A new method for non-invasive, manoeuvre-free determination of 'static' pressure-volume curves during dynamic/therapeutic mechanical ventilation

Background: Lung mechanics are usually measured using static or quasistatic methods, abandoning normal ventilatory treatment. We have developed a method to calculate the alveolar pressure during dynamic/therapeutic conditions, 'the dynostatic pressure' (P(dyn)), using airway pressure (P) measured in the trachea and volume (V) and flow (V̇) at the Y-piece. Methods: P(dyn) is calculated according to the formula P(dyn)=(P(insp) x V̇(exp)-P(exp) x V̇(insp))/(V̇(exp)-V̇(insp)), making the assumption that inspiratory and expiratory resistances are equal at isovolume. The method was evaluated in a lung model during dynamic conditions comparing measured alveolar pressure (P(alv)) and P(dyn) at equal and unequal inspiratory and expiratory resistances and P/V-curves obtained during static and dynamic conditions. The algorithm was then applied in patients with acute lung injury (ALI). Results: When inspiratory and expiratory resistances were equal there was an excellent agreement between the P(dyn) and the P(alv), irrespective of ventilator settings, r²=0.995 (range 0.981-0.999). P(dyn) derived compliance was equal to static values. When the ratio between inspiratory and expiratory resistance was varied between 2.3:1 and 1:2.3 the r² was above 0.95 (range 0.952-0.996). Development of intrinsic PEEP and overdistension was easily revealed in patients, as shown by the dynostatic P/V-curve. Conclusion: The dynostatic method gives a breath-by-breath reflection of the interaction between ventilatory settings and lung mechanics in patients during ordinary ventilator treatment. It is only marginally affected by the moderate differences in inspiratory versus expiratory resistances present in patients with ALI. (C) Acta Anaesthesiologica Scandinavica 44 (2000).