Background - Upright posture reduces venous return, stroke volume and cardiac output (CO) while causing reflex sinus rate (HR) increase. Yet, in inappropriate sinus tachycardia (IST), postural tachycardia syndrome (POTS), and vasovagal syncope (VVS) symptomatic excessive HR occurs. We hypothesized CO reaches maximum as function of HR in all. Methods - We recruited 12 healthy controls, 9 IST, 30 VVS and 30 POTS patients (13-23years) selected randomly by disorder not by HR, each fulfilled appropriate diagnostic criteria. Subjects were instrumented for electrocardiography, beat-to-beat blood pressure, respiratory rate, CO-Modelflow algorithm, and central blood volume (CBV) from impedance cardiography; 10min data was collected supine; subjects were tilted head-up for =/<10min. We computed phase differences, ΔΦ, between fluctuations of HR (ΔHR) and CO (ΔCO) tabulating data when phases were synchronized, determined by a squared nonlinear phase synchronization index (PhSI) >0.5, describing extent/validity of CO/HR coupling. We graphed results supine, 1min-post-tilt-up, mid-tilt, and pre-tilt-down using polar coordinates (HR - radius, ΔΦ - angle) plotting cos(ΔΦ) vs HR to determine if transition HR exists at which in-phase shifts to anti-phase above which CO decreases when HR further increases. Results - At baseline HR, diastolic and mean arterial pressure in IST and POTS were higher vs controls. Upright HR increased most in POTS then IST and VVS, with diverse changes in CO, SVR, and CBV. Each patient grouping was separately and collectively analyzed for HR change showing transition from in-phase to anti-phase (ΔΦ) as HR increased: HRtransition =115±6(IST),123±8(POTS),124±7(VVS), p=ns. Controls never reached transitional HR. Conclusions - Excessive HR independently and equivalently reduces upright CO, in IST, POTS and VVS.