Vol. 15 •Issue 18 • Page 5
Tips on Practice
Prone Positioning Often Helps Improve Oxygenation
Intensivists sometimes resort to very aggressive and complicated measures, such as super-PEEP, inverse-ratio ventilation, and high-frequency oscillation ventilation in an attempt to treat refractory oxygenation. It’s surprising that something as “simple” as turning patients onto their bellies can often improve oxygenation, sometimes dramatically. Proning should be considered a first-line measure to help improve blood oxygenation.
If most intensivists are aware of the potential benefits of this maneuver, then why is it not used more frequently? Probably because it’s not always easy to put a critically ill patient into a prone position. Think of the logistics. It can be a nightmare getting all of the lines, cords, hoses, tubes, catheters, wires, probes, sensors, circuits, cables and other gizmos safely turned over (still intact!) with the patient. It’s no wonder that prone positioning is often a last-ditch effort.
GIVE IT A WHIRL?
One of the drawbacks with proning is that it doesn’t always help with oxygenation. In fact, it may even prove to be deleterious. What’s more, oxygenation usually worsens initially, then gradually gets better over the next 1 to 2 hours of proning. The only way to find out if proning will help is to give it a whirl. A patient’s life may even depend on it.
There are numerous concerns that clinicians may have when considering prone positioning. For instance, what if something important becomes disconnected, dislodged, kinked, or pulled out (yikes!)? What about being able to continue caring for bandages and dressings on the front of the patient? Is the patient going to tolerate lying on his stomach for hours or days at a time? What if the patient codes? How about the thought of turning over patients who weigh 200 pounds or more?
So it’s not surprising why many nurses and therapists are somewhat reluctant to recommend prone positioning. Many facilities do, however, successfully use it on a regular basis. Yes, really! These institutions have a protocol or a specific algorithm to help guide their ambitious staff through the successful use of prone positioning.1 The procedure usually requires three or more people, with one person assigned to closely monitor the lines and the endotracheal tube.
Sedation is often needed with proning to prevent excessive agitation of the patient. Paralytics, however, should be used sparingly because of their deleterious effects on the diaphragm. Facial edema is also common, but can be minimized with a combined reverse Trendelenberg position. Careful attention is also needed to help prevent bedsores on the skin.
HOW DOES IT HELP IMPROVE OXYGENATION?
A number of theories have been proposed about how prone positioning improves blood oxygenation. It used to be thought that it increases end-expiratory lung volume, but this theory has been disproved.
There are actually several major physiologic changes that occur with prone positioning that are potentially beneficial. One of the most profound is a significant reduction in the amount of lung being compressed under the weight of the heart. In the supine position, 7 to 42 percent of the lung is under the weight of the heart.2 In the prone position, however, none of the lung is under the heart, which can have a dramatic effect on relieving compression of the lungs by the weight of the heart.
The other major change is a significant reduction in chest wall compliance. Lying supine, the anterior ribs are able to move outward and upward with each inspiration. The thoracic compliance decreases in the prone position because the posterior ribs become upright and have significantly less ability for movement than the anterior ribs when supine. Meanwhile, in the prone position, the anterior ribs are being pressed against the mattress and have restricted movement. The result is that in the prone position there is increased ventilation into the anterior lungs because of the reduced thoracic compliance.
HOMOGENOUS VENTILATION AND PERFUSION
Prone positioning causes a significant improvement in regional lung inflation as well as a redistribution of perfusion. In comparison to lying supine, the prone position causes a more homogenous distribution of ventilation and perfusion, rather than most of the perfusion going to the gravity-dependent region and ventilation going mostly into the non-dependent regions.
During mechanical ventilation of supine patients with acute respiratory distress syndrome (ARDS), ventilation is diverted into the non-dependent region, and the dependent regions often collapse or become consolidated. The situation is confounded by hypoxic vasoconstriction, pulmonary edema and vessel compression or obliteration. In the prone position, however, ventilation and perfusion have a more homogenous distribution.
Air beds and soft mattresses do not achieve the same effect as a firm mattress for reducing chest compliance. Also be aware that with the use of pressure ventilation and prone positioning, the mechanical tidal volume will drop and the vent will need to be adjusted accordingly. Likewise, tidal volume will increase when the patient is returned to the supine position.
HOW MUCH DOES OXYGENATION IMPROVE?
Patients who have a 20 percent or greater improvement in their PaO2/FIO2 ratio are considered “responders” to prone positioning. Studies have demonstrated that 50-100 percent of patients evaluated were responders because they had a significant improvement in oxygenation.
HOW LONG TO PRONE?
Since proning is a complicated maneuver, it is usually done for at least 12 hours or more at a time. It might be best to change positions 2 to 3 hours before a change of shift. This will enable the patient and nurse to stabilize before the new shift takes over. On the other hand, there may be more willing and enthusiastic staff available during a change of shift, which will permit more people to assist with turning the patient onto his stomach.
1. Messerole E, Peine P, et al. The pragmatics of prone positioning. Am J Respir Crit Care Med. (2002;165:1359-63).
2. Albert RK, Hubmayr RD. The prone position eliminates compression of the lungs by the heart. Am J Respir Crit Care Med. (2000;161:1660-65).
Michael Hahn is a California practitioner.