Abstract
Perfusion at microvascular level involves the contribution of both local and central regulators, under a complex vascular signaling frame. The venoarteriolar reflex (VAR) is one of such regulatory responses, of particular relevance in the lower limb to prevent edema. Although known for quite some time, many of the complex interactions involving all of these regulatory mechanisms still need clarification. Our objective was to look deeper into VAR through modern photoplethymography (PPG). Twelve healthy subjects (both sexes, 26.0 ± 5.0 y.o.) were enrolled in this study after informed written consent. Subjects were submitted to a leg lowering maneuver while lying supine to evoke the VAR, involving three phases–10 min baseline register, both legs extended, 10 min challenge, with one randomly chosen leg (test) pending 50 cm below heart level, while the contralateral (control) remained in place, and 10 minutes recovery, resuming the initial position. PPG signals were collected from both feet and treated by the wavelet transform (WT) revealing six spectral bands in frequency intervals comprising the cardiac [1.6–0.7 Hz], respiratory [0.4–0.26 Hz], myogenic [0.26–0.1 Hz], neurogenic/sympathetic [0.1–0.045 Hz], endothelial NO-dependent (NOd) [0.045–0.015 Hz], and NO-independent (NOi) [0.015–0.007 Hz] activities. For the first time, this approach revealed that, with VAR, perfusion significantly decreased in both limbs, although the change was more pronounced in the test foot. Here, a significant decrease in myogenic, neurogenic and NOd, were noted, while the control foot recorded a decrease in neurogenic and an increase in NOd. These results confirm the utility of WT spectral analysis for flowmotion. Further, it strongly suggests that VAR results from a complex cooperation between local myogenic-endothelial responses, where a central neurogenic reflex might also be involved.
Highlights
Microcirculation is critical for nutrient delivery, for waste removal, contributing to peripheral vascular resistance through various complex mechanisms acting on local Starling forces (Levy et al, 2001)
Our main objective in this study is to look deeper into the physiological mechanism involved in the vascular response to a classic venoarteriolar reflex (VAR) by a tested experimental setup that simplifies procedures, reduces variability sources (Rathbun et al, 2008; Silva et al, 2015; Rocha et al, 2017), using modern PPG, and explores new analytical directions through the wavelet transform
It has been recently proposed that the modulation of PPG light intensity may be attributed to the compression/decompression of the capillary bed caused by varying arterial transmural pressure (Kamshilin et al, 2015), and to changes in RBCs orientation in the capillaries (Volkov et al, 2017)
Summary
Microcirculation is critical for nutrient delivery, for waste removal, contributing to peripheral vascular resistance through various complex mechanisms acting on local Starling forces (Levy et al, 2001). The VAR refers to the reduction in limb blood perfusion in the dependent position due to an increase in pre-capillary vascular resistance. Changing the limb position immediately reduces blood perfusion (∼40%) by increasing venous transmural pressure, which evokes arteriolar vasoconstriction (Hassan and Tooke, 1988; Rathbun et al, 2008). This arteriolar vasoconstriction, in turn, decreases regional blood flow and reduces capillary pressure, protecting the dependent limb against blood accumulation and edema (Gabrielsen and Norsk, 2007). An impaired venoarterial reflex may explain swelling in patients with metabolic impairment (e.g., diabetics) or in special physiological periods (e.g., edema during the luteal phase of menstrual cycle) (Rayman et al, 1986; Hassan et al, 1990)
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