Key messages

• Mortality is a function of both the number of failing organ systems and the severity of physiological derangement within each

• The Multiple Organ Dysfunction (MOD) score variables reflect physiological perturbation rather than therapeutic intervention.

• The MOD score is calibrated against mortality and may provide a measure of admission severity, intensity of therapeutic intervention, and global intensive care unit morbidity.

• The MOD score may be useful as an outcome measure in clinical trials. Introduction

The multiple organ dysfunction syndrome (Bone.etal 1992) is an outgrowth of the development of the intensive care unit (ICU). It exists because the ICU permits prolonged survival in the face of otherwise lethal physiological derangement; its conceptual evolution reflects the organ system-based supportive approach that the ICU evolved to provide.

The first reports of specific organ dysfunction in the ICU described states that were either present or absent as determined by arbitrary and variable criteria. Two features of this poorly understood process were apparent. Its development followed overwhelming infection, injury, ischemia, or other stimuli associated with activation of a host inflammatory response, and mortality rose sharply as its severity, reflected in the number of failing systems, increased.

Both the systems studied and the criteria used to define failure differed significantly between studies. A systematic review of 30 reports published between 1969 and 1993 showed that the respiratory, renal, hepatic, cardiovascular, gastrointestinal, hematological, and neurological systems were the most commonly evaluated systems in published reports (Marshall 1995).

The recognition that ICU prognosis is a function not only of the number of failing systems but also of the severity of derangement within each system, and that organ dysfunction is a potentially preventable complication of critical illness whose quantification provides a surrogate measure of clinical morbidity, has stimulated intense interest in the development of valid and objective systems for quantifying organ dysfunction as an outcome measure. Several such systems have emerged ( Marshall etal 1995; ,Le...Galj.efla/ 1996; Vin.cent.efal 1996). The similarities of these are more striking than their differences, reflecting an implicit and evolving consensus on descriptive principles. The Multiple Organ Dysfunction (MOD) score ( T§b.!e...1.) (M§Esha.!ie.t,..a/ 19.9.5) was structured using a methodologically rigorous approach to maximize the ultimate validity of the measure. Because the description of a complex entity such as the multiple organ dysfunction syndrome involves considerably more than the simple prediction of mortality, this process will be described in greater detail.

Table 1 The MOD score

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