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

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

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

Overall, the absolute mean difference between the set RR and RR_P was 2.8 bpm (95% CI: 2.3-3.2). Twenty-nine (52%) patients had prolonged RC_EXP (>0.8 s), mean difference between set RR and RR_P of 3.1 bpm (95% CI: 2.3-3.8; p < 0.0001) and significantly higher PEEPi compared to those with RC_EXP ≤ 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.

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