Outcome
Type |
Measure |
Description |
Time frame |
Safety issue |
Primary |
Muscle Sympathetic Nerve Activity (MSNA) |
MSNA is recorded continuously at rest and during O/LBNP protocols with a tungsten microelectrode (0.2 diameters insulated shaft tapered to an uninsulated tip of ~1-5 µm) inserted percutaneously into the median nerve. Multiunit, postganglionic MSNA is distinguished from other sources of nerve activity by the presence of spontaneous pulse synchronous bursts, increased activity when the subject holds their breath or clenches their first (stretches forearm muscles around median nerve), or if activity does not change when the skin is stimulated by light brushing or stroking (indicates lack of skin nerve activity). Bursts of MSNA at rest and during LBNP are quantified as total activity (AU/min), burst frequency (bursts/min) and burst incidence (bursts/100 heartbeats). The MSNA burst incidence is expressed in bursts/100 heartbeats to normalize for differences in heart rate among subjects. |
3 hours |
|
Primary |
Systolic Blood Pressure (SBP) |
SBP is measured continuously with a Finometer beat-to-beat blood pressure measurement system at rest and during O/LBNP protocols. Units are mm Hg. |
3 hours |
|
Primary |
Diastolic Blood Pressure (DBP) |
DBP is measured continuously with a Finometer beat-to-beat blood pressure measurement system at rest and during O/LBNP protocols. Units are mm Hg. |
3 hours |
|
Primary |
Mean Arterial Pressure (MAP) |
MAP at rest and during O/LBNP protocols is calculated as 1/3(SBP-DBP)+DBP using the beat-to-beat SBP and DBP measurements taken with the Finometer system. Units are mm Hg. |
3 hours |
|
Primary |
Sympathetic Baroreflex (BR) Gain |
Sympathetic BR gain is used as an index of sympathetic (neural) control of blood pressure, where gain is determined by the slope of the linear relationship between MSNA (burst/100 heartbeats) and DBP (mm Hg) at rest and during LBNP. The relationship produces a negative slope in that for a given subject, when DBP falls below the normal level sustained at rest, sympathetic activity increases to restore DBP back to that normal level. Thus, more MSNA bursts are observed at lower levels of DBP and vice versa. A steeper (more negative) slope suggests greater sensitivity of the baroreceptors to changes in blood pressure, and thereby more effective sympathetic control of blood pressure. A flatter slope suggests impaired BR sensitivity and blood pressure dysfunction, which is expected for the trans men group compared to cisgender women controls. |
3 hours |
|
Primary |
Frequency in the Neural Cardiovascular Control |
The frequency of SBP, DBP and heart rate during OLBNP is analyzed in Matlab in a linear time-invariant system (LTI) framework. Heart rate and BP measures taken during OLBNP are low-pass filtered (0.05 Hz estimated example) and corrected for end-effects using a Hamming window. The derived impulse responses (IRs) are 1/3rd octave smoothed before calculating the frequency power spectra (0.001-0.05 Hz estimated example). The power of DBP and SBP at the OLBNP frequency are anticipated to be independently inversely related to the efficiency of BP control. Further, the difference in heart rate and SBP, and in heart rate and DBP, power at those frequencies reflects the degree of autonomic function, analogous to baroreflex gain. |
0.5 hours |
|
Primary |
Electrocardiogram (ECG) |
A 3-lead ECG is continuously recorded at rest and during the O/LBNP protocols. The R-wave of the QRS complex of the ECG recording will be used for measures of pulse interval by determining the amount of time in seconds between the R-waves of 2 consecutive QRS complexes. |
3 hours |
|
Secondary |
Brachial Artery Mean Blood Flow Velocity |
Mean blood flow velocity in the brachial artery will be examined at rest and during LBNP using Doppler ultrasound velocimetry. Blood flow velocity spectra and arterial diameter are measured simultaneously with a Doppler probe and SonoScape S2 ultrasound system. The brachial artery is imaged with a 6.0-MHz linear array probe positioned at a 45° angle over the skin at the distal one-third of the upper arm. Brachial blood flow is expected to decrease as the level of LBNP increases and induces a greater orthostatic challenge. For example, blood flow would be highest at the -10LBNP stage and continually decrease during the following stages of -20, -30 and finally -40LBNP, where blood flow would be the lowest. |
3 hours |
|
Secondary |
Cardiovagal Baroreflex (BR) Gain |
Cardiovagal BR gain is determined by the slope of the linear relationship between R-R Interval (seconds) and SBP (mm Hg) at rest and during LBNP. R-R Intervals are obtained from the ECG recording and SBP is obtained from the beat-to-beat measures taken by the Finometer. |
3 hours |
|