Volume May (Or May Not) Impact Mortality in ICU Patients

Abstracts & Commentary

By Leslie A. Hoffman, RN, PhD, Department of Acute/Tertiary Care, School of Nursing, University of Pittsburgh. Dr. Hoffman reports no financial relationship to this field of study.
This article originally appeared in the November 2006 issue of Critical Care Alert. It was edited by David J. Pierson, MD, and peer reviewed by William Thompson, MD. Dr. Pierson is Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, and Dr. Thompson is Staff Pulmonologist, VA Medical Center; Associate Professor of Medicine, University of Washington,. Dr. Pierson and Dr. Thompson report no financial relationships relevant to this field of study.

Synopsis: In 2 studies, mechanical ventilation in a hospital with a high case volume was associated with reduced mortality for nonsurgical patients, either regardless of severity of illness (Kahn et al) or only in high-risk patients (Glance et al).

Sources: Kahn JM, et al. Hospital volume and the outcomes of mechanical ventilation. N Engl J Med. 2006;355:41-50; Glance LG, et al. Impact of patient volume on the mortality rate of adult intensive care unit patients. Crit Care Med. 2006;34:1925-1934.

Associations between the number of patients managed (hospital volume) and improved patient survival have been extensively documented in the surgical literature—such as cardiac surgery, ruptured aortic aneurysm, and several types of cancer surgery—and suggested for selected medical conditions, such as acute myocardial infarction and the acquired immunodeficiency syndrome. Two recent studies attempted to determine whether high hospital volume was associated with reduced mortality following mechanical ventilation (MV).

Kahn et al examined data from 20,241 patients admitted to 37 ICUs in the Acute Physiology and Chronic Health Evaluation (APACHE) clinical information system who underwent MV from 2002 through 2003. The sample excluded surgical patients (as determined by admitting diagnosis code) and also admissions to eight "outlier" hospitals with very low (< 50 patients/year; n = 7) or very high (> 1000 patients/year; n = 1) volumes of patients receiving mechanical ventilation. The analysis controlled for severity of illness (APACHE III score), admission diagnosis, preadmission location, academic status of the hospital, type of ICU, geographic region, and presence of intensivists. The primary outcome variables were ICU and hospital mortality. An increase in hospital volume was associated with improved survival in the ICU and in the hospital. Admission to a hospital with the highest quartile volume (> 400 ventilated patients/year) was associated with a 37% reduction in odds of death in the ICU as compared with admission to a hospital in the lowest quartile (£ 150 ventilated patients/year) (P < .001). The absolute risk of death in a low-volume hospital was 34.2% vs 25.5% in a high-volume hospital.

Glance et al examined data from 70,757 patients admitted to 92 ICUs in the Project IMPACT (Society of Critical Care Medicine) database from 2001 to 2003. After controlling for patient risk factors and ICU characteristics, patients admitted to high-volume ICUs had improved outcomes (P = .025). However, the mortality benefit was only seen in high-risk patients managed in ICUs treating high volumes of high-risk patients, defined as a Simplified Acute Physiology Score (SAPS II) > 41. There was no association between ICU volume and outcomes when the volume calculation was based on all ICU admissions.

Commentary

Several expert task forces sponsored by the National Institutes of Health, the Society of Critical Care Medicine, and the American College of Chest Physicians have proposed that critical care medicine be regionalized, with the goal of providing improved patient outcomes at lower cost. The premises behind these recommendations are that patients who require mechanical ventilation need complex care that is best provided by a team of highly experienced clinicians, and such teams are more likely to be found in high-volume institutions. Accordingly, the findings of these two studies have important health policy implications.

Study results suggest a volume-outcome association, but do not clarify whether this association is restricted to high-risk patients managed in ICUs that admit large numbers of such patients, or applies to all nonsurgical patients admitted to a high-volume institution, regardless of their risk status. Several differences in methodology likely explain these somewhat divergent findings. First, the databases used to identify subjects differed, with one study using the APACHE III database (which collects data from hospitals in the United States), and the second the Project IMPACT database (which includes hospitals in the United States, as well as Australia, Brazil, Puerto Rico, and Canada). The types of patients enrolled also differed, with one study restricting entry to nonsurgical patients, based on the rationale that a volume-outcome relationship had already been established for many surgical procedures, and the second including surgical patients. The scoring systems used to assess severity of illness (APACHE III, SAPS II) were different, and one study focused on hospital volume whereas the second focused on ICU volume. In addition, the methods used for statistical analysis differed in regard to the techniques used for model development and variables introduced to control for potential confounders.

There are many potential causes for improved survival in patients who require mechanical ventilation. These include differences in organizational structure, access to an intensivist, nurse-to-patient ratio, and patient management. The last of these would include differences with respect to evidence-based care, the use of protocols for such things as sedation, mechanical ventilation and glycemic control, and other aspects of care. Clinicians in high-volume hospitals are likely to have more experience in the care of critically ill patients, which should translate into improved outcomes. However, high-volume hospitals are also likely to be challenged by the numbers of patients seen, frequent rotation of coverage, inconsistency in attending and nurse staffing, and organizational directives that at times seem adverse to optimal care practices.

Findings of these studies suggest that hospital volume may be an important determinant of outcome among critically ill patients who require mechanical ventilation, especially if they are high risk and managed in an ICU that admits large number of such patients.