The Prone Position in the Treatment of Patients with ARDS: Problems and Real Utility



Fig. 1.1
The figure illustrates a simplified model that shows the influence of lung shape on the distribution of aeration in the parenchyma during supine and prone position. If we consider an elliptical-shaped lung, the portion of well-aerated tissue in non-dependent regions is similar between supine and prone position. As the amount of open and closed parenchyma will be the same in both positions, the aeration distribution will remain the same. If we consider a triangle-shaped lung, the portion of well-aerated tissue in non-dependent regions increases during the turning process as the result of recruitment in dorsal zones is higher than the derecruitment in ventral zones. As a consequence, parenchyma aeration will increase and lung compliance will improve





1.4.2 Blood Flow Distribution


Historically, pulmonary perfusion has been described using a gravitational model in which uneven distribution of blood flow can be explained by the hydrostatic pressure difference between blood vessels in dependent and non-dependent lung regions. In non-dependent regions, intravascular pressure (in pulmonary arterioles and venules) falls below alveolar pressure (zone 1 according to West). If this occurs, capillaries are squashed flat and no flow is possible, resulting in ventilated but not perfused alveoli (alveolar dead space). In intermediate regions (zone 2 according to West), alveolar pressure is lower than arteriole pressure, but higher than venule pressure and blood flow is dependent on pressure difference between arterioles and venules. In the most dependent regions (zone 3 according to West), intravascular pressure is always greater than alveolar pressure, both in arterioles and venules, so these are the best perfused lung regions [17]. According to the West model, lung segments that are at the same height should receive the same perfusion. However, studies on pulmonary perfusion performed with the radioactive microsphere emboli technique showed that the craniocaudal perfusion gradient is maintained both in supine and lateral body position [18, 19]. Further studies using the same technique pointed out that differences in perfusion between lung segments at the same height may be even ten times greater than the differences existing between lung segments at different heights [20]. It follows that gravitational forces are minor determiners of nonhomogeneous pulmonary perfusion distribution and that blood vessel architecture might be the leading cause of regional perfusion differences; [21] this last model gives a more complete picture of what might happen during prone positioning. In supine position pulmonary perfusion is distributed along a ventral-dorsal gradient; however, in prone position, this gradient doesn’t shift and dorsal regions remain well perfused. Between all the body positions that were tested, prone position is the one associated with the best distribution of pulmonary perfusion.


1.4.3 Gas Exchange


As previously described, in ARDS patient, prone positioning is associated with increased arterial oxygenation. This data was confirmed by countless clinical trials and by experimental animal models of acute respiratory failure. This increase is justified by the pathophysiological mechanisms previously described: in ARDS prone positioning causes a recruitment of dorsal lung regions that is greater than the derecruitment of ventral lung regions; also, recruited dorsal regions are better perfused than the ventral regions and this induces an improvement in ventilation/perfusion relationship and reduces blood shunt, the major cause of hypoxemia [15]. However, an increase in dead space and a resulting decrease in CO2 elimination seem to be the major determiner of patient outcome in ARDS [22]. In prone positioning the greater recruitment of dorsal lung regions compared to the ventral region collapse reduces dead space, improves CO2 elimination at even total ventilation, and reduces overdistension of ventral lung regions that are inclined to overinflation and inadequate ventilation [15]. Prone positioning is not always associated with a decrease in arterial CO2 because the presence of aerated alveoli (and the associated improved oxygenation) doesn’t necessarily mean that they are well ventilated. It has been described that the variations in arterial CO2 after pronation are independent from oxygenation response and a decrease in PaCO2 is associated with a greater lung recruitment and a better outcome [23, 24].



1.5 Prone Positioning and Ventilator-Induced Lung Injury


Many trials demonstrated that there is a greater homogeneity in ventilation during prone position than during supine position, and experimental studies showed that prone position has a role in modulating ventilator-induced lung injury (VILI). VILI originates from the repeated application to lung tissue of forces generated and amplified between consolidated and ventilated zones and from cyclical opening-closing of ventilated alveoli (atelectrauma). These areas are prevalent in dependent lung regions; for this reason dorsal regions are mainly exposed to the risk of VILI in supine position and consequently more protected from it in prone position. The mechanism involved in reducing lung injury implies the reduction of interfaces between consolidated and ventilated areas and the decrease and better distribution of transpulmonary pressure (the driving force distending the lung) due to increase in compliance that follows lung recruitment. So if prone positioning is successful in recruiting lung parenchyma, mechanical ventilation should be less harmful, thanks to the reduction in global “stress” (transpulmonary pressure) and “strain” (tidal volume/functional residual capacity ratio) of overinflated areas. Atelectrauma (injury due to opening-closing of lung units) seems to be reduced particularly in patients with high potential for lung recruitment ventilated with high PEEP. Also, prone position reduces the injury due to inflammation (a significant reduction in pro-inflammatory cytokine concentration has been indeed shown) and the development of hemorrhagic pulmonary edema [25]. It also improves transpulmonary pressure distribution and contributes in preventing VILI thanks to mechanical factor and to the simple gas exchange improvement (particularly PaO2), reducing in this way iatrogenic intervention to sustain oxygenation. In fact, all common procedures that enhance oxygenation or improve ventilation induce lung injury; so oxygenation improvement allows to reduce FiO2 and mean airway pressure, both determinants of VILI progression. Another important factor is that prone positioning potentially increases drainage of oropharyngeal and airways secretions: this can improve gas exchange and reduces incidence of ventilator-associated pneumonia (VAP) [26]. The main determiner in reducing mortality, besides maneuver per se (considered a life-saving treatment in the most severe cases of hypoxemia), is probably related to the reduced incidence of VILI.


1.6 Patient Selection



1.6.1 Indication


Prone positioning has to be considered a life-saving treatment in severe hypoxemia, especially when the procedure is standardized and performed by a skilled team. As is known, prone positioning is indicated in patients with “severe ARDS”; however the definition of “severe ARDS” has been formalized only recently in the Berlin definition of ARDS [8]. In clinical practice the severity of ARDS is established by PaO2/FiO2 ratio, but it’s obvious that it might change on the base of the ventilator strategy (i.e., PEEP level) or applied FiO2. Despite this variability, based on existing literature from randomized trials and meta-analysis, long-term pronation in severe ARDS (defined by a PaO2/FiO2 ratio < 100 mmHg) is recommended. Instead, it is not recommended in mild ARDS (PaO2/FiO2 300–200 mmHg) because it doesn’t provide any benefit in terms of survival. The use of prone positioning in moderate ARDS (PaO2/FiO2 200–100 mmHg) is still debated: recent meta-analysis suggests that prone positioning should be considered in moderate ARDS with PaO2/FiO2 lower than 150 mmHg, with PEEP level set on 5 cmH2O or higher and FiO2 set on 0.6 or higher. This is why it is reasonable to use prone positioning in ARDS patients with PaO2/FiO2 lower than 150 mmHg measured at least with a PEEP level of 5 cmH2O [15, 27]. It is important to highlight that the efficacy of prone positioning is maximum when performed in an early stage of the disease in which edema, lung recruitability, and absence of structural alterations of the lung are most represented. This means that the benefits of prone positioning in minimizing VILI in early stage of ARDS are probably greater than those obtained in late stage when lung injury is already induced.


1.6.2 Contraindications and Complication


There are only few absolute contraindications to prone positioning and they are mainly represented by spinal instability and unmonitored intracranial hypertension, because the procedure may require to turn the head leftward or rightward and this might cause compression of jugular veins and reduce cerebral venous outflow; it is possible to overcome the problem by using specifically designed beds to prone patients, maintaining the head in neutral position. Relative contraindications include severe hemodynamic instability, open abdominal wounds, multiple unstabilized fractures, pregnancy, difficult airway management, and presence of invasive line monitoring (included dialysis catheter), even though the latter should not be considered a real contraindication to the maneuver since firmly securing them to the patient and monitoring them during the procedure should be sufficient. Regarding the decision whether to pronate or not the patient, it is important to consider the team expertise and to balance risks and benefits associated with a life-saving procedure not free from potential complications. Complications, such as extubation, catheter displacement, and transient hypotension or desaturation, mainly related to the proning maneuver itself, have been described in literature. Another series of complications, such as pressure ulcers, vomiting, and need for increased sedation, are associated with the duration of staying prone. Particularly harmful although extremely rare is the compression of optic nerve and retinal vessels and the resulting blindness. The incidence of adverse effects decreases with experience gained by a team routinely using this intervention and with the use of preventive measures or special devices that facilitate the mechanics of safe proning and prevent duration-related complications.


1.7 Application



1.7.1 Positioning


Prone positioning requires the cooperation of more people from the intensive care unit staff. A standardized procedure does not exist; hence a lot of centers apply prone position based on sequence of movements used for log-rolling maneuver (by the way, materials that facilitate the procedure and beds specifically designed to facilitate prone positioning in intensive care unit and minimize associated risks are available (Table 1.1)). Independently from the technique used, it is essential for the staff performing the procedure to be adequately skilled to avoid injuries to the patients and central catheter dislocation. It is necessary that support zones are protected to avoid bedsore formation. The number of people it takes to safely prone the patient depends on the size of the patient itself, on the number and the position of invasive lines, and on the choice of material that will be used. An accurate preparation of the patient before prone positioning and a special attention to endotracheal tube and vascular lines are crucial issues; we recommend having, during the turning process, one person assigned only to do nothing more than monitoring the endotracheal tube and central lines (if placed on upper thorax). Because of the accidental extubation risk, the endotracheal tube must be secured and it is always recommendable to have promptly available all the materials for an emergency re-intubation of the patient before starting the procedure. Another issue is represented by the position of the endotracheal tube compared to the carina; during prone positioning, the endotracheal tube might move and be displaced, so it is important for the distal tip of the tube to be placed 2–4 cm above the carina in order to avoid extubation or right mainstem bronchus intubation. Regarding airway management, prone positioning can result in such copious drainage of airway secretions that ventilation becomes impaired; hence we recommend to have promptly available endotracheal suction equipment. During prone positioning the presence of tracheostomy may represent a major problem; however, various devices allow full support to the patient, avoiding any contact of tracheostomy cannula to the bed or to the foam pads and sparing excessive head and neck rotation. The head is usually rotated leftward or rightward to minimize any orbital or facial pressure and to avoid lip or nasal trauma caused by endotracheal tube. This lateral rotation may be difficult to accomplish in elderly patients who have stiff cervical spines or in those with cervical disk disease. In this case it is possible to use foam donuts that suspend the head off of the bed without any lateral rotation. However these devices may result in greater facial trauma. Furthermore, it is important that any pillow or device potentially useful to support the head or other body parts during prone positioning is promptly available before beginning the procedure [28]. Regarding vascular line management, the patency of all catheters should be monitored before and after the turning process, especially when vasopressor agents are being administered.
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Sep 22, 2016 | Posted by in ANESTHESIA | Comments Off on The Prone Position in the Treatment of Patients with ARDS: Problems and Real Utility

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