Physiological effects of reduced tidal volume at constant minute ventilation and inspiratory flow rate in acute respiratory distress syndrome

Objective To assess the effect of changes in tidal volume (V_T) with a constant inspiratory flow and minute ventilation on gas exchange and oxygen transport in acute respiratory distress syndrome (ARDS).Design A crossover study of threeV_T in two study groups, using patients as their own controls.Setting: A medical-surgical intensive care unit in a tertiary care center.Patients Eight patients with ARDS and seven postoperative cardiac surgery patients with uncomplicated recoveries were studied during volume-controlled mechanical ventilation.Interventions During controlled mechanical ventilation, patients were first ventilated with aV_T of 9–11 ml/kg.V_T was then increased to 12–14 ml/kg (+25%) for 30 min and subsequently decreased to 6–8 ml/kg (–25%) for 30 min by adjusting the respiratory rate (RR) while the inspiratory flow rate,, and inspiratory duty cycle (T_L/T_TOT) were kept constant. At the end, patients were ventilated with the baseline settings for another 30 min.Measurements and results V_E, carbon dioxide production and oxygen consumption were measured continuously with a gas exchange monitor, and cardiac output and arterial and mixed venous blood samples were taken at the end of each 30-min period to assess CO₂ removal and oxygen transport. Alveolar minute ventilation and the deadspace to tidal volume ratio (V_D/V_T) were calculated from the Bohr equation. Despite large changes inV_T, arterial oxygenation (P_aO₂) and oxygen transport were unchanged throughout the study. WhenV_T was increased, physiologicalV_D increased from 448±34 ml to 559±46 ml (mean±SE) in ARDS (P<0.001) and from 281±22 ml to 357±35 ml in CABG (P<0.05). With the smallV_T,V_D decreased to 357±22 ml in ARDS (P<0.01), and to 234±24 ml in CABG (P<0.05). In ARDS,V_D/V_T decreased from 0.57±0.03 to 0.55±0.03 (P<0.05) with the largeV_T, and increased to 0.60±0.03 (P<0.01), whenV_T was reduced. In CABG,V_D/V_T did not change significantly. ARDS patients had a higherP_aCO₂ than cardiac patients (P<0.001), and only minor changes inP_aCO₂ were observed (for ARDS and CABG respectively, baseline 5.9±0.3 kPa and 4.1±0.1 kPa, largeV_T 5.7±0.3 kPa and 4.1±0.2 kPa, smallV_T 6.2±0.3 kPa and 4.2±0.2 kPa;P<0.05).Conclusions Tidal volumes can be reduced to 6–8 ml/kg in ARDS patients without compromising oxygen transport, while adequate CO₂ elimination can be maintained.