The principle of measuring left atrial pressures through the pulmonary capillaries rests on the premise that capillary resistance is very low, and with the baloon inflated there is no flow, leaving only the capillary pressure to act on the transducer.

The relationship of pressure, flow and resistance

Behold the Tube.

In it, a nameless fluid flows, with a flow rate of Q.

It flows because there is a difference in pressure from one end of the tube to the other.

This is the Delta P.

The walls of the tube offer a resistance to the flow, expressed as R.

This equation describes the rate of flow:

flow pressure and resstance equation

To rearrange it, so we describe the difference in pressure:

flow pressure and resiatnce equation rearranged

That is to say, the change in pressure from one end of the tube to the other is proportional to the product of the flow and the resistance.

How does this relate to the PA catheter?

In the pulmonary circulation, the Delta P we are interested in is the difference in pressure between the pulmonary capillaries and the left atrium.

PAWP diagram pulmonary wedge pressure difference equation

Now, if you inflate the balloon and stop the pulmonary arterial flow,

Q is now zero.

This is the key feature.

why PAWP equals PLA


This a full-text version of the seminal paper from 1970:

Swan HJ, Ganz W, Forrester J, Marcus H, Diamond G, Chonette D (August 1970). "Catheterization of the heart in man with use of a flow-directed balloon-tipped catheter"N. Engl. J. Med. 283 (9): 447–51.

The PA catheter section from The ICU Book by Paul L Marino (3rd edition, 2007) is a valuable read.