The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

Monitoring Pressure/Volume Curves

Alex B. Adams MPH RRT

A static pressure-volume (p-v) relationship of the lungs was determined for normal individuals in classic physiology studies more than 40 years ago. This relationship becomes markedly abnormal in victims of acute lung injury (ALI) and ARDS. Current care in the mechanical ventilation of ALI/ARDS patients includes a pressure guarding strategy. This strategy requires a more precise setting of tidal volume and PEEP with inspiratory p-v curves used to guide these settings. From the p-v curve, a lower inflection point, and upper inflection point, and a measure of inspiratory system compliance can be made. Tidal volume and PEEP can then be set in reference to these determinations. Methods available to obtain p-v curves include the "gold standard" super syringe method, a slow flow technique and a multiple tidal volume (Levy) method. There are small differences between the results from these methods, but of greater concern at this time, is the relevance of static p-v curves to dynamic conditions. Under dynamic conditions, the patient may well be ventilated closer to the deflation limb of the static curve. In addition, the ventilatory strategy in use and the recent history of lung recruitment may cause an elevated lung volume status--above that predicted from the static curve. Thus, the static p-v curve as a guide to setting the ventilator, may be of less value than previously expected.

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