Electrocardiographic Electrode Misplacement, Misconnection, and Artifact

Richard A. Harrigan, MD; Theodore C. Chan, MD; William J. Brady, MD

Disclosures

J Emerg Med. 2012;43(6):1038-1044. 

In This Article

Discussion

Electrode Misplacement and Misconnection

Limb Electrode Misplacement. The four limb electrodes (right arm, left arm, and left leg, with the right leg electrode being a ground wire) may at times be inadvertently misplaced, leading to unexpected changes on the 12-lead ECG; most often this involves right/left confusion on the part of the technician, although arm/leg reversal may also occur. Misplacement of these electrodes will affect the waveforms in some, but usually not all, of the standard (I, II, and III) and augmented (aVR, aVL, and aVF) leads. Such electrode reversal results in several characteristic findings.

Right/Left Reversal. Right/left reversal involves switching either the arm or the leg electrodes. Arm reversal (right arm/left arm) is probably the most common electrocardiographic electrode misconnection; although it is reasonable to assume that leg reversal (right leg/left leg) would be equally common.[1–3] Due to the function of the right leg electrode as a ground wire, and the distance of the leg electrodes from the heart, reversal of the leg electrodes does not cause distinguishable changes on the ECG; the potentials at each leg are essentially the same (4). However, reversal of the arm electrodes (right arm/left arm) results in significant changes in the ECG (Figure 1). Key findings for right arm/left arm reversal include the following: 1) inverted P-QRS-T waves in lead I; 2) "normal" appearance of lead aVR (i.e., upright P-QRS-T in lead aVR as opposed to the expected inversion of these waveforms in a normal tracing); 3) QRS vector in lead I does not match that of lead V6. An inverted P-QRS-T in lead I will result in an unexpected QRS axis deviation (either rightward or extreme axis deviation, depending upon the major vector of the QRS complex in lead aVF), another indicator of electrode reversal. Inversion of the P-QRS-T complex in lead I occurs with dextrocardia as well, but dextrocardia will also demonstrate a lack of normal precordial R-wave progression from leads V1–V6, whereas arm electrode reversal will not affect the precordial leads. Because the principal vector of lead I (zero degrees in the frontal plane, or toward the patient's left) approximates that of the principal vector of lead V6 (also leftward), these two leads logically should have similarly oriented QRS complexes.

Figure 1.

Arm electrode reversal. Note the inverted P wave, QRS complex, and T wave in lead I—in the absence of dextrocardia, this is pathognomonic of arm electrode reversal. As a result, the major vector of the QRS complex in lead I (downward) does not match that of lead V6 (upward), despite the fact that these two leads are similarly oriented on the patient. Lastly, note the unexpected "normal" appearance of the P-QRS-T waveforms in lead aVR—another telltale sign of arm electrode reversal.

Arm/Leg Reversal. Arm/leg reversal involves the inadvertent switching of the arm and leg electrodes, yet usually preserves sidedness, resulting in right arm/right leg reversal or left arm/left leg reversal. Right arm/right leg reversal (Figure 2) results in a series of somewhat complex morphologic changes in the standard and augmented leads, but also reliably causes an abnormality that fortunately serves as a telltale sign: a near-"flat line" appearance in one of the three standard limb leads (in the case of right arm/right leg reversal, this occurs in lead II).[3,4] This change results from the placement of the right leg (ground) lead—which the electrocardiographic processor recognizes as having no potential difference from the left leg—in the right arm position. If two electrodes have no potential difference between them, then the waveform generated for the lead they represent will be nearly isoelectric, or flat. Thus, the electrocardiograph "sees" the right leg electrode in the right arm position, and inscribes a nearly flat waveform in lead II—the lead that represents the potential difference between the right arm position (with the right leg electrode mistakenly attached) and the left leg position. Taking this a step further, if the right leg electrode is switched with the left arm electrode (an unlikely event, because this would require simultaneously confusing both the limbs and sidedness), a near-flat line would appear in lead III—because lead III records the potential difference between electrodes placed in the left arm position and the left leg position.

Figure 2.

Right arm/right leg electrode reversal. The most striking finding on this electrocardiogram (ECG) (A) beyond the rhythm (atrial fibrillation) is the nearly isoelectric recording in lead II—also evident in the lead II rhythm strip at the bottom of the tracing. This finding disappeared once the ECG was repeated with attention to proper electrode placement (B).

Left Arm/Left Leg Reversal. Left arm/left leg reversal is the most difficult limb electrode misplacement to detect in the absence of a comparison, or baseline, ECG (Figure 3). The changes that result do not seem overtly abnormal, such as with arm electrode reversal or misplacement of the right leg electrode as described above. Even comparison with an old ECG does not necessarily invoke the conclusion that the electrodes have been misplaced—the difference at first glance appears to possibly represent ischemic change (Figure 3). Reversing the arm and leg electrodes on the left side changes the findings in all limb electrodes except lead aVR—thus, lead aVR maintains its "normally abnormal" appearance (i.e., upside-down P-QRS-T complex in most cases), thereby removing one route to detection of this electrode reversal. What results is the following: lead II becomes lead I; lead aVF becomes lead aVL; and lead III is inverted. Alternately stated, the lateral limb leads (I and aVL) become inferior, and two of the inferior leads (II and aVF) become lateral—thus a quick perusal of the tracing does not betray the reversal to the untrained eye because regionalism is preserved. The inversion of lead III, although detectable when scrutinizing a comparison ECG, is not blatantly abnormal, because the principal QRS complex vector ordinarily may be positive or negative in this lead.

Figure 3.

Left arm/left leg electrode reversal. At first glance, this electrocardiogram (ECG) tracing does not appear abnormal (A), but it does appear different than that seen (B), which was recorded moments later in the same patient. (A) was recorded with the inadvertent reversal of the left arm and leg electrodes; (B) demonstrates proper electrode positioning. Closer inspection reveals that lead I in one tracing is lead II in the other; similarly, lead aVF in one tracing is lead aVL in the other. Lead III is the inverted image of the comparison tracing, but either lead III appears grossly acceptable. The key to discovering that the tracing in (A) is a left arm/left leg reversal lies in noticing that the P wave in lead I is bigger than that in lead II—this is unusual in most people and should trigger consideration that left-sided electrode reversal has occurred. Note also that the QRS axis has shifted: it is closer to +30 in (A) yet closer to +60 in (B). Such an axis shift should prompt consideration of electrode reversal as well as body position changes and other pathologic entities. One final observation should be made with respect to the precordial leads; in (A) there is the appearance of poor R-wave progression across the precordium; indeed, the computer read this as anterior infarction, age indeterminate. (B) reflects the proper placement of the precordial electrodes, the growth of an R wave in lead V3, and smooth R-wave progression across the precordium—no longer giving the appearance of an indeterminate-age anterior infarction. This tracing was typical of the patient's prior ECGs. This is an example of precordial electrode misplacement (see Figure 4).

So how does one detect left arm/left leg reversal? One key is analysis of the P wave in leads I, II, and III. In a case series of 70 sinus rhythm tracings, finding a P wave that was smaller in lead II than in lead I, or an upward terminal phase in a biphasic, lead III P wave (i.e., P-wave deflection that is down/up, rather than up/down as would be expected in this relatively right-sided lead) correctly predicted reversal of arm and leg electrodes on the left side in 90% of cases.[5] This is the key clue to detecting left arm/left leg reversal without a comparison tracing. One should also look for unexpected shifts in the QRS axis (and P-wave axis); although this is not specific for limb electrode reversal, it is one cause to be considered when an axis change is evident during comparison of old and new ECGs.

Precordial Electrode Misplacement. If the precordial electrodes (those that define leads V1–V6) are slightly misplaced, it is extremely difficult to detect by analyzing a single tracing. Not surprisingly, this happens fairly commonly, as precordial electrode positioning is relative to anatomic landmarks (Table 1). Obesity and the feminine anatomy are two common variables that make consistent precordial electrode positioning difficult. Thus, new T-wave changes in the precordial leads seen in comparison to old tracings must be viewed in the context of the QRS complexes of those same leads. If the R/S ratio in a given precordial lead's QRS complex is vastly different from baseline, not reflective of that which occurs with infarction, and occurs in isolation, then it is inappropriate to compare that lead's complexes to those of the baseline tracing and draw inferences about ischemic change.

Improper positioning of the precordial electrodes may result in a pseudoinfarction pattern (Figure 4). If the clinical history is not consistent with myocardial infarction during the time between new and old/baseline tracings, electrode misplacement must be considered in addition to silent myocardial infarction. Repeating the tracing with meticulous attention to precordial electrode positioning (Table 1) will usually eradicate the problem if it is due to improper positioning.

Figure 4.

Pseudoinfarction due to misplacement of the precordial electrodes. The computer read this electrocardiogram (ECG) in (A) as "possible anterior infarct, age indeterminate," due to the poor R-wave progression across the precordium; the R waves are small and static as you move though leads V1–V3, with the sudden emergence of an R>S wave amplitude in lead V4. The clinical history did not match this finding; this patient was sent from the preoperative testing suite to the Emergency Department for "acute ECG changes," yet there was an absence of clinical complaints. After the ECG was repeated with careful attention to precordial electrode placement (B), this finding disappeared.

Precordial Electrode Misconnection. The placement of two precordial electrodes also may be reversed, but this is usually easy to detect. Normally, there should be a smooth transition of R-wave growth and S-wave regression moving from the rightward chest leads (V1/V2/V3) to the leftward ones (V4/V5/V6). An interruption in this smooth transition will result if two precordial electrodes are reversed. Cardiac ischemia virtually never occurs in one lead, so such an aberration is suggestive of precordial electrode reversal.

Electrocardiographic Artifact

Artifact on the ECG may be obvious, but may simulate clinical pathology (Figure 5). It can be classified into non-physiologic and physiologic artifact; the former is due to equipment problems or interference from neighboring electrical devices, whereas the latter results from muscle activity or skin interference.[6] At times, physiologic interference is easy to discern, but it can mimic true dysrhythmia and lead to unnecessary testing and therapy.[7] One study surveying physicians presented with rhythm strips simulating ventricular tachycardia (yet containing evidence of artifact—e.g., visible notching through the "dysrhythmia" that matched the preceding sinus rhythm interval) found that 94% of internists, 58% of cardiologists, and 38% of electrophysiologists failed to recognize the pseudodysrhythmia as artifact, in most cases recommending an invasive procedure for further evaluation or treatment (8). Artifact is likely if the baseline is unstable before or after the dysrhythmia in question, or if a normal QRS complex appears immediately after cessation of the apparent dysrhythmia at an interval that is too short to be physiologic. Other evidence suggestive of electrocardiographic artifact includes lack of hemodynamic deterioration, or at least symptomatology, during the event, and association or reproduction of the rhythm in question with movement.[8,9] The "notches sign" (Figure 6) should be sought—evidence of black notches marching through the dysrhythmia at an interval corresponding to the sinus RR interval preceding the "dysrhythmia".[8,10]

Figure 5.

Artifact mimicking atrial flutter. The lead II rhythm strip in this patient (A) features regular atrial-like deflections at an interval of approximately 300 beats/min—simulating atrial flutter. This artifact is less pronounced in lead III as well as across the precordium (B)—where an underlying sinus rhythm is more apparent. This patient had clinical findings (tremor) due to Parkinson disease.

Figure 6.

Notching of sinus rhythm through pseudodysrhythmia. This is a rhythm strip from a patient with pneumonia and rigors who exhibited an apparent wide complex dysrhythmia. Mapping the RR interval of the sinus rhythm back through the dysrhythmia reveals that this is artifact. The lower strip is marked to illustrate the "notching" of native normal ventricular complexes (*) through the pseudodysrhythmia.

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