La

Figure 11-38: Identification of improper connections of a single cable from the electrocardiographic machine to the corresponding electrodes placed on the patient's limbs. Note that aVR, aVL, and aVp invariably refer to whatever morphology is recorded when, while the ECG is being obtained, the corresponding knobs are turned in this order (regardless of whether the cables were attached properly or improperly). On the other hand, RA (right arm), LA (left arm), and LL (left leg) or LF (left foot) correspond to the normal morphology recorded by the cables so labeled. This method, based solely on the analysis of the unipolar extremity leads, is simpler than the method based on the study of the bipolar standard leads but is useful only when a single cable is misconnected. A. Normal. B. Since LA appears in aVR and RA appears in aVR (with LF being in its normal position), the right arm and left arm cables must have been switched. C. Since LF appears in aVR and RA appears in aVF (with LA in its normal position), the right arm and left leg cables must have been switched. D. Since LA appears in aVF and LF appears in aVL (with RA in its normal position), the left arm and left leg cables must have been switched.

Order of Appearance while recording

Healthy Coronary Artery

Figure 11-39: Identification of improper connections of the right leg (RL) (ground) cable. C can be regarded as almost equal to the control tracing because the RL (ground) and left leg (LL) cables were switched. The corresponding morphologies are not identical to the control morphologies because a very small difference in potential between both legs does exist. The latter is seen in A. Because the RL and RA cables were switched, lead II (RA-LL) records the difference in potential between both legs, which seems to be approximately 0.15 mV. The latter results in an almost straight line interrupted by a small blip. In addition, lead I represents the mirror image of normal lead III, and lead III is the normal lead III. In B, where the LA and RL cables have been switched, lead III (LA-LL) records almost a straight line. In addition, lead I is the normal lead II, and lead II is the normal lead II. [From Castellanos A, Saoudi NC, Schwartz A, et al. Electrocardiographic patterns resulting from improper connection of the right leg (ground) cable. PACE 1985; 8:364-368. Reproduced with permission from the publisher and authors.]

Figure 11-39: Identification of improper connections of the right leg (RL) (ground) cable. C can be regarded as almost equal to the control tracing because the RL (ground) and left leg (LL) cables were switched. The corresponding morphologies are not identical to the control morphologies because a very small difference in potential between both legs does exist. The latter is seen in A. Because the RL and RA cables were switched, lead II (RA-LL) records the difference in potential between both legs, which seems to be approximately 0.15 mV. The latter results in an almost straight line interrupted by a small blip. In addition, lead I represents the mirror image of normal lead III, and lead III is the normal lead III. In B, where the LA and RL cables have been switched, lead III (LA-LL) records almost a straight line. In addition, lead I is the normal lead II, and lead II is the normal lead II. [From Castellanos A, Saoudi NC, Schwartz A, et al. Electrocardiographic patterns resulting from improper connection of the right leg (ground) cable. PACE 1985; 8:364-368. Reproduced with permission from the publisher and authors.]

Variations in Precordial-Lead Placement

This is a problem more common now than when, in 1961, Simonson noted the considerable variation in chest lead placement in the same patient by different technicians and even by the same technician in several ECGs in the same patient.137 Simonson also found that in a controlled study, placement of the V2 electrode varied 10 cm vertically and 8 cm horizontally in 103 healthy subjects.137 Moreover, Kerwin et al.138 found a rather large error in placement of chest electrodes (2 to 3 cm in both the horizontal and vertical directions) in repeated trials in the same patients by the same technicians.138 Perhaps the frequency of precordial-lead misplacement is greater than that of somatic tremor. In our institution, the most frequent cause of "poor" r-wave progression in the anteroseptal leads (often misinterpreted by the computer as indicative of anteroseptal MI) is misplacements of leads V2 and V3.

False Variations in Voltage

Garson139 noticed how, in several patients, ECGs taken weeks apart showed markedly different QRS voltages. The latter were sometimes of enough magnitude to cause a pseudonormalization of a ventricular hypertrophy pattern. There had been no changes in hemodynamics, but different types of ECGs were used. A study of this problem demonstrated that electrocardiographic data had a different voltage depending on whether they were recorded and displayed on an analog electrocardiograph or on a digital electrocardiograph. Thus, if there is a statistically significant difference among ECGs, the serial comparisons must be done with the same machine. Moreover, criteria for voltage are only applicable to the type of instrument with which the data were gathered.

In addition, overshooting, overdamping, and running down of the standardization battery can cause significant changes in QRS voltage and ST segments.

How Should an ECG Be Performed?

This question is appropriate in view of the many artifacts and technical (machine and human) problems occurring when ECGs are recorded. The Task Force of the American College of Cardiology (ACC)-American Heart Association (AHA) in their Guidelines for Electrocardiography4 have stated that the ECG should be performed and interpreted in accordance with the guidelines for optimal electrocardiography described in the ACC Tenth Bethesda Conference Report,140 the guidelines for training described in the ACC Seventeenth Bethesda Conference Report on Cardiology Training,141 the recommendations for standardization of leads and specifications for instruments in electrocardiography and vectorcardiography of the AHA,142 and the recommendations for standardization and specifications for automated electrocardiography of the AHA.143

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