The following is a summary of “Determining respiratory rate using measured expiratory time constant: A prospective observational study” published in the October 2022 issue of Critical Care by Depta, et al.

Potential negative implications linked with the high respiratory rate (RR) are intrinsic positive end-expiratory pressure (PEEPi) production, cardiovascular depression, and maybe ventilator-induced lung injury. Despite these drawbacks, however, the optimal RR is still a mystery. Researchers hypothesized that physician settings of RR may exceed the frequency needed for efficient lung emptying if they did not take into account the dynamics of lung emptying (i.e., the expiratory time constant [RCEXP]).

In this prospective, multicenter observational study, RCEXP was assessed in 56 ICU patients using pressure-controlled ventilation. They analyzed the difference between the actual RR and the RCEXP prediction (RRP). The subset of patients who experienced an extended RCEXP was also examined. The absolute mean difference between the set RR and RRP was 2.8 bpm (95% CI: 2.3-3.2). About 29 patients (52%) exhibited prolonged RCEXP (>0.8 s), with a mean difference between set RR and RRP of 3.1 bpm (95% CI: 2.3-3.8; P<0.0001) and substantially higher PEEPi than those with RCEXP  less than equal to 0.8 s: 4.4 (95% CI: 3.6-5.2) versus 1.5 (95% CI: 0.9-2.0) cmH2O respectively, P less than 0.0001. 

Applying RRP based on actual RCEXP showed that the RR chosen by clinicians was higher than what was anticipated by RCEXP in the vast majority of cases. When adjusting the RR for obligatory mechanical ventilation, measuring RCEXP seems to be a relevant variable.