Equation 21.5 •Fig. 21.7 The relationship between transpulmonary pressure (PL) and the pleural (Ppl), alveolar (PA), and elastic recoil (Pel) pressures of the lung is depicted. Alveolar pressure is the sum of pleural pressure and elastic recoil pressure, while transpulmonary pressure is the difference between alveolar pressure and pleural pressure.

In general, PL represents the pressure that expands 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 lung's elastic properties, indicating how easily the lung can be expanded. It is defined as the change in lung volume resulting from a 1–cm H2O change in the pressure that expands the lung. Compliance is expressed in milliliters (or liters) per centimeter of water. A higher lung compliance implies easier distensibility, while a lower lung compliance signifies a stiff lung that is not easily distended. The mathematical expression for lung compliance (CL) is provided in Equation 21.6, where ΔV represents the change in volume and ΔP represents the change in pressure.

Graphically, lung compliance is represented by the slope of the line connecting two points on the deflation limb of the pressure-volume loop (Fig. 21.8). The compliance of a typical human lung is approximately 0.2 L/cm H2O, but this value varies with lung volume. Notably, lung distensibility decreases at higher lung volumes. To account for this, compliance is adjusted for the lung volume at which it is measured, known as specific compliance (Fig. 21.9). However, measuring compliance for clinical purposes is not common due to the requirement of placing an esophageal balloon. The esophageal balloon, connected to a pressure transducer, serves as a reliable surrogate marker for pleural pressure, which is challenging to directly measure. The change in pleural pressure (Ppl) is assessed based on the change in lung volume, as represented by the equations CL = ΔV/ΔPpl or ΔPpl = ΔCL.

Surfactant, in addition to the lung's elastic properties, is another significant determinant of lung compliance (Fig. 21.8).
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.