Prone positioning and ARDS
Dr John Griffiths DICM MRCP FRCA MA
Focus on prone positioning and oxygenation
The importance of improving oxygenation is to try and improve the long-term outcome for the acute respiratory distress syndrome (ARDS) patient. Previous studies have shown improvements in oxygenation in approximately 60-70% of patients with ARDS when positioned prone. However, many of the case series in the systematic reviews did not address the effect of prone positioning on mortality. Moreover, there is also considerable heterogeneity between many of the studies in the definition of response and the duration of prone positioning.
Various mechanisms have been proposed to explain the improvement of oxygenation when a patient is turned prone. These include:
- redirecting the compressive forces of the heart weight on the lungs
- redistribution of secretions, atelectasis and interstitial oedema away from the posterior areas
- redistribution of pulmonary perfusion to less oedematous areas
- increasing alveolar recruitment
- increasing the end-expiratory volume
- changes in regional diaphragmatic excursion
Clearly, positioning a patient prone is labour intensive and is not without risk. Areas of concern include cardiovascular instability, line displacement and accidental extubation.
Focus on the role of prone positioning on outcome from ARDS
Two large randomised controlled trials have compared the effect of prone versus supine positioning on mortality in ARDS. In an Italian multi-centre trial, Gattinoni et al. found that the mortality rate did not differ significantly between the prone (21%) and supine (25%) groups at the end of the 10-day study period, at discharge or at the 6-month follow-up (51% versus 48%). Prone positioning improved oxygenation in more than 70% of the patients. The rates of displacement of endotracheal tubes, vascular catheters and thoractomy tubes were similar in the two groups. However, the trial was underpowered and the authors noted the progressive difficulty they experienced in recruiting patients, presumably because physicians were unwilling to forego the prone position as a treatment option in the face of refractory hypoxaemia. Moreover, the data were analysed on an intention-to-treat basis, and 12 of the supine group were turned prone for severe hypoxaemia. It may be disingenuous to count being turned prone in the “supine” group and missing one prone episode in the “prone” group as equivalent. Interestingly, a post hoc analysis showed that the 10-day mortality rate was almost half that of the control group for patients with high illness severity scores, and/or lowered PaO2/FIO2 ratios and/or higher tidal volumes. Moreover, a PaCO2 response (decrease of ≥1 mm Hg or 0.133 kPa after 6 hours of the first pronation) was associated with increased survival, whereas a change in PaO2/FIO2 was not. Increased carbon dioxide clearance is likely to have been caused by recruitment of unventilated lung in responders, for which there are several plausible explanations. However, it is worth noting that the mean tidal volume used to ventilate these patients (10.4 ml/kg ideal body weight) would now be considered to be undesirably high, given the findings of the ARDSnet trial. Despite the trial’s limitations, the authors feel that the post hoc analysis findings may indicate that the duration of prone positioning employed in the trial was insufficient or that pronation simply delayed an inevitable outcome.
A subsequent study by Guerin et al. essentially repeated the findings of Gattinoni et al. in patients with hypoxaemic respiratory failure. This study also demonstrated a lower incidence of ventilator-associated pneumonia in the prone-positioning group. Importantly, pressure sores, selective intubation and endotracheal tube placement occurred more frequently in the prone group.
Key learning points
- The timing and duration of prone positioning as an intervention in ARDS and refractory hypoxaemia remains unclear at the present time
- Prone positioning invariably improves oxygenation in ARDS
- Prone positioning does not improve patient survival
- There are clear risks to putting a critically ill patient in the prone position and it has considerable implications on staffing requirements
- Further randomised controlled trials are required to clarify the true role of prone positioning on patient outcome from ARDS
Key references Fridrich P, Krafft P, Hochleuthner H, Mauritz W.
The effects of long-term prone positioning in patients with trauma-induced adult respiratory distress syndrome.
Anesth Analg 1996; 83: 1206-1211.
Chatte G, Sab JM, Dubois JM, Sirodot M, Gaussorgues P, Robert D.
Prone position in mechanically ventilated patients with severe acute respiratory failure.
Am J Respir Crit Care Med 1997; 155: 473-478.
Ware LB, Matthay MA.
The acute respiratory distress syndrome.
N Engl J Med 2000; 342: 1334-1339.
Rialp G, Betbese AJ, Perez-Marquez M, Mancebo J.
Short-term effects of inhaled nitric oxide and prone position in pulmonary and extrapulmonary acute respiratory distress syndrome.
Am J Respir Crit Care Med 2001; 164: 243-249.
Gattinoni L, Tognoni G, Pesenti A, et al; Prone-Supine Study Group.
Effect of prone positioning on the survival of patients with acute respiratory failure.
N Engl J Med 2001; 345: 568-573. CCUK paper review
Ward NS. Effects of prone position ventilation in ARDS.
An evidence-based review of the literature.
Crit Care Clin 2002; 18: 35-44.
Guerin C, Gaillard S Lemasson S et al.
Effects of systematic prone positioning in hypoxaemic acute respiratory failure: a randomized controlled trial.
JAMA 2004; 292: 2379-2387.