Student Here. I'm currently learning about the mechanisms of right heart catheterization. I know that PCWP is essentially an indirect measurement of LAP, but I cannot figure out why this is. Can someone shed some light on this? Thanks
LAP is the pressure that is transmitted back through the pulmonary vasculature that must be overcome by the R ventricle in order for blood to flow and arrive at the left atrium.
The PA catheter floats out an artery (via the balloon) and occludes pulsatile flow from behind it that is generated by the R ventricle. Thus it's wedged in there. So the only pressure left that it is seeing is the pressure infront of it, which is transmitted back through the pulmonary system by the left atrium.
There is a relationship between the PAWP(or PCWP, same thing) and the pulmonary artery diastolic pressure: A PAWP higher than the PADP indicates that the PAWP measurement may be erroneous. On the other hand, if the PADP exceeds the PAWP by 6.0 mm Hg or more, the patient has probably developed pulmonary hypertension.
That's all I got.
Just think of lighting a bright flashlight in a dark hallway, the flashlight being the inflated balloon of the swan wedged in the pulmonary artery. You can see everything in front of the light, and you can't see anything behind the flashlight. Now imagine seeing a yellow balloon at the end of the hallway, and a similar balloon near the flash light. You know they both are yellow balloons.
The yellow balloon closest to the flashlight is your PAP, and the yellow balloon distal to the flashlight is your wedge pressure. In a regular healthy person with no valve disease or pulmonary disease both pressures should be within 5mmhg of each other to truly represent a close to accurate reading. That's why when a swan is first floated into the pulmonary artery, it's very important to document your wedge pressure and your PAP to see any differences when trending your pressures.
I and many others have abandoned the practice of "wedging" the catheter because of the extremely rare yet lethal complication of pulmonary artery rupture. That number is mostly useless anyway and not worth the risks.
I had thought wedge pressure readings had pretty well gone the way of the Dodo bird. The last couple places I've worked prohibited wedge pressures outright, the places before that highly discouraged them. They don't really offer anything to decision making in a way that is likely to actually affect outcomes, and they are relatively dangerous.
The incidence of pulmonary artery rupture is reported as high as 1 in every 500 balloon inflations, and half of those patients will die before the rupture can be repaired. That's a lot of risk for something that provides relatively little treatment-guiding hemodynamic data.