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Low Tidal Volume Ventilation Improves Outcomes in Elective Surgery
Using low tidal volumes (6-8 mL/kg ideal body weight, or about 500 mL in the average man) during mechanical ventilation has been known for more than a decade to be lifesaving for people with acute respiratory distress syndrome (ARDS). Since reducing mortality by a relative 22% in ARDSNet, low tidal volume ventilation for ARDS has become one of the few universally accepted beneficial therapies in critical care medicine (although that doesn't mean everyone actually does it, in practice).
In the operating room, on the other hand, tidal volumes of 10-15 mL/kg have been advised in order to prevent atelectasis and hypoxemia in surgical patients under general anesthesia. Although that advice came from 1963, it's still common today for patients undergoing surgery to receive these high tidal volumes (about 750 mL in the average man), and no PEEP while ventilated. PEEP, or positive end-expiratory pressure, reduces shear stress on alveoli during mechanical ventilation.
Animal research suggests that higher tidal volumes can injure healthy lungs, stimulate the release of inflammatory chemicals and predispose animals to organ damage through so-called ventilator-induced lung injury. Observational studies and small randomized trials have not shown clear excess risks among people receiving high tidal volumes while undergoing routine surgery, and while some have found evidence of preclinical ventilatory-induced lung injury, others have argued that low tidal volume ventilation may actually be harmful in surgical patients. Still others say we haven't found the harm of "physiologic" tidal volumes during surgery because we haven't looked that hard (i.e., with large sample sizes and follow up for at least several days after surgery).
More than 230 million people wordlwide receive general anesthesia and mechanical ventilation for major surgery each year. On this scale, even a very small clinical effect determined by tidal volumes delivered during surgery could prove to be important. So Emmanuel Futier et al conducted the IMPROVE Study to find out what impact low tidal volumes would have on clinical outcomes after elective abdominal surgery. Their findings are in the August 1, 2013 New England Journal of Medicine.
What They Did
Authors randomized 400 people undergoing elective abdominal surgery at 7 French academic centers to receive either:
- Low tidal volume mechanical ventilation -- 6-8 mL/kg ideal body weight; PEEP 6-8 cm H2O; recruitment maneuvers every 30 minutes;
- Ordinary mechanical ventilation -- tidal volume 10-12 mL/kg; PEEP and recruitment maneuvers were not protocolized but could be provided at the anesthesiologist's discretion.
A large majority in each group (78 and 80%) were undergoing laparotomy for cancer resection. Patients had equivalent intraoperative experiences (length of surgery, blood loss, use of epidural analgesia vs. general anesthesia, etc.) and were treated identically during surgery (volume of fluids administered, etc.), according to the recorded data. Anesthesiologists were not blinded to treatment group.
In both groups, anesthesiologists tried to keep plateau pressures under 30 cm H2O. All other ventilation procedures were the same in both groups.
The primary end point was reached if any of a list of major complications occurred in the first week after surgery:
- Pneumonia, by predefined criteria
- Need for invasive or noninvasive ventilation for acute respiratory failure
- Sepsis (of any severity)
There were numerous prespecified endpoints as well (length of stay, mortality, ARDS, etc).
What They Found
Major postoperative complications occurred within one week after abdominal surgery in 27.5% of the patients receiving larger tidal volumes, but only in 10.5% of those receiving low tidal volume ventilation.
Low tidal volume ventilation during surgery prevented postoperative acute respiratory failure: 5% of patients in the low tidal volume ventilation group required invasive or noninvasive ventilation in the week after surgery, compared to 17% in the conventional ventilation group -- a 69% relative risk reduction.
17.5% of patients receiving low tidal volume ventilation had pulmonary complications of any type, compared to 36% in the conventional ventilation group. In addition to pneumonia and respiratory failure, these could include atelectasis, acute lung injury or ARDS.
30-day mortality was similar in both groups (~3%), but hospital stays were slightly shorter in those receiving low tidal volume ventilation. There was no apparent harm from low tidal volume mechanical ventilation.
What It Means
An alternative way to view low tidal volume ventilation for ARDS is not as a beneficial therapy, but an avoidance of a harmful practice. For decades prior to the ARDSNet trials, standard care included routinely, harmfully overdistending people's injured lungs, and fortunately this harm was finally recognized. The positive framing of the ARDSNet findings as a discovery of a new beneficial treatment, akin to a lifesaving drug, is more pleasant than dwelling on the sad corollary, that thousands of people have needlessly died from lung injuries induced or worsened by what were best practices for mechanical ventilation.
I'm not aware of what the harm from low tidal volume ventilation might be -- a little more hypoxemia during the case, probably -- but unless they're more problematic than I realize, this trial would seem to tip the balance of controversy toward providing low tidal volumes during abdominal surgery, and perhaps all surgeries, whenever possible.
Don't expect a tidal wave of reduced tidal volumes in response to the IMPROVE study, though. Ventilator-induced lung injury is a perfect example of the type of medical harm whose causative practices are stubbornly resistant to change. Its effects are subtle, slow, usually sublethal, and virtually untraceable back to individual members of the health care team who could have prevented them. (See Atul Gawande's latest piece in the New Yorker for similar examples of clinical inertia throughout medical history, and throughout the developing world today.)
And lest anyone conclude this is a problem isolated to operating theaters, consider that adherence with low tidal volume ventilation for people with acute lung injury and ARDS is less than 50%, even in ICUs at top academic centers. Ironically, the task of protecting lungs may be made more difficult by the proliferation of highly computerized ventilator modes like adaptive support ventilation: these gadgets, promoted as "auto-weaning," typically deliver inappropriately high tidal volumes to patients with ARDS. If the IMPROVE study's findings are real, and low tidal volumes aren't just for ARDS patients anymore, is it possible that these staff-friendly, hands-off ventilators are imperceptibly hurting a small but significant fraction of our patients, right under our noses?
Although not conclusive, other data suggests that low tidal volumes can be beneficial (or that high tidal volumes can be harmful) in patients without evidence of ARDS or acute lung injury. A large meta-analysis combining randomized trials and observational studies published late 2012 in JAMA showed a strong trend toward reduced mortality, lung infections, and ARDS in more than 2,000 people treated with low tidal volumes with a diverse spectrum of illness.
Clinical Takeaway: Low tidal volume ventilation appears beneficial in patients undergoing abdominal surgery, and it's possible that traditionally acceptable high tidal volumes are more harmful than previously realized. It's prudent to at least keep in mind these round-number target low tidal volumes: 500 mL for an average-height man (5'9"), and 380 mL for an average-height woman (5'4"). Use an online calculator or download a mobile app to help you determine target low tidal volume at the point of care.
Emmanuel Futier et al (IMPROVE Study Group). A Trial of Intraoperative Low-Tidal-Volume Ventilation in Abdominal Surgery. N Engl J Med 2013; 369:428-437.