In the prehospital setting, when you’re obligated to be both treating (mostly) fully-clothed patients and moving them through various public spaces, the logistics of the ECG are slightly more complicated than in the hospital setting.
Although the anatomical landmarks of the precordial leads are known and usually utilized by prehospital providers, there is significantly greater variation in the placement of the limb leads; whether driven by circumstance or preference, the limb leads (RA, RL, LA, LL) may be placed in a number of places ranging from the anterior chest wall and abdomen to distal wrists and ankles. BLS providers seeking to help their paramedic partner frequently ask their preference; however, there is a clinically correct answer.
The Question: Where should the limb leads be placed for a standard 12-lead?
The Short Answer: Limb leads should be placed on the limbs, distal to the shoulders and hips but not necessarily on the wrists and ankles.
The Long Answer: A 2007 article in Circulation outlines the AHA’s recommendations for standardization of the ECGs. The authors reference a 1975 statement released by the AHA that recommends limb lead placement “on the arms and legs distal to the shoulders and hips, and thus not necessarily on the wrists and ankles” as the leads had been placed traditionally. **Note– despite my best efforts, including a thorough search of the Circulation archives, I cannot find this original reference; the closest thing I can find is a 1967 recommendation on the standardization of ECGs, in there is no discussion of lead placement.**
The authors continue to explain that one study found variations in limb lead placement (on a given limb) can alter the ECG, specifically the amplitude and duration. They go on to explain that, because there isn’t great evidence surrounding the question, we don’t know if the differences are significant enough to alter diagnostic criteria.
Then there is the more important question of limb lead placement on the torso, which is also called the Mason-Likar lead position. (Lead positioning in this system has the arm electrodes placed in the infraclavicular fossae medial to the deltoid insertions with the left leg electrode placed midway between the costal margin and iliac crest in the left anterior axillary).
While the AHA acknowledges that rhythm diagnosis is not adversely affected by lead placement on the torso, tracings that use this position differ significantly from the standard 12-lead ECG. Specifically, “electrodes placed on the trunk do not provide standard limb leads, and distortion of the central terminal alters the augmented limb leads and the precordial leads.” Differences in QRS morphology and repolarization may then lead to false-negative and/or false-positive infarction criteria.
So despite the fact that placement on the torso may reduce artifact, “ECGs recorded with torso placement of the extremity electrodes cannot be considered equivalent to standard ECGs for all purposes and should not be used interchangeably with standard ECGs for serial comparison.”
The Takeaway: If, as a prehospital provider, you place your electrodes on the patient’s limbs distal to the shoulders and hips, your prehospital tracings can be considered a part of the serial-ECG monitoring of patients with ACS. The same is not true with torso limb lead placement.
BONUS QUESTION: Should precordial electrodes be placed above or underneath the breast of a large-breasted woman?
AHA Answer: Despite one study that said that “reproducibility of ECG measurements is slightly increased when electrodes are positioned on top of the breast,” the under-breast placement leads to more intuitive lead placement and would reduce amplitude attenuation caused by torso impedance. So, until further studies say otherwise, under-boob placement it is.