Flow Index: a novel, non-invasive, continuous, quantitative method to evaluate patient inspiratory effort during pressure support ventilation

Filippo Albani,Luigi Pisani,Barbara Buizza,Gianni Ciabatti,Francesco Murgolo,Nicola Latronico,Antonio Rosano,Eros Aniballi,Giuseppe Natalini,Massimo Antonelli,Salvatore Grasso,Valeria Lippolis,Federica Fusina,Anna Granato

doi:10.1186/s13054-021-03624-3

Abstract

BackgroundThe evaluation of patient effort is pivotal during pressure support ventilation, but a non-invasive, continuous, quantitative method to assess patient inspiratory effort is still lacking. We hypothesized that the concavity of the inspiratory flow-time waveform could be useful to estimate patient’s inspiratory effort. The purpose of this study was to assess whether the shape of the inspiratory flow, as quantified by a numeric indicator, could be associated with inspiratory effort during pressure support ventilation.MethodsTwenty-four patients in pressure support ventilation were enrolled. A mathematical relationship describing the decay pattern of the inspiratory flow profile was developed. The parameter hypothesized to estimate effort was named Flow Index. Esophageal pressure, airway pressure, airflow, and volume waveforms were recorded at three support levels (maximum, minimum and baseline). The association between Flow Index and reference measures of patient effort (pressure time product and pressure generated by respiratory muscles) was evaluated using linear mixed effects models adjusted for tidal volume, respiratory rate and respiratory rate/tidal volume.ResultsFlow Index was different at the three pressure support levels and all group comparisons were statistically significant. In all tested models, Flow Index was independently associated with patient effort (p < 0.001). Flow Index prediction of inspiratory effort agreed with esophageal pressure-based methods.ConclusionsFlow Index is associated with patient inspiratory effort during pressure support ventilation, and may provide potentially useful information for setting inspiratory support and monitoring patient-ventilator interactions.

Highlights

The evaluation of patient effort is pivotal during pressure support ventilation, but a non-invasive, continuous, quantitative method to assess patient inspiratory effort is still lacking
PSmin = minimal pressure support, PSbase = baseline pressure support, PSmax = maximal pressure support, rapid shallow breathing index (RSBI) = Rapid Shallow Breathing Index obtained by the ratio of respiratory rate and tidal volume (VT), PTPpt,minute = Pressure–time product for the patient in one minute, PTPpt,breath = Pressure–time product for the patient in a single breath, PTPratio,breath = PTPpt,breath/(PTPpt, breath + PTPvent, breath), Pmusc = pressure generated by respiratory muscles
Flow Index was compared in the different quartiles of inspiratory effort estimates (PTPpt,breath, PTPratio,breath and Pmusc) with analysis of variance (ANOVA) for repeated measures and post-hoc Tukey’s honestly significant difference (HSD) test

Summary

Introduction

The evaluation of patient effort is pivotal during pressure support ventilation, but a non-invasive, continuous, quantitative method to assess patient inspiratory effort is still lacking. The purpose of this study was to assess whether the shape of the inspiratory flow, as quantified by a numeric indicator, could be associated with inspiratory effort during pressure support ventilation. During pressure support ventilation (PSV), assessing patient inspiratory effort could allow the titration of respiratory assistance in order to minimize over- and underassistance [1,2,3]. Other methods include monitoring of the diaphragm’s electrical activity and ultrasound assessment of inspiratory diaphragmatic displacement and thickness. Both are associated with drawbacks, such as availability, costs, non-continuous monitoring and operator-dependency [16]

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