Correlation between peripheral vascular resistance and time to peak flow during reactive hyperaemia

Impaired peripheral microvascular reactivity in patients with nonobstructive coronary artery disease

OBJECTIVE

This study aimed to determine whether peripheral microvascular reactivity is impaired in patients with nonobstructive coronary artery disease (NOCAD).

METHODS

Stable patients presenting with angina were recruited and, based on results from coronary angiography, were categorized into OCAD (coronary stenosis of ≥50%) and NOCAD (stenosis <50%) groups. A control group with no history of angina was also recruited. Forearm skin microvascular reactivity was measured using the laser Doppler blood perfusion monitor and the process of postocclusive skin reactive hyperemia (PORH).

RESULTS

Patients were categorized into OCAD (n = 42), NOCAD (n = 40), and control (n = 39) groups. Compared with the control group, the PORH perfusion percent change (PORH% change) was significantly lower in the OCAD and NOCAD groups. No significant differences were noted between the OCAD and NOCAD groups. Additionally, the NOCAD group without any coronary obstruction takes a longer time to reach peak perfusion and had lower PORH% change compared with the nonangina control group.

CONCLUSION

Angina patients with NOCAD have microvascular dysfunction as demonstrated by reduced magnitude of reperfusion with an ischemic stimulus. NOCAD patients without coronary obstruction also displayed a slower response to reperfusion.

Reproducibility and normalization of reactive hyperemia using laser speckle contrast imaging

Background

Impaired perfusion indices signal potential microvascular dysfunction preceding atherosclerosis and other cardiometabolic pathologies. Post-occlusive reactive hyperemia (PORH), a vasodilatory response following a mechanically induced ischemia, is a transient increase in perfusion and can assess microvascular function. The greatest blood flow change corresponding to the first minute of hyperemia (represented by time-to-peak, hyperemic velocity, AUC within 1^st min) has been shown to indicate microvascular dysfunction. However, the reproducibility of these temporal kinetic indices of the PORH response is unknown. Our aim was to examine the inter- and intra-day reproducibility and standardization of reactive hyperemia, with emphasis on the kinetic indices of PORH, using laser speckle contrast imaging (LSCI) technique.

Methods and results

Seventeen healthy adults (age = 24 ± 3 years) completed three PORH bouts over two lab visits. LSCI region of interest was a standardized 10 cm region on the dominant ventral forearm. A 5-min brachial artery occlusion period induced by inflating an arm cuff to 200 mmHg, preceded a 4-min hyperemic period. Inter- and intra-day reliability and reproducibility of cutaneous vascular conductance (LSCI flux / mean arterial pressure) were determined using intraclass correlation (ICC) and coefficient of variation (CV%). Maximal flow and area under the curve standardized to zero perfusion showed intra- and inter-day reliability (ICC > 0.70). Time to maximal flow (TMF) was not reproducible (inter-day CV = 18%). However, alternative kinetic indices such as 1-min AUC and overshoot rate-of-change (ORC), represented as a piecewise function (at 5s, 10s, 15s, and 20s into hyperemia), were reproducible (CV< 11%). Biological zero was a reliable normalization point.

Conclusion

PORH measured with LSCI is a reliable assessment of microvascular function. However, TMF or its derived hyperemic velocity are not recommended for longitudinal assessment. Piecewise ORC and 1-min AUC are reliable alternatives to assess the kinetic response of PORH.

BOLD MRI applied to a murine model of peripheral artery disease

Peripheral artery disease (PAD) is the narrowing or complete occlusion of vessels due to the progression of atherosclerosis. Ultimately, the reduction in blood supply, due to a reduced lumen diameter, results in a functional deficit, e.g., reduced mobility. Because function is closely tied to blood flow through large-caliber vessels, therapeutic development to treat PAD has recently focused on arteriogenesis rather than angiogenesis. Optimally, the preclinical investigations related to such therapeutic development would take place in murine models of PAD to allow for future studies utilizing transgenic strains. However, it can be challenging to quantify functional recovery of the peripheral vascular network in murine models. The purpose of this work is to provide a protocol of temporally and spatially resolved methods for functional assessment of arteriogenesis in a murine model.

Reactive hyperemia and BOLD MRI demonstrate that VEGF inhibition, age, and atherosclerosis adversely affect functional recovery in a murine model of peripheral artery disease

PURPOSE

To develop magnetic resonace imaging (MRI) methods for functional assessment of arteriogenesis in a murine model of peripheral artery disease to quantify the influences of vascular endothelial growth factor (VEGF), age, and atherosclerosis.

MATERIALS AND METHODS

Reactive hyperemia (RH), which was induced using a device designed for remote and transient occlusion of the aorta and vena cava, was measured by blood-oxygen-level-dependent MRI. Twenty-eight days after femoral artery ligation, peak height (PH) and time to peak (TTP) of the RH response was compared with sham-operated animals in 10-week-old C57Bl6, 9-month-old C57Bl6, and 9-month-old Ldlr(-/-)Apobec(-/-) mice. The contribution of VEGF to functional recovery was assessed in young mice. Angiogenesis was quantified using an anti-PECAM1 radioimmunoassay.

RESULTS

In young animals, angiogenesis was maximal 7 days after ligation, whereas functional recovery took 28 days. Inhibition of VEGF eliminated the angiogenesis seen at 7 days and reduced RH (PH, P < 0.05). At day 28, RH was altered in old (TTP, P < 0.05) and atherosclerotic (PH, P < 0.05; TTP, P < 0.05) animals. RH was different in young, old, and atherosclerotic sham animals. Old and atherosclerotic mice showed reduced angiogenesis.

CONCLUSION

The method presented herein can provide a sensitive assay for the functional assessment of arteriogenesis and highlights the contribution of VEGF, age, and atherosclerosis to this process.

Lumped parameter thermal model for the study of vascular reactivity in the fingertip

Vascular reactivity (VR) denotes changes in volumetric blood flow in response to arterial occlusion. Current techniques to study VR rely on monitoring blood flow parameters and serve to predict the risk of future cardiovascular complications. Because tissue temperature is directly impacted by blood flow, a simplified thermal model was developed to study the alterations in fingertip temperature during arterial occlusion and subsequent reperfusion (hyperemia). This work shows that fingertip temperature variation during VR test can be used as a cost-effective alternative to blood perfusion monitoring. The model developed introduces a function to approximate the temporal alterations in blood volume during VR tests. Parametric studies are performed to analyze the effects of blood perfusion alterations, as well as any environmental contribution to fingertip temperature. Experiments were performed on eight healthy volunteers to study the thermal effect of 3 min of arterial occlusion and subsequent reperfusion (hyperemia). Fingertip temperature and heat flux were measured at the occluded and control fingers, and the finger blood perfusion was determined using venous occlusion plethysmography (VOP). The model was able to phenomenologically reproduce the experimental measurements. Significant variability was observed in the starting fingertip temperature and heat flux measurements among subjects. Difficulty in achieving thermal equilibration was observed, which indicates the important effect of initial temperature and thermal trend (i.e., vasoconstriction, vasodilatation, and oscillations).

Reproducibility of different laser Doppler fluximetry parameters of postocclusive reactive hyperemia in human forearm skin

INTRODUCTION

Postocclusive reactive hyperemia in forearm skin is a commonly used model for studying microvascular reactivity function, particularly in the assessment of vascular effect of topically applied pharmacological substances. In this study, we investigated the reproducibility of several different laser-Doppler-derived parameters in the measurement of postocclusive reactive hyperemia at forearm skin in healthy subjects.

METHODS

Eighteen young healthy male volunteers were recruited and studied in a supine position while fasted. Forearm blood flow was occluded at suprasystolic pressure for 3 min. Microvascular perfusion was measured continuously using laser Doppler fluximetry. Parameters studied were maximum increase in hyperemia perfusion (PORHmax), time-to-peak (Tp), amplitude of peak perfusion (PORHpeak), percentage of hyperemic response (PORH%) and mean velocity of the hyperemia increase (PORHmax/Tp). Measurement was performed twice within each study day for 2 study days. Coefficient of variation and intraclass correlation coefficient (ICC; with 95% confidence interval) were calculated for each parameter. An ICC value above 0.75 was interpreted as "excellent reproducibility".

RESULTS

ICC analysis showed that all studied parameters, except for PORH%, demonstrated excellent reproducibility for both within- and between-day measurements. Satisfactory intraday and interday coefficients of variation (<10%) were also obtained for these parameters.

CONCLUSION

Laser-Doppler-derived PORHmax, Tp, PORHpeak and PORHmax/Tp were highly reproducible parameters for measuring microvascular reactivity during reactive hyperemia, with PORHmax shown as the most reproducible index. PORH% is, however, less reproducible. These findings have implications for the use of laser Doppler fluximetry coupled with 3-min-occlusion PORHmax as a useful and reliable noninvasive clinical measurement index of microvascular function.

Albumin excretion rate and cardiovascular risk: could the association be explained by early microvascular dysfunction?

Elevated albumin excretion rate (AER) independently predicts total and cardiovascular mortality in a variety of conditions, although the exact mechanisms are unknown. Laser Doppler fluximetry was used to study associations with risk factors and renal damage (AER calculated from a timed overnight urine collection) in 188 people without diabetes and 117 individuals with diabetes. Skin flow (flux) in response to arterial occlusion (ischemia) was measured. Three distinct patterns of postischemic peak flow were observed: 1) gradual rise to peak (normal), 2) nondominant early peak, and 3) dominant early peak. Those with a dominant early peak were more likely to have diabetes (P = 0.01), hypertension (P = 0.001), and obesity (P < 0.001) and had a higher AER (12.6 microg/min [95% CI 7.8-20.2] vs. 7.2 [5.5-9.5] nondominant early peak group and 3.7 [3.2-4.1] normal group; P < 0.001 for trend). This could not be accounted for by conventional cardiovascular risk factors (P < 0.001 after adjustment). A rapid peak flow response after ischemia is associated with an elevated AER and increased cardiovascular risk. This may represent shared mechanistic pathways and causative or con-sequential changes in the microvasculature and supports the hypothesis that microvascular dysfunction may contribute to large vessel pathophysiology.

The association between laser Doppler reactive hyperaemia curves and the distribution of peripheral arterial disease

OBJECTIVES

To determine whether postocclusive laser Doppler fluxmetry (LDF) curves can be related to the arteriographic distribution of disease.

DESIGN

Prospective study.

MATERIALS

Sixty-nine patients with symptomatic peripheral ischaemia and 15 healthy subjects.

METHODS

Laser Doppler fluxmetry (LDF) was monitored on the dorsum of the symptomatic foot following 2 min of arterial occlusion at the ankle. During reperfusion three patterns of LDF were identified (types I-III). All patients subsequently underwent arteriography which was reported independent of LDF results. The distribution of disease, particularly patency of below-knee vessels, was related to the type of LDF curve observed during reactive hyperaemia.

RESULTS

Type I curves were observed in all healthy subjects and 75% of patients with a single arterial lesion. Type II curves were found in 78% of patients with multiple lesions above the knee. The presence of either a type I or II curve was associated with a continuous vessel from knee to ankle (positive predictive value 83%, p < 0.01), whilst type III curve was associated with discontinuous infrapopliteal run-off (positive predictive value 86%, p < 0.01).

CONCLUSIONS

This pilot study suggests that post-occlusive LDF curves may identify the distribution of arterial disease and may be useful in the non-invasive management of peripheral ischaemia.

The predictive value of laser Doppler fluxmetry and transcutaneous oximetry for clinical outcome in patients undergoing revascularisation for severe leg ischaemia

OBJECTIVES

To investigate the predictive value of laser Doppler fluxmetry and transcutaneous oximetry in 41 patients undergoing technically successful revascularisation for severe leg ischaemia.

DESIGN

Toe and ankle systolic arterial pressures, transcutaneous oxygen tension (tcpO2), and stressed laser Doppler fluxmetry at the foot (time to peak laser Doppler flux following 2 min arterial occlusion, tp LDF, and the response of LDF to raising the leg 40 cm, the elevated:supine LDF ratio) were measured before revascularisation. Six months later these measurements were compared in those patients who were clinically improved, and those who still had symptoms of severe ischaemia or had lost their limb.

SETTING

Vascular Laboratory, St. George's Hospital, London SW17, U.K.

RESULTS

Six months following revascularisation 30 (73%) of the 41 patients had partial or complete symptomatic relief. Six (15%) had undergone major amputation and five (12%) still had symptoms of severe ischaemia. Before intervention toe and ankle systolic pressures were similar in the 30 who were improved and the 11 who had lost, or were still at risk of losing, their leg. Pre-revascularisation tcpO2 was significantly lower (18.3 mm Hg vs 33.5 mm Hg; p < 0.05) and tp LDF significantly longer (140 s vs 92 s; P < 0.05) in the 11 patients who were not clinically improved at 6 months. Whilst pre-revascularisation toe and ankle pressures below 30 mm Hg and 50 mm Hg respectively identified only 55% of those patients who were not improved 6 months later, a tp LDF in excess of 100 s identified 82% (p < 0.05) and was noted in five of the six amputees.

CONCLUSION

Microcirculatory assessments performed in patients with limb-threatening ischaemia are likely to be more deranged in those patients who suffer clinical failure or amputation despite an apparently successful revascularisation procedure.