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Objectives:  This study aims to investigate the diurnal variability of heart rate (HR) increment after standing (ΔHR) in pediatric postural tachycardia syndrome (POTS) and explore appropriate cutoff values of ΔHR at different times for the POTS diagnosis.   Materials and Methods:  Seventy-eight patients (9–14 years) who presented with orthostatic intolerance symptoms were enrolled. Forty-three patients were diagnosed as POTS (ΔHR ≥40 bpm), and 35 patients were assigned to the non-POTS group (ΔHR <40 bpm). Twenty-six healthy children served as the control group. All subjects completed three standing tests in the morning, afternoon, and evening. Orthostatic HR parameters were analyzed to predict the diagnosis of POTS. Additionally, 41 patients were recruited as an external validation group.   Results:  Orthostatic HR increments in both the POTS and non-POTS groups exhibited diurnal variability, which was markedly larger in the morning ( P  < 0.05), whereas it did not differ with the time of day in the control group. Among the POTS patients, 100% met the diagnostic criteria for POTS in the morning, 44.2% in the afternoon, and 27.9% in the evening. Almost half of the POTS patients (51.2%) displayed a positive result only in the morning standing test. However, in the three standing tests at different times, ΔHR from 1 to 10 min after standing and ΔHR max  were the highest in the POTS group compared with in the non-POTS and control groups ( P  < 0.05). Furthermore, the maximum ΔHR (ΔHR max ) and ΔHR at 5 and 10 min in the afternoon and evening standing tests yielded moderate predictive values for the POTS diagnosis. The external validation test showed that the afternoon ΔHR max  ≥30 bpm to diagnose POTS yielded sensitivity, specificity, and accuracy of 85, 71.4, and 78%, respectively, and the evening ΔHR max  ≥25 bpm yielded sensitivity, specificity, and accuracy of 85, 76.2, and 80.5%, respectively.   Conclusions:  The orthostatic HR increment exhibits diurnal variability in children and adolescents with POTS that may affect the diagnosis of POTS. Supplementary criteria are proposed for the POTS diagnosis based on diurnal variability.

Diagnostic Value of Diurnal Variability of Orthostatic Heart Rate Increment in Children and Adolescents With POTS