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False abnormality of precordial electrocardiograms due to the effect of changes of potential at the remote electrode
FALSE
ABNORMALITY OF PRECORDIAL ELECTROCARDIOGRAMS DUE TO THE EFFECT OF CHANGES OF POTENTIAL AT THE REMOTE ELECTRODE EDGAR HULL,
M.D., HERBERT DE N.TUCKEII, J. 0. WEILBAECHER, JR., M.D. NEW ORLEANS,
M,D., AND
LA.
the use of precordial leads often enables the electrocardiographer A LTHOUGH better to determine the location and extent of myocardial lesions, their employment also increases the complexity of his task and without doubt has led to an increase in the incidence of incorrect interpretations. An important factor in the misinterpretation of precordial electrocardiograms is distortion of the true precordial pattern by changes of potential which occur in the region of the remote, supposedly indifferent, electrode. Whenever a single point on the body is used as the site of the “indifferent” electrode, the precordial pattern is distorted; occasionally, the distortion is of such order that abnormality may be simulated or truly abnormal patterns masked or exaggerated. Error due to this source is best avoided by leading from the precordium to a central terminal which is connected through resistances of 5000 ohms to the three extremities used for the standard leads; Wilson and associates, who devised this method of leading,l have shown that only minimal changes of potential occur at the central terminaL2 A less satisfactory practice is to estimate (from the direction and size of the deflections in the standard leads’) the sign and magnitude of the changes of potential at the extremity to which the remote electrode is attached and to allow for these changes in the interpretation of the precordial electrocardiogram. During inscription of the major deflections of the ventricular complexes of normal electrocardiograms, the potential at the right arm is negative; that at the left leg, positive. Therefore, in CR leads positive (upward) deflections are esaggerated, negative (downward) deflections are decreased or abolished; this explains the alleged superiority of CR leads in recording shifts of the S-T segment due to pericarditis. Conversely, in CF leads positive deflections are reduced, abolished, or even reversed, whereas negative deflections are exaggerated. True abnormality of precordial electrocardiograms is most often manifested by decrease in the amplitude of upward deflections or by the presence of abnormal downward deflections (Q waves and inverted T waves). This being the case, From the Department of Medicirm, Louisiana State University School of Medicine, and thr C’harity Hospital of Louisiana at New Orleans. Rwxived for publication July 23, 1947. 135
136
z\MEHICAN
HEART
JOURNAL
abnormality is most likely to be simulated in CF leads in cases in which relatively large positive potentials are developed at the left leg. The occurrence of high positive potentials at the left leg is manifested in the standard three lead electrocardiogram by large upward deflections in J,eads I J and J I J. In such cases, CF leads may appear abnormal while leads employing the central terminal (17 leads) show normal patterns. Depending upon the position of the heart in the thorax, high positive potentials may develop at the left leg during inscription of the QRS complex, or the In order to illustrate distortion of the CF leads, four electroT wave, or both. cardiograms have been selected in which both the R and the T waves are of considerable amplitude in Leads II and III.
Fig.
I.-See
text
for
explanation.
Fig.
3.-See
text
for
explanation.
138
AMERICAN
ILLUSTRATIVE
HEART
JOURNAL
ELECTROCARDIOGRAMS
The electrocardiograms of Fig. 1 are those of a white woman, 28 years of age, who on the evening of Sept. 8, 1946, after a trying day of household duties ant1 social activity, experienced a sensation in the chest which she said felt as though she had “swallowed something that wouldn’t go clown.” The symptom lasted about an hour, and did not recur. No signs of heart disease were detected, nor did fever, leucocytosis, or elevation of the sedimentation rate occur. On September 9, the electrocardiogram of Fig. 1,~ was recorded, in which the T waves of the CF leads appear abnormal. In the V leads (September 10, Fig. l,b), however, the T waves are normal in each precordial position. Unipolar extremity leads were not taken, but it is estimated that the maximum positive potential developed at the left leg during inscription of the T wave is nearly 0.25 millivolt. Fig. 2 is the electrocardiogram of a thin, 68-year-old Negro man who had chronic osteomyelitis of the right femur, the sequel of an injury sustained five years before his admission to the Charity Hospital. There were no signs of heart disease; the electrocardiogram if as taken as part of a general checkup prior to amputation of the diseased member. The standard leads are normal, but in I,ead CFs the R wave is very small, and in Leads CE‘, and CFG the T waves are inverted. In the V leads, however, the precordial patterns are normal. The reason for the differences between the CF and the V leads is seen in the unipolar lead from the left leg (Vr), in which a positive potential of 0.6 millivolt is manifested during inscription of the QRS complex, and of 0.2 millivolt at the height of the T wave. Amputation was performed without incident, ancl the postoperative course \vas uneventful. The electrocardiogram of Fig. 3 is that of a thin, white woman, 26 years of age, who had bronchiectasis of the right lower lobe for which lobectomy was perwas taken on September 3. formed on Sept. 9, 1946. The electrocardiogram The standard leads are normal, but in ho signs of heart disease were present. Lead CF, the T wave is inverted, and in the CF leads from the Ieft side of the precordium the amplitudes of the waves are very small; the T wave is inverted is suggested. The cause of the in Lead CFG. In the V leads no abnormality “abnormality” of the CF leacls is apparent in the VP lead, which manifests maximum positive potentials of 0.6 and 0.3 millivolt during inscription of the R and T waves, respectively. The postoperative course was without event. Fig. 4 is the electrocardiogram of a Negro man, 53 years of age, who had general paresis. There were no signs of heart disease; the electrocardiogram was taken as a matter of routine before the patient was given a course of fever therapy The standard leads are normal. The which he took without notable incident. T waves are inverted in Leads CF, and CE’,, and in the latter lead the R wave is of “abnormally” small amplitude; the V leads, however, are all normal. In this case the pattern of the QRS in all of the CF leads is rather markedly distorted by a large positive potential (1.2 millivolts at the height of the R wave) at the left leg.
Fig.
4.--Bee
test
for
explanation.
Precordial leads are employed for the purpose of recording, as accurately as possible, the changes of potential that occur in the region of a single electrode placed upon the surface of the body near to the heart. This purpose is not always fulfilled, even in a practical sense, by leading from the precordium to one of the extremities. Changes of potential at the extremity may be sufficiently great to distort the precordial electrocardiogram in such a way as to simulate or to mask abnormality. In order to emphasize this source of error in the interpretation of electrocardiograms, four normal records are reproduced, in which abnormality of the CF leads is simulated by distortion of normal precordial patterns.
140
AMERICAN
HEART
JOURNAL
REFERENCES
1.
2.
Wilson,
F. N., Johnston, F. D., Macleod, A. G., and Barker, P. S.: Electrocardiograms That Represent the Potential Variations of a Single Electrode, AM. HEART J. 9: 447, 19.34. Wilson, F. N., Johnston, F. D., Kosenbaum, F. F.. and Barker, P. S.: On Einthoven’s Triangle, the Theory of Unipolar Electrocardiographic Leads and the Interpretation of the Precordial Electrocardiogram, AY. HEART J. 32: 277, 1946.
ABNORMALITY OF PRECORDIAL ELECTROCARDIOGRAMS DUE TO THE EFFECT OF CHANGES OF POTENTIAL AT THE REMOTE ELECTRODE EDGAR HULL,
M.D., HERBERT DE N.TUCKEII, J. 0. WEILBAECHER, JR., M.D. NEW ORLEANS,
M,D., AND
LA.
the use of precordial leads often enables the electrocardiographer A LTHOUGH better to determine the location and extent of myocardial lesions, their employment also increases the complexity of his task and without doubt has led to an increase in the incidence of incorrect interpretations. An important factor in the misinterpretation of precordial electrocardiograms is distortion of the true precordial pattern by changes of potential which occur in the region of the remote, supposedly indifferent, electrode. Whenever a single point on the body is used as the site of the “indifferent” electrode, the precordial pattern is distorted; occasionally, the distortion is of such order that abnormality may be simulated or truly abnormal patterns masked or exaggerated. Error due to this source is best avoided by leading from the precordium to a central terminal which is connected through resistances of 5000 ohms to the three extremities used for the standard leads; Wilson and associates, who devised this method of leading,l have shown that only minimal changes of potential occur at the central terminaL2 A less satisfactory practice is to estimate (from the direction and size of the deflections in the standard leads’) the sign and magnitude of the changes of potential at the extremity to which the remote electrode is attached and to allow for these changes in the interpretation of the precordial electrocardiogram. During inscription of the major deflections of the ventricular complexes of normal electrocardiograms, the potential at the right arm is negative; that at the left leg, positive. Therefore, in CR leads positive (upward) deflections are esaggerated, negative (downward) deflections are decreased or abolished; this explains the alleged superiority of CR leads in recording shifts of the S-T segment due to pericarditis. Conversely, in CF leads positive deflections are reduced, abolished, or even reversed, whereas negative deflections are exaggerated. True abnormality of precordial electrocardiograms is most often manifested by decrease in the amplitude of upward deflections or by the presence of abnormal downward deflections (Q waves and inverted T waves). This being the case, From the Department of Medicirm, Louisiana State University School of Medicine, and thr C’harity Hospital of Louisiana at New Orleans. Rwxived for publication July 23, 1947. 135
136
z\MEHICAN
HEART
JOURNAL
abnormality is most likely to be simulated in CF leads in cases in which relatively large positive potentials are developed at the left leg. The occurrence of high positive potentials at the left leg is manifested in the standard three lead electrocardiogram by large upward deflections in J,eads I J and J I J. In such cases, CF leads may appear abnormal while leads employing the central terminal (17 leads) show normal patterns. Depending upon the position of the heart in the thorax, high positive potentials may develop at the left leg during inscription of the QRS complex, or the In order to illustrate distortion of the CF leads, four electroT wave, or both. cardiograms have been selected in which both the R and the T waves are of considerable amplitude in Leads II and III.
Fig.
I.-See
text
for
explanation.
Fig.
3.-See
text
for
explanation.
138
AMERICAN
ILLUSTRATIVE
HEART
JOURNAL
ELECTROCARDIOGRAMS
The electrocardiograms of Fig. 1 are those of a white woman, 28 years of age, who on the evening of Sept. 8, 1946, after a trying day of household duties ant1 social activity, experienced a sensation in the chest which she said felt as though she had “swallowed something that wouldn’t go clown.” The symptom lasted about an hour, and did not recur. No signs of heart disease were detected, nor did fever, leucocytosis, or elevation of the sedimentation rate occur. On September 9, the electrocardiogram of Fig. 1,~ was recorded, in which the T waves of the CF leads appear abnormal. In the V leads (September 10, Fig. l,b), however, the T waves are normal in each precordial position. Unipolar extremity leads were not taken, but it is estimated that the maximum positive potential developed at the left leg during inscription of the T wave is nearly 0.25 millivolt. Fig. 2 is the electrocardiogram of a thin, 68-year-old Negro man who had chronic osteomyelitis of the right femur, the sequel of an injury sustained five years before his admission to the Charity Hospital. There were no signs of heart disease; the electrocardiogram if as taken as part of a general checkup prior to amputation of the diseased member. The standard leads are normal, but in I,ead CFs the R wave is very small, and in Leads CE‘, and CFG the T waves are inverted. In the V leads, however, the precordial patterns are normal. The reason for the differences between the CF and the V leads is seen in the unipolar lead from the left leg (Vr), in which a positive potential of 0.6 millivolt is manifested during inscription of the QRS complex, and of 0.2 millivolt at the height of the T wave. Amputation was performed without incident, ancl the postoperative course \vas uneventful. The electrocardiogram of Fig. 3 is that of a thin, white woman, 26 years of age, who had bronchiectasis of the right lower lobe for which lobectomy was perwas taken on September 3. formed on Sept. 9, 1946. The electrocardiogram The standard leads are normal, but in ho signs of heart disease were present. Lead CF, the T wave is inverted, and in the CF leads from the Ieft side of the precordium the amplitudes of the waves are very small; the T wave is inverted is suggested. The cause of the in Lead CFG. In the V leads no abnormality “abnormality” of the CF leacls is apparent in the VP lead, which manifests maximum positive potentials of 0.6 and 0.3 millivolt during inscription of the R and T waves, respectively. The postoperative course was without event. Fig. 4 is the electrocardiogram of a Negro man, 53 years of age, who had general paresis. There were no signs of heart disease; the electrocardiogram was taken as a matter of routine before the patient was given a course of fever therapy The standard leads are normal. The which he took without notable incident. T waves are inverted in Leads CF, and CE’,, and in the latter lead the R wave is of “abnormally” small amplitude; the V leads, however, are all normal. In this case the pattern of the QRS in all of the CF leads is rather markedly distorted by a large positive potential (1.2 millivolts at the height of the R wave) at the left leg.
Fig.
4.--Bee
test
for
explanation.
Precordial leads are employed for the purpose of recording, as accurately as possible, the changes of potential that occur in the region of a single electrode placed upon the surface of the body near to the heart. This purpose is not always fulfilled, even in a practical sense, by leading from the precordium to one of the extremities. Changes of potential at the extremity may be sufficiently great to distort the precordial electrocardiogram in such a way as to simulate or to mask abnormality. In order to emphasize this source of error in the interpretation of electrocardiograms, four normal records are reproduced, in which abnormality of the CF leads is simulated by distortion of normal precordial patterns.
140
AMERICAN
HEART
JOURNAL
REFERENCES
1.
2.
Wilson,
F. N., Johnston, F. D., Macleod, A. G., and Barker, P. S.: Electrocardiograms That Represent the Potential Variations of a Single Electrode, AM. HEART J. 9: 447, 19.34. Wilson, F. N., Johnston, F. D., Kosenbaum, F. F.. and Barker, P. S.: On Einthoven’s Triangle, the Theory of Unipolar Electrocardiographic Leads and the Interpretation of the Precordial Electrocardiogram, AY. HEART J. 32: 277, 1946.