Specificity of head-up tilt testing in adolescents: effect of various degrees of tilt challenge in normal control subjects.

OBJECTIVES: This study sought to determine the specificity of commonly used tilt protocols in children. BACKGROUND: Tilt table testing is commonly utilized in the evaluation of children and adolescents with syncope despite a lack of uniformity in tilt protocols and a lack of studies of specificity in normal control subjects. METHODS: Sixty-nine normal control volunteers (12 to 18 years old, 38 male, 31 female) with no previous history of syncope, presyncope or arrhythmia underwent tilting to 80 degrees, 70 degrees or 60 degrees for a maximum of 30 min on a motorized table with a footboard support. Autonomic maneuvers, including deep breathing, carotid massage, Valsalva maneuver and diving reflex, were performed before tilt testing to determine whether the response to these maneuvers could identify subjects prone to fainting during tilt testing. RESULTS: Symptoms of presyncope and frank syncope were elicited in 24 of 69 subjects (13 male, 11 female): 6 (60%) of 10 were tilted at 80 degrees, 9 (29%) of 31 at 70 degrees and 9 (32%) of 28 at 60 degrees. Tilt testing at 80 degrees was terminated after the tenth subject by the institutional review board. The mean time to a positive test response was 10.5 min at 80 degrees, 14.2 min at 70 degrees and 13.2 min at 60 degrees. In the 80 degrees tilt, 4 of 10 subjects had a positive response within 10 minutes, whereas only 3 of 31 and 2 of 28 had a positive response within < 10 min at 70 degrees and 60 degrees tilt angles, respectively. Subjects with and without a positive response to tilt testing were similar with respect to age; gender; PR, QRS and QT intervals; and baseline heart rate and blood pressure. Likewise, responses to other autonomic function tests performed were similar in tilt-positive and tilt-negative patients. The power for detecting a significant difference between patients tilted at 80 degrees versus 60 degrees and 70 degrees was 0.45 and for detecting differences in autonomic tone between tilt-positive (n = 24) and tilt-negative (n = 45) subjects was 0.8. CONCLUSIONS: Children appear to be more susceptible to orthostatic stress than adults. Therefore, tilt protocols commonly used in adults lack specificity in teenage patients. A specificity > 85% may be obtained by performing the tilt test at 60 degrees or 70 degrees for no longer than 10 min.

The virtual crossmatch--a screening tool for sensitized pediatric heart transplant recipients.

Heart transplantation in the setting of human leukocyte antigen (HLA) sensitization is challenging, as a time-consuming prospective crossmatch (XM) may be required, severely limiting the number of potential donors. We evaluated a 'virtual XM', defining a positive virtual XM as the presence of recipient pre-formed anti-HLA antibodies to the prospective donor HLA type, and compared the virtual XM to a standard direct XM. Bead-based flow cytometric analysis was used to identify anti-HLA antibody (Ab) present in a child listed for heart transplantation. Using recipient serum, direct-flow cytometric T- and B-cell XM were run for potential donors against whose HLA type the recipient had specific antibodies (group 1, n = 7) and for potential donors with predicted compatible HLA types by virtual XM (group 2, n = 7). Results were expressed as median channel difference (MCD) between the control and recipient serum. A positive T-cell XM was defined as MCD > 50, whereas MCD > 100 constituted a positive B-cell result. The rate of T-cell reactivity was significantly less in group 2 than in group 1 (29% vs. 100%, p = 0.02); similarly, B-cell reactivity was also less for group 2 (14% vs. 100%, p = 0.005). The virtual XM was 100% sensitive in detecting positive flow cytometric XM results for T and B cells. Although only 72% specific in predicting a negative T-cell XM, and 86% specific for negative B-cell XM, the false negatives were weakly positive and would probably have been clinically acceptable. Currently, potentially suitable donor organs are often declined for lack of a prospective XM; these organs may ultimately be allocated to more distant recipients or perhaps not used at all. While further studies are needed, virtual XM has the potential to improve availability of organs for sensitized patients and improve the overall allocation process.