Sympathetic control of hemodynamics during moderate head-up tilt in human subjects

To confirm sympathetic control of hemodynamics during postural change, sympathetic nerve activity supplying the anterior tibial muscle (MSNA) was measured by microneurography and muscle blood flow (MBF) by the xenon washout method during graded head-up tilt from 0 degrees horizontal to 30 degrees in 5 degrees increments. MSNA was represented as the increase in burst rate with increased tilt angle. The increments of MSNA from 0 degree horizontal were statistically significant at all tilt angles. MBF decreased gradually with increasing tilt angle, with no difference noted from 5 to 15 degrees, but a significant decrease between 20 and 30 degrees. Mean blood pressure remained unchanged during graded tilt. The heart rate did not increase until a tilt angle of 10 degrees, with increments becoming significant beyond 15 degrees. These results demonstrated that MSNA plays an important role in hemodynamic regulation during incremental changes in posture, while tilt angle increased HR and other control mechanisms that may be involves.