In patients with AHRF, HFNC exerts multiple physiologic effects including less inspiratory effort and improved lung volume and compliance. These benefits might underlie the clinical efficacy of HFNC.

Close correlations exist between bedside assessment of positive end-expiratory pressure-induced changes in lung inflation and recruitment by the helium dilution and electrical impedance tomography techniques. Higher positive end-expiratory pressure exerts mixed effects on the regional determinants of ventilator-induced lung injury; these merit close monitoring.

BACKGROUND: Quantification of patient effort during spontaneous breathing is important to tailor ventilatory assistance. Because a correlation between inspiratory muscle pressure (P mus ) and electrical activity of the diaphragm (EA di ) has been described, we aimed to assess the reliability of surface electromyography (EMG) of the respiratory muscles for monitoring diaphragm electrical activity and subject effort during assisted ventilation. METHODS: At a general ICU of a single university-affiliated hospital, we enrolled subjects who were intubated and on pressure support ventilation (PSV) and were on mechanical ventilation for > 48 h. The subjects were studied at 3 levels of pressure support. Airway flow and pressure; esophageal pressure; EA di ; and surface EMG of the diaphragm (surface EA di ), intercostal, and sternocleidomastoid muscles were recorded. Respiratory cycles were sampled for off-line analysis. The P mus /EA di index (PEI) was calculated by relying on EA di and surface EA di (surface PEI) from an airway pressure drop during end-expiratory occlusions performed every minute. RESULTS: surface EA di well correlated with EA di and P mus , in particular, after averaging breaths into deciles (R ‫؍‬ 0.92 and R ‫؍‬ 0.84). When surface PEI was used with surface EA di , it provided a reliable estimation of P mus (R ‫؍‬ 0.94 in comparison with measured P mus ). CONCLUSIONS: During assisted mechanical ventilation, EA di can be reliably monitored by both EA di and surface EMG. The measurement of P mus based on the calibration of EA di was also feasible by the use of surface EMG. Key words: mechanical ventilation; pressuresupport ventilation; electrical activity of the diaphragm; surface electromyography; esophageal pressure.

BackgroundPositive end-expiratory pressure (PEEP) is a key element of mechanical ventilation. It should optimize recruitment, without causing excessive overdistension, but controversy exists on the best method to set it. The purpose of the study was to test the feasibility of setting PEEP with electrical impedance tomography in order to prevent lung de-recruitment following a recruitment maneuver. We enrolled 16 patients undergoing mechanical ventilation with PaO2/FiO2 <300 mmHg. In all patients, under constant tidal volume (6–8 ml/kg) PEEP was set based on the PEEP/FiO2 table proposed by the ARDS network (PEEPARDSnet). We performed a recruitment maneuver and monitored the end-expiratory lung impedance (EELI) over 10 min. If the EELI signal decreased during this period, the recruitment maneuver was repeated and PEEP increased by 2 cmH2O. This procedure was repeated until the EELI maintained a stability over time (PEEPEIT).ResultsThe procedure was feasible in 87% patients. PEEPEIT was higher than PEEPARDSnet (13 ± 3 vs. 9 ± 2 cmH2O, p < 0.001). PaO2/FiO2 improved during PEEPEIT and driving pressure decreased. Recruited volume correlated with the decrease in driving pressure but not with oxygenation improvement. Finally, regional alveolar hyperdistention and collapse was reduced in dependent lung layers and increased in non-dependent lung layers.ConclusionsIn hypoxemic patients, a PEEP selection strategy aimed at stabilizing alveolar recruitment guided by EIT at the bedside was feasible and safe. This strategy led, in comparison with the ARDSnet table, to higher PEEP, improved oxygenation and reduced driving pressure, allowing to estimate the relative weight of overdistension and recruitment.Electronic supplementary materialThe online version of this article (doi:10.1186/s13613-017-0299-9) contains supplementary material, which is available to authorized users.

Background The amount of extracorporeal carbon dioxide removal may influence respiratory drive in acute respiratory distress syndrome (ARDS) patients undergoing extracorporeal membrane oxygenation (ECMO). The authors evaluated the effects of different levels of extracorporeal carbon dioxide removal in patients recovering from severe ARDS undergoing pressure support ventilation (PSV) and neurally adjusted ventilatory assist (NAVA). Methods The authors conducted a prospective, randomized, crossover study on eight spontaneously breathing ARDS patients undergoing venovenous ECMO since 28 ± 20 days. To modulate carbon dioxide extraction, ECMO gas flow (GF) was decreased from baseline resting protective conditions (i.e., GF100%, set to obtain pressure generated in the first 100 ms of inspiration against an occluded airway less than 2 cm H2O, respiratory rate less than or equal to 25 bpm, tidal volume less than 6 ml/kg, and peak airway pressure less than 25 cm H2O) to GF50%-GF25%-GF0% during both PSV and NAVA (random order for ventilation mode). Continuous recordings of airway pressure and flow and esophageal pressure were obtained and analyzed during all study phases. Results At higher levels of extracorporeal carbon dioxide extraction, pressure generated in the first 100 ms of inspiration against an occluded airway decreased from 2.8 ± 2.7 cm H2O (PSV, GF0%) and 3.0 ± 2.1 cm H2O (NAVA, GF0%) to 0.9 ± 0.5 cm H2O (PSV, GF100%) and 1.0 ± 0.8 cm H2O (NAVA, GF100%; P &lt; 0.001) and patients’ inspiratory muscle pressure passed from 8.5 ± 6.3 and 6.5 ± 5.5 cm H2O to 4.5 ± 3.1 and 4.2 ± 3.7 cm H2O (P &lt; 0.001). In time, decreased inspiratory drive and effort determined by higher carbon dioxide extraction led to reduction of tidal volume from 6.6 ± 0.9 and 7.5 ± 1.2 ml/kg to 4.9 ± 0.8 and 5.3 ± 1.3 ml/kg (P &lt; 0.001) and of peak airway pressure from 21 ± 3 and 25 ± 4 cm H2O to 21 ± 3 and 21 ± 5 cm H2O (P &lt; 0.001). Finally, transpulmonary pressure linearly decreased when the amount of carbon dioxide extracted by ECMO increased (R2 = 0.823, P &lt; 0.001). Conclusions In patients recovering from ARDS undergoing ECMO, the amount of carbon dioxide removed by the artificial lung may influence spontaneous breathing. The effects of carbon dioxide removal on spontaneous breathing during the earlier acute phases of ARDS remain to be elucidated.