Background: Carbon dioxide elimination (V̇CO2) at steady state corresponds to the metabolic rate. A change in tidal ventilation will lead to a transient response in V̇CO2 if other determinants of V̇CO2 are constant. This principle may be applied in the critical care unit to reset ventilators. Objective: To define and characterize the transient response of V̇CO2 to a well-defined change in ventilation. Methods: Forty-four patients in stable condition receiving volume-controlled mechanical ventilation had trend recordings of ventilator pressures, flow, volumes, V̇CO2, and end- tidal CO2 (ETCO2) for 20 min. At time t0, the minute ventilation was either increased (n=22) or decreased (n=22) by 10% after which these parameters were monitored over 30 min. Blood gas values were measured 5 and 20 min after the change in ventilation and the dead space fractions were computed using the single breath-CO2 test. Data analysis: The first ten breaths (till t1) after a change in ventilation were excluded. The time constant (τ) of the relative change in V̇CO2 (ΔV̇CO2) was calculated by fitting exponential regressions tn ΔV̇CO2 for periods up to 20 min after t1. Results: The ΔV̇CO2 at t1 was proportional to the relative change in tidal volume (ΔVT). The proportionality decreased gradually during 20 min. The proportionality of the relative change in ETCO2 (ΔETCO2) or PaCO2 (ΔPaCO2) with ΔVT was minimal at t1 and increased during the 20 min. τ increased progressively when calculated over longer periods (p<0.001). τ was similar in the groups with increased and decreased ventilation up to 5 min, after which it was longer in the group with decreased ventilation (p<0.05). The ΔPaCO2 after 20 min correlated best with ΔV̇CO2 at t1 (r= -0.8) and with ΔETCO2 at the end of 20 min (r=0.8). Conclusions: Noninvasively monitored V̇CO2 provides an instantaneous indication of the change in alveolar ventilation in well-sedated, mechanically ventilated patients in stable condition without significant cardiopulmonary disease.
- CO elimination
- mechanical ventilation
- ventilator resetting
ASJC Scopus subject areas
- Pulmonary and Respiratory Medicine
- Critical Care and Intensive Care Medicine
- Cardiology and Cardiovascular Medicine