Experimental Protocol. The subject laid supine on a table and his/her MAP was continuously monitored by a finger plethysmograph (Figure 3.1). The finger sensor for MAP measurements was placed on the subject’s middle finger of left hand with his/her left arm fully extended at the same level of the heart. A DCS flowmeter was used to continuously monitoring CBF. A fiber-optic probe containing a multimode source fiber (x 1) and a single mode detector fiber (x 1) was taped on the middle of forehead for CBF measurement. An elastic bandage was used to wrap around the forehead to secure the probe and reduce the influence of room light. The measured light intensity was limited by the use of single- mode fiber for DCS detection, thus restricting the use of large S-D separation. The separation was set at 2.5 cm which allowed for the detection of cerebral hemodynamics in adult prefrontal cortex [25, 38, 61, 63, 64, 68, 108]. The sampling rate for CBF and MAP was 0.64 Hz (1/1.56 s). After baseline measurements for 5 minutes, both thighs of the subject were occluded by two cuff occlusion machines (Aspen ATS1000, Xxxxxx Inc., IN, USA) at a pressure of 230 mm Hg for 5 minutes. The pressures on both thighs were then rapidly and concurrently released.
Experimental Protocol. Relative changes of rCBF, Δ[HbO2], Δ[Hb], and ΔTHC during HUT were monitored by the custom-designed dual-wavelength DCS flow-oximeter. Here “r” denotes the relative change with the unit %, while “Δ” indicates the absolute change in unit of μM. Details about the DCS flow-oximeter can be found in Chapter 2.5. A fiber- optic probe was taped in the middle of forehead at the position of 1 cm above the eyebrows for the optical measurements of cerebral hemodynamic parameters. The separation between source and detector fibers was 2.5 cm. To fix the probe on the frontal head, a self-adhesive elastic band was used to wrap the probe tightly around the forehead. The noninvasive finger plethysmograph (Portapres, FMS Inc., Netherlands) was used to continuously monitor the beat-to-beat MAP in unit of mm Hg and heart rate (HR) in unit of beats-per-minute (BPM). The plethysmograph finger sensor on the middle finger of left hand was maintained at the heart level. Data sampling rate for both optical and MAP/HR measurements is 1 Hz. A 30-minute 70° HUT protocol was adopted in the present study, which is similar to the 10-minute 70° HUT protocol described in Chapter 4. This protocol was selected because previous studies had found that it may induce VVS in healthy subjects [159, 169]. More specifically, the subject was asked to lay supine on a tilt table. To immobilize the body, two Velcro strips were placed over the chest and thighs respectively. Following the supine control for ~10 minutes, the subject was tilted up to 70° and instructed to remain still for 30 minutes if possible. However, when presyncopal symptoms appeared during standing at 70°, the subject was tilted back to supine position immediately for recovery. The recovery period at supine position was ~5 minutes.
