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

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 (+/- 7.61 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.