How electrode placement affects ECGs
The accuracy of ECGs are directly related to the condition of the electrodes, and how they come in contact with the patient
The electrocardiogram (ECG) allows medical personnel to both monitor the electrical signals produced by the heart and interpret their meaning. The quality and accuracy of the prehospital ECG are directly related to the quality and condition of the electrodes.
At the beginning of the shift, paramedics must insure that electrode packages have current expiration dates. Expired electrodes may have dried or faulty conduction gel which will adversely affect the quality of the ECG tracing.
EMS personnel should replace any package of electrodes that has an expired date code. For best results, ECG electrodes should be composed of silver/silver chloride combination (Ag/AgCl). These types of electrodes have a faster screen recovery time following defibrillation than other types of electrodes.
Tips for improving electrode conduction
In order for proper sensing and conduction of the ECG signal, the electrode must be in full contact with the patient’s skin, and EMS personnel must remove any hair that interferes with electrode adhesion. If the skin is oily, cleanse the area with an alcohol pad and dry the site with a brisk towel rub. Avoid placing electrodes over tendons or major muscle masses.
Attach the lead wires to the electrode before placing it on the patient’s chest. As you peel the electrode from its protective cover, insure the conducting gel is intact. Apply the electrode flat on the patient’s skin, smoothing the electrode from the center towards the edges, and avoid pressing on the center of the electrode as this can disrupt the integrity of the conduction gel reservoir.
When placing the electrodes on female patients (or very obese male patients,) place the lateral chest electrodes under the breast tissue, not on top.
Researchers recently demonstrated that alternative placement of ECG electrodes did not significantly influence ECG quality or rhythm interpretation. The case series involved patients with significant burn percentages that made electrode placement in the conventional locations impossible.
Owing to the fact that Einthoven's law makes no assumptions about geometric placement of the three electrodes, the care team placed the left and right arm electrodes on the patient’s left and right cheek, respectively. Physicians noted the morphology of the resulting ECG tracing was acceptable for rhythm interpretation, but caution against substituting interval measurements obtained from modified electrode placement for those obtained with conventional placement.
Generally speaking, precordial electrode placement that varies less than one centimeter from the recommended location produces negligible alterations in morphology or interval measurements.
However, placement variation of more than two centimeters results in significant morphology changes. Whether these changes influence a provider’s ability to interpret the ECG is unclear.
Early defibrillation is one of the most important interventions for improving survival from cardiac arrest. Self-adhesive defibrillation electrodes provide an effective way to deliver this electrical therapy. Generally, larger defibrillation electrodes (> 50 cm2) are more effective than smaller pads.
The American Heart Association acknowledges the reasonableness of placing the defibrillation electrodes in any one of four pad positions: anterolateral, anteroposterior, anterior-left infrascapular, and anterior-right- infrascapular. One position has not proven superior to any others.
As with ECG electrodes, rescuers should not place defibrillation electrodes directly on the breast tissue of females. If using the anterolateral position, place the lateral pad on the patient’s thorax instead of the breast tissue.
In males, excessive chest hair can prevent optimal contact between the patient’s skin and the conduction gel. This may prevent effective defibrillation. If such a condition is present, rescuers should quickly shave the area before attempting to apply the electrode.
The American Heart Association also makes some very specific recommendations regarding the placement and use of defibrillation electrodes.
It is reasonable to avoid placing the electrode directly over an implanted cardioverter defibrillator. Instead, place the electrode about 3 inches away from the border of the implanted device.
If the location of a transdermal medication patch interferes with electrode placement, rescuers should quickly remove the patch and wipe the area before attaching the electrode.
For patients lying in water, it is reasonable to remove the patient from the water before applying the electrodes.On the other hand, there is no need to move the victim who is lying on snow or ice before applying the electrode.
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