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[ Home ] [ Up ] [ Exercise Intolerance- the Exercise Pressor Reflex in POTS ] [ Abnormal Skeletal Muscle Pump ] [ Normal Leg Venous Capacitance ] [ 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 ] [ Neuronal Nitric Oxide is Specifically Decreased in Low Flow POTS ] [ Local Vascular Responses in POTS ] [ Microvascular Filtration in High Flow POTS ] [ POTS as Thoracic Hypovolemia ] |
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translocates thoracic blood volume into the dependent body. The skeletal
muscle pump participates in preventing orthostatic intolerance by
enhancing venous return. We investigated the hypothesis that skeletal
muscle pump function is impaired in postural tachycardia (POTS) associated
with low calf blood flow (“low flow POTS”) and depends in general on
muscle blood flow. Twelve low flow POTS subjects were compared with 10
controls, and with 7 POTS patients with normal calf blood flow using
strain gauge plethysmography to measure peripheral blood flow, venous
capacitance, and calf muscle pump function. Blood volume was estimated by
dye dilution. We found that calf circumference was reduced in low flow
POTS (32±1 vs 39±3, 43±3 cm) and, compared to control and POTS patients
with normal blood flow, is related to reduced fraction of calf venous
capacity emptied during voluntary muscle contraction (ejection fraction,
0.52±.07 vs 0.76±.07, 0.80±.06). We found that blood flow was linearly
correlated (rp=0.69) with calf circumference (used as a
surrogate for muscle mass). Blood volume was 2.2±0.3 in low flow POTS vs
2.6±0.5 in controls (p=0.17) and 2.4±0.7 in normal flow POTS patients.
Decreased calf blood flow may reduce calf size in POTS, impairing the
upright ejectile ability of the skeletal muscle pump and further
contributing to overall reduced blood flow and orthostatic intolerance in
these patients.
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| Diagrammatic
representation of typical recording of plethysmographically measured
changes in calf volume during a sequence of leg lift, standing , and
tiptoe exercise. The patient is first supine position with the leg
elevated, then standing with weight on nonexamined leg, then performs
several single tiptoe movements followed by 2 ten tiptoe movement
sequences returning to quiet standing position in between. The
complete venous volume is designated VV, the ejection volume of a tiptoe
is designated EV, and the residual volume is RV. RV was defined
from an average of the residual volume during multiple tiptoes. Ejection
fraction is defined as the ratio EV/VV.
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relation between supine blood flow and calf size is shown. Low flow POTS patients are shown as crosses,
control subjects are shown as triangles, normal flow POTS patients are
shown as squares. Calf size increases (rp=0.74) with blood flow
suggesting that the mass of the calf increases with blood flow to the
calf.
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| The
relation between calf ejection fraction = ejection volume (EV)/ venous
volume (VV) and supine calf blood flow measured by venous occlusion strain
gauge plethysmography. Low flow POTS patients are shown as crosses,
control subjects are shown as triangles, normal flow POTS are shown as
squares. Ejection fraction increases (rp=0.69) with blood flow
suggesting that muscle pump efficacy increases with calf muscle perfusion.
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Abnormal Skeletal Muscle Pump