Using Concurrent Cardiovascular Information to Augment Survival Time Data from Orthostatic Tilt Tests

• Activity Number: 323
• Type: Topic Contributed
• Date/Time: Tuesday, August 5, 2014 : 10:30 AM to 12:20 PM
• Sponsor: Biometrics Section
• Author(s): Alan H. Feiveson*+ and James Fiedler and Stuart Lee and Christian Westby and Michael Stenger and Steven H. Platts
• Companies: NASA Johnson Space Center and Universities Space Research Association and Wyle and Universities Space Research Association and NASA Johnson Space Center and NASA
• Keywords: survival ; functional data ; convex hull ; orthostatic intolerance ; tilt test ; non-parametric

Abstract

Tilt tests are used by NASA to evaluate astronauts' degree of orthostatic intolerance (OI) before and after spaceflight. The method for evaluating OI is to have subjects lie flat on a table and then tilt the table up to 80 deg for a short time. During the test, several aspects (X) of cardiovascular function, such as heart rate, blood pressure, and stroke volume, are recorded concurrently. The tilt position is maintained until the subject becomes presyncopal (e.g . experiences lightheadedness, fatigue, or nausea) or until a pre-set time has passed. If the test is stopped early, the time to presyncope is the survival time; otherwise the survival time is censored. Because of high demand for astronauts' time just after landing, the maximum time allowed for testing is short, and censoring rates are often too high to permit effective statistical inference on OI treatment effectiveness from survival times alone. As a consequence, the time-trend of X becomes an important additional source of information. In particular, we will show how the area or volume of convex hulls traced out by X in time can effectively augment survival analysis for the assessment of OI in these types of studies.