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PA catheter- physiology of pulmonary capillary hydrostatic pressure
What is pulmonary capillary hydrostatic pressure, and what is its significance?
Pulmonary
Artery
Pulmonary
capillaries
Left
Atrium
PLA = left atrial pressure
This is the pressure which forces fluid out of
the pulmonary capillaries into the interstitium,
causing pulmonary oedema.
It can be measured by analysis of a pressure
transient after an acute PA occlusion
PC = capillary pressure
PA = alveolar air pressure
PA
R = resistance in pulmonary arteries
Q = flow in pulmonary arteries
PC
Q=0
When the balloon is deflated, PAWP is equivalent to Pcap
ONLY if the resistance to flow is negligible
PC - PAWP(instead of PLA) = Q x R
Pa
PLA
60%
40%
Thus, when R = 0
PC – PAWP = 0
And thus
Assumed
Pressure drop
PC = PAWP
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 P cap are not equal.
Normally, because of the venous resistance, P cap will be higher than PAWP.
Converting PAWP into P cap
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.
PC – PAWP = 0.4 x (Pa – PAWP)
Thus,
PC = PAWP + 0.4 x (Pa – PAWP)
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.
From Bersten and Soni’s” Oh's Intensive Care Manual”, 6th Edition, as well as the PA catheter section from The ICU Book by Paul L Marino (3rd edition, 2007)