Data were extracted for 129,647 admissions to 128 adult, general critical care units (ICUs including combined intensive care/high dependency units) from the Case Mix Programme Database (CMPD), covering the period from December 1995 to August 2001. Data were collected locally by trained data collectors according to precise rules and definitions, and the data underwent extensive validation before incorporation into the CMPD. The process of data validation and cleaning was described in detail previously 11.

A standard coding method that classifies information describing the reasons for admission was used to identify the patients, and has been described previously 12. Surgical patients were identified and excluded when the primary or secondary reason for admission was a surgical code or if the patient was admitted from theatre having undergone all or part of a surgical procedure. Nonsurgical admissions were selected if they had a primary reason for admission of COPD, exacerbation of COPD or emphysema, or any of these three conditions as the secondary reason for admission when pneumonia, right ventricular failure, or left ventricular failure was the primary reason. Patients were assigned to one of three subgroups of COPD (pneumonia in a patient with COPD, right ventricular failure in a patient with COPD, or left ventricular failure in a patient with COPD) if the primary or secondary reason for admission was pneumonia, right ventricular failure, or left ventricular failure, respectively, in a patient in whom the other reason for admission was COPD, exacerbation of COPD, or emphysema. The primary and secondary reasons for admission were the diagnoses made by the clinicians and recorded in the patient notes. As such the diagnoses were not arrived at by applying preset diagnostic criteria but by using the working clinical diagnoses reached during the management of the patients, and will reflect the uncertainty inherent in clinical practice.

In addition, the intubation status of the COPD admissions was categorized on the basis of whether they were definitely intubated within the first 24 hours of admission to the Case Mix Programme (CMP) unit, probably intubated in the first 24 hours, or not intubated in the first 24 hours. The CMPD does not contain a response field solely for the identification of intubation status, but patients with an intubated fractional inspired oxygen (FiO₂) recorded and those defined as intubated in the field describing the intubation status associated with the lowest arterial oxygen tension (PaO₂) were identified as definitely intubated. Patients who were probably intubated had a response of 'yes' in the field for mechanical ventilation at admission, or a non-zero value for the lowest or highest ventilated respiratory rate. Patients not intubated in the first 24 hours in the CMP unit fulfilled none of the above criteria.

Data were extracted on case mix, outcome and activity for nonsurgical COPD admissions to CMP units. Detailed description of the definitions used in the CMPD are described elsewhere 11, but the definitions of particular relevance to the COPD patients are outlined below.

Age at admission and sex were extracted. A history of respiratory functional impairment and comorbidity was recorded using data collected as part of the chronic health evaluation section of the APACHE II score 13. The respiratory functional impairment has two aspects: 'severe respiratory disease' is defined as patients with permanent shortness of breath with light activity due to pulmonary disease; and 'home ventilation' specifies whether a patient has used or uses home ventilation (excluding continuous positive airway pressure ventilation). Patients with liver disease included those with an episode of hepatic encephalopathy in the previous 6 months or who had evidence of portal hypertension. Patients with cardiovascular comorbidity included patients with fatigue, claudication or angina at rest, in whom any activity increased the symptoms and where the symptoms were ascribed to myocardial or peripheral vascular disease. Patients with chronic renal failure were those requiring chronic renal replacement therapy. Patients with haematological diseases included those with acute myelogenous leukaemia, acute lymphocytic leukaemia, multiple myeloma, chronic myelogenous leukaemia, chronic lymphocytic leukaemia, and lymphoma. Patients with immunosuppression included those with a congenital immunohumoral or cellular immune deficiency state and those receiving chemotherapy defined as the receipt of a drug that can lower resistance to infection and those who have received radiotherapy. AIDS patients were those meeting the current World Health Organization definition. Patients with metastatic cancer had distant metastases documented by surgery, imaging, or biopsy. History of steroid treatment was defined as the receipt of 0.3 mg/kg, or greater, prednisolone (or equivalent) daily for the 6 months before critical care admission. Patients receiving cardiopulmonary resuscitation within 24 hours before unit admission were also identified.

The following physiological variables, selected a priori, were extracted from the data collected during the first 24 hours in the CMP unit: lowest pH; arterial carbon dioxide tension from the arterial blood gas with the lowest pH; PaO₂/FiO₂ gradient from the arterial blood gas with the lowest PaO₂; and lowest serum albumin. Organ failures were defined according to Knaus 14 using data describing the patient's status during the first 24 hours in the CMP unit. Acute severity was also measured with the acute physiology scores from APACHE II and APACHE III, and the APACHE II and APACHE III scores.

Survival data were collected at discharge from the CMP unit and at ultimate discharge from an acute hospital.

Length of stay in the CMP unit was calculated, as fraction of days, from the date/time of admission to the CMP unit and date/time of discharge from the CMP unit or death in the CMP unit. Total length of stay in hospital was calculated, in days, from the date of original admission to hospital and date of ultimate discharge from hospital or death in hospital. Transfers in from (out to) another critical care unit were identified as admissions whose source of admission to the CMP unit (destination following discharge from the unit) was any ICU or high dependency unit in the same or another hospital. Readmissions to the CMP unit in the same hospital stay were identified from the postcode, date of birth and sex, and confirmed by the participating units.

The overall proportion of admissions to CMP units with nonsurgical COPD was calculated along with the proportion by subgroups: COPD with pneumonia; COPD with right ventricular failure; and COPD with left ventricular failure. Case mix, outcome and activity were described for the overall group and for the subgroups. Admissions of patients younger than 16 years, admissions of patients who stayed for less than 8 hours in critical care, and readmissions within the same hospital stay and transfers from other ICUs were excluded from the calculation of the APACHE II acute physiology score and APACHE II score. Admissions of patients younger than 16 years, admissions of patients who stayed fewer than 4 hours in critical care, and readmissions within the same hospital stay were excluded from the calculation of the APACHE III acute physiology score and APACHE III score.

The effects of case mix factors, specified a priori, on ultimate hospital mortality for nonsurgical admissions with COPD were investigated using logistic regression.

Readmissions within the same hospital stay and admissions for whom hospital outcome data were missing were excluded from all analyses relating case mix factors to ultimate hospital mortality. In addition to univariate analyses, the factors were entered into a multiple logistic regression model. Adjusted subgroup effects were calculated for the three specified subgroups from the multiple logistic regression model.

All analyses were performed using Stata 8.0 (StataCorp, College Station, TX, USA).

Of 129,647 admissions to 128 adult, general critical care units in the CMPD, 3755 (2.9%) of admissions were identified as nonsurgical COPD patients. Table 1 describes measures of case mix for all patients with COPD and the subgroups with pneumonia, right ventricular failure and left ventricular failure. Overall, 35% of patients had severe respiratory disease before hospital admission and 9% had received daily steroids in the 6 months before admission, although only 2% were receiving home ventilation. Comorbidity was relatively rare, with fewer than 1% of patients having any comorbidity other than cardiovascular disease, and with cardiovascular comorbidity present in 2.2% of the group as a whole, 5.9% of those with right ventricular failure and 7.6% of those with left ventricular failure. Patients spent a median of 1 day in hospital before admission to the CMP unit, and 8.3% of patients received CPR in the 24 hours before admission to the CMP unit. Of the group as a whole, 2775 (73.9%) were definitely intubated in the first 24 hours, with an additional 278 (7.4%) probably intubated in the first 24 hours. Patients were acidotic, hypercapnic and hypoxic, and had low serum albumin (Table 1). For the group as a whole the median (interquartile range [IQR]) APACHE II and APACHE III scores were 18 (14–23) and 62 (49–78), respectively. Of all admissions, 12.2% had no organ failures, 49.4% had single organ failure (the majority, 45.2% of all admissions, being respiratory), 27.6% had two organ failures, and had 8.2% three or more organ failures.

Overall, 827 (23.1%) patients died in the admitting CMP unit and 1322 (38.3%) died during the hospital admission (Table 2). The median (IQR) length of stay in the CMP unit was 4.0 (1.6–9.4) days and in hospital it was 16 (9–29) days. Readmissions within the same hospital stay accounted for 4.5% of all admissions. A decision was made to withdraw all active treatment in 422 (11.3%) patients, with the withdrawal decision being made a median (IQR) of 3.7 (1.2–7.9) days after admission to the CMP unit. The 2765 patients who were definitely intubated in the first 24 hours in the CMP unit had a median (IQR) stay in the CMP unit of 5.7 (2.5–11.4) days, as compared with 1.3 (0.6–2.6) days for the 697 patients who were definitely not intubated in the first 24 hours (Table 3).

Patients without any organ failure had a hospital mortality (95% confidence interval [CI]) of 33.8% (29.4–38.4%) whereas patients with single organ respiratory failure had a hospital mortality of 29.8% (27.6–32.1%; Table 4). As the number of organ failures increased the hospital mortality increased, with patients who had respiratory failure and three additional organ failures having a hospital mortality of 83.3% (65.3–94.4%). Eleven characteristics were found to have a significant independent association with increased hospital mortality in the multivariate analysis (Table 5): increased age; presence of severe respiratory disease; increased length of stay in hospital before admission to the CMP unit; cardiopulmonary resuscitation within 24 hours before admission; intubation status in the first 24 hours in the CMP unit; low pH; low PaO₂/FiO₂ gradient; low serum albumin; cardiovascular organ failure; neurological organ failure; and renal organ failure. Respiratory organ failure had a significant independent association with decreased hospital mortality (Table 5).

Case mix adjusted comparison of the three subgroups pneumonia, right ventricular failure and left ventricular failure gave adjusted odds ratios for ultimate hospital mortality (95% CI) of 1.04 (0.86–1.27), 1.59 (1.06–2.38) and 0.77 (0.50–1.18), respectively. This showed patients with right ventricular failure to have a significantly increased risk for hospital mortality after case mix adjustment.