Epidemiology Incidence

The incidence of ALI and ARDS has been difficult to establish. Most studies were conducted before the NAECC definition was proposed and used different criteria to enrol patients. Defining the population at risk in a given study has been equally problematic. Accurate measurement of disease incidence requires knowledge of the number of people with the disease within a defined population at risk for developing it. Prospective trials must account for the catchment area of the hospitals studied; each hospital's catchment area may overlap with that of several other hospitals.

In 1972 the National Heart and Lung Institute (NHLI) task force estimated an incidence of 75 cases of ARDS per 100 000 population per year.27 Several subsequent studies have estimated a much lower annual incidence of 1.5-13.5 cases per 100 000 population (table 5.5).28-32 There are several reasons why the number of cases may have been overestimated. The task force predated the widespread acceptance of the definition of ARDS and used a broad definition of lung injury that included conditions such as renal failure and volume overload. In addition, the population at risk was not clearly

Table 5.5 Annual incidence of the acute respiratory distress syndrome (ARDS) and acute lung injury (ALI) in different clinical studies

Study

Criteria used to diagnose ALI/ARDS

Incidence (cases/1GG GGG population/year)

Study limitations

NHLI task force27

75

• Broad definition of respiratory distress syndrome including patients with volume overload

Canary Islands31

PaO2/FiO2 <110 PaO2/FiO2 <150

1.5 3.5

• Mean age of study population was 32

• Non-urban setting

Utah30

PaO2/FiO2 <110

4.8-8.3

• Incomplete sampling of hospitals

• Use of ICD-9 codes to diagnose ARDS

Berlin28

Lung injury score >2.5 Lung injury score >1.75-<2.5

3.0 17.1

• 2 month study may miss seasonal variation in incidence of ALI/ARDS

• No correction for migration in and out of study population

PaO22/FiO22 <200 Lung injury score >2.5

17.9 13.5 7.6

• 2 month study may miss seasonal variation in incidence of ALI/ARDS

• No correction for migration in and out of study population

defined.33 34 A three year study conducted in the Canary Islands was unique in that all patients requiring mechanical ventilation were cared for at one hospital.29 Using a Pao2/Fio2 ratio of <110 to define ARDS, the population incidence was 1.5 cases per 100 000 population. A Pao2/Fio2 cut off of <150 resulted in an incidence of 3.5 cases per 100 000 population. Although the study strictly defined the population at risk, extrapolation of incidence data from this young population (average age 32) to an urban setting is questionable. A prospective three year study in Utah using a Pao2/PAo2 ratio of <0.2 (corresponding to a Pao2/Fio2 ratio of <110) to define ARDS estimated an annual incidence of 4.8-8.3 cases per 100 000 population. The authors recorded all cases of ARDS in six Utah hospitals and estimated the number of cases in the remaining 34 acute care hospitals using ICD-9 codes.30 Although the investigators made corrections for the population migrating in and out of Utah as well as for visitors developing ARDS, the incomplete sampling of hospitals and the use of ICD-9 codes weakened the study. Also, the incidence of alcohol and tobacco use is probably much less in Utah than in other states. A 2 month prospective study in Berlin found an annual incidence of 3 cases per 100 000 population using the expanded definition proposed by Murray et al.28 The population of Berlin was the study population and 97% of all intensive care units were surveyed. No corrections were made for migration in and out of the catchment area. Finally, a prospective 2 month study in Sweden, Denmark, and Iceland using the NAECC definition found an annual incidence of ARDS and of ALI of 13.5 per 100 000 and 17.9 cases per 100 000, respectively.29 The annual incidence of ARDS using the LIS was 7.6 cases per 100 000 population. Interestingly, only 71 of 110 patients who met the criteria for ARDS using the LIS also met the criteria using a Pao2/Fio2 ratio of <200.

The true incidence of ALI/ARDS is currently unknown, but may not be as high as the 1972 NHLI estimate nor as low as estimates made in the Canary Islands or Berlin. A definitive study using the NAECC definition has been completed at the University of Washington and preliminary results suggest that the original NHLI estimate may have been reasonable.33

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