Estimation of the diaphragm neuromuscular efficiency index in mechanically ventilated critically ill patients

Abstract

BackgroundDiaphragm dysfunction develops frequently in ventilated intensive care unit (ICU) patients. Both disuse atrophy (ventilator over-assist) and high respiratory muscle effort (ventilator under-assist) seem to be involved. A strong rationale exists to monitor diaphragm effort and titrate support to maintain respiratory muscle activity within physiological limits. Diaphragm electromyography is used to quantify breathing effort and has been correlated with transdiaphragmatic pressure and esophageal pressure. The neuromuscular efficiency index (NME) can be used to estimate inspiratory effort, however its repeatability has not been investigated yet. Our goal is to evaluate NME repeatability during an end-expiratory occlusion (NMEoccl) and its use to estimate the pressure generated by the inspiratory muscles (Pmus).MethodsThis is a prospective cohort study, performed in a medical-surgical ICU. A total of 31 adult patients were included, all ventilated in neurally adjusted ventilator assist (NAVA) mode with an electrical activity of the diaphragm (EAdi) catheter in situ. At four time points within 72 h five repeated end-expiratory occlusion maneuvers were performed. NMEoccl was calculated by delta airway pressure (ΔPaw)/ΔEAdi and was used to estimate Pmus. The repeatability coefficient (RC) was calculated to investigate the NMEoccl variability.ResultsA total number of 459 maneuvers were obtained. At time T = 0 mean NMEoccl was 1.22 ± 0.86 cmH2O/μV with a RC of 82.6%. This implies that when NMEoccl is 1.22 cmH2O/μV, it is expected with a probability of 95% that the subsequent measured NMEoccl will be between 2.22 and 0.22 cmH2O/μV. Additional EAdi waveform analysis to correct for non-physiological appearing waveforms, did not improve NMEoccl variability. Selecting three out of five occlusions with the lowest variability reduced the RC to 29.8%.ConclusionsRepeated measurements of NMEoccl exhibit high variability, limiting the ability of a single NMEoccl maneuver to estimate neuromuscular efficiency and therefore the pressure generated by the inspiratory muscles based on EAdi.Electronic supplementary materialThe online version of this article (10.1186/s13054-018-2172-0) contains supplementary material, which is available to authorized users.

Monitoring diaphragm function in the ICU

“…However, the repeatability of the measurements of the neuromechanical efficiency ratio recently has been questioned, as these measurements currently exhibit marked within-subject variability (37). This poor repeatability could potentially constitute a major limitation and clinicians must use several measurements to select occlusions with the lowest variability in order to estimate inspiratory muscle effort from EAdi (37). The transformation of EAdi into VT can also be studied by calculating the neuroventilatory efficiency index (VT/EAdi).…”

Physiology of the Respiratory Drive in ICU Patients: Implications for Diagnosis and Treatment

“…As EA di amplitude varies considerably between individuals and normal values are unknown, recordings are mainly used to evaluate changes in respiratory drive in the same patient. EA di during tidal breathing is often standardized to respiratory muscle pressure (i.e., neuromechanical efficiency index) [34] or to that observed during a maximum inspiratory contraction (i.e., EA di%max ) [7]. Although the latter was shown to correlate with the intensity of breathlessness in non-ventilated patients with chronic obstructive pulmonary disease (COPD) [35], it is generally not feasible to perform maximum inspiratory maneuvers in ICU patients.…”

“…In addition, recruitment of accessory respiratory muscles is not reflected in the EA di signal. Finally, suboptimal filtering of the raw electromyography signal may affect validity to quantify drive with EA di [34].…”

A brief airway occlusion is sufficient to measure the patient’s inspiratory effort/electrical activity of the diaphragm index (PEI)

“…A previous report showed a high within-patient variability of the neuromuscular efficiency index, an index similar to the PEI, over repeated measures [16]. Thus, we focused on the median value, rather than the mean value, of the measurements collected in every patient to estimate the PEI during brief occlusions.…”