Low flow inflation pressure-time curve in patients with acute respiratory distress syndrome

Objective: In mechanically ventilated patients with ARDS, determination of the lower (LIP) and upper (UIP) inflection points of the static pressure-vo lume curve (PV) is crucial for planning ventilatory strategies. Recently, a simple new method was proposed for measuring the P-V curve by inflating th e lung with constant low flow [14]. We hypothesized that during low now inf lation LIP and UIP might be determined using the pressure-time curve (P-T) instead of P-V. Methods: Eleven paralyzed patients with ARDS were studied. During volume control ventilation the patients were allowed to reach passiv e functional residual capacity (FRC) and then ventilator frequency, inspira tory to total breath duration ratio and tidal volume (V-T) were set to 5 br eaths/min, 80% and 500 or 1500 ml, respectively. With these settings, const ant inspiratory flow (V'(I)) was administered for 9.6 s and ranged, dependi ng on V-T, between 0.05 and 0.15 l/s. P-V and P-T were obtained at two leve ls of positive end-expiratory pressure (PEEP; 0 and 10 cmH(2)O), with V'(I) being achieved either fast (< 0.1 s, minimum delay) or slowly (0.4 s, maxi mum delay). Results: With minimum flow delay for a given experimental condi tion, the shape of the P-T did not differ from that of P-V. In all cases P- T correctly identified the presence of LIP and UIP which did not differ sig nificantly between P-T and P-V. With maximum now delay, compared to P-V, th e initial part of P-T was significantly shifted to the left. P-T did not id entify the presence of UIP and LIP in one and two cases, respectively. Conc lusions: Provided that constant flow is given relatively fast, P-T accurate ly determines the shape of P-V, as well as the LIP and UIP. Plow delay caus es a leftward shift of the initial part of P-T, masking the presence of LIP and UIP in some cases.