INCIDENCE AND MORTALITY AFTER ACUTE RESPIRATORY FAILURE AND ACUTE RESPIRATORY DISTRESS SYNDROME IN SWEDEN, DENMARK, AND ICELAND

To the Editor :

We are grateful to Luhr and colleagues (1) for their contribution to the outcome and epidemiological data for ARDS, but for a variety of reasons feel that the apparently low mortality rate may be misleading.

First of all, it seems that they have an atypical ICU population because only 12% of their patients required  40% oxygen by mask. This may help to explain how 39 of their 221 ARDS patients avoid intubation. Those patients receiving relatively low concentrations of oxygen via a face mask (for example PO₂ 75 mm Hg on 40% oxygen), with bilateral infiltrates, do indeed fulfill the American-European Consensus diagnostic criteria for ARDS. However, these patients represent a very different entity from those patients with respiratory failure severe enough to require intubation and ventilation with high concentrations of oxygen and PEEP.

It is also slightly odd that those patients with ARDS have almost exactly the same APACHE II score as those with other forms of respiratory failure. As the ARDS patients have significantly worse gas exchange, it follows that their general physiological state is in fact better than those without ARDS.

We should also point out that the APACHE II scores presented were recorded around the time of diagnosis and not necessarily within the first 24 h of intensive care for which they were validated. However, the reported mean scores of 19.2 are still only marginally worse than our Scottish ICU population mean score of 18.3 for all patients.

In addition, 78% of these ARDS patients had an underlying diagnosis of pulmonary origin, which has recently been reported to carry a substantially lower mortality than a nonpulmonary diagnosis (2).

The final and perhaps most surprising point is the exclusion of all those patients (a total of 303) who died in the first 24 h in ICU. These patients were presumably the most unwell in their population. We cannot know how many of these patients met the criteria for ARDS, but even if only 10% of them did, the published mortality figure would look much less impressive.

Studies such as this are helpful in understanding more about present outcome in ARDS. We do feel, however, that the headline mortality figure is unlikely to be translated into other ICU populations.

Department of Anaesthesia, Royal Infirmary, Glasgow, Scotland, United Kingdom

I. S. Grant

Intensive Care Unit, Western General Hospital, Edinburgh, Scotland, United Kingdom

F. N. MacKirdy

Scottish Intensive Care Society Audit Group, Victoria Infirmary, Glasgow, Scotland, United Kingdom

1. Luhr, O. R., K. Antonsen, M. Karlsson, S. Aardal, P. Thorsteinsson, C. G. Frostell, J. Bonde, and the ARF Study Group. 1999. Incidence and mortality after acute respiratory failure and acute respiratory distress syndrome in Sweden, Denmark and Iceland. Am. J. Respir. Crit. Care Med. 159: 1849-1861 [Abstract/Free Full Text].

2. Jardin, F., J.-L. Fellahi, A. Beauchet, A. Vieillard-Baron, Y. Loubieres, and B. Page. 1999. Improved prognosis of the acute respiratory distress syndrome 15 years on. ICM 25: 936-941 .

From the Authors:

We disagree with the conclusion of Drs. Hughes and Grant that the mortality rate we reported (1) is falsely low due to an "atypical" ICU study population.

Our study has again demonstrated the extent to which age and acute physiological score influence mortality in ARDS, and that "average" mortality of a non-stratified group of ARDS patients gives little information. If Hughes and colleagues would like to directly compare mortality figures they need to stratify their patients first, an almost impossible task.

In order to avoid diluting our material with postoperative cases (delayed extubations) we included patients treated  24 h. It is possible that some patients who died soon after ICU admission would have fulfilled ARDS criteria, but in our experience, which is well supported in the literature, it is rare that patients with ARDS die during the initial phase of the disease. It can also be argued that the screening of all patients for ARDS in the first few hours of illness would also require screening in emergency rooms and operating theaters. That overall 12% of the patients required  40% oxygen is not remarkable. Many small ICUs were represented in the study with admission criteria that were different than those of larger referral centers. To give an example, the ratio of included patients to total ICU admissions in seven multidisciplinary Swedish University ICUs was 20% compared with 8% in the group of smaller nonuniversity ICUs. In the University ICUs the ARDS mortality was 45%.

The strict use of a new definition of ARDS, a study population representing almost all admissions within three countries at the time and our high incidence, renders it unlikely that the mortality rates we reported are a result of missed subpopulations of ARDS patients. A trend towards a lower mortality in ARDS was discussed several years ago (2).

However, the lack of a precise definition of "acute onset" in the new ARDS definition is unfortunate. Does a patient with severe hypoxemia upon arrival in the emergency room, on the operating table, or upon ICU admission have ARDS if titration of PEEP, aggressive recruitment attempts, prone positioning, or treatment with NO or ECMO a few hours later improve oxygenation to an extent to which ARDS criteria are no longer fulfilled?

We would like to encourage reports on epidemiology and clinical outcome after respiratory failure and ARDS on a wide basis, rather than limit the discussion to case series from University ICUs.

OWE LUHR

Division of Anesthesia and Intensive Care

Karolinska Institute at Danderyd Hospital

Stockholm, Sweden

KRISTIAN ANTONSEN

JAN BONDE

Department of Anaesthesiology and Intensive Care

Herlev Hospital

Herlev, Denmark

ADALBJÖRN THORSTEINSSON

Division of Anesthesia and Intensive Care

Landspitalinn Hospital

Reykjavik, Iceland

CLAES FROSTELL

Department of Paediatric Intensive Care

Astrid Lindgren Childrens Hospital

  at Karolinska Hospital

Stockholm, Sweden

1. Luhr, O. R., K. Antonsen, M. Karlsson, S. Aardal, A. Thorsteinsson, C. G. Frostell, J. Bonde, and the ARF Study Group. 1999. Incidence and mortality after acute respiratory failure and acute respiratory distress syndrome in Sweden, Denmark and Iceland. Am. J. Respir. Crit. Care Med. 159: 1849-1861 .

2. Milberg, J. A., D. R. Davis, K. P. Steinberg, and L. D. Hudson. 1995. Improved survival of patients with acute respiratory distress syndrome (ARDS): 1983-1993. J.A.M.A. 273: 306-309 [Abstract].