Causes of Hypoxemia in COVID-19 Acute Respiratory Distress Syndrome: A Combined Multiple Inert Gas Elimination Technique and Dual-energy Computed Tomography Study

BACKGROUND

Despite the fervent scientific effort, a state-of-the art assessment of the different causes of hypoxemia (shunt, ventilation-perfusion mismatch, and diffusion limitation) in COVID-19 acute respiratory distress syndrome (ARDS) is currently lacking. In this study, the authors hypothesized a multifactorial genesis of hypoxemia and aimed to measure the relative contribution of each of the different mechanism and their relationship with the distribution of tissue and blood within the lung.

METHODS

In this cross-sectional study, the authors prospectively enrolled 10 patients with COVID-19 ARDS who had been intubated for less than 7 days. The multiple inert gas elimination technique (MIGET) and a dual-energy computed tomography (DECT) were performed and quantitatively analyzed for both tissue and blood volume. Variables related to the respiratory mechanics and invasive hemodynamics (PiCCO [Getinge, Sweden]) were also recorded.

RESULTS

The sample (51 ± 15 yr; Pao2/Fio2, 172 ± 86 mmHg) had a mortality of 50%. The MIGET showed a shunt of 25 ± 16% and a dead space of 53 ± 11%. Ventilation and perfusion were mismatched (LogSD, Q, 0.86 ± 0.33). Unexpectedly, evidence of diffusion limitation or postpulmonary shunting was also found. In the well aerated regions, the blood volume was in excess compared to the tissue, while the opposite happened in the atelectasis. Shunt was proportional to the blood volume of the atelectasis (R2 = 0.70, P = 0.003). V˙A/Q˙T mismatch was correlated with the blood volume of the poorly aerated tissue (R2 = 0.54, P = 0.016). The overperfusion coefficient was related to Pao2/Fio2 (R2 = 0.66, P = 0.002), excess tissue mass (R2 = 0.84, P < 0.001), and Etco2/Paco2 (R2 = 0.63, P = 0.004).

CONCLUSIONS

These data support the hypothesis of a highly multifactorial genesis of hypoxemia. Moreover, recent evidence from post-mortem studies (i.e., opening of intrapulmonary bronchopulmonary anastomosis) may explain the findings regarding the postpulmonary shunting. The hyperperfusion might be related to the disease severity.

EDITOR’S PERSPECTIVE

Estimation of normal lung weight index in healthy female domestic pigs

INTRODUCTION

Lung weight is an important study endpoint to assess lung edema in porcine experiments on acute respiratory distress syndrome and ventilatory induced lung injury. Evidence on the relationship between lung-body weight relationship is lacking in the literature. The aim of this work is to provide a reference equation between normal lung and body weight in female domestic piglets.

MATERIALS AND METHODS

177 healthy female domestic piglets from previous studies were included in the analysis. Lung weight was assessed either via a CT-scan before any experimental injury or with a scale after autopsy. The animals were randomly divided in a training (n = 141) and a validation population (n = 36). The relation between body weight and lung weight index (lung weight/body weight, g/kg) was described by an exponential function on the training population. The equation was tested on the validation population. A Bland-Altman analysis was performed to compare the lung weight index in the validation population and its theoretical value calculated with the reference equation.

RESULTS

A good fit was found between the validation population and the exponential equation extracted from the training population (RMSE = 0.060). The equation to determine lung weight index from body weight was: [Formula: see text] At the Bland and Altman analyses, the mean bias between the real and the expected lung weight index was - 0.26 g/kg (95% CI - 0.96-0.43), upper LOA 3.80 g/kg [95% CI 2.59-5.01], lower LOA - 4.33 g/kg [95% CI = - 5.54-(- 3.12)].

CONCLUSIONS

This exponential function might be a valuable tool to assess lung edema in experiments involving 16-50 kg female domestic piglets. The error that can be made due to the 95% confidence intervals of the formula is smaller than the one made considering the lung to body weight as a linear relationship.

Computation of contrast-enhanced perfusion using only two CT scan phases: a proof-of-concept study on abdominal organs

Background

Computed tomography perfusion imaging (CTPI) by repeated scanning has clinical relevance but implies relatively high radiation exposure. We present a method to measure perfusion from two CT scan phases only, considering tissue enhancement, feeding vessel (aortic) peak enhancement, and bolus shape.

Methods

CTPI scans (each with 40 frames acquired every 1.5 s) of 11 patients with advanced hepatocellular carcinoma (HCC) enrolled between 2012 and 2016 were retrospectively analysed (aged 69 ± 9 years, 8/11 males). Perfusion was defined as the maximal slope of the time-enhancement curve divided by the peak enhancement of the feeding vessel (aorta). Perfusion was computed two times, first using the maximum slope derived from all data points and then using the peak tissue enhancement and the bolus shape obtained from the aortic curve.

Results

Perfusion values from the two methods were linearly related (r2 = 0.92, p < 0.001; Bland–Altman analysis bias -0.12). The mathematical model showed that the perfusion ratio of two ROIs with the same feeding vessel (aorta) corresponds to their peak enhancement ratio (r2 = 0.55, p < 0.001; Bland–Altman analysis bias -0.68). The relationship between perfusion and tissue enhancement is predicted to be linear in the clinical range of interest, being only function of perfusion, peak feeding vessel enhancement, and bolus shape.

Conclusions

This proof-of-concept study showed that perfusion values of HCC, kidney, and pancreas could be computed using enhancement measured only with two CT scan phases, if aortic peak enhancement and bolus shape are known.

Lung response to a higher positive end-expiratory pressure in mechanically ventilated patients with COVID-19

Background

International guidelines suggest using a higher (> 10 cm H₂O) positive end-expiratory pressure (PEEP) in patients with moderate-to-severe ARDS due to COVID-19. However, even if oxygenation generally improves with a higher PEEP, compliance, and Paco₂ frequently do not, as if recruitment was small.

Research Question

Is the potential for lung recruitment small in patients with early ARDS due to COVID-19?

Study Design and Methods

Forty patients with ARDS due to COVID-19 were studied in the supine position within 3 days of endotracheal intubation. They all underwent a PEEP trial, in which oxygenation, compliance, and Paco₂ were measured with 5, 10, and 15 cm H₂O of PEEP, and all other ventilatory settings unchanged. Twenty underwent a whole-lung static CT scan at 5 and 45 cm H₂O, and the other 20 at 5 and 15 cm H₂O of airway pressure. Recruitment and hyperinflation were defined as a decrease in the volume of the non-aerated (density above −100 HU) and an increase in the volume of the over-aerated (density below −900 HU) lung compartments, respectively.

Results

From 5 to 15 cm H₂O, oxygenation improved in 36 (90%) patients but compliance only in 11 (28%) and Paco₂ only in 14 (35%). From 5 to 45 cm H₂O, recruitment was 351 (161-462) mL and hyperinflation 465 (220-681) mL. From 5 to 15 cm H₂O, recruitment was 168 (110-202) mL and hyperinflation 121 (63-270) mL. Hyperinflation variably developed in all patients and exceeded recruitment in more than half of them.

Interpretation

Patients with early ARDS due to COVID-19, ventilated in the supine position, present with a large potential for lung recruitment. Even so, their compliance and Paco₂ do not generally improve with a higher PEEP, possibly because of hyperinflation.

Using Artificial Intelligence for Automatic Segmentation of CT Lung Images in Acute Respiratory Distress Syndrome

Knowledge of gas volume, tissue mass and recruitability measured by the quantitative CT scan analysis (CT-qa) is important when setting the mechanical ventilation in acute respiratory distress syndrome (ARDS). Yet, the manual segmentation of the lung requires a considerable workload. Our goal was to provide an automatic, clinically applicable and reliable lung segmentation procedure. Therefore, a convolutional neural network (CNN) was used to train an artificial intelligence (AI) algorithm on 15 healthy subjects (1,302 slices), 100 ARDS patients (12,279 slices), and 20 COVID-19 (1,817 slices). Eighty percent of this populations was used for training, 20% for testing. The AI and manual segmentation at slice level were compared by intersection over union (IoU). The CT-qa variables were compared by regression and Bland Altman analysis. The AI-segmentation of a single patient required 5–10 s vs. 1–2 h of the manual. At slice level, the algorithm showed on the test set an IOU across all CT slices of 91.3 ± 10.0, 85.2 ± 13.9, and 84.7 ± 14.0%, and across all lung volumes of 96.3 ± 0.6, 88.9 ± 3.1, and 86.3 ± 6.5% for normal lungs, ARDS and COVID-19, respectively, with a U-shape in the performance: better in the lung middle region, worse at the apex and base. At patient level, on the test set, the total lung volume measured by AI and manual segmentation had a R² of 0.99 and a bias −9.8 ml [CI: +56.0/−75.7 ml]. The recruitability measured with manual and AI-segmentation, as change in non-aerated tissue fraction had a bias of +0.3% [CI: +6.2/−5.5%] and −0.5% [CI: +2.3/−3.3%] expressed as change in well-aerated tissue fraction. The AI-powered lung segmentation provided fast and clinically reliable results. It is able to segment the lungs of seriously ill ARDS patients fully automatically.

Machine Learning to Predict In-Hospital Mortality in COVID-19 Patients Using Computed Tomography-Derived Pulmonary and Vascular Features

Pulmonary parenchymal and vascular damage are frequently reported in COVID-19 patients and can be assessed with unenhanced chest computed tomography (CT), widely used as a triaging exam. Integrating clinical data, chest CT features, and CT-derived vascular metrics, we aimed to build a predictive model of in-hospital mortality using univariate analysis (Mann-Whitney U test) and machine learning models (support vectors machines (SVM) and multilayer perceptrons (MLP)). Patients with RT-PCR-confirmed SARS-CoV-2 infection and unenhanced chest CT performed on emergency department admission were included after retrieving their outcome (discharge or death), with an 85/15% training/test dataset split. Out of 897 patients, the 229 (26%) patients who died during hospitalization had higher median pulmonary artery diameter (29.0 mm) than patients who survived (27.0 mm, p < 0.001) and higher median ascending aortic diameter (36.6 mm versus 34.0 mm, p < 0.001). SVM and MLP best models considered the same ten input features, yielding a 0.747 (precision 0.522, recall 0.800) and 0.844 (precision 0.680, recall 0.567) area under the curve, respectively. In this model integrating clinical and radiological data, pulmonary artery diameter was the third most important predictor after age and parenchymal involvement extent, contributing to reliable in-hospital mortality prediction, highlighting the value of vascular metrics in improving patient stratification.

The impact of ventilation-perfusion inequality in COVID-19: a computational model

COVID-19 infection may lead to acute respiratory distress syndrome (CARDS) where severe gas exchange derangements may be associated, at least in the early stages, only with minor pulmonary infiltrates. This may suggest that the shunt associated to the gasless lung parenchyma is not sufficient to explain CARDS hypoxemia. We designed an algorithm (Vent_riQ_lar), based on the same conceptual grounds described by J.B. West in 1969. We set 498 ventilation-perfusion (V_A/Q) compartments and, after calculating their blood composition (PO₂, PCO₂, and pH), we randomly chose 10⁶ combinations of five parameters controlling a bimodal distribution of blood flow. The solutions were accepted if the predicted PaO₂ and PaCO₂ were within 10% of the patient's values. We assumed that the shunt fraction equaled the fraction of non-aerated lung tissue at the CT quantitative analysis. Five critically-ill patients later deceased were studied. The PaO₂/FiO₂ was 91.1 ± 18.6 mmHg and PaCO₂ 69.0 ± 16.1 mmHg. Cardiac output was 9.58 ± 0.99 L/min. The fraction of non-aerated tissue was 0.33 ± 0.06. The model showed that a large fraction of the blood flow was likely distributed in regions with very low V_A/Q (Q_mean = 0.06 ± 0.02) and a smaller fraction in regions with moderately high V_A/Q. Overall LogSD, Q was 1.66 ± 0.14, suggestive of high V_A/Q inequality. Our data suggest that shunt alone cannot completely account for the observed hypoxemia and a significant V_A/Q inequality must be present in COVID-19. The high cardiac output and the extensive microthrombosis later found in the autopsy further support the hypothesis of a pathological perfusion of non/poorly ventilated lung tissue.NEW & NOTEWORTHY Hypothesizing that the non-aerated lung fraction as evaluated by the quantitative analysis of the lung computed tomography (CT) equals shunt (V_A/Q = 0), we used a computational approach to estimate the magnitude of the ventilation-perfusion inequality in severe COVID-19. The results show that a severe hyperperfusion of poorly ventilated lung region is likely the cause of the observed hypoxemia. The extensive microthrombosis or abnormal vasodilation of the pulmonary circulation may represent the pathophysiological mechanism of such V_A/Q distribution.

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.

Understanding Lactatemia in Human Sepsis. Potential Impact for Early Management

Rationale: Hyperlactatemia in sepsis may derive from a prevalent impairment of oxygen supply/demand and/or oxygen use. Discriminating between these two mechanisms may be relevant for the early fluid resuscitation strategy.Objectives: To understand the relationship among central venous oxygen saturation (Scv_O2), lactate, and base excess to better determine the origin of lactate.Methods: This was a post hoc analysis of baseline variables of 1,741 patients with sepsis enrolled in the multicenter trial ALBIOS (Albumin Italian Outcome Sepsis). Variables were analyzed as a function of sextiles of lactate concentration and sextiles of Scv_O2. We defined the "alactic base excess," as the sum of lactate and standard base excess.Measurements and Main Results: Organ dysfunction severity scores, physiologic variables of hepatic, metabolic, cardiac, and renal function, and 90-day mortality were measured. Scv_O2 was lower than 70% only in 35% of patients. Mortality, organ dysfunction scores, and lactate were highest in the first and sixth sextiles of Scv_O2. Although lactate level related strongly to mortality, it was associated with acidemia only when kidney function was impaired (creatinine >2 mg/dl), as rapidly detected by a negative alactic base excess. In contrast, positive values of alactic base excess were associated with a relative reduction of fluid balance.Conclusions: Hyperlactatemia is powerfully correlated with severity of sepsis and, in established sepsis, is caused more frequently by impaired tissue oxygen use, rather than by impaired oxygen transport. Concomitant acidemia was only observed in the presence of renal dysfunction, as rapidly detected by alactic base excess. The current strategy of fluid resuscitation could be modified according to the origin of excess lactate.

Determinants of the esophageal-pleural pressure relationship in humans

Esophageal pressure has been suggested as adequate surrogate of the pleural pressure. We investigate after lung surgery the determinants of the esophageal and intrathoracic pressures and their differences. The esophageal pressure (through esophageal balloon) and the intrathoracic/pleural pressure (through the chest tube on the surgery side) were measured after surgery in 28 patients immediately after lobectomy or wedge resection. Measurements were made in the nondependent lateral position (without or with ventilation of the operated lung) and in the supine position. In the lateral position with the nondependent lung, collapsed or ventilated, the differences between esophageal and pleural pressure amounted to 4.4 ± 1.6 and 5.1 ± 1.7 cmH₂O. In the supine position, the difference amounted to 7.3 ± 2.8 cmH₂O. In the supine position, the estimated compressive forces on the mediastinum were 10.5 ± 3.1 cmH₂O and on the iso-gravitational pleural plane 3.2 ± 1.8 cmH₂O. A simple model describing the roles of chest, lung, and pneumothorax volume matching on the pleural pressure genesis was developed; modeled pleural pressure = 1.0057 × measured pleural pressure + 0.6592 (r² = 0.8). Whatever the position and the ventilator settings, the esophageal pressure changed in a 1:1 ratio with the changes in pleural pressure. Consequently, chest wall elastance (E_cw) measured by intrathoracic (E_cw = ΔPpl/tidal volume) or esophageal pressure (E_cw = ΔPes/tidal volume) was identical in all the positions we tested. We conclude that esophageal and pleural pressures may be largely different depending on body position (gravitational forces) and lung-chest wall volume matching. Their changes, however, are identical.NEW & NOTEWORTHY Esophageal and pleural pressure changes occur at a 1:1 ratio, fully justifying the use of esophageal pressure to compute the chest wall elastance and the changes in pleural pressure and in lung stress. The absolute value of esophageal and pleural pressures may be largely different, depending on the body position (gravitational forces) and the lung-chest wall volume matching. Therefore, the absolute value of esophageal pressure should not be used as a surrogate of pleural pressure.

Reclassifying Acute Respiratory Distress Syndrome

RATIONALE

The ratio of Pa_O2 to Fi_O2 (P/F) defines acute respiratory distress syndrome (ARDS) severity and suggests appropriate therapies.

OBJECTIVES

We investigated 1) whether a 150-mm-Hg P/F threshold within the range of moderate ARDS (100-200 mm Hg) would define two subgroups that were more homogeneous; and 2) which criteria led the clinicians to apply extracorporeal membrane oxygenation (ECMO) in severe ARDS.

METHODS

At the 150-mm-Hg P/F threshold, moderate patients were split into mild-moderate (n = 50) and moderate-severe (n = 55) groups. Patients with severe ARDS (Fi_O2 not available in three patients) were split into higher (n = 63) and lower (n = 18) Fi_O2 groups at an 80% Fi_O2 threshold.

MEASUREMENTS AND MAIN RESULTS

Compared with mild-moderate ARDS, patients with moderate-severe ARDS had higher peak pressures, Pa_CO2, and pH. They also had heavier lungs, greater inhomogeneity, more noninflated tissue, and greater lung recruitability. Within 84 patients with severe ARDS (P/F < 100 mm Hg), 75% belonged to the higher Fi_O2 subgroup. They differed from the patients with severe ARDS with lower Fi_O2 only in Pa_CO2 and lung weight. Forty-one of 46 patients treated with ECMO belonged to the higher Fi_O2 group. Within this group, the patients receiving ECMO had higher Pa_CO2 than the 22 non-ECMO patients. The inhomogeneity ratio, total lung weight, and noninflated tissue were also significantly higher.

CONCLUSIONS

Using the 150-mm-Hg P/F threshold gave a more homogeneous distribution of patients with ARDS across the severity subgroups and identified two populations that differed in their anatomical and physiological characteristics. The patients treated with ECMO belonged to the severe ARDS group, and almost 90% of them belonged to the higher Fi_O2 subgroup.

Prevention of Lung Bacterial Colonization With a Leak-Proof Endotracheal Tube Cuff: An Experimental Animal Study

BACKGROUND

Endotracheal tubes with standard polyvinyl chloride cuffs create folds on inflation into the trachea, which lead to potential leakage of subglottic secretions into the lower airways and cause lung colonization and pneumonia. The use of a double-layer prototype leak-proof cuff has shown effective prevention of the fluid leakage across the cuff. We hypothesized that the use of such a leak-proof cuff could prevent lung bacterial colonization in vivo.

METHODS

To simulate patients in the ICU, 13 pigs were placed in the semirecumbent position, intubated, and mechanically ventilated for 72 h. Five animals were prospectively intubated with an endotracheal tube with a leak-proof cuff (leak-proof cuff group). Data from 8 animals previously intubated with an endotracheal tube with a standard polyvinyl chloride cuff (standard cuff group) were retrospectively analyzed. Leakage of tracheal secretions across the leak-proof cuff was tested by the macroscopic methylene blue evaluation. Arterial blood gas exchanges and microbiology were tested in all the pigs at necropsy.

RESULTS

In the standard cuff group, all the pigs showed heavy bacterial colonization of the lungs after 72 h of mechanical ventilation, with an overall proportion of colonized lung lobes of 92% (44/48 lobes, 8/8 animals) compared with 27% (8/30 lobes, 5/5 animals) in the leak-proof cuff group (P < .001). These results were strengthened by the absence of methylene blue in the tracheal secretions below the leak-proof cuff. Furthermore, no hypoxemia was demonstrated in the pigs in the leak-proof cuff group after the 72-h experiment (P_aO2 /F_IO2 change from baseline, leak-proof cuff group vs standard cuff group; median difference 332, 95% CI 41-389 mm Hg; P = .030).

CONCLUSIONS

A new leak-proof cuff for endotracheal intubation prevented macroscopic leakage of subglottic secretions along the airways. This mechanism led to the reduction of lung bacterial colonization, which could contribute to the prevention of hypoxemia in the pigs on mechanical ventilation while in the semirecumbent position.

Septic shock-3 vs 2: an analysis of the ALBIOS study

Background

A reanalysis of the ALBIOS trial suggested that patients with septic shock - defined by vasopressor-dependent hypotension in the presence of severe sepsis (Shock-2) - had a survival benefit when treated with albumin. The new septic shock definition (Shock-3) added the criterion of a lactate threshold of 2 mmol/L. We investigated how the populations defined according to Shock-2 and Shock-3 differed and whether the albumin benefit would be confirmed.

Methods

This is a retrospective analysis of the ALBIOS study, a randomized controlled study conducted between 2008 and 2012 in 100 intensive care units in Italy comparing the administration of 20% albumin and crystalloids versus crystalloids alone in patients with severe sepsis or septic shock. We analyzed data from 1741 patients from ALBIOS with serum lactate measurement available at baseline. We compared group size, physiological variables and 90-day mortality between patients defined by Shock-2 and Shock-3 and between the albumin and crystalloid treatment groups.

Results

We compared the Shock-2 and the Shock-3 definitions and the albumin and crystalloid treatment groups in terms of group size and physiological, laboratory and outcome variables. The Shock-3 definition reduced the population with shock by 34%. The Shock-3 group had higher lactate (p < 0.001), greater resuscitation-fluid requirement (p = 0.014), higher Simplified Acute Physiology Score II (p < 0.001) and Sepsis-related Organ Failure Assessment scores (p = 0.022), lower platelet count (p = 0.002) and higher 90-day mortality (46.7% vs 51.9%; p = 0.031). Albumin decreased mortality in Shock-2 patients compared to crystalloids (43.5% vs 49.9%; 12.6% relative risk reduction; p = 0.04). In patients defined by Shock-3 a similar benefit was observed for albumin with a 11.3% relative risk reduction (48.7% vs 54.9%; 11.3% relative risk reduction; p = 0.22).

Conclusions

The Sepsis-3 definition reduced the size of the population with shock and showed a similar effect size in the benefits of albumin. The Shock-3 criteria will markedly slow patients’ recruitment rates, in view of testing albumin in septic shock.

Trial registration

ClinicalTrials.gov, number NCT00707122. Registered on 30 June 2008.

A Morphological and Quantitative Analysis of Lung CT Scan in Patients With Acute Respiratory Distress Syndrome and in Cardiogenic Pulmonary Edema

BACKGROUND

The acute respiratory distress syndrome (ARDS) and cardiogenic pulmonary edema (CPE) are both characterized by an increase in lung edema that can be measured by computed tomography (CT). The aim of this study was to compare possible differences between patients with ARDS and CPE in the morphologic pattern, the aeration, and the amount and distribution of edema within the lung.

METHODS

Lung CT was performed at a mean positive end-expiratory pressure level of 5 cm H₂O in both groups. The morphological evaluation was performed by two radiologists, while the quantitative evaluation was performed by a dedicated software.

RESULTS

A total of 60 patients with ARDS (20 mild, 20 moderate, 20 severe) and 20 patients with CPE were enrolled. The ground-glass attenuation regions were similarly present among the groups, 8 (40%), 8 (40%), 14 (70%), and 10 (50%), while the airspace consolidations were significantly more present in ARDS. The lung gas volume was significantly lower in severe ARDS compared to CPE (830 [462] vs 1120 [832] mL). Moving from the nondependent to the dependent lung regions, the not inflated lung tissue significantly increased, while the well inflated tissue decreased (ρ = 0.96-1.00, P < .0001). Significant differences were found between ARDS and CPE mostly in dependent regions. In severe ARDS, the estimated edema was significantly higher, compared to CPE (757 [740] vs 532 [637] g).

CONCLUSIONS

Both ARDS and CPE are characterized by a similar presence of ground-glass attenuation and different airspace consolidation regions. Acute respiratory distress syndrome has a higher amount of not inflated tissue and lower amount of well inflated tissue. However, the overall regional distribution is similar within the lung.

Pathogenic Link Between Postextubation Pneumonia and Ventilator-Associated Pneumonia: An Experimental Study

BACKGROUND

The presence of an endotracheal tube is the main cause for developing ventilator-associated pneumonia (VAP), but pneumonia can still develop in hospitalized patients after endotracheal tube removal (postextubation pneumonia [PEP]). We hypothesized that short-term intubation (24 hours) can play a role in the pathogenesis of PEP. To test such hypothesis, we initially evaluated the occurrence of lung colonization and VAP in sheep that were intubated and mechanically ventilated for 24 hours. Subsequently, we assessed the incidence of lung colonization and PEP at 48 hours after extubation in sheep previously ventilated for 24 hours.

METHODS

To simulate intubated intensive care unit patients placed in semirecumbent position, 14 sheep were intubated and mechanically ventilated with the head elevated 30° above horizontal. Seven of them were euthanized after 24 hours (Control Group), whereas the remaining were euthanized after being awaken, extubated, and left spontaneously breathing for 48 hours after extubation (Awake Group). Criteria of clinical diagnosis of pneumonia were tested. Microbiological evaluation was performed on autopsy in all sheep.

RESULTS

Only 1 sheep in the Control Group met the criteria of VAP after 24 hours of mechanical ventilation. However, heavy pathogenic bacteria colonization of trachea, bronchi, and lungs (range, 10-10 colony-forming unit [CFU]/g) was reported in 4 of 7 sheep (57%). In the Awake Group, 1 sheep was diagnosed with VAP and 3 developed PEP within 48 hours after extubation (42%), with 1 euthanized at 30 hours because of respiratory failure. On autopsy, 5 sheep (71%) confirmed pathogenic bacterial growth in the lower respiratory tract (range, 10-10 CFU/g).

CONCLUSIONS

Twenty-four hours of intubation and mechanical ventilation in semirecumbent position leads to significant pathogenic colonization of the lower airways, which can promote the development of PEP. Strategies directed to prevent pathogenic microbiological colonization before and after mechanical ventilation should be considered to avert the onset of PEP.

Driving pressure and mechanical power: new targets for VILI prevention

Several factors have been recognized as possible triggers of ventilator-induced lung injury (VILI). The first is pressure (thus the 'barotrauma'), then the volume (hence the 'volutrauma'), finally the cyclic opening-closing of the lung units ('atelectrauma'). Less attention has been paid to the respiratory rate and the flow, although both theoretical considerations and experimental evidence attribute them a significant role in the generation of VILI. The initial injury to the lung parenchyma is necessarily mechanical and it could manifest as an unphysiological distortion of the extracellular matrix and/or as micro-fractures in the hyaluronan, likely the most fragile polymer embedded in the matrix. The order of magnitude of the energy required to break a molecular bond between the hyaluronan and the associated protein is 1.12×10^-16 Joules (J), 70-90% higher than the average energy delivered by a single breath of 1L assuming a lung elastance of 10 cmH₂O/L (0.5 J). With a normal statistical distribution of the bond strength some polymers will be exposed each cycle to an energy large enough to rupture. Both the extracellular matrix distortion and the polymer fractures lead to inflammatory increase of capillary permeability with edema if a pulmonary blood flow is sufficient. The mediation analysis of higher vs. lower tidal volume and PEEP studies suggests that the driving pressure, more than tidal volume, is the best predictor of VILI, as inferred by increased mortality. This is not surprising, as both tidal volume and respiratory system elastance (resulting in driving pressure) may independently contribute to the mortality. For the same elastance driving pressure is a predictor similar to plateau pressure or tidal volume. Driving pressure is one of the components of the mechanical power, which also includes respiratory rate, flow and PEEP. Finding the threshold for mechanical power would greatly simplify assessment and prevention of VILI.

Randomized, multicenter trial of lateral Trendelenburg versus semirecumbent body position for the prevention of ventilator-associated pneumonia

PURPOSE

The lateral Trendelenburg position (LTP) may hinder the primary pathophysiologic mechanism of ventilator-associated pneumonia (VAP). We investigated whether placing patients in the LTP would reduce the incidence of VAP in comparison with the semirecumbent position (SRP).

METHODS

This was a randomized, multicenter, controlled study in invasively ventilated critically ill patients. Two preplanned interim analyses were performed. Patients were randomized to be placed in the LTP or the SRP. The primary outcome, assessed by intention-to-treat analysis, was incidence of microbiologically confirmed VAP. Major secondary outcomes included mortality, duration of mechanical ventilation, and intensive care unit length of stay.

RESULTS

At the second interim analysis, the trial was stopped because of low incidence of VAP, lack of benefit in secondary outcomes, and occurrence of adverse events. A total of 194 patients in the LTP group and 201 in the SRP group were included in the final intention-to-treat analysis. The incidence of microbiologically confirmed VAP was 0.5% (1/194) and 4.0% (8/201) in LTP and SRP patients, respectively (relative risk 0.13, 95% CI 0.02-1.03, p = 0.04). The 28-day mortality was 30.9% (60/194) and 26.4% (53/201) in LTP and SRP patients, respectively (relative risk 1.17, 95% CI 0.86-1.60, p = 0.32). Likewise, no differences were found in other secondary outcomes. Six serious adverse events were described in LTP patients (p = 0.01 vs. SRP).

CONCLUSIONS

The LTP slightly decreased the incidence of microbiologically confirmed VAP. Nevertheless, given the early termination of the trial, the low incidence of VAP, and the adverse events associated with the LTP, the study failed to prove any significant benefit. Further clinical investigation is strongly warranted; however, at this time, the LTP cannot be recommended as a VAP preventive measure. CLINICALTRIALS.

GOV IDENTIFIER

NCT01138540.

Airway driving pressure and lung stress in ARDS patients

Background

Lung-protective ventilation strategy suggests the use of low tidal volume, depending on ideal body weight, and adequate levels of PEEP. However, reducing tidal volume according to ideal body weight does not always prevent overstress and overstrain. On the contrary, titrating mechanical ventilation on airway driving pressure, computed as airway pressure changes from PEEP to end-inspiratory plateau pressure, equivalent to the ratio between the tidal volume and compliance of respiratory system, should better reflect lung injury. However, possible changes in chest wall elastance could affect the reliability of airway driving pressure. The aim of this study was to evaluate if airway driving pressure could accurately predict lung stress (the pressure generated into the lung due to PEEP and tidal volume).

Methods

One hundred and fifty ARDS patients were enrolled. At 5 and 15 cmH₂O of PEEP, lung stress, driving pressure, lung and chest wall elastance were measured.

Results

The applied tidal volume (mL/kg of ideal body weight) was not related to lung gas volume (r² = 0.0005 p = 0.772). Patients were divided according to an airway driving pressure lower and equal/higher than 15 cmH₂O (the lower and higher airway driving pressure groups). At both PEEP levels, the higher airway driving pressure group had a significantly higher lung stress, respiratory system and lung elastance compared to the lower airway driving pressure group. Airway driving pressure was significantly related to lung stress (r² = 0.581 p < 0.0001 and r² = 0.353 p < 0.0001 at 5 and 15 cmH₂O of PEEP). For a lung stress of 24 and 26 cmH₂O, the optimal cutoff value for the airway driving pressure were 15.0 cmH₂O (ROC AUC 0.85, 95 % CI = 0.782–0.922); and 16.7 (ROC AUC 0.84, 95 % CI = 0.742–0.936).

Conclusions

Airway driving pressure can detect lung overstress with an acceptable accuracy. However, further studies are needed to establish if these limits could be used for ventilator settings.

Electronic supplementary material

The online version of this article (doi:10.1186/s13054-016-1446-7) contains supplementary material, which is available to authorized users.

Effect of body mass index in acute respiratory distress syndrome

BACKGROUND

Obesity is associated in healthy subjects with a great reduction in functional residual capacity and with a stiffening of lung and chest wall elastance, which promote alveolar collapse and hypoxaemia. Likewise, obese patients with acute respiratory distress syndrome (ARDS) could present greater derangements of respiratory mechanics than patients of normal weight.

METHODS

One hundred and one ARDS patients were enrolled. Partitioned respiratory mechanics and gas exchange were measured at 5 and 15 cm H2O of PEEP with a tidal volume of 6-8 ml kg(-1) of predicted body weight. At 5 and 45 cm H2O of PEEP, two lung computed tomography scans were performed.

RESULTS

Patients were divided as follows according to BMI: normal weight (BMI≤25 kg m(-2)), overweight (BMI between 25 and 30 kg m(-2)), and obese (BMI>30 kg m(-2)). Obese, overweight, and normal-weight groups presented a similar lung elastance (median [interquartile range], respectively: 17.7 [14.2-24.8], 20.9 [16.1-30.2], and 20.5 [15.2-23.6] cm H2O litre(-1) at 5 cm H2O of PEEP and 19.3 [15.5-26.3], 21.1 [17.4-29.2], and 17.1 [13.4-20.4] cm H2O litre(-1) at 15 cm H2O of PEEP) and chest elastance (respectively: 4.9 [3.1-8.8], 5.9 [3.8-8.7], and 7.8 [3.9-9.8] cm H2O litre(-1) at 5 cm H2O of PEEP and 6.5 [4.5-9.6], 6.6 [4.2-9.2], and 4.9 [2.4-7.6] cm H2O litre(-1) at 15 cm H2O of PEEP). Lung recruitability was not affected by the body weight (15.6 [6.3-23.4], 15.7 [9.8-22.2], and 11.3 [6.2-15.6]% for normal-weight, overweight, and obese groups, respectively). Lung gas volume was significantly lower whereas total superimposed pressure was significantly higher in the obese compared with the normal-weight group (1148 [680-1815] vs 827 [686-1213] ml and 17.4 [15.8-19.3] vs 19.3 [18.6-21.7] cm H2O, respectively).

CONCLUSIONS

Obese ARDS patients do not present higher chest wall elastance and lung recruitability.

Bowel ultrasound imaging in patients with cystic fibrosis: Relationship with clinical symptoms and CFTR genotype

BACKGROUND

Ultrasound imaging is used to assess bowel abnormalities in gastrointestinal diseases. We aimed to assess the rate of predefined bowel ultrasound signs and their relationship with gastrointestinal symptoms and the cystic fibrosis transmembrane conductance regulator (CFTR) genotype in cystic fibrosis patients in regular follow-up.

METHODS

Prospective study of 70 consecutive patients with cystic fibrosis and 45 controls who underwent abdominal ultrasound; pertinent findings were related to gastrointestinal symptoms and, in cystic fibrosis patients, to pancreatic status, malabsorption degree, lipase intake, CFTR genotype (classified as severe or mild against functional class of CFTR mutations).

RESULTS

96% patients showed at least one abnormal bowel ultrasound sign. Most frequent signs were lymph node enlargement (64%), bowel loop dilatation (55%), thick corpuscular intraluminal content (49%), bowel wall hypervascularization (26%), thickened bowel wall (22%) and intussusception (17%). Patients with recurrent abdominal pain showed more bowel wall hypervascularization than patients without recurrent pain (47% vs. 19%, respectively; p = 0.02) and intussusception (58% vs. 17%, respectively; p < 0.01). Genotype was not associated to specific bowel ultrasound signs. Patients with bowel loop intussusception showed greater lipase intake than those without intussusception (8.118 ± 2.083 vs. 5.994 ± 4.187, respectively; p < 0.01).

CONCLUSION

Cystic fibrosis patients present a higher rate of bowel ultrasound abnormalities than controls. Bowel ultrasound abnormalities are associated with abdominal symptoms.

Hemostasis changes during veno-venous extracorporeal membrane oxygenation for respiratory support in adults

BACKGROUND

We investigated the coagulation system in patients during extracorporeal membrane oxygenation (ECMO) initiated for respiratory failure and the influence of the ECMO circuit on coagulation tests; we compared different coagulation tests for monitoring unfractionated heparin (UH) therapy; we investigated whether or not coagulation parameters were predictive of bleeding during ECMO.

METHODS

Pilot study on twelve consecutive adult patients admitted at our general ICU for acute respiratory failure and placed on ECMO from November 2011 to October 2012. Coagulation tests were performed before ECMO start and daily, including day of circuit change and day of circuit removal. UH was monitored with activated partial thromboplastin time (APTT) ratio, at a therapeutic range of 1.5-2.0.

RESULTS

We observed no effect of ECMO circuit on coagulation parameters measured pre- and postlung, but platelet count decreased significantly over time (-82x10(3)/mmc, 95%CI 40-123). APTT showed a correlation with antifactor Xa activity, whereas other global coagulation tests such as activated clotting time, thromboelastography and endogenous thrombin potential did not. Major bleeding occurred in three patients but no difference in any coagulation parameter was observed between them and those who did not bleed.

CONCLUSIONS

This pilot study shows that ECMO initiated for respiratory support in adults does not change coagulation parameters. Over time a statistically significant reduction of platelet count was observed, possibly due to consumption within the circuit, consumption microangiopathy or the underlying patients' diseases. Although APTT was appropriate to monitor UH, major bleedings occurred and a lower therapeutic range may be advisable.

Lung inhomogeneities, inflation and [18F]2-fluoro-2-deoxy-D-glucose uptake rate in acute respiratory distress syndrome

The aim of the study was to determine the size and location of homogeneous inflamed/noninflamed and inhomogeneous inflamed/noninflamed lung compartments and their association with acute respiratory distress syndrome (ARDS) severity.

In total, 20 ARDS patients underwent 5 and 45 cmH2O computed tomography (CT) scans to measure lung recruitability. [18F]2-fluoro-2-deoxy-d-glucose ([18F]FDG) uptake and lung inhomogeneities were quantified with a positron emission tomography-CT scan at 10 cmH2O. We defined four compartments with normal/abnormal [18F]FDG uptake and lung homogeneity.

The homogeneous compartment with normal [18F]FDG uptake was primarily composed of well-inflated tissue (80±16%), double-sized in nondependent lung (32±27% versus 16±17%, p<0.0001) and decreased in size from mild, moderate to severe ARDS (33±14%, 26±20% and 5±9% of the total lung volume, respectively, p=0.05). The homogeneous compartment with high [18F]FDG uptake was similarly distributed between the dependent and nondependent lung. The inhomogeneous compartment with normal [18F]FDG uptake represented 4% of the lung volume. The inhomogeneous compartment with high [18F]FDG uptake was preferentially located in the dependent lung (21±10% versus 12±10%, p<0.0001), mostly at the open/closed interfaces and related to recruitability (r2=0.53, p<0.001).

The homogeneous lung compartment with normal inflation and [18F]FDG uptake decreases with ARDS severity, while the inhomogeneous poorly/not inflated compartment increases. Most of the lung inhomogeneities are inflamed. A minor fraction of healthy tissue remains in severe ARDS.

Assessment of Fibrinolysis in Sepsis Patients with Urokinase Modified Thromboelastography

INTRODUCTION

Impairment of fibrinolysis during sepsis is associated with worse outcome. Early identification of this condition could be of interest. The aim of this study was to evaluate whether a modified point-of-care viscoelastic hemostatic assay can detect sepsis-induced impairment of fibrinolysis and to correlate impaired fibrinolysis with morbidity and mortality.

METHODS

This single center observational prospective pilot study was performed in an adult Intensive Care Unit (ICU) of a tertiary academic hospital. Forty consecutive patients admitted to the ICU with severe sepsis or septic shock were included. Forty healthy individuals served as controls. We modified conventional kaolin activated thromboelastography (TEG) adding urokinase to improve assessment of fibrinolysis in real time (UK-TEG). TEG, UK-TEG, plasminogen activator inhibitor (PAI)-1, thrombin-activatable fibrinolysis inhibitor (TAFI), d-dimer, DIC scores and morbidity (rated with the SOFA score) were measured upon ICU admission. Logistic regression was used to calculate odds ratios (ORs) and 95% confidence intervals (95% CIs) of mortality at ICU discharge.

RESULTS

UK-TEG revealed a greater impairment of fibrinolysis in sepsis patients compared to healthy individuals confirmed by PAI-1. TAFI was not different between sepsis patients and healthy individuals. 18/40 sepsis patients had fibrinolysis impaired according to UK-TEG and showed higher SOFA score (8 (6-13) vs 5 (4-7), p = 0.03), higher mortality (39% vs 5%, p = 0.01) and greater markers of cellular damage (lactate levels, LDH and bilirubin). Mortality at ICU discharge was predicted by the degree of fibrinolysis impairment measured by UK-TEG Ly30 (%) parameter (OR 0.95, 95% CI 0.93-0.98, p = 0.003).

CONCLUSIONS

Sepsis-induced impairment of fibrinolysis detected at UK-TEG was associated with increased markers of cellular damage, morbidity and mortality.

Lung recruitability is better estimated according to the Berlin definition of acute respiratory distress syndrome at standard 5 cm H2O rather than higher positive end-expiratory pressure: a retrospective cohort study

OBJECTIVES

The Berlin definition of acute respiratory distress syndrome has introduced three classes of severity according to PaO2/FIO2 thresholds. The level of positive end-expiratory pressure applied may greatly affect PaO2/FIO2, thereby masking acute respiratory distress syndrome severity, which should reflect the underlying lung injury (lung edema and recruitability). We hypothesized that the assessment of acute respiratory distress syndrome severity at standardized low positive end-expiratory pressure may improve the association between the underlying lung injury, as detected by CT, and PaO2/FIO2-derived severity.

DESIGN

Retrospective analysis.

SETTING

Four university hospitals (Italy, Germany, and Chile).

PATIENTS

One hundred forty-eight patients with acute lung injury or acute respiratory distress syndrome according to the American-European Consensus Conference criteria.

INTERVENTIONS

Patients underwent a three-step ventilator protocol (at clinical, 5 cm H2O, or 15 cm H2O positive end-expiratory pressure). Whole-lung CT scans were obtained at 5 and 45 cm H2O airway pressure.

MEASUREMENTS AND MAIN RESULTS

Nine patients did not fulfill acute respiratory distress syndrome criteria of the novel Berlin definition. Patients were then classified according to PaO2/FIO2 assessed at clinical, 5 cm H2O, or 15 cm H2O positive end-expiratory pressure. At clinical positive end-expiratory pressure (11±3 cm H2O), patients with severe acute respiratory distress syndrome had a greater lung tissue weight and recruitability than patients with mild or moderate acute respiratory distress syndrome (p<0.001). At 5 cm H2O, 54% of patients with mild acute respiratory distress syndrome at clinical positive end-expiratory pressure were reclassified to either moderate or severe acute respiratory distress syndrome. In these patients, lung recruitability and clinical positive end-expiratory pressure were higher than in patients who remained in the mild subgroup (p<0.05). When patients were classified at 5 cm H2O, but not at clinical or 15 cm H2O, lung recruitability linearly increases with acute respiratory distress syndrome severity (5% [2-12%] vs 12% [7-18%] vs 23% [12-30%], respectively, p<0.001). The potentially recruitable lung was the only CT-derived variable independently associated with ICU mortality (p=0.007).

CONCLUSIONS

The Berlin definition of acute respiratory distress syndrome assessed at 5 cm H2O allows a better evaluation of lung recruitability and edema than at higher positive end-expiratory pressure clinically set.

Spatial orientation and mechanical properties of the human trachea: a computed tomography study

BACKGROUND

The literature generally describes the trachea as oriented toward the right and back, but there is very little detailed characterization. Therefore, the aim of this study was to precisely determine the spatial orientation and to better characterize the physical properties of the human trachea.

METHODS

We analyzed lung computed tomography scans of 68 intubated and mechanically ventilated subjects suffering from acute lung injury/ARDS at airway pressures (Paw) of 5, 15, and 45 cm H2O. At each Paw, the inner edge of the trachea from the subglottal space to the carina was captured. Tracheal length and diameter were measured. Tracheal orientation and compliance were estimated from processing barycenter and surface tracheal sections.

RESULTS

Tracheal orientation at a Paw of 5 cm H2O showed a 4.2 ± 5.3° angle toward the right and a 20.6 ± 6.9° angle downward toward the back, which decreased significantly while increasing Paw (19.4 ± 6.9° at 15 cm H2O and 17.1 ± 6.8° at 45 cm H2O, P < .001). Tracheal compliance was 0.0113 ± 0.0131 mL/cm H2O/cm of trachea length from 5 to 15 cm H2O and 0.004 ± 0.0041 mL/cm H2O/cm of trachea length from 15 to 45 cm H2O (P < .001). Tracheal diameter was 19.6 ± 3.4 mm on the medial-lateral axis and 21.0 ± 4.3 mm on the sternal-vertebral axis.

CONCLUSIONS

The trachea is oriented downward toward the back at a 20.6 ± 6.9° angle and slightly toward the right at a 4.2 ± 5.3° angle. Understanding tracheal orientation may help in enhancing postural drainage and respiratory physiotherapy, and knowing the physical properties of the trachea may aid in endotracheal tube cuff design.

Validation of computed tomography for measuring lung weight

Background

Lung weight characterises severity of pulmonary oedema and predicts response to mechanical ventilation. The aim of this study was to evaluate the accuracy of quantitative analysis of thorax computed tomography (CT) for measuring lung weight in pigs with or without pulmonary oedema.

Methods

Thirty-six pigs were mechanically ventilated with different tidal volumes and positive end-expiratory pressures that did or did not induce pulmonary oedema. After 54 h, they underwent thorax CT (CT_{in vivo}) and were then sacrificed and exsanguinated. Fourteen pigs underwent a second thorax CT (CT_post-exsang.) after exsanguination. Lungs were excised and weighed with a balance (balance_post-exsang.). Agreement between lung weights measured with the balance (considered as reference) and those estimated by quantitative analysis of CT was assessed with Bland-Altman plots.

Results

One animal unexpectedly died before CT_{in vivo}. In 35 pigs, lung weight measured with balance_post-exsang. was 371 ± 184 g and that estimated with CT_{in vivo} was 481 ± 189 g (p < 0.001). Bias between methods was −111 g (−35%) and limits of agreement were −176 (−63%) and −46 g (−8%). Measurement error was similar in animals with (−112 ± 45 g; n = 11) or without (−110 ± 27 g; n = 24) pulmonary oedema (p = 0.88). In 14 pigs with thorax CT after exsanguination, lung weight measured with balance_post-exsang. was 342 ± 165 g and that estimated with CT_post-exsang. was 352 ± 160 g (p = 0.02). Bias between methods was −9 g (−4%) and limits of agreement were −36 (−11%) and 17 g (3%). Measurement errors were similar in pigs with (−1 ± 26 g; n = 11) or without (−12 ± 7 g; n = 3) pulmonary oedema (p = 0.12).

Conclusions

Compared to the balance, CT obtained in vivo constantly overestimated the lung weight, as it included pulmonary blood (whereas the balance did not). By contrast, CT obtained after exsanguination provided accurate and reproducible results.

Friday night ventilation: a safety starting tool kit for mechanically ventilated patients

We wish to report here a practical approach to an acute respiratory distress syndrome (ARDS) patient as devised by a group of intensivists with different expertise. The referral scenario is an intensive care unit of a Community Hospital with limited technology, where a young doctor, alone, must deal with this complicate syndrome during the night. The knowledge of pulse oximetry at room air and at 100% oxygen allows to estimate the PaO2 and the cause of hypoxemia, shunt vs. VA/Q maldistribution. The ARDS severity (mild [200<PaO2/FiO2≤300], moderate [100<PaO2/FiO2≤200] and severe [PaO2/FiO2≤100]) must be immediately assessed. Noninvasive ventilation should be attempted in mild ARDS only. Possible errors due to inappropriate premature intubation are preferable to a delayed intubation. In moderate and severe ARDS tracheal intubation associated with heavy sedation/muscle relaxation allows to fully characterize the patient. A tidal volume of 6 mL/kg predicted body weight is recommended, either in pressure or volume control ventilation. Tailoring tidal volume on residual functional capacity, however, is preferable. Plateau pressure greater than 30 cmH2O is acceptable only if chest wall compliance is decreased. In this case maximal attention must be devoted to the hemodynamics. PEEP from 5 to 10, from 10 to 15 and greater than 15 cmH2O should be set in mild, moderate and severe ARDS, respectively. Prone position should be applied in severe ARDS, if experience is available. In case of unchanged conditions or increased ARDS severity a referral center should be contacted.

Lung inhomogeneity in patients with acute respiratory distress syndrome

RATIONALE

Pressures and volumes needed to induce ventilator-induced lung injury in healthy lungs are far greater than those applied in diseased lungs. A possible explanation may be the presence of local inhomogeneities acting as pressure multipliers (stress raisers).

OBJECTIVES

To quantify lung inhomogeneities in patients with acute respiratory distress syndrome (ARDS).

METHODS

Retrospective quantitative analysis of CT scan images of 148 patients with ARDS and 100 control subjects. An ideally homogeneous lung would have the same expansion in all regions; lung expansion was measured by CT scan as gas/tissue ratio and lung inhomogeneities were measured as lung regions with lower gas/tissue ratio than their neighboring lung regions. We defined as the extent of lung inhomogeneities the fraction of the lung showing an inflation ratio greater than 95th percentile of the control group (1.61).

MEASUREMENTS AND MAIN RESULTS

The extent of lung inhomogeneities increased with the severity of ARDS (14 ± 5, 18 ± 8, and 23 ± 10% of lung volume in mild, moderate, and severe ARDS; P < 0.001) and correlated with the physiologic dead space (r(2) = 0.34; P < 0.0001). The application of positive end-expiratory pressure reduced the extent of lung inhomogeneities from 18 ± 8 to 12 ± 7% (P < 0.0001) going from 5 to 45 cm H2O airway pressure. Lung inhomogeneities were greater in nonsurvivor patients than in survivor patients (20 ± 9 vs. 17 ± 7% of lung volume; P = 0.01) and were the only CT scan variable independently associated with mortality at backward logistic regression.

CONCLUSIONS

Lung inhomogeneities are associated with overall disease severity and mortality. Increasing the airway pressures decreased but did not abolish the extent of lung inhomogeneities.