Ventilator-induced injury to the lungs can contribute to prolongedrespiratory failure and even death among patients with acute respiratorydistress syndrome (ARDS). Even post-operative patients with previouslyhealthy lungs, who require temporary mechanical ventilation, are at riskof ventilator-induced lung injury. Such injuries have been reducedtremendously during the last 20 years since studies had demonstratedventilators should be set to deliver lower breath volumes (tidalvolumes) so as to reduce the stress and strain on fragile alveolidamaged by ARDS. According to a new study, however, we may have muchfurther to go to reduce ventilator-induced lung injury.
"It is ironic, because for a large number of patients with ARDS it isthe treatment, rather the syndrome, which ends up killing them" saysLuciano Gattinoni, M.D., lead researcher on the study.
The study was published in the second issue for August of the AmericanJournal of Respiratory and Critical Care Medicine, by the AmericanThoracic 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 attributesthe bulk of that improvement to improved understanding and refinement ofmechanical ventilation protocol. Currently, to assess lung strain,clinicians estimate tidal volume based on patients' weight. "Now weapply tidal volumes normalized per ideal body weight of six to eightmL/kg, down from 12 to 15," he says. Currently, to assess lung strain inorder to ensure that even a lower set tidal volume does not causeexcessive stress, clinicians can only measure end-inspiratory plateaupressure generated by the delivered tidal volume.
Assessing patients' lung stress and strain accurately could mean thedifference between life and death. Overestimating stress and deliveringtoo low of a tidal volume may lead to carbon dioxide build-up in theblood and atelectasis-lung tissue collapse. Underestimating stress anddelivering too high of a tidal volume may enhance the risk ofventilator-induced lung injury.
To determine whether measurement of plateau pressure is an accuratesurrogate measure for lung stress and strain in ARDS patients, Dr.Gattinoni and colleagues directly measured actual stress- the internalcounterforce that reacts to an external load- and strain- the structuralchange associated with stress- in a total 80 patients, includingpost-surgical patients, patients with ARDS, patients with acute lunginjury (ALI) and patients with a medical disease. They used a number ofmeasurements to measure lung stress and strain, primarily esophagealpressure and lung volume assessment with helium dilution technique, andfound that there was little correlation between plateau pressure and settidal volume with the actual lung stress and strain in all four groups.
While plateau pressures and tidal volumes may be reflective of the chestwall elastance and lung volume of the population as a whole, incircumstances where patients require mechanical ventilation, thosegeneral guidelines are inadequate to assess the individual's lung stressand strain. For example, there are certain clear indicators that thechest wall elastance may be altered, e.g. severe obesity. In this casethe plateau pressure would overestimate the stress and encouragephysicians to set the tidal volume too low.
"The consequences are, of course, potentially more dangerous in patientsin which the chest wall elastance is more compromised and the lungvolume is more reduced," said Dr. Gattinoni. "The immediate clinicalimplications are that clinicians should not trust the conventionalmeasurements."
Going forward, Dr. Gattinoni and colleagues would like to see improvedmeasures of lung stress and strain, including routine assessment ofesophageal pressure and lung volumes to compute stress and strain inlarge populations of mechanically ventilated patients.
"If we could decrease, with more refined measurements, the mortalityrate due to mechanical ventilation by 4-5 percent this could save up to7500 lives in the U.S., if we accept that ALI/ARDS has an incidence of150000/year," said Dr. Gattinoni.
Source: American Thoracic Society