Early severe acute respiratory distress syndrome: What’s going on? Part I: pathophysiology

Abstract

Severe acute respiratory distress syndrome (ARDS, PaO 2 /FiO 2 < 100 on PEEP ≥ 5 cm H 2 O) is treated using controlled mechanical ventilation (CMV), recently combined with muscle relaxation for 48 h and prone positioning. While the amplitude of tidal volume appears set £ 6 mL kg -1 , the level of positive end-expiratory pressure (PEEP) remains controversial. This overview summarizes several salient points, namely: a) ARDS is an oxygenation defect: consolidation/difuse alveolar damage is reversed by PEEP and/or prone positioning, at least during the early phase of ARDS b) ARDS is a dynamic disease and partially iatrogenic. This implies that the management of the ventilator may be a life-saver by reducing the duration of mechanical ventilation, or detrimental by extending this duration, leading into critical care-acquired diseases. Indeed, a high PEEP (10−24 cm H 2 O) appears to be a life-saver in the context of early severe diffuse ARDS; c) tidal volume and plateau pressure cannot be identical for all patients; d) the only remaining rationale for CMV and muscle relaxation is to suppress patient-ventilator asynchrony and to lower VO 2 , during the acute cardio-ventilatory distress. Therefore, in early severe diffuse ARDS, this review argues for a combination of a high PEEP (preferably titrated on transpulmonary pressure) with spontaneous ventilation + pressure support (or newer modes of ventilation). However, conditionalities are stringent: upfront circulatory optimization, upright positioning, lowered VO 2 , lowered acidotic and hypercapnic drives, sedation without ventilatory depression and without lowered muscular tone. As these propositions require evidence-based demonstration, the accepted practice remains, in 2016, controlled mechanical ventilation, muscle relaxation, and prone position.

Effect of facemask oxygenation with and without positive pressure ventilation on gastric volume during anesthesia induction in patients undergoing laparoscopic cholecystectomy or partial hepatectomy: a randomized controlled trial

“…On the other hand, pulmonary vascular shunt caused by atelectasis leads mixed venous blood with higher CO 2 content to flow directly to the arterial side [27], exacerbating accumulation. However, transient respiratory acidosis and hypercapnia due to the accumulation of CO 2 is permitted, even rapid hypercapnia does not result in significant hemodynamic disturbances [28][29][30]. Futhermore, it can be quickly redressed by mechanical ventilation after endotracheal intubation.…”

Pneumolysis: a fundamental concept in COVID-19 lung disease

“…As shown by the following photomicrographs, the most important histopathological findings were the changes associated with diffuse alveolar damage (DAD). The latter is a general term used concerning ARDS (Petitjeans et al, 2016), but since we are now dealing with a new highly lethal pathology, new precise terminology is required. This is what we define as pneumolysis, an inflammatory process characterized by lung tissue derangement with alveolar collapse, hyaline tissue formation, some microhemorrhages, and micro-abscess possibly due to superinfection, with polymorphonuclear cells and monocytes infiltration.…”

Hypothesis: Fever control, a niche for alpha-2 agonists in the setting of septic shock and severe acute respiratory distress syndrome?

“…As delineated [110][111][112], after correction of one factor after the other factor generating the acute cardio-ventilatory distress [108], the present hypothesis is that spontaneous ventilation combined to alpha-2 agonists helps in the setting of severe diffuse ARDS. An "analytical" bundle separates the factors involved in the ARDS conundrum, using an itemized, step by step, process: a) Position: upright position improves oxygenation [113,114] especially when severe ARDS is considered ( Figure 4; panel B in electronic supplement [115]), as delineated [111,116]. Intraabdominal pressure is to be lowered.…”

“…In the setting of ARDS [119], a pH>7.23-29 is required to use spontaneous ventilation (synchronized intermittent [126][127][128]. Under spontaneous ventilation-high PEEP-low pressure support, the objective is only to recruit a "penumbra area" [116] and increase P/F%50 to P/F%150-200. Schematically, the issue is to move from the worst case scenario (FiO2 = 1, PEEP = 24, P/F = 50) to an acceptable scenario (FiO2 = 0.4, PEEP = 10, P/F = 150): after having used the above mentioned bundle (combined if appropriate with veno-venous ECMO), the sole stimulus on the respiratory generator is hypoxemia.…”