Generation of a single pulmonary pressure-volume curve does not durably affect oxygenation in patients with acute respiratory distress syndrome

INTRODUCTION: It is possible that taking a static pressure-volume (PV) measurement could durably affect oxygenation and thus interfere with early evaluation of a therapeutic intervention delivered just after that measurement. The aim of the present study was to investigate the effects over time of a single static PV measurement on gas exchange and haemodynamics; the PV measurements were taken using a super syringe and by using the constant flow method in patients with acute respiratory distress syndrome. METHOD: We conducted a prospective, randomized and controlled interventional study in an intensive care unit. The study was conducted in 17 patients with early acute respiratory distress syndrome ventilated with a tidal volume of 6.9 ± 1.0 ml/kg, a plateau pressure of 27 ± 7 cmH(2)O and a positive end-expiratory pressure [PEEP] of 10 cmH(2)O. They were all evaluated for 1 hour after each of the following two measurements was taken and during a control period (in a randomized order): generation of a PV curve using a 2 l super syringe (PV(SS); insufflated volume = 1824 ± 381 ml, plateau pressure = 46 ± 9 cmH(2)O); and generation of a PV curve using the constant flow method on the ventilator (PV(CF); insufflated volume = 1120 ± 115 ml in zero end-expiratory pressure after 20 s expiratory pause, plateau pressure = 46 ± 11 cmH(2)O). The maximal airway pressure allowed during PV measurement was 60 cmH(2)O. PEEP was set to 10 cmH(2)O immediately after PV measurement. Partial arterial oxygen tension (Pao(2)), partial carbon dioxide tension (Paco(2)) and mean arterial pressure were recorded each minute. RESULTS: PV measurement did not significantly affect Pao(2), Paco(2), mean arterial pressure and lung mechanics. Two patients exhibited a sustained increase in Pao(2 )by more than 20% after PV(CF )(>60 minutes). Two patients exhibited a decrease in Pao(2 )by more than 20% after PV(SS), which was sustained in one. These latter patients had an upper inflection point identified on the PV curve. After PV(SS), Paco(2 )increased by more than 10 mmHg in two patients and returned to baseline values after 15 minutes. One patient exhibited a decrease in mean arterial pressure by more than 10 mmHg for less than 5 minutes after PV(SS )and one patient after PV(CF). CONCLUSION: Evaluation of the effects of a strategy aimed at improving oxygenation can be reliably recorded early after a single PV measurement that is not followed by a change in PEEP level. PV measurement using the constant flow method improves oxygenation in a limited number of patients.