Interpretation and bias in case-crossover studies.

The case-crossover design is an innovative epidemiologic technique with distinct strengths and limitations. We review the fundamental logic of this self-matching non-randomized design and direct attention to 15 concerns related to the available data, unavailable data, analytic technique, quantitative statistics, and etiologic model. Implications for each concern are discussed in the context of a recent report on whether cellular telephone calls are associated with an increased risk of a motor vehicle collision. We suggest that an understanding of the case-crossover design may help investigators explore selected questions in behavioral medical research.

Association between cellular-telephone calls and motor vehicle collisions.

BACKGROUND: Because of a belief that the use of cellular telephones while driving may cause collisions, several countries have restricted their use in motor vehicles, and others are considering such regulations. We used an epidemiologic method, the case-crossover design, to study whether using a cellular telephone while driving increases the risk of a motor vehicle collision. METHODS: We studied 699 drivers who had cellular telephones and who were involved in motor vehicle collisions resulting in substantial property damage but no personal injury. Each person's cellular-telephone calls on the day of the collision and during the previous week were analyzed through the use of detailed billing records. RESULTS: A total of 26,798 cellular-telephone calls were made during the 14-month study period. The risk of a collision when using a cellular telephone was four times higher than the risk when a cellular telephone was not being used (relative risk, 4.3; 95 percent confidence interval, 3.0 to 6.5). The relative risk was similar for drivers who differed in personal characteristics such as age and driving experience; calls close to the time of the collision were particularly hazardous (relative risk, 4.8 for calls placed within 5 minutes of the accident, as compared with 1.3 for calls placed more than 15 minutes before the accident; P<0.001); and units that allowed the hands to be free (relative risk, 5.9) offered no safety advantage over hand-held units (relative risk, 3.9; P not significant). Thirty-nine percent of the drivers called emergency services after the collision, suggesting that having a cellular telephone may have had advantages in the aftermath of an event. CONCLUSIONS: The use of cellular telephones in motor vehicles is associated with a quadrupling of the risk of a collision during the brief time interval involving a call. Decisions about regulation of such telephones, however, need to take into account the benefits of the technology and the role of individual responsibility.

A randomised trial of compassionate care for the homeless in an emergency department.

Homeless adults often visit emergency departments and often leave dissatisfied. We tested whether compassionate care, by improving patient satisfaction, can alter subsequent use of emergency services. We identified 133 consecutive homeless adults visiting one inner-city emergency department who were not acutely psychotic, extremely intoxicated, unable to speak English, or medically unstable. Half were randomly assigned to receive compassionate contact from trained volunteers. All patients otherwise had usual care and were followed for repeat visits to emergency departments. We found that rates of use were high, with patients making an average of seven visits a year (0.60 per month). More than a third of all patients made two or more visits within two days of each other. The average number of visits per month after intervention was significantly lower for patients who received compassionate care (0.43 vs 0.65, p = 0.018). Analyses adjusting for each patient's previous rate of use confirmed that compassionate care led to a one third reduction in the number of return visits within one month (95% CI 14 to 40%). Compassionate management of selected homeless adults decreases repeat visits to the emergency department. One explanation is that patients tend to return frequently until they are satisfied with their treatment.

Warm induction blood cardioplegia in the infant. A technique to avoid rapid cooling myocardial contracture.

The use of profound hypothermia and total circulatory arrest for repair of heart defects in neonates usually involves a period of systemic and myocardial bypass cooling. Rapid cooling of muscle (skeletal, smooth, and myocardial) can result in contracture through elevation of cytosolic calcium levels. The increased myocardial tone caused by cooling might render the heart more vulnerable to a subsequent period of cardioplegic ischemic arrest. Infants may be more susceptible to contracture because their small body mass allows more rapid myocardial temperature change when prearrest bypass cooling is used. The influence of avoiding rapid myocardial cooling before induced cardioplegic arrest was analyzed in a group of infants weighing less than 6 kg at the time of open cardiac operation. Myocardial ischemic arrest by warm (37 degrees C) induction blood cardioplegia was used in 57 infants and compared with results in 440 infants treated with standard blood cardioplegia. Multivariate logistic regression analysis revealed that patient diagnosis, weight, and age at operation were significant risk factors for operative mortality. The use of warm induction blood cardioplegia had a strongly positive independent effect on survival (p = 0.0003) for any patient weight, age, or diagnostic group. We recommend the avoidance of rapid myocardial cooling on bypass in all patients before induction of cardioplegic ischemic arrest.

A family of proportional- and additive-hazards models for survival data.

A family of proportional- and additive-hazards models for the analysis of grouped survival data is developed. This family generalizes the unpublished work of F.J. Aranda-Ordaz and follows Holford (1976, Biometrics 32, 227-237). It contains the proportional-hazards model for grouped data, as well as additive-hazards models with time trends. The time trends prove to be useful in an example in which the hazards of the two groups cross.