Monitoring pressure within the pulmonary capillary bed in the critically ill patient is important for several reasons. First, and most importantly, true pulmonary capillary pressure represents the midpoint of the capillary bed and is therefore the hydrostatic pressure that drives the formation of edema in the lungs. The rate of filtration in the pulmonary capillary bed is given by the Starling equation as follows:

where Q=flow rate, K=filtration coefficient of plasma, Pcap=pressure within the pulmonary capillaries at the site of filtration, Pint =pulmonary interstitial pressure, σ= reflection coefficient of the capillary endothelium to molecules within plasma, and πPcap and πint = oncotic pressures within the pulmonary capillaries and interstitium, respectively.

As can be seen from this equation, the pressure within the pulmonary capillary is a major determinant of fluid flux across the capillary endothelium into the pulmonary interstitium. In fact, because of the movement of serum proteins into the interstitium and the efficiency of the protein clearance by the lymphatic pump, hydrostatic pressure is the main determinant of interstitial edema within the lung. Although the rate of protein movement across the endothelium may vary in disease states, the efficiency of the lymphatic clearance mechanism serves as a protective mechanism to limit the accumulation of water in the interstitium. However, regardless of serum oncotic pressure and the efficiency of the lymphatic pump, interstitial fluid accumulation occurs beyond a certain level of pulmonary capillary hydrostatic pressure. Pulmonary capillary pressure (Pcap) is the average hydrostatic pressure in the small pulmonary vessels where filtration occurs. Pcap is thus the filtration point in the capillary bed. Ultimately, therefore, Pcap determines interstitial fluid accumulation within the lung.

Because of the close relationship between pulmonary capillary hydrostatic pressure and interstitial edema and the obvious clinical relevance of this variable, many attempts have been made to develop a reliable means for determining Pcap. Unfortunately, it is impossible to measure pulmonary capillary hydrostatic pressure directly in the intact lung; therefore, only indirect measurements are clinically possible.