Visually estimating the pulmonary capillary hydrostatic pressure using the PAWP waveform
At the point of occlusion, there is a rapid drop, followed by a more gradual drop.
The point where the drop becomes gradual is the inflection point.
You can gawk at the trace and identify the inflection point visually, or you can use an interpolated curve to arrive at the Pcap mathematically.
Neither method is accurate, and both will provide you with a number which is ultimately meaningless.
Calculating the pulmonary capillary hydrostatic pressure using the PAWP waveform
The rate of pressure drop is estimated from serial measurements in the time period following the inflation of the ballon, and this is made into an exponential curve of best fit;
The Pcap is estimated from the pressure along this curve at the time of occlusion.
The physiology underlying pulmonary capillary hydrostatic pressure
So, all of this works only when pulmonary resistance to flow is negligible.
When would pulmonary venous resistance ever be zero?
It wouldn’t be. Pulmonary veins contribute 40% to the total pulmonary resistance.
Plus, ICU patients have a lot of stuff going on to promote pulmonary venoconstriction. ARDS and suchlike.
Multiple factors conspire to make sure the PAWP and the Pcap are not equal.
Normally, because of the venous resistance, Pcap will be higher than PAWP.
Converting PAWP into Pcap
Lets just assume that of the total pressure drop between the pulmonary artery and the left atrium the drop between capillaries and the left atrium is 40%. PAWP here is a substitute for LA pressure.
The factor of 0.4 could change: in ARDS, pulmonary venoconstriction accounts for more of the pressure drop, and this factor could be as high as 0.6 or 0.8
Is this useful?
Probably not. In ICU, capillary leakiness is rarely a purely hydrostatic thing. What of sepsis, DIC etc?...
In short, what you are measuring might be pulmonary capillary hydrostatic pressure, but it is not relevant to your decisionmaking.