Prone Positioning
A There is currently only one indication for prone positioning: Acute respiratory distress syndrome (ARDS).
B The specific level of oxygenation deficit indicating a need for prone positioning has not been precisely defined.
C However, many would agree that if the following is required and the Pao2 is still <70 mm Hg, prone positioning should be considered.
D Animal studies indicate that prone positioning is lung protective (i.e., ventilator-induced lung injuries are less severe in the prone position than in the supine position).
II Contraindications of Prone Positioning
B Recent thoracic or abdominal surgery
F However, correcting hemodynamic and airway instability eliminates the contraindication.
A In the supine position, especially in ARDS because of the increased weight of the lung, there is a large transpulmonary pressure gradient between nondependent and dependent lung.
B The reasons for this gradient when supine are the following.
1. Approximately 60% of the lung is in the dependent position. This is because the lung in the supine position is essentially triangular shaped with the base of the triangular most dependent.
2. The heart and great vessels sit on top of the lungs.
3. The posterior basilar segments of the lungs are under the diaphragm.
C In ARDS the force exerted by these factors pulls the lung from the anterior chest wall, creating a negative anterior transpulmonary pressure gradient and a positive posterior or dependent transpulmonary pressure gradient.
D This causes most of the functional residual capacity (FRC) to occupy the nondependent lung and most of the tidal volume (Vt) to go to the nondependent lung.
E A large V/Q mismatch is created because most perfusion in the supine position goes to the dependent lung.
F On positioning in the prone position many of these issues are reversed.
1. The majority of the lung is now nondependent.
2. The heart and great vessels are now dependent.
3. The impact of the abdomen on the posterior-basilar lung segments is eliminated.
4. Prone positioning also decreases chest wall compliance.
5. As a result, the transpulmonary pressure gradient is more evenly distributed from nondependent to dependent lung.
6. Blood flow also is more evenly distributed from nondependent to dependent lung.
7. In the prone position a greater percentage of the FRC volume is dependent and Vt is more evenly distributed.
8. All of this results in improved V/Q matching and Pao2 increases.
IV Response to Prone Positioning
