Performance of the Pediatric Index of Mortality 2 for pediatric cardiac surgery patients

Division of Critical Care, Department of Pediatrics, Denver Children's Hospital, University of Colorado, Denver, CO, USA.
Pediatric Critical Care Medicine (Impact Factor: 2.34). 03/2011; 12(2):184-9. DOI: 10.1097/PCC.0b013e3181e89694
Source: PubMed


To evaluate the performance of the Pediatric Index of Mortality 2 (PIM-2) for pediatric cardiac surgery patients admitted to the pediatric intensive care unit (PICU).
: Retrospective cohort analysis.
Multi-institutional PICUs.
Children whose PICU admission had an associated cardiac surgical procedure.
Performance of the PIM-2 was evaluated with both discrimination and calibration measures. Discrimination was assessed with a receiver operating characteristic curve and associated area under the curve measurement. Calibration was measured across defined groups based on mortality risk, using the Hosmer-Lemeshow goodness-of-fit test. Analyses were performed initially, using the entire cohort, and then based on operative status (perioperative defined as procedure occurring within 24 hrs of PICU admission and preoperative as occurring >24 hrs from the time of PICU admission). A total of 9,208 patients were identified as cardiac surgery patients with 8,391 (91%) considered as perioperative. Average age of the entire cohort was 3.3 yrs (median, 10 mos, 0-18 yrs), although preoperative children tended to be younger (median, <1 month). Preoperative patients also had longer PICU median lengths of stay than perioperative patients (12 days [1-375 days] vs. 3 days [1-369 days], respectively). For the entire cohort, the PIM-2 had fair discrimination power (area under the curve, 0.80; 95% confidence interval, 0.77-0.83) and poor calibration (p < .0001). Its predictive ability was similarly inadequate for quality assessment (standardized mortality ratio, 0.81; 95% confidence interval, 0.72-0.90) with significant overprediction in the highest-decile risk group. For the subpopulations, the model continued to perform poorly with low area under the curves for preoperative patients and poor calibration for both groups. PIM-2 tended to overpredict mortality for perioperative patients and underpredict for preoperative patients (standardized mortality ratios, 0.69 [95% confidence interval, 0.59-0.78] and 1.48 [95% confidence interval, 1.27-1.70], respectively).
The PIM-2 demonstrated poor performance with fair discrimination, poor calibration, and predictive ability for pediatric cardiac surgery population and thus cannot be recommended in its current form as an adequate adjustment tool for quality measurement in this patient group.

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Available from: Matthew C. Scanlon, Nov 20, 2014
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    • "However, its performance has recently been questioned for pediatric cardiac surgery patients, as a study carried out in the United States showed that in this group discrimination was lower than previously reported, and calibration was poor [11]. Whether or not the same result would be obtained in pediatric patients treated in a specialized dedicated cardiac ICU was, according to the Authors [11], an open question. "
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    ABSTRACT: Mortality rate of patients admitted to Intensive Care Units is a widely adopted outcome indicator. Because of large case-mix variability, comparisons of mortality rates must be adjusted for the severity of patient illness at admission. The Pediatric Index of Mortality 2 (PIM-2) has been widely adopted as a tool for adjusting mortality rate by patients' case mix. The objective of this study was to assess the performance of PIM-2 in children admitted to intensive care units after cardiac surgery, other surgery, or for other reasons. This was a prospective cohort study, conducted in a 607 inpatient-bed tertiary-care pediatric hospital in Italy, with three pediatric intensive care Units (PICUs) and one cardiac Unit (CICU). In 2009--11, all consecutive admissions to PICUs/CICU of children aged 0--16 years were included in the study. Discrimination and calibration measures were computed to assess PIM-2 performance. Multivariable logistic regression analysis was used to assess the association of patients' main reason for intensive care admission (cardiac-surgical, other-surgical, medical), age, Unit and year with observed mortality, adjusting for PIM-2 score. PIM-2 data collection was completed for 91.2% of total PICUs/CICU patient admissions (2912), and for 94.8% of patients who died in PICUs/CICU (129). Overall observed mortality was 4.4% (95%CI, 3.7-5.2), compared to 6.4% (95%CI, 5.5-7.3) expected mortality. Standardised mortality ratio was 0.7 (95% CI: 0.6-0.8). PIM-2 discrimination was fair (area under the curve, 0.79; 95%CI: 0.75-0.83). Calibration was less satisfactory, mainly because of the over two-fold overprediction of deaths in the highest risk group (114.7 vs 53; p < 0.001), and particularly in cardiac-surgical patients. Multivariable logistic analysis showed that risk of death was significantly reduced in cardiac-surgical patients and in those aged 1 month to 12 years, independently from PIM-2. The children age distribution and the proportion of cardiac-surgical patients should be taken into account when interpreting SMRs estimated using the PIM-2 prediction model in different Units. A new calibration study of PIM-2 score might be needed, and more appropriate cardiac-focused risk-adjustment models should be developed. The role of age on risk of death needs to be further explored.
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