Femoral pulse during CPR - Arterial or Venous?

There are many uncertainties about how each of us will die.

Based on my experience in the ED, however, many of us will spend our last minutes on Earth with a stranger's hand shoved into our groin.

Your own hand? Fine.

I'm talking about the checking the femoral pulse during CPR, of course. It's a well-enshrined part of our resuscitation practices, but what it is it actually telling us?

The Theory
The idea, of course, is that effective CPR will produce an arterial pulse, albeit weak, that will generate a cardiac output about 20% of normal. Palpating a femoral pulse during compressions supposedly verifies that the CPR is being effectively delivered.

Evidence that the femoral pulse is venousHowever, there is some uncertainty about what a palpable femoral pulse actually represents. Hilty used ultrasound in a study of central line placement during cardiac arrest, and noted that 9/20 patients appeared to have femoral venous pulsations, rather than arterial.

Note that this was in the dark ages of ED ultrasound (1997), back before Christian Doppler was born. 

Based on this finding, as well as on a case report of two kids getting open-chest cardiac compressions, many people now believe that the femoral pulse during CPR is just the venous back-flow. A video posted by an emergency physician in Qatar supports this view, where he demonstrates interrogation of the femoral vessels with power doppler during CPR:

The femoral vein shows much brighter signal than the artery, suggesting that the venous flow far exceed the arterial. Of course, since this is power doppler, we don't know the direction of the flow.

Evidence that the femoral pulse is arterial?
Cardiac arrest, asystole when EMS arrived in the ED, the LUCAS dutifully chugging along.  

Probe placed in the right groin, angled cephalad slightly. First, with color doppler:

The femoral artery seems to show a fairly well-defined arterial pulse, while the femoral vein has a turbulent, almost "yin-yang-like" character, that does not suggest effective flow. So based on this clip, it seems like the femoral pulse indeed reflects arterial impulses, not venous.

(BTW, here's a clip of the same view, while the LUCAS was taking a break. Just so you know that there was no spontaneous cardiac activity mucking up the doppler.)

So, what does the femoral pulse tell us about CPR quality?
Not much, probably. The scanty and conflicting "evidence" (i.e. collection of anecdotes) reviewed here doesn't make it clear if pulsations in the groin are coming from the artery, the vein, or perhaps even both. This looks like a promising avenue for an emergency ultrasound study!

In the meantime, assessing the quality of CPR is likely best done with end-tidal CO2, although a recent Ultrasound Podcast episode suggested using focused echo  to optimize compressions.