Equation 21.5 •Fig. 21.7 Relationshipbetweentranspulmonarypressure(PL)andthepleural(Ppl),alveolar(PA),andelasticrecoil(Pel)pressuresofthelung.Alveolarpressureisthesumofpleuralpressureandelasticrecoilpressure.Transpulmonarypressureisthedifferencebetweenalveolarpressureandpleuralpressure.
In general, PL is the pressure distending the lung, whereas Pel is the pressure that tends to collapse the lung. Lung elastic recoil increases as the lung inflates.
Lung compliance (CL) is a measure of the elastic properties of the lung. It reflects how easily the lung is distended. Lung compliance is defined as the change in lung volume resulting from a 1–cm H2O change in the distending pressure of the lung. The units of compliance are in milliliters (or liters) per centimeter of water. When lung compliance is high, the lung is readily distended. When lung compliance is low (“stiff” lung), the lung is not easily distended. The compliance of the lung (CL) is expressed as
Equation 21.6 where ΔV is the change in volume and ΔP is the change in pressure. Graphically, lung compliance is the slope of the line between any two points on the deflation limb of the pressurevolume loop (
Fig. 21.8 ). The compliance of a normal human lung is approximately 0.2 L/cm H2O, but it varies with lung volume. Note that the lung is less distensible at high lung volumes. For this reason, compliance is corrected for the lung volume at which it is measured (specific compliance;
Fig. 21.9 ). Compliance is not often measured for clinical purposes because it requires placement of an esophageal balloon. The esophageal balloon, which is connected to a pressure transducer, is an excellent surrogate marker for pleural pressure, which is very difficult to measure directly. The change in pleural pressure (Ppl) is measured as a function of the change in lung volume; that is, CL =ΔV/ΔPplor .ΔPpl =ΔCL.
In addition to the elastic properties of the lungs, another major determinant of lung compliance is surfactant and its •Fig. 21.8 Deflation Pressure-Volume Curve. Thepatientinhalestototallungcapacity,andtranspulmonarypressureismeasuredwiththeuseofanesophagealballoon(whichmeasurespleuralpressure).Thepatientthenexhalesslowly,andpressureismeasuredatpointsofnoairflow,whentherespiratorymusclesarerelaxed.Thepressure-volumecurveofthelungisnotthesameininspiration(notshown)andexhalation.Thisdifferenceiscalledhysteresis, anditiscausedbytheactionofsurfactant.Inaccordancewithconvention,thedeflationpressure-volumecurveisusedformeasurements.Complianceatanypointalongthiscurveisthechangeinvolumeperchangeinpressure.Thecurvedemonstratesthatlungcompliancevarieswithlungvolume.Alinecanbedrawnbetweentwodifferentvolumesonthecurve,andtheslopeofthislinerepresentsthechangeinvolume(ΔV)forachangeinpressure(ΔP).Comparethecomplianceatline1versusline 2.Theslopeofline2islesssteepthantheslopeofline1,andsothecomplianceislessatthishigherlungvolumethanitisatthelowerlungvolume.Inaccordancewithconvention,lungcomplianceisthechangeinpressurefromfunctionalresidualcapacity(FRC)toFRC+1L.MV,minimalvolume;RV,residualvolume;TLC,totallungcapacity.