Flow Index as a Noninvasive Method to Evaluate Inspiratory Effort in Patients on Pressure Support Ventilation

Background: The Flow Index was recently developed as a bedside method based on flow waveforms to assess patient inspiratory effort during invasive mechanical ventilation. The aim of this study is to externally validate the Flow Index by assessing its ability to identify low and high inspiratory effort breaths. Methods: Secondary analysis of a randomized controlled trial. The association between Flow Index and patient inspiratory effort (pressure generated by the respiratory muscles [ΔP_mus] and pressure-time product from the start of inspiratory flow [PTP_insp]) was evaluated using linear mixed effects models. The discrimination capacity (area under the curve [AUC]) of the Flow Index to identify low and high inspiratory effort breaths was analyzed. Results: A total of 1,095 breaths from 38 subjects were included in the analysis. Flow Index had moderate discriminatory power in identifying low inspiratory effort breaths (AUC of 0.73 and 0.77 for low inspiratory effort defined with ΔP_mus and PTP_insp, respectively). Discriminatory power in identifying high inspiratory effort breaths was low (AUC of 0.68 and 0.65 for ΔP_mus and PTP_insp, respectively). Conclusions: Flow Index demonstrated moderate discriminatory power in identifying low inspiratory effort breaths, whereas discriminatory power in identifying high inspiratory effort breaths was low.