The main findings of this study, the largest study of CAP admissions to ICU published worldwide, are of the progressive rise in the number of admissions with time and the survival of half of all cases. While CAP accounts for only a small proportion of total ICU admissions, the rise in the number of CAP cases has been disproportionately large in comparison to the rise in ICU admissions overall. Illness severity, whether judged by admission gas exchange parameters, APACHE score, septic shock, length of hospital stay or mortality, does not appear to have altered sufficiently to explain this large increase, suggesting that the rise is either due to an increase in the total number of severely ill CAP cases presenting to hospital or the accommodation of a previously unmet need. There is evidence to support both explanations and it is likely that a combination of the two has contributed. In support of the former is the disproportionate rise in those aged >74 years (which could also be due to previous unmet need) and the increase in overall hospital admissions for CAP . The previously unmet need theory is supported by the fall in the proportion that are paralysed and sedated on ICU arrival, perhaps reflecting a recent increase in the use of non-invasive ventilation (although this was not specifically recorded in the database). Statistically significant, but clinically small, rises/changes in admission source, steroid treatment, renal replacement therapy, severe respiratory problems, pneumonia type, and secondary admission reason are unlikely to explain these changes.
The overall ICU mortality of 35% is lower than the figures of 48%, 57% and 58% reported in previous UK studies [3–6], and is more similar to reports from New Zealand and other European countries, where seven studies have reported mortalities of 40% or less [10–16]. This apparent improvement in outcome may be due to improved clinical practice (supported perhaps by the fall in admissions only after CPR, which were 7, 13 and 25% in earlier studies [3, 5, 6]), but may also be due to differing admission policies and also the potential for bias in the previous UK studies. These studies were all small (maximum 62 cases) and two were based in single centres and may, therefore, not be representative of the UK as a whole. The much higher ultimate hospital mortality emphasises the value of this as a more relevant outcome measure. Nevertheless, that approximately 50% of patients survived to leave hospital vindicates the use of ICU resources in the management of patients with CAP. The much higher ultimate hospital mortality is not unique to CAP .
The small rise in mortality between the two periods of the study may be because sicker patients were admitted or because of the increase in the proportion of elderly cases in whom comorbidity may be more common and the decision not to augment treatment might be taken at a different threshold to that in younger, fitter individuals. Hospital care, especially identification of the severely ill patient, prior to ICU admission may affect outcome [17, 18]. It is not possible to comprehensively assess pre-ICU care from this database. The fall in the prior CPR frequency and the low CPR frequency compared to previous UK studies [3, 5, 6], might suggest better pre-ICU management. The increase in the pre-ICU length of stay might, however, suggest delay in ICU referral. The admission of more than half of cases within 48 hours of hospital admission supports previous research suggesting that patients with severe illness are usually severely ill at hospital admission and it is at this time that alertness to severity of illness markers is especially important. In previous studies, the proportion of patients admitted early to the ICU has varied markedly. In a Spanish study , 80% reached the ICU within 24 hours of hospital admission, whereas in a British multicentre study , only 48% reached the ICU within this time. The latter figure is similar to the present case series and may reflect a difference in practice between countries, but may also be artefactually related to the small number in such previous studies. The higher mortality in those admitted later to the ICU is a cause for concern. The precise reasons for this cannot be identified from this database, but late referral could include failure to recognise markers of severe illness [17, 18], high use of intermediate care facilities, high ICU bed occupancy limiting ICU access and shortage of ICU beds in the UK. Some late referrals may have had nosocomial pneumonia, which is discussed in detail later. A detailed analysis of factors, including existing severity scoring models, related to outcome is the subject of a separate paper in preparation.
The main weaknesses of this study are its retrospective design and, in particular, that patients with CAP were not separately identified. While it is not possible to be certain that some patients with other diagnoses (such as, nosocomial pneumonia, pneumonia in the immuno-compromised and non-pneumonic exacerbations of chronic obstructive pulmonary disease (COPD)) might not have been included, we believe that the steps taken to exclude such cases and the large size of the study population mean that any such cases that were mistakenly still included are unlikely to have significantly biased the study findings. In particular, the exclusion of postoperative cases and those with specified diagnoses linked with immunosuppression is likely to have minimised inclusion of nosocomial cases and pneumonia in the immuno-compromised; however, nosocomial pneumonia arising in patients admitted for another medical reason might still have been included, but such numbers are likely to be small. If such cases have been inadvertently included, this is most likely to have been in those only admitted to the ICU after seven days in hospital. That we have been successful in excluding even this group is suggested by the absence of any rise in the frequency of cases with secondary admission reasons of myocardial infarction, cardiac failure and COPD in those admitted to the ICU only after two days compared to the less than two days group. The high frequency of a raised PaCO2 is not in our opinion a sign of misclassification of patients with exacerbations of COPD. In a recent study, 21% of CAP patients had a PaCO2 of >45 mmHg at hospital admission , a proportion that is likely to be significantly higher in those admitted to the ICU. The low frequency of 'obstructive airways disease' as a secondary reason for ICU admission also suggests that large numbers of non-pneumonic COPD exacerbations have not been included.