Effects of mechanical ventilation on the interstitial extracellular matrix in healthy lungs and lungs affected by acute respiratory distress syndrome: a narrative review

BACKGROUND

Mechanical ventilation, a lifesaving intervention in critical care, can lead to damage in the extracellular matrix (ECM), triggering inflammation and ventilator-induced lung injury (VILI), particularly in conditions such as acute respiratory distress syndrome (ARDS). This review discusses the detailed structure of the ECM in healthy and ARDS-affected lungs under mechanical ventilation, aiming to bridge the gap between experimental insights and clinical practice by offering a thorough understanding of lung ECM organization and the dynamics of its alteration during mechanical ventilation.

MAIN TEXT

Focusing on the clinical implications, we explore the potential of precise interventions targeting the ECM and cellular signaling pathways to mitigate lung damage, reduce inflammation, and ultimately improve outcomes for critically ill patients. By analyzing a range of experimental studies and clinical papers, particular attention is paid to the roles of matrix metalloproteinases (MMPs), integrins, and other molecules in ECM damage and VILI. This synthesis not only sheds light on the structural changes induced by mechanical stress but also underscores the importance of cellular responses such as inflammation, fibrosis, and excessive activation of MMPs.

CONCLUSIONS

This review emphasizes the significance of mechanical cues transduced by integrins and their impact on cellular behavior during ventilation, offering insights into the complex interactions between mechanical ventilation, ECM damage, and cellular signaling. By understanding these mechanisms, healthcare professionals in critical care can anticipate the consequences of mechanical ventilation and use targeted strategies to prevent or minimize ECM damage, ultimately leading to better patient management and outcomes in critical care settings.

Stress & strain in mechanically nonuniform alveoli using clinical input variables: a simple conceptual model

Clinicians currently monitor pressure and volume at the airway opening, assuming that these observations relate closely to stresses and strains at the micro level. Indeed, this assumption forms the basis of current approaches to lung protective ventilation. Nonetheless, although the airway pressure applied under static conditions may be the same everywhere in healthy lungs, the stresses within a mechanically non-uniform ARDS lung are not. Estimating actual tissue stresses and strains that occur in a mechanically non-uniform environment must account for factors beyond the measurements from the ventilator circuit of airway pressures, tidal volume, and total mechanical power. A first conceptual step for the clinician to better define the VILI hazard requires consideration of lung unit tension, stress focusing, and intracycle power concentration. With reasonable approximations, better understanding of the value and limitations of presently used general guidelines for lung protection may eventually be developed from clinical inputs measured by the caregiver. The primary purpose of the present thought exercise is to extend our published model of a uniform, spherical lung unit to characterize the amplifications of stress (tension) and strain (area change) that occur under static conditions at interface boundaries between a sphere's surface segments having differing compliances. Together with measurable ventilating power, these are incorporated into our perspective of VILI risk. This conceptual exercise brings to light how variables that are seldom considered by the clinician but are both recognizable and measurable might help gauge the hazard for VILI of applied pressure and power.

The Respiratory Mechanics of COVID-19 Acute Respiratory Distress Syndrome-Lessons Learned?

Acute respiratory distress syndrome (ARDS) is a well-defined clinical entity characterized by the acute onset of diffuse pulmonary injury and hypoxemia not explained by fluid overload. The COVID-19 pandemic brought about an unprecedented volume of patients with ARDS and challenged our understanding and clinical approach to treatment of this clinical syndrome. Unique to COVID-19 ARDS is the disruption and dysregulation of the pulmonary vascular compartment caused by the SARS-CoV-2 virus, which is a significant cause of hypoxemia in these patients. As a result, gas exchange does not necessarily correlate with respiratory system compliance and mechanics in COVID-19 ARDS as it does with other etiologies. The purpose of this review is to relate the mechanics of COVID-19 ARDS to its underlying pathophysiologic mechanisms and outline the lessons we have learned in the management of this clinic syndrome.

The place of positive end expiratory pressure in ventilator-induced lung injury generation

PURPOSE OF REVIEW

Describe the rationale for concern and accumulating pathophysiologic evidence regarding the adverse effects of high-level positive end expiratory pressure (PEEP) on excessive mechanical stress and ventilator-induced lung injury (VILI).

RECENT FINDINGS

Although the inclusion of PEEP in numerical estimates of mechanical power may be theoretically debated, its potential to increase stress, strain, and mean airway pressure are not. Recent laboratory data in a variety of animal models demonstrate that higher levels of PEEP coupled with additional fluids needed to offset its impediment of hemodynamic function are associated with increased VILI. Moreover, counteracting end-tidal hyperinflation by external chest wall pressure may paradoxically improve respiratory mechanics, indicating that lower PEEP helps protect the small 'baby lung' of advanced acute respiratory distress syndrome (ARDS).

SUMMARY

The potentially adverse effects of PEEP on VILI can be considered in three broad categories. First, the contribution of PEEP to total mechanical energy expressed through mechanical power, raised mean airway pressure, and end-tidal hyperinflation; second, the hemodynamic consequences of altered cardiac loading, heightened pulmonary vascular stress and total lung water; and third, the ventilatory consequences of compromised carbon dioxide eliminating efficiency. Minimizing ventilation demands, optimized body positioning and care to avoid unnecessary PEEP are central to lung protection in all stages of ARDS.

Optimized ventilation power to avoid VILI

The effort to minimize VILI risk must be multi-pronged. The need to adequately ventilate, a key determinant of hazardous power, is reduced by judicious permissive hypercapnia, reduction of innate oxygen demand, and by prone body positioning that promotes both efficient pulmonary gas exchange and homogenous distributions of local stress. Modifiable ventilator-related determinants of lung protection include reductions of tidal volume, plateau pressure, driving pressure, PEEP, inspiratory flow amplitude and profile (using longer inspiration to expiration ratios), and ventilation frequency. Underappreciated conditional cofactors of importance to modulate the impact of local specific power may include lower vascular pressures and blood flows. Employed together, these measures modulate ventilation power with the intent to avoid VILI while achieving clinically acceptable targets for pulmonary gas exchange.

From pressure to tension: a model of damaging inflation stress

Although the stretch that generates ventilator-induced lung injury (VILI) occurs within the peripheral tissue that encloses the alveolar space, airway pressures and volumes monitor the gas within the interior core of the lung unit, not its cellular enclosure. Measured pressures (plateau pressure, positive end-expiratory pressure, and driving pressure) and tidal volumes paint a highly relevant but incomplete picture of forces that act on the lung tissues themselves. Convenient and clinically useful measures of the airspace, such as pressure and volume, neglect the partitioning of tidal elastic energy into the increments of tension and surface area that constitute actual stress and strain at the alveolar margins. More sharply focused determinants of VILI require estimates of absolute alveolar dimension and morphology and the lung's unstressed volume at rest. We present a highly simplified but informative mathematical model that translates the radial energy of pressure and volume of the airspace into its surface energy components. In doing so it elaborates conceptual relationships that highlight the forces tending to cause end-tidal hyperinflation of aerated units within the 'baby lung' of acute respiratory distress syndrome (ARDS).

High- versus Low-Flow Extracorporeal Respiratory Support in Experimental Hypoxemic Acute Lung Injury

Rationale: In the EOLIA (ECMO to Rescue Lung Injury in Severe ARDS) trial, oxygenation was similar between intervention and conventional groups, whereas [Formula: see text]e was reduced in the intervention group. Comparable reductions in ventilation intensity are theoretically possible with low-flow extracorporeal CO2 removal (ECCO2R), provided oxygenation remains acceptable. Objectives: To compare the effects of ECCO2R and extracorporeal membrane oxygenation (ECMO) on gas exchange, respiratory mechanics, and hemodynamics in animal models of pulmonary (intratracheal hydrochloric acid) and extrapulmonary (intravenous oleic acid) lung injury. Methods: Twenty-four pigs with moderate to severe hypoxemia (PaO2:FiO2 ⩽ 150 mm Hg) were randomized to ECMO (blood flow 50-60 ml/kg/min), ECCO2R (0.4 L/min), or mechanical ventilation alone. Measurements and Main Results: [Formula: see text]o2, [Formula: see text]co2, gas exchange, hemodynamics, and respiratory mechanics were measured and are presented as 24-hour averages. Oleic acid versus hydrochloric acid showed higher extravascular lung water (1,424 ± 419 vs. 574 ± 195 ml; P < 0.001), worse oxygenation (PaO2:FiO2 = 125 ± 14 vs. 151 ± 11 mm Hg; P < 0.001), but better respiratory mechanics (plateau pressure 27 ± 4 vs. 30 ± 3 cm H2O; P = 0.017). Both models led to acute severe pulmonary hypertension. In both models, ECMO (3.7 ± 0.5 L/min), compared with ECCO2R (0.4 L/min), increased mixed venous oxygen saturation and oxygenation, and improved hemodynamics (cardiac output = 6.0 ± 1.4 vs. 5.2 ± 1.4 L/min; P = 0.003). [Formula: see text]o2 and [Formula: see text]co2, irrespective of lung injury model, were lower during ECMO, resulting in lower PaCO2 and [Formula: see text]e but worse respiratory elastance compared with ECCO2R (64 ± 27 vs. 40 ± 8 cm H2O/L; P < 0.001). Conclusions: ECMO was associated with better oxygenation, lower [Formula: see text]o2, and better hemodynamics. ECCO2R may offer a potential alternative to ECMO, but there are concerns regarding its effects on hemodynamics and pulmonary hypertension.

Metabolic Support in Acute Respiratory Distress Syndrome: A Narrative Review

Nutritional support for acute respiratory distress syndrome (ARDS) patients shares metabolic notions common to other critically ill conditions. Nevertheless, it generates specific concern regarding the primary limitation of oxygen supply and the complications of carbon dioxide elimination, as well as the significant metabolic alterations due to the body's response to illness. In the present narrative review, after briefly summarizing the pathophysiology of critical illness stress response and patients' metabolic requirements, we focus on describing the characteristics of metabolic and artificial nutrition in patients with acute respiratory failure. In patients with ARDS, several aspects of metabolism assume special importance. The physiological effects of substrate metabolism are described for this setting, particularly regarding energy consumption, diet-induced thermogenesis, and the price of their clearance, transformation, and storage. Moreover, we review the possible direct effects of macronutrients on lung tissue viability during ARDS. Finally, we summarize the noteworthy characteristics of metabolic control in critically ill patients with ARDS and offer a suggestion as to the ideal methods of metabolic support for this problem.

Practical assessment of risk of VILI from ventilating power: a conceptual model

At the bedside, assessing the risk of ventilator-induced lung injury (VILI) requires parameters readily measured by the clinician. For this purpose, driving pressure (DP) and end-inspiratory static 'plateau' pressure ([Formula: see text]) of the tidal cycle are unquestionably useful but lack key information relating to associated volume changes and cumulative strain. 'Mechanical power', a clinical term which incorporates all dissipated ('non-elastic') and conserved ('elastic') energy components of inflation, has drawn considerable interest as a comprehensive 'umbrella' variable that accounts for the influence of ventilating frequency per minute as well as the energy cost per tidal cycle. Yet, like the raw values of DP and [Formula: see text], the absolute levels of energy and power by themselves may not carry sufficiently precise information to guide safe ventilatory practice. In previous work we introduced the concept of 'damaging energy per cycle'. Here we describe how-if only in concept-the bedside clinician might gauge the theoretical hazard of delivered energy using easily observed static circuit pressures ([Formula: see text] and positive end expiratory pressure) and an estimate of the maximally tolerated (threshold) non-dissipated ('elastic') airway pressure that reflects the pressure component applied to the alveolar tissues. Because its core inputs are already in use and familiar in daily practice, the simplified mathematical model we propose here for damaging energy and power may promote deeper comprehension of the key factors in play to improve lung protective ventilation.

Ventilatory ratio, dead space, and venous admixture in patients with acute respiratory distress syndrome

BACKGROUND

Ventilatory ratio (VR) has been proposed as an alternative approach to estimate physiological dead space. However, the absolute value of VR, at constant dead space, might be affected by venous admixture and CO₂ volume expired per minute (VCO₂).

METHODS

This was a retrospective, observational study of mechanically ventilated patients with acute respiratory distress syndrome (ARDS) in the UK and Italy. Venous admixture was either directly measured or estimated using the surrogate measure Pa_O2/FiO₂ ratio. VCO₂ was estimated through the resting energy expenditure derived from the Harris-Benedict formula.

RESULTS

A total of 641 mechanically ventilated patients with mild (n=65), moderate (n=363), or severe (n=213) ARDS were studied. Venous admixture was measured (n=153 patients) or estimated using the Pa_O2/FiO₂ ratio (n=448). The VR increased exponentially as a function of the dead space, and the absolute values of this relationship were a function of VCO₂. At a physiological dead space of 0.6, VR was 1.1, 1.4, and 1.7 in patients with VCO₂ equal to 200, 250, and 300, respectively. VR was independently associated with mortality (odds ratio [OR]=2.5; 95% confidence interval [CI], 1.8-3.5), but was not associated when adjusted for V_D/V_Tphys, VCO₂, Pa_O2/FiO₂ (OR_adj=1.2; 95% CI, 0.7-2.1). These three variables remained independent predictors of ICU mortality (V_D/V_Tphys [OR_adj=17.9; 95% CI, 1.8-185; P<0.05]; VCO₂ [OR_adj=0.99; 95% CI, 0.99-1.00; P<0.001]; and Pa_O2/FiO₂ (OR_adj=0.99; 95% CI, 0.99-1.00; P<0.001]).

CONCLUSIONS

VR is a useful aggregate variable associated with outcome, but variables not associated with ventilation (VCO₂ and venous admixture) strongly contribute to the high values of VR seen in patients with severe illness.

Paradoxical response to chest wall loading predicts a favorable mechanical response to reduction in tidal volume or PEEP

Background

Chest wall loading has been shown to paradoxically improve respiratory system compliance (CRS) in patients with moderate to severe acute respiratory distress syndrome (ARDS). The most likely, albeit unconfirmed, mechanism is relief of end-tidal overdistension in ‘baby lungs’ of low-capacity. The purpose of this study was to define how small changes of tidal volume (VT) and positive end-expiratory pressure (PEEP) affect CRS (and its associated airway pressures) in patients with ARDS who demonstrate a paradoxical response to chest wall loading. We hypothesized that small reductions of VT or PEEP would alleviate overdistension and favorably affect CRS and conversely, that small increases of VT or PEEP would worsen CRS.

Methods

Prospective, multi-center physiologic study of seventeen patients with moderate to severe ARDS who demonstrated paradoxical responses to chest wall loading. All patients received mechanical ventilation in volume control mode and were passively ventilated. Airway pressures were measured before and after decreasing/increasing VT by 1 ml/kg predicted body weight and decreasing/increasing PEEP by 2.5 cmH2O.

Results

Decreasing either VT or PEEP improved CRS in all patients. Driving pressure (DP) decreased by a mean of 4.9 cmH2O (supine) and by 4.3 cmH2O (prone) after decreasing VT, and by a mean of 2.9 cmH2O (supine) and 2.2 cmH2O (prone) after decreasing PEEP. CRS increased by a mean of 3.1 ml/cmH2O (supine) and by 2.5 ml/cmH2O (prone) after decreasing VT. CRS increased by a mean of 5.2 ml/cmH2O (supine) and 3.6 ml/cmH2O (prone) after decreasing PEEP (P < 0.01 for all). Small increments of either VT or PEEP worsened CRS in the majority of patients.

Conclusion

Patients with a paradoxical response to chest wall loading demonstrate uniform improvement in both DP and CRS following a reduction in either VT or PEEP, findings in keeping with prior evidence suggesting its presence is a sign of end-tidal overdistension. The presence of ‘paradox’ should prompt re-evaluation of modifiable determinants of end-tidal overdistension, including VT, PEEP, and body position.

Chest wall loading in the ICU: pushes, weights, and positions

Clinicians monitor mechanical ventilatory support using airway pressures—primarily the plateau and driving pressure, which are considered by many to determine the safety of the applied tidal volume. These airway pressures are influenced not only by the ventilator prescription, but also by the mechanical properties of the respiratory system, which consists of the series-coupled lung and chest wall. Actively limiting chest wall expansion through external compression of the rib cage or abdomen is seldom performed in the ICU. Recent literature describing the respiratory mechanics of patients with late-stage, unresolving, ARDS, however, has raised awareness of the potential diagnostic (and perhaps therapeutic) value of this unfamiliar and somewhat counterintuitive practice. In these patients, interventions that reduce resting lung volume, such as loading the chest wall through application of external weights or manual pressure, or placing the torso in a more horizontal position, have unexpectedly improved tidal compliance of the lung and integrated respiratory system by reducing previously undetected end-tidal hyperinflation. In this interpretive review, we first describe underappreciated lung and chest wall interactions that are clinically relevant to both normal individuals and to the acutely ill who receive ventilatory support. We then apply these physiologic principles, in addition to published clinical observation, to illustrate the utility of chest wall modification for the purposes of detecting end-tidal hyperinflation in everyday practice.

Energy dissipation during expiration and Ventilator Induced Lung Injury: an experimental animal study

The amount of energy delivered to the respiratory system is recognized as a cause of Ventilator Induced Lung Injury (VILI). How energy dissipation within the lung causes damage is still a matter of debate. Expiratory flow control has been proposed as a strategy to reduce the energy dissipated into the respiratory system during expiration and, possibly, VILI. We studied 22 healthy pigs (29±2 kg), which were randomized into a control (n=11) and a valve group (n=11), where the expiratory flow was controlled through a variable resistor. Both groups were ventilated with the same tidal volume, PEEP and inspiratory flow. Electric impedance tomography was continuously acquired. At completion, lung weight, wet to dry ratios and histology were evaluated. The total mechanical power was similar in the control and valve groups (8.54±0.83 J min^-1 and 8.42±0.54 J min^-1 respectively, p=0.552). The total energy dissipated within the whole system (circuit + respiratory system) was remarkably different (4.34±0.66 vs 2.62±0.31 J/min, p<0.001). However, most of this energy was dissipated across the endotracheal tube (2.87±0.3 vs 1.88±0.2 J/min, p<0.001). The amount dissipated into the respiratory system averaged 1.45±0.5 in controls vs 0.73±0.16 J min^-1 in the valve group, p<0.001. Although respiratory mechanics, gas exchange, hemodynamics, wet to dry ratios and histology were similar in the two groups, the decrease of end-expiratory lung impedance was significantly greater in the control group (p=0.02). We conclude that with our experimental conditions, the reduction of energy dissipated in the respiratory system did not lead to appreciable differences in VILI.

Paradoxical response to chest wall loading predicts a favorable mechanical response to reduction in tidal volume or PEEP

BACKGROUND

Chest wall loading has been shown to paradoxically improve respiratory system compliance (C_RS) in patients with moderate to severe acute respiratory distress syndrome (ARDS). The most likely, albeit unconfirmed, mechanism is relief of end-tidal overdistension in 'baby lungs' of low-capacity. The purpose of this study was to define how small changes of tidal volume (V_T) and positive end-expiratory pressure (PEEP) affect C_RS (and its associated airway pressures) in patients with ARDS who demonstrate a paradoxical response to chest wall loading. We hypothesized that small reductions of V_T or PEEP would alleviate overdistension and favorably affect C_RS and conversely, that small increases of V_T or PEEP would worsen C_RS.

METHODS

Prospective, multi-center physiologic study of seventeen patients with moderate to severe ARDS who demonstrated paradoxical responses to chest wall loading. All patients received mechanical ventilation in volume control mode and were passively ventilated. Airway pressures were measured before and after decreasing/increasing V_T by 1 ml/kg predicted body weight and decreasing/increasing PEEP by 2.5 cmH₂O.

RESULTS

Decreasing either V_T or PEEP improved C_RS in all patients. Driving pressure (DP) decreased by a mean of 4.9 cmH₂O (supine) and by 4.3 cmH₂O (prone) after decreasing V_T, and by a mean of 2.9 cmH₂O (supine) and 2.2 cmH₂O (prone) after decreasing PEEP. C_RS increased by a mean of 3.1 ml/cmH₂O (supine) and by 2.5 ml/cmH₂O (prone) after decreasing V_T. C_RS increased by a mean of 5.2 ml/cmH₂O (supine) and 3.6 ml/cmH₂O (prone) after decreasing PEEP (P < 0.01 for all). Small increments of either V_T or PEEP worsened C_RS in the majority of patients.

CONCLUSION

Patients with a paradoxical response to chest wall loading demonstrate uniform improvement in both DP and C_RS following a reduction in either V_T or PEEP, findings in keeping with prior evidence suggesting its presence is a sign of end-tidal overdistension. The presence of 'paradox' should prompt re-evaluation of modifiable determinants of end-tidal overdistension, including V_T, PEEP, and body position.

Intracycle power distribution in a heterogeneous multi-compartmental mathematical model: possible links to strain and VILI

BACKGROUND

Repeated expenditure of energy and its generation of damaging strain are required to injure the lung by ventilation (VILI). Mathematical modeling of passively inflated, single-compartment lungs with uniform parameters for resistance and compliance indicates that standard clinical modes (flow patterns) differ impressively with respect to the timing and intensity of energy delivery-the intracycle power (ICP) that determines parenchymal stress and strain. Although measures of elastic ICP may accurately characterize instantaneous rates of global energy delivery, how the ICP component delivered to a compartment affects the VILI-linked variable of strain is determined by compartmental mechanics, compartmental size and mode of gas delivery. We extended our one-compartment model of ICP to a multi-compartment setting that varied those characteristics.

MAIN FINDINGS

The primary findings of this model/simulation indicate that: (1) the strain and strain rate experienced within a modeled compartment are nonlinear functions of delivered energy and power, respectively; (2) for a given combination of flow profile and tidal volume, resting compartmental volumes influence their resulting maximal strains in response to breath delivery; (3) flow profile is a key determinant of the maximal strain as well as maximal strain rate experienced within a multi-compartment lung. By implication, different clinician-selected flow profiles not only influence the timing of power delivery, but also spatially distribute the attendant strains of expansion among compartments with diverse mechanical properties. Importantly, the contours and magnitudes of the compartmental ICP, strain, and strain rate curves are not congruent; strain and strain rate do not necessarily follow the compartmental ICP, and the hierarchy of amplitudes among compartments for these variables may not coincide.

CONCLUSIONS

Different flow patterns impact how strain and strain rate develop as compartmental volume crests to its final value. Notably, as inflation proceeds, strain rate may rise or fall even as total strain, a monotonic function of volume, steadily (and predictably) rises. Which flow pattern serves best to minimize the maximal strain rate and VILI risk experienced within any sector, therefore, may strongly depend on the nature and heterogeneity of the mechanical properties of the injured lung.

End-Tidal to Arterial PCO2 Ratio as Guide to Weaning from Veno-Venous Extra-Corporeal Membrane Oxygenation

RATIONALE

Weaning from veno-venous extracorporeal membrane oxygenation (VV-ECMO) is based on oxygenation and not on carbon dioxide elimination.

OBJECTIVE

To predict readiness to wean from VV-ECMO Methods: In this multicenter study of mechanically ventilated adults with severe acute respiratory distress syndrome (ARDS) receiving VV-ECMO, we investigated a variable based on CO2 elimination. The study included a prospective interventional study of a physiological cohort (n=26), and a retrospective clinical cohort (n=638).

MEASUREMENTS AND MAIN RESULTS

Weaning failure in the clinical and physiological cohorts were respectively 37% and 42%. The main cause of failure in the physiological cohort was high inspiratory effort or respiratory rate. All patients exhaled similar amounts of CO2 but in patients who failed the weaning trial minute ventilation was higher to maintain the PaCO2 unchanged. The effort to eliminate one unit-volume of CO2, was double in failing patients [68·9 (42·4-123) vs. 39 (20·1-57) [cmH2O/(L/min)], p=0.007], owing to the higher physiological dead space [68 (58.73) % vs. 54 (41,.64) %; p=0.012]. PetCO2/PaCO2 ratio was a clinical variable strongly associated with weaning outcome at baseline was the, AUC: 0.87 (95%CI 0·71 - one). Similarly, the PetCO2/PaCO2 ratio was associated with weaning outcome in the "clinical cohort" both pre-weaning trial (OR 4·14; 95% CI 1·32 - 12·2; p=0·015), and at a sweep gas flow of zero (OR 13·1; 95% CI 4-44·4; p<0·001).

CONCLUSION

The primary reason for weaning failure from VV-ECMO is high effort to eliminate CO2. A higher PetCO2/PaCO2 ratio was associated with greater likelihood of weaning from VV-ECMO.

Mechanical power thresholds during mechanical ventilation: An experimental study

The extent of ventilator-induced lung injury may be related to the intensity of mechanical ventilation--expressed as mechanical power. In the present study, we investigated whether there is a safe threshold, below which lung damage is absent. Three groups of six healthy pigs (29.5 ± 2.5 kg) were ventilated prone for 48 h at mechanical power of 3, 7, or 12 J/min. Strain never exceeded 1.0. PEEP was set at 4 cmH2 O. Lung volumes were measured every 12 h; respiratory, hemodynamics, and gas exchange variables every 6. End-experiment histological findings were compared with a control group of eight pigs which did not undergo mechanical ventilation. Functional residual capacity decreased by 10.4% ± 10.6% and 8.1% ± 12.1% in the 7 J and 12 J groups (p = 0.017, p < 0.001) but not in the 3 J group (+1.7% ± 17.7%, p = 0.941). In 3 J group, lung elastance, PaO2 and PaCO2 were worse compared to 7 J and 12 J groups (all p < 0.001), due to lower ventilation-perfusion ratio (0.54 ± 0.13, 1.00 ± 0.25, 1.78 ± 0.36 respectively, p < 0.001). The lung weight was lower (p < 0.001) in the controls (6.56 ± 0.90 g/kg) compared to 3, 7, and 12 J groups (12.9 ± 3.0, 16.5 ± 2.9, and 15.0 ± 4.1 g/kg, respectively). The wet-to-dry ratio was 5.38 ± 0.26 in controls, 5.73 ± 0.52 in 3 J, 5.99 ± 0.38 in 7 J, and 6.13 ± 0.59 in 12 J group (p = 0.03). Vascular congestion was more extensive in the 7 J and 12 J compared to 3 J and control groups. Mechanical ventilation (with anesthesia/paralysis) increase lung weight, and worsen lung histology, regardless of the mechanical power. Ventilating at 3 J/min led to better anatomical variables than at 7 and 12 J/min but worsened the physiological values.

Static and Dynamic Measurements of Compliance and Driving Pressure: A Pilot Study

Rationale

Monitoring tidal cycle mechanics is key to lung protection. For this purpose, compliance and driving pressure of the respiratory system are often measured clinically using the plateau pressure, obtained after imposing an extended end-inspiratory pause, which allows for relaxation of the respiratory system and redistribution of inflation volume (method A). Alternative methods for estimating compliance and driving pressure utilize the measured pressure at the earliest instance of zero flow (method B), the inspiratory slope of the pressure-time tracing during inflation with constant flow (method C), and the expiratory time constant (method D).

Methods

Ten passive mechanically ventilated subjects, at a large tertiary referral center, underwent measurements of compliance and driving pressure using the four different methods. The inspiratory tidal volume, inspiratory to expiratory ratio, and positive end expiratory pressures were then adjusted from baseline and the measurements re-obtained.

Results

Method A yielded consistently higher compliance and lower driving pressure calculations compared to methods B and C. Methods B and C most closely approximated one another. Method D did not yield a consistent reliable pattern.

Conclusion

Static measurements of compliance and driving pressure using the plateau pressure may underestimate the maximum pressure experienced by the most vulnerable lung units during dynamic inflation. Utilizing the pressure at zero flow as a static measurement, or the inspiratory slope as a dynamic measurement, may calculate a truer estimate of the maximum alveolar pressure that generates stress upon compromised lung units.

A more gradual positive end-expiratory pressure increase reduces lung damage and improves cardiac function in experimental acute respiratory distress syndrome

Increases in positive end-expiratory pressure (PEEP) or recruitment maneuvers may increase stress in lung parenchyma, extracellular matrix, and lung vessels; however, adaptative responses may occur. We evaluated the effects of PEEP on lung damage and cardiac function when increased abruptly, gradually, or more gradually in experimental mild/moderate acute respiratory distress syndrome (ARDS) induced by Escherichia coli lipopolysaccharide intratracheally. After 24 h, Wistar rats (n = 48) were randomly assigned to four mechanical ventilation strategies according to PEEP levels: 1) 3 cmH2O for 2 h (control); 2) 3 cmH2O for 1 h followed by an abrupt increase to 9 cmH2O for 1 h (no adaptation time); 3) 3 cmH2O for 30 min followed by a gradual increase to 9 cmH2O over 30 min then kept constant for 1 h (shorter adaptation time); and 4) more gradual increase in PEEP from 3 cmH2O to 9 cmH2O over 1 h and kept constant thereafter (longer adaptation time). At the end of the experiment, oxygenation improved in the shorter and longer adaptation time groups compared with the no-adaptation and control groups. Diffuse alveolar damage and expressions of interleukin-6, club cell protein-16, vascular cell adhesion molecule-1, amphiregulin, decorin, and syndecan were higher in no adaptation time compared with other groups. Pulmonary arterial pressure was lower in longer adaptation time than in no adaptation (P = 0.002) and shorter adaptation time (P = 0.025) groups. In this model, gradually increasing PEEP limited lung damage and release of biomarkers associated with lung epithelial/endothelial cell and extracellular matrix damage, as well as the PEEP-associated increase in pulmonary arterial pressure.NEW & NOTEWORTHY In a rat model of Escherichia coli lipopolysaccharide-induced mild/moderate acute respiratory distress syndrome, a gradual PEEP increase (shorter adaptation time) effectively mitigated histological lung injury and biomarker release associated with lung inflammation, damage to epithelial cells, endothelial cells, and the extracellular matrix compared with an abrupt increase in PEEP. A more gradual PEEP increase (longer adaptation time) decreased lung damage, pulmonary vessel compression, and pulmonary arterial pressure.

Reliable Estimates of Power Delivery During Mechanical Ventilation Utilizing Easily Obtained Bedside Parameters

BACKGROUND

Ventilator-induced lung injury (VILI) requires repetitive transfer of energy from the ventilator to the compromised lung. To understand this phenomenon, 2 sets of equations have been developed to partition total inflation energy into harmless and hazardous components using an arbitrary level of alveolar pressure as a threshold beyond which further energy increments may become damaging. One set of equations uses premeasured resistance and compliance as inputs to predict the energy that would be delivered by typical ventilator settings, whereas the other equation set uses observed output values for end-inspiratory peak and plateau pressure of an already completed inflation.

METHODS

Our aim for this study was to compare the relative accuracy of these equation sets against the performance of a physical one-compartment model of the respiratory system, programmed with information readily available at the bedside and ventilated using both constant and decelerating flow profiles. Accordingly, equations of each set were compared against the corresponding energy areas measured by digital planimetry of pressure-volume curves for 76 ventilator and patient parameter combinations and over 500 power calculations.

RESULTS

With few exceptions, all equations strongly correlated with their corresponding measurements by planimetry.

CONCLUSIONS

This validation of threshold-partitioned energy equations suggests their potential utility for implementing practical strategies for VILI avoidance.

COVID-19 pneumonia: pathophysiology and management

Coronavirus disease 2019 (COVID-19) pneumonia is an evolving disease. We will focus on the development of its pathophysiologic characteristics over time, and how these time-related changes determine modifications in treatment. In the emergency department: the peculiar characteristic is the coexistence, in a significant fraction of patients, of severe hypoxaemia, near-normal lung computed tomography imaging, lung gas volume and respiratory mechanics. Despite high respiratory drive, dyspnoea and respiratory rate are often normal. The underlying mechanism is primarily altered lung perfusion. The anatomical prerequisites for PEEP (positive end-expiratory pressure) to work (lung oedema, atelectasis, and therefore recruitability) are lacking. In the high-dependency unit: the disease starts to worsen either because of its natural evolution or additional patient self-inflicted lung injury (P-SILI). Oedema and atelectasis may develop, increasing recruitability. Noninvasive supports are indicated if they result in a reversal of hypoxaemia and a decreased inspiratory effort. Otherwise, mechanical ventilation should be considered to avert P-SILI. In the intensive care unit: the primary characteristic of the advance of unresolved COVID-19 disease is a progressive shift from oedema or atelectasis to less reversible structural lung alterations to lung fibrosis. These later characteristics are associated with notable impairment of respiratory mechanics, increased arterial carbon dioxide tension (P_aCO2), decreased recruitability and lack of response to PEEP and prone positioning.

COVID-19 pneumonia cannot be correctly described, analysed and treated if the time-factor is not taken into accounthttps://bit.ly/3AOKxc4

Intracycle power and ventilation mode as potential contributors to ventilator-induced lung injury

Background

High rates of inflation energy delivery coupled with transpulmonary tidal pressures of sufficient magnitude may augment the risk of damage to vulnerable, stress-focused units within a mechanically heterogeneous lung. Apart from flow amplitude, the clinician-selected flow waveform, a relatively neglected dimension of inflation power, may distribute inflation energy of each inflation cycle non-uniformly among alveoli with different mechanical properties over the domains of time and space. In this initial step in modeling intracycle power distribution, our primary objective was to develop a mathematical model of global intracycle inflation power that uses clinician-measurable inputs to allow comparisons of instantaneous ICP profiles among the flow modes commonly encountered in clinical practice: constant, linearly decelerating, exponentially decelerating (pressure control), and spontaneous (sinusoidal).

Methods

We first tested the predictions of our mathematical model of passive inflation with the actual physical performance of a mechanical ventilator–lung system that simulated ventilation to three types of patients: normal, severe ARDS, and severe airflow obstruction. After verification, model predictions were then generated for 5000 ‘virtual ARDS patients’. Holding constant the tidal volume and inflation time between modes, the validated model then varied the flow profile and quantitated the resulting intensity and timing of potentially damaging ‘elastic’ energy and intracycle power (pressure–flow product) developed in response to random combinations of machine settings and severity levels for ARDS.

Results

Our modeling indicates that while the varied flow patterns ultimately deliver similar total amounts of alveolar energy during each breath, they differ profoundly regarding the potentially damaging pattern with which that energy distributes over time during inflation. Pressure control imposed relatively high maximal intracycle power.

Conclusions

Flow amplitude and waveform may be relatively neglected and modifiable determinants of VILI risk when ventilating ARDS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40635-021-00420-9.

Role of total lung stress on the progression of early COVID-19 pneumonia

Purpose

We investigated if the stress applied to the lung during non-invasive respiratory support may contribute to the coronavirus disease 2019 (COVID-19) progression.

Methods

Single-center, prospective, cohort study of 140 consecutive COVID-19 pneumonia patients treated in high-dependency unit with continuous positive airway pressure (n = 131) or non-invasive ventilation (n = 9). We measured quantitative lung computed tomography, esophageal pressure swings and total lung stress.

Results

Patients were divided in five subgroups based on their baseline PaO₂/FiO₂ (day 1): non-CARDS (median PaO₂/FiO₂ 361 mmHg, IQR [323–379]), mild (224 mmHg [211–249]), mild-moderate (173 mmHg [164–185]), moderate-severe (126 mmHg [114–138]) and severe (88 mmHg [86–99], p < 0.001). Each subgroup had similar median lung weight: 1215 g [1083–1294], 1153 [888–1321], 968 [858–1253], 1060 [869–1269], and 1127 [937–1193] (p = 0.37). They also had similar non-aerated tissue fraction: 10.4% [5.9–13.7], 9.6 [7.1–15.8], 9.4 [5.8–16.7], 8.4 [6.7–12.3] and 9.4 [5.9–13.8], respectively (p = 0.85).

Treatment failure of CPAP/NIV occurred in 34 patients (24.3%). Only three variables, at day one, distinguished patients with negative outcome: PaO₂/FiO₂ ratio (OR 0.99 [0.98–0.99], p = 0.02), esophageal pressure swing (OR 1.13 [1.01–1.27], p = 0.032) and total stress (OR 1.17 [1.06–1.31], p = 0.004). When these three variables were evaluated together in a multivariate logistic regression analysis, only the total stress was independently associated with negative outcome (OR 1.16 [1.01–1.33], p = 0.032).

Conclusions

In early COVID-19 pneumonia, hypoxemia is not linked to computed tomography (CT) pathoanatomy, differently from typical ARDS. High lung stress was independently associated with the failure of non-invasive respiratory support.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00134-021-06519-7.

The impact of fluid status and decremental PEEP strategy on cardiac function and lung and kidney damage in mild-moderate experimental acute respiratory distress syndrome

BACKGROUND

We evaluated the effects of abrupt versus gradual PEEP decrease, combined with standard versus high-volume fluid administration, on cardiac function, as well as lung and kidney damage in an established model of mild-moderate acute respiratory distress syndrome (ARDS).

METHODS

Wistar rats received endotoxin intratracheally. After 24 h, they were treated with Ringer's lactate at standard (10 mL/kg/h) or high (30 mL/kg/h) dose. For 30 min, all animals were mechanically ventilated with tidal volume = 6 mL/kg and PEEP = 9 cmH2O (to keep alveoli open), then randomized to undergo abrupt or gradual (0.2 cmH2O/min for 30 min) PEEP decrease from 9 to 3 cmH2O. Animals were then further ventilated for 10 min at PEEP = 3 cmH2O, euthanized, and their lungs and kidneys removed for molecular biology analysis.

RESULTS

At the end of the experiment, left and right ventricular end-diastolic areas were greater in animals treated with high compared to standard fluid administration, regardless of PEEP decrease rate. However, pulmonary arterial pressure, indicated by the pulmonary acceleration time (PAT)/pulmonary ejection time (PET) ratio, was higher in abrupt compared to gradual PEEP decrease, independent of fluid status. Animals treated with high fluids and abrupt PEEP decrease exhibited greater diffuse alveolar damage and higher expression of interleukin-6 (a pro-inflammatory marker) and vascular endothelial growth factor (a marker of endothelial cell damage) compared to the other groups. The combination of standard fluid administration and gradual PEEP decrease increased zonula occludens-1 expression, suggesting epithelial cell preservation. Expression of club cell-16 protein, an alveolar epithelial cell damage marker, was higher in abrupt compared to gradual PEEP decrease groups, regardless of fluid status. Acute kidney injury score and gene expression of kidney injury molecule-1 were higher in the high versus standard fluid administration groups, regardless of PEEP decrease rate.

CONCLUSION

In the ARDS model used herein, decreasing PEEP abruptly increased pulmonary arterial hypertension, independent of fluid status. The combination of abrupt PEEP decrease and high fluid administration led to greater lung and kidney damage. This information adds to the growing body of evidence that supports gradual transitioning of ventilatory patterns and warrants directing additional investigative effort into vascular and deflation issues that impact lung protection.

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.

Personalized mechanical ventilation in acute respiratory distress syndrome

A personalized mechanical ventilation approach for patients with adult respiratory distress syndrome (ARDS) based on lung physiology and morphology, ARDS etiology, lung imaging, and biological phenotypes may improve ventilation practice and outcome. However, additional research is warranted before personalized mechanical ventilation strategies can be applied at the bedside. Ventilatory parameters should be titrated based on close monitoring of targeted physiologic variables and individualized goals. Although low tidal volume (VT) is a standard of care, further individualization of VT may necessitate the evaluation of lung volume reserve (e.g., inspiratory capacity). Low driving pressures provide a target for clinicians to adjust VT and possibly to optimize positive end-expiratory pressure (PEEP), while maintaining plateau pressures below safety thresholds. Esophageal pressure monitoring allows estimation of transpulmonary pressure, but its use requires technical skill and correct physiologic interpretation for clinical application at the bedside. Mechanical power considers ventilatory parameters as a whole in the optimization of ventilation setting, but further studies are necessary to assess its clinical relevance. The identification of recruitability in patients with ARDS is essential to titrate and individualize PEEP. To define gas-exchange targets for individual patients, clinicians should consider issues related to oxygen transport and dead space. In this review, we discuss the rationale for personalized approaches to mechanical ventilation for patients with ARDS, the role of lung imaging, phenotype identification, physiologically based individualized approaches to ventilation, and a future research agenda.

Prevalence and outcome of silent hypoxemia in COVID-19

BACKGROUND

In the early stages of COVID-19 pneumonia, hypoxemia has been described in absence of dyspnea ("silent" or "happy" hypoxemia). Our aim was to report its prevalence and outcome in a series of hypoxemic patients upon Emergency Department admission.

METHODS

In this retrospective observational cohort study we enrolled a study population consisting of 213 COVID-19 patients with PaO<inf>2</inf>/FiO<inf>2</inf> ratio <300 mmHg at hospital admission. Two groups (silent and dyspneic hypoxemia) were defined. Symptoms, blood gas analysis, chest X-ray (CXR) severity, need for intensive care and outcome were recorded.

RESULTS

Silent hypoxemic patients (68-31.9%) compared to the dyspneic hypoxemic patients (145-68.1%) showed greater frequency of extra respiratory symptoms (myalgia, diarrhea and nausea) and lower plasmatic LDH. PaO<inf>2</inf>/FiO<inf>2</inf> ratio was 225±68 mmHg and 192±78 mmHg in silent and dyspneic hypoxemia respectively (P=0.002). Eighteen percent of the patients with PaO<inf>2</inf>/FiO<inf>2</inf> from 50 to 150 mmHg presented silent hypoxemia. Silent and dyspneic hypoxemic patients had similar PaCO<inf>2</inf> (34.2±6.8 mmHg vs. 33.5±5.7 mmHg, P=0.47) but different respiratory rates (24.6±5.9 bpm vs. 28.6±11.3 bpm respectively, P=0.002). Even when CXR was severely abnormal, 25% of the population was silent hypoxemic. Twenty-six point five percent and 38.6% of silent and dyspneic patients were admitted to the ICU respectively (P=0.082). Mortality rate was 17.6% and 29.7% (log-rank P=0.083) in silent and dyspneic patients.

CONCLUSIONS

Silent hypoxemia is remarkably present in COVID-19. The presence of dyspnea is associated with a more severe clinical condition.

Elastic Power of Mechanical Ventilation in Morbid Obesity and Severe Hypoxemia

BACKGROUND

To minimize ventilator-induced lung injury, the primary clinical focus is currently expanding from measuring static indices of the individual tidal cycle (eg, plateau pressure and tidal volume) to more inclusive indicators of energy load, such as total power and its elastic components. Morbid obesity may influence these components. We characterized the relative values of elastic subcomponents of total power (ie, driving power and dynamic power) in subjects with severe hypoxemia, morbid obesity, or their combination.

METHODS

We analyzed data from subjects receiving mechanical ventilation divided into 4 groups. [Formula: see text]/[Formula: see text] < 150 mm Hg (severe hypoxemia) indicated probable reduction of lung compliance while body mass index > 40 kg/m² (morbid obesity) suggested a possible contribution to reduced respiratory system compliance from the chest wall. Group 1 included subjects with no expected abnormality of lung compliance or chest wall compliance; Group 2 included subjects with expected reduction of lung compliance on the basis of severe hypoxemia but with no morbid obesity; Group 3 included subjects with morbid obesity without severe hypoxemia; and Group 4 included subjects with morbid obesity and severe hypoxemia. All ventilator-induced lung injury predictors were compared among groups using mixed-effects linear models.

RESULTS

Groups 1-4 included 61, 52, 49, and 51 subjects, respectively. Mean body mass index averaged 28.7 kg/m² for nonobese subjects and 52.1 kg/m² for morbidly obese subjects. Mean driving pressure, dynamic power, and driving power of Groups 2 and 3 exceeded the corresponding values of Group 1 but fell into similar ranges when compared with each other. These values were highest in Group 4 subjects. In Group 2, mean dynamic power and driving power values were comparable to those in Group 3.

CONCLUSIONS

In mechanically ventilated subjects, stress and energy-based ventilator-induced lung injury indicators are influenced by the relative contributions of chest wall and lung to overall respiratory mechanics. Numerical guidelines for ventilator-induced lung injury risk must strongly consider adjustment for these elastic characteristics in morbid obesity.

Estimating the Damaging Power of High-Stress Ventilation

Redirection of our clinical attention from the pressures and volumes of the individual cycle to the broader and more inclusive considerations of energy load and power has untapped potential to reduce iatrogenic risk from ventilation (ie, ventilator-induced lung injury). Power is the product of breathing frequency and inflation energy per breath. Yet, while feasible to calculate at the bedside, measuring total power may not prove to be precise enough for accurate prediction of ventilator-induced lung injury, even if normalized to lung capacity (ie, specific power). The same power value can be reached by a multitude of frequency and tidal volume combinations, not all of which carry equal risk of damage. If some arbitrary level of alveolar pressure were accepted as a sharply defined hazard boundary, a rather straightforward geometric analysis theoretically would allow partitioning of overall tidal energy into components above and below a damage threshold. In this discussion, we introduce the concept of quantitative power partitioning and illustrate how tidal energy and power might be deconstructed into their key parts.

Physiological and quantitative CT-scan characterization of COVID-19 and typical ARDS: a matched cohort study

Purpose

To investigate whether COVID-19-ARDS differs from all-cause ARDS.

Methods

Thirty-two consecutive, mechanically ventilated COVID-19-ARDS patients were compared to two historical ARDS sub-populations 1:1 matched for PaO₂/FiO₂ or for compliance of the respiratory system. Gas exchange, hemodynamics and respiratory mechanics were recorded at 5 and 15 cmH₂O PEEP. CT scan variables were measured at 5 cmH₂O PEEP.

Results

Anthropometric characteristics were similar in COVID-19-ARDS, PaO₂/FiO₂-matched-ARDS and Compliance-matched-ARDS. The PaO₂/FiO₂-matched-ARDS and COVID-19-ARDS populations (both with PaO₂/FiO₂ 106 ± 59 mmHg) had different respiratory system compliances (Crs) (39 ± 11 vs 49.9 ± 15.4 ml/cmH₂O, p = 0.03). The Compliance-matched-ARDS and COVID-19-ARDS had similar Crs (50.1 ± 15.7 and 49.9 ± 15.4 ml/cmH₂O, respectively) but significantly lower PaO₂/FiO₂ for the same Crs (160 ± 62 vs 106.5 ± 59.6 mmHg, p < 0.001). The three populations had similar lung weights but COVID-19-ARDS had significantly higher lung gas volume (PaO₂/FiO₂-matched-ARDS 930 ± 644 ml, COVID-19-ARDS 1670 ± 791 ml and Compliance-matched-ARDS 1301 ± 627 ml, p < 0.05). The venous admixture was significantly related to the non-aerated tissue in PaO₂/FiO₂-matched-ARDS and Compliance-matched-ARDS (p < 0.001) but unrelated in COVID-19-ARDS (p = 0.75), suggesting that hypoxemia was not only due to the extent of non-aerated tissue. Increasing PEEP from 5 to 15 cmH₂O improved oxygenation in all groups. However, while lung mechanics and dead space improved in PaO₂/FiO₂-matched-ARDS, suggesting recruitment as primary mechanism, they remained unmodified or worsened in COVID-19-ARDS and Compliance-matched-ARDS, suggesting lower recruitment potential and/or blood flow redistribution.

Conclusions

COVID-19-ARDS is a subset of ARDS characterized overall by higher compliance and lung gas volume for a given PaO₂/FiO₂, at least when considered within the timeframe of our study.

Electronic supplementary material

The online version of this article (10.1007/s00134-020-06281-2) contains supplementary material, which is available to authorized users.