Hyper-Oxygenation Attenuates the Rapid Vasodilatory Response to Muscle Contraction and Compression

Reduction in peripheral arterial stiffness after reactive hyperemia is dependent on increases in blood flow

The acute reduction in peripheral arterial stiffness during reactive hyperemia is assumed to be flow-mediated; however, the mechanism remains unproven. We hypothesized that restricting the blood flow increase during reactive hyperemia would abolish the reduction in peripheral arterial stiffness. Fourteen healthy young adults (5 females, 25 ± 5 years, mean ± SD) underwent reactive hyperemia with a rapid-release cuff on the upper arm inflated to 220 mmHg for 5 min: once with unrestricted blood flow and once with restricted blood flow by manually applying pressure to the brachial artery. Brachial-radial pulse wave velocity (PWV) was measured with tonometers over brachial and radial arteries before cuff inflation and at 5, 15, and 30 min after release. Brachial blood flow was monitored with Doppler ultrasound. Baseline brachial-radial PWV was similar between conditions (10.3 ± 1.8 vs. 10.7 ± 1.7 m/s). With unrestricted flow, PWV decreased 5 min post-reactive hyperemia (8.6 ± 1.1 m/s; p < 0.05) and returned near baseline at 15 and 30 min post (p < 0.05). With restricted flow, PWV did not change (p > 0.05) post-reactive hyperemia. Reactive hyperemia acutely reduced peripheral arterial stiffness, but not when brachial artery blood flow increase was restricted. This suggests that the reduction in peripheral arterial stiffness during reactive hyperemia depends on increased blood flow.

The Cardiac Caval Index: Improving Noninvasive Assessment of Cardiac Preload

OBJECTIVES

Inferior vena cava (IVC) pulsatility quantified by the Caval Index (CI) is characterized by poor reliability, also due to the irregular magnitude of spontaneous respiratory activity generating the major pulsatile component. The aim of this study was to test whether the IVC cardiac oscillatory component could provide a more stable index (Cardiac CI-CCI) compared to CI or respiratory CI (RCI).

METHODS

Nine healthy volunteers underwent long-term monitoring in supine position of IVC, followed by 3 minutes passive leg raising (PLR). CI, RCI, and CCI were extracted from video recordings by automated edge-tracking and CCI was averaged over each respiratory cycle (aCCI). Cardiac output (CO), mean arterial pressure (MAP) and heart rate (HR) were also recorded during baseline (1 minutes prior to PLR) and PLR (first minute).

RESULTS

In response to PLR, all IVC indices decreased (P < .01), CO increased by 4 ± 4% (P = .055) while HR and MAP did not vary. The Coefficient of Variation (CoV) of aCCI (13 ± 5%) was lower than that of CI (17 ± 5%, P < .01), RCI (26 ± 7%, P < .001) and CCI (25 ± 7%, P < .001). The mutual correlations in time of the indices were 0.81 (CI-RCI), 0.49 (CI-aCCI) and 0.2 (RCI-aCCI).

CONCLUSIONS

Long-term IVC monitoring by automated edge-tracking allowed us to evidence that 1) respiratory and averaged cardiac pulsatility components are uncorrelated and thus carry different information and 2) the new index aCCI, exhibiting the lowest CoV while maintaining good sensitivity to blood volume changes, may overcome the poor reliability of CI and RCI.

Differential control of blood flow in masseter and biceps brachii muscles during stress

OBJECTIVE

The present study aimed to compare sympathetic hemodynamic effects in masticatory and limb muscles in response to different stressors.

DESIGN

Twelve healthy participants were subjected to a randomized series of stressors, including cold pressor test (CPT), mental arithmetic test, apnea, isometric handgrip (IHG) and post-handgrip muscle ischemia (PHGMI), while in the supine position. Spatially-resolved near-infrared spectroscopy was used to measure relative changes in blood volume and oxygenation (TOI) of the resting masseter and biceps muscles. Cardiac output, heart rate, and arterial blood pressure (ABP) were also monitored.

RESULTS

Except apnea, all tests increased ABP. Different response patterns were observed in the 2 muscles: TOI significantly increased during contralateral IHG (1.24 ± 1.17%) but markedly decreased during CPT (-4.84 ± 4.09%) and PHGMI (-6.65 ± 5.31%) in the biceps muscle, while exhibiting consistent increases in the masseter (1.88 ± 1.85%; 1.60 ± 1.75%; 1.06 ± 3.29%, respectively) (p < 0.05).

CONCLUSIONS

The results allow us to infer differential control of blood flow in head and limb muscles. In general, the masseter appears more prone to dilatation than the biceps, exhibiting opposite changes in response to painful stimuli (CPT and PHGMI). Several mechanisms may mediate this effect, including reduced sympathetic outflow to the extracranial vasculature of the head, generally exposed to lower hydrostatic loads than the rest of the body.

Evidence that large vessels do affect near infrared spectroscopy

The influence of large vessels on near infrared spectroscopy (NIRS) measurement is generally considered negligible. Aim of this study is to test the hypothesis that changes in the vessel size, by varying the amount of absorbed NIR light, could profoundly affect NIRS blood volume indexes. Changes in haemoglobin concentration (tHb) and in tissue haemoglobin index (THI) were monitored over the basilic vein (BV) and over the biceps muscle belly, in 11 subjects (7 M - 4 F; age 31 ± 8 year) with simultaneous ultrasound monitoring of BV size. The arm was subjected to venous occlusion, according to two pressure profiles: slow (from 0 to 60 mmHg in 135 s) and rapid (0 to 40 mmHg maintained for 30 s). Both tHb and THI detected a larger blood volume increase (1.7 to 4 fold; p < 0.01) and exhibited a faster increase and a greater convexity on the BV than on the muscle. In addition, NIRS signals from BV exhibited higher correlation with changes in BV size than from muscle (r = 0.91 vs 0.55, p < 0.001 for THI). A collection of individual relevant recordings is also included. These results challenge the long-standing belief that the NIRS measurement is unaffected by large vessels and support the concept that large veins may be a major determinant of blood volume changes in multiple experimental conditions.

Vascular reactivity of cutaneous circulation to brief compressive stimuli, in the human forearm

PURPOSE

A brief compressive stimulus is known to induce a rapid hyperemia in skeletal muscles, considered to contribute to the initial phase of functional hyperemia. Whether the same mechano-sensitivity characterizes the cutaneous circulation is debated. This study aims to investigate whether a rapid hyperemic response to compressive stimuli is also expressed by skin blood flow in humans.

METHODS

In 12 subjects, brief compressive stimuli were delivered to the forearm at varying pressures/durations (50/2, 100/2, 200/2, 200/1, 200/5 mmHg/s); the sequence was randomized and repeated with the arm above and below heart level. Laser Doppler flowmetry technique was used to monitor skin blood flow. The response was described in terms of peak skin blood flow normalized to baseline (nSBFpeak), time-to-peak from the release of compression, and excess blood volume (EBV, expressed in terms of seconds of basal flow, s-bf) received during the response.

RESULTS

The results consistently evidenced the occurrence of a compression-induced hyperemic response, with nSBFpeak = 2.9 ± 1.1, EBV = 17.0 ± 6.6 s-bf, time-to-peak = 7.0 ± 0.7 s (200 mmHg, 2 s, below heart level). Both nSBFpeak and EBV were significantly reduced (by about 50%) above compared to below heart level (p < 0.01). In addition, EBV slightly increased with increasing pressure (p < 0.05) and duration (p < 0.01) of the stimulus.

CONCLUSIONS

For the first time, the rapid dilatator response to compressive stimuli was demonstrated in human cutaneous circulation. The functional meaning of this response remains to be elucidated.

Objective Assessment of Venous Pulse Wave Velocity in Healthy Humans

Central venous pressure and volume status are relevant parameters for characterization of a patient's hemodynamic condition; however, their invasive assessment is affected by various risks while non-invasive approaches provide limited and subjective indications. Here we explore the possibility of assessing venous pulse wave velocity (vPWV), a potential indicator of venous pressure changes. In eight healthy patients, pressure pulses were generated artificially in the leg veins by rapid compression of the foot, and their propagation was detected at the level of the superficial femoral vein with Doppler ultrasound. Changes in leg venous pressure were obtained by raising the trunk from the initial supine position by 30° and 60°. vPWV increased from 1.78 ± 0.06 m/s (supine) to 2.26 ± 0.19 m/s (60°) (p < 0.01) and exhibited an overall linear relationship with venous pressure. These results indicate that vPWV can be easily assessed, and is a non-invasive indicator of venous pressure changes.