We studied 15 patients with POTS and 13 age-matched (all females) control subjects. Baseline characteristics were similar between the two groups and are summarized in ().
Baseline Renal and Systemic Hemodynamic Measurements
All baseline measurements were done in the supine position over 1 hour. POTS patients had a higher baseline heart rate than control subjects (72±2 bpm vs. 59±2 bpm; P=<0.001). The systolic BP (SBP), diastolic BP (DBP) and mean BP (MBP) were all similar between POTS and control subjects (SBP: 97±2 mmHg vs. 99±1 mmHg, P=0.51; DBP: 63±2 mmHg vs. 63±1 mmHg, P=0.91; MBP: 76±2 mmHg vs. 76±1 mmHg; P=0.88). Baseline effective renal plasma flow was not different between the two groups (641±30 ml/min per 1.73m2 vs. 639±39 mmHg, P=0.97). The renal blood flow and renal vascular resistance were also similar at baseline ().
Renal and Systemic Hemodynamic Responses to Ang II Infusion
Infusion of Ang II increased blood pressure in both POTS and control subjects. Compared to controls, POTS patients had a diminished response to Ang II infusion (), with a blunted increase in MAP (10±1 mmHg vs. 14±1 mmHg; P=0.01; ), and in DBP (9±1 mmHg vs. 13±1 mmHg; P=0.01; ), but not SBP (13±2 mmHg vs. 15±2 mmHg; P=0.40; ). The heart rate change in response to Ang II infusion was minimal in both POTS and control subjects (2±0.7 bpm vs. −1±0.9 bpm; P=0.002). Post Ang II infusion, renal plasma flow decreased to a similar extent in POTS patients and controls (−166±20 vs. −181±17 mL/min/1.73 kg/m2; P=0.58;
). Renal blood flow and renal vascular resistance were not different between the two groups following Ang II infusion (data not shown).
Figure 1 The net increase in systolic blood pressure (SBP; panel A), diastolic blood pressure (DBP; panel B) and mean arterial pressure (MAP; panel C) from baseline in response to Ang II infusion in POTS patients (solid line) and control subjects (dashed line). (more ...)
Figure 2 The change in renal plasma flow (RPF; panel A) and urine sodium excretion corrected for creatinine (panel B) at baseline (base) and in response to Ang II infusion (post). Ang II decreased RPF and increased sodium reabsorption in both POTS patients (black (more ...)
Hormonal (PRA, Aldosterone, Angiotensin species, Cortisol) Baseline & Responses to Ang II Infusion
Baseline PRA was similar (), and it was suppressed to a similar extent in the two groups following infusion (−0.9±0.2 vs. − 0.6±0.2 ng/mL/h; P=0.43; ). Baseline aldosterone level was similar in POTS vs. controls (), and the increment in response to Ang II infusion was similar between the two groups (17±1 vs. 15±2 pg/ml; P=0.34; ). Cortisol decreased similarly in both POTS and control groups (−0.7±0.6 vs. −1.4±0.6 ng/mL/h; P=0.43).
The response of plasma renin activity (PRA; panel A) and aldosterone (panel B) were similar between POTS (black bars) and controls (gray bars) at baseline (base) and post-infusion (post).
Consistent with our previous findings, baseline plasma Ang II levels were 2 fold higher in POTS compared to control (). With Ang II infusion, the plasma Ang II level increased equally in both groups ().
Plasma Ang II level at baseline was 2 fold higher in POTS compared to control (Panel A). The increase in plasma Ang II levels following Ang II infusion (Panel B) was similar between POTS (black bars) and controls (gray bars).
Urine Sodium Response to Ang II Infusion
One hour of Ang II infusion decreased urine sodium from baseline to a similar extent in both POTS and controls (), these experiments were carried out at the same time of the day to avoid diurnal variation in sodium excretion.
The spontaneous baroreflex sensitivity (BRS) calculated by the sequence technique for down-slopes of SBP (BRS-sequence down) at baseline was significantly lower in POTS compared to healthy controls (13.7±3.5 vs. 26.0±2.6 ms/mmHg, P=0.01). Ang II infusion decreased BRS in POTS (from 13.7±3.5 to 10.4±2.3 ms/mmHg, P=0.04) and in controls (from 26.0±2.6 to 18.9±2.2 ms/mmHg, P=0.005) (). BRS calculated as mean value of the transfer function between SBP and pulse intervals in the LF band demonstrated similar results. Baseline BRS-LF was lower in POTS vs. controls (10.1±1.2 vs. 16.8±1.5 ms/mmHg, P=0.003). Ang II infusion decreased BRS in POTS (from 10.1±1.2 to 8.7±1.3 ms/mmHg, P=0.07) and in controls (from 16.8±1.5 to 13.0±1.3 ms/mmHg, P=0.01). The mean reduction in BRS between the two groups with both techniques was not statistically significant.
Figure 5 Baseline baroreflex sensitivity (BRS) calculated with the sequence technique (BRS-down) was significantly lower at baseline in POTS patients compared to control. In response to Ang II infusion the BRS decreased significantly in both groups. The mean reduction (more ...)
Correlation of BRS and Ang II level
To examine the relationship between BRS and Ang II level, we performed correlation analysis between BRS values and Ang II levels using Pearson test. There was significant negative correlation between baseline Ang II level and baroreflex sensitivity in POTS but not in controls using the sequence method (BRS- sequence down) (r = −0.69, P= 0.009 vs. r = 0.40, P= 0.32). The decrease in BRS correlated with the increase in Ang II levels in patients with POTS. In contrast, there was no correlation in healthy controls.