Low-flow mediated constriction: environmental modifiers and the role of shear rate in young males

Alali, Mohammad H ORCID: 0000-0003-3530-2079 (2023). Low-flow mediated constriction: environmental modifiers and the role of shear rate in young males. University of Birmingham. Ph.D.

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Abstract

Low-flow mediated constriction (L-FMC) is a recently developed method of assessing conduit artery vasoconstrictor function. L-FMC is suggested to provide complimentary information to that provided by the more traditional flow mediated dilation (FMD) technique. However, unlike FMD, the mechanisms underlining the L-FMC response and the influence of acute environmental and physiological stimuli is not completely understood. The aim of this thesis was to determine the impact of environmental modifiers and shear rate (SR) manipulation on radial artery L-FMC. Three experimental studies investigated the L-FMC response to heat stress, leg cycling exercise and simulated hyper/hypotension (i.e., lower body pressure).

The first experimental study examined the influence of the acute whole-body passive heat stress (+1°C core temperature), and manipulation of shear rate via partial wrist cuff (WC) inflation, on radial artery function. Heat significantly increased radial artery anterograde and mean SR, blood flow, and diameter (P < 0.05). Heat + WC diminished the heat-induced increase in mean and anterograde SR (P > 0.05), but markedly increased retrograde SR (P < 0.05), while increases in radial artery diameter and blood flow were reduced (heat + WC vs. heat, P < 0.05). Heat diminished FMD (8.6 ± 1.2% vs. 2.2 ± 1.4%, P < 0.05), however no change in FMD was observed in heat + WC (7.8± 1.2% vs. 10.8± 1.2%, P > 0.05). In contrast, L-FMC was not different in either trial (P > 0.05).

In study two, the acute impact of leg cycling exercise and SR manipulation on radial artery functional characteristics was determined. Exercise improved radial artery anterograde and mean SR, along with the blood flow, diameter, velocity, and conductance (P < 0.05). By design, Exercise + WC reduced the exercise-induced increase in anterograde and mean SR (P > 0.05), whereas it increased retrograde SR (P < 0.05). Moreover, increases in radial artery diameter and blood flow were reduced during Exercise + WC (Exercise + WC vs. Exercise, P < 0.05). After Exercise, L-FMC was increased (-4.4 ± 1.4 vs. -13.1 ± 1.6%, P < 0.05), whereas no change in L-FMC after Exercise + WC (-5.2 ± 2.0 vs. -3.0 ± 1.6%, P > 0.05).

The final experimental study determined the impact of acute increases and decreases in sympathetic nerve activity (SNA), utilised by lower body negative pressure (LBNP) and lower body positive pressure (LBPP) respectively, on radial artery function. Results indicated that radial artery characteristics (blood flow, velocity and mean SR) decreased (Time, P<0.05) acutely following both trials. However, radial artery L-FMC was not altered following either LBNP or LBPP (P>0.05).

Collectively, these findings demonstrate the diverse impact of environmental stimuli (heat, exercise, lower body pressure), and the mediating effects of SR, on conduit artery function.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):