An evaluation of ventilator reliability: a multivariate, failure time analysis of 5 common ventilator brands

Mechanical ventilator failures expose patients to unacceptable risks and are expensive. By identifying factors that correlate with the amount of time between consecutive ventilator failures, we might reduce patient risk, save money, and shed light on a number of important questions concerning whether reliability changes as a function of time.

Investigate the correlation between several explanatory variables and the time between consecutive ventilator failures and address the following questions: (1) Are ventilators as safe and reliable following repairs as they were before failing? (2) Does reliability change significantly as a ventilator is used or ages? (3) Does a hospital's particular operating environment play a role in ventilator reliability? (4) Are ventilator service contracts worth the money?

A retrospective review was conducted using repair and maintenance records from 2 hospitals: a 570-bed teaching hospital and a 410-bed local community hospital. Records were examined from a total of 66 individual ventilators, of 5 different brands, used between July 1, 1991, and January 3, 2001. The ventilators included 13 Tyco-Mallinckrodt Infant Star, 10 Bird VIP, 11 Bird 6400ST, 16 Bird 8400STi, and 16 Tyco-Mallinckrodt 7200ae. The dependent variable was the operating time between or before unexpected mechanical failures; this was determined by the difference between hours logged on the ventilator hour meter at the time of failure and that recorded when the study began, or when the ventilator was new. Thereafter (when applicable), the time before failure was the difference in hours at consecutive failures. Seven independent explanatory covariates were selected and analyzed as potential correlates with time between failures. Another independent variable, the site of ventilator use (community or teaching hospital), was also tested for significance. Data were analyzed using the Cox proportional hazard model, the multiple-groups survival statistic, and the Cox-Mantel test.

In 2,567,365 hours of ventilator operation, 290 observations were recorded (226 failures and 64 censored observations). Two of the 7 covariates were judged time-dependent, excluded from the Cox model, and evaluated using other techniques. Of the 5 remaining covariates, 2 were significantly related to reliability, both indirectly. There was no difference in reliability, regardless of how many times a ventilator had been previously repaired, but hospital environment did significantly affect reliability.

Ventilator reliability depends on a number of factors. This study indicates that, on average, ventilator reliability improves the more a ventilator is used and the longer the brand has been commercially available. The number of previous ventilator repairs did not affect reliability, but the hospital environment did. These data, if validated, should help to enhance our understanding of ventilator reliability and could eventually have profound economic and safety implications as well.