This is the latest in a series of posts I am dedicating to achieving excellent data quality for prehospital 12 lead ECGs, particularly when they are being transmitted to the emergency department for physician interpretation (and early activation of the cardiac cath lab).
Lately I have noticed that when an ECG shows artifact in the limb leads, most paramedics start aimlessly checking the wires between the various electrodes and the ECG machine.
In my series on axis determination, we discussed Einthoven’s triangle, and how leads I, II and III are derived from the white, black and red electrodes.
Let’s look at the following ECG.
Since we’re in monitor mode, we’re not in the standard 12 lead ECG format. In other words, lead II is on top, lead III is in the middle, and lead I is on the bottom.
Where is the artifact on this tracing? Leads III and I. How about lead II? That looks fine. So which electrode is responsible for the poor data quality?
Let’s think about it. For lead II, the negative is the white electrode on the right shoulder. The positive is the red electrode on the left leg. Since lead II looks fine, we can deduce that the white and red electrodes are not responsible for this poor data quality. Which electrode do leads III and I have in common? The negative for lead III is the black electrode on the left shoulder. It also happens to be the positive electrode for lead I.
Ding, ding, ding! We have a winner! Or should I say, we have found our culprit. Check the black electrode. If necessary, peel it off, wipe the skin, and replace it with a new one.
Here’s another example.
I have a confession to make. I induced this abnormality on an emergency call the other day by partially peeling back one of the electrodes. But which one?
Lead III looks fine. The negative for lead III is the black electrode on the left shoulder. The positive for lead III is the red electrode on the left leg. We can speculate that the black and red leads are not responsible for this poor data quality. What’s left? The white electrode on the right shoulder. Do leads II and I share this electrode? You bet.
Here’s the coup de grace.
This is from an actual emergency call.
The paramedics were called to the scene of an elderly male who wasn’t answering the phone at his apartment. The son went to check on him, and found him unresponsive and not breathing. When paramedics arrived at the scene, it appeared to be an obvious death (although there was no rigor mortis in the fingers and no dependent lividity was noted).
The cardiac monitor was attached and showed this tracing. The paramedics were surprised to see VF on the monitor. It seemed strange that lead II showed asystole, but CPR was initiated, the patient was defibrillated (many times), and the patient was ultimately transported lights and sirens to the emergency department.
Afterward, the paramedic in charge of the call faxed this ECG to me and asked my opinion as to why lead II showed flat line, when leads III and I showed VF.
My answer was simple. Did you check the black electrode?
What is the more likely scenario? That the VF was isoelectric in lead II (a theory a physician rendered at an ACLS class where this strip was shown) or something was wrong with the black electrode?
Let’s look at the history. This was an unwitnessed cardiac arrest! My money is on asystole and a bad black electrode!
Guide to Understanding ECG Artifact at ACLS Medical Training