Ventilator-related causes of lung injury: the mechanical power

PURPOSE

We hypothesized that the ventilator-related causes of lung injury may be unified in a single variable: the mechanical power. We assessed whether the mechanical power measured by the pressure-volume loops can be computed from its components: tidal volume (TV)/driving pressure (∆P aw), flow, positive end-expiratory pressure (PEEP), and respiratory rate (RR). If so, the relative contributions of each variable to the mechanical power can be estimated.

METHODS

We computed the mechanical power by multiplying each component of the equation of motion by the variation of volume and RR: [Formula: see text]where ∆V is the tidal volume, ELrs is the elastance of the respiratory system, I:E is the inspiratory-to-expiratory time ratio, and R aw is the airway resistance. In 30 patients with normal lungs and in 50 ARDS patients, mechanical power was computed via the power equation and measured from the dynamic pressure-volume curve at 5 and 15 cmH2O PEEP and 6, 8, 10, and 12 ml/kg TV. We then computed the effects of the individual component variables on the mechanical power.

RESULTS

Computed and measured mechanical powers were similar at 5 and 15 cmH2O PEEP both in normal subjects and in ARDS patients (slopes = 0.96, 1.06, 1.01, 1.12 respectively, R (2) > 0.96 and p < 0.0001 for all). The mechanical power increases exponentially with TV, ∆P aw, and flow (exponent = 2) as well as with RR (exponent = 1.4) and linearly with PEEP.

CONCLUSIONS

The mechanical power equation may help estimate the contribution of the different ventilator-related causes of lung injury and of their variations. The equation can be easily implemented in every ventilator's software.

Ventilator-induced Lung Injury

Prevention of ventilator-induced lung injury (VILI) can attenuate multiorgan failure and improve survival in at-risk patients. Clinically significant VILI occurs from volutrauma, barotrauma, atelectrauma, biotrauma, and shear strain. Differences in regional mechanics are important in VILI pathogenesis. Several interventions are available to protect against VILI. However, most patients at risk of lung injury do not develop VILI. VILI occurs most readily in patients with concomitant physiologic insults. VILI prevention strategies must balance risk of lung injury with untoward side effects from the preventive effort, and may be most effective when targeted to subsets of patients at increased risk.

Respiratory physiology of COVID-19-induced respiratory failure compared to ARDS of other etiologies

BACKGROUND

Whether respiratory physiology of COVID-19-induced respiratory failure is different from acute respiratory distress syndrome (ARDS) of other etiologies is unclear. We conducted a single-center study to describe respiratory mechanics and response to positive end-expiratory pressure (PEEP) in COVID-19 ARDS and to compare COVID-19 patients to matched-control subjects with ARDS from other causes.

METHODS

Thirty consecutive COVID-19 patients admitted to an intensive care unit in Rome, Italy, and fulfilling moderate-to-severe ARDS criteria were enrolled within 24 h from endotracheal intubation. Gas exchange, respiratory mechanics, and ventilatory ratio were measured at PEEP of 15 and 5 cmH2O. A single-breath derecruitment maneuver was performed to assess recruitability. After 1:1 matching based on PaO2/FiO2, FiO2, PEEP, and tidal volume, COVID-19 patients were compared to subjects affected by ARDS of other etiologies who underwent the same procedures in a previous study.

RESULTS

Thirty COVID-19 patients were successfully matched with 30 ARDS from other etiologies. At low PEEP, median [25th-75th percentiles] PaO2/FiO2 in the two groups was 119 mmHg [101-142] and 116 mmHg [87-154]. Average compliance (41 ml/cmH2O [32-52] vs. 36 ml/cmH2O [27-42], p = 0.045) and ventilatory ratio (2.1 [1.7-2.3] vs. 1.6 [1.4-2.1], p = 0.032) were slightly higher in COVID-19 patients. Inter-individual variability (ratio of standard deviation to mean) of compliance was 36% in COVID-19 patients and 31% in other ARDS. In COVID-19 patients, PaO2/FiO2 was linearly correlated with respiratory system compliance (r = 0.52 p = 0.003). High PEEP improved PaO2/FiO2 in both cohorts, but more remarkably in COVID-19 patients (p = 0.005). Recruitability was not different between cohorts (p = 0.39) and was highly inter-individually variable (72% in COVID-19 patients and 64% in ARDS from other causes). In COVID-19 patients, recruitability was independent from oxygenation and respiratory mechanics changes due to PEEP.

CONCLUSIONS

Early after establishment of mechanical ventilation, COVID-19 patients follow ARDS physiology, with compliance reduction related to the degree of hypoxemia, and inter-individually variable respiratory mechanics and recruitability. Physiological differences between ARDS from COVID-19 and other causes appear small.

Quantifying unintended exposure to high tidal volumes from breath stacking dyssynchrony in ARDS: the BREATHE criteria

PURPOSE

Breath stacking dyssynchrony generates higher tidal volumes than intended, potentially increasing lung injury risk in acute respiratory distress syndrome (ARDS). Lack of validated criteria to quantify breath stacking dyssynchrony contributes to its under-recognition. This study evaluates performance of novel, objective criteria for quantifying breath stacking dyssynchrony (BREATHE criteria) compared to existing definitions and tests if neuromuscular blockade eliminates high-volume breath stacking dyssynchrony in ARDS.

METHODS

Airway flow and pressure were recorded continuously for up to 72 h in 33 patients with ARDS receiving volume-preset assist-control ventilation. The flow-time waveform was integrated to calculate tidal volume breath-by-breath. The BREATHE criteria considered five domains in evaluating for breath stacking dyssynchrony: ventilator cycling, interval expiratory volume, cumulative inspiratory volume, expiratory time, and inspiratory time.

RESULTS

The observed tidal volume of BREATHE stacked breaths was 11.3 (9.7-13.3) mL/kg predicted body weight, significantly higher than the preset volume [6.3 (6.0-6.8) mL/kg; p < 0.001]. BREATHE identified more high-volume breaths (≥2 mL/kg above intended volume) than the other existing objective criteria for breath stacking [27 (7-59) vs 19 (5-46) breaths/h; p < 0.001]. Agreement between BREATHE and visual waveform inspection was high (raw agreement 96.4-98.1 %; phi 0.80-0.92). Breath stacking dyssynchrony was near-completely eliminated during neuromuscular blockade [0 (0-1) breaths/h; p < 0.001].

CONCLUSIONS

The BREATHE criteria provide an objective definition of breath stacking dyssynchrony emphasizing occult exposure to high tidal volumes. BREATHE identified high-volume breaths missed by other methods for quantifying this dyssynchrony. Neuromuscular blockade prevented breath stacking dyssynchrony, assuring provision of the intended lung-protective strategy.

The Effects of Aging on Lung Structure and Function

Growth of the segment of the population older than 65 years has led to intensified interest in understanding the biology of aging. This article is focused on age-related alterations in lung structure that produce predictable changes in physiologic function, both at rest and during exercise. Increased insight into the physiology of the healthy aging lung should ultimately lead to improved methods of lung function assessment in the elderly (defined as those older than 65 years) as well as better understanding of the manifestations and possibly even the treatment of geriatric lung disease.

Recruitability and effect of PEEP in SARS-Cov-2-associated acute respiratory distress syndrome

Background

A large proportion of patients with a SARS-Cov-2-associated respiratory failure develop an acute respiratory distress syndrome (ARDS). It has been recently suggested that SARS-Cov-2-associated ARDS may differ from usual non-SARS-Cov-2-associated ARDS by higher respiratory system compliance (C_RS), lower potential for recruitment with positive end-expiratory pressure (PEEP) contrasting with severe shunt fraction. The purpose of the study was to systematically assess respiratory mechanics and recruitability in SARS-Cov-2-associated ARDS.

Methods

Gas exchanges, C_RS and hemodynamics were assessed at 2 levels of PEEP (15 cmH₂O and 5 cmH₂O) within 36 h (day1) and from 4 to 6 days (day 5) after intubation. The recruited volume was computed as the difference between the volume expired from PEEP 15 to 5 cmH₂O and the volume predicted by compliance at PEEP 5 cmH₂O (or above airway opening pressure). The recruitment-to-inflation (R/I) ratio (i.e. the ratio between the recruited lung compliance and C_RS at PEEP 5 cmH₂O) was used to assess lung recruitability. A R/I ratio value higher than or equal to 0.5 was used to define highly recruitable patients.

Results

The R/I ratio was calculated in 25 of the 26 enrolled patients at day 1 and in 15 patients at day 5. At day 1, 16 (64%) were considered as highly recruitable (R/I ratio median [interquartile range] 0.7 [0.55–0.94]) and 9 (36%) were considered as poorly recruitable (R/I ratio 0.41 [0.31–0.48]). The PaO₂/FiO₂ ratio at PEEP 15 cmH₂O was higher compared to PEEP 5 cmH₂O only in highly recruitable patients (173 [139–236] vs 135 [89–167] mmHg; p < 0.01). Neither PaO₂/FiO₂ or C_RS measured at PEEP 15 cmH₂O or at PEEP 5 cmH₂O nor changes in PaO₂/FiO₂ or C_RS in response to PEEP changes allowed to identify highly or poorly recruitable patients.

Conclusion

In this series of 25 patients with SARS-Cov-2 associated ARDS, 64% were considered as highly recruitable and only 36% as poorly recruitable based on the R/I ratio performed on the day of intubation. This observation suggests that a systematic R/I ratio assessment may help to guide initial PEEP titration to limit harmful effect of unnecessary high PEEP in the context of Covid-19 crisis.

Restoring pulmonary surfactant membranes and films at the respiratory surface

Pulmonary surfactant is a complex of lipids and proteins assembled and secreted by the alveolar epithelium into the thin layer of fluid coating the respiratory surface of lungs. There, surfactant forms interfacial films at the air-water interface, reducing dramatically surface tension and thus stabilizing the air-exposed interface to prevent alveolar collapse along respiratory mechanics. The absence or deficiency of surfactant produces severe lung pathologies. This review describes some of the most important surfactant-related pathologies, which are a cause of high morbidity and mortality in neonates and adults. The review also updates current therapeutic approaches pursuing restoration of surfactant operative films in diseased lungs, mainly through supplementation with exogenous clinical surfactant preparations. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Personalized medicine for ARDS: the 2035 research agenda

In the last 20 years, survival among patients with acute respiratory distress syndrome (ARDS) has increased substantially with advances in lung-protective ventilation and resuscitation. Building on this success, personalizing mechanical ventilation to patient-specific physiology for enhanced lung protection will be a top research priority for the years ahead. However, the ARDS research agenda must be broader in scope. Further understanding of the heterogeneous biology, from molecular to mechanical, underlying early ARDS pathogenesis is essential to inform therapeutic discovery and tailor treatment and prevention strategies to the individual patient. The ARDSne(x)t research agenda for the next 20 years calls for bringing personalized medicine to ARDS, asking simultaneously both whether a treatment affords clinically meaningful benefit and for whom. This expanded scope necessitates standard acquisition of highly granular biological, physiological, and clinical data across studies to identify biologically distinct subgroups that may respond differently to a given intervention. Clinical trials will need to consider enrichment strategies and incorporate long-term functional outcomes. Tremendous investment in research infrastructure and global collaboration will be vital to fulfilling this agenda.

Inhaled sulfur dioxide causes pulmonary and systemic inflammation leading to fibrotic respiratory disease in a rat model of chemical-induced lung injury

Inhalation of high concentrations of sulfur dioxide (SO₂) affects the lungs and can be immediately dangerous to life. We examined the development of acute and long-term effects after exposure of SO₂ in Sprague-Dawley rats, in particular inflammatory responses, airway hyperresponsiveness (AHR) and lung fibrosis. Animals were subjected to a single exposure of 2200ppm SO₂ during 10min and treated with a single dose of the anti-inflammatory corticosteroid dexamethasone 1h following exposure. Exposed rats showed labored breathing, decreased body-weight and an acute inflammation with neutrophil and macrophage airway infiltrates 5h post exposure. The acute effects were characterized by bronchial damage restricted to the larger bronchi with widespread injured mucosal epithelial lining. Rats displayed hyperreactive airways 24h after exposure as indicated by increased methacholine-induced respiratory resistance. The inflammatory infiltrates remained in lung tissue for at least 14 days but at the late time-point the dominating granulocyte types had changed from neutrophils to eosinophils. Analysis of immunoregulatory and pro-inflammatory cytokines in serum and airways implicated mixed macrophage phenotypes (M1/M2) and T helper cell activation of both T_H1 and T_H2 subtypes. Increased expression of the pro-fibrotic cytokine TGFβ1 was detected in airways 24h post exposure and remained increased at the late time-points (14 and 28 days). The histopathology analysis confirmed a significant collagen deposition 14 days post exposure. Treatment with dexamethasone significantly counteracted the acute inflammatory response but was insufficient for complete protection against SO₂-induced adverse effects, i.e. treatment only provided partial protection against AHR and the long-term fibrosis.

Coupled and reduced dimensional modeling of respiratory mechanics during spontaneous breathing

In this paper, we develop a total lung model based on a tree of 0D airway and acinar models for studying respiratory mechanics during spontaneous breathing. This model utilizes both computer tomography-based geometries and artificially generated lobe-filling airway trees to model the entire conducting region of the lung. Beyond the conducting airways, we develop an acinar model, which takes into account the alveolar tissue resistance, compliance, and the intrapleural pressure. With this methodology, we compare four different 0D models of airway mechanics and determine the best model based on a comparison with a 3D-0D coupled model of the conducting airways; this methodology is possible because the majority of airway resistance is confined to the lower generations, that is, the trachea and the first few bronchial generations. As an example application of the model, we simulate the flow and pressure dynamics under spontaneous breathing conditions, that is, at flow conditions driven purely by pleural space pressure. The results show good agreement, both qualitatively and quantitatively, with reported physiological values. One of the key advantages of this model is the ability to provide insight into lung ventilation in the peripheral regions. This is often crucial because this is where information, specifically for studying diseases and gas exchange, is needed. Thus, the model can be used as a tool for better understanding local peripheral lung mechanics without excluding the upper portions of the lung. This tool will be also useful for in vitro investigations of lung mechanics in both health and disease.

Partition of respiratory mechanics in patients with acute respiratory distress syndrome and association with outcome: a multicentre clinical study

PURPOSE

In acute respiratory distress syndrome (ARDS), physiological parameters associated with outcome may help defining targets for mechanical ventilation. This study aimed to address whether transpulmonary pressures (P_L), including transpulmonary driving pressure (DP_L), elastance-derived plateau P_L, and directly-measured end-expiratory P_L, are better associated with 60-day outcome than airway driving pressure (DP_aw). We also tested the combination of oxygenation and stretch index [PaO₂/(FiO₂*DP_aw)].

METHODS

Prospective, observational, multicentre registry of ARDS patients. Respiratory mechanics were measured early after intubation at 6 kg/ml tidal volume. We compared the predictive power of the parameters for mortality at day-60 through receiver operating characteristic (ROC) and assessed their association with 60-day mortality through unadjusted and adjusted Cox regressions. Finally, each parameter was dichotomized, and Kaplan-Meier survival curves were compared.

RESULTS

385 patients were enrolled 2 [1-4] days from intubation (esophageal pressure and arterial blood gases in 302 and 318 patients). As continuous variables, DP_aw, DP_L, and oxygenation stretch index were associated with 60-day mortality after adjustment for age and Sequential Organ Failure Assessment, whereas elastance-derived plateau P_L was not. DP_aw and DP_L performed equally in ROC analysis (P = 0.0835). DP_aw had the best-fit Cox regression model. When dichotomizing the variables, DP_aw ≥ 15, DP_L ≥ 12, plateau P_L ≥ 24, and oxygenation stretch index < 10 exhibited lower 60-day survival probability. Directly measured end-expiratory P_L ≥ 0 was associated with better outcome in obese patients.

CONCLUSION

DP_L was equivalent predictor of outcome than DP_aw. Our study supports the soundness of limiting lung and airway driving pressure and maintaining positive end-expiratory P_L in obese patients.

Static compliance of the respiratory system in COVID-19 related ARDS: an international multicenter study

BACKGROUND

Controversies exist on the nature of COVID-19 related acute respiratory distress syndrome (ARDS) in particular on the static compliance of the respiratory system (Crs). We aimed to analyze the association of Crs with outcome in COVID-19-associated ARDS, to ascertain its determinants and to describe its evolution at day-14.

METHODS

In this observational multicenter cohort of patients with moderate to severe Covid-19 ARDS, Crs was measured at day-1 and day-14. Association between Crs or Crs/ideal body weight (IBW) and breathing without assistance at day-28 was analyzed with multivariable logistic regression. Determinants were ascertained by multivariable linear regression. Day-14 Crs was compared to day-1 Crs with paired t-test in patients still under controlled mechanical ventilation.

RESULTS

The mean Crs in 372 patients was 37.6 ± 13 mL/cmH2O, similar to as in ARDS of other causes. Multivariate linear regression identified chronic hypertension, low PaO2/FiO2 ratio, low PEEP, and low tidal volume as associated with lower Crs/IBW. After adjustment on confounders, nor Crs [OR 1.0 (CI 95% 0.98-1.02)] neither Crs/IBW [OR 0.63 (CI 95% 0.13-3.1)] were associated with the chance of breathing without assistance at day-28 whereas plateau pressure was [OR 0.93 (CI 95% 0.88-0.99)]. In a subset of 108 patients, day-14 Crs decreased compared to day-1 Crs (31.2 ± 14.4 mL/cmH2O vs 37.8 ± 11.4 mL/cmH2O, p < 0.001). The decrease in Crs was not associated with day-28 outcome.

CONCLUSION

In a large multicenter cohort of moderate to severe COVID-19 ARDS, mean Crs was decreased below 40 mL/cmH2O and was not associated with day-28 outcome. Crs decreased between day-1 and day-14 but the decrease was not associated with day-28 outcome.

Respiratory mechanics in brain injury: A review

Several clinical and experimental studies have shown that lung injury occurs shortly after brain damage. The responsible mechanisms involve neurogenic pulmonary edema, inflammation, the harmful action of neurotransmitters, or autonomic system dysfunction. Mechanical ventilation, an essential component of life support in brain-damaged patients (BD), may be an additional traumatic factor to the already injured or susceptible to injury lungs of these patients thus worsening lung injury, in case that non lung protective ventilator settings are applied. Measurement of respiratory mechanics in BD patients, as well as assessment of their evolution during mechanical ventilation, may lead to preclinical lung injury detection early enough, allowing thus the selection of the appropriate ventilator settings to avoid ventilator-induced lung injury. The aim of this review is to explore the mechanical properties of the respiratory system in BD patients along with the underlying mechanisms, and to translate the evidence of animal and clinical studies into therapeutic implications regarding the mechanical ventilation of these critically ill patients.

The occlusion tests and end-expiratory esophageal pressure: measurements and comparison in controlled and assisted ventilation

Background

Esophageal pressure is used as a reliable surrogate of the pleural pressure. It is conventionally measured by an esophageal balloon placed in the lower part of the esophagus. To validate the correct position of the balloon, a positive pressure occlusion test by compressing the thorax during an end-expiratory pause or a Baydur test obtained by occluding the airway during an inspiratory effort is used. An acceptable catheter position is defined when the ratio between the changes in esophageal and airway pressure (∆Pes/∆Paw) is close to unity. Sedation and paralysis could affect the accuracy of esophageal pressure measurements. The aim of this study was to evaluate, in mechanically ventilated patients, the effects of paralysis, two different esophageal balloon positions and two PEEP levels on the ∆Pes/∆Paw ratio measured by the positive pressure occlusion and the Baydur tests and on the end-expiratory esophageal pressure and respiratory mechanics (lung and chest wall).

Methods

Twenty-one intubated and mechanically ventilated patients (mean age 64.8 ± 14.0 years, body mass index 24.2 ± 4.3 kg/m², PaO₂/FiO₂ 319.4 ± 117.3 mmHg) were enrolled. In step 1, patients were sedated and paralyzed during volume-controlled ventilation, and in step 2, they were only sedated during pressure support ventilation. In each step, two esophageal balloon positions (middle and low, between 25–30 cm and 40–45 cm from the mouth) and two levels of PEEP (0 and 10 cmH₂O) were applied. The ∆Pes/∆Paw ratio and end-expiratory esophageal pressure were evaluated.

Results

The ∆Pes/∆Paw ratio was slightly higher (+0.11) with positive occlusion test compared with Baydur’s test. The level of PEEP and the esophageal balloon position did not affect this ratio. The ∆Pes and ∆Paw were significantly related to a correlation coefficient of r = 0.984 during the Baydur test and r = 0.909 in the positive occlusion test. End-expiratory esophageal pressure was significantly higher in sedated and paralyzed patients compared with sedated patients (+2.47 cmH₂O) and when esophageal balloon was positioned in the low position (+2.26 cmH₂O). The esophageal balloon position slightly influenced the lung elastance, while the PEEP reduced the chest wall elastance without affecting the lung and total respiratory system elastance.

Conclusions

Paralysis and balloon position did not clinically affect the measurement of the ∆Pes/∆Paw ratio, while they significantly increased the end-expiratory esophageal pressure.

In vivo thorax 3D modelling from costovertebral joint complex kinematics

BACKGROUND

The costovertebral joint complex is mechanically involved in both respiratory function and thoracic spine stability. The thorax has been studied for a long time to understand its involvement in the physiological mechanism leading to specific gas exchange. Few studies have focused on costovertebral joint complex kinematics, and most of them focused on experimental in vitro analysis related to loading tests or global thorax and/or lung volume change analysis. There is however a clinical need for new methods allowing to process in vivo clinical data. This paper presents results from in vivo analysis of the costovertebral joint complex kinematics from clinically-available retrospective data.

METHODS

In this study, in vivo spiral computed tomography imaging data were obtained from 8 asymptomatic subjects at three different lung volumes (from total lung capacity to functional residual capacity) calibrated using a classical spirometer. Fusion methods including 3D modelling and kinematic analysis were used to provide 3D costovertebral joint complex visualization for the true ribs (i.e., first seven pairs of ribs).

FINDINGS

The 3D models of the first seven pairs of costovertebral joint complexes were obtained. A continuous kinematics simulation was interpolated from the three discrete computerized tomography positions. Helical axis representation was also achieved.

INTERPRETATION

Preliminary results show that the method leads to meaningful and relevant results for clinical and pedagogical applications. Research in progress compares data from a sample of healthy volunteers with data collected from patients with cystic fibrosis to obtain new insights about the costovertebral joint complex range of motion and helical axis assessment in different pathological conditions.

Improving lung compliance by external compression of the chest wall

As exemplified by prone positioning, regional variations of lung and chest wall properties provide possibilities for modifying transpulmonary pressures and suggest that clinical interventions related to the judicious application of external pressure may yield benefit. Recent observations made in late-phase patients with severe ARDS caused by COVID-19 (C-ARDS) have revealed unexpected mechanical responses to local chest wall compressions over the sternum and abdomen in the supine position that challenge the clinician's assumptions and conventional bedside approaches to lung protection. These findings appear to open avenues for mechanism-defining research investigation with possible therapeutic implications for all forms and stages of ARDS.

Longitudinal changes in compliance, oxygenation and ventilatory ratio in COVID-19 versus non-COVID-19 pulmonary acute respiratory distress syndrome

Background

Differences in physiology of ARDS have been described between COVID-19 and non-COVID-19 patients. This study aimed to compare initial values and longitudinal changes in respiratory system compliance (C_RS), oxygenation parameters and ventilatory ratio (VR) in patients with COVID-19 and non-COVID-19 pulmonary ARDS matched on oxygenation.

Methods

135 patients with COVID-19 ARDS from two centers were included in a physiological study; 767 non-COVID-19 ARDS from a clinical trial were used for the purpose of at least 1:2 matching. A propensity-matching was based on age, severity score, oxygenation, positive end-expiratory pressure (PEEP) and pulmonary cause of ARDS and allowed to include 112 COVID-19 and 198 non-COVID pulmonary ARDS.

Results

The two groups were similar on initial oxygenation. COVID-19 patients had a higher body mass index, higher C_RS at day 1 (median [IQR], 35 [28–44] vs 32 [26–38] ml cmH₂O⁻¹, p = 0.037). At day 1, C_RS was correlated with oxygenation only in non-COVID-19 patients; 61.6% and 68.2% of COVID-19 and non-COVID-19 pulmonary ARDS were still ventilated at day 7 (p = 0.241). Oxygenation became lower in COVID-19 than in non-COVID-19 patients at days 3 and 7, while C_RS became similar. VR was lower at day 1 in COVID-19 than in non-COVID-19 patients but increased from day 1 to 7 only in COVID-19 patients. VR was higher at days 1, 3 and 7 in the COVID-19 patients ventilated using heat and moisture exchangers compared to heated humidifiers. After adjustment on PaO₂/FiO₂, PEEP and humidification device, C_RS and VR were found not different between COVID-19 and non-COVID-19 patients at day 7. Day-28 mortality did not differ between COVID-19 and non-COVID-19 patients (25.9% and 23.7%, respectively, p = 0.666).

Conclusions

For a similar initial oxygenation, COVID-19 ARDS initially differs from classical ARDS by a higher C_RS, dissociated from oxygenation. C_RS become similar for patients remaining on mechanical ventilation during the first week of evolution, but oxygenation becomes lower in COVID-19 patients.

Trial registration: clinicaltrials.gov NCT04385004

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-021-03665-8.

N-acetyl cysteine improves the effects of corticosteroids in a mouse model of chlorine-induced acute lung injury

Chlorine (Cl2) causes tissue damage and a neutrophilic inflammatory response in the airways manifested by pronounced airway hyperreactivity (AHR). The importance of early anti-inflammatory treatment has previously been addressed. In the previous study, both high-dose and low-dose of dexamethasone (DEX) decreased the risk of developing delayed effects, such as persistent lung injuries, while only high-dose treatment could significantly counteract acute-phase effects. One aim of this study was to evaluate whether a low-dose of DEX in combination with the antioxidant N-acetyl cysteine (NAC) and if different treatments (Triptolide, Reparixin and Rolipram) administered 1h after Cl2-exposure could improve protection against acute lung injury in Cl2-exposed mice. BALB/c mice were exposed to 300 ppm Cl2 during 15 min. Assessment of AHR and inflammatory cells in bronchoalveolar lavage was analyzed 24h post exposure. Neither of DEX nor NAC reduced the AHR and displayed only minor effects on inflammatory cell influx when given as separate treatments. When given in combination, a protective effect on AHR and a significant reduction in inflammatory cells (neutrophils) was observed. Neither of triptolide, Reparixin nor Rolipram had an effect on AHR but Triptolide had major effect on the inflammatory cell influx. Treatments did not reduce the concentration of either fibrinogen or plasminogen activator inhibitor-1 in serum, thereby supporting the theory that the inflammatory response is not solely limited to the lung. These results provide a foundation for future studies aimed at identifying new concepts for treatment of chemical-induced lung injury. Studies addressing combination of anti-inflammatory and antioxidant treatment are highly motivated.

Diaphragmatic shear modulus at various submaximal inspiratory mouth pressure levels

This study assessed the shear modulus of the diaphragm at various submaximal inspiratory mouth pressure levels by ultrasound shear wave elastography. In 14 healthy male subjects, diaphragmatic shear moduli were assessed at end expiration during resting breathing and at submaximal inspiratory tasks at 15, 30, 45, 60, and 75% of the maximal inspiratory mouth pressure. The shear modulus increased along with mouth pressure, and the mouth-pressure-shear-modulus relationship fit the second-order polynomial regression equation (r² = 0.99 ± 0.01; all subjects, r² ≥ 0.95) better than it did the simple linear regression equation (r² = 0.94 ± 0.05; 8/14 subjects, r² ≥ 0.95). The second regression coefficient in the second-order polynomial equation was a negative value in 10 of 14 subjects, which indicates that the second-order polynomial regression equation opened downwards. These findings suggest that the diaphragmatic shear modulus increases along with inspiratory mouth pressure, but the rate of increase slows when the pressure reaches higher levels.

Predictors of asynchronies during assisted ventilation and its impact on clinical outcomes: The EPISYNC cohort study

PURPOSE

To investigate if respiratory mechanics and other baseline characteristics are predictors of patient-ventilator asynchrony and to evaluate the relationship between asynchrony during assisted ventilation and clinical outcomes.

METHODS

We performed a prospective cohort study in patients under mechanical ventilation (MV). Baseline measurements included severity of illness and respiratory mechanics. The primary outcome was the Asynchrony Index (AI), defined as the number of asynchronous events divided by the number of ventilator cycles and wasted efforts. We recorded ventilator waveforms throughout the entire period of MV.

RESULTS

We analyzed 11,881 h of MV from 103 subjects. Median AI during the entire period of MV was 5.1% (IQR:2.6-8.7). Intrinsic PEEP was associated with AI (OR:1.72, 95%CI:1.1-2.68), but static compliance and airway resistance were not. Simplified Acute Physiology Score 3 (OR:1.03, 95%CI:1-1.06) was also associated with AI. Median AI was higher during assisted (5.4%, IQR:2.9-9.1) than controlled (2%, IQR:0.6-4.9) ventilation, and 22% of subjects had high incidence of asynchrony (AI≥10%). Subjects with AI≥10% had more extubation failure (33%) than patients with AI<10% (6%), p = .01.

CONCLUSIONS

Predictors of high incidence of asynchrony were severity of illness and intrinsic PEEP. High incidence of asynchrony was associated with extubation failure, but not mortality.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT02687802.

The effect on respiratory mechanics when using a Jackson surgical table in the prone position during spinal surgery

Background

Respiratory dynamics may be monitored and evaluated indirectly by measuring the peak inspiratory pressure and plateau pressure. In this study, the respiratory dynamics of patients undergoing spinal surgery using a Jackson surgical table were observed with a device after converting their position from supine to prone. The effects of the dynamic compliance and airway resistance were observed from the changes in peak inspiratory pressure and plateau.

Methods

Twenty five patients were selected as subjects scheduled to undergo lumbar spine surgery. After intubation, the patients were ventilated mechanically with a tidal volume of 10 ml/kg and a respiration rate of 10/min. Anesthesia was maintained with sevoflurane 1.5%, nitrous oxide 2 L/min and oxygen 2 L/min. The peak inspiratory pressure, plateau pressure, resistance, compliance, arterial oxygen tension, carbon dioxide tension, heart rate and arterial blood pressure were measured at 10 minutes after the induction of anesthesia. These parameters were measured again 10 minutes after placing the patient in the prone position.

Results

The prone position did not significantly affect the arterial oxygen tension, carbon dioxide tension, blood pressure and heart rate, but significantly increased the peak inspiratory pressure and resistance and decreased the dynamic compliance.

Conclusions

The peak inspiratory pressure was increased using a Jackson surgical table to minimize the abdominal pressure when converting from the supine to prone position. This might be due to a decrease in lung and chest compliance as well as an increase in airway resistance.

Noninvasive ventilation during weaning from prolonged mechanical ventilation

BACKGROUND AND OBJECTIVE

Non invasive ventilation (NIV) is currently employed for weaning from invasive ventilation (IMV) in the acute setting but its use for weaning from prolonged ventilation is still occasional and not standardized. We wanted to evaluate whether a combined protocol of NIV and decannulation in tracheostomized patients needing prolonged mechanical ventilation was feasible and what would be the one-year outcome.

METHODS

We studied patients still dependent from invasive mechanical ventilation with the following inclusion criteria: a) tolerance of at least 8h of unsupported breathing, b) progressive hypercapnia/acidosis after invasive ventilation discontinuation, c) good adaptation to NIV, d) favorable criteria for decannulation. These patients were switched from IMV to NIV and decannulated; then they were discharged on home NIV and followed-up for one year in order to evaluate survival and complications rate.

RESULTS

Data from patients consecutively admitted to a weaning unit were prospectively collected between 2005 and 2018. Out of 587 patients admitted over that period, 341 were liberated from prolonged mechanical ventilation. Fifty-one out of 147 unweaned patients (35%) were eligible for the protocol but only 46 were enrolled. After a mean length of stay of 35 days they were decannulated and discharged on domiciliary NIV. After one year, 38 patients were still alive (survival rate 82%) and 37 were using NIV with good adherence (only one patient was switched again to invasive ventilation).

CONCLUSIONS

NIV applied to patients with failed weaning from prolonged IMV is feasible and can facilitate the decannulation process. Patients successfully completing this process show good survival rates and few complications.

Latent class analysis to predict intensive care outcomes in Acute Respiratory Distress Syndrome: a proposal of two pulmonary phenotypes

Background

Acute respiratory distress syndrome remains a heterogeneous syndrome for clinicians and researchers difficulting successful tailoring of interventions and trials. To this moment, phenotyping of this syndrome has been approached by means of inflammatory laboratory panels. Nevertheless, the systemic and inflammatory expression of acute respiratory distress syndrome might not reflect its respiratory mechanics and gas exchange.

Methods

Retrospective analysis of a prospective cohort of two hundred thirty-eight patients consecutively admitted patients under mechanical ventilation presenting with acute respiratory distress syndrome. All patients received standardized monitoring of clinical variables, respiratory mechanics and computed tomography scans at predefined PEEP levels. Employing latent class analysis, an unsupervised structural equation modelling method, on respiratory mechanics, gas-exchange and computed tomography-derived gas- and tissue-volumes at a PEEP level of 5cmH₂O, distinct pulmonary phenotypes of acute respiratory distress syndrome were identified.

Results

Latent class analysis was applied to 54 respiratory mechanics, gas-exchange and CT-derived gas- and tissue-volume variables, and a two-class model identified as best fitting. Phenotype 1 (non-recruitable) presented lower respiratory system elastance, alveolar dead space and amount of potentially recruitable lung volume than phenotype 2 (recruitable). Phenotype 2 (recruitable) responded with an increase in ventilated lung tissue, compliance and PaO₂/FiO₂ ratio (p < 0.001), in addition to a decrease in alveolar dead space (p < 0.001), to a standardized recruitment manoeuvre. Patients belonging to phenotype 2 (recruitable) presented a higher intensive care mortality (hazard ratio 2.9, 95% confidence interval 1.7–2.7, p = 0.001).

Conclusions

The present study identifies two ARDS phenotypes based on respiratory mechanics, gas-exchange and computed tomography-derived gas- and tissue-volumes. These phenotypes are characterized by distinctly diverse responses to a standardized recruitment manoeuvre and by a diverging mortality. Given multicentre validation, the simple and rapid identification of these pulmonary phenotypes could facilitate enrichment of future prospective clinical trials addressing mechanical ventilation strategies in ARDS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-021-03578-6.

Respiratory mechanics in infants with severe bronchiolitis on controlled mechanical ventilation

Background

Analysis of respiratory mechanics during mechanical ventilation (MV) is able to estimate resistive, elastic and inertial components of the working pressure of the respiratory system. Our aim was to discriminate the components of the working pressure of the respiratory system in infants on MV with severe bronchiolitis admitted to two PICU’s.

Methods

Infants younger than 1 year old with acute respiratory failure caused by severe bronchiolitis underwent neuromuscular blockade, tracheal intubation and volume controlled MV. Shortly after intubation studies of pulmonary mechanics were performed using inspiratory and expiratory breath hold. The maximum inspiratory and expiratory flow (QI and QE) as well as peak inspiratory (PIP), plateau (PPL) and total expiratory pressures (tPEEP) were measured. Inspiratory and expiratory resistances (RawI and RawE) and Time Constants (K_TI and K_TE) were calculated.

Results

We included 16 patients, of median age 2.5 (1–5.8) months. Bronchiolitis due to respiratory syncytial virus was the main etiology (93.8%) and 31.3% had comorbidities. Measured respiratory pressures were PIP 29 (26–31), PPL 24 (20–26), tPEEP 9 [8–11] cmH2O. Elastic component of the working pressure was significantly higher than resistive and both higher than threshold (tPEEP – PEEP) (P < 0.01). QI was significantly lower than QE [5 (4.27–6.75) v/s 16.5 (12–23.8) L/min. RawI and RawE were 38.8 (32–53) and 40.5 (22–55) cmH2O/L/s; K_TI and K_TE [0.18 (0.12–0.30) v/s 0.18 (0.13–0.22) s], and K_TI:K_TE ratio was 1:1.04 (1:0.59–1.42).

Conclusions

Analysis of respiratory mechanics of infants with severe bronchiolitis receiving MV shows that the elastic component of the working pressure of the respiratory system is the most important. The elastic and resistive components in conjunction with flow profile are characteristic of restrictive diseases. A better understanding of lung mechanics in this group of patients may lead to change the traditional ventilatory approach to severe bronchiolitis.

Electronic supplementary material

The online version of this article (10.1186/s12890-017-0475-6) contains supplementary material, which is available to authorized users.

Respiratory mechanics in patients with acute respiratory distress syndrome

Despite the recognition of its iatrogenic potential, mechanical ventilation remains the mainstay of respiratory support for patients with acute respiratory distress syndrome (ARDS). The low volume ventilation has been recognized as the only method to reduce mortality of ARDS patients and plateau pressure as the lighthouse for delivering safe ventilation. Recent investigations suggest that a ventilation based on lung mechanics (tidal ventilation tailored to the available lung volume able to receive it, i.e., driving pressure) is a successful approach to improve outcome. However, currently available bedside mechanical variables do not consider regional mechanical properties of ARDS affected lungs, which include the role of local stress risers at the boundaries of areas with different aeration. A unifying approach considers lung-related causes and ventilation-related causes of lung injury. These last may be incorporated in the mechanical power (i.e., amount of mechanical energy transferred per unit of time). Ventilation-induced lung injury (which includes the self-inflicted lung injury of a spontaneously breathing patient) can therefore be prevented by the adoption of measures promoting an increase of ventilable lung and its homogeneity and by delivering lower levels of mechanical power. Prone position promotes lung homogeneity without increasing the delivered mechanical power. This review describes the recent developments on respiratory mechanics in ARDS patients, providing both bedside and research insights from the most updated evidence.