A Perspective on Tidal Volume Reduction in Acute Respiratory Distress Syndrome
By Rich Kallet, MS, RRT, FAARC, FCCM
Director of Quality Assurance, Respiratory Care Services, San Francisco General Hospital
Mr. Kallet reports no financial relationships relevant to this field of study.
SYNOPSIS: Less than 20% of acute respiratory distress syndrome patients were treated at any point during mechanical ventilation with a tidal volume at or below the target used during the NIH ARDSNet trials.
SOURCE: Weiss CH, Baker DW, Weiner S, et al. Low tidal volume use in acute respiratory distress syndrome. Crit Care Med 2016 March 31 [Epub ahead of print].
Four Chicago area hospitals (one academic, three community facilities) were the subject of a six-month retrospective study reviewing the management of 362 patients meeting the Berlin Definition of acute respiratory distress syndrome (ARDS). The electronic medical record at each institution provided continuous ventilator data from the entire course of mechanical ventilation from intubation to extubation or death. The primary outcome was the percentage of patients who demonstrated at least one tidal volume (VT) measurement meeting the ARDSNet goal without reference to plateau pressure (Pplat), which was not measured at 50% of the hospitals. Among the secondary outcomes were the percentage of time patients were managed with a VT < 6.5 mL/kg predicted body weight (PBW) and the time from ARDS onset to the first VT of < 6.5 mL/kg PBW.
Of the 362 ARDS patients, approximately 20% were classified as severe and 40% as either moderate or mild cases each. Overall, only 19% of patients ever demonstrated a VT < 6.5 mL/kg, and only 26% among those with severe ARDS. There was no difference between academic vs. community hospitals in this regard. In patients who were managed with the ARDSNet VT, only about 60% of the time did they actually receive a VT < 6.5 mL/kg. Only 40% of severe ARDS cases received a VT < 6.5 mL/kg at some point during mechanical ventilation. Finally, among all ARDS patients who received an ARDSNet VT, one-third had a delay of more than three days in reaching that threshold.
This study reflects the inadequacies of utilizing lung-protective ventilation (LPV) 16 years after the landmark ARDSNet trial. By 2013, none of the hospitals had incorporated an LPV protocol or order set as a standard of care. Only 6% of attending physicians initiated rigorous LPV (VT < 6.5 mL/kg PBW) within a day of ARDS onset in at least 50% of their patients. In contrast, more than half of attending physicians never initiated rigorous LPV within 24 hours of ARDS onset. Furthermore, Pplat had not been universally mandated to guide VT adjustments. This is very troubling, as alveolar driving pressure (Pplat - PEEP, or positive end-expiratory pressure) is a crucial driver in the development of ventilator-induced lung injury, cor pulmonale, and mortality.
Among the factors the authors cited while explaining the inadequate achievement of LPV, three hint at a common theme of clinical compromise. First, the failure to recognize ARDS in a timely manner partly may reflect a long-standing disagreement with how ARDS was redefined in the mid-1990s. Overcoming this obstacle requires physicians to recognize that most patients intubated for acute respiratory failure usually feature two inherent risk factors for ARDS: a pro-inflammatory state and a reduction in functional residual capacity that magnifies the risk for iatrogenic lung injury. Acknowledging these risk factors renders the interpretation of chest radiographic opacities much less important than expediting effective LPV.
Second, we should acknowledge that in times of high census and acuity, it’s unrealistic to expect physicians to manage every aspect of multiple critically ill patients without inadvertently overlooking important details, at least temporarily. The ARDSNet ventilator and fluid management studies demonstrated that respiratory therapists and nurses are eminently capable of executing highly intricate protocols. During the ARDSNet studies, both professions welcomed this responsibility with great pride. The emphasis on patient-centered outcomes driving 21st century healthcare requires an evolution in care delivery that incorporates and develops these professional resources.
The third issue is patient-ventilator asynchrony, which was a topic of frequent discussion during the ARDSNet ARMA study. I distinctly remember Dr. Brower remarking,“eight is easy, six is much harder,” an observation that was affirmed many years later.1 VT mismatching increases work of breathing and provokes dyspnea. It has long been recognized as a barrier to LPV. Because generous sedation negatively affects patient outcomes, the tendency to liberalize VT to 7-8 mL/kg appears to be a reasonable compromise, but only in less severe cases and during the recovery phase. However, in the early phase of moderate-to-severe ARDS, strict adherence to LPV goals is likely crucial to improving outcomes and should outweigh other concerns. Finally, the fact most patients were managed at a VT < 9 mL/kg and only a tiny fraction at > 12 mL/kg is noteworthy. Regardless of the shortcomings found in this study, it still represents a substantial change in practice from 20 years ago.
- Kallet RH, Campbell AR, Dicker RA, et al. The effects of tidal volume demand on work of breathing during lung-protective ventilation in patients with acute lung injury and acute respiratory distress syndrome. Crit Care Med 2006;34:8-14.
Less than 20% of acute respiratory distress syndrome patients were treated at any point during mechanical ventilation with a tidal volume at or below the target used during the NIH ARDSNet trials.
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