Abstract
Objective
To assess the effect of changes in tidal volume (V T) with a constant inspiratory flow and minute ventilation\((\dot V_E )\) 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,\(\dot V_E \), and inspiratory duty cycle (TL/TTOT) 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\((\dot VCO_2 )\) and oxygen consumption\((\dot VO_2 )\) 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\(\dot V_A \) 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\((\dot DO_2 )\) 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.
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Kiiski, R., Kaitainen, S., Karppi, R. et al. Physiological effects of reduced tidal volume at constant minute ventilation and inspiratory flow rate in acute respiratory distress syndrome. Intensive Care Med 22, 192–198 (1996). https://doi.org/10.1007/BF01712236
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DOI: https://doi.org/10.1007/BF01712236