Evaluating outcomes in intensive care

There is no dispute that, because of their high internal validity, rigorously conducted experiments or randomized controlled trials of sufficient statistical power are the gold-standard research design for detecting the important beneficial or harmful effects of existing or new interventions. In such experiments, individuals are assigned prospectively to one of two (or more) alternative interventions using a random method of allocation.

With rigorous random allocation, the investigator can assume that the two (or more) groups studied have similar distributions of both the known and unknown confounding factors, i.e. the characteristics that could affect both their selection and risk for a particular outcome. Therefore interpretation of the results of randomized controlled trials can assume that the groups being compared were similar in all respects apart from the intervention(s) under study.

In practice, however, there are a number of situations where random allocation is not possible for either ethical or logistical reasons ( B.l.a..c.k..1.9..96.). For example, it is deemed unethical to randomly allocate severely ill patients to receive intensive care or general ward care. Although it might be possible to obtain ethical approval to randomly allocate less severely ill patients, it is likely that neither clinicians nor patients and their relatives would find this acceptable. Low recruitment to such a trial would jeopardize the ability to generalize from the results.

There are also questions about the way that services are organized that preclude evaluation by an experimental method. For example, it is difficult to conceive how the supposed benefits of a regionalized service could be compared with a non-regionalized service within a randomized study design.

In certain situations, even specific medical interventions may not be amenable to a randomized controlled trial. This is because, even though clinicians may vary in how they treat similar patients, each clinician may be quite certain of the correctness of his or her own practice. In other words, although collective equipoise may exist, individual equipoise may be absent (Liilford.. „and...J.ackson 1995).

The alternative in such a situation is to use observational methods in which the outcome of care that patients receive as part of their 'natural' treatment is studied. In other words, no attempt is made by the investigators to assign patients on a random basis to particular treatments. Instead, naturally occurring variations in how patients are treated form the basis of any comparisons. Before drawing inferences from the outcomes of treatment for such groups of patients, the characteristics of the patients admitted to intensive care have to be taken into account.

The collective term which has been applied to these patient characteristics is case mix, and the term case-mix adjustment has been applied to the process of attempting to account for the presence of these risk factors when comparing hospital death rates following intensive care.

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