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

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

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

PURPOSE OF REVIEW: Describe the rationale for concern and accumulating pathophysiologic evidence regarding the adverse effects of high-level positive end expiratory pressure (PEEP) on excessive mechanical stress and ventilator-induced lung injury (VILI). RECENT FINDINGS: Although the inclusion of PEEP in numerical estimates of mechanical power may be theoretically debated, its potential to increase stress, strain, and mean airway pressure are not. Recent laboratory data in a variety of animal models demonstrate that higher levels of PEEP coupled with additional fluids needed to offset its impediment of hemodynamic function are associated with increased VILI. Moreover, counteracting end-tidal hyperinflation by external chest wall pressure may paradoxically improve respiratory mechanics, indicating that lower PEEP helps protect the small 'baby lung' of advanced acute respiratory distress syndrome (ARDS). SUMMARY: The potentially adverse effects of PEEP on VILI can be considered in three broad categories. First, the contribution of PEEP to total mechanical energy expressed through mechanical power, raised mean airway pressure, and end-tidal hyperinflation; second, the hemodynamic consequences of altered cardiac loading, heightened pulmonary vascular stress and total lung water; and third, the ventilatory consequences of compromised carbon dioxide eliminating efficiency. Minimizing ventilation demands, optimized body positioning and care to avoid unnecessary PEEP are central to lung protection in all stages of ARDS.

Ventilatory Parameters Measured After One Week of Mechanical Ventilation and Survival in COVID-19-Related ARDS.

BACKGROUND: Ventilatory parameters measured soon after initiation of mechanical ventilation have limited ability to predict outcome of COVID-19-related ARDS. We hypothesized that ventilatory parameters measured after one week of mechanical ventilation might differ between survivors and non-survivors. METHODS: One hundred twenty-seven subjects with COVID-related ARDS had gas exchange and lung mechanics assessed on the day of intubation and one week later. The main parameters of interest were PaO2 /FIO2 , ventilatory ratio (VR), respiratory system compliance (CRS), and a composite score that was calculated as (PaO2 /FIO2 /100) × CRS/VR. The primary outcome was death in the ICU. RESULTS: Of the 127 subjects, 42 (33%) died in the ICU and 85 (67%) were successfully extubated. On the day of intubation, PaO2 /FIO2 , CRS, and composite score of survivors and non-survivors were similar, but survivors had a lower VR. At one week, as compared to survivors, non-survivors had a significantly higher VR (2.04 ± 0.76 vs 1.60 ± 0.43, P < .001), lower CRS (27.4 ± 6.4 mL/cm H2O vs 32.4 ± 9.3 mL/cm H2O, P = .002), and lower composite score (20.6 ± 11.9 vs 34.5 ± 18.6, P < .001), with no statistically significant difference in PaO2 /FIO2 (137 ± 49 vs 155 ± 48, P = .08). CONCLUSIONS: In subjects with COVID ARDS, parameters that reflect dead space (VR), lung mechanics (CRS), and a combined score that included PaO2 /FIO2 , VR, and CRS differed between survivors and non-survivors after one week of mechanical ventilation but with considerable overlap of values between survivors and non-survivors.

Naloxone-associated pulmonary edema following recreational opioid overdose.

OBJECTIVE: Describe a series of patients who developed naloxone-associated pulmonary edema after recreational opioid use. DESIGN: Single center retrospective case series of patients who developed pulmonary edema following the prehospital administration of naloxone. SETTING: Academic, urban safety-net hospital. PATIENTS: Adults with recreational opioid overdose who developed naloxone-associated pulmonary edema, defined as the acute onset of respiratory distress, hypoxemia, and radiographic pulmonary edema after naloxone administration for opioid intoxication, provided that gas exchange and chest imaging rapidly improved and pulmonary aspiration of gastric contents was not clinically suspected. MEASUREMENTS AND MAIN RESULTS: Ten adults (median age 23 years, 90% male) met our case definition for naloxone-associated pulmonary edema. Implicated opioids were heroin in 8 patients and methadone and oxycodone in 1 patient each. The median total dose of naloxone was 4.25 mg (interquartile range [IQR] 3.3-9.8) prior to the onset of clinically-apparent pulmonary edema. Seven patients received invasive mechanical ventilation for a median of two days (IQR 0.8-5), one of whom received veno-venous extracorporeal membrane oxygenation support, and all survived to hospital discharge. CONCLUSIONS: Severe acute pulmonary edema may follow naloxone administration after recreational opioid overdose. Acute care clinicians should be aware of this potentially life-threatening adverse effect of naloxone.

CD8αα intraepithelial lymphocytes arise from two main thymic precursors.

TCRαß+CD4-CD8α+CD8ß- intestinal intraepithelial lymphocytes (CD8αα IELs) are an abundant population of thymus-derived T cells that protect the gut barrier surface. We sought to better define the thymic IEL precursor (IELp) through analysis of its maturation, localization and emigration. We defined two precursor populations among TCRß+CD4-CD8- thymocytes by dependence on the kinase TAK1 and rigorous lineage-exclusion criteria. Those IELp populations included a nascent PD-1+ population and a T-bet+ population that accumulated with age. Both gave rise to intestinal CD8αα IELs after adoptive transfer. The PD-1+ IELp population included more strongly self-reactive clones and was largely restricted by classical major histocompatibility complex (MHC) molecules. Those cells localized to the cortex and efficiently emigrated in a manner dependent on the receptor S1PR1. The T-bet+ IELp population localized to the medulla, included cells restricted by non-classical MHC molecules and expressed the receptor NK1.1, the integrin CD103 and the chemokine receptor CXCR3. The two IELp populations further differed in their use of the T cell antigen receptor (TCR) α-chain variable region (Vα) and ß-chain variable region (Vß). These data provide a foundation for understanding the biology of CD8αα IELs.