Esophageal Manometry

Phenotypes of esophageal pressure response to the change of positive end-expiratory pressure in patients with moderate acute respiratory distress syndrome

Background

Esophageal pressure (Pes) has been used as a surrogate of pleural pressure (Ppl) to titrate positive end-expiratory pressure (PEEP) in acute respiratory distress syndrome (ARDS) patients. The relationship between Pes and PEEP remains undetermined.

Methods

A gastric tube with a balloon catheter was inserted to monitor Pes in moderate to severe ARDS patients who underwent invasive mechanical ventilation. To assess the end-expiratory Pes response (ΔPes) to PEEP changes (ΔPEEP), the PEEP level was decreased and increased subsequently (with an average change of 3 cmH2O). The patients underwent the following two series of PEEP adjustment: (I) from PEEP-3 cmH2O to PEEPbaseline; and (II) from PEEPbaseline to PEEP+3 cmH2O. The patients were classified as "PEEP-dependent type" if they had ΔPes ≥30% ΔPEEP and were otherwise classified as "PEEP-independent type" (ΔPes <30% ΔPEEP in any series).

Results

In total, 54 series of PEEP adjustments were performed in 18 ARDS patients. Of these patients, 12 were classified as PEEP-dependent type, and six were classified as PEEP-independent type. During the PEEP adjustment, end-expiratory Pes changed significantly in the PEEP-dependent patients, who had a Pes of 10.8 (7.9, 12.3), 12.5 (10.5, 14.9), and 14.5 (13.1, 18.3) cmH2O at PEEP-3 cmH2O, PEEPbaseline, and PEEP+3 cmH2O, respectively (median and quartiles; P<0.0001), while end-expiratory transpulmonary pressure (PL) was maintained at an optimal range [-0.1 (-0.7, 0.4), 0.1 (-0.6, 0.5), and 0.3 (-0.3, 0.7) cmH2O, respectively]. In the PEEP-independent patients, the Pes remained unchanged, with a Pes of 15.4 (11.4, 17.8), 15.5 (11.6, 17.8), and 15.4 (11.7, 18.30) cmH2O at each of the three PEEP levels, respectively. Meanwhile, end-expiratory PL significantly improved [from -5.5 (-8.5, -3.4) at PEEP-3 cmH2O to -2.5 (-5.0, -1.6) at PEEPbaseline to -0.5 (-1.8, 0.3) at PEEP+3 cmH2O; P<0.01].

Conclusions

Two types of Pes phenotypes were identified according to the ΔPes to ΔPEEP. The underlying mechanisms and implications for clinical practice require further exploration.

Evaluation of Optimal Esophageal Catheter Balloon Inflation Volume in Mechanically Ventilated Children

BACKGROUND

Accuracy of esophageal pressure measured by an air-filled esophageal balloon catheter is dependent on balloon filling volume. However, this has been understudied in mechanically ventilated children. We sought to study the optimal filling volume in children receiving ventilation by using previously reported calibration methods. Secondary objectives included to examine the difference in pressure measurements at individualized optimal filling volume versus a standardized inflation volume and to study if a static hold during calibration is required to identify the optimal filling volume.

METHODS

An incremental inflation calibration procedure was performed in children receiving ventilation, <18 y, instrumented with commercially available catheters (6 or 8 French) who were not breathing spontaneously. The balloon was manually inflated by 0.2 to 1.6 mL (6 French) or 2.6 mL (8 French). Esophageal pressure (Pes) and airway pressure tracings were recorded during the procedure. Data were analyzed offline by using 2 methods: visual determination of filling range with the calculation of the highest difference between expiratory and inspiratory Pes and determination of a correctly filled balloon by calculating the esophageal elastance.

RESULTS

We enrolled 40 subjects with median (interquartile range [IQR]) age 6.8 (2-25) months. The optimal filling volume ranged from 0.2 to 1.2 mL (median [IQR] 0.6 [0.2-1.0] mL) in the subjects with a 6 French catheter and 0.2-2.0 mL (median [IQR] 0.7 [0.5-1.2] mL) for 8 French catheters. Inflating the balloon with 0.6 mL (median computed from the whole cohort) gave an absolute difference in transpulmonary pressure that ranged from -4 to 7 cm H2O compared with the personalized volume. Pes calculated over 5 consecutives breaths differed with a maximum of 1 cm H2O compared to Pes calculated during a single inspiratory hold. The esophageal elastance was correlated with weight, age, and sex.

CONCLUSIONS

The optimal balloon inflation volume was highly variable, which indicated the need for an individual calibration procedure. Pes was not overestimated when an inspiratory hold was not applied.

Pulmonary vascular pressure respiratory swings in COPD and ILD candidates for lung transplantation: Large but different

We analyzed the effect of respiratory swings on interpreting intravascular pulmonary vascular pressures (PVPs) in chronic obstructive pulmonary disease (COPD) and interstitial lung disease (ILD) candidates for lung transplantation (LTx) and the role of the alterations in pulmonary function tests on the dynamic respiratory variations. Twenty-eight consecutive patients were included. All patients underwent a complete hemodynamic study (right atrial, mean pulmonary arterial, and pulmonary arterial occlusion pressures [RAP, mPAP, and PAOP]-) and pulmonary function testing (force vital capacity [FVC], forced expiratory volume in the first second [FEV1], and residual volume [RV]). A subgroup of 10 patients underwent simultaneous esophageal pressure (PES). All hemodynamic parameters and PES were collected during apnea after an unforced expiration (ee) and during spontaneous breathing averaging five respiratory cycles (mrc). The respiratory swing (osc) was estimated as the difference between maximum-minimum values of pressures during the respiratory cycle. Intravascular RAPee, mPAPee, and PAOPee were higher than mrc values (p < 0.05), leading to 11% of pulmonary hypertension (PH) misdiagnosis and 37% of postcapillary PH misclassification. PAOPosc of COPD was higher than ILD patients and RAPosc (p < 0.05). Only PAOPosc correlated with FVC, FEV1, and RV (p < 0.05). ILD PESmrc was lower than COPD (p < 0.05), and it was associated with a significantly higher transmural than intravascular RAPmrc, mPAPmrc, and PAOPmrc. PESmrc was significantly correlated with FVC. Transmural mPAPmrc and PAOPmrc readings determined around 20% of reclassification of the patients compared to ee measurements. Candidates for LTx showed large respiratory swings in PVP, which were correlated with pulmonary function alterations. mrc PVP would be more closely approximated to the true transmural PVP leading to PH reclassification. Adjusting PVP for PES should be considered in COPD and ILD candidates of LTx with severe alterations in pulmonary functional tests and suspicion of a PESmrc far from 0. PES respiratory swings could be different in ILD to COPD patients.

State of the art: Monitoring of the respiratory system during veno-venous extracorporeal membrane oxygenation

Monitoring the patient receiving veno-venous extracorporeal membrane oxygenation (VV ECMO) is challenging due to the complex physiological interplay between native and membrane lung. Understanding these interactions is essential to understand the utility and limitations of different approaches to respiratory monitoring during ECMO. We present a summary of the underlying physiology of native and membrane lung gas exchange and describe different tools for titrating and monitoring gas exchange during ECMO. However, the most important role of VV ECMO in severe respiratory failure is as a means of avoiding further ergotrauma. Although optimal respiratory management during ECMO has not been defined, over the last decade there have been advances in multimodal respiratory assessment which have the potential to guide care. We describe a combination of imaging, ventilator-derived or invasive lung mechanic assessments as a means to individualise management during ECMO.

Airway and Transpulmonary Driving Pressure by End-Inspiratory Holds During Pressure Support Ventilation

BACKGROUND

The precision of quasi-static airway driving pressure (ΔP) assessed in pressure support ventilation (PSV) as a surrogate of tidal lung stress is debatable because persistent muscular activity frequently alters the readability of end-inspiratory holds. In this study, we used strict criteria to discard excessive muscular activity during holds and assessed the accuracy of ΔP in predicting global lung stress in PSV. Additionally, we explored whether the physiological effects of high PEEP differed according to the response of respiratory system compliance (CRS).

METHODS

Adults with ARDS undergoing PSV were enrolled. An esophageal catheter was inserted to calculate lung stress through transpulmonary driving pressure (ΔPL). ΔP and ΔPL were assessed in PSV at PEEP 5, 10, and 15 cm H2O by end-inspiratory holds. CRS was calculated as tidal volume (VT)/ΔP. We analyzed the effects of high PEEP on pressure-time product per minute (PTPmin), airway pressure at 100 ms (P0.1), and VT over PTP per breath (VT/PTPbr) in subjects with increased versus decreased CRS at high PEEP.

RESULTS

Eighteen subjects and 162 end-inspiratory holds were analyzed; 51/162 (31.5%) of the holds had ΔPL ≥ 12 cm H2O. Significant association between ΔP and ΔPL was found at all PEEP levels (P < .001). ΔP had excellent precision to predict ΔPL, with 15 cm H2O being identified as the best threshold for detecting ΔPL ≥ 12 cm H2O (area under the receiver operating characteristics 0.99 [95% CI 0.98-1.00]). CRS changes from low to high PEEP corresponded well with lung compliance changes (R2 0.91, P < .001) When CRS increased, a significant improvement of PTPmin and VT/PTPbr was found, without changes in P0.1. No benefits were observed when CRS decreased.

CONCLUSIONS

In subjects with ARDS undergoing PSV, high ΔP assessed by readable end-inspiratory holds accurately detected potentially dangerous thresholds of ΔPL. Using ΔP to assess changes in CRS induced by PEEP during assisted ventilation may inform whether higher PEEP could be beneficial.

Esophageal Pressure Measurement: A Primer

Over the last decade, the literature exploring clinical applications for esophageal manometry in critically ill patients has increased. New mechanical ventilators and bedside monitors allow measurement of esophageal pressures easily at the bedside. The bedside clinician can now evaluate the magnitude and timing of esophageal pressure swings to evaluate respiratory muscle activity and transpulmonary pressures. The respiratory therapist has all the tools to perform these measurements to optimize mechanical ventilation delivery. However, as with any measurement, technique, fidelity, and accuracy are paramount. This primer highlights key knowledge necessary to perform measurements and highlights areas of both uncertainty and ongoing development.

The oesophageal balloon for respiratory monitoring in ventilated patients: updated clinical review and practical aspects

There is a well-recognised importance for personalising mechanical ventilation settings to protect the lungs and the diaphragm for each individual patient. Measurement of oesophageal pressure (P oes) as an estimate of pleural pressure allows assessment of partitioned respiratory mechanics and quantification of lung stress, which helps our understanding of the patient's respiratory physiology and could guide individualisation of ventilator settings. Oesophageal manometry also allows breathing effort quantification, which could contribute to improving settings during assisted ventilation and mechanical ventilation weaning. In parallel with technological improvements, P oes monitoring is now available for daily clinical practice. This review provides a fundamental understanding of the relevant physiological concepts that can be assessed using P oes measurements, both during spontaneous breathing and mechanical ventilation. We also present a practical approach for implementing oesophageal manometry at the bedside. While more clinical data are awaited to confirm the benefits of P oes-guided mechanical ventilation and to determine optimal targets under different conditions, we discuss potential practical approaches, including positive end-expiratory pressure setting in controlled ventilation and assessment of inspiratory effort during assisted modes.

Acute Respiratory Distress Syndrome, Mechanical Ventilation, and Inhalation Injury in Burn Patients

Respiratory failure occurs with some frequency in seriously burned patients, driven by a combination of inflammatory and infection factors. Inhalation injury contributes to respiratory failure in some burn patients via direct mucosal injury and indirect inflammation. In burn patients, respiratory failure leading to acute respiratory distress syndrome, with or without inhalation injury, is effectively managed using principles evolved for non-burn critically ill patients.

Ultrasound Imaging for Diaphragm Function in a Population of Healthy Infants: A Short Observational Report

Introduction: Diaphragm ultrasound is increasingly used in adults, and more recently in pediatric practice. However, normal diaphragm parameters in healthy infants are unknown. This was a prospective observational pilot study aiming to define the normal diaphragm ultrasound characteristics in healthy infants during the first 6 months of life. Methods: We recruited healthy neonates at 7 to 15 days of life, who were followed until the sixth month of life, undergoing five assessments in different time points. The measurements included diaphragm thickness at end expiration (TEE) and at end inspiration (TEI). The thickening fraction (TF) was calculated as (TEI-TEE)/TEE and expressed as a percentage, and as (TEI-TEE)/TEI. Results: A total of 37 toddlers, 16 of which were females (43.2%), were enrolled. Thirty-four children (91.9%) were of Caucasian ethnicity and the median gestational age was 38.4 (35.7–40) weeks. Normal TEE, TEI, and TF have been provided for each time point. Conclusion: We provided new insight regarding data about thickness and thickening function in healthy children to be used for future physiologic and pathologic pediatric studies.

Relationship of Extravascular Lung Water and Pulmonary Vascular Permeability to Respiratory Mechanics in Patients with COVID-19-Induced ARDS

During acute respiratory distress syndrome (ARDS), the increase in pulmonary vascular permeability and lung water induced by pulmonary inflammation may be related to altered lung compliance. A better understanding of the interactions between respiratory mechanics variables and lung water or capillary permeability would allow a more personalized monitoring and adaptation of therapies for patients with ARDS. Therefore, our main objective was to investigate the relationship between extravascular lung water (EVLW) and/or pulmonary vascular permeability index (PVPI) and respiratory mechanic variables in patients with COVID-19-induced ARDS. This is a retrospective observational study from prospectively collected data in a cohort of 107 critically ill patients with COVID-19-induced ARDS from March 2020 to May 2021. We analyzed relationships between variables using repeated measurements correlations. We found no clinically relevant correlations between EVLW and the respiratory mechanics variables (driving pressure (correlation coefficient [CI 95%]: 0.017 [-0.064; 0.098]), plateau pressure (0.123 [0.043; 0.202]), respiratory system compliance (-0.003 [-0.084; 0.079]) or positive end-expiratory pressure (0.203 [0.126; 0.278])). Similarly, there were no relevant correlations between PVPI and these same respiratory mechanics variables (0.051 [-0.131; 0.035], 0.059 [-0.022; 0.140], 0.072 [-0.090; 0.153] and 0.22 [0.141; 0.293], respectively). In a cohort of patients with COVID-19-induced ARDS, EVLW and PVPI values are independent from respiratory system compliance and driving pressure. Optimal monitoring of these patients should combine both respiratory and TPTD variables.

Intraoperative Ventilator Management of the Critically Ill Patient

Strategies for the intraoperative ventilator management of the critically ill patient focus on parameters used for lung protective ventilation with acute respiratory distress syndrome, preventing or limiting the deleterious effects of mechanical ventilation, and optimizing anesthetic and surgical conditions to limit postoperative pulmonary complications for patients at risk. Patient conditions such as obesity, sepsis, the need for laparoscopic surgery, or one-lung ventilation may benefit from intraoperative lung protective ventilation strategies. Anesthesiologists can use risk evaluation and prediction tools, monitor advanced physiologic targets, and incorporate new innovative monitoring techniques to develop an individualized approach for patients.

Respiratory Monitoring During Mechanical Ventilation: The Present and the Future

The increased application of mechanical ventilation, the recognition of its harms and the interest in individualization raised the need for an effective monitoring. An increasing number of monitoring tools and modalities were introduced over the past 2 decades with growing insight into asynchrony, lung and chest wall mechanics, respiratory effort and drive. They should be used in a complementary rather than a standalone way. A sound strategy can guide a reduction in adverse effects like ventilator-induced lung injury, ventilator-induced diaphragm dysfunction, patient-ventilator asynchrony and helps early weaning from the ventilator. However, the diversity, complexity, lack of expertise, and associated cost make formulating the appropriate monitoring strategy a challenge for clinicians. Most often, a big amount of data is fed to the clinicians making interpretation difficult. Therefore, it is fundamental for intensivists to be aware of the principle, advantages, and limits of each tool. This analytic review includes a simplified narrative of the commonly used basic and advanced respiratory monitors along with their limits and future prospective.

Evaluation of the accuracy of established patient inspiratory effort estimation methods during mechanical support ventilation

There is a clinical need for monitoring inspiratory effort to prevent lung- and diaphragm injury in patients who receive supportive mechanical ventilation in an Intensive Care Unit. Different pressure-based techniques are available to estimate this inspiratory effort at the bedside, but the accuracy of their effort estimation is uncertain since they are all based on a simplified linear model of the respiratory system, which omits gas compressibility of air, and the viscoelasticity and nonlinearities of the respiratory system. The aim of this in-silico study was to provide an overview of the pressure-based estimation techniques and to evaluate their accuracy using a more sophisticated model of the respiratory system and ventilator. The influence of the following parameters on the accuracy of the pressure-based estimation techniques was evaluated using the in-silico model: 1) the patient's respiratory mechanics 2) PEEP and the inspiratory pressure of the ventilator 3) gas compressibility of air 4) viscoelasticity of the respiratory system 5) the strength of the inspiratory effort. The best-performing technique in terms of accuracy was the whole breath occlusion. The average error and maximum error were the lowest for all patient archetypes. We found that the error was related to the expansion of gas in the breathing set and lungs and respiratory compliance. However, concerns exist that other factors not included in the model, such as a changed muscle-force relation during an occlusion, might influence the true accuracy. The estimation techniques based on the esophageal pressure showed an error related to the viscoelastic element in the model which leads to a higher error than the occlusion. The error of the esophageal pressure-based techniques is therefore highly dependent on the pathology of the patient and the settings of the ventilator and might change over time while a patient recovers or becomes more ill.

The calibration of esophageal pressure by proper esophageal balloon filling volume: A clinical study

Background

Esophageal pressure (Pes) can be used as a reliable surrogate for pleural pressure, especially in critically ill patients requiring personalized mechanical ventilation strategies. How to choose the proper esophageal balloon filling volume and then find the optimal value of esophageal pressure remains a challenge. The study aimed to assess the feasibility of catheters for Pes monitoring in mechanically ventilated patients.

Materials and methods

Twelve patients under pressure-controlled mechanical ventilation were included in this study. Raw esophageal pressure was recorded at different balloon filling volumes. Then, the P-V curves were determined. V _WORK was the intermediate linear section on the end-expiratory P-V curve, and V _BEST was the filling volume providing the maximum difference between Pes at end-inspiration and end-expiration. The raw value of Pes was recorded, and the calibrated values of Pes were calculated by calculating the esophageal wall pressure (Pew) and esophageal elastance (Ees).

Results

Twenty-four series of Pes measurements were performed. The mean V _MIN and V _MAX were 2.17 ± 0.49 ml (range, 1.0-3.0 ml) and 6.79 ± 0.83 ml (range, 5.0-9.0 ml), respectively, whereas V _BEST was 4.69 ± 0.16 ml (range, 2.0-8.0 ml). Ees was 1.35 ± 0.51 cm H₂O/ml (range, 0.26-2.38 cm H₂O/ml). The estimated Pew at V _BEST was 3.16 ± 2.19 cm H₂O (range, 0-7.97 cm H₂O). Patients with a body mass index (BMI) ≥ 25 kg/m² had a significantly lower V _MAX (5.88 [5.25-6] vs. 7.25 [7-8] ml, p = 0.006) and a significantly lower V _BEST (3.69 [2.5-4.38] vs. 5.19 [4-6] ml, p = 0.036) than patients with a BMI < 25 kg/m². Patients with positive end-expiratory pressure (PEEP) ≥ 10 cm H₂O had a lower V _MIN and V _BEST than patients with PEEP < 10 cm H₂O, P > 0.05. Patients in the supine position had a higher esophageal pressure than those in the prone position with the same balloon filling volume.

Conclusions

Calibration of esophageal pressure to identify the best filling volume of esophageal balloon catheters is feasible. The esophageal pressure can be influenced by BMI, PEEP, and position. It is necessary to titrate the optimal inflation volume again when the PEEP values or the positions change.

Spontaneous breathing promotes lung injury in an experimental model of alveolar collapse

Vigorous spontaneous breathing has emerged as a promotor of lung damage in acute lung injury, an entity known as “patient self-inflicted lung injury”. Mechanical ventilation may prevent this second injury by decreasing intrathoracic pressure swings and improving regional air distribution. Therefore, we aimed to determine the effects of spontaneous breathing during the early stage of acute respiratory failure on lung injury and determine whether early and late controlled mechanical ventilation may avoid or revert these harmful effects. A model of partial surfactant depletion and lung collapse was induced in eighteen intubated pigs of 32 ±4 kg. Then, animals were randomized to (1) SB‐group: spontaneous breathing with very low levels of pressure support for the whole experiment (eight hours), (2) Early MV-group: controlled mechanical ventilation for eight hours, or (3) Late MV-group: first half of the experiment on spontaneous breathing (four hours) and the second half on controlled mechanical ventilation (four hours). Respiratory, hemodynamic, and electric impedance tomography data were collected. After the protocol, animals were euthanized, and lungs were extracted for histologic tissue analysis and cytokines quantification. SB-group presented larger esophageal pressure swings, progressive hypoxemia, lung injury, and more dorsal and inhomogeneous ventilation compared to the early MV-group. In the late MV-group switch to controlled mechanical ventilation improved the lung inhomogeneity and esophageal pressure swings but failed to prevent hypoxemia and lung injury. In a lung collapse model, spontaneous breathing is associated to large esophageal pressure swings and lung inhomogeneity, resulting in progressive hypoxemia and lung injury. Mechanical ventilation prevents these mechanisms of patient self-inflicted lung injury if applied early, before spontaneous breathing occurs, but not when applied late.

High-Flow Nasal Cannula Oxygen Therapy: Physiological Mechanisms and Clinical Applications in Children

High-flow nasal cannula (HFNC) oxygen therapy has rapidly become a popular modality of respiratory support in pediatric care. This is undoubtedly due to its ease of use and safety, which allows it to be used in a wide variety of settings, ranging from pediatric intensive care to patients' homes. HFNC devices make it possible to regulate gas flow and temperature, as well as allowing some nebulized drugs to be administered, features very useful in children, in which the balance between therapeutic effectiveness and adherence to treatment is pivotal. Although the physiological effects of HFNC are still under investigation, their mechanisms of action include delivery of fixed concentration of oxygen, generation of positive end-expiratory pressure, reduction of the work of breathing and clearance of the nasopharyngeal dead space, while providing optimal gas conditioning. Nevertheless, current evidence supports the use of HFNC mainly in moderate-to-severe bronchiolitis, whereas for asthma exacerbations and breath sleeping disorders there is a lack of randomized controlled trials comparing HFNC to continuous positive airway pressure (CPAP) and non-invasive ventilation (NIV), which are essentials for the identification of response and non-response predictors. In this regard, the development of clinical guidelines for HFNC, including flow settings, indications, and contraindications is urgently needed.

2021 Acute Respiratory Distress Syndrome Update, With Coronavirus Disease 2019 Focus

Acute respiratory distress syndrome (ARDS) is a heterogeneous lung disease responsible for significant morbidity and mortality among critically ill patients, including those infected with severe acute respiratory syndrome coronavirus 2, the virus responsible for coronavirus disease 2019. Despite recent advances in pathophysiology, diagnostics, and therapeutics, ARDS is dangerously underdiagnosed, and supportive lung protective ventilation and prone positioning remain the mainstay interventions. Rescue therapies, including neuromuscular blockade and venovenous extracorporeal membrane oxygenation, remain a key component of clinical practice, although benefits are unclear. Even though coronavirus disease 2019 ARDS has some distinguishing features from traditional ARDS, including delayed onset, hyperinflammatory response, and pulmonary microthrombi, it clinically is similar to traditional ARDS and should be treated with established supportive therapies.

Prevalence of Different Types of Primary Esophageal Motility Disorders and Their Associated Factors in Patients Referring to Shariati Hospital during 2018-2019

BACKGROUND: Esophageal motility disorders (EMDs) are common in patients with dysphagia and are effectively diagnosed with high-resolution manometry (HREM). In this study, we aimed to evaluate the prevalence of different types of primary EMDs in patients referred for HREM and to further investigate the factors associated with EMDs. METHODS: In this cross-sectional study, all patients referred to the endoscopy section of Shariati Hospital during 2018-2019 (279 patients) were subjected to HREM and were evaluated according to their diagnosis, and the effect of each factor and each symptom on motility disorders was investigated. RESULTS: 84.5% (235) of the participants were diagnosed with at least one esophageal motility disorder; of them, achalasia was the most common form (52.6%). None of the predictive factors showed a statistically significant correlation with EMDs. However, regarding the symptoms, regurgitation and nocturnal cough were significantly more common in patients with EMD (P=0.001 and 0.009, respectively). CONCLUSION: This study demonstrates the high prevalence of EMDs in patients undergoing manometry. None of the factors studied, such as age, sex, diabetes, hypothyroidism, smoking, and alcohol and opium consumption, had a statistically significant correlation with EMDs.

The physiological underpinnings of life-saving respiratory support

Treatment of respiratory failure has improved dramatically since the polio epidemic in the 1950s with the use of invasive techniques for respiratory support: mechanical ventilation and extracorporeal respiratory support. However, respiratory support is only a supportive therapy, designed to "buy time" while the disease causing respiratory failure abates. It ensures viable gas exchange and prevents cardiorespiratory collapse in the context of excessive loads. Because the use of invasive modalities of respiratory support is also associated with substantial harm, it remains the responsibility of the clinician to minimize such hazards. Direct iatrogenic consequences of mechanical ventilation include the risk to the lung (ventilator-induced lung injury) and the diaphragm (ventilator-induced diaphragm dysfunction and other forms of myotrauma). Adverse consequences on hemodynamics can also be significant. Indirect consequences (e.g., immobilization, sleep disruption) can have devastating long-term effects. Increasing awareness and understanding of these mechanisms of injury has led to a change in the philosophy of care with a shift from aiming to normalize gases toward minimizing harm. Lung (and more recently also diaphragm) protective ventilation strategies include the use of extracorporeal respiratory support when the risk of ventilation becomes excessive. This review provides an overview of the historical background of respiratory support, pathophysiology of respiratory failure and rationale for respiratory support, iatrogenic consequences from mechanical ventilation, specifics of the implementation of mechanical ventilation, and role of extracorporeal respiratory support. It highlights the need for appropriate monitoring to estimate risks and to individualize ventilation and sedation to provide safe respiratory support to each patient.

Effect of Lowering Vt on Mortality in Acute Respiratory Distress Syndrome Varies with Respiratory System Elastance

Rationale: If the risk of ventilator-induced lung injury in acute respiratory distress syndrome (ARDS) is causally determined by driving pressure rather than by Vt, then the effect of ventilation with lower Vt on mortality would be predicted to vary according to respiratory system elastance (Ers). Objectives: To determine whether the mortality benefit of ventilation with lower Vt varies according to Ers. Methods: In a secondary analysis of patients from five randomized trials of lower- versus higher-Vt ventilation strategies in ARDS and acute hypoxemic respiratory failure, the posterior probability of an interaction between the randomized Vt strategy and Ers on 60-day mortality was computed using Bayesian multivariable logistic regression. Measurements and Main Results: Of 1,096 patients available for analysis, 416 (38%) died by Day 60. The posterior probability that the mortality benefit from lower-Vt ventilation strategies varied with Ers was 93% (posterior median interaction odds ratio, 0.80 per cm H₂O/[ml/kg]; 90% credible interval, 0.63-1.02). Ers was classified as low (<2 cm H₂O/[ml/kg], n = 321, 32%), intermediate (2-3 cm H₂O/[ml/kg], n = 475, 46%), and high (>3 cm H₂O/[ml/kg], n = 224, 22%). In these groups, the posterior probabilities of an absolute risk reduction in mortality ≥ 1% were 55%, 82%, and 92%, respectively. The posterior probabilities of an absolute risk reduction ≥ 5% were 29%, 58%, and 82%, respectively. Conclusions: The mortality benefit of ventilation with lower Vt in ARDS varies according to elastance, suggesting that lung-protective ventilation strategies should primarily target driving pressure rather than Vt.

A narrative review of diaphragmatic ultrasound in pediatric critical care

The use of point of care ultrasound (POCUS) at the bedside has increased dramatically within emergency medicine and in critical care. Applications of POCUS have spread to include diaphragmatic assessments in both adults and children. Diaphragm POCUS can be used to assess for diaphragm dysfunction (DD) and atrophy or to guide ventilator titration and weaning. Quantitative, semi-quantitative and qualitative measurements of diaphragm thickness, diaphragm excursion, and diaphragm thickening fraction provide objective data related to DD and atrophy. The potential for quick, noninvasive, and repeatable bedside diaphragm assessments has led to a growing amount of literature on diaphragm POCUS. To date, there are no reviews of the current state of diaphragm POCUS in pediatric critical care. The aims of this narrative review are to summarize the current literature regarding techniques, reference values, applications, and future innovations of diaphragm POCUS in critically ill children. A summary of current practice and future directions will be discussed.

Transpulmonary Pressure-Guided Lung-Protective Ventilation Improves Pulmonary Mechanics and Oxygenation Among Obese Subjects on Mechanical Ventilation

BACKGROUND

Transpulmonary pressure (P_L) is used to assess pulmonary mechanics and guide lung-protective mechanical ventilation (LPV). P_L is recommended to individualize LPV settings for patients with high pleural pressures and hypoxemia. We aimed to determine whether P_L-guided LPV settings, pulmonary mechanics, and oxygenation improve and differ from non-P_L-guided LPV among obese patients after 24 h on mechanical ventilation. Secondary outcomes included classification of hypoxemia severity, count of ventilator-free days, ICU length of stay, and overall ICU mortality.

METHODS

This is a retrospective analysis of data. Ventilator settings, pulmonary mechanics, and oxygenation were recorded on the initial day of P_L measurement and 24 h later. P_L-guided LPV targeted inspiratory P_L < 20 cm H₂O and expiratory P_L of 0-6 cm H₂O. Comparisons were made to repeat measurements.

RESULTS

Twenty subjects (13 male) with median age of 49 y, body mass index 47.5 kg/m², and SOFA score of 8 were included in our analysis. Fourteen subjects received care in a medical ICU. P_L measurement occurred 16 h after initiating non-P_L-guided LPV. P_L-guided LPV resulted in higher median PEEP (14 vs 18 cm H₂O, P = .009), expiratory P_L (-3 vs 1 cm H₂O, P = .02), respiratory system compliance (30.7 vs 44.6 mL/cm H₂O, P = .001), and [Formula: see text] (156 vs 240 mm Hg, P = .002) at 24 h. P_L-guided LPV resulted in lower [Formula: see text] (0.53 vs 0.33, P < .001) and lower P_L driving pressure (10 vs 6 cm H₂O, P = .001). Tidal volume (420 vs 435 mL, P = .64) and inspiratory P_L (7 vs 7 cm H₂O, P = .90) were similar. Subjects had a median of 7 ventilator-free days, and median ICU length of stay was 14 d. Three of 20 subjects died within 28 d after ICU admission.

CONCLUSIONS

P_L-guided LPV resulted in higher PEEP, lower [Formula: see text], improved pulmonary mechanics, and greater oxygenation when compared to non-P_L-guided LPV settings in adult obese subjects.

Pathophysiology of light phenotype SARS-CoV-2 interstitial pneumonia: from histopathological features to clinical presentations

Little is known about the light phenotype of SARS-CoV-2 pneumonia, which behaves in an unusual way, unlike other known respiratory diseases. We believe that the histopathological features of early COVID-19 could be considered the pathophysiological hallmark of this disease. Lung cryobiopsies show almost pristine alveoli, enlarged/hyperplasic alveolar capillaries along with dilatation of the post capillary pulmonary venules. Hypoxemia could therefore be explained by a reduction of the normal V/Q ratio, due to blood overflow around well ventilated alveoli. This could clarify typical manifestations of type L COVID-19, such as happy hypoxemia, response to awake prone positioning, response to PEEP/CPAP and platypnea orthodeoxia.