While prone positioning can produce an increase in oxygenation in ARDS, further studies are needed to determine its clinical value.
By Jorge Pedroza, MD, and Guillermo Domínguez-Cherit, MD
Since the first description of adult respiratory distress syndrome (ARDS) in 1967 by Ashbaugh et al, there has been continuing growth in the understanding of its origins, pathophysiology, and clinical course, but few advances in decreasing ARDS mortality (which is approximately 50% in the United States and Europe).
Indeed, a recent consensus conference produced no conclusions regarding this issue; there is no definitive treatment for ARDS patients, especially where mechanical ventilation settings are concerned. In the literature, some new interventions are promising; these include the open-lung approach of Amato et al, nitric oxide, and prone positioning.
It is widely accepted that ARDS is a nonhomogeneous disease that predominantly affects the dependent areas of the lung. Lung volume is essentially conserved in ARDS patients; the tissue volume of ARDS lungs appears increased, when compared with ARDS-free control lungs. The ratio of gas to tissue is also lower in ventral areas than in dorsal areas in ARDS lungs, as seen in CT images. This nonhomogeneous distribution of gas can be reduced when positive end-expiratory pressure (PEEP) is applied, but the dependent areas have lower compliance. PEEP may be diverted toward normal, nondependent acini with higher compliance and lower flow resistance, producing alveolar overdistention, capillary compression, and a dead-space effect (with deterioration in oxygenation and hemodynamics, especially at high PEEP levels).
The main effect of prone positioning is to increase oxygenation to varying degrees without affecting carbon-dioxide removal. Theories that have been presented to explain this clinical finding include the achievement of increased oxygenation in the prone position because, when dorsal lung areas become nondependent, their ventilation improves, with increased alveolar recruitment that is unrelated to the PEEP level. This increased ventilation is attributed to decreases in the hydrostatic forces that oppose the passive diaphragmatic motion of the dorsal area and to reduced positive pleural pressure in dorsal regions in the presence of lung injury. The prone position also promotes the distribution of perfusion in dorsal areas, and this is not influenced by gravity. Added to increased ventilation, this can correct the ventilation-perfusion mismatching in these units that is observed in the supine position. The addition of PEEP, which redirects blood flow to dependent areas in the supine position and sometimes increases ventilation-perfusion mismatching, appears to promote even perfusion in dependent and nondependent regions in the prone position, reducing hypoxemia.
Statistically significant differences in hemodynamics between supine and prone patients have not been found.11,12 Some patients have transient arterial hypotension, when turned to a lateral position, but this corrects itself when a prone or supine position is resumed; it is thought to result from diminished venous return due to rapid changes in intrathoracic pressure.13
Indications for Use
All clinical studies record increased oxygenation during prone positioning, but most recent studies refer to patients as responders or nonresponders. A responder, when prone, must show an increase in oxygenation (defined as an increase in PaO2 of 10 mm Hg or more or an increase in PaO2 divided by fraction of inspired oxygen [Fio2] of 20 mm Hg or more) 30 to 60 minutes after turning. Studies [11-12, 14-16] have found that 57% to 60% of patients respond to prone positioning and that nonresponders experience no deterioration. When comparing both groups, Jolliet et al found that a trial of 30 minutes detected most responders, but some took 2 hours to show an improvement in oxygenation. Although an initial trial detected most responders and nonresponders, some responders who were returned to a supine position after 12 hours prone showed no improvement in oxygenation when turned to a prone position for the second time, and one nonresponder showed increased oxygenation when placed prone for the second time. For the most part, however, response to the initial trial predicted response to subsequent trials, and no difference between the two groups could be used to predict clinical behavior.
Blanch et al found that nonresponders had been turned prone longer after ARDS onset (32.8±42 days) than responders (11.8±16 days), which could mean that responders were turned early in the course of ARDS and nonresponders were turned during the fibroproliferative stage of ARDS.
A lack of information makes understanding and applying prone positioning more difficult. There is no consensus on the optimal moment for turning a patient. It appears that all ARDS patients can be given a trial of prone positioning to determine whether they are responders. Earlier turning also seems better, but all work to date has focused on ARDS; there has been no study of prone positioning in acute lung injury, a recognized part of the ARDS continuum.
Whether it is better to alternate prone and supine positioning or leave patients prone as long as clinical improvement is maintained is unknown. Repositioning intervals that have been investigated also varied from 1 to 37 hours. At the Instituto Nacional de la Nutrición Dr Salvador Zubirán, Mexico City, some of our patients have been kept prone continuously for as long as a week, with persistent improvement in oxygenation and with no major complications. For prone patients, there is no standard mode of mechanical ventilation. There are reports [11-12,14-16,18-19] on volume-cycled ventilation, pressure-limited ventilation, application of variable levels of PEEP, low and high FiO2 levels, inverse-ratio ventilation, extracorporeal carbon dioxide removal, and inhaled nitric oxide (which may have an additive effect in the prone position). Some investigators have turned patients using special face and/or body frames, but most have not.
No reports focus on the optimal time to end prone positioning as a clinical tool. Although, logically speaking, the best time would be when oxygenation indices remain normal or close to normal, the values for PEEP, FiO2 or other mechanical ventilation variables that should indicate the termination of the maneuver remain unknown.
Contraindications and Complications
There are no clear, absolute contraindications; high-risk patients include those with nonmobile injuries (traction or head/spinal trauma), open thoracic or abdominal wounds, hemodynamic instability, and increased intracranial pressure.21 A patient in prone positioning can be resuscitated without turning using closed-chest compressions with a fist under the anterior chest wall and compressions over the spine.22
Major complications are accidental extubation, loss of vascular access, and nerve injury. Minor and more frequent complications are pressure sores (on the face, shoulders, knees, and ankles) and dislocation of artificial intraocular lenses.22,23
Unfortunately, no studies report the effect of prone positioning on mortality. The causes of death for patients in the prone position are the same as those of other ARDS patients (septic shock and multiple organ dysfunction syndrome [MODS]). As for other ARDS therapies, the effect of prone positioning on mortality will be clear only when there are treatments available for septic shock and MODS. No study analyzes the impact of prone positioning on weaning or on the time spent on mechanical ventilation. A prospective, controlled, randomized study seems mandatory for the evaluation of these issues.
Our center has no special devices for turning patients, so special care in turning them is required. All vital signs should be recorded before and after turning, since some patients may develop hypoxemia or hemodynamic instability in the lateral decubitus position; this resolves when a prone or supine position is resumed. In our intensive care unit (ICU), we prefer to increase the Fio2 to 100% 15 minutes before turning the patient. For patient comfort, we recommend heavy sedation and/or paralysis before and during prone positioning (although cases of conscious nonintubated patients being turned face down have been reported).
Intravenous (IV) lines and endotracheal tubes should be watched carefully during turning. In our ICU, we turn patients using four to five staff members: one takes care of the patient’s head and airway, one cares for IV and monitoring lines, and two or three turn the patient. First, we turn the patient to the lateral decubitus position with the arms along the sides of the body. Immediately after reaching prone positioning, we record results for pulse oximetry, blood-pressure measurement, and an electrocardiogram obtained by placing the electrodes on the patient’s back.
To reduce the occurrence of skin lesions, we cushion the patient’s face, shoulders, arms, legs, and ankles; we also apply gel-foam pads to high-pressure sites. Although these measures diminish the occurrence of pressure sores considerably, they may still develop, especially in association with edema or long-term prone positioning.
Prone positioning is an easy maneuver that can produce an increase in oxygenation in ARDS patients. It can be performed at low risk and without substantial increases in costs in the ICU. Complications are few and can be minimized with daily care. Unfortunately, until the results of prospective studies become available, the exact value of prone positioning remains undetermined.
Jorge Pedroza, MD, is staff physician, Critical Care Medicine Division, Instituto Nacional de la Nutrición “Dr Salvador Zubirán,” Mexico City.
Guillermo Domínguez-Cherit, MD, is chief of the intensive care unit of the institute.
ACKNOWLEDGMENT: The authors wish to thank Alejandra Valdivia for her contribution and collaboration.
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