Diurnal variability in orthostatic tachycardia: implications for the postural tachycardia syndrome

Abstract

Patients with POTS (postural tachycardia syndrome) have excessive orthostatic tachycardia (>30 beats/min) when standing from a supine position. HR (heart rate) and BP (blood pressure) are known to exhibit diurnal variability, but the role of diurnal variability in orthostatic changes of HR and BP is not known. In the present study, we tested the hypothesis that there is diurnal variation of orthostatic HR and BP in patients with POTS and healthy controls. Patients with POTS (n=54) and healthy volunteers (n=26) were admitted to the Clinical Research Center. Supine and standing (5 min) HR and BP were obtained in the evening on the day of admission and in the following morning. Overall, standing HR was significantly higher in the morning (102±3 beats/min) than in the evening (93±2 beats/min; P<0.001). Standing HR was higher in the morning in both POTS patients (108±4 beats/min in the morning compared with 100±3 beats/min in the evening; P=0.012) and controls (89±3 beats/min in the morning compared with 80±2 beats/min in the evening; P=0.005) when analysed separately. There was no diurnal variability in orthostatic BP in POTS. A greater number of subjects met the POTS HR criterion in the morning compared with the evening (P=0.008). There was significant diurnal variability in orthostatic tachycardia, with a great orthostatic tachycardia in the morning compared with the evening in both patients with POTS and healthy subjects. Given the importance of orthostatic tachycardia in diagnosing POTS, this diurnal variability should be considered in the clinic as it may affect the diagnosis of POTS.

Figure 1. Evening and Morning Heart Rate Parameters

Orthostatic increase in heart rate (delta heart rate [HR]) from supine to standing (Panel A), standing HR at 5 minutes (Panel B) and supine HR (Panel C) are shown for patients with Postural Tachycardia Syndrome (POTS; black bars) and Control subjects (gray bars) in the evening (solid bars) and the morning (hashed bars). Reported P values were generated using paired t-tests.

Figure 2. Evening and Morning Blood Pressure Parameters

Orthostatic changes in systolic blood pressure (delta systolic blood pressure [SBP]) from supine to standing (Panel A), standing SBP (Panel B) and supine SBP (Panel C) are shown for patients with Postural Tachycardia Syndrome (POTS; black bars) and Control subjects (gray bars) in the evening (solid bars) and the morning (hashed bars). Similar data for diastolic blood pressure (DBP) is shown in Panels D–F. Reported P values were generated using paired t-tests.

Figure 3. POTS Heart Rate Criterion Met by Time of Day

These histograms show the percentage of subjects with an increase in heart rate >30 bpm with a 5 minute stand (from a supine body position) and not the 10 minute stand that is used to diagnose Postural Tachycardia Syndrome (POTS). Data are shown for patients with POTS (black bars) and Control subjects (gray bars) in the evening (solid bars) and the morning (hashed bars). McNemar's test was used to determine if there was a difference in the number of combined subjects meeting this criterion between the 2 time points.