Postural hypocapnic hyperventilation is associated with enhanced peripheral vasoconstriction in postural tachycardia syndrome with normal supine blood flow

Abstract

Previous investigations have demonstrated a subset of postural tachycardia syndrome (POTS) patients characterized by normal peripheral resistance and blood volume while supine but thoracic hypovolemia and splanchnic blood pooling while upright secondary to splanchnic hyperemia. Such "normal-flow" POTS patients often demonstrate hypocapnia during orthostatic stress. We studied 20 POTS patients (14-23 yr of age) and compared them with 10 comparably aged healthy volunteers. We measured changes in heart rate, blood pressure, heart rate and blood pressure variability, arm and leg strain-gauge occlusion plethysmography, respiratory impedance plethysmography calibrated against pneumotachography, end-tidal partial pressure of carbon dioxide (Pet(CO2)), and impedance plethysmographic indexes of blood volume and blood flow within the thoracic, splanchnic, pelvic (upper leg), and lower leg regional circulations while supine and during upright tilt to 70 degrees. Ten POTS patients demonstrated significant hyperventilation and hypocapnia (POTS(HC)) while 10 were normocapnic with minimal increase in postural ventilation, comparable to control. While relative splanchnic hypervolemia and hyperemia occurred in both POTS groups compared with controls, marked enhancement in peripheral vasoconstriction occurred only in POTS(HC) and was related to thoracic blood flow. Variability indexes suggested enhanced sympathetic activation in POTS(HC) compared with other subjects. The data suggest enhanced cardiac and peripheral sympathetic excitation in POTS(HC).

Postural Hyperventilation as a Cause of Postural Tachycardia Syndrome: Increased Systemic Vascular Resistance and Decreased Cardiac Output When Upright in All Postural Tachycardia Syndrome Variants

“…Nevertheless, intermittent hyperventilation, presenting as postural hypocapnic hyperpnea, has been reported 14, 15, 16, 17. We observed hypocapnic hyperpnea in POTS in response to a rapid initial orthostatic decrease in CO and CBV 15.…”

“…The relationship of hyperventilation to orthostasis has been revived by recent reports from our laboratory14, 15 and a report from the Mayo autonomic group,17 contemporary with the definition of POTS 3. More recently, “hyperventilation syndrome” (but not “postural hyperventilation”) has been subsumed under the aegis of “dysfunctional breathing” treated with variable success by breathing retraining 42…”

“…The respiratory pump (increased intra‐abdominal and decreased intrathoracic pressures) does not improve CO in subjects with POTS14 because reduced blood volume and extrathoracic pooling of blood severely hamper venous return. On the contrary, if hyperpnea occurs at high operating lung volumes,53 thoracic blood volume may be even further reduced 54…”

“…Impedance plethysmography was measured using a tetrapolar, high‐resolution, 4‐channel digital impedance plethysmograph, as previously described 14, 27. These quantities were obtained within 4 anatomic segments, defined by electrode placement on anatomic landmarks that delimit respective regional circulations, designated the thoracic segment or CBV (supraclavicular area to xyphoid process), the splanchnic segment (xyphoid process to iliac crest), the pelvic segment, including lower pelvis to the knee (iliac crest to knee), and the leg or calf segment (upper calf just below the knee to the ankle).…”

Indirect measures of human vagal withdrawal during head-up tilt with and without a respiratory acidosis

“…Reduced variability may suggest increased risk of mortality [9,10], although regular physical exercise may increase HRV [10][11][12]. In humans, it is well established that moving from supine to an upright posture induces an increase in heart rate and a small decrease in end-tidal partial pressure of CO 2 (PET CO 2 ) [13][14][15][16][17]. Both postural change and hypercapnia are known to affect HRV, whereby the HF component has consistently been shown to decrease with head-up tilt (HUT) in normoxic healthy humans [14,[18][19][20].…”