Ventilatory Mechanics in the Patient With Obesity

Luigi Grassi, M.D.; Robert Kacmarek, Ph.D.; Lorenzo Berra, M.D.

Disclosures

Anesthesiology. 2020;132(5):1246-1256. 

In This Article

Mechanics During Artificial Ventilation With Lung Injury

Obesity has been often a criterion of exclusion in studies focused on lung injury in the intensive care unit. Consequently, to date, hard evidence that can help understanding the complex interaction between the high abdominal load and the injured lung is lacking. For example, the driving pressure of the respiratory system (plateau pressure – total end-expiratory pressure), which is the force needed to overcome the elastic pressures during inflation,[48] is known to directly correlate with mortality in lean patients with ARDS, but the same correlation is not observed for the obese patients.[49] The driving pressure has been associated with the concept of strain (the ratio of the tidal volume to the end-expiratory lung volume), which describes the dynamic force applied to the lung parenchyma during tidal elongation, and is one key mechanism of volutrauma, a type of ventilator-induced lung injury in ARDS.[50] Contrary to what is generally believed, high pleural pressure does not protect against ventilator-induced lung injury if the compliance of the chest wall is normal, as seems to be the case in the obese patient (see discussion in the previous paragraph). In fact, given the partitioned driving pressure of the lung as dPlung = (plateau pressure – PEEP) – (pleural pressureend-inspiration – pleural pressureend-exhalation),[51] it is evident as high absolute values of pleural pressure with a small difference between end inspiration and end expiration expose the lung to strain and hence volutrauma, if a sufficient PEEP is not provided. It is true, on the other hand, that high pleural pressure can have a protective effect against static stress across the lung parenchyma, especially at the end of inflation (i.e., a decreased transpulmonary pressure during an inspiratory pause, resulting in less alveolar overstretching, particularly in nondependent lung zones). It is difficult to understand what effect is more important in the single patient; it is worth noting that dynamic strain has been associated with greater injury than static stress in some studies.[52] In this context, it would be wise to implement a personalized physiologic approach complementary to the protective ventilatory strategy that has been shown to improve outcomes in ARDS.[53] This approach allows choosing the PEEP corresponding to the best compliance of the respiratory system, after the performance of a recruitment maneuver to reopen collapsed areas of the lung parenchyma, and, in obese patients with a diagnosis of ARDS, has been shown to improve oxygenation and lung mechanics when compared to pressure-oxygenation tables.[54] Decremental PEEP titration based on best compliance may result in a higher PEEP than the one set with predefined tables (with the benefit of a decreased driving pressure), and the use of recruitment maneuvers implies a progressive rise in airway pressures as high as 50 cm H2O.[55] Such high pressures could raise concerns of important side effects, especially from a hemodynamic point of view,[56] and a recent large randomized trial showed an increase in mortality, barotrauma, and hypotension when decremental PEEP titration was applied to a general population with ARDS.[57] Excessive pressure at end exhalation can decrease cardiac output by decreasing venous return (mainly through an increase in venous resistance[58]) and increased pulmonary vascular resistance, thus impairing the function of the right ventricle. However, pulmonary vascular resistance is augmented at the extremes of lung volumes.[59] That is, the right ventricle is exposed to increased afterload if the lung volumes are too high or too low. Patients with obesity and ARDS theoretically experience very low lung volumes (given that both the conditions are associated with decreased FRC) and may require a higher PEEP to restore a functional residual capacity compatible with a normal resistance in the pulmonary circulation. Hence, the application of high pressures in the obese patient with ARDS would have a strong rationale, once granted an optimal volemic status in order to prevent decreases in venous return. However, future physiologic studies should yield more knowledge about the lung-heart interaction dynamics, and randomized trials are needed to demonstrate any benefit in clinical outcomes of recruitment maneuvers and decremental PEEP titration in this specific population.

Although technically demanding, prone positioning is another therapeutic opportunity to consider: it can be undergone safely in obese patients and has been proven to be beneficial in terms of physiologic outcomes when obesity is associated with severe refractory hypoxemia.[60] In the lean person, pronation works through redistribution of ventilation, releasing the pressure imposed by lung edema on the most dependent lung zones. More studies are needed to prospectively investigate its mechanisms and effects on clinical outcomes in the obese population.

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