Dead Space Fraction Changes During PEEP Titration Following Lung Recruitment in Patients With ARDS

BACKGROUND: Elevated dead space fraction (the ratio of dead space to tidal volume [V(D)/V(T)]) is a feature of ARDS. PEEP can partially reverse atelectasis, prevent alveoli recollapse, and improve lung compliance and gas exchange in patients with ARDS. However, whether V(D)/V(T) variables have a close relationship with PEEP and collapse alveolar recruitment remains under recognized. Meanwhile, few clinicians titrate PEEP in consideration of changes in V(D)/V(T). Therefore, we performed the study to evaluate V(D)/V(T), arterial oxygenation, and compliance changes during PEEP titration following lung recruitment in ARDS patients. METHODS: Twenty-three ARDS patients ventilated in volume-controlled mode were enrolled in the study. Sustained inflation (40 cm H₂O, 30 s) was used as a recruitment maneuver, followed by decremental PEEP changes from 20 to 6 cm H₂O, in steps of 2 cm H₂O, and then to 0 cm H₂O. V(D)/V(T), pulmonary mechanics parameters, gas exchange parameters, and hemodynamic parameters were recorded after 20 min at each PEEP step. RESULTS: Compared with V(D)/V(T) at the PEEP levels of 20 cm H₂O and 0 cm H₂O, V(D)/V(T) was significantly lower at 12 cm H₂O (P = .02), and compliance of the static respiratory system (C(RS)) was significantly higher at pressure step 12/10 cm H₂O (P < .001). Compared with P(aCO₂) at the PEEP level of 20 cm H₂O, P(aCO₂) was significantly lower at 12 cm H₂O (P < .001). Arterial oxygenation values and functional residual capacity were reduced gradually during PEEP, decreasing from 20 cm H₂O to 0 cm H₂O. CONCLUSIONS: A significant change of V(D)/V(T), compliance and arterial oxygenation could be induced by PEEP titration in subjects with ARDS. Optimal PEEP in these subjects was 12 cm H₂O, because at this pressure level the highest compliance in conjunction with the lowest V(D)/V(T) indicated a maximum amount of effectively expanded alveoli. Monitoring of V(D)/V(T) was useful for detecting lung collapse and for establishing open-lung PEEP after a recruitment maneuver.