Kaitlin M. McLay

Vascular responsiveness determined by near-infrared spectroscopy measures of oxygen saturation.

What is the central question of this study? Can the near‐infrared spectroscopy (NIRS)‐derived reperfusion rate (slope 2) of tissue oxygen saturation (StO2) be correlated with flow‐mediated dilation (FMD), the commonly used method to assess vascular endothelial function? What is the main finding and its importance? The present data were able to establish a correlation between the reperfusion rate of StO2 and percentage FMD in healthy young men. These data suggest that NIRS‐derived slope 2 StO2 can be used as a measure of vascular endothelial function.

Repeatability of vascular responsiveness measures derived from near‐infrared spectroscopy

Near‐infrared spectroscopy (NIRS)‐derived measures of tissue oxygen saturation (StO2) have been recently shown to significantly correlate with the widely used method for noninvasively assessing vascular endothelial function, flow‐mediated dilation (FMD). The purpose of this study was to examine the intraday and interday reliability of the reperfusion slope of StO2 (slope 2 StO2) and compare it to FMD. Ultrasound‐derived FMD was quantified following 5 min of distal cuff occlusion of the popliteal artery in nine healthy young men (26 ± 3 years). An FMD test was performed each of 4 days, with a fifth involving three tests. FMD was calculated as the greatest percent change in diameter from baseline (%FMD). StO2 was measured using NIRS throughout each test, with slope 2 StO2 being calculated as the upslope of 10‐sec following cuff release. Reliability was determined using repeatability, intraclass correlation coefficients (ICC), and coefficient of variation (CV). Repeatability of slope 2 StO2 was better than %FMD for both intraday (0.43 and 5.65, respectively) and interday (0.48 and 4.82, respectively) comparisons; approximately 30% of mean values for slope 2 StO2 could be attributed to measurement error, whereas 100% of mean FMD could be for both intraday and interday comparisons. Similarly, ICC and CV values indicated stronger reliability of slope 2 StO2 compared to %FMD for both intraday (ICC 0.92 and 0.36, respectively; CV 9 ± 4% and 44 ± 24%, respectively) and interday (ICC 0.94 and 0.25, respectively; CV 14 ± 5% and 40 ± 22%, respectively) comparisons. In conclusion, NIRS‐derived slope 2 StO2 can be used as a reliable measure of vascular reactivity.

Vascular responsiveness measured by tissue oxygen saturation reperfusion slope is sensitive to different occlusion durations and training status

What is the central question of this study? Is the near‐infrared spectroscopy‐derived measure of tissue oxygen saturation ( StO2 ) reperfusion slope sensitive to a range of ischaemic conditions, and do differences exist between trained and untrained individuals? What is the main finding and its importance? The StO2 reperfusion rate is sensitive to different occlusion durations, and changes in the reperfusion slope in response to a variety of ischaemic challenges can be used to detect differences between two groups. These data indicate that near‐infrared spectroscopy‐derived measures of StO2 , specifically the reperfusion slope following a vascular occlusion, can be used as a sensitive measure of vascular responsiveness.

Similar pattern of change in V̇o2 kinetics, vascular function, and tissue oxygen provision following an endurance training stimulus in older and young adults

The purpose of this study was to examine the time course of changes in the oxygen uptake (V̇o2) kinetics response subsequent to short-term exercise training (i.e., 24, 48, 72, and 120 h posttraining) and examine the relationship with the time course of changes in microvascular [deoxygenated hemoglobin concentration ([HHb])-to-V̇o2 ratio ([HHb])/V̇o2)] and macrovascular [flow-mediated dilation (FMD)] O2 delivery to the active tissues/limbs. Seven healthy older [OA; 74 ± 6 (SD) yr] and young men (YA; 25 ± 3 yr) completed three endurance cycling exercise training sessions at 70% V̇o2peak Moderate-intensity exercise on-transient V̇o2 (measured breath by breath) and [HHb] (measured by near-infrared spectroscopy) were modeled with a monoexponential and normalized (0-100% of response), and the [HHb])/V̇o2 was calculated. Ultrasound-derived FMD of the popliteal artery was assessed after 5 min of cuff occlusion. %FMD was calculated as the greatest percent change in diameter from baseline. Time constant of V̇o2 (τV̇o2) was significantly reduced in both OA (~18%) and YA (~23%) at 24 h (P < 0.001) posttraining and remained decreased at 48 h before returning toward pretraining (PRE) values. Both groups showed a significant decrease in the [HHb])/V̇o2 at 24, 48, and 72 h (P = 0.001, 0.01, and 0.03, respectively) posttraining before returning toward PRE values at 120 h. %FMD followed a similar time course to that of changes in the [HHb])/V̇o2, being significantly greater in both OA (by ~64%) and YA (by ~26%) at 24 h (P < 0.001), remaining increased at 48 and 72 h (P = 0.02 and 0.03, respectively), and returning toward PRE values at 120 h. These data suggest the rate of adjustment of V̇o2 may be constrained by O2 availability in the active tissues.

Allometric scaling of flow-mediated dilation: is it always helpful?

Flow‐mediated dilation (FMD) is calculated as the greatest percent change in arterial diameter following an ischaemic challenge. This Traditional %FMD calculation is thought to have statistical bias towards baseline diameter (Dbase), which is reduced by allometric scaling. This study examined whether allometric scaling FMD influenced the difference between a group of healthy young and older adults compared to the Traditional %FMD, and to determine whether a New (allometric) scaling %FMD improved the ability to obtain individually scaled FMD. Popliteal artery FMD was assessed in 18 young (26 ± 3 years) and 17 older adults (77 ± 5 years). ‘Corrected’ mean FMD was generated from a log‐linked ANCOVA model. Individual %FMD was evaluated using three calculations: (1) Traditional %FMD calculation; (2) Atkinson (allometric) scaling %FMD (peak diameter (Dpeak)/(Dbasescalingexponent) ); and (3) New scaling %FMD ((Dpeak−Dbase)/(Dbasescalingexponent)) . Traditional %FMD was significantly larger in young (5·82 ± 2·58%) versus old (3·72 ± 1·26%). ‘Corrected’ FMD means (Y: 5·97 ± 2·12%; O: 3·98 ± 2·06%) were similar to Traditional %FMD; however, the logarithmic transformation prevents statistical interpretation of group differences. Individually scaled %FMD using the Atkinson scaling resulted in values that were corrected for variations in Dbase but that were twofold to threefold larger than those of the Traditional calculation. New scaling %FMD resulted in values that were similar to values expected (Y: 6·21 ± 2·75%; O: 3·98 ± 1·36%); however, it did not effectively correct for variation in Dbase. Recommendations regarding the advantages of allometrically scaling %FMD should be made with caution until research clearly establishes the benefits of this approach.

Response to Letter from Tremblay & King: Near-infrared spectroscopy: can it measure conduit artery endothelial function?: Response to Letter

We thank Tremblay & King (2016) for their interest in our publication (McLay et al. 2016a), which has since been followed up with studies by McLay and colleagues (McLay et al. 2016c,b). In these studies, it is suggested that the near-infrared spectroscopy (NIRS)-derived measure of tissue oxygen saturation (StO2 ), specifically the reperfusion rate (slope 2) following a brief period of ischaemia, can be easily applied as a reliable, non-invasive measure of vascular reactivity (McLay et al. 2016a,b,c). As mentioned by Tremblay & King (2016), the StO2 reperfusion rate, being measured distal to the site of occlusion, provides insight on microvascular reperfusion, and it was an error in our wording that wrongfully implied that the NIRS measure of slope 2 could be used as a surrogate for conduit artery endothelium-dependent vasodilatation. We appreciate this opportunity to clarify the interpretation of the present study to avoid further confusion on this topic. The primary concern raised by Tremblay & King (2016) is that microvascular and macrovascular function are different entities, and they conclude that the significant correlation observed in the present study is likely to reflect the fact that the StO2 reperfusion rate is related to the stimulus for flow-mediated dilation (FMD). This is true and was discussed in our paper; refer to paragraph 2 of the Discussion in McLay et al. (2016a). We accept that our statement in the Abstract and New Findings section that ‘NIRS-derived slope 2 StO2 can be used as a measure of vascular endothelial function’ was a poorly crafted sentence, as we offered no evidence to support an endothelium-dependent mechanism from this NIRS-derived measure. However, it is evident to us that there are different mechanisms governing the responses being measured by the two methods, and that the StO2 reperfusion slope is a microvascular response, whereas %FMD is measuring the arterial vascular responsiveness of a conduit artery (i.e. macrovascular response). Indeed, we stated that the significant correlation between the two measures was most likely to be a result of the relationship between what is being measured at the microvascular level by NIRS and the stimulus for FMD. The focus of this paper (McLay et al. 2016a) was not FMD, which was used primarily as an indirect means of validation, but instead to discuss the new NIRS technique for assessing vascular reactivity. Owing to space limitations, we chose not to emphasize and discuss in detail the shear stress stimulus for FMD. It is unfortunate that our understanding of the different mechanisms at play for the two measures was not clearly delivered, and we appreciate the opportunity to discuss them in more detail here. Although other researchers using similar NIRS measurements have concluded that the slope 2 represents endothelial function, we tried not to do so because we are aware that measurements distal and proximal to the site of occlusion evoke different mechanisms of vasodilation, even at the same level of the vasculature. The FMD of a conduit artery is an endothelium-dependent response, and the mechanisms governing this response have been heavily researched. As mentioned by Tremblay & King (2016), the majority of studies consider this dilatory response to an ischaemic challenge to be largely nitric oxide (NO) dependent; however, some studies have been unable to abolish a vasodilatory response through pharmacological blockades targeting the NO pathway (Pyke et al. 2009). Although we would accept that the FMD response to ischaemia is largely attributable to a single dilatory pathway, the mechanisms governing the control of microvascular blood flow distribution are not fully understood, and there are multiple different mechanisms influencing the microvascular response. The endothelium appears to play a major role both by detecting reductions in local oxygen availability and by inducing vasodilatation in the microcirculation by releasing NO (Blitzer et al. 1996; Justice et al. 2000). However, additional pathways that may be influencing the StO2 reperfusion rate may include metabolites, and even red blood cells themselves have been identified as regulators of oxygen delivery and distribution rather than merely transporters (Bergfeld & Forrester, 1992). Thus, even though endothelium-dependent mechanisms cannot be directly linked to the StO2 reperfusion responses, some lines of research suggest that they might have a certain degree of participation. Further research is needed to elucidate the mechanistic components that control the StO2 response following a period of blood flow occlusion. With this in mind, we would like to point out that we do not consider the reperfusion rate of the StO2 signal (as assessed by the graphical display of the slope 2 over a given period of time) simply as an indicator of the stimulus for conduit artery FMD but also as an autonomous indicator of vascular reperfusion within the microcirculation. It seems evident that this measure reflects vascular responsiveness within the microvasculature (independently of the mechanisms that control it) and, given that the microcirculation plays a critical role in the redistribution of blood flow, evaluating vascular responsiveness within the microcirculation is crucial. In other words, measures of FMD have been widely used as these vessels are easily accessible in humans. However, we argue that the main focus of interest in terms of vascular responsiveness lies within the microcirculation. Although measurements of FMD provide information on the dilator function of peripheral conduit arteries, which relates to the function of coronary arteries and clinical macrovascular events, a measurement of responsiveness in the microcirculation provides important information on ‘functional’ perfusion at the ever-important capillaries. In expressing their hesitation for the use of NIRS to measure vascular responsiveness, Tremblay & King (2016) also addressed a follow-up investigation that should be done to correlate the StO2 reperfusion rate with the stimulus for FMD by measuring blood velocity to calculate the postischaemic shear rate response. As our NIRS technique is very much in its infancy, we agree that there are several studies that should, and

Response to letter from Tremblay & King: NIRS: can it measure conduit artery endothelial function?

We thank Tremblay & King (2016) for their interest in our publication (McLay et al. 2016a), which has since been followed up with studies by McLay and colleagues (McLay et al. 2016c,b). In these studies, it is suggested that the near-infrared spectroscopy (NIRS)-derived measure of tissue oxygen saturation (StO2 ), specifically the reperfusion rate (slope 2) following a brief period of ischaemia, can be easily applied as a reliable, non-invasive measure of vascular reactivity (McLay et al. 2016a,b,c). As mentioned by Tremblay & King (2016), the StO2 reperfusion rate, being measured distal to the site of occlusion, provides insight on microvascular reperfusion, and it was an error in our wording that wrongfully implied that the NIRS measure of slope 2 could be used as a surrogate for conduit artery endothelium-dependent vasodilatation. We appreciate this opportunity to clarify the interpretation of the present study to avoid further confusion on this topic. The primary concern raised by Tremblay & King (2016) is that microvascular and macrovascular function are different entities, and they conclude that the significant correlation observed in the present study is likely to reflect the fact that the StO2 reperfusion rate is related to the stimulus for flow-mediated dilation (FMD). This is true and was discussed in our paper; refer to paragraph 2 of the Discussion in McLay et al. (2016a). We accept that our statement in the Abstract and New Findings section that ‘NIRS-derived slope 2 StO2 can be used as a measure of vascular endothelial function’ was a poorly crafted sentence, as we offered no evidence to support an endothelium-dependent mechanism from this NIRS-derived measure. However, it is evident to us that there are different mechanisms governing the responses being measured by the two methods, and that the StO2 reperfusion slope is a microvascular response, whereas %FMD is measuring the arterial vascular responsiveness of a conduit artery (i.e. macrovascular response). Indeed, we stated that the significant correlation between the two measures was most likely to be a result of the relationship between what is being measured at the microvascular level by NIRS and the stimulus for FMD. The focus of this paper (McLay et al. 2016a) was not FMD, which was used primarily as an indirect means of validation, but instead to discuss the new NIRS technique for assessing vascular reactivity. Owing to space limitations, we chose not to emphasize and discuss in detail the shear stress stimulus for FMD. It is unfortunate that our understanding of the different mechanisms at play for the two measures was not clearly delivered, and we appreciate the opportunity to discuss them in more detail here. Although other researchers using similar NIRS measurements have concluded that the slope 2 represents endothelial function, we tried not to do so because we are aware that measurements distal and proximal to the site of occlusion evoke different mechanisms of vasodilation, even at the same level of the vasculature. The FMD of a conduit artery is an endothelium-dependent response, and the mechanisms governing this response have been heavily researched. As mentioned by Tremblay & King (2016), the majority of studies consider this dilatory response to an ischaemic challenge to be largely nitric oxide (NO) dependent; however, some studies have been unable to abolish a vasodilatory response through pharmacological blockades targeting the NO pathway (Pyke et al. 2009). Although we would accept that the FMD response to ischaemia is largely attributable to a single dilatory pathway, the mechanisms governing the control of microvascular blood flow distribution are not fully understood, and there are multiple different mechanisms influencing the microvascular response. The endothelium appears to play a major role both by detecting reductions in local oxygen availability and by inducing vasodilatation in the microcirculation by releasing NO (Blitzer et al. 1996; Justice et al. 2000). However, additional pathways that may be influencing the StO2 reperfusion rate may include metabolites, and even red blood cells themselves have been identified as regulators of oxygen delivery and distribution rather than merely transporters (Bergfeld & Forrester, 1992). Thus, even though endothelium-dependent mechanisms cannot be directly linked to the StO2 reperfusion responses, some lines of research suggest that they might have a certain degree of participation. Further research is needed to elucidate the mechanistic components that control the StO2 response following a period of blood flow occlusion. With this in mind, we would like to point out that we do not consider the reperfusion rate of the StO2 signal (as assessed by the graphical display of the slope 2 over a given period of time) simply as an indicator of the stimulus for conduit artery FMD but also as an autonomous indicator of vascular reperfusion within the microcirculation. It seems evident that this measure reflects vascular responsiveness within the microvasculature (independently of the mechanisms that control it) and, given that the microcirculation plays a critical role in the redistribution of blood flow, evaluating vascular responsiveness within the microcirculation is crucial. In other words, measures of FMD have been widely used as these vessels are easily accessible in humans. However, we argue that the main focus of interest in terms of vascular responsiveness lies within the microcirculation. Although measurements of FMD provide information on the dilator function of peripheral conduit arteries, which relates to the function of coronary arteries and clinical macrovascular events, a measurement of responsiveness in the microcirculation provides important information on ‘functional’ perfusion at the ever-important capillaries. In expressing their hesitation for the use of NIRS to measure vascular responsiveness, Tremblay & King (2016) also addressed a follow-up investigation that should be done to correlate the StO2 reperfusion rate with the stimulus for FMD by measuring blood velocity to calculate the postischaemic shear rate response. As our NIRS technique is very much in its infancy, we agree that there are several studies that should, and

Response to Letter from Tremblay & King: Near‐infrared spectroscopy: can it measure conduit artery endothelial function?

We thank Tremblay & King (2016) for their interest in our publication (McLay et al. 2016a), which has since been followed up with studies by McLay and colleagues (McLay et al. 2016c,b). In these studies, it is suggested that the near-infrared spectroscopy (NIRS)-derived measure of tissue oxygen saturation (StO2 ), specifically the reperfusion rate (slope 2) following a brief period of ischaemia, can be easily applied as a reliable, non-invasive measure of vascular reactivity (McLay et al. 2016a,b,c). As mentioned by Tremblay & King (2016), the StO2 reperfusion rate, being measured distal to the site of occlusion, provides insight on microvascular reperfusion, and it was an error in our wording that wrongfully implied that the NIRS measure of slope 2 could be used as a surrogate for conduit artery endothelium-dependent vasodilatation. We appreciate this opportunity to clarify the interpretation of the present study to avoid further confusion on this topic. The primary concern raised by Tremblay & King (2016) is that microvascular and macrovascular function are different entities, and they conclude that the significant correlation observed in the present study is likely to reflect the fact that the StO2 reperfusion rate is related to the stimulus for flow-mediated dilation (FMD). This is true and was discussed in our paper; refer to paragraph 2 of the Discussion in McLay et al. (2016a). We accept that our statement in the Abstract and New Findings section that ‘NIRS-derived slope 2 StO2 can be used as a measure of vascular endothelial function’ was a poorly crafted sentence, as we offered no evidence to support an endothelium-dependent mechanism from this NIRS-derived measure. However, it is evident to us that there are different mechanisms governing the responses being measured by the two methods, and that the StO2 reperfusion slope is a microvascular response, whereas %FMD is measuring the arterial vascular responsiveness of a conduit artery (i.e. macrovascular response). Indeed, we stated that the significant correlation between the two measures was most likely to be a result of the relationship between what is being measured at the microvascular level by NIRS and the stimulus for FMD. The focus of this paper (McLay et al. 2016a) was not FMD, which was used primarily as an indirect means of validation, but instead to discuss the new NIRS technique for assessing vascular reactivity. Owing to space limitations, we chose not to emphasize and discuss in detail the shear stress stimulus for FMD. It is unfortunate that our understanding of the different mechanisms at play for the two measures was not clearly delivered, and we appreciate the opportunity to discuss them in more detail here. Although other researchers using similar NIRS measurements have concluded that the slope 2 represents endothelial function, we tried not to do so because we are aware that measurements distal and proximal to the site of occlusion evoke different mechanisms of vasodilation, even at the same level of the vasculature. The FMD of a conduit artery is an endothelium-dependent response, and the mechanisms governing this response have been heavily researched. As mentioned by Tremblay & King (2016), the majority of studies consider this dilatory response to an ischaemic challenge to be largely nitric oxide (NO) dependent; however, some studies have been unable to abolish a vasodilatory response through pharmacological blockades targeting the NO pathway (Pyke et al. 2009). Although we would accept that the FMD response to ischaemia is largely attributable to a single dilatory pathway, the mechanisms governing the control of microvascular blood flow distribution are not fully understood, and there are multiple different mechanisms influencing the microvascular response. The endothelium appears to play a major role both by detecting reductions in local oxygen availability and by inducing vasodilatation in the microcirculation by releasing NO (Blitzer et al. 1996; Justice et al. 2000). However, additional pathways that may be influencing the StO2 reperfusion rate may include metabolites, and even red blood cells themselves have been identified as regulators of oxygen delivery and distribution rather than merely transporters (Bergfeld & Forrester, 1992). Thus, even though endothelium-dependent mechanisms cannot be directly linked to the StO2 reperfusion responses, some lines of research suggest that they might have a certain degree of participation. Further research is needed to elucidate the mechanistic components that control the StO2 response following a period of blood flow occlusion. With this in mind, we would like to point out that we do not consider the reperfusion rate of the StO2 signal (as assessed by the graphical display of the slope 2 over a given period of time) simply as an indicator of the stimulus for conduit artery FMD but also as an autonomous indicator of vascular reperfusion within the microcirculation. It seems evident that this measure reflects vascular responsiveness within the microvasculature (independently of the mechanisms that control it) and, given that the microcirculation plays a critical role in the redistribution of blood flow, evaluating vascular responsiveness within the microcirculation is crucial. In other words, measures of FMD have been widely used as these vessels are easily accessible in humans. However, we argue that the main focus of interest in terms of vascular responsiveness lies within the microcirculation. Although measurements of FMD provide information on the dilator function of peripheral conduit arteries, which relates to the function of coronary arteries and clinical macrovascular events, a measurement of responsiveness in the microcirculation provides important information on ‘functional’ perfusion at the ever-important capillaries. In expressing their hesitation for the use of NIRS to measure vascular responsiveness, Tremblay & King (2016) also addressed a follow-up investigation that should be done to correlate the StO2 reperfusion rate with the stimulus for FMD by measuring blood velocity to calculate the postischaemic shear rate response. As our NIRS technique is very much in its infancy, we agree that there are several studies that should, and