Which brings up the next question: Is this really left atrial pressure? That basically means that the pressure you end up measuring is the pressure of the next point in the circulation where there is some flow, which would usually be a junction of pulmonary veins. Because there is no flow distal to the wedged catheter, a static column of fluid connects it to the rest of the pulmonary venous circulation. Why is this discussion of flow and "venous junctions" important? Observe: it is connected to a part of the pulmonary venous circulation by a static column of fluid, and provided that fluid column remains unbroken, the catheter should measure the pressure at the nearest venous junction (i.e. Observe: when the catheter is wedged, flow distally top it ceases i.e. If you had to explain this without multiple equations, you could certainly do so. Now, if you inflate the balloon and stop the pulmonary arterial flow, Q becomes zero. This is the key feature. Without flow, there can be no resistance. To rearrange this equation for these pressures of interest, So, how does this relate to the PA catheter? In the pulmonary circulation, the ΔP we are interested in is the difference in pressure between the pulmonary capillaries and the left atrium. 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. To rearrange it, so we describe the difference in pressure: Thus, this equation describes the rate of flow: The walls of the tube offer resistance to the flow, expressed as R. It flows because there is a difference in pressure from one end of the tube to the other. This is the ΔP. In it, a nameless fluid flows, with a flow rate of Q. The relationship of pressure, flow and resistance
Pulmonary capillary wedge pressure professional#
In practice, various caveats decrease the validity of this measurement, and the practice of PAWP-guided haemodynamic management has fallen into disfavour.įor a more professional overview, one might instead want to read O'Quinn (1983). The idea of measuring LA pressure by means of a pulmonary capillary wedge is ancient (older than Swan and Ganz), but they specifically designed their catheter for this purpose, and so their original article is probably still worth reading.Left atrial pressure is a determinant of preload, and could theoretically be used to direct fluid therapy.The pressure in the resulting static column of blood, therefore, represents the pressure in the pulmonary venous circulation, which is close to left atrial pressure.In the absence of flow, the measured pressure is no longer affected by any contribution from pulmonary vascular resistance.This occlusion results in the absence of flow distal to the catheter balloon.The pulmonary artery wedge pressure is the pressure which is measured from the PA catheter when its balloon is inflated, occluding the pulmonary artery.
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