Pulmonary lesions associated with oxygen therapy and artificial ventilation

The role of oxygen in the development and treatment of bronchopulmonary dysplasia

Oxygen (O2) is crucial for both the development and treatment of one of the most important consequences of prematurity: bronchopulmonary dysplasia (BPD). In fetal life, the hypoxic environment is important for alveolar development and maturation. After birth, O2 becomes a double-edged sword. While O2 is needed to prevent hypoxia, it also causes oxidative stress leading to a plethora of morbidities, including retinopathy and BPD. The advent of continuous O2 monitoring with pulse oximeters has allowed clinicians to recognize the narrow therapeutic margins of oxygenation for the preterm infant, but more knowledge is needed to understand what these ranges are at different stages of the preterm infant's life, including at birth, in the neonatal intensive care unit and after hospital discharge. Future research, especially in innovative technologies such as automated O2 control and remote oximetry, will improve the understanding and treatment of the O2 needs of infants with BPD.

Mechanical Ventilation: Negative to Positive and Back Again

Critical care and mechanical ventilation have a relatively brief history in medicine. Premises existed through the seventeenth to nineteenth centuries but modern mechanical ventilation started in the twentieth century. Noninvasive ventilation techniques had started both in the intensive care unit and for home ventilation at the end of the 1980s and the 1990s. The need for mechanical ventilation is increasingly influenced worldwide by the spread of respiratory viruses, and the last coronavirus disease 2019 pandemic has seen a massive successful use of noninvasive ventilation.

Oxygenation target in acute respiratory distress syndrome

Determining oxygenation targets in acute respiratory distress syndrome (ARDS) remains a challenge. Although oxygenation targets have been used since ARDS was first described, they have not been investigated in detail. However, recent retrospective and prospective trials have evaluated the optimal oxygenation threshold in patients admitted to the general intensive care unit. In view of the lack of prospective data, clinicians continue to rely on data from the few available trials to identify the optimal oxygenation strategy. Assessment of the cost-benefit ratio of the fraction of inspired oxygen (FiO₂) to the partial pressure of oxygen in the arterial blood (PaO₂) is an additional challenge. A high FiO₂ has been found to be responsible for respiratory failure and deaths in numerous animal models. Low and high PaO₂ values have also been demonstrated to be potential risk factors in experimental and clinical situations. The findings from this literature review suggest that PaO₂ values ranging between 80 mmHg and 90 mmHg are acceptable in patients with ARDS. The costs of rescue maneuvers needed to reach these targets have been discussed. Several recent papers have highlighted the risk of disagreement between arterial oxygen saturation (SaO₂) and peripheral oxygen saturation (SpO₂) values. In order to avoid discrepancies and hidden hypoxemia, SpO₂ readings need to be compared with those of SaO₂. Higher SpO₂ values may be needed to achieve the recommended PaO₂ and SaO₂ values.

Conservative oxygen supplementation during Helmet CPAP therapy in patients with COVID-19 and respiratory failure: A pilot study

Background

Respiratory failure is a severe complication in COVID-19 pneumonia that, in addition to oxygen therapy, may require CPAP support. It has been postulated that COVID-19 lung injury may share some features with those observed in HALI. Thus, a correct target PaO2during oxygen supplementation may be crucial to protect the lung from further tissue damage. Aims of the study were: 1) to evaluate the effects of conservative oxygen supplementation during Helmet CPAP therapy on mortality and ICU admission in patients with COVID-19 and respiratory failure; 2) to evaluate the effect of conservative oxygen supplementation on new-onset organ failure and secondary pulmonary infections.

Methods

This was a single-center, historically controlled study of patients with severe respiratory failure due to COVID-19 pneumonia, receiving either conservative or non-conservative oxygen supplementation during Helmet CPAP. A cohort receiving conservative oxygen supplementation was studied prospectively in which oxygen supplementation was administered with a target PaO2<100 mmHg. Results of this cohort were compared with those of a cohort who had received liberal oxygen supplementation.

Results

Seventy-one patients were included in the conservative cohort and 75 in the non-conservative cohort. Mortality rate was lower in the conservative cohort (22.5%versus62.7%, p<0.001). Rates of ICU admission and new-onset rate organ failure were lower in conservative cohort (14.1%versus37.3%, p=0.001, and 9.9%versus45.3% p<0.001, respectively).

Conclusions

In patients with COVID-19 and severe respiratory failure, conservative oxygen supplementation during Helmet CPAP was associated to improved survival, lower ICU admission rate and less new-onset organ failure.

Effect of Hypoxemia on Outcome in Respiratory Failure Supported With Extracorporeal Membrane Oxygenation: A Cardinality Matched Cohort Study

Venovenous extracorporeal membrane oxygenation (ECMO) is recommended in adult patients with refractory acute respiratory failure (ARF), but there is limited evidence for its use in patients with less severe hypoxemia. Prior research has suggested a lower PaO 2 /FiO 2 at cannulation is associated with higher short-term mortality, but it is unclear whether this is due to less severe illness or a potential benefit of earlier ECMO support. In this exploratory cardinality-matched observational cohort study, we matched 668 patients who received venovenous ECMO as part of a national severe respiratory failure service into cohorts of "less severe" and "very severe" hypoxemia based on the median PaO 2 /FiO 2 at ECMO institution of 68 mmHg. Before matching, ICU mortality was 19% in the 'less severe' hypoxemia group and 28% in the "very severe" hypoxemia group (RR for mortality = 0.69, 95% CI 0.54-0.88). After matching on key prognostic variables including underlying diagnosis, this difference remained statistically present but smaller: (23% vs. 30%, RR = 0.76, 95% CI 0.59-0.99). This may suggest the observed survival benefit of venovenous ECMO is not solely due to reduced disease severity. Further research is warranted to examine the potential role of ECMO in ARF patients with less severe hypoxemia.

Unilateral acute lung injury in pig: a promising animal model

Background

Acute lung injury (ALI) occurs in 23% unilateral. Models of unilateral ALI were developed and used previously without clearly demonstrating the strictly unilateral nature and severity of lung injury by the key parameters characterizing ALI as defined by the American Thoracic Society (ATS). Thus, the use of unilateral ALI remained rare despite the innovative approach. Therefore, we developed a unilateral model of ALI and focused on the crucial parameters characterizing ALI. This model can serve for direct comparisons between the injured and intact lungs within single animals, thus, reducing the number of animals required for valid experimental conclusions.

Methods

We established the model in nine pigs, followed by an evaluation of key parameters in six pigs (main study). Pigs were ventilated using an adapted left double-lumen tube for lung separation and two ventilators. ALI was induced in the left lung with cyclic rinsing (NaCl 0.9% + Triton® X-100), after which pigs were ventilated for different time spans to test for the timing of ALI onset. Ventilatory and metabolic parameters were evaluated, and bronchoalveolar lavage (BAL) was performed for measurements of inflammatory mediators. Finally, histopathological specimens were collected and examined in respect of characteristics defining the lung injury score (LIS) as suggested by the ATS.

Results

After adjustments of the model (n = 9) we were able to induce strictly left unilateral ALI in all six pigs of the evaluation study. The median lung injury score was 0.72 (IQR 0.62–0.79) in the left lung vs 0.14 (IQR 0.14–0.16; p < 0.05) in the right lung, confirming unilateral ALI. A significant and sustained drop in pulmonary compliance (Cdyn) of the left lung occurred immediately, whereas Cdyn of the right lung remained unchanged (p < 0.05). BAL fluid concentrations of interleukin-6 and -8 were increased in both lungs.

Conclusions

We established a model of unilateral ALI in pigs, confirmed by histopathology, and typical changes in respiratory mechanics and an inflammatory response. This thoroughly evaluated model could serve as a basis for future studies and for comparing pathophysiological and pharmacological changes in the uninjured and injured lung within the same animal.

Oxygen as an Essential Medicine

Supplemental oxygen is an essential medication in critical care. The optimal oxygen dose delivery system remains unclear, however. The "dose" and "delivery" of oxygen carry significant importance for resource-limited settings, such as low- and middle-income countries (LMICs). Regrettably, LMICS often experience significant inequities in oxygen supply and demand, with major impacts on preventable mortality. These inequities have become particularly prominent during the global COVID-19 pandemic, highlighting the need for additional investment and research into the best methods to utilize supplemental oxygen and ensure stable access to medical oxygen.

Continuous Titration of Inspired Oxygen Using Oxygen Reserve Index to Decrease Oxygen Exposure During One-Lung Ventilation: A Randomized Controlled Trial

BACKGROUND

A high fraction of inspired oxygen (Fio2) is administered during one-lung ventilation (OLV). However, a high Fio2 is not physiologic and may lead to various complications. We hypothesized that continuous titration of Fio2 using the oxygen reserve index (ORI) reduces oxygen exposure compared to conventional management during OLV.

METHODS

In this randomized, double-blinded trial, patients undergoing thoracic surgery were assigned to an ORI (n = 64) or a control group (n = 60). In the ORI group, ORI was continuously displayed using multiwavelength pulse co-oximetry (Masimo) between 0 and 1 (0, no reserve; 1, maximum reserve), and Fio2 was titrated for a target ORI of 0.21 at 5-minute intervals during OLV. In the control group, Fio2 was adjusted using arterial blood gas analysis measured at 15 minutes after OLV initiation. The primary end point was the time-weighted average Fio2 during OLV.

RESULTS

Overall, time-weighted average Fio2 did not differ between the groups (control versus ORI: median [interquartile range], 0.87 [0.73-1.00] vs 0.82 [0.68-0.93]; P = .09). However, in a subgroup analysis, the ORI group reduced time-weighted average Fio2 after pulmonary vascular ligation compared to the control group (control versus ORI: median [interquartile range], 0.75 [0.70-1.00] vs 0.72 [0.59-0.89]; P = .0261). The incidence of intraoperative hypoxia (arterial oxygen saturation [Spo2] <94%; control versus ORI: 32% [19/60; 95% confidence interval (CI), 20-45] vs 19% [12/64; 95% CI, 10-31]; P = .09), and postoperative complications within the first 7 days did not differ between the groups.

CONCLUSIONS

ORI-guided continuous Fio2 titration does not reduce overall oxygen exposure during OLV.

Prevalence of Diaphragmatic Dysfunction in the Long-Term Acute Care Setting and Its Effects on Ventilator Weaning Outcomes: A Retrospective Cohort Study

OBJECTIVE

To determine the prevalence of undiagnosed diaphragmatic dysfunction in a long-term acute care hospital setting in patients on prolonged mechanical ventilation and its association with weaning outcomes.

DESIGN

This is a single-center, retrospective cohort study including 451 patients on prolonged mechanical ventilation admitted to a long-term acute care hospital facility between 2012 and 2017. Diaphragmatic dysfunction was assessed using fluoroscopy.

RESULTS

Three hundred nineteen patients on prolonged mechanical ventilation were assessed for diaphragmatic dysfunction. Nine patients were diagnosed with diaphragmatic dysfunction before admission. Eighty (72.7%) without diaphragmatic dysfunction were successfully weaned and 30 (27.3%) failed to wean, whereas 51 participants (31.9%) with diaphragmatic dysfunction were successfully weaned and 109 (68.1%) failed to wean (P < 0.001). When analyzing days to wean, the median was 13 days for those with no diaphragmatic dysfunction, 19 days with unilateral diaphragmatic dysfunction, and 28 days with bilateral diaphragmatic dysfunction (P < 0.001). Weaning success was not statistically associated with generalized neuromuscular disorders, age, sex, body mass index, smoking history, or diabetes.

CONCLUSIONS

Diaphragmatic dysfunction was found to be strongly associated with time to wean and weaning success in the long-term acute care hospital setting. Very few patients despite being on prolonged mechanical ventilation were diagnosed with diaphragmatic dysfunction before long-term acute care hospital admission. Given this information, early diagnosis of diaphragmatic dysfunction among prolonged mechanical ventilation patients in the long-term acute care hospital setting is paramount in preventing secondary complications associated with mechanical ventilation.

Clinical Benefit of Extubation in Patients on Venoarterial Extracorporeal Membrane Oxygenation

OBJECTIVES

Although patients on venoarterial extracorporeal membrane oxygenation for refractory cardiogenic shock are usually supported with mechanical ventilation, it is not clear whether sedation cessation and extubation might improve outcomes.

DESIGN

Retrospective cohort study with propensity score overlap weighting analysis.

SETTING

Three ICUs in a 1,500-bed tertiary university hospital.

PATIENTS

From an overall cohort of 641 patients with venoarterial-extracorporeal membrane oxygenation support, the primary analysis was performed in 344 patients who had been successfully decannulated in order to reduce immortal time bias.

MEASUREMENTS AND MAIN RESULTS

Seventy-five patients (22%) were extubated during extracorporeal membrane oxygenation support and were subsequently decannulated alive. Forty-nine percent received noninvasive ventilation, and 25% had emergency reintubation for respiratory, neurologic, or hemodynamic reasons. Higher Simplified Acute Physiology Score II at admission (odds ratio, 0.97; 95% CI [0.95-0.99]; p = 0.008) was associated with a lower probability of extubation, whereas cannulation in cardiac surgery ICU (odds ratio, 3.14; 95% CI [1.21-8.14]; p = 0.018) was associated with an increased probability. Baseline characteristics were well balanced after propensity score overlap weighting. The number of ICU-free days within 30 days of extracorporeal membrane oxygenation decannulation was significantly higher among extubated patients compared with nonextubated patients (22 d [11-26 d] vs 18 d [7-25 d], respectively; p = 0.036). There were no differences in other outcomes including ventilator-associated pneumonia (odds ratio, 0.96; 95% CI [0.51-1.82]; p = 0.90) and all-cause mortality within 30 days of extracorporeal membrane oxygenation decannulation (5% vs 17%; hazard ratio, 0.54; 95% CI [0.19-1.59]; p = 0.27).As a secondary analysis, outcomes were compared in the overall cohort of 641 venoarterial extracorporeal membrane oxygenation-supported patients. Results were consistent with the primary analysis as extubated patients had a higher number of ICU-free days (18 d [0-24 d] vs 0 d [0-18 d], respectively; < 0.001) and a lower risk of death within 30 days of extracorporeal membrane oxygenation cannulation (hazard ratio, 0.45; 95% CI [0.29-0.71]; p = 0.001).

CONCLUSIONS

Extubation during venoarterial-extracorporeal membrane oxygenation support is safe, feasible, and associated with greater ICU-free days.

Pulmonary Interstitial Matrix and Lung Fluid Balance From Normal to the Acutely Injured Lung

This review analyses the mechanisms by which lung fluid balance is strictly controlled in the air-blood barrier (ABB). Relatively large trans-endothelial and trans-epithelial Starling pressure gradients result in a minimal flow across the ABB thanks to low microvascular permeability aided by the macromolecular structure of the interstitial matrix. These edema safety factors are lost when the integrity of the interstitial matrix is damaged. The result is that small Starling pressure gradients, acting on a progressively expanding alveolar barrier with high permeability, generate a high transvascular flow that causes alveolar flooding in minutes. We modeled the trans-endothelial and trans-epithelial Starling pressure gradients under control conditions, as well as under increasing alveolar pressure (Palv) conditions of up to 25 cmH2O. We referred to the wet-to-dry weight (W/D) ratio, a specific index of lung water balance, to be correlated with the functional state of the interstitial structure. W/D averages ∼5 in control and might increase by up to ∼9 in severe edema, corresponding to ∼70% loss in the integrity of the native matrix. Factors buffering edemagenic conditions include: (i) an interstitial capacity for fluid accumulation located in the thick portion of ABB, (ii) the increase in interstitial pressure due to water binding by hyaluronan (the "safety factor" opposing the filtration gradient), and (iii) increased lymphatic flow. Inflammatory factors causing lung tissue damage include those of bacterial/viral and those of sterile nature. Production of reactive oxygen species (ROS) during hypoxia or hyperoxia, or excessive parenchymal stress/strain [lung overdistension caused by patient self-induced lung injury (P-SILI)] can all cause excessive inflammation. We discuss the heterogeneity of intrapulmonary distribution of W/D ratios. A W/D ∼6.5 has been identified as being critical for the transition to severe edema formation. Increasing Palv for W/D > 6.5, both trans-endothelial and trans-epithelial gradients favor filtration leading to alveolar flooding. Neither CT scan nor ultrasound can identify this initial level of lung fluid balance perturbation. A suggestion is put forward to identify a non-invasive tool to detect the earliest stages of perturbation of lung fluid balance before the condition becomes life-threatening.

Volume-Targeted Ventilation

Volume-targeted ventilation (VTV) has been increasingly used in neonatology. In systematic reviews, VTV has been shown to reduce the risk of neonatal morbidities and improve long-term outcomes. It is adaptive ventilation using complex computer algorithms to deliver ventilator inflations with expired tidal volumes close to a target set by clinicians. Significant endotracheal tube leak and patient-ventilator interactions may complicate VTV and make ventilator parameters and waveforms difficult to interpret. In this article, we review the rationale for using VTV and the evidence supporting its use and provide practical advice for clinicians ventilating newborn infants.

Bronchopulmonary dysplasia-A historical perspective

Bronchopulmonary dysplasia (BPD) was first described by Northway et al in 1967. This article describes the evolution of our understanding of the pathophysiology of BPD and the approaches to treatments of this illness developed over the past fifty years. These interventions had their roots in the understanding of the principles of the surface tension present at air-liquid interfaces, which were developed over 150 years before BPD's initial description. Improving outcomes in neonatal care have led to greater survival of preterm and very preterm infants, and to an evolution of the pathogenesis and pathology of BPD, from an illness caused primarily by barotrauma and oxygen toxicity to one of interruption of lung development. While the incidence of BPD has remained about the same in recent decades, this is because survival of infants born at lower gestational ages is increasing. Understanding of molecular, genetic and physiologic mechanisms has led to newer treatments that have mitigated some of the harmful effects of prolonged mechanical ventilation. Recognition of BPD as a chronic multi-system disease has resulted in further improvements in care after discharge from neonatal intensive care. Since many of the origins of chronic obstructive lung disease in adults are based in childhood respiratory illnesses, improving outcomes of BPD in infancy and childhood will undoubtedly lead to improved respiratory outcomes in the adults that these children will become.

Mechanical ventilation in COVID-19: A physiological perspective

New Findings

What is the topic of this review?

This review presents the fundamental concepts of respiratory physiology and pathophysiology, with particular reference to lung mechanics and the pulmonary phenotype associated with severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) infection and subsequent coronavirus disease 2019 (COVID‐19) pneumonia.

What advances does it highlight?

The review provides a critical summary of the main physiological aspects to be considered for safe and effective mechanical ventilation in patients with severe COVID‐19 in the intensive care unit.

Abstract

Severe respiratory failure from coronavirus disease 2019 (COVID‐19) pneumonia not responding to non‐invasive respiratory support requires mechanical ventilation. Although ventilation can be a life‐saving therapy, it can cause further lung injury if airway pressure and flow and their timing are not tailored to the respiratory system mechanics of the individual patient. The pathophysiology of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) infection can lead to a pattern of lung injury in patients with severe COVID‐19 pneumonia typically associated with two distinct phenotypes, along a temporal and pathophysiological continuum, characterized by different levels of elastance, ventilation‐to‐perfusion ratio, right‐to‐left shunt, lung weight and recruitability. Understanding the underlying pathophysiology, duration of symptoms, radiological characteristics and lung mechanics at the individual patient level is crucial for the appropriate choice of mechanical ventilation settings to optimize gas exchange and prevent further lung injury. By critical analysis of the literature, we propose fundamental physiological and mechanical criteria for the selection of ventilation settings for COVID‐19 patients in intensive care units. In particular, the choice of tidal volume should be based on obtaining a driving pressure < 14 cmH₂O, ensuring the avoidance of hypoventilation in patients with preserved compliance and of excessive strain in patients with smaller lung volumes and lower lung compliance. The level of positive end‐expiratory pressure (PEEP) should be informed by the measurement of the potential for lung recruitability, where patients with greater recruitability potential may benefit from higher PEEP levels. Prone positioning is often beneficial and should be considered early. The rationale for the proposed mechanical ventilation settings criteria is presented and discussed.

Supplemental oxygen therapy in trauma patients: An exploratory registry-based study

BACKGROUND

Supplemental oxygen (SO) is one of the most commonly administered drugs in trauma patients and is recommended by guidelines. However, evidence supporting uniform administration is sparse, and excess oxygen use has been shown to be harmful in other patient populations. We hypothesized that SO may be harmful in patients with oxygen saturation > 97%.

METHODS

Patients with available information on SO-therapy in the American Trauma Quality Improvement Program 2017 database were included. Patients were categorized into 3 groups according to Emergency Department (ED) oxygen saturation: (1) saturation < 94%; (2) saturation 94%-97%; (3) saturation 98%-100%. Primary outcome was in-hospital mortality with comparisons made between patients who received SO or not. Secondary outcome was acute respiratory distress syndrome (ARDS). Patients were compared after propensity score matching.

RESULTS

Overall, 864 340 patients were identified. Mean age was 47.4 ± 24.4 years, and median injury severity score was 9. SO was associated with an increased risk of in-hospital mortality: (all patients: adjusted odds ratio [aOR] with 95% confidence interval [CI] 3.07 [2.92-3.22], ED saturation <94%: 2.63 [2.38-2.91], ED saturation 94%-97%: 2.71 [2.47-2.97], ED saturation >97%: 3.38 [3.16-3.61]. Same pattern was seen for in-hospital ARDS: (aOR 1.79, 95% CI [1.59-2.02], ED saturation <94%: aOR 1.75, 95% CI [1.37-2.24], ED saturation 94%-97%: aOR 1.81, 95% CI [1.43-2.29, ED saturation >97%: aOR 2.31, 95% CI [1.92-2.79]).

CONCLUSION

Based on propensity matched, registry data for trauma patients, the administration of SO was associated with a higher incidence of in-hospital mortality and ARDS. The highest risk was found in patients with an ED saturation >97%.

Rat model of smoke inhalation-induced acute lung injury

BACKGROUND

Acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) is a lethal disease with limited therapeutic options and an unacceptably high mortality rate. Understanding the complex pathophysiological processes involved in the development of ALI/ARDS is critical for developing novel therapeutic strategies. Smoke inhalation (SI) injury is the leading cause of morbidity and mortality in patients with burn-associated ALI/ARDS; however, to our knowledge few reliable, reproducible models are available for pure SI animal model to investigate therapeutic options for ALI/ARDS without the confounding variables introduced by cutaneous burn or other pathology.

OBJECTIVE

To develop a small animal model of pure SI-induced ALI and to use this model for eventual testing of novel therapeutics for ALI.

METHODS

Rats were exposed to smoke using a custom-made smoke generator. Peripheral oxygen saturation (SpO₂), heart rate, arterial blood gas, and chest X-ray (CXR) were measured before and after SI. Wet/dry weight (W/D) ratio, lung injury score and immunohistochemical staining of cleaved caspase 3 were performed on harvested lung tissues of healthy and SI animals.

RESULTS

The current study demonstrates the induction of ALI in rats after SI as reflected by a significant, sustained decrease in SpO₂ and the development of diffuse bilateral pulmonary infiltrates on CXR. Lung tissue of animals exposed to SI showed increased inflammation, oedema and apoptosis as reflected by the increase in W/D ratio, injury score and cleaved caspase 3 level of the harvested tissues compared with healthy animals.

CONCLUSION

We have successfully developed a small animal model of pure SI-induced ALI. This model is offered to the scientific community as a reliable model of isolated pulmonary SI-induced injury without the confounding variables of cutaneous injury or other systemic pathology to be used for study of novel therapeutics or other investigation.

A novel large animal model of smoke inhalation-induced acute respiratory distress syndrome

Background

Acute respiratory distress syndrome (ARDS) is multifactorial and can result from sepsis, trauma, or pneumonia, amongst other primary pathologies. It is one of the major causes of death in critically ill patients with a reported mortality rate up to 45%. The present study focuses on the development of a large animal model of smoke inhalation-induced ARDS in an effort to provide the scientific community with a reliable, reproducible large animal model of isolated toxic inhalation injury-induced ARDS.

Methods

Animals (n = 21) were exposed to smoke under general anesthesia for 1 to 2 h (median smoke exposure = 0.5 to 1 L of oak wood smoke) after the ultrasound-guided placement of carotid, pulmonary, and femoral artery catheters. Peripheral oxygen saturation (SpO₂), vital signs, and ventilator parameters were monitored throughout the procedure. Chest x-ray, carotid, femoral and pulmonary artery blood samples were collected before, during, and after smoke exposure. Animals were euthanized and lung tissue collected for analysis 48 h after smoke inhalation.

Results

Animals developed ARDS 48 h after smoke inhalation as reflected by a decrease in SpO₂ by approximately 31%, PaO₂/FiO₂ ratio by approximately 208 (50%), and development of bilateral, diffuse infiltrates on chest x-ray. Study animals also demonstrated a significant increase in IL-6 level, lung tissue injury score and wet/dry ratio, as well as changes in other arterial blood gas (ABG) parameters.

Conclusions

This study reports, for the first time, a novel large animal model of isolated smoke inhalation-induced ARDS without confounding variables such as cutaneous burn injury. Use of this unique model may be of benefit in studying the pathophysiology of inhalation injury or for development of novel therapeutics.

Oxygen Toxicity in Critically Ill Adults

Oxygen supplementation is one of the most common interventions in critically ill patients. Despite over a century of data suggesting both beneficial and detrimental effects of supplemental oxygen, optimal arterial oxygenation targets in adult patients remain unclear. Experimental animal studies have consistently showed that exposure to a high fraction of inspired oxygen causes respiratory failure and early death. Human autopsy studies from the 1960s purported to provide histologic evidence of pulmonary oxygen toxicity in the form of diffuse alveolar damage. However, concomitant ventilator-induced lung injury and/or other causes of acute lung injury may explain these findings. While some observational studies in general populations of critically adults showed higher mortality in association with higher oxygen exposures, this finding has not been consistent. For some specific populations, such as those with cardiac arrest, studies have suggested harm from targeting supraphysiologic PaO2s. More recently, randomized clinical trials of arterial oxygenation targets in narrower physiologic ranges were conducted in critically ill adult patients. Though two smaller trials came to opposite conclusions, the two largest of these trials showed no differences in clinical outcomes in study groups that received conservative versus liberal oxygen targets, suggesting that either strategy is reasonable. It is possible that some strategies are of benefit in some sub-populations, and this remains an important ongoing area of research. Because of the ubiquity of oxygen supplementation in critically ill adults, even small treatment effects could have a large impact on a global scale.

Interstitial Lung Disease Worsens Short- and Long-Term Outcomes of Systemic Rheumatic Disease Patients Admitted to the ICU: A Multicenter Study

Critically ill patients with systemic rheumatic diseases (SRDs) have a fair prognosis, while those with interstitial lung disease (ILD) have a poorer outcome. However, the prognosis of SRD patients with ILD admitted to the intensive care unit (ICU) remains unclear. We conducted a case–control study to investigate the outcomes of critically ill SRD-ILD patients. Consecutive SRD-ILD patients admitted to five ICUs from January 2007 to December 2017 were compared to SRD patients without ILD. Mortality rates were compared between groups, and prognostic factors were then identified. One hundred and forty critically ill SRD patients were included in the study. Among the 70 patients with SRD–ILD, the SRDs were connective tissue diseases (56%), vasculitis (29%), sarcoidosis (13%), and spondylarthritis (3%). Patients were mainly admitted for acute exacerbation of SRD-ILD (36%) or infection (34%). ICU, in-hospital, and one-year mortality rates in SRD-ILD patients were higher than in SRD patients without ILD (n = 70): 40% vs. 16% (p < 0.01), 49% vs. 19% (p < 0.01), and 66% vs. 40% (p < 0.01), respectively. Hypoxemia, high sequential organ failure assessment (SOFA) score, and admission for ILD acute exacerbation were associated with ICU mortality. In conclusion, ILD worsened the outcomes of SRD patients admitted to the ICU. Admissions related to SRD-ILD acute exacerbation and the severity of the acute respiratory failure were associated with ICU mortality.

Impact of Using a Non-Rebreathing Mask in Patients With Respiratory Failure

BACKGROUND

Liberal oxygen therapy might increase the mortality rate of patients. Non-rebreathing masks (NRM) are a high-flow, non-invasive oxygen device that can provide oxygen concentration up to 95%. This study aimed to determine the impact of using NRM in patients with respiratory failure.

METHODS

This retrospective cohort study was conducted in four medical institutions in Taiwan from January 2010 to December 2016. The association between mortality and NRM use before receiving ventilator support in patients with respiratory failure in the emergency department was analyzed. Patients were divided into the NRM treatment and no NRM treatment groups. A 1:4 propensity score matching was conducted. Regarding the duration of NRM use, treatments were grouped as 0 h, 0-1 h, 1-2 h, and >2 h.

RESULTS

A total of 18,749 patients were included, with 1074 using NRM. After propensity score matching, 1028 patients using NRM (0-1 h: 508, 1-2 h: 193, and >2 h: 327) and 4112 patients not using NRM were analyzed. The 30-day mortality rates were 29.1%, 28.5%, 27.5%, and 35.5% in the 0 h, 0-1 h, 1-2 h, and >2 h treatment groups, respectively. Patients with respiratory failure due to pulmonary disease using NRM over 2 h had a higher mortality rate than patients not using NRM (hazard ratio: 1.3, 95% CI: 1.01-1.66).

CONCLUSIONS

Prolonged use of NRM in patients with respiratory failure due to pulmonary disease possibly increases mortality.

Interpretation of the transpulmonary pressure in the critically ill patient

Mechanical ventilation is a life-saving procedure, which takes over the function of the respiratory muscles while buying time for healing to take place. However, it can also promote or worsen lung injury, so that careful monitoring of respiratory mechanics is suggested to titrate the level of support and avoid injurious pressures and volumes to develop. Standard monitoring includes flow, volume and airway pressure (Paw). However, Paw represents the pressure acting on the respiratory system as a whole, and does not allow to differentiate the part of pressure that is spent di distend the chest wall. Moreover, if spontaneous breathing efforts are allowed, the Paw is the sum of that applied by the ventilator and that generated by the patient. As a consequence, monitoring of Paw has significant shortcomings. Assessment of esophageal pressure (Pes), as a surrogate for pleural pressure (Ppl), may allow the clinicians to discriminate between the elastic behaviour of the lung and the chest wall, and to calculate the degree of spontaneous respiratory effort. In the present review, the characteristics and limitations of airway and transpulmonary pressure monitoring will be presented; we will highlight the different assumptions underlying the various methods for measuring transpulmonary pressure (i.e., the elastance-derived and the release-derived method, and the direct measurement), as well as the potential application of transpulmonary pressure assessment during both controlled and spontaneous/assisted mechanical ventilation in critically ill patients.

Admission Hyperoxia Is a Risk Factor for Mortality in Pediatric Intensive Care

OBJECTIVES

To determine whether the association between hyperoxia and increased risk-adjusted mortality in adult intensive care patients is also observed in a pediatric intensive care population.

DESIGN

Single-center retrospective analysis of admissions to ICU over a 5-year period commencing January 1, 2012, examining the relationship between PaO2 measured within the first hour of admission and risk-adjusted mortality. Standardized mortality rates were calculated using the Pediatric Index of Mortality-3, and patients were grouped into 50 mm Hg (6.67 kPa) PaO2 bands to assess the relationship between initial PaO2 and risk-adjusted mortality.

SETTING

Tertiary PICU with 17 beds and 1,100 annual admissions located in metropolitan Sydney, Australia.

PATIENTS

A total of 1,447 patients 0-18 years old with PaO2 recorded at admission to the ICU.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

There were 5,176 patients admitted to the ICU during the study period and 1,447 (28%) with PaO2 recorded at admission. A U-shaped relationship between raw mortality and admission PaO2 was observed, with lowest mortality (2.3% and 2.6%, respectively) observed in the 101-150 (13.5-20.0 kPa) and 151-200 mm Hg (20.1-26.7 kPa) bands and the highest mortality observed in patients with PaO2 less than 50 mm Hg (6.67 kPa) with mortality of 5.3%, or greater than 350 mm Hg (46.7 kPa) with mortality of 18.2%. Hyperoxia at admission was associated with an increase in risk-adjusted mortality, with polynomial regression indicating a strong correlation between PaO2 band and risk-adjusted outcome (r = 0.845). When included in a multivariate model that included the Pediatric Index of Mortality-3 variables, the odds ratio for hyperoxia (defined as PaO2 > 250 mm Hg [33.3 kPa]) predicting death was 2.66 (p = 0.047).

CONCLUSIONS

In this single-center study, hyperoxia at admission to the PICU was highly correlated with increased risk-adjusted mortality. Further investigation of these observations in a large multicenter cohort is warranted.

Acute Lung Injury

A wide variety of insults can produce acute lung damage, inclusive of those that injure the lungs directly. The clinical syndrome of acute onset respiratory distress, dyspnea, and bilateral infiltrates is referred to as acute respiratory distress syndrome. The histologic counterpart of acute respiratory distress syndrome is diffuse alveolar damage, classically characterized by hyaline membranes. Other histologic features of acute lung injury include intraalveolar fibrin, organization, interstitial edema, and reactive pneumocytes. Diffuse alveolar damage and other histologic features of acute lung injury are nonspecific as to etiology, and once identified require the pathologist to search the biopsy for further features that may help identify a specific etiology. This chapter reviews the temporal sequence of acute lung injury and explores the large variety of specific etiologic causes with emphasis on helpful histologic features to identify.

Fifty Years of Research in ARDS. Respiratory Mechanics in Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome is a multifactorial lung injury that continues to be associated with high levels of morbidity and mortality. Mechanical ventilation, although lifesaving, is associated with new iatrogenic injury. Current best practice involves the use of small Vt, low plateau and driving pressures, and high levels of positive end-expiratory pressure. Collectively, these interventions are termed "lung-protective ventilation." Recent investigations suggest that individualized measurements of pulmonary mechanical variables rather than population-based ventilation prescriptions may be used to set the ventilator with the potential to improve outcomes beyond those achieved with standard lung protective ventilation. This review outlines the measurement and application of clinically applicable pulmonary mechanical concepts, such as plateau pressures, driving pressure, transpulmonary pressures, stress index, and measurement of strain. In addition, the concept of the "baby lung" and the utility of dynamic in addition to static measures of pulmonary mechanical variables are discussed.

Mechanical Ventilation: State of the Art

Mechanical ventilation is the most used short-term life support technique worldwide and is applied daily for a diverse spectrum of indications, from scheduled surgical procedures to acute organ failure. This state-of-the-art review provides an update on the basic physiology of respiratory mechanics, the working principles, and the main ventilatory settings, as well as the potential complications of mechanical ventilation. Specific ventilatory approaches in particular situations such as acute respiratory distress syndrome and chronic obstructive pulmonary disease are detailed along with protective ventilation in patients with normal lungs. We also highlight recent data on patient-ventilator dyssynchrony, humidified high-flow oxygen through nasal cannula, extracorporeal life support, and the weaning phase. Finally, we discuss the future of mechanical ventilation, addressing avenues for improvement.

WHO Needs High FIO2?

World Health Organization and the United States Center for Disease Control have recently recommended the use of 0.8 FIO₂ in all adult surgical patients undergoing general anaesthesia, to prevent surgical site infections. This recommendation has arisen several discussions: As a matter of fact, there are numerous studies with different results about the effect of FIO₂ on surgical site infection. Moreover, the clinical effects of FIO₂ are not limited to infection control. We asked some prominent authors about their comments regarding the recent recommendations.

Fifty Years of Research in ARDS. Setting Positive End-Expiratory Pressure in Acute Respiratory Distress Syndrome

Positive end-expiratory pressure (PEEP) has been used during mechanical ventilation since the first description of acute respiratory distress syndrome (ARDS). In the subsequent decades, many different strategies for optimally titrating PEEP have been proposed. Higher PEEP can improve arterial oxygenation, reduce tidal lung stress and strain, and promote more homogenous ventilation by preventing alveolar collapse at end expiration. However, PEEP may also cause circulatory depression and contribute to ventilator-induced lung injury through alveolar overdistention. The overall effect of PEEP is primarily related to the balance between the number of alveoli that are recruited to participate in ventilation and the amount of lung that is overdistended when PEEP is applied. Techniques to assess lung recruitment from PEEP may help to direct safer and more effective PEEP titration. Some PEEP titration strategies attempt to weigh beneficial effects on arterial oxygenation and on prevention of cyclic alveolar collapse with the harmful potential of overdistention. One method for PEEP titration is a PEEP/FiO2 table that prioritizes support for arterial oxygenation. Other methods set PEEP based on mechanical parameters, such as the plateau pressure, respiratory system compliance, or transpulmonary pressure. No single method of PEEP titration has been shown to improve clinical outcomes compared with other approaches of setting PEEP. Future trials should focus on identifying individuals who respond to higher PEEP with recruitment and on clinically important outcomes (e.g., mortality).

Acute respiratory distress syndrome

Acute respiratory distress syndrome presents as hypoxia and bilateral pulmonary infiltrates on chest imaging in the absence of heart failure sufficient to account for this clinical state. Management is largely supportive, and is focused on protective mechanical ventilation and the avoidance of fluid overload. Patients with severe hypoxaemia can be managed with early short-term use of neuromuscular blockade, prone position ventilation, or extracorporeal membrane oxygenation. The use of inhaled nitric oxide is rarely indicated and both β2 agonists and late corticosteroids should be avoided. Mortality remains at approximately 30%.

Biotrauma and Ventilator-Induced Lung Injury: Clinical Implications

The pathophysiological mechanisms by which mechanical ventilation can contribute to lung injury, termed "ventilator-induced lung injury" (VILI), is increasingly well understood. "Biotrauma" describes the release of mediators by injurious ventilatory strategies, which can lead to lung and distal organ injury. Insights from preclinical models demonstrating that traditional high tidal volumes drove the inflammatory response helped lead to clinical trials demonstrating lower mortality in patients who underwent ventilation with a lower-tidal-volume strategy. Other approaches that minimize VILI, such as higher positive end-expiratory pressure, prone positioning, and neuromuscular blockade have each been demonstrated to decrease indices of activation of the inflammatory response. This review examines the evolution of our understanding of the mechanisms underlying VILI, particularly regarding biotrauma. We will assess evidence that ventilatory and other "adjunctive" strategies that decrease biotrauma offer great potential to minimize the adverse consequences of VILI and to improve the outcomes of patients with respiratory failure.

High oxygen modifies vasodilator effect of cysteine via enhanced oxidative stress and thromboxane production in the rat mesenteric artery

Whether high oxygen is harmful to the vascular function is unclear. The present study examined if high oxygen modifies vasodilator effect of cysteine via enhanced oxidative stress and thromboxane production. Rat mesenteric arteries with endothelium at 95 or 50 % oxygen were subjected to isometric force recordings, measurement of thromboxane B2 levels, determination of superoxide and peroxynitrite levels and evaluation of NADPH oxidase subunit protein expression, respectively. L-cysteine (0.01-3 mM) constricted or dilated arteries at 95 and 50 % oxygen, respectively. Thromboxane receptor antagonist SQ-29,548 (1 μM) abolished the constriction at 95 % oxygen. L-cysteine (3 mM) increased levels of thromboxane B2 in arteries upon 95 % oxygen application. L-cysteine relaxed arteries treated with superoxide inhibitor tiron (2 mM) or NADPH oxidase inhibitor gp91ds-tat (1 μM) irrespective of the oxygen concentration while ATP-sensitive K(+) channel inhibitor glibenclamide (1 μM) and cystathionine-γ-lyase (CSE) inhibitor DL-propargylglycine (10 mM) similarly abolished the relaxation. L-cysteine (3 mM) with 95 % oxygen augmented levels of superoxide as well as nitrotyrosine within the artery, concomitantly with enhanced membrane protein expression of NADPH oxidase subunit p47phox. The higher concentration of oxygen attenuates L-cysteine-induced vasodilation via superoxide production mediated by NADPH oxidase along with thromboxane A2 production, resulting in vasoconstriction. The increased levels of superoxide, as well as peroxynitrite, coexist with the impaired vasodilation related to ATP-sensitive K(+) channels and CSE. Higher oxygen with plasma cysteine may cause oxidative stress and vasoconstrictor prostanoid production in blood vessels.

Analytic Reviews : Oxygen Toxicity

Oxygen therapy is administered to patients to decrease tissue hypoxia and to relieve arterial hypoxemia. High concentrations of oxygen often are used for short pe riods of time in patients with acute respiratory illnesses, and concentrations only slightly higher than ambient levels are administered for much longer time periods to patients with chronic respiratory diseases. Supplying oxygen to plants, animals, or bacteria has long been known to produce varying amounts of tissue damage; toxicity increases as concentrations of oxygen or the pressure used during exposure increases. End-organ damage from hyperoxia depends on both the concentra tion of oxygen administered and the pressure during the exposure. Prolonged exposure to hyperbaric oxygen (> 2.5 atmosphere of pressure) causes both central nervous system and pulmonary toxicity that results in atelectasis, pulmonary edema, and seizures. Lung dam age as a result of normobaric hyperoxia is the predomi nant manifestation of toxicity. A severe retinopathy (re trolental fibroplasia) also can occur in neonates during oxygen exposures at ambient pressure, and cases have been reported to occur with only modest increases in inspired oxygen concentrations. For these reasons, the lowest possible concentration of oxygen that relieves tissue hypoxia is administered to patients, and the oxy gen concentration is stabilized when the desired thera peutic goals are accomplished.

Clinical challenges in mechanical ventilation

Mechanical ventilation supports gas exchange and alleviates the work of breathing when the respiratory muscles are overwhelmed by an acute pulmonary or systemic insult. Although mechanical ventilation is not generally considered a treatment for acute respiratory failure per se, ventilator management warrants close attention because inappropriate ventilation can result in injury to the lungs or respiratory muscles and worsen morbidity and mortality. Key clinical challenges include averting intubation in patients with respiratory failure with non-invasive techniques for respiratory support; delivering lung-protective ventilation to prevent ventilator-induced lung injury; maintaining adequate gas exchange in severely hypoxaemic patients; avoiding the development of ventilator-induced diaphragm dysfunction; and diagnosing and treating the many pathophysiological mechanisms that impair liberation from mechanical ventilation. Personalisation of mechanical ventilation based on individual physiological characteristics and responses to therapy can further improve outcomes.

Hyperoxia in intensive care, emergency, and peri-operative medicine: Dr. Jekyll or Mr. Hyde? A 2015 update

This review summarizes the (patho)-physiological effects of ventilation with high FiO₂ (0.8–1.0), with a special focus on the most recent clinical evidence on its use for the management of circulatory shock and during medical emergencies. Hyperoxia is a cornerstone of the acute management of circulatory shock, a concept which is based on compelling experimental evidence that compensating the imbalance between O₂ supply and requirements (i.e., the oxygen dept) is crucial for survival, at least after trauma. On the other hand, “oxygen toxicity” due to the increased formation of reactive oxygen species limits its use, because it may cause serious deleterious side effects, especially in conditions of ischemia/reperfusion. While these effects are particularly pronounced during long-term administration, i.e., beyond 12–24 h, several retrospective studies suggest that even hyperoxemia of shorter duration is also associated with increased mortality and morbidity. In fact, albeit the clinical evidence from prospective studies is surprisingly scarce, a recent meta-analysis suggests that hyperoxia is associated with increased mortality at least in patients after cardiac arrest, stroke, and traumatic brain injury. Most of these data, however, originate from heterogenous, observational studies with inconsistent results, and therefore, there is a need for the results from the large scale, randomized, controlled clinical trials on the use of hyperoxia, which can be anticipated within the next 2–3 years. Consequently, until then, “conservative” O₂ therapy, i.e., targeting an arterial hemoglobin O₂ saturation of 88–95 % as suggested by the guidelines of the ARDS Network and the Surviving Sepsis Campaign, represents the treatment of choice to avoid exposure to both hypoxemia and excess hyperoxemia.

On the practical identifiability of a two-parameter model of pulmonary gas exchange

Background

Successful application of mechanical ventilation as a life-saving therapy implies appropriate ventilator settings. Decision making is based on clinicians’ knowledge, but can be enhanced by mathematical models that determine the individual patient state by calculating parameters that are not directly measurable. Evaluation of models may support the clinician to reach a defined treatment goal. Bedside applicability of mathematical models for decision support requires a robust identification of the model parameters with a minimum of measuring effort. The influence of appropriate data selection on the identification of a two-parameter model of pulmonary gas exchange was analyzed.

Methods

The model considers a shunt as well as ventilation-perfusion-mismatch to simulate a variety of pathologic pulmonary gas exchange states, i.e. different severities of pulmonary impairment. Synthetic patient data were generated by model simulation. To incorporate more realistic effects of measurement errors, the simulated data were corrupted with additive noise. In addition, real patient data retrieved from a patient data management system were used retrospectively to confirm the obtained findings. The model was identified to a wide range of different FiO₂ settings. Just one single measurement was used for parameter identification. Subsequently prediction performance was obtained by comparing the identified model predicted oxygen level in arterial blood either to exact data taken from simulations or patients measurements.

Results

Structural identifiability of the model using one single measurement for the identification process could be demonstrated. Minimum prediction error of blood oxygenation depends on blood gas level at the time of system identification i.e. the measurement situation. For severe pulmonary impairment, higher FiO₂ settings were required to achieve a better prediction capability compared to less impaired pulmonary states. Plausibility analysis with real patient data could confirm this finding.

Discussion and conclusions

Dependent on patients’ pulmonary state, the influence of ventilator settings (here FiO₂) on model identification of the gas exchange model could be demonstrated. To maximize prediction accuracy i.e. to find the best individualized model with as few data as possible, best ranges of FiO₂-settings for parameter identification were obtained. A less effort identification process, which depends on the pulmonary state, can be deduced from the results of this identifiability analysis.

Targeting normoxemia in acute respiratory distress syndrome may cause worse short-term outcomes because of oxygen toxicity

It was suggested that targeting normoxemia (PaO2 85-110 mm Hg) in patients with acute respiratory distress syndrome (ARDS) might prevent neurocognitive dysfunction in survivors. However, targeting normoxemia may cause detrimental effects to the lungs from oxygen toxicity. Some have suggested that oxygen is not harmful to the lungs at FiO2 (fraction of inspired oxygen) levels less than 0.6-0.7, but contrasting evidence in normal humans suggests that there can be untoward effects of moderate FiO2 levels. Furthermore, in experimental models of the acute respiratory distress syndrome, coexisting lung inflammation increases susceptibility to oxygen toxicity. Coexisting lung inflammation may lower the threshold for oxygen toxicity in patients with ARDS or in other acute illnesses in the lung. Moreover, physicians frequently prescribe higher FiO2 levels than are necessary to achieve their arterial oxygenation goal, further increasing the risk of oxygen toxicity. Targeting normoxemia in patients with ARDS may prevent some long-term neurocognitive deficits in survivors, but it may increase lung inflammation and cause worse short-term clinical outcomes. We advocate for a clinical trial in patients with ARDS to determine more appropriate goals for arterial oxygenation.

Gas exchange in the respiratory distress syndromes

This article describes the gas exchange abnormalities occurring in the acute respiratory distress syndrome seen in adults and children and in the respiratory distress syndrome that occurs in neonates. Evidence is presented indicating that the major gas exchange abnormality accounting for the hypoxemia in both conditions is shunt, and that approximately 50% of patients also have lungs regions in which low ventilation-to-perfusion ratios contribute to the venous admixture. The various mechanisms by which hypercarbia may develop and by which positive end-expiratory pressure improves gas exchange are reviewed, as are the effects of vascular tone and airway narrowing. The mechanisms by which surfactant abnormalities occur in the two conditions are described, as are the histological findings that have been associated with shunt and low ventilation-to-perfusion.

Oxygen therapy in critical illness: precise control of arterial oxygenation and permissive hypoxemia

OBJECTIVE

The management of hypoxemia in critically ill patients is challenging. Whilst the harms of tissue hypoxia are well recognized, the possibility of harm from excess oxygen administration, or other interventions targeted at mitigating hypoxemia, may be inadequately appreciated. The benefits of attempting to fully reverse arterial hypoxemia may be outweighed by the harms associated with high concentrations of supplemental oxygen and invasive mechanical ventilation strategies. We propose two novel related strategies for the management of hypoxemia in critically ill patients. First, we describe precise control of arterial oxygenation involving the specific targeting of arterial partial pressure of oxygen or arterial hemoglobin oxygen saturation to individualized target values, with the avoidance of significant variation from these levels. The aim of precise control of arterial oxygenation is to avoid the harms associated with inadvertent hyperoxia or hypoxia through careful and precise control of arterial oxygen levels. Secondly, we describe permissive hypoxemia: the acceptance of levels of arterial oxygenation lower than is conventionally tolerated in patients. The aim of permissive hypoxemia is to minimize the possible harms caused by restoration of normoxemia while avoiding tissue hypoxia. This review sets out to discuss the strengths and limitations of precise control of arterial oxygenation and permissive hypoxemia as candidate management strategies in hypoxemic critically ill patients.

DESIGN

We searched PubMed for references to "permissive hypoxemia/hypoxaemia" and "precise control of arterial oxygenation" as well as reference to "profound hypoxemia/hypoxaemia/hypoxia," "severe hypoxemia/hypoxaemia/hypoxia." We searched personal reference libraries in the areas of critical illness and high altitude physiology and medicine. We also identified large clinical studies in patients with critical illness characterized by hypoxemia such as acute respiratory distress syndrome.

SUBJECTS

Studies were selected that explored the physiology of hypoxemia in healthy volunteers or critically ill patients.

SETTING

The data were subjectively assessed and combined to generate the narrative.

RESULTS

Inadequate tissue oxygenation and excessive oxygen administration can be detrimental to outcome but safety thresholds lack definition in critically ill patients. Precise control of arterial oxygenation provides a rational approach to the management of arterial oxygenation that reflects recent clinical developments in other settings. Permissive hypoxemia is a concept that is untested clinically and requires robust investigation prior to consideration of implementation. Both strategies will require accurate monitoring of oxygen administration and arterial oxygenation. Effective, reliable measurement of tissue oxygenation along with the use of selected biomarkers to identify suitable candidates and monitor harm will aid the development of permissive hypoxemia as viable clinical strategy.

CONCLUSIONS

Implementation of precise control of arterial oxygenation may avoid the harms associated with excessive and inadequate oxygenation. However, at present there is no direct evidence to support the immediate implementation of permissive hypoxemia and a comprehensive evaluation of its value in critically ill patients should be a high research priority.

Hyperoxic acute lung injury

Prolonged breathing of very high F(IO(2)) (F(IO(2)) ≥ 0.9) uniformly causes severe hyperoxic acute lung injury (HALI) and, without a reduction of F(IO(2)), is usually fatal. The severity of HALI is directly proportional to P(O(2)) (particularly above 450 mm Hg, or an F(IO(2)) of 0.6) and exposure duration. Hyperoxia produces extraordinary amounts of reactive O(2) species that overwhelms natural anti-oxidant defenses and destroys cellular structures through several pathways. Genetic predisposition has been shown to play an important role in HALI among animals, and some genetics-based epidemiologic research suggests that this may be true for humans as well. Clinically, the risk of HALI likely occurs when F(IO(2)) exceeds 0.7, and may become problematic when F(IO(2)) exceeds 0.8 for an extended period of time. Both high-stretch mechanical ventilation and hyperoxia potentiate lung injury and may promote pulmonary infection. During the 1960s, confusion regarding the incidence and relevance of HALI largely reflected such issues as the primitive control of F(IO(2)), the absence of PEEP, and the fact that at the time both ALI and ventilator-induced lung injury were unknown. The advent of PEEP and precise control over F(IO(2)), as well as lung-protective ventilation, and other adjunctive therapies for severe hypoxemia, has greatly reduced the risk of HALI for the vast majority of patients requiring mechanical ventilation in the 21st century. However, a subset of patients with very severe ARDS requiring hyperoxic therapy is at substantial risk for developing HALI, therefore justifying the use of such adjunctive therapies.