Blood volume deficit in postural orthostatic tachycardia syndrome assessed by semiautomated carbon monoxide rebreathing

Purpose: The semiautomated carbon monoxide (CO) rebreathing method has been introduced as a noninvasive and radiation-free blood volume estimation method. We tested whether the semiautomated CO rebreathing method can detect the blood volume deficit in postural orthostatic tachycardia syndrome (POTS). In addition, we explored the relationship between blood volume estimated from CO rebreathing and body impedance.

Patients and methods: We recruited 53 subjects (21 female patients with POTS, 19 healthy female participants, and 13 healthy male participants) to record blood volumes and hemodynamic data. Blood volumes were measured by CO rebreathing and segmental body impedance. Linear regression models to predict normal values of red blood cell volume (RBCV), plasma volume (PV), and total blood volume (BV) were developed. Percentage deviations from the predicted normal volumes were calculated.

Results: Patients with POTS had lower RBCV (25.18 ± 3.95 versus 28.57 ± 3.68 mL/kg, p = 0.008, patients with POTS versus healthy female participants), BV (64.53 ± 10.02 versus 76.78 ± 10.00 mL/kg, p < 0.001), and BV deviation (-13.92 ± 10.38% versus -0.02 ± 10.18%, p < 0.001). Patients with POTS had higher supine heart rate (HR) (84 ± 14 versus 69 ± 11 bpm, p < 0.001) and upright HR (123 ± 23 versus 89 ± 22 bpm, p < 0.001). We found a correlation between BV deviation and upright HR in patients with POTS (r = -0.608, p = 0.003), but not in healthy participants. Volumes from the CO rebreathing and body impedance were well correlated (r = 0.629, p < 0.001).

Conclusions: The CO rebreathing method can detect BV deficit, as well as the RBCV deficit in patients with POTS. The negative correlation between BV deviation and upright HR indicates that hypovolemia is one of the pathophysiological causes of POTS. Correlations between body impedance and CO rebreathing volume suggest its usefulness for measurements of volume changes.