Pulmonary capillary pressure: a review

To demonstrate the importance of a) measuring effective pulmonary capillary pressure and b) evaluating the longitudinal distribution of pulmonary vascular resistance relative to pre- and postcapillary resistances. To review the development of methods used to determine pulmonary capillary pressure in experimental animal and clinical studies.

Human, animal, and modeling studies published since 1966 identified through MEDLINE and a review of bibliographies of relevant articles.

All studies identified were reviewed with an emphasis on recent studies and those studies identifying various methodologies used to determine capillary pressure. Experimental studies were selected for their historical value and applicability to the clinical setting.

Different models of the pulmonary circulation have been proposed. The electrical circuit model, which incorporated capacitance elements and two or four resistive elements, has been the basis for the determination of pulmonary capillary pressure in isolated lungs and in situ lungs in animals and patients. Methods used to determine pulmonary capillary pressure from a pulmonary arterial pressure tracing after balloon occlusion are: a) division of waveform into two components and logarithmic extrapolation of the slow component to occlusion time; b) visual determination of the pressure inflection point of the pulmonary arterial pressure tracing; and c) computer processing of the total arterial pressure transient. Both ease of calculations and difficulties can arise when each method is used.

Pulmonary capillary hydrostatic pressure is an important determinant of pulmonary edema especially in the setting of pulmonary hypertension and adult respiratory distress syndrome. Hypoxia, sepsis, cardiac valvular disease, and inflammatory mediators produce variable changes in the longitudinal distribution of pulmonary vascular resistance so that an increased capillary pressure cannot be predicted by the pulmonary arterial or occlusion pressure. For proper therapy aimed at decreasing pulmonary vascular resistance, it is important to determine whether or not the particular therapy increases capillary pressure. Pulmonary capillary pressure is the most important determinant of lung fluid balance and is the major physiologic parameter that should be measured when various forms of plasma volume expansion and pulmonary vasodilators are used in the critically ill patient.