Dynamics of carbon dioxide elimination following ventilator resetting

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.