POTS as Thoracic Hypovolemia

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Variants of postural tachycardia syndrome (POTS) are associated with increased (“high flow” POTS, HFP), decreased (“low flow  POTS”, LFP) and normal (“normal flow POTS”, NFP) blood flow measured in the lower extremities while supine. We propose that postural tachycardia is related to thoracic hypovolemia during orthostasis but that the patterns of peripheral blood flow relate to different mechanisms for thoracic hypovolemia. We studied 37 POTS patients aged 14-21 years: 14 LFP, 15 NFP and 8 HFP patients and 12 healthy control subjects. Peripheral blood flow was measured supine by venous occlusion strain gauge plethysmography of the forearm and calf in order to subgroup patients. Using indocyanine green techniques we showed decreased cardiac index (CI) and increased total peripheral resistance (TPR) in LFP, increased CI and decreased TPR in HFP, and unchanged CI and TPR in NFP while supine compared to control subjects. Blood volume tended to be decreased in LFP compared to control subjects. We used impedance plethysmography to assess regional blood volume redistribution during upright tilt. Thoracic blood volume decreased while splanchnic, pelvic and leg blood volumes increased for all subjects during orthostasis, but were markedly lower than control for all POTS groups.  Splanchnic volume was increased in NFP and LFP. Pelvic blood volume was increased in HFP only.  Calf volume was increased above control in HFP and LFP.  The results support the hypothesis of [at least] three pathophysiologic variants of POTS distinguished by peripheral blood flow related to characteristic changes in regional circulations. The data demonstrate enhanced thoracic hypovolemia during upright tilt and confirm that POTS is related to inadequate cardiac venous return during orthostasis.
The figure shows changes in thoracic, splanchnic, pelvic, and leg percent volume changes during upright tilt averaged over subject groups. High flow POTS is red, low flow POTS is green, normal flow POTS is blue and control is black. Splanchnic changes dominate normal flow POTS. Low flow POTS patients have widespread blood collection. High flow POTS have blood pooling in the dependent body parts
Coda: Data for Hyperadrenergic POTS are less clear in terms of regional displacements of blood volume. It is likely that most hyperadrenergic patients have an associated absolute hypovolemia - meaning that there overasll blood volume is reduced even while supine. This may be reflected in a resting tachycardia which may also relate to adrenergic activation in these patients. 
 

 


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Exercise Intolerance- the Exercise Pressor Reflex in POTS
Skeletal Muscle Pump
Normal Leg Venous Capacitance
Postural Neurocognitive
Splanchnic Pooling in Normal Flow POTS
Nitric Oxide Dysfunction in Low Flow POTS
Angiotensin-II in POTS
Decreased Upright Cerebral Blood Flow and Cerebral Autoregulation in POTS
Postural Hyperpnea
Nitric Oxide is Decreased in Angiotensin-II dependent Low flow POTS but increased along with Splanchnic pooling Neuropathic POTS
Local Vascular Responses in POTS
Microvascular Filtration in High Flow POTS
POTS as Thoracic Hypovolemia