When a straight line is drawn tangent to the rapid component, Pcap can be estimated as the point at which the pressure deviates from the rapid portion of the pressure (there are more sophisticated methods but we don’t need them)

For a rough estimate directly from the monitor screen, just freeze the pressure trace when measuring the pulmonary artery occlusion pressure

Take-home messages:

The pulmonary artery occlusion pressure or left atrial pressure underestimate the pulmonary capillary pressure

The pulmonary artery occlusion pressure is not the same pressure as the pulmonary capillary pressure"