Page 122 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

88 PART 1: An Overview of the Approach to and Organization of Critical Care

APACHE II model, the developers selected those variables they thought The Mortality Probability Model (MPM II) was developed from
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relevant to patient outcome and then arbitrarily weighted each variable. 19,124 ICU admissions in 12 countries. MPM II is not disease specific.
In the development of MPM II, SAPS II, and APACHE III, statistical MPM is the only severity-of-illness scoring system that was derived
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techniques identified agnostically variables independently associated at ICU admission and can therefore be used at ICU admission. MPM
with death. These variables were then further refined by use of linear dis- II does not yield a score, but rather a direct probability of survival.
criminant function and stepwise logistic regression analysis, and the final Burn, coronary care, and cardiac surgery patients are excluded. MPM
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set of variables were then weighted by statistical methods and presented includes three physiologic variables, three chronic diagnoses, five acute
as a cumulative score to predict mortality. diagnoses, and three other variables: cardiopulmonary resuscitation
The Acute Physiology and Chronic Health Evaluation II (APACHE II) prior to admission, mechanical ventilation, and medical or unscheduled
system is the most commonly used clinical severity-of-illness scor- surgery admission (see Table 13-2). Each variable is scored as absent
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ing system in North America. APACHE II is a disease-specific scoring or present and is allocated a coefficient. The sum of these coefficients
https://kat.cr/user/tahir99/
system. It uses age, type of admission, chronic health evaluation, and constitutes the logit that is used to calculate the probability of hospital
12 physiologic variables (acute physiology score or APS) to predict hos- mortality.
pital mortality (see Table 13-2). The 12 physiologic variables are defined The MPM 24 20 was designed to be calculated for patients who remained
as the most abnormal values during the 24 hours after ICU admission. in the ICU for 24 hours or longer. MPM includes 13 variables, 5 of
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The predicted hospital death rate is computed from the weighted sum which are used in the MPM . In the validation data set, the area under
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of APACHE II score, a variable determined by whether the patient had the ROC curve was 0.82 and 0.84 for the MPM and MPM , respectively.
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emergency surgery, and the specific diagnostic category weight. The The Simplified Acute Physiology Score II (SAPS II) was developed
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original publication in 1985 of APACHE II score was validated in 5815 from a sample of 13,152 admissions from 12 countries, based on a
ICU admissions from 13 hospitals. The correlation of APACHE II and European/North American multicenter database. SAPS II is not disease
predicted mortality is likely not accurate after nearly 30 years of progress specific. SAPS II uses 17 variables (see Table 13-2) that were selected
including changes in ICU design and administration, wide presence of by logistic regression: 12 physiology variables, age, type of admis-
trained intensivists, introduction of many new therapies and proto- sion (scheduled surgical, unscheduled surgical, or medical), and three
colized care. In the original APACHE II report, the correct classification underlying disease variables (acquired immunodeficiency syndrome,
rate for a 50% predicted risk of death was 85%. metastatic cancer, and hematologic malignancy). The area under the
APACHE III extended APACHE II by improving calibration and ROC curve was 0.86 in the validation sample. The probability of hospital
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discrimination through the use of a much larger derivation and valida- mortality is calculated from the score. SAPS is widely used in Europe
tion patient sample. However, at this time, APACHE III is a proprietary and less widely used in North America.
commercial product. The Sequential Organ Failure Assessment (SOFA) was originally
The main disadvantages of the APACHE II system are its failure to developed as a descriptor of a continuum of organ dysfunction in
compensate for lead-time bias, the requirement to select only one critically ill patients over the course of their ICU stay. The SOFA
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clinical diagnosis, inaccuracies in clinical subsets, which produce poor score is composed of scores from six organ systems, graded from 0 to 4
interobserver reliability and its derivation from cohorts nearly 30 years according to the degree of dysfunction/failure. The score was primar-
ago. To reemphasize, the derivation set of APACHE II is now nearly ily designed to describe morbidity; however, a retrospective analysis of
30 years old and so the absolute predictions are quite inaccurate the relationship between the SOFA score and mortality was developed
and dated. Nonetheless, comparison of contemporaneously collected using the European/North American Study of Severity System data-
patients (such as a treatment and control group in an RCT) may be base. Subsequently, SOFA was evaluated as a predictor of outcome in
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compared for similarity of APACHE II scores. If there are differences a prospective Belgium study. SOFA score on admission was not a good
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in APACHE II scores between cases and controls at baseline, that raises predictor of mortality (area under the ROC curve 0.79); however, mean
concerns about interpretation of the main study results (because dif- SOFA score and highest SOFA score had better discrimination (area
ferences in baseline risk of death could explain differences between under the ROC curve 0.88 and 0.90, respectively). Independent of the
treatment groups in observed mortality rate). A lack of balance between initial value, an increase in the SOFA score during the first 48 hours of
treatment groups at baseline drives the need for an adjusted analysis ICU admission predicts a mortality rate of at least 50%. SOFA is quite
in which APACHE II score and treatment group are covariates in the commonly used in assessing balance at baseline of treatment groups in
analyses. In spite of these shortcomings, APACHE II remains the most RCTs, especially in Europe.
well known and most widely used severity of illness scoring system. 24
admissions in 40 US hospitals. Eighteen variables (see Table 13-2) were ■ DYNAMIC SEVERITY OF ILLNESS SCORING SYSTEMS
APACHE III is a disease-specific score that was developed from 17,440
included, and their respective weights were derived by logistic regression All severity-of-illness scoring systems at ICU admission have rela-
modeling. To improve the accuracy of assessment of neurologic function, tively high rates of misclassification of survivors and nonsurvivors.
the GCS score was changed, because reliability testing suggested the need Misclassifications may be caused by (1) inadvertent exclusion of strong
to eliminate similar GCS scores that could occur in patients who had outcome risk factors that cannot be measured or were not measured at
different neurologic presentations. The APACHE III score sums physiol- ICU admission, (2) inadvertent exclusion of complications that occur
ogy, age, and data variables from seven potential comorbid conditions. during ICU stay, and/or (3) inadvertent exclusion of treatment effects
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The final APACHE III score can vary between 0 and 300. Risk estimate that modify outcome. Scoring systems applied over the course of the
equations for hospital mortality are calculated from the weighted sum ICU stay can diminish the impact of these factors. However, discrimi-
of disease category (78 diagnostic categories are included), a coefficient nation of scoring systems applied during the ICU course is lower than
related to prior treatment location, and the APACHE III score. In the discrimination of scoring systems evaluating outcome at the time of
original derivation sample, estimates of mortality for the first day in initial admission to the ICU.
the ICU had an area under the ROC curve of 0.90, and the correct clas- MPM and MPM 72 27 were developed to estimate the probability of
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sification at 50% mortality risk level was 88%. Although APACHE III hospital mortality at 48 and 72 hours in the ICU. MPM and MPM
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scores can be calculated from published information, weights to con- have the same 13 variables and coefficients that are used in MPM , but
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vert the score to probability of death are proprietary; therefore, the full the models differ in the constant terms, which reflect the increasing
commercial APACHE III system has not been widely accepted or used. probability of mortality with increasing length of ICU stay, even if physi-
However, some trials groups (eg, ARDSnet) use APACHE III raw scores ologic parameters are constant. In the validation group, the areas under
to compare treatment groups at baseline in their trials. the ROC curves of MPM and MPM were 0.80 and 0.75, respectively.
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