Prospective targeting and control of end‐tidal CO2 and O2 concentrations

Abstract

Current methods of forcing end‐tidal P_CO2 (P_ETCO2) and P_O2 (P_ETO2) rely on breath‐by‐breath adjustment of inspired gas concentrations using feedback loop algorithms. Such servo‐control mechanisms are complex because they have to anticipate and compensate for the respiratory response to a given inspiratory gas concentration on a breath‐by‐breath basis. In this paper, we introduce a low gas flow method to prospectively target and control P_ETCO2 and P_ETO2 independent of each other and of minute ventilation in spontaneously breathing humans. We used the method to change P_ETCO2 from control (40 mmHg for P_ETCO2 and 100 mmHg for P_ETO2) to two target P_ETCO2 values (45 and 50 mmHg) at iso‐oxia (100 mmHg), P_ETO2 to two target values (200 and 300 mmHg) at normocapnia (40 mmHg), and P_ETCO2 with P_ETO2 simultaneously to the same targets (45 with 200 mmHg and 50 with 300 mmHg). After each targeted value, P_ETCO2 and P_ETO2 were returned to control values. Each state was maintained for 30 s. The average difference between target and measured values for P_ETCO2 was ± 1 mmHg, and for P_ETO2 was ± 4 mmHg. P_ETCO2 varied by ± 1 mmHg and P_ETO2 by ± 5.6 mmHg (s.d.) over the 30 s stages. This degree of control was obtained despite considerable variability in minute ventilation between subjects (± 7.6 l min⁻¹). We conclude that targeted end‐tidal gas concentrations can be attained in spontaneously breathing subjects using this prospective, feed‐forward, low gas flow system.