Individual response in patient's effort and driving pressure to variations in assistance during pressure support ventilation.

BACKGROUND: During Pressure Support Ventilation (PSV) an inspiratory hold allows to measure plateau pressure (Pplat), driving pressure (∆P), respiratory system compliance (Crs) and pressure-muscle-index (PMI), an index of inspiratory effort. This study aims [1] to assess systematically how patient's effort (estimated with PMI), ∆P and tidal volume (Vt) change in response to variations in PSV and [2] to confirm the robustness of Crs measurement during PSV. METHODS: 18 patients recovering from acute respiratory failure and ventilated by PSV were cross-randomized to four steps of assistance above (+ 3 and + 6 cmH2O) and below (-3 and -6 cmH2O) clinically set PS. Inspiratory and expiratory holds were performed to measure Pplat, PMI, ∆P, Vt, Crs, P0.1 and occluded inspiratory airway pressure (Pocc). Electromyography of respiratory muscles was monitored noninvasively from body surface (sEMG). RESULTS: As PSV was decreased, Pplat (from 20.5 ± 3.3 cmH2O to 16.7 ± 2.9, P < 0.001) and ∆P (from 12.5 ± 2.3 to 8.6 ± 2.3 cmH2O, P < 0.001) decreased much less than peak airway pressure did (from 21.7 ± 3.8 to 9.7 ± 3.8 cmH2O, P < 0.001), given the progressive increase of patient's effort (PMI from -1.2 ± 2.3 to 6.4 ± 3.2 cmH2O) in line with sEMG of the diaphragm (r = 0.614; P < 0.001). As ∆P increased linearly with Vt, Crs did not change through steps (P = 0.119). CONCLUSION: Patients react to a decrease in PSV by increasing inspiratory effort-as estimated by PMI-keeping Vt and ∆P on a desired value, therefore, limiting the clinician's ability to modulate them. PMI appears a valuable index to assess the point of ventilatory overassistance when patients lose control over Vt like in a pressure-control mode. The measurement of Crs in PSV is constant-likely suggesting reliability-independently from the level of assistance and patient's effort.

Waveforms-guided cycling-off during pressure support ventilation improves both inspiratory and expiratory patient-ventilator synchronisation.

OBJECTIVE: To test the performance of a software able to control mechanical ventilator cycling-off by means of automatic, real-time analysis of ventilator waveforms during pressure support ventilation. DESIGN: Prospective randomised crossover study. SETTING: University Intensive Care Unit. PATIENTS: Fifteen difficult-to-wean patients under pressure support ventilation. INTERVENTIONS: Patients were ventilated using a G5 ventilator (Hamilton Medical, Bonaduz, Switzerland) with three different cycling-off settings: standard (expiratory trigger sensitivity set at 25% of peak inspiratory flow), optimised by an expert clinician and automated; the last two settings were tested at baseline pressure support and after a 50% increase in pressure support. MEASUREMENTS AND MAIN RESULTS: Ventilator waveforms were recorded and analysed by four physicians experts in waveforms analysis. Major and minor asynchronies were detected and total asynchrony time computed. Automation compared to standard setting reduced cycling delay from 407 ms [257-567] to 59 ms [22-111] and ineffective efforts from 12.5% [3.4-46.4] to 2.8% [1.9-4.6]) at baseline support (p < 0.001); expert optimisation performed similarly. At high support both cycling delay and ineffective efforts increased, mainly in the case of expert setting, with the need of reoptimisation of expiratory trigger sensitivity. At baseline support, asynchrony time decreased from 39.9% [27.4-58.7] with standard setting to 32% [22.3-39.4] with expert optimisation (p < 0.01) and to 24.4% [19.6-32.5] with automation (p < 0.001). Both at baseline and at high support, asynchrony time was lower with automation than with expert setting. CONCLUSIONS: Cycling-off guided by automated real-time waveforms analysis seems a reliable solution to improve synchronisation in difficult-to-wean patients under pressure support ventilation.

Characterization of compliance phenotypes in COVID-19 acute respiratory distress syndrome.

BACKGROUND: Coronavirus disease 2019-associated acute respiratory distress syndrome (COVID-19 ARDS) seems to differ from the "classic ARDS", showing initial significant hypoxemia in the face of relatively preserved compliance and evolving later in a scenario of poorly compliant lungs. We tested the hypothesis that in patients with COVID-19 ARDS, the initial value of static compliance of respiratory system (Crs) (1) depends on the previous duration of the disease (i.e., the fewer days of illness, the higher the Crs and vice versa) and (2) identifies different lung patterns of time evolution and response to prone positioning. METHODS: This was a single-center prospective observational study. We enrolled consecutive mechanically ventilated patients with a diagnosis of COVID-19 who met ARDS criteria, admitted to intensive care unit (ICU). Patients were divided in four groups based on quartiles of initial Crs. Relationship between Crs and the previous duration of the disease was evaluated. Respiratory parameters collected once a day and during prone positioning were compared between groups. RESULTS: We evaluated 110 mechanically ventilated patients with a diagnosis of COVID-19 who met ARDS criteria admitted to our ICUs. Patients were divided in groups based on quartiles of initial Crs. The median initial Crs was 41 (32-47) ml/cmH2O. No association was found between the previous duration of the disease and the initial Crs. The Crs did not change significantly over time within each quartile. Positive end-expiratory pressure (PEEP) and driving pressure were respectively lower and greater in patients with lower Crs. Prone positioning significantly improved PaO2/FiO2 in the 4 groups, however it increased the Crs significantly only in patients in lower quartile of Crs. CONCLUSIONS: In our cohort, the initial Crs is not dependent on the previous duration of COVID-19 disease. Prone positioning improves oxygenation irrespective to initial Crs, but it ameliorates respiratory mechanics only in patients with lower Crs.

Timing of inspiratory muscle activity detected from airway pressure and flow during pressure support ventilation: the waveform method.

BACKGROUND: Whether respiratory efforts and their timing can be reliably detected during pressure support ventilation using standard ventilator waveforms is unclear. This would give the opportunity to assess and improve patient-ventilator interaction without the need of special equipment. METHODS: In 16 patients under invasive pressure support ventilation, flow and pressure waveforms were obtained from proximal sensors and analyzed by three trained physicians and one resident to assess patient's spontaneous activity. A systematic method (the waveform method) based on explicit rules was adopted. Esophageal pressure tracings were analyzed independently and used as reference. Breaths were classified as assisted or auto-triggered, double-triggered or ineffective. For assisted breaths, trigger delay, early and late cycling (minor asynchronies) were diagnosed. The percentage of breaths with major asynchronies (asynchrony index) and total asynchrony time were computed. RESULTS: Out of 4426 analyzed breaths, 94.1% (70.4-99.4) were assisted, 0.0% (0.0-0.2) auto-triggered and 5.8% (0.4-29.6) ineffective. Asynchrony index was 5.9% (0.6-29.6). Total asynchrony time represented 22.4% (16.3-30.1) of recording time and was mainly due to minor asynchronies. Applying the waveform method resulted in an inter-operator agreement of 0.99 (0.98-0.99); 99.5% of efforts were detected on waveforms and agreement with the reference in detecting major asynchronies was 0.99 (0.98-0.99). Timing of respiratory efforts was accurately detected on waveforms: AUC for trigger delay, cycling delay and early cycling was 0.865 (0.853-0.876), 0.903 (0.892-0.914) and 0.983 (0.970-0.991), respectively. CONCLUSIONS: Ventilator waveforms can be used alone to reliably assess patient's spontaneous activity and patient-ventilator interaction provided that a systematic method is adopted.

Reliability of plateau pressure during patient-triggered assisted ventilation. Analysis of a multicentre database.

PURPOSE: An inspiratory hold during patient-triggered assisted ventilation potentially allows to measure driving pressure and inspiratory effort. However, muscular activity can make this measurement unreliable. We aim to define the criteria for inspiratory holds reliability during patient-triggered breaths. MATERIAL AND METHODS: Flow, airway and esophageal pressure recordings during patient-triggered breaths from a multicentre observational study (BEARDS, NCT03447288) were evaluated by six independent raters, to determine plateau pressure readability. Features of "readable" and "unreadable" holds were compared. Muscle pressure estimate from the hold was validated against other measures of inspiratory effort. RESULTS: Ninety-two percent of the recordings were consistently judged as readable or unreadable by at least four raters. Plateau measurement showed a high consistency among raters. A short time from airway peak to plateau pressure and a stable and longer plateau characterized readable holds. Unreadable plateaus were associated with higher indexes of inspiratory effort. Muscular pressure computed from the hold showed a strong correlation with independent indexes of inspiratory effort. CONCLUSION: The definition of objective parameters of plateau reliability during assisted-breath provides the clinician with a tool to target a safer assisted-ventilation and to detect the presence of high inspiratory effort.

Coagulopathy and COVID-19.

SARS-CoV-2 infection is associated with frequent thrombotic events, at the micro and macro-vascular level, due to the perpetuation of a state of hypercoagulability. The so-called 'COVID-19 associated coagulopathy' (CAC) represents a key aspect in the genesis of organ damage from SARS-CoV-2. The main coagulative alterations described in the literature are represented by high levels of D-dimer and fibrinogen. Although CAC has some common features with disseminated intravascular coagulation and sepsis-induced coagulopathy, there are important differences between these clinical pictures and the phenotype of CAC is unique. The pathogenesis of CAC is complex and is affected by the strong interconnection between the inflammatory system and coagulation, in the phenomenon of immunothrombosis and thrombo-inflammation. Several mechanisms come into play, such as inflammatory cytokines, neutrophils, the complement system as well as an alteration of the fibrinolytic system. Finally, an altered platelet function and especially endothelial dysfunction also play a central role in the pathophysiology of CAC. Heparin has several potential effects in CAC, in fact in addition to the anticoagulant effect, it could have a direct antiviral effect and anti-inflammatory properties. The high incidence of thrombo-embolic phenomena despite the use of antithrombotic prophylaxis have led some experts to recommend the use of anticoagulant doses of heparin, but at present the optimal anticoagulant regimen remains to be determined.

Technical aspects of bedside respiratory monitoring of transpulmonary pressure.

Transpulmonary pressure, that is the difference between airway pressure (Paw) and pleural pressure, is considered one of the most important parameters to know in order to set a safe mechanical ventilation in acute respiratory distress syndrome (ARDS) patients but also in critically ill obese patients, in abdominal pathologies or in pathologies affecting the chest wall itself. Transpulmonary pressure should rely on the assessment of intrathoracic pleural pressure. Esophageal pressure (Pes) is considered the best surrogate of pleural pressure in critically ill patients, but concerns about its reliability exist. The aim of this article is to describe the technique of Pes measurement in mechanically ventilated patients: the catheter insertion, the proper balloon placement and filling, the validation test and specific procedures to remove the main artifacts will be discussed.