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In 1929 von Neergaard demonstrated that surface forces are responsible for a large part of the elastic recoil pressure of the lungs.1 This was evidenced by recordings of elastic recoil during deflation of air- and liquid-filled lungs. On the basis of similar experiments, extended to include inflation (fig 1), Radford laid down concepts which still form the basis for the interpretation of elastic pressure–volume (Pel–V) curves in today’s intensive care units.2 In this review these concepts will be analysed. The relevance of Pel–V curves as guidelines in managing ventilation to avoid lung trauma will be discussed. Furthermore, techniques for recording and analysis of Pel–V curves will be briefly commented upon.
Pulmonary pressure–volume curves obtained during inflation and deflation with air and saline.2 The higher elastic recoil pressure during air deflation shows that surface tension contributes to lung recoil. It can be seen that during inflation with air a lower inflection point is followed by a steep, nearly linear, segment.
Features of the elastic pressure–volume curve
Pel–V curves are often recorded during an insufflation of gas which is preceded by an expiration to the elastic equilibrium volume. An example of an inspiratory Pel–V curve recorded from a patient with acute lung injury (ALI) is shown in fig 2. The features of the curve are well known.3 ,4 The curve can be considered to consist of three segments: an initial flat segment reflects a very low compliance, indicating collapse of peripheral airways and/or lung units preventing lung inflation; there then follows a segment with a steeper slope (that is, greater compliance); the transition between these two segments, which may be more or less abrupt, can be denoted the lower inflection point (LIP). Compliance remains stable over the second “linear” segment, as shown …