SINCE the description in the 1970s of external positive end-expiratory pressure for acute respiratory distress syndrome (ARDS),1the optimum level of external positive end-expiratory pressure remains unresolved.2In the 1990s, the lower inflection point, an inspiratory phenomenon on the low-flow pressure–volume curve, was defined as the point above which external positive end-expiratory pressure should be set to ensure full opening of the lung, e.g. , open lung approach.3The significance of the lower inflection point is, however, in part explained by expiratory flow limitation in ARDS patients4unmasked by the presence of intrinsic positive end-expiratory pressure (PEEPi). Interestingly, the lower inflection point has been observed in morbidly obese patients without ARDS.5Prone positioning has been applied to ARDS patients, especially those with severe hypoxemia. We report a morbidly obese ARDS patient exhibiting PEEPi–lower inflection point interdependence after a period of prone positioning.
A 56-yr-old man with a body mass index of 42 kg/m2was admitted to our intensive care unit with the diagnosis of febrile acute respiratory failure. He had a medical history of arterial hypertension and diabetes mellitus and was a nonsmoker. At initial examination, the patient was conscious, was hemodynamically stable, had bilateral crackles predominating on the left side, and had an arterial oxygen saturation of 92% while on 15 l/min oxygen. The initial workup revealed positive urinary antigen for legionella pneumophila, leading to the administration of intravenous erythromycin and rifampicin. After the first 24 h, the intensive care severity score–new simplified acute physiology score (SAPS II)6was 52 points.
On day 2, the patient became even more dyspneic, with encephalopathy leading to tracheal intubation, deep sedation, and paralysis. A ventilatory protective pressure limited (< 30 cm H2O) strategy was implemented with a Servoiventilator (Maquet Critical Care AB, Solna, Sweden) with the following values on 100% oxygen: pH, 7.24; arterial partial pressure of oxygen, 83 mmHg; arterial partial pressure of carbon dioxide, 59 mmHg; bicarbonate, 23 mm; base excess, −5 mm; arterial oxygen saturation, 96%. A chest radiograph revealed bilateral alveolointerstitial infiltrates. This clinical picture was compatible with ARDS as defined by American–European consensus conference7and a Lung Injury Severity Score of 2.75.8
Because of hemodynamic instability, transesophageal echocardiography was performed, showing acute cor pulmonale. Thereafter, hemodynamic stabilization was rapidly achieved with norepinephrine infusion.
On day 7, ARDS persisted, and a lung computed tomography scan (fig. 1) revealed bilateral posterior alveolar consolidation. At that time, standard ventilator software was used to perform the following respiratory mechanics measurements: an expiratory occlusion technique showing a PEEPi of 12 cm H2O and a low-flow (9 l/min) inspiratory pressure–volume curve exhibiting a lower inflection point of 14 cm H2O (fig. 2). The patient was then positioned prone with upper chest and pelvic support to ensure free movement of the abdomen. After 12 h of prone positioning, the PEEPi was 2 cm H2O, and the low flow inspiratory pressure-volume curve was repeated, showing a lower inflection point of 3 cm H2O (fig. 3). Respiratory settings, mechanics, and arterial blood gas tension measurements before and after the first 12 h of prone positioning are reported in table 1. Considering this improvement, the patient was turned from supine to prone and was kept prone 12 h per day, for 3 consecutive days.
On day 19, the patient was successfully weaned from the ventilator. On day 24, the patient was discharged from the intensive care unit to the ward, then to a rehabilitation facility on day 41, and finally to home on day 60.
Acute respiratory distress syndrome management in morbidly obese patients is challenging, with few previous reports.9Expiratory flow limitation has been one of the key findings in these patients. One of the explanations has been small airway closure10with atelectasis. Furthermore, it is well known that functional residual capacity decreases when a supine position is assumed,11especially in morbidly obese patients5because of unopposed intraabdominal pressure.
While supine, our patient displayed an expiratory flow limitation with a large PEEPi. Because of the distribution of consolidation on lung computed tomography scan, we reasoned that prone positioning would be helpful. Prone positioning has been shown to improve respiratory system mechanics in ARDS12and morbidly obese patients.13As shown in our patient, the response was impressive, with almost disappearance of PEEPi with the concomitant diminution of lower inflection point. At the same time, we observed an improvement in alveolar ventilation, an already reported marker of good prognosis in ARDS patients.14From a physiologic point of view, spontaneously breathing nonobese individuals11had greater functional residual capacity in a prone position compared with a supine one, and this was also reported in morbidly obese patients during general anesthesia.13In our patient, prone positioning improved alveolar ventilation comparable with what has been previously reported in nonobese ARDS patients,12 e.g. , homogenization of tidal ventilation. This improvement was prompted by unloading of the abdominal contents and relief of pressure on the diaphragm, thereby opening small airways and finally the dependent parts of the lungs.
By applying an “open lung approach,” this patient would have been ventilated with external positive end-expiratory pressure of at least 16 cm H2O, leading to a reduction in tidal volume and consequent respiratory acidosis, and possible hemodynamic instability, especially in this patient with ARDS related acute cor pulmonale.
This clinical case report questions the physiologic basis of the open lung approach compared with the prone positioning strategy when applied to morbidly obese patients, and at the same time highlights interdependence between PEEPi and the lower inflection point, especially in morbidly obese ARDS patients. Obesity is a major health problem,15and it would be of great public health interest to launch a clinical trial testing the impact of prone positioning in severely hypoxemic, morbidly obese ARDS patients. A previous randomized trial of prone positioning conducted in normal-size patients was negative,16but a post hoc analysis showed a significantly lower 10-day mortality rate in the prone group compared with the supine group in those with the lowest arterial oxygen pressure to fraction of inspired oxygen ratio. From these results, we speculate that a randomized trial of prone positioning in severely hypoxemic morbidly obese ARDS patients might be positive.