of the heart in the frontal plane, as it appears in a standard PA chest x-ray. The precordial leads yield a composite view of the heart in the transverse plane, as seen in a CT scan or MRI projection. Note, however, that the ECG does not give an actual image per se; rather, the electrical tracing can be interpreted to show the status of various parts of the heart.
21.A
Feedback: An intraesophageal lead is placed by having the patient swallow an ECG electrode. This allows ECG exploration of the heart electrically.
22.B
Feedback: The heart’s conduction system consists of highly specialized tissue that starts and transmits electrical impulses that make the heart beat. The pacemaker of the heart is the sinoatrial (SA) node, a small nubbin of tissue in the upper part of the right atrium. The SA node is almost always called “the sinus.”
23.C
Feedback: The SA node’s electrical impulse causes the atria to contract; this impulse next reaches the AV node, which is specialized tissue located in the lower part of the right atrium.
24.E
Feedback: The electrical impulse generated by the SA node triggers a wave of depolarization that spreads over both atria, causing them to contract. When this impulse reaches the atrioventricular (AV) node, which is another mass of specialized tissue located low in the right atrium, it is picked up and transmitted down the atrioventricular bundle (bundle of His), a tract of specialized conducting fibers, to the septum between the two ventricles.
25.C
Feedback: The right and left bundle branches are located in the interventricular septum and take the electrical impulse from the bundle of His and pass them on to the anterior and posterior fascicles and then to the Purkinje fibers.
26.D
Feedback: The electrical events in the normal cardiac cycle cause a series of waves to appear on the ECG tracing, which detects electrical activity, not muscular contraction or movement.
27.A
Feedback: Not all of these waves appear in every lead, and some vary in amplitude (size) and polarity (above or below the baseline) from one lead to another.
28.C
Feedback: R waves predominate in V6 due to the position of the lead in relation to the heart, so that is a normal finding. However, inverted T waves in lead I or lead II usually indicate deficient blood supply to heart muscle, and a deep, wide Q wave in any lead is evidence of myocardial infarction. Absence of P waves from all leads indicates that the sinoatrial node is not functioning as a pacemaker.
29.List 6 principal uses of electrocardiography.
A.Identifying and analyzing abnormalities in heart rate and rhythm.
B.Identifying and analyzing abnormalities in the size or shape of the heart or its chambers.
C.Identifying and analyzing effects of problems with circulation of blood through the heart.
D.Diagnosing inflammation of the tissue surrounding the heart (pericarditis).
E.Verifying correct function of artifical pacemakers.
F.Assessing various disorders that affect heart function.
30.List 4 values that physicians determine from ECG tracings.
A.Atrial rate (the number of P waves per minute)
B.Ventricular rate (the number of R waves per minute)
C.PR interval (time elapsed between the beginning of the P wave and beginning of the R wave, representing the interval between the start of atrial depolarization and the start of ventricular depolarization)
D.QRS interval (time duration of a QRS complex, which represents the whole process of ventricular depolarization)
31.E
Feedback: The axis is also recorded by the physician in the ECG reading. It is an imaginary line that represents the direction along which the maximum electrical activity passes through the heart in each cardiac cycle. It is determined by reviewing the first 6 leads of the ECG—the lead whose recording electrode is most in line with the electrical axis will show the highest R waves, while the lead at nearly a right angle to the electrical axis will show the lowest R waves. The axis is expressed in degrees, with a perfectly horizontal line being 0 degrees—if, as on a clock face, the axis at 3 o’clock is 0 degrees, then the one at 6 o’clock would be at 90 degrees.
32.E
Feedback: A heart whose electrical axis lies above the horizontal 0° line (for example, one with an axis of minus 60°) is said to show left axis deviation, and a heart whose electrical axis lies to the right of the vertical 90° line (for example, one with an axis of 150°) is said to show right axis deviation. Significant axis deviation indicates either abnormality in the shape of the heart (left or right ventricular hypertrophy) or some deflection of the normal depolarizing wave by a block somewhere in the conducting system.
33.A
Feedback: The rhythm of the heart, as paced by the sinoatrial node, is called regular sinus rhythm (RSR) or normal sinus rhythm.
34.E
Feedback: The term arrhythmia denotes any deviation from regular sinus rhythm and includes bradycardia (heart rate below 60/min) and tachycardia (heart rate above 100/min), even with regular sinus rhythm, known as sinus bradycardia and sinus tachycardia. An arrhythmia can occur with a normal pulse rate (60-100/min), with bradycardia (bradyarrhythmia), or with tachycardia (tachyarrhythmia). An arrhythmia can be present with an apparently regular pulse, with an irregular but patterned pulse (regular irregularity), or with an entirely chaotic pulse (irregular irregularity).
35.D
Feedback: WPW is relatively common, and occurs when an accessory band of muscle fibers, the bundle of Kent, connects the atria directly to the ventricles. Impulses from the SA node pass through this bundle, bypassing the normal pathway. This results in early ventricular depolarization (pre-excitation). The PR interval is abnormally short, and the R wave, instead of rising sharply, is slurred and prolonged in what is called a delta wave.
36.C
Feedback: The term heart block refers to any delay or obstruction to the passage of an impulse through the conducting system of the heart. Since the passage of a depolarizing wave through the conducting system is not reflected in the ECG, the existence and location of a block must be inferred from indirect evidence. Heart blocks are classified according to location as sinoatrial (between the sinoatrial node and the atria), atrioventricular (at the atrioventricular node), and intraventricular (in the bundle of His or one or more of its branches).
37.C
Feedback: Atrioventricular blocks are of three degrees. In first-degree block the PR intervals are prolonged, reflecting a delay of impulse transmission, but every impulse gets through to the ventricles and causes a beat.
38.A
Feedback: Third-degree block does not necessarily result in the heart stopping. Even in the presence of sinus arrest (complete cessation of impulse formation at the SA node), some ectopic focus in the atria or the ventricles often spontaneously takes over the pacemaker function. With a junctional (often called “nodal”) rhythm the pulse is 50-60 and QRS complexes are normal. With an idioventricular rhythm (one arising from an ectopic pacemaker at the ventricular level) the pulse is 40-50 and QRS complexes are wide and bizarre.
39.List and define the 3 levels or grades of severity of reduction of coronary blood flow as found on an ECG.
A. Ischemia—the heart muscle is not receiving oxygen but no irreversible damage has occurred, and the patient may have angina pectoris
B. Injury—cell damage has occurred but healing is still possible; serum cardiac
enzymes such as troponins and CK-MB are elevated.
C. Infarction—irreversible myocardial damage has occurred; if the patient lives, the infarcted area will be replaced by scar tissue that cannot contract. Infarcted tissue is dead tissue.
Matching Exercise
Instructions: Match the definitions (1 through 21, below) with the names of the diseases and conditions that follow (letters A through T).
C 1.Repeated travel of impulses in a circular path, as seen in Wolff-Parkinson-White syndrome
Q 2.Cardiac impulse is conducted normally to the left ventricle but not to the right ventricle, with delayed polarization of the RV until the impulse spreads to it from the LV
A3.Cardiac standstill; absence of ventricular activity.
D 4.A group of ECG changes caused by administration of cardiac glycosides, including bradycardia, AV block, and distortion of ST segment
P 5.A runaway artificial pacemaker firing at an inappropriately rapid rate
R6.Asymptomatic myocardial infarction diagnosed only on the basis of ECG findings
O7.An R wave from a PVC that appears immediately on top of the T wave of the previous contraction.
T8.Syncopal episode caused by transitory ventricular asystole
S9.Jaggedness or fuzziness of the ECG baseline due to tremors or tension of the subject’s muscles
G10. A deflection occurring between the onset of the Q wave and the peak of the R wave in standard electrocardiography
B11. Regular 1:1 alternation of normal cardiac contractions and ectopic beats, usually of ventricular origin
F12. A premature heartbeat, also called an extra systole
E13. A heartbeat that is able to “escape” the usual pacemaking dominance of the SA node due to some disturbance in the conducting system
I14. The point in an ECG tracing where the QRS complex ends and the ST segment begins
J15. A large P wave due to right atrial enlargement, occurring in right heart disease due to pulmonary disease such as emphysema
M16. An ECG pattern suggesting MI but actually due to a conduction defect or similar abnormality
U 17. Ventricular tachycardia in which variations in the height of R waves cause an undulating or scalloped contour on the tracing; literally, “twisting of points”
N18. Pulse in which every other beat is stronger; alternating strong and weak pulses during a sinus rhythm.
H19. An ECG wave that goes below the baseline (records a negative voltage)
L20. A narrow spike in the ECG tracing indicating an electrical impulse from an artificial pacemaker
K21. Caused by left atrial hypertrophy due to mitral stenosis, this is a large, notched P wave
Cardiology, The SUM Program Advanced Medical Transcription Unit, 2nd ed.
Health Professions Institute