An increase in blood flow associated shear stress results in an endothelial dependent increase in vessel diameter (flow mediated vasodilation (FMD)). Assessment of FMD can provide an index of endothelial function. The stimulus profiles that have been used to investigate FMD in human conduit arteries fall into two categories: reactive hyperemia and sustained stimuli. Stimulus-response specificity proposes that the stimulus characteristics (e.g. magnitude, pattern) are essential determinants of the response characteristics (e.g. magnitude, mechanisms). Purpose: To investigate four specific aspects of FMD stimulus-response specificity: 1) The relative importance of the peak vs. the duration of reactive hyperemia in determining FMD response magnitude. 2) The nitric oxide (NO) dependence of FMD following different durations of reactive hyperemia. 3) The impact of sustained shear stress stimulus magnitude on FMD response dynamics and magnitude. 4) FMD dynamics and magnitude in response to steady vs. oscillatory shear stress evoked passively or via exercise. Methods: Doppler ultrasound was applied to the brachial or radial artery to measure blood flow velocity. Vessel diameter was measured with automated edge detection software. Shear rate, an estimate of shear stress was calculated as the blood flow velocity/vessel diameter. Results: 1) The duration of reactive hyperemia is an important determinant of peak FMD magnitude while the independent contribution of the peak shear to FMD is minimal. 2) NO is not obligatory to FMD following either a five or a ten minute duration occlusion. 3) FMD in response to a sustained stimulus is characterized by a generally bi-phasic response with a fast first phase followed by a slower final phase. 4) The endothelium transduces the mean shear stress when it is exposed passive or handgrip exercise induced oscillations in shear stress. Conclusions: The results indicate that future reactive hyperemia studies must account for the stimulus duration when interpreting FMD results. Further, they demonstrate that the role of NO in FMD is unclear and caution against oversimplified conceptual models of FMD mechanisms. FMD in response to sustained stimuli provides information distinct from reactive hyperemia investigations and exercise may provide a valuable stimulus creation technique. / Thesis (Ph.D, Kinesiology & Health Studies) -- Queen's University, 2007-08-16 18:11:18.941

Flow-mediated dilation (FMD) can be used to assess the risk of atherosclerosis; however, an inverse relationship between vessel size and FMD has been identified using reactive hyperemia (RH) to create a shear stress (SS) stimulus in human conduit arteries. RH creates a transient and uncontrolled SS stimulus that is inversely related to baseline arterial diameter. It is therefore unclear whether differences in FMD between groups with non-uniform artery sizes are indicative of differences in vascular health or due to the creation of a greater SS stimulus in smaller vessels. Unlike RH, exercise can effectively create sustained and controlled increases in conduit artery SS. The purpose of this study was to compare the FMD responses of two differently sized upper limb arteries (brachial (BA) and radial artery (RA)) to matched graded levels of SS. Using exercise, three distinct sustained shear rate stimuli were created ((SR)=blood flow velocity/vessel diameter; estimate of SS) in the RA and BA. Artery diameter and mean blood flow velocity were assessed with echo and Doppler ultrasound respectively in 15 healthy male subjects (19-25yrs). Data are means ±SE. Subjects performed 6-min each of adductor pollicis and handgrip exercise to increase SR in the RA and BA respectively. Exercise intensity was modulated to achieve uniformity of SR between the RA and BA at three SR targets (40s-1, 60s-1, 80s-1). Three distinct SR levels were successfully created (steady state exercise: 39.8±0.6s-1, 57.3±0.7s-1, 72.4±1.2s-1; p<0.001 between SR levels). The %FMD at the end of exercise was greater in the RA vs. BA (SR40 RA: 5.4±0.8%, BA: 1.0±0.2%; SR60 RA: 9.8±1.0%, BA: 2.5±0.5%; SR80 RA: 15.7±1.5%, BA: 5.4±0.7%; p<0.001). The mean slope of the within-subject FMD-SR dose-response regression lines was significantly greater in the RA (RA: 0.33±0.04, BA: 0.13±0.02; p<0.001) and a strong within-subjects relationship between FMD and SR was observed in both arteries (RA r2: 0.92±0.02; BA r2: 0.90±0.03). These findings suggest that the response to SS is not uniform across differently sized vessels, which is in agreement with previous RH studies. Future research is required to investigate the potential mechanisms that mediate the functional differences observed between differently sized vessels. / Thesis (Master, Kinesiology & Health Studies) -- Queen's University, 2010-09-22 11:01:26.028

Shear stress is the frictional force exerted on the vascular wall by blood flowing through an artery. It is a major regulator of endothelial cell function, which is essential for vasoprotection and local regulation of vascular tone. Using handgrip exercise (HGEX) to increase shear stress is an increasingly popular method for assessing brachial artery (BA) endothelial cell function via flow-mediated dilation (FMD, dilation which increases with improved endothelial function). However, different exercise duty-cycles (ratio of handgrip relaxation to contraction in seconds) produce different patterns of BA shear stress with different anterograde and retrograde flow magnitudes. PURPOSE: To determine the impact of HGEX duty-cycle on BA %FMD while maintaining a constant mean shear stress. METHODS: N=16 healthy males. BA diameter (BAD) and blood velocity (BV) were assessed via echo and Doppler ultrasound. Shear stress was estimated as shear rate (SR=BV/BAD) and reported as mean SR during the last minute of baseline (target 10 s 1) and each minute of HGEX (75 s-1). Subjects performed 3 six minute HGEX trials on each of 2 separate days (like trials averaged). Each trial was one of 3 randomly ordered HGEX duty-cycles (1:1, 3:1, 5:1). %FMD was calculated as the increase in BAD from baseline to the end of HGEX and at each minute (subset N=10) during HGEX. RESULTS: Data are means ± SD. As intended, mean SR was similar between duty-cycles (main effect, p=0.835), despite significant differences in anterograde and retrograde SR (P<0.001). There was no impact of duty cycle on blood pressure (p=0.188) or heart rate (p=0.131) responses. End exercise %FMD (4.0 ± 1.3%, 4.1 ± 2.2%, 4.2 ± 1.4%, p=0.860) and minute by minute %FMD (main effect p=0.939; interaction, p=0.545) were also not different between duty-cycles. CONCLUSION: Distinct HGEX duty-cycles create markedly different shear stress patterns in the BA. However, duty cycle had no impact on %FMD magnitude suggesting that mean shear stress is the most important stimulus for FMD in the BA. Using a 5:1 duty cycle may yield the best vessel image and diameter measurement quality due to the long period of arm stability between contractions. / Thesis (Master, Kinesiology & Health Studies) -- Queen's University, 2012-09-06 14:31:50.467

Flow mediated dilation (FMD) is a non-invasive test that assesses endothelial health and nitric oxide bioavailability; it is commonly used to examine changes in vascular health due to disease or de-conditioning. Currently, a wide variety of protocols are being used to assess upper and lower extremity conduit artery health. The current project was embarked upon to gain a better understanding of the FMD protocols currently being used to asses conduit artery FMD and how these results impact our understanding of a participant’s vascular health. More specifically occlusion duration, cuff placement and artery location were examined in three commonly examined conduit arteries. The FMD responses in the brachial artery (BA), superficial femoral artery (SFA), and popliteal artery (PA) of ten healthy men, mean age of 27, after five and/or two-minutes of distal occlusion were examined. When the two-minute protocol was performed on the SFA and PA, low-resistance static calf exercise was added to augment the shear stimulus. It was hypothesized that percent FMD and shear stress responses of the SFA and PA would not be significantly different after five-minutes of occlusion, thereby allowing leg conduit artery FMD to be performed on either artery. It was further hypothesized that there would be no significant differences between the shear stress and percent FMD responses of the leg conduit arteries after five or two-minutes of occlusion; inferring that shorter occlusion durations when combines with ischemic muscle contractions can be used to assess SFA or PA FMD. With regards to comparisons between arm and leg conduit arteries, it was hypothesized that there would be significant between limb differences in baseline diameter, FMD and shear stress post five-minutes of distal occlusion. These differences will be used to better understand the effects of artery location and size on conduit artery FMD IV responses. Limitations with the traditional edge-detection method of determining arterial diameter prompted the creation of a new method of measuring artery diameter, the center-based method. It was hypothesized that there would be no significant differences in the percent FMD and time to FMD after five-minutes of BA occlusion (n=7). The results of the current study demonstrated that five-minutes of calf occlusion elicited a significant PA FMD but not a significant SFA FMD. FMD post two-minutes of PA occlusion with exercise was not significantly different than that produced by five-minutes of occlusion. Conversely, two-minutes of calf occlusion with exercise was unable to elicit a SFA FMD response. Significant differences in shear stress and FMD were reported between arm and leg conduit arteries, demonstrating different responses to five-minutes of distal occlusion due to artery size and location. Finally, no significant differences were noted between FMD and time to FMD when the center-based or edge-detection method was used. This study has demonstrated that the calf occlusion protocol was unable to elicit a FMD response in the SFA FMD; this occlusion location is only able to elicit a PA FMD response. Furthermore, two-minutes of occlusion with one-minute of exercise can be used in place of the five-minute protocol to examine PA FMD but not SFA FMD. Differences between arm and leg conduit arteries are noted and it has been suggested that this is likely due to leg conduit artery adaptations to gravity. Lastly, preliminary data suggest that the center-based method is an appropriate method of measuring conduit artery diameter.

Flow mediated dilation (FMD) is a non-invasive test that assesses endothelial health and nitric oxide bioavailability; it is commonly used to examine changes in vascular health due to disease or de-conditioning. Currently, a wide variety of protocols are being used to assess upper and lower extremity conduit artery health. The current project was embarked upon to gain a better understanding of the FMD protocols currently being used to asses conduit artery FMD and how these results impact our understanding of a participant’s vascular health. More specifically occlusion duration, cuff placement and artery location were examined in three commonly examined conduit arteries. The FMD responses in the brachial artery (BA), superficial femoral artery (SFA), and popliteal artery (PA) of ten healthy men, mean age of 27, after five and/or two-minutes of distal occlusion were examined. When the two-minute protocol was performed on the SFA and PA, low-resistance static calf exercise was added to augment the shear stimulus. It was hypothesized that percent FMD and shear stress responses of the SFA and PA would not be significantly different after five-minutes of occlusion, thereby allowing leg conduit artery FMD to be performed on either artery. It was further hypothesized that there would be no significant differences between the shear stress and percent FMD responses of the leg conduit arteries after five or two-minutes of occlusion; inferring that shorter occlusion durations when combines with ischemic muscle contractions can be used to assess SFA or PA FMD. With regards to comparisons between arm and leg conduit arteries, it was hypothesized that there would be significant between limb differences in baseline diameter, FMD and shear stress post five-minutes of distal occlusion. These differences will be used to better understand the effects of artery location and size on conduit artery FMD IV responses. Limitations with the traditional edge-detection method of determining arterial diameter prompted the creation of a new method of measuring artery diameter, the center-based method. It was hypothesized that there would be no significant differences in the percent FMD and time to FMD after five-minutes of BA occlusion (n=7). The results of the current study demonstrated that five-minutes of calf occlusion elicited a significant PA FMD but not a significant SFA FMD. FMD post two-minutes of PA occlusion with exercise was not significantly different than that produced by five-minutes of occlusion. Conversely, two-minutes of calf occlusion with exercise was unable to elicit a SFA FMD response. Significant differences in shear stress and FMD were reported between arm and leg conduit arteries, demonstrating different responses to five-minutes of distal occlusion due to artery size and location. Finally, no significant differences were noted between FMD and time to FMD when the center-based or edge-detection method was used. This study has demonstrated that the calf occlusion protocol was unable to elicit a FMD response in the SFA FMD; this occlusion location is only able to elicit a PA FMD response. Furthermore, two-minutes of occlusion with one-minute of exercise can be used in place of the five-minute protocol to examine PA FMD but not SFA FMD. Differences between arm and leg conduit arteries are noted and it has been suggested that this is likely due to leg conduit artery adaptations to gravity. Lastly, preliminary data suggest that the center-based method is an appropriate method of measuring conduit artery diameter.

The endothelium is an integral mediator of vascular homeostasis and a dysfunctional endothelium is now recognized as an early marker of atherosclerosis. Importantly, the non-invasive measurement of endothelial function by flow-mediated dilation (FMD) predicts future cardiovascular events. However, the appropriate method of its assessment and the mechanisms that govern FMD are still poorly understood. We investigated alternative parameters and methods of FMD measurement in healthy volunteers and cardiovascular disease patients. We found time to peak FMD to be highly variable both within and between individuals. Accordingly, continuous arterial diameter measurement post-cuff release was more sensitive in discriminating between health and disease compared to the measurement of diameter at 60’’ post-cuff release. Reperfusion to an ischemic tissue can paradoxically contribute to endothelial dysfunction development and further tissue damage, in a phenomenon known as ischemia and reperfusion (IR) injury. Previous exposure to sublethal ischemia (ischemic preconditioning (IPC)) can reduce sensitivity to IR injury and pharmacologic agents have since been shown to mimic this response. Using the FMD technique, we investigated various preconditioning strategies to prevent IR-induced endothelial dysfunction in the forearm vasculature of healthy volunteers. The sodium-hydrogen exchanger inhibitor amiloride and the angiotensin-converting enzyme inhibitor captopril were found not to provide endothelial protection from IR. In contrast, potent protection from IR-induced endothelial dysfunction was observed during the high-estrogen, late follicular phase of the menstrual cycle in pre-menopausal women. Finally, daily episodes of IPC were found to provide endothelial protection equipotent to an acute episode of IPC. The findings from the FMD methodological study highlight the importance of continuous arterial diameter measurement post-cuff deflation, and provide mechanistic insight that may contribute to measurement standardization and normalization. The results of the preconditioning studies improve our understanding of potential approaches to mitigate the detrimental effects of IR on the endothelium in humans.

The endothelium is an integral mediator of vascular homeostasis and a dysfunctional endothelium is now recognized as an early marker of atherosclerosis. Importantly, the non-invasive measurement of endothelial function by flow-mediated dilation (FMD) predicts future cardiovascular events. However, the appropriate method of its assessment and the mechanisms that govern FMD are still poorly understood. We investigated alternative parameters and methods of FMD measurement in healthy volunteers and cardiovascular disease patients. We found time to peak FMD to be highly variable both within and between individuals. Accordingly, continuous arterial diameter measurement post-cuff release was more sensitive in discriminating between health and disease compared to the measurement of diameter at 60’’ post-cuff release. Reperfusion to an ischemic tissue can paradoxically contribute to endothelial dysfunction development and further tissue damage, in a phenomenon known as ischemia and reperfusion (IR) injury. Previous exposure to sublethal ischemia (ischemic preconditioning (IPC)) can reduce sensitivity to IR injury and pharmacologic agents have since been shown to mimic this response. Using the FMD technique, we investigated various preconditioning strategies to prevent IR-induced endothelial dysfunction in the forearm vasculature of healthy volunteers. The sodium-hydrogen exchanger inhibitor amiloride and the angiotensin-converting enzyme inhibitor captopril were found not to provide endothelial protection from IR. In contrast, potent protection from IR-induced endothelial dysfunction was observed during the high-estrogen, late follicular phase of the menstrual cycle in pre-menopausal women. Finally, daily episodes of IPC were found to provide endothelial protection equipotent to an acute episode of IPC. The findings from the FMD methodological study highlight the importance of continuous arterial diameter measurement post-cuff deflation, and provide mechanistic insight that may contribute to measurement standardization and normalization. The results of the preconditioning studies improve our understanding of potential approaches to mitigate the detrimental effects of IR on the endothelium in humans.

Flow-mediated dilation (FMD) refers to the relaxation of vascular smooth muscle and the subsequent dilation of the vessel in response to increases in shear stress on the endothelial lining accompanying increases in blood flow. The phenomenon has been shown to be endothelium dependent and as such is used clinically and experimentally as an index of endothelial health. FMD can be assessed by imaging a conduit artery with ultrasound during a period of reactive hyperaemia, typically following a period of prior blood flow occlusion achieved by the inflation of a pneumatic cuff around the limb distal to the imaging site. Previous studies have shown that the health of the endothelium is predictive of the health of the cardiovascular system as a whole. This thesis set out to scrutinize the FMD test as a marker for endothelial health by testing the following five hypotheses: 1. A short burst of high shear is not adequate to elicit the FMD response. 2. Brachial artery dilation following 15 minutes of occlusion is a clearer indicator of endothelium dependent FMD than 5 minutes of occlusion with exercise. 3. Oscillating the post occlusion shear stress will decrease FMD compared to unidirectional shear). 4. Heavy dynamic hand grip exercise 6 minutes before an occlusion-only FMD protocol will result in an enhanced FMD response. 5. Long term bed-rest inactivity will attenuate the FMD response and an exercise program will preserve endothelial function. The experiments documented in Chapter 2 found that a 20-s shear stress stimulus following 15 min of forearm circulatory occlusion was not adequate to induce an FMD response compared to longer durations of shear and there was a progressive reduction in FMD when the magnitude of the initial peak shear was reduced by limiting the duration of prior occlusion. Also, the FMD response was correlated with the total shear to time of peak diameter for all shear durations and peaks that were studied while the same was not true of peak shear. In Chapter 3 it was revealed that an uncoupling of the shear-to-dilation ratio occurred when dynamic exercise was added to the FMD test as both 15 min of occlusion (15OC) and 5 min of occlusion with 1 min of exercise (1EXin5OC) yielded similar FMD responses, even though the shear stimulus was increased with the addition of exercise. Increased plasma nitrite during hyperaemia was observed only in the 15OC protocol, suggesting that the exercise in the 1EXin5OC protocol initiates dilatory mechanisms that are not as heavily reliant on the shear sensitive nitric oxide pathway . In Chapter 4 it was shown that 5 min of intense dynamic hand grip exercise (5EX) produced a greater dilation than either continuous (15OC) or intermittent (IO) shear following 15OC. Total shear to the time of peak diameter (AUCshear) and peak shear were both correlated to %dilation following 15OC; however this relationship was lost during 5EX and IO. The results of this study echoed the suggestion in Chapter 3 that there was an uncoupling of the intensity of the shear stimulus and the magnitude of vasodilatation when exercise was introduced, and adds that it may be in part due to the oscillatory nature of the shear profile during exercise. The acute effects of local exercise on the FMD response following 15OC were examined in Chapter 5. FMD in the brachial artery was blunted following dynamic hand grip exercise, even though the shear stimulus was greater during PostEX. Nitrite was significantly elevated in CON at 15s while PostEX nitrite was significantly elevated at 30s post cuff release but not different from CON at 15s. The results of this study suggested that prior exercise had a negative effect on FMD which may be related to exercise blunting post occlusion endothelial N ̇O production. Chapter 6 examined the effect of 56 days of head-down tilt bed rest (HDBR) and an exercise countermeasure on conduit artery FMD following release of distal limb ischemia and NMD following sublingual administration of 0.3 mg of nitroglycerin. HDBR without EX decreased the resting diameter of the popliteal artery while EX increased the diameter. HDBR had no effect on the resting diameter of the brachial artery. FMD was elevated in all groups for the brachial but only in the non-exercisers for the popliteal. When change in resting diameter was taken into account the preserved FMD in EX was removed. NMD was not altered by HDBR in any group. There was enhanced endothelial function relative to intrinsic dilatory capacity in both the brachial and popliteal arteries post HDBR. The results from Chapter 2 support hypothesis 1, showing that a 20 second burst of high shear stimulus was not adequate to elicit the FMD response during reactive hyperaemia. It is not clear whether hypothesis 2 was supported or not given that the results from Chapter 3 showed on the one hand that the %FMD did not change with the addition of exercise in the occlusion but on the other hand the shear to dilation ratio was altered. The finding, in Chapter 4, that FMD was not reduced when the hyperaemia was intermittent does not support hypothesis 3. In opposition to hypothesis 4, Chapter 5 showed that %FMD was reduced following bouts of heavy hand grip exercise; however the absolute magnitude of vessel diameter was similar in both post exercise and control tests. Finally, hypothesis 5 was also contradicted, with Chapter 6 showing that long term bed-rest enhanced rather than attenuated the FMD response in both arm and leg arteries, while an exercise countermeasure preserved pre-bed-rest FMD in the legs only. In addition to the specific hypotheses tested, there was evidence that acute exercise evoked dilatory mechanisms in the conduit arteries that were not shear/endothelium dependent given that the shear to dilation relationship was uncoupled during, following, and in occlusion protocols that include exercise. The precise mechanisms by which this is achieved are still unknown, but it may be partially due to the oscillatory nature of the elevated blood flow during exercise. I conclude that inference of cardiovascular health from endothelial function by the evaluation of %FMD should be approached with caution, especially in the event that physical activity is involved.

Flow-mediated dilation (FMD) refers to the relaxation of vascular smooth muscle and the subsequent dilation of the vessel in response to increases in shear stress on the endothelial lining accompanying increases in blood flow. The phenomenon has been shown to be endothelium dependent and as such is used clinically and experimentally as an index of endothelial health. FMD can be assessed by imaging a conduit artery with ultrasound during a period of reactive hyperaemia, typically following a period of prior blood flow occlusion achieved by the inflation of a pneumatic cuff around the limb distal to the imaging site. Previous studies have shown that the health of the endothelium is predictive of the health of the cardiovascular system as a whole. This thesis set out to scrutinize the FMD test as a marker for endothelial health by testing the following five hypotheses: 1. A short burst of high shear is not adequate to elicit the FMD response. 2. Brachial artery dilation following 15 minutes of occlusion is a clearer indicator of endothelium dependent FMD than 5 minutes of occlusion with exercise. 3. Oscillating the post occlusion shear stress will decrease FMD compared to unidirectional shear). 4. Heavy dynamic hand grip exercise 6 minutes before an occlusion-only FMD protocol will result in an enhanced FMD response. 5. Long term bed-rest inactivity will attenuate the FMD response and an exercise program will preserve endothelial function. The experiments documented in Chapter 2 found that a 20-s shear stress stimulus following 15 min of forearm circulatory occlusion was not adequate to induce an FMD response compared to longer durations of shear and there was a progressive reduction in FMD when the magnitude of the initial peak shear was reduced by limiting the duration of prior occlusion. Also, the FMD response was correlated with the total shear to time of peak diameter for all shear durations and peaks that were studied while the same was not true of peak shear. In Chapter 3 it was revealed that an uncoupling of the shear-to-dilation ratio occurred when dynamic exercise was added to the FMD test as both 15 min of occlusion (15OC) and 5 min of occlusion with 1 min of exercise (1EXin5OC) yielded similar FMD responses, even though the shear stimulus was increased with the addition of exercise. Increased plasma nitrite during hyperaemia was observed only in the 15OC protocol, suggesting that the exercise in the 1EXin5OC protocol initiates dilatory mechanisms that are not as heavily reliant on the shear sensitive nitric oxide pathway . In Chapter 4 it was shown that 5 min of intense dynamic hand grip exercise (5EX) produced a greater dilation than either continuous (15OC) or intermittent (IO) shear following 15OC. Total shear to the time of peak diameter (AUCshear) and peak shear were both correlated to %dilation following 15OC; however this relationship was lost during 5EX and IO. The results of this study echoed the suggestion in Chapter 3 that there was an uncoupling of the intensity of the shear stimulus and the magnitude of vasodilatation when exercise was introduced, and adds that it may be in part due to the oscillatory nature of the shear profile during exercise. The acute effects of local exercise on the FMD response following 15OC were examined in Chapter 5. FMD in the brachial artery was blunted following dynamic hand grip exercise, even though the shear stimulus was greater during PostEX. Nitrite was significantly elevated in CON at 15s while PostEX nitrite was significantly elevated at 30s post cuff release but not different from CON at 15s. The results of this study suggested that prior exercise had a negative effect on FMD which may be related to exercise blunting post occlusion endothelial N ̇O production. Chapter 6 examined the effect of 56 days of head-down tilt bed rest (HDBR) and an exercise countermeasure on conduit artery FMD following release of distal limb ischemia and NMD following sublingual administration of 0.3 mg of nitroglycerin. HDBR without EX decreased the resting diameter of the popliteal artery while EX increased the diameter. HDBR had no effect on the resting diameter of the brachial artery. FMD was elevated in all groups for the brachial but only in the non-exercisers for the popliteal. When change in resting diameter was taken into account the preserved FMD in EX was removed. NMD was not altered by HDBR in any group. There was enhanced endothelial function relative to intrinsic dilatory capacity in both the brachial and popliteal arteries post HDBR. The results from Chapter 2 support hypothesis 1, showing that a 20 second burst of high shear stimulus was not adequate to elicit the FMD response during reactive hyperaemia. It is not clear whether hypothesis 2 was supported or not given that the results from Chapter 3 showed on the one hand that the %FMD did not change with the addition of exercise in the occlusion but on the other hand the shear to dilation ratio was altered. The finding, in Chapter 4, that FMD was not reduced when the hyperaemia was intermittent does not support hypothesis 3. In opposition to hypothesis 4, Chapter 5 showed that %FMD was reduced following bouts of heavy hand grip exercise; however the absolute magnitude of vessel diameter was similar in both post exercise and control tests. Finally, hypothesis 5 was also contradicted, with Chapter 6 showing that long term bed-rest enhanced rather than attenuated the FMD response in both arm and leg arteries, while an exercise countermeasure preserved pre-bed-rest FMD in the legs only. In addition to the specific hypotheses tested, there was evidence that acute exercise evoked dilatory mechanisms in the conduit arteries that were not shear/endothelium dependent given that the shear to dilation relationship was uncoupled during, following, and in occlusion protocols that include exercise. The precise mechanisms by which this is achieved are still unknown, but it may be partially due to the oscillatory nature of the elevated blood flow during exercise. I conclude that inference of cardiovascular health from endothelial function by the evaluation of %FMD should be approached with caution, especially in the event that physical activity is involved.

Flow-mediated dilation (FMD) is used as a surrogate marker for endothelial function, a subclinical indicator of coronary artery disease (CAD) and for that reason; FMD is commonly used to compare endothelial function across groups differing in age and number and/or type of CAD risk factors. The traditional calculation of FMD involves arterial diameter prior to cuff inflation and then peak arterial diameter following cuff release. Generally, arterial response during cuff inflation is not taken into consideration. The aims of the present study were to determine 1) if there were differences in brachial artery response, more specifically vasoconstriction, during cuff inflation in a diverse population of subjects, 2) if variability existed, the resulting impact on the calculation of traditional FMD, and 3) if arterial stiffness was a physiological determinant in this process. A total of 84 subjects, varying in age (18-62 years) and CAD risk factor profiles were studied. Low flow-mediated constriction (L-FMC), during cuff inflation, traditional FMD, and modified FMD, which accounts for L-FMC, were calculated to investigate brachial artery response during all three stages of the FMD measurement. Subjects ≥ 50 years old had lower FMD response compared with those ≤ 35 years old but only the modified FMD was statistically significant. The same effect was seen when comparing healthy subjects to those with multiple risk factors for CAD; there was an attenuated FMD response that only reached statistical significance with modified FMD. L-FMC was modestly but significantly associated with FMD. L-FMC was weakly but positively correlated with brachial pulse wave velocity (PWV). Our results indicate that modified FMD, which takes into consideration brachial response to cuff inflation, may be a more sensitive indicator of endothelial dysfunction and that arterial stiffening may be a physiological determinant in this process. / text