The mortality in patients  with ARDS has only shown a gradual decline over the last two decades.   Outline  why the observed decline in mortality has not been greater in magnitude.

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College Answer

A number of factors need to be considered, in particular the large amount of background noise making accurate assessment of improvements near impossible.   Indeed, the studies that have actually shown benefit may not be extrapolatable to the majority of the ARDS population seen in Intensive Care.
The mortality of ARDS is not usually due to respiratory disease per se, but instead to multiple organ dysfunction.   This in turn is due to a multiplicity of factors (including the underlying disease process that resulted in  ARDS [eg. pancreatitis, sepsis, burns], inflammatory response due to ARDS, nosocomial infections.  No single specific therapy is likely to prevent the cascade of events that result in inflammation.  Insufficient studies have been performed to consistently demonstrate one technique has benefits, let alone which combinations of therapies may be useful.
ARDS is also the end result of a large number of predisposing insults.   The outcomes vary dramatically between subgroups (eg. trauma versus pneumonia).  More specific classification or stratification may allow more accurate comparisons.
As a result of better general supportive care, patients that would not previously been considered salvageable could now be going on to develop ARDS, and are more likely to have an adverse outcome.  Potential risk factors as they are discovered are continually being treated/corrected, decreasing the likelihood of less severe/complex cases developing ARDS. It is probably impossible to accurately compare outcomes now with decades ago, given the inability to control for the many factors that influence outcome.

Discussion

In comparison to more recent questions, this subtly hypothetical question calls upon the candidate to speculate about why the current state-of-the-art in ARDS management has failed to produce significant improvements.

This is explored in greater detail in the chapter on outcomes in adult ARDS.

Some points worth mentioning are:

  • Breakthroughs in ARDS management happen very infrequently.It would be unreasonable to expect major improvements in survival if there have been no major improvements in management.
  • Many widely-accepted therapies which were expected to improve mortality have failed to do so, but people kept using them anyway.
  • The trend towards improvement does not seem to be related with any specific breakthroughs in ventilation management
  • We expect an unrealistic improvement in mortality in a condition which is associated with such morbidity.
  • We are measuring mortality badly and inconsistently, especially when it comes to early studies.
  • ARDS patients die of multi-organ system failure rather than hypoxia, and MOSF management has not improved dramatically in recent history.
  • The ARDS patients today are sicker than they were 20 years ago.
  • There are fewer cases of  "mild" survivable ARDS due to aggressive early management of ARDS-associated conditions
  • Difficult to treat causes of ARDS (eg. sepsis and SIRS) have become more prevalent

That said, mortality in ARDS seems to have been declining steadily, at 1.1% per year (at least between 1994 and 2006).

References

References

Abel, S. J. C., et al. "Reduced mortality in association with the acute respiratory distress syndrome (ARDS)." Thorax 53.4 (1998): 292-294.

Zambon, Massimo, and Jean-Louis Vincent. "Mortality rates for patients with acute lung injury/ARDS have decreased over time." CHEST Journal 133.5 (2008): 1120-1127.

ARDS Definition Task Force. "Acute Respiratory Distress Syndrome." Jama307.23 (2012): 2526-2533.

De Campos, T. "Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network." N Engl J Med342.18 (2000): 1302-130g.

Villar, Jesús, et al. "A Clinical Classification of the Acute Respiratory Distress Syndrome for Predicting Outcome and Guiding Medical Therapy*." Critical care medicine 43.2 (2015): 346-353.

 

Erickson, Sara E., et al. "Recent trends in acute lung injury mortality: 1996-2005." Critical care medicine 37.5 (2009): 1574.

 

Zambon, Massimo, and Jean-Louis Vincent. "Mortality rates for patients with acute lung injury/ARDS have decreased over time." CHEST Journal 133.5 (2008): 1120-1127.

 

Milberg, John A., et al. "Improved survival of patients with acute respiratory distress syndrome (ARDS): 1983-1993." Jama 273.4 (1995): 306-309.

 

Stapleton, Renee D., et al. "Causes and timing of death in patients with ARDS." CHEST Journal 128.2 (2005): 525-532.

 

Ferring, Martine, and Jean Louis Vincent. "Is outcome from ARDS related to the severity of respiratory failure?." European Respiratory Journal 10.6 (1997): 1297-1300.

 

Pierrakos, Charalampos, and Jean-Louis Vincent. "The changing pattern of acute respiratory distress syndrome over time: a comparison of two periods." European Respiratory Journal 40.3 (2012): 589-595.

Villar, Jesús, Demet Sulemanji, and Robert M. Kacmarek. "The acute respiratory distress syndrome: incidence and mortality, has it changed?." Current opinion in critical care 20.1 (2014): 3-9.

Amato, Marcelo BP, et al. "Driving pressure and survival in the acute respiratory distress syndrome." New England Journal of Medicine 372.8 (2015): 747-755.