Intra- and inter-day reproducibility of low-flow mediated constriction response in young adults

PURPOSE

When assessing brachial endothelial function by reactive hyperaemia, stopping blood flow creates a period of low-flow-mediated constriction (L-FMC). As little is known about how this parameter influences flow-mediated vasodilation (FMD), the purpose of this study was to better understand this relationship and to determine the intra- and interday reproducibility of brachial L-FMC.

METHODS

Brachial L-FMC and FMD were measured on 26 healthy, young adults (13 males, 13 females; 24·6 ± 2·7 years). Each participant had two assessments conducted on two separate visits, separated by a minimum of seven days. Brachial artery baseline diameter was imaged during rest. Continuous imaging of the artery was performed during the last 20 s of cuff-occlusion to 180 s postcuff release. An L-FMC was considered present if the relative change from pre-occlusion baseline to L-FMC artery diameter was less than -0·1%.

RESULTS

Overall, there was a strong, positive correlation between increased brachial L-FMC and blunted FMD (visit 1 test 1: r = 0·758, P<0·001; visit 1 test 2: r = 0·706, P<0·001; visit 2 test 1: r = 0·836, P<0·001; visit 2 test 2: r = 0·857, P<0·001). The reproducibility of intra- and interday L-FMC diameter was intraclass correlation coefficients (ICC) = 0·627, coefficient of variation (CV) = 54·4% and ICC = 0·734, CV = 43·5%, respectively.

CONCLUSION

Vasoconstriction to low-flow conditions influences the subsequent maximal dilation during reactive hyperaemia. However, L-FMC is variable as evidenced by the weak intra- and interday reproducibility of the measure. Further research should study brachial L-FMC reproducibility among varying populations and the implications L-FMC has on the interpretation of FMD results.

Endothelial dysfunction as assessed with magnetic resonance imaging - A major determinant in chronic heart failure

Chronic heart failure (CHF) is a clinical syndrome resulting from interaction of different structure and functional disturbances leading to decreased heart ability to ensure adequate supply of oxygenized blood to tissues and ensure adequate metabolic needs in the cases of normal or increased afterload. Endothelial dysfunction (ED) is a pathological condition characterized by general imbalance of all major endothelial mechanisms with key role in development and progression of atherosclerotic disease. ED has been associated with most cardiovascular risk factors. There is increasing interest in assessing endothelial function non-invasively, leading to development and evaluation of new diagnostic methods. We suggest that MRI is safe and reliable test that offers important advantages over ultrasound for the detection of ED and monitoring of the expected therapeutic effect. We believe that ED plays a pivotal role in chronic heart failure development and progression, regardless of its etiology, and that MRI should be introduced as a "gold standard" in diagnostic procedure and treatment.

Brachial artery flow-mediated dilation is not affected by pregnancy or regular exercise participation

Whether brachial artery FMD (flow-mediated dilation) is altered in pregnancy by 28-35 weeks compared with non-pregnant women remains controversial. The controversy may be due to limitations of previous studies that include failing to: (i) test non-pregnant controls in the mid-late luteal phase, (ii) account for effects of pregnancy on the dilatory shear stimulus, (iii) account for physical activity or (iv) control for inter-individual variation in the time to peak FMD. In the present study, brachial artery FMD was measured in 17 active and eight sedentary pregnant women (34.1±1.6 weeks of gestation), and in 19 active and 11 sedentary non-pregnant women (mid-late luteal phase). Decreased vascular tone secondary to increased shear stress contributes minimally to pregnancy-induced increases in baseline brachial artery diameter, as shear stress removal during distal cuff inflation in pregnant women did not reduce diameter to baseline levels observed in non-pregnant controls. Neither the shear stimulus nor the percentage FMD was affected by pregnancy or regular exercise. Continuous diameter measurements are required to control for delayed peak dilation during pregnancy (57±15 compared with 46±15 s; P=0.012), as post-release diameter measured at 60 or 55-65 s post-release underestimated FMD to a greater extent in non-pregnant than in pregnant women.

Endothelial function assessment: flow-mediated dilation and constriction provide different and complementary information on the presence of coronary artery disease

AIMS

A number of risk factors for atherosclerosis have been identified, but it remains difficult, on an individual patient basis, to predict how these factors interact in determining the development of coronary artery disease (CAD). It also remains unclear whether the study of endothelial function provides information that is additive to that of traditional risk factors.

METHODS AND RESULTS

Flow-mediated dilation (FMD) and low-flow-mediated constriction (L-FMC) were measured in 451 consecutive patients before coronary angiography. Low-flow-mediated constriction (P< 0.0001) and FMD (P=0.0005) progressively decreased with the number of diseased vessels, and L-FMC showed a significant linear correlation with the SYNTAX score (R=0.38; P< 0.0001). Logistic regression analysis confirmed the association between endothelial function parameters and CAD (P=0.001 for L-FMC, P=0.02 for FMD). Receiver operating characteristic analysis demonstrated that the addition of L-FMC alone and of the combination of FMD and L-FMC improved the predictive power of a model based on traditional risk factors for CAD (area under the curve of the risk factor model=0.716; risk factor model + FMD=0.734, P=0.1 compared with risk factor model; risk factor model + L-FMC=0.771, P=0.004; risk factor model + L-FMC + FMD=0.779, P=0.002). Reclassification statistics showed that the introduction of FMD to the model based on the traditional risk factors correctly reclassified an additional 5% of patients, and that the introduction of L-FMC net correctly reclassified 19% of the patients. There was no correlation between different parameters of endothelial function.

CONCLUSION

Endothelial function assessment provides modest but statistically significant additional information in predicting the presence of CAD.

Low flow-mediated constriction occurs in the radial but not the brachial artery in healthy pregnant and nonpregnant women

Radial artery diameter decreases when a wrist cuff is inflated to stop blood flow to distal tissue. This phenomenon, referred to as low flow-mediated vasoconstriction (L-FMC), was proposed as a vascular function test. Recommendations that L-FMC be measured concurrently with flow-mediated dilation (FMD) were based on radial artery data. However, cardiovascular disease prediction studies traditionally measure brachial artery FMD. Therefore, studies should determine whether L-FMC occurs in the brachial artery. The hypothesis that reduced shear causes L-FMC has not been tested. Brachial and radial artery L-FMC and FMD were assessed in active nonpregnant (n=17), inactive nonpregnant (n=10), active pregnant (n=15, 34.1+/-1.2 wk gestation), and inactive pregnant (n=8, 34.2+/-2.2 wk gestation) women. Radial artery diameter decreased significantly during occlusion in all groups (nonpregnant, -4.4+/-4.2%; pregnant, -6.4+/-3.2%). Brachial artery diameter did not change in active and inactive nonpregnant, and inactive pregnant women; however, the small decrease in active pregnant women was significant. Occlusion decreased shear rate in both arteries, yet L-FMC only occurred in the radial artery. Radial artery L-FMC was not correlated with the reduction in shear rate. L-FMC occurs in the radial but not the brachial artery and is not related to changes in shear rate. Positive correlations between L-FMC (negative values) and FMD (positive values) suggest that radial artery FMD may be reduced among women who experience greater L-FMC. Studies should clarify the underlying stimulus and mechanisms regulating L-FMC, and test the hypothesis that endothelial dysfunction is manifested as enhanced brachial artery L-FMC, but attenuated radial artery L-FMC.