“Switched” Precordial Leads
Most physicians and technicians have, at one time or another, inadvertently “switched” the extremity leads in preparation for the recording of an ECG. This is so common that some modern ECG machines advise the operator that he or she has switched the leads.
Misplacement of the electrode positions is the most common error related to recording the chest leads. The next most common cause of error is probably switching the V1 and V2 electrode positions on the chest. This error is easy to make because the wires attached to the lead selector box are adjacent to each other.
During our cardiology morning report, I was shown several ECGs. Two of them were quite unusual. I could not construct the spatial direction of the mean QRS vectors; the frontal plan calculation was easily diagrammed, but the anterior-posterior direction could not be constructed. One of these tracings is shown in Figure 1⇓. There was a tall R wave in lead V1, a small R wave in lead V3, and tall R waves in leads V4 through V6. The resultantly negative QRS complex at position V3 was out of place. It appeared that the QRS complex in lead V3 was more likely recorded from the V1 position and the QRS complex in lead V1 was recorded from the V3 position. If this was true, how did it happen?
I inspected the ECG machine that was used by the new night technician. Inspection of the lead selection box yielded the explanation. The lead wire that was labeled at its end as V1 was plugged into the V3 receptacle of the lead selector box and the lead wire labeled V3 was plugged into the V1 receptacle of the lead selector box. The new night technician had undoubtedly looked only at the labeling at the end of wires. The corrected ECG is shown in Figure 2⇓.
How could this happen? The wire that connects the lead selector box to the electrode placed on the chest may occasionally break near the clamp used to attach the wire to the electrode. When this happens, the wire must be replaced. In this case, it appears that 2 wires were replaced. The individual who made the replacement plugged lead V1 into the V3 position on the box and plugged V3 into the V1 position on the box (see Figure 3⇓).
There are other abnormalities in the tracing. They are not discussed here because the purpose of this communication is to point out what must be an uncommon type of “switched” chest leads. The computer, of course, failed to recognize the error.
The editor of Images in Cardiovascular Medicine is Hugh A. McAllister, Jr, MD, Chief, Department of Pathology, St Luke’s Episcopal Hospital and Texas Heart Institute, and Clinical Professor of Pathology, University of Texas Medical School and Baylor College of Medicine.
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