High inspired CO₂ target accuracy in mechanical ventilation and spontaneous breathing using the Additional CO₂ method

Introduction: Cerebrovascular reactivity imaging (CVR) is a diagnostic method for assessment of alterations in cerebral blood flow in response to a controlled vascular stimulus. The principal utility is the capacity to evaluate the cerebrovascular reserve, thereby elucidating autoregulatory functioning. In CVR, CO₂ gas challenge is the most prevalent method, which elicits a vascular response by alterations in inspired CO₂ concentrations. While several systems have been proposed in the literature, only a limited number have been devised to operate in tandem with mechanical ventilation, thus constraining the majority CVR investigations to spontaneously breathing individuals.

Methods: We have developed a new method, denoted Additional CO₂, designed to enable CO₂ challenge in ventilators. The central idea is the introduction of an additional flow of highly concentrated CO₂ into the respiratory circuit, as opposed to administration of the entire gas mixture from a reservoir. By monitoring the main respiratory gas flow emanating from the ventilator, the CO₂ concentration in the inspired gas can be manipulated by adjusting the proportion of additional CO₂. We evaluated the efficacy of this approach in (1) a ventilator coupled with a test lung and (2) in spontaneously breathing healthy subjects. The method was evaluated by assessment of the precision in attaining target inspired CO₂ levels and examination of its performance within a magnetic resonance imaging environment.

Results and discussion: Our investigations revealed that the Additional CO₂ method consistently achieved a high degree of accuracy in reaching target inspired CO₂ levels in both mechanical ventilation and spontaneous breathing. We anticipate that these findings will lay the groundwork for a broader implementation of CVR assessments in mechanically ventilated patients.