Fig. 4.1
The greater lung expansion in ventral regions, due to shape matching between the lung and thorax, counteracts the gravitational forces when they act on those ventral regions, as in the prone position. This leads to a more homogeneous inflation of alveoli along the ventral-to-dorsal axis in the prone position, as compared to supine (Adapted from Gattinoni et al. [5] with permission of the American Thoracic Society. Copyright © 2014 American Thoracic Society)
Improvement in oxygenation however does not seem to be the primary mechanism of mortality reduction by PP. Indeed, a retrospective analysis of data from the PROSEVA trial has shown that the reduction in mortality observed in ARDS patients receiving prone ventilation was not dependent on whether PP improved gas exchange [18].
The survival benefit may be rather attributed to the prevention of VILI [2, 5, 6, 16, 18, 19], whose major determinants are lung overdistension (volutrauma), pertaining to increase in transpulmonary pressure (lung stress), and cyclic opening and closing of the small airways (atelectrauma) [1, 16]. Accordingly, the aforementioned more uniform distribution of the gravitational transpulmonary pressure gradient, as well as of both V T and end-expiratory lung volume, results in a homogenization of the strain (i.e., the V T to end-expiratory lung volume ratio) imposed by mechanical ventilation and, consequently, in a reduction of the resulting stress [2, 5, 16]. Finally, a more uniformly distributed V T translates into a reduced atelectrauma [18], and improvements in PaO2/FiO2 ratio resulting from PP may itself indirectly contribute to the prevention of VILI by reducing the need for iatrogenic interventions to sustain oxygenation [5].
4.4 Therapeutic Use
There is now clear evidence that PP, together with protective ventilation, is indicated as first-line therapy in severe ARDS. Probably, its use should be also extended to patients with moderate ARDS (according to the Berlin definition [20]) when PaO2/FiO2 ratio is below 150 mmHg.
In order to be effective in reducing mortality, PP should be initiated early and maintained for at least 10–12 h per day (even if maybe >16 is better) until stable improvement in oxygenation is achieved (optimal duration of PP has yet to be established [11]). In fact, since the presumed mechanism of improved survival is the prevention of VILI, PP should start before the onset of structural damage due to mechanical ventilation, and a longer daily time spent in prone position may result in a lower injury [5].
Contraindications are few and not well defined: conditions such as spinal instability, open wounds/burns on the ventral body surface, nonstabilized fractures, increased intracranial pressure, hemodynamic instability, serious cardiac arrhythmias, and pregnancy should preclude PP or, at least, impose a careful evaluation of the risks/benefits balance [5, 6].
The technical features of PP are quite complex. Thus, a skilled and well-coordinated team is needed in order to avoid major complications [3, 5, 6, 11, 14]. Adequate patient preparation (e.g., check the correct positioning of the distal end of the tracheal tube 2–4 cm above the carina in order to prevent extubation or mainstem bronchus intubation) and direct visual monitoring of devices (primarily, endotracheal tube and central lines) are pivotal [6]. In fact, the most common potentially serious complications involve airway problems, such as endotracheal tube displacement, kinking or obstruction, and vascular lines kinking/removal [5, 16].
Despite both Sud et al. [3] and Lee et al. [11], in line with what was said before, reported an increased risk of airway complications with PP, no difference between the two groups was found in the PROSEVA trial [14], maybe due to the high experience with PP of all centers involved in that study. Moreover, none of the previous RCTs reported death from airway problems [11]. Regarding vascular access, PP seems to be safe even during extracorporeal membrane oxygenation (ECMO) [21–23]. Finally, a higher risk of pressure ulcers was reported by previous trials, as well as by the latest meta-analyses [3, 11], and was also confirmed in an ancillary study of the PROSEVA trial [24]. However, it is not clear whether such increase in the incidence of pressure ulcers is due to PP itself or to the greater survival that results from PP [16, 24].
Clinical summary
Drug/technique | Indications | Cautions | Side effects | Dose | Notes |
|---|---|---|---|---|---|
Prone positioning | Severe ARDS Moderate to severe ARDS (PaO2/FiO2 < 150 mmHg) | Possible contraindications: Spinal instability
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