ABSTRACT
Pulmonary blood flow is driven by a pressure difference between an inflow pressure, or mean pulmonary artery
pressure (mPAP), and an outflow pressure, or mean left atrial pressure (LAP). This is an extrapolation of the HagenPoiseuille law that governs laminar flows within rigid straight and cylindrical capillary tubes of Newtonian fluids. Thus the functional state of the pulmonary circulation can be approximated by a single number, pulmonary vascular resistance (PVR), which depends on the ratio between (mPAP – LAP) and pulmonary blood flow, reasonably assumed equal to cardiac output:
PVR = (mPAP – LAP)/CO
In clinical practice, measurements of pulmonary vascular pressures and flows are usually performed during a catheterization of the right heart with a triple-lumen fluid-filled balloon and thermistance-tipped catheter introduced by Swan, Ganz, Forrester, and colleagues in the early 1970s (1,2) (Figure 2.1). The procedure allows for the estimation of LAP from a balloon-occluded (PAOP) or wedged PAP (PAWP). This is because the fractal structure of the pulmonary arterial and venous trees allows for a stop-flow phenomenon downstream of occlusion and de facto extension of the fluid-filled lumen of the catheter to the same diameter pulmonary veins. With injection of 5 to 10 mL of cooled saline into a lumen opening into the right atrium, a temperature change over time in the pulmonary artery is registered, and CO is calculated from this thermal dilution curve.
