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 (VT) with a constant inspiratory flow and minute ventilation (VE) on gas exchange and oxygen transport in acute respiratory distress syndrome (ARDS).

DESIGN: A crossover study of three VT 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 a VT of 9-11 ml/kg. VT 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, VE, and inspiratory duty cycle (TL/T(TOT)) were kept constant. At the end, patients were ventilated with the baseline settings for another 30 min.

MEASUREMENTS AND RESULTS: VE, carbon dioxide production (VCO2) and oxygen consumption (VO2) 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 (VA) and the deadspace to tidal volume ratio (VD/VT) were calculated from the Bohr equation. Despite large changes in VT, arterial oxygenation (PaO2) and oxygen transport (DO2) were unchanged throughout the study. When VT was increased, physiological VD 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 small VT, VD decreased to 357 +/- 22 ml in ARDS (P < 0.01), and to 234 +/- 24 ml in CABG (P < 0.05). In ARDS, VD/VT decreased from 0.57 +/- 0.03 to 0.55 +/- 0.03 (P < 0.05) with the large VT, and increased to 0.60 +/- 0.03 (P < 0.01), when VT was reduced. In CABG, VD/VT did not change significantly. ARDS patients had a higher PaCO2 than cardiac patients (P < 0.001), and only minor changes in PaCO2 were observed (for ARDS and CABG respectively, baseline 5.9 +/- 0.3 kPa and 4.1 +/- 0.1 kPa, large VT 5.7 +/- 0.3 kPa and 4.1 +/- 0.2 kPa, small VT 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.