In their prospective trial on PEEP titration following recruitment maneuver in patients with ARDS, Fengmei et al provided a well controlled study on the effects of the ratio of dead space to tidal volume (VD/VT).1 Changes in compliance, functional residual capacity, and PaO2/FIO2 after the recruitment maneuver were also reported. In the clinical arena it is sometimes difficult to obtain correct information on which PEEP level works best for a patient when there are interruptions on maintaining the patient in a “steady state,” such as suctioning, turns, and general care. Fengmei et al mentioned in the discussion that the PEEP level corresponding to the maximum compliance after the recruitment maneuver was found to be 10 cm H2O, which was lower than the PEEP corresponding to the minimal VD/VT. They also mentioned that the higher PEEP may increase VD/VT, leading to over-distention of well ventilated alveoli, or may reduce cardiac output, which may not be detected. This adds to the clinical relevance that too much PEEP may become just as detrimental to the lungs. Clearly, the lowest VD/VT should not be the end point of the exercise. The risks were explained in studies from several years ago.2,3 The conclusion in this trial was that managing VD/VT may be clinically useful, but that particular parameter was one of the least statistically significant in their Table 2.
The trial gives insight into recruitment maneuvers and PEEP titration, and their subject group compares similarly to the outcomes of VD/VT from Raurich et al in terms of mortality and PaO2/FIO2.4 The bedside clinician may not be able to spend the time or have the tools to obtain the best functional residual capacity and VD/VT, but may see a combination of increases and decreases to certain values from the devices that were described (NICO and Engström Carestation). When I am trying to find an optimal PEEP for the patient, especially if the patient is unstable, I look for the best of a combination of parameters. Certainly, optimizing compliance and VD/VT, as well as watching for any decrease in mean arterial pressure < 60 mm Hg, assists me in determining the best PEEP for the situation and avoiding over-distention or reduced cardiac output just to obtain the lowest VD/VT. Capnography or functional residual capacity is useful in applying information gathering to estimate respiratory quotient and VD/VT breath by breath, which utilizes the equations described in the paper and appears robust enough to reduce the frequency to obtain multiple arterial blood gas values.5 Some separate parameters of the study could be mentioned that could influence the strength of their results.
I am interested to know if Fengmei et al collected data for oxygen delivery, carbon dioxide clearance, and alveolar tidal volume, to see if their statistical information can be taken one step further. As the PEEP level is increased, those values are expected to rise until a certain point, which may indicate over-distention. The equations described for the NICO and Engström Carestation use similar samples from expiratory alveolar tidal volume, end-tidal carbon dioxide, oxygen delivery, and carbon dioxide clearance to obtain an estimated respiratory quotient. If multiple regression analysis can be done to determine the correlations between VD/VT, compliance, oxygen delivery, carbon dioxide clearance, alveolar tidal volume, and functional residual capacity, my suspicion is that r2 will still be quite high for most parameters, as most are derived under the same equation. The strength of many parameters may be just as useful as stand alone parameters. The values in Fengmei et al's Figure 1 seem to show that optimal PEEP is between 10 and 12 cm H2O. It may help the bedside clinician to look at several values, including changes in VD/VT, in the decision making.
Footnotes
Ms Couture has disclosed a relationship with Respironics.
- Copyright © 2013 by Daedalus Enterprises
References
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