title: The interaction of environment and exercise on systemic vascular function
Creators: name: Miller, G
Publisher: name: Liverpool John Moores University
Year: 2021

description: The incidences of cerebrovascular events, such as stroke, appear to have seasonal variation, increasing in the winter months, particularly in older adults. Whilst, the cerebrovascular mechanisms underlying the winter increase in stroke with cold has not been investigated. There is some evidence that a cold stress such as skin surface cooling causes an increase in cerebral blood flow (CBFv) which could contribute to an increase risk of stroke. Conversely, an increase in CBFv may be useful in instances where CBFv has declined (e.g. with age) or where increases in CBFv are beneficial (e.g. during exercise) to cerebrovascular health. The overarching aim of this thesis was to investigate whether the responses to an acute cold stress differ in older individuals and whether the combination of exercising in a cold environment could alter cerebral, conduit and micro-vascular function in young healthy individuals.

In study 1, 12 young (25±5 years) and 9 older healthy individuals (62±6 years) were recruited. CBFv was quantified in the middle cerebral artery using transcranial Doppler, and function assessed via cerebrovascular reactivity to CO2 (CVRCO2; rebreathing gas concentration of 5% CO2), dynamic cerebral autoregulation (dCA; manipulations of BP with squat-stand manoeuvres at 0.1Hz.min-1). Conduit endothelial function was assessed via flow mediated dilation (FMD) whilst participants wore a tube-lined suit perfused with 34oC water (thermoneutral condition). Subsequently, the suit was perfused with 12oC water for 30 mins. CBFv, temperature and haemodynamics were monitored during cooling and were followed by repeat measurements of cerebrovascular and endothelial function (cold condition). Cerebrovascular reactivity and autoregulation was not altered by the cold and this was similar in both age groups. FMD reduced by 2.6% (-5.1, -0.2, P=0.05), peak artery diameter was reduced by 0.02 cm (-0.03, 0.00, P=0.05), and SRAUC was reduced by 10 (-19, -2, P=0.02) following cooling. During cooling skin temperature reduced by 6.2oC (-6.9, -5.4, P&lt;0.001) but CBFv did not significantly increase and was similar in both the young and older individuals.
Study 2 employed a randomised control design where 21 (16 males, 22±5 years) individuals were randomly allocated to either exercise training in a cold (5º) or thermoneutral (15º) environment. Exercise consisted of 50-minutes cycling at 70% heart rate max (HRmax), 3 times per week for 8 weeks. CBFv, cerebrovascular reactivity and autoregulation were assessed prior to and following the exercise intervention. FMD, skin microvascular function (gradual local heating) and cardiorespiratory fitness (VO2peak) were also assessed. During one session at the midpoint of the training intervention CBFv, temperature and haemodynamics were measured continuously during an exercise bout. Cardiorespiratory fitness improved (2.9 ml.min.kg-1, 95%CI 0.5, 5.3; P=0.02), regardless of environment. Neither exercise intervention had an impact on CBFv, cerebrovascular reactivity, FMD or skin microvascular function (P&gt;0.05). There was a significant time*condition interaction for normalised gain (marker of autoregulation), with evidence of a 0.192 %cm.s-1.%mmHg-1 (95%CI -0.318, -0.065) decrease following training in the cold. There was also evidence of an increase in dCA phase by 0.072 radians (95%CI -0.007, 0.152) following training in the cold (P=0.02). Despite lower Tsk during the acute exercise bout in the cold (P&lt;0.001), CBFv, blood pressure, skin temperature or skin blood flow were not different between environmental conditions (P&gt;0.05). 
These novel findings indicate that (1) skin surface cooling does not influence cerebrovascular haemodynamics and function, although causes a reduction in peripheral vascular function in both young and older individuals and (2) exercise training in a cold environment may improve cerebral autoregulation more than training in a thermoneutral environment.