Figure 4.

ANS (left) and HRV (right) responses from test N to test N + 1, where midodrine was introduced prior to test N and discontinued before test N + 1. Each data point represents a parameter response to a 5-minute seated to a 5-minute standing postural change. Each pair of points shows the change in the postural response from a pair of consecutive tests, “test N” to a follow-up “test N + 1” where midodrine was discontinued immediately after test N. Test N + 1 is approximately 3 months later. (Left) Sympathetic (LFa) and parasympathetic (RFa) measures, sympathovagal balance (SB = LFa/RFa), systolic BP, and mean HR. (Right) Normalized low- and high-frequency parameters (LFnu and HFnu, respectively), total spectral power (TSP), and range HR. See text for details.