## Materials and Methods  
### Subjects and Clinical Investigation Protocol  
All recordings were obtained at the Almazov National Medical Research Centre in accordance with the ethical standards presented in the Declaration of Helsinki. The study protocol was reviewed and approved by the Ethics Committee of the Almazov National Medical Research Centre (Ref. No. 110, approval date 12 June 2010) before the beginning of the study. All patients and volunteers provided their informed consent in written form prior to their participation in the study. The study included 95 subjects subdivided into three groups:  
* Group 1 included 33 patients aged \(39.4\pm 17.5\) years old with either classic or gradual orthostatic hypotension with orthostatic systolic blood pressure (SBP) reduction by at least 20 mmHg and/or by diastolic blood pressure reduction by at least 10 mmHg within three minutes of the orthostatic compared to the initial (supine) position. Functional tests performed prior to the tilt test indicated that most patients in this group demonstrated moderate autonomic dysfunction (see Table 1 for the functional autonomic tests results);
* Group 2 included 34 diabetes mellitus patients aged \(52.7\pm 10.9\) years old with autonomic neuropathy diagnosed by functional autonomic tests as indicated in [31; 32] that were performed prior to the tilt test. Only patients with at least two (out of seven) reduced functional autonomic test responses (for further details, see the functional tests specification below) were considered. All diabetes patients received standard combination therapy of blood sugar reducing drugs, including glitzones, gliptins, metformin and sulfonylurea, while receiving no specific therapy of autonomic function within one month before the study;
* Group 3 contained of 28 healthy volunteers aged \(41.2\pm 11.1\) years old. Prior to the tilt-test all patients and volunteers underwent standard functional autonomic tests. A comprehensive assessment of autonomic regulation of blood circulation included the following tests:  
1. Valsalva maneuver (cardiac component);
2. Valsalva maneuver (vasomotor component);
3. Deep breathing test;
4. Hand-grip test;
5. Cold-stress induced vasoconstriction for evaluation reaction of the forearm vessels while cooling the skin of the upper chest for 2 min;
6. Spontaneous arterial baroreflex (ABR);
7. Spectral power of heart rate variability. Next all 95 subjects and patients underwent a head-up tilt-test (table tilt 70\({}^{\circ}\), duration of orthostatic position up to 30 min unless stopped earlier due to syncope response) [33], see also [34] that was performed under identical conditions between 10 am and 1 pm. The recording in the initial supine position and the initial fragment of the orthostatic phase recording (both around 10 min duration) were used in further analysis. Hemodynamic parameters we measured continuously using the Finometer-Pro blood pressure monitor (Finapres Medical Systems, Enschede, The Netherlands) with parallel electrocardiogram (ECG) recording. The forearm blood flow was measured by venous occlusion plethysmography using Dohn air-filled cuff. The overall study design is summarized in Figure 1.  
Detailed characteristic of the studied patients' groups are summarized in Table 1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Attribute/Parameter/Quantity** & **Group 1** & **Group 2** \\ \hline Age, years & \(39.4\pm 17.5\) & \(52.7\pm 10.9\) \\ \hline Concomitant cardiovascular disease (arterial hypertension, coronary heart disease, heart failure), \(n\) (\%) & 21 (64) & 32 (94) \\ \hline Orthostatic decrease of systolic/diastolic blood pressure, mmHg & \(34\pm 13\)/\(17\pm 7\) & \(17\pm 14\)/\(2\pm 16\) \\ \hline Reduced indicators of functional autonomic tests (out of seven), \(n\) & \(2.5\pm 1.1\) & \(5.0\pm 1.2\) \\ \hline Diabetes duration, years & \(n\)/\(a\) & \(10.1\pm 3.9\) \\ \hline Diabetes complications, \(n\) (\%) & \(n\)/\(a\) & 34 (100) \\ \hline \hline \end{tabular}
\end{table}