The prevalence of chronic obstructive pulmonary disease (COPD) in the UK has been estimated to be 1%, increasing to 5% in men aged 65–74 years and 10% in men older than 75 years 1. COPD accounted for more than 100,000 hospital admissions in England in 2000/2001 and is the fifth most common cause of mortality in the UK, with 30,000 deaths each year 2. Exacerbations of COPD may lead to respiratory failure requiring ventilatory support either noninvasively or invasively following endotracheal intubation. A recent UK study 3 suggested that clinicians involved in the emergency care of COPD patients had made a median of 10 intensive care unit (ICU) gatekeeping decisions for COPD patients in the preceding 12 months.

COPD is a progressive disorder that spans a continuum from early mild disease to severe and disabling terminal disease, and prognostic estimates can be important in informing critical care admission decisions 4. Factors found to have an association with mortality have included those related to diminished functional reserve resulting from the COPD, such as impairments in activities of daily living (ADL) and factors related to the aetiology of respiratory failure 5; severity of the acute illness, measured using either the acute physiology score of APACHE (Acute Physiology and Chronic Health Evaluation) II 6 or APACHE III 57; and the presence of nonrespiratory organ failures 78.

The majority of studies reporting COPD ICU outcomes originate from the USA, and there have only been two recent studies from the UK; one reported on 42 intubated patients in a single centre 9 and another reported hospital mortality of 277 COPD ICU admissions from 24 ICUs in one region of the UK 10.

This report examines the outcomes of COPD patients admitted to ICUs across England, Wales and Northern Ireland, identified using a high quality clinical database. The case mix at ICU admission, outcome and activity associated with these admissions are described. The effect of factors, determined a priori, on hospital mortality is investigated.

This study describes the outcomes of 3752 COPD patients admitted to UK ICUs over a 5-year period and includes analyses that describe the association between patient characteristics available within the first 24 hours of ICU admission and hospital outcome.

Overall, patients with COPD admitted to the 128 ICUs in this study had a 77% chance of surviving to leave ICU and a 61% chance of surviving to leave hospital. Comparisons of ICU outcomes between studies are difficult to interpret because ICU beds are used in different ways in different countries and subtle differences in inclusion criteria can make populations differ in prognostically important ways. The SUPPORT (Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatment) study 5 recruited consecutive hypercapnic COPD patients admitted to five US hospitals. Of the 1016 hospitalized hypercapnic COPD patients, 526 (51.8%) were admitted to ICU and 348 (66.2%) of those admitted to the ICU were intubated. This intubation rate contrasts with 81.1% being probably or definitely intubated in the first 24 hours in the present study. Caution must also be exercised in comparisons focusing on the apparently more homogeneous intubated groups. For example, although the study conducted by Seneff and coworkers 7 included patients with COPD exacerbations, patients who had right ventricular failure or pneumonia were excluded. In our study, patients with right ventricular failure had a significant, independent increased risk for death, and the exclusion of these patients may go some way to explaining the difference in hospital mortality in intubated patients between the Seneff study (31.8%, 95% CI 24.8–39.3%) and this study (40.6%, 95% CI 38.8–42.3%). Although almost all studies report hospital mortality, measures such as 30- or 60-day mortality are more robust for comparison purposes because the definition of hospital mortality may be influenced by local discharge practices. For example, Nevins and Epstein 6 reported a comparatively low hospital mortality of 27.7% (95% CI 21.1–35.2%) in 166 intubated COPD patients from one US centre, but 38% of patients were transferred to chronic care facilities. In contrast, Ely and coworkers 15 reported a hospital mortality of 38.6% (95% CI 24.4–54.5%) for 44 intubated COPD patients from a US academic centre, which is similar to that observed in the present study. Esteban and coworkers 16 studied 522 patients with COPD receiving mechanical ventilation in 361 ICUs in 20 countries and found an overall hospital mortality of 28% (95% CI 24–32%) with a median (IQR) Simplified Acute Physiology Score II of 38 (31–49), as compared with 42 (35–52) for the intubated patients in the present study.

Differences in length of stay between studies are likely to be influenced by the case mix and supply side factors discussed above. The 522 mechanically ventilated COPD patients from the study by Esteban and coworkers 16 had a median (IQR) length of ICU stay of 8 (5–13) days, as compared with 4.0 (1.6–9.4) days observed in our study. However, the total hospital length of stay was similar in both studies, with the Esteban study having a median (IQR) hospital stay of 17 (10–27) days versus 16 (9–29) days in the present study. Longer ICU stay (median 7 days, IQR 4–14) and similar hospital stay (median 14 days, IQR 9–29) are also apparent in the study by Nevins and Epstein 6. The greater proportion of the total hospital stay spent in European and North American ICUs compared with the UK may well reflect the lower proportion of all hospital beds designated for critical care in the UK.

This study demonstrates that age has an independent relationship with hospital death. This increased risk is similar to that observed in the SUPPORT study, which calculated the independent relative hazard of death to be 1.22 (95% CI 1.05–1.41) per 10-year increase 5. A number of studies of COPD outcome 68917 did not find a relationship between age and hospital mortality. There is potential for prognostic studies to produce conflicting results because of inaccuracy in the measurement of patient characteristics, inadequate measurement of and adjustment for confounding, and lack of power 1819. Most commentators suggest that a minimum of between 15 and 20 deaths will be required per risk factor analyzed 2021. In three of the studies that did not find age to predict mortality 689, age was analyzed as a univariate predictor with between 20 and 46 deaths. In the study by Breen and coworkers 17, in which age was analyzed in a multivariate logistic regression, there were only 15 hospital deaths.

Of chronic diseases existing before hospitalization, only severe respiratory disease had a significant independent relationship with hospital death. Patients with severe respiratory disease have shortness of breath performing most ADL, and the increased risk for mortality for patients with severe respiratory disease is consistent with the increased hazard of death associated with impairment in ADL identified in SUPPORT (relative hazard per additional ADL impairment: 1.14, 95% CI 1.03–1.26) 5. Prior cardiovascular disease was only present in 78 patients (2% of the total) and did not have a significant independent association with outcome. Although a 2% prevalence of cardiovascular comorbidity may seem surprisingly low in a COPD population, it is should be remembered that, to qualify for this comorbidity, patients had to have impairment equivalent to New York Heart Association functional class IV. A total of 294 patients (7.8%) had received 0.3 mg/kg steroid daily for the 6 months prior to admission. Oral steroids have been suggested to confer an increased risk for hospital mortality in a small study conducted in intubated COPD patients using univariate analysis 9. It is possible that the lack of association of oral steroids with death in this study may represent a true absence of association. However, the use of oral steroids in a heterogeneous population, which might have included both COPD patients with a marked asthmatic component and a better than average prognosis, as well as very severe COPD patients with a worse than average prognosis, could lead to no net effect being detected, as of course could incomplete documentation of steroid use in the population as a whole.

In a study of 362 COPD admissions to US critical care units 22 the number of days in hospital before critical care admission was reported to be significantly associated with hospital mortality, although the increased risk per day was not reported. This study confirms an association between days in hospital before ICU and hospital mortality, but it shows the independent risk to be relatively small with patients having only a 2% (95% CI 1–3%) increased odds of hospital death for each day in hospital before ICU admission.

Brochard and coworkers 23 suggested that invasive ventilation was associated with increased mortality when compared with noninvasive ventilation, but there were differences in severity between groups. Seneff and coworkers 7 found that intubation on day 1 was not an independent predictor of hospital mortality (P = 0.07) in a multiple regression model, with 170 patients intubated on day 1 and 54 deaths among intubated patients. Afessa and coworkers 8 also failed to find an independent association between intubation and death in a study of 250 episodes of respiratory failure in 180 COPD patients, in which 153 patients episodes involved intubation and 31 patients died. In this study, 3052 patients were definitely or probably intubated on day 1 and intubation on day 1 was associated with an independent odds ratio (95% CI) of 1.36 (1.07–1.73) for hospital mortality. Given that the risk for ventilator-associated pneumonia has been estimated to be 3% per day 24, it would not be surprising to find increased hospital mortality associated with intubation, and it is possible that the Seneff and Afessa studies were underpowered to detect this effect.

Cardiovascular, neurological and renal organ failures were all associated with a significant, independent increased risk for hospital mortality. Treatment withdrawal was reported to occur in 11.3% of admissions, and if the development of organ failure were to precipitously trigger withdrawal decisions then interpretation of the prognosis associated with organ failure would be difficult. Treatment withdrawal was most common in patients with acute renal failure, but treatment withdrawal accounted for fewer than one-third of the deaths, and recalculation of the risk for death after removing all patients with withdrawal decisions resulted in little change in the OR for death associated with acute renal failure. Respiratory organ failure had a significant independent association with lower hospital mortality, with patients with isolated respiratory failure having a hospital mortality of 29.8%, as compared with 33.8% for patients without any organ failures. It is possible that patients admitted to ICU with COPD but without respiratory organ failure had more severe illness in other organ systems. For example, although the regression model adjusts for the presence or absence of acute renal failure, the severity of renal failure may still constitute residual confounding. Afessa and coworkers 8 also used the Knaus organ failure classification 14 and found an independent increased odds of hospital death of 5.5 per organ failure (excluding respiratory organ failure). Seneff and coworkers 7 found that the development of nonrespiratory organ dysfunction was the major predictor of hospital mortality in 362 COPD admissions to US ICUs. This is also consistent with a study of patients with heterogeneous causes of respiratory failure 25, in which other organ failures were found to be more important than the severity of the respiratory failure.

Although practice may have changed over the period during which data were sampled, for example because increasing use of noninvasive ventilation, this analysis was not conceived to explore, detect and report secular trends. This may be a suitable topic for further research in this area.

When patients with obstructive lung disease present with decompensated type II respiratory failure, it can be very difficult to distinguish those with 'pure' COPD (without any significant reversibility) from those with a mixture of COPD and asthma (patients with a major component of fixed obstruction that coexists with some reversibility). It is likely that patients with greater reversibility will tend to be younger, possibly respond better to steroids and have a better overall prognosis. Unfortunately, the CMP data do not allow this distinction to be explored, and there is no real agreement about how this distinction should be made even when a patient has had full lung function – a distinction that is even harder in the acute setting when information regarding lung function is rarely available. Additional prospective research may be required to further illuminate this problem.

This study demonstrates that patients admitted to ICU with COPD and single organ respiratory failure have a 70% chance of leaving hospital alive and a median length of stay in hospital of 16 days. Although patients entering critical care will represent a selected group of all those patients admitted to hospital with respiratory failure, it is reassuring that the independent risk factors for mortality have much in common with those identified in the SUPPORT cohort, which included all COPD admissions with hypercapnic respiratory failure irrespective of whether the patient was admitted to ICU.

• Non-surgical COPD patients account for 2.9% of all ICU admissions in the UK

• Intubated patients have a median stay in intensive care of 6 days

• COPD patients admitted to ICU have a mortality of 23% in the admitting ICU and 38% in hospital, which is similar to the mortality of patients with all diagnoses admitted to ICU

ADL = activities of daily living; APACHE = Acute Physiology and Chronic Health Evaluation; CI = confidence interval; CMP = Case Mix Programme; CMPD = Case Mix Programme Database; COPD = chronic obstructive pulmonary disease; FiO₂ = fractional inspired oxygen; ICU = intensive care unit; IQR = interquartile range; PaO₂ = arterial oxygen tension.

The author(s) declare that they have no competing interests.

MW drafted the manuscript. DH performed the analyses and contributed to drafting the manuscript. All authors participated in the design and interpretation of the study, and critical revision of the manuscript, and read and approved the final manuscript.