Ventilator-induced injury to the lungs can contribute to prolonged respiratory failure and even death among patients with acute respiratory distress syndrome (ARDS). Even post-operative patients with previously healthy lungs, who require temporary mechanical ventilation, are at risk of ventilator-induced lung injury.
Such injuries have been reduced tremendously during the last 20 years since studies had demonstrated ventilators should be set to deliver lower breath volumes (tidal volumes) so as to reduce the stress and strain on fragile alveoli damaged by ARDS. According to a new study, however, we may have much further to go to reduce ventilator-induced lung injury.
"It is ironic, because for a large number of patients with ARDS it is the treatment, rather the syndrome, which ends up killing them" says Luciano Gattinoni, M.D., lead researcher on the study.
The study was published in the second issue for August of the American Journal of Respiratory and Critical Care Medicine, by the American Thoracic Society.
In the early 1970's, the mortality rate for ARDS was up to 90 percent. Now, that rate has fallen to about 40 percent. Dr. Gattinoni attributes the bulk of that improvement to improved understanding and refinement of mechanical ventilation protocol. Currently, to assess lung strain, clinicians estimate tidal volume based on patients' weight. "Now we apply tidal volumes normalized per ideal body weight of six to eight mL/kg, down from 12 to 15," he says. Currently, to assess lung strain in order to ensure that even a lower set tidal volume does not cause excessive stress, clinicians can only measure end-inspiratory plateau pressure generated by the delivered tidal volume.
Assessing patients' lung stress and strain accurately could mean the difference between life and death. Overestimating stress and delivering too low of a tidal volume may lead to carbon dioxide build-up in the blood and atelectasis-lung tissue collapse. Underestimating stress and delivering too high of a tidal volume may enhance the risk of ventilator-induced lung injury.
To determine whether measurement of plateau pressure is an accurate surrogate measure for lung stress and strain in ARDS patients, Dr. Gattinoni and colleagues directly measured actual stress- the internal counterforce that reacts to an external load- and strain- the structural change associated with stress- in a total 80 patients, including post-surgical patients, patients with ARDS, patients with acute lung injury (ALI) and patients with a medical disease. They used a number of measurements to measure lung stress and strain, primarily esophageal pressure and lung volume assessment with helium dilution technique, and found that there was little correlation between plateau pressure and set tidal volume with the actual lung stress and strain in all four groups.
While plateau pressures and tidal volumes may be reflective of the chest wall elastance and lung volume of the population as a whole, in circumstances where patients require mechanical ventilation, those general guidelines are inadequate to assess the individual's lung stress and strain. For example, there are certain clear indicators that the chest wall elastance may be altered, e.g. severe obesity. In this case the plateau pressure would overestimate the stress and encourage physicians to set the tidal volume too low.
"The consequences are, of course, potentially more dangerous in patients in which the chest wall elastance is more compromised and the lung volume is more reduced," said Dr. Gattinoni. "The immediate clinical implications are that clinicians should not trust the conventional measurements."
Going forward, Dr. Gattinoni and colleagues would like to see improved measures of lung stress and strain, including routine assessment of esophageal pressure and lung volumes to compute stress and strain in large populations of mechanically ventilated patients.
"If we could decrease, with more refined measurements, the mortality rate due to mechanical ventilation by 4-5 percent this could save up to 7500 lives in the U.S., if we accept that ALI/ARDS has an incidence of 150000/year," said Dr. Gattinoni.
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