Measurements based on airway pressure
In this method, it is assumed that similar amounts of work are required to inflate the respiratory system during controlled and assisted ventilation ( Marini eta[ 1985).
The area enclosed within an inflation Prs-volume curve is measured during controlled ventilation ( ManDi.§L§L 1985). Another curve is obtained with the patient assisting the ventilator under conditions of inspiratory flow, tidal volume, respiratory frequency, and end-expiratory pressure identical with those during controlled ventilation. The active work being performed by the patient's inspiratory muscles can be calculated by subtracting the area of the 'active' curve from that of the 'passive' curve (MMnl..§t§/; 1985).
Unless the ideal conditions defined above are met, recordings of Prs are insufficient for measurement of respiratory work in a patient who is assisting the ventilator. Instead, an estimate of pleural pressure (Ppl) is required, which is usually achieved by recording esophageal pressure ( Pes). The graphical approach to the analysis of Pes-volume loops introduced by Campbell allows work to be separated into several components (Tobin.1.and.Van...De Graaff.1994).
During unassisted breathing, the total resistive work done on the lungs in each breath is obtained by integrating the area subtended by Pes and lung volume during a complete respiratory cycle (Fig . .3). Work is partitioned into its inspiratory and expiratory resistive components by drawing a line between points of zero flow, i.e. the dynamic pulmonary compliance line. In Fig 3, the area enclosed by the ellipse to the left of this line represents inspiratory resistive work while the area to the right represents expiratory resistive work.
Fig. 3 A plot of esophageal pressure versus volume during (a) unassisted breathing and (b) passive ventilation. The CLdyn line between the points of zero flow (the dynamic pulmonary compliance line) partitions resistive work into its inspiratory and expiratory components. Note the clockwise direction in which actively generated Pes-volume curves are formed. The Ccw line connecting the points of zero flow during passive ventilation is the static chest wall compliance line. Note the counterclockwise direction in which passively generated Pes-volume curves are formed. (Reproduced with permission from Tob.i.D and VaD...„d® Graaif.1.1994.).)
During unassisted breathing, work performed by the inspiratory muscles against elastic recoil of the lungs and chest wall can be calculated by constructing a Campbell diagram. This method requires measurement of the static compliance of the chest wall ( Ccw), which can be closely estimated by recording Pes and volume during controlled ventilation while the respiratory muscles are completely relaxed ( Fig, 3). The difference between Pes and Pcw indicates the pressure developed by the respiratory muscles in expanding the chest wall (Pmus).
The chest wall compliance line is fitted to the pressure-volume curve by passing it through the end-expiratory elastic recoil pressure of the chest wall, which coincides with Pes at zero flow (Fig, 4). The chest wall compliance line is generally assumed to be linear and is extrapolated to the tidal volume of each breath. Elastic inspiratory work is then measured as the area between the dynamic pulmonary compliance line ( CLdyn) and the static chest wall compliance line (Ccw) within the tidal volume range (Fig:.4).
Fig. 4 Plot of esophageal pressure versus volume during unassisted ventilation. The static chest wall compliance line ( Ccw) is fitted at the end-expiratory elastic recoil pressure of the chest wall. Inspiratory elastic work is calculated as the shaded area of the pressure-volume curve subtended by the Ccw and CLdyn lines (see Fig 3).
(Reproduced with permission from T.obin,.,a.n.d...yaQ.,d.e Graaff. . . (.1994).)
During quiet breathing, expiration is passive and the work of breathing is usually performed entirely by the inspiratory muscles. However, when ventilatory demands increase or expiratory resistance is markedly increased, the expiratory muscles are recruited. Such expiratory muscle activity increases Pes, producing Pes-volume values to the right of the chest wall relaxation line and outside the elastic work area on a Campbell diagram, i.e. Pes is higher than the static elastic recoil pressure of the chest wall. This can be quantified as the area enclosed by the expiratory portion of the Pes-volume loop to the right of the chest wall relaxation line ( Fig, 5).
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Fig. 5 Plot of esophageal pressure versus volume in a patient exhibiting expiratory muscle activity. The chest wall compliance line is measured and fitted to the pressure-volume curve as described in Fig, 3 and Fig.:...,.!. Expiratory work is quantified as the shaded area enclosed by the portion of the pressure-volume loop lying to the right of the Ccw line. (Reproduced with permission from T.obin,.,a.n.d...yaQ.,d.e G,raa,ff.i199!).)
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