Regional distribution of gas and tissue in acute respiratory distress syndrome. II. Physiological correlations and definition of an ARDS Severity Score

Abstract

In patients with ARDS, differences in lung morphology are associated with differences in outcome and lung mechanics. A new ARDS Severity Score based on CT lung morphology and cardiorespiratory parameters accurately identified patients with the most severe forms of ARDS and a mortality rate above 60%.

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

“…However, increased lung water generates also intrapulmonary shunting [13] without necessitating a true collapse. Schematically, focal vs. diffuse ARDS may be opposed with different prognostics ("focal vs. "patchy" vs. "diffuse" ARDS [87,88]; Fig. 1 in [89] and Fig.…”

“…2 in [87]; mortality in diffuse ARDS: 75%; focal ARDS: 42%). Nevertheless, this very convenient description [87] from a pathophysiologic and a therapeutic (Fig. 2 in [88]) perspective was not incorporated within the Berlin definition [19] (footnote in Table 1).…”

“…When combined with the transmission of a high IAP on the dependent lung, this effect can explain the severity of the ARDS observed in supine morbidly obese patients presenting with mild to moderate pulmonary infection located on the peri-diaphragmatic areas (basis of the lung: "focal" [87] ARDS, acute hypoxemic non-hypercapnic failure). [Thus, focal ARDS in a morbidly obese patient requires: a) from a physiological point of view, a low PEEP (5−10 cm H 2 O) to reopen the dependent atelectasis itself [87] (Fig. 2 in [87]); and b) from a practical point of view, a high PEEP to withstand the high IAP [120,121].…”

“…Now, electrical impedance tomography allows one to avoid transportation of unstable patients to the CT scan [92]. In the setting of early ARDS (< 10 days) a) 27% of the patients present lower lobes with mechanical/compression atelectasis (pure loss of aeration) while 73% of the patients present inflammatory atelectasis (loss of aeration + excess of lung tissue) b) ARDS may be characterized by evenly distributed diffuse alterations (23% of the patients; mortality rate: 75%; lower inflection point: 8 ± 2 cm H 2 O) vs. lobar attenuations predominating in the lower lobes (36%; mortality: 42%; LIP: 5 ± 2 cm H 2 O) vs. patchy attenuations unevenly distributed in the 2 lungs (40%; mortality: 41%; LIP: 6 ± 3 cm H 2 O) [87,89]. This classification was simplified to «diffuse» vs. «focal» acute non-hypercapnic hypoxemic failure [86,88].…”

“…Within the severe ARDS group exists a subgroup (15% of all ARDS patients) with a 52% mortality (48−56%) with: P/F < 100, compliance of the respiratory system: Crs < 20 mL cm H 2 O -1 , or standardized minute ventilation at PaCO 2 = 40 (VE corr = minute ventilation*PaCO 2 /40) > 13 L min -1 Refractory hypoxia is defined as PaO 2 < 70 mm Hg on FiO 2 = 0.8−1 (P/F: 70−87), PEEP >10 cm H 2 O for > 12−24 h [30]. Surprisingly, this is an infrequent cause of death: only 15% of all ARDS deaths are caused by refractory hypoxia [30] a Chest X-ray or CT scan; note that the Berlin definition exludes "focal" ARDS caused by atelectasis [87] b If altitude higher than 1000 m, correction factor should be made as follows: PaO 2 /FiO 2 *(barometric pressure/760) c This may be delivered non-invasively in the mild ARDS group Note: 1) to adjust Vt, the formula to calculate predicted body weight (PBW) is: Male: ideal body weight = 50 + 0. 91 3) the early template of the Berlin definition considered minute ventilation standardized at PaCO 2 = 40 mm Hg (VE corrected = minute ventilation*PaCO 2 /40).…”