Regional lung viscoelastic properties in supine and prone position in a porcine model of acute respiratory distress syndrome

The effects of prone position ventilation on patients with acute respiratory distress syndrome after cardiac surgery

BACKGROUND

This study aimed to investigate the effects of prone position ventilation treatment on patients with acute respiratory distress syndrome (ARDS) after cardiac surgery.

METHODS

Clinical data were collected from 93 hospitalized patients with acute respiratory distress syndrome (ARDS) in the intensive care unit (ICU) of cardiology from February 2021 to February 2023. Patients were divided into supine position group (n = 45) and prone position group (n = 48). The difference in 28-days survival rates, blood gas indicators, respiratory mechanics indicators, and adverse events before and after treatment was analyzed.

RESULTS

We found that within 28 days of admission, 8.33% of ARDS patients in prone position group and 11.11% in supine position group died of all causes (p > .05). After treatment, the levels of arterial PaO2 (103.25 ± 9.44 in prone position group and 91.62 ± 9.18 in supine position group), PaCO2 (30.26 ± 5.54 and 36.56 ± 6.37), blood LAC (1.35 ± 0.37 and 1.68 ± 0.42), oxygenation (232.23 ± 28.56 and 205.13 ± 31.34) and diffusion index (453.48 ± 63.30 and 395.18 ± 58.54) in both groups were improved (p < .001). Moreover, the increase in prone position group was more remarkable. After treatment, the respiratory mechanics indexes of the lung compliance as well as respiratory resistance were improved (p < .05). Moreover, the increase in supine position group was more remarkable (p < .05). The incidence of atelectasis in prone position group was lower than that in supine position group (p < .05). Additionally, the alteration in other adverse events showed no significant difference between the two groups (p > .05).

CONCLUSIONS

Taken together, prone position ventilation in patients with ARDS after cardiac surgery improved blood gas indexes, hypoxemia, and respiratory mechanics indexes, as well as reduced the incidence of atelectasis.

Ventilation Mechanics

A fundamental task of the respiratory system is to operate as a mechanical gas pump ensuring that fresh air gets in close contact with the blood circulating through the lung capillaries to achieve O2 and CO2 exchange. To ventilate the lungs, the respiratory muscles provide the pressure required to overcome the viscoelastic mechanical load of the respiratory system. From a mechanical viewpoint, the most relevant respiratory system properties are the resistance of the airways (R aw), and the compliance of the lung tissue (C L) and chest wall (C CW). Both airflow and lung volume changes in spontaneous breathing and mechanical ventilation are determined by applying the fundamental mechanical laws to the relationships between the pressures inside the respiratory system (at the airway opening, alveolar, pleural, and muscular) and R aw, C L, and C CW. These relationships also are the basis of the different methods available to measure respiratory mechanics during spontaneous and artificial ventilation. Whereas a simple mechanical model (R aw, C L, and C CW) describes the basic understanding of ventilation mechanics, more complex concepts (nonlinearity, inhomogeneous ventilation, or viscoelasticity) should be employed to better describe and measure ventilation mechanics in patients.

LUNG AND CHEST WALL MECHANICAL PROPERTIES IN METFORMIN-TREATED AND UNTREATED MODELS OF TYPE 2 DIABETES

The adverse respiratory consequences of type-2 diabetes mellitus (T2DM) may reflect compromised lung function and/or alterations of the chest wall because of skeletal muscle stiffening. We assessed the separate contributions of these compartments to respiratory complications in diabetes and explored the effects of metformin on respiratory abnormalities. Experiments were performed in untreated rats (control, n=7), high-fat diet-fed rats receiving streptozotocin (T2DM, n=7), and metformin-treated diabetic rats (MET, n=6). Newtonian resistance, tissue damping, and elastance were separately assessed for lung and chest wall components by measuring the esophageal pressure during forced oscillations at low (0 cmH₂O), medium (3 cmH₂O), and high positive end-expiratory pressure (PEEP) (6 cmH₂O). Tissue hysteresivity was calculated as damping/elastance. Blood gas parameters were used to assess gas exchange, and lung histology was performed to characterize collagen expression. T2DM at low PEEP compromised airway and lung tissue mechanics in association with gas-exchange defects and collagen overexpression. Abnormal chest wall mechanics in T2DM was indicated only by decreased tissue hysteresivity. No difference in lung or chest wall mechanics, gas exchange, or lung histology was observed between the MET and control groups. These findings suggest the primary involvement of the pulmonary system in the respiratory consequences of T2DM, with chest wall properties only disturbed by a shift toward the dominance of elastic forces at low PEEP. The adequacy of metformin to treat the adverse respiratory consequences of diabetes was also revealed, in addition to its well-established beneficial effects on other organs.