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APACHE II ("Acute Physiology, Age, Chronic Health Evaluation II") is a severity-of-disease classification system (Knaus et al., 1985), one of several ICU scoring systems. It is applied within 24 hours of admission of a patient to an intensive care unit (ICU): an integer score from 0 to 71 is computed based on several measurements; higher scores correspond to more severe disease and a higher risk of death. The first APACHE model was presented by Knaus et al. in 1981.
APACHE II was designed to measure the severity of disease for adult patients admitted to intensive care units. It has not been validated for use in children or young people aged under 16.
This scoring system is used in many ways which include:
- Some procedures or some medicine is only given to patients with a certain APACHE II score
- APACHE II score can be used to describe the morbidity of a patient when comparing the outcome with other patients.
- Predicted mortalities are averaged for groups of patients in order to specify the group's morbidity.
Even though newer scoring systems, such as APACHE II, have replaced APACHE II in many places, APACHE II continues to be used extensively because so much documentation is based on it.
The point score is calculated from a patient's age and 12 routine physiological measurements:
- AaDO2 or PaO2 (depending on FiO2)
- Temperature (rectal)
- Mean arterial pressure
- pH arterial
- Heart rate
- Respiratory rate
- Sodium (serum)
- Potassium (serum)
- White blood cell count
- Glasgow Coma Scale
These were measured during the first 24 hours after admission, and utilized in addition to information about previous health status (recent surgery, history of severe organ insufficiency, immunocompromised state) and baseline demographics such as age. The calculation method is optimized for paper schemas, by using integer values and reducing the number of options so that data fits on a single-sheet paper form.
The score is not recalculated during the stay; it is by definition an admission score. If a patient is discharged from the ICU and readmitted, a new APACHE II score is calculated.
In the original research paper that described the APACHE II score (see references), patient prognosis (specifically, predicted mortality) was computed based on the patient's APACHE II score in combination with the principal diagnosis at admission.
A method to compute a refined score known as APACHE III was published in 1991.
The score was validated on the dataset from 17,440 adult medical/surgical intensive care unit (ICU) admissions at 40 US hospitals.
The prognostic system of APACHE III has two options:
- APACHE III Score
- This provides an initial risk classification of severely ill hospitalized patients in defined groups.
- APACHE III predictive equation
- This uses APACHE III Score with a number of additional variables including the primary reason for ICU admission (from a reference list of 212 conditions classified according to etiology, major organ involved, and distinction between surgical/medical categories); age, sex, race and preexisting comorbidities; and location prior to ICU admission (operating room, recovery or emergency department, transfer or readmission from another hospital or ICU).
When possible, data about the interval time between the patient's arrival to hospital and the ICU admission time are collected.
To measure severity of disease 20 physiologic variable were selected.
APACHE III scores range from 0 to 299.
APACHE IV, published in 2006, is the latest version.  The model was developed using data from 104 intensive care units (ICUs) in 45 U.S. hospitals and could be recommended to use in U.S. ICUs.
- Knaus WA, Draper EA, Wagner DP, Zimmerman JE (1985). "APACHE II: a severity of disease classification system". Critical Care Medicine. 13 (10): 818–29. doi:10.1097/00003246-198510000-00009. PMID 3928249. (This is the first published description of the APACHE II scoring system)
- Knaus WA, Zimmerman JE, Wagner DP, Draper EA, Lawrence DE (1981). "APACHE-acute physiology and chronic health evaluation: a physiologically based classification system". Critical Care Medicine. 9 (8): 591–7. doi:10.1097/00003246-198108000-00008. PMID 7261642.
- Knaus WA, Wagner DP, Draper EA, Zimmerman JE, Bergner M, Bastos PG, Sirio CA, Murphy DJ, Lotring T, Damiano A, et al. (1991). "The APACHE III prognostic system. Risk prediction of hospital mortality for critically ill hospitalized adults". Chest. 100 (6): 1619–36. doi:10.1378/chest.100.6.1619. PMID 1959406..
- Zimmerman JE, Kramer AA, McNair DS, Malila FM, et al. (2006). "Acute Physiology and Chronic Health Evaluation (APACHE) IV: hospital mortality assessment for today's critically ill patients". Critical Care Medicine. 34 (5): 1297–310. doi:10.1097/01.CCM.0000215112.84523.F0. PMID 16540951..