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 [RC]) clinician settings of RR may exceed the frequency needed for optimal lung emptying.

Materials And Methods: This prospective multicenter observational study measured RC in 56 intensive care patients receiving pressure-controlled ventilation. We compared set RR to the one predicted with RC (RR). Also, the subgroup of patients with prolonged RC was analyzed.

Results: Overall, the absolute mean difference between the set RR and RR was 2.8 bpm (95% CI: 2.3-3.2). Twenty-nine (52%) patients had prolonged RC (>0.8 s), mean difference between set RR and RR of 3.1 bpm (95% CI: 2.3-3.8; p < 0.0001) and significantly higher PEEPi compared to those with RC ≤ 0.8 s: 4.4 (95% CI: 3.6-5.2) versus 1.5 (95% CI: 0.9-2.0) cmHO respectively, p < 0.0001.

Conclusions: Use of RR based on measured RC revealed that the clinician-set RR exceeded that predicted by RC in the majority of patients. Measuring RC appears to be a useful variable for adjusting the RR during mandatory mechanical ventilation.