Determining respiratory rate using measured expiratory time constant: A prospective observational study.

PURPOSE:Potential negative implications associated with high respiratory rate (RR) are intrinsic positive end-expiratory pressure (PEEPi) generation, cardiovascular depression and possibly ventilator induced lung injury. Despite these negative consequences, optimal RR remains largely unknown. We hypothesized that without consideration of dynamics of lung emptying (i.e., the expiratory time constant [RCEXP]) clinician settings of RR may exceed the frequency needed for optimal lung emptying. MATERIALS AND METHODS:This prospective multicenter observational study measured RCEXP in 56 intensive care patients receiving pressure-controlled ventilation. We compared set RR to the one predicted with RCEXP (RRP). Also, the subgroup of patients with prolonged RCEXP was analyzed. RESULTS:Overall, the absolute mean difference between the set RR and RRP was 2.8 bpm (95% CI: 2.3-3.2). Twenty-nine (52%) patients had prolonged RCEXP (>0.8 s), mean difference between set RR and RRP of 3.1 bpm (95% CI: 2.3-3.8; p < 0.0001) and significantly higher PEEPi compared to those with RCEXP ≤ 0.8 s: 4.4 (95% CI: 3.6-5.2) versus 1.5 (95% CI: 0.9-2.0) cmH2O respectively, p < 0.0001. CONCLUSIONS:Use of RRP based on measured RCEXP revealed that the clinician-set RR exceeded that predicted by RCEXP in the majority of patients. Measuring RCEXP appears to be a useful variable for adjusting the RR during mandatory mechanical ventilation.