Echocardiographic assessment of coronary microvascular dysfunction: Basic concepts, technical aspects, and clinical settings

Abstract Coronary flow reserve is the capacity of the coronary circulation to augment the blood flow in response an increase in myocardial metabolic demands and has a powerful prognostic significance in different clinical situations. It might assess with invasive and noninvasive technique. Transthoracic echocardiography Doppler is an emerging diagnostic technique, noninvasive, highly feasible, safe for patient and physician, without radiation, and able to detect macrovascular and microvascular anomalies in the coronary circulation. This review aims to describe the benefit and limits of echocardiographic assessment of coronary flow reserve.


| INTRODUC TI ON
Coronary flow reserve (CFR), assessed by transthoracic echocardiography Doppler (TTE), is a noninvasive diagnostic technique able to reflect presence of macrovascular as well as microvascular disease in the coronary circulation, with the advantages of being highly feasible, safe for patient and physician, and not associated with any radiation.
It may represent a diagnostic tool for coronary microvascular dysfunction (CMD), while in the setting of ischemic cardiac disease is useful in identification and assessment of functional significance of coronary lesions as well as for the follow-up of patients after coronary interventions. In addition, CFR has also showed a powerful prognostic significance in different clinical situations.

| CFR AND MI CRO CIRCUL ATI ON: G ENER AL CON CEP TS AND PHYS I OLOGY
Coronary circulation is characterized by complex morphology and physiology. Coronary arteries bifurcate into smaller vessels, 1 divided into large arteries (diameter >500 µm) with capacitance function, small arteries, or prearterioles (diameter between 100 and 500 µm), that represent the intermediate compartment with a measurable pressure drop along their length and arterioles (diameter <100 µm). 1 The coronary microcirculation includes vessels with diameters below ∼300 µm ranging from small arterioles to venules and is the site of regulation of flow resistance. 1 The coronary microcirculation plays a key role in the myocardial perfusion.
Myocardial perfusion is predominantly diastolic with unique profile of coronary blood flow (CBF) velocity. 2 Various mechanisms (metabolic, neurohumoral, and myogenic) intervene in the regulation of coronary flow in the prearteriolar and arteriolar microcirculation that allow stable flow across a large range of perfusion pressures. 3 The myogenic autoregulation consists in the change of distal prearterioles diameter in response to flow changes and to increased pressure. 3 The presence of functional and/or structural abnormalities of this circulatory pathway may impair the myocardial perfusion, a condition referred as coronary microvascular dysfunction (CMD). This term was referred to a large number of clinical scenarios characterized by evidence of a reduced CFR in the absence of obstructive epicardial disease. 4 CMD includes any pathology that may damage the microvasculature, including endothelial dysfunction, coronary spasm, inflammation, and atherosclerosis. 5 The coronary microvasculature cannot be directly visualized in vivo (vessels <300 µm), but the CMD may be assessed by many invasive and noninvasive technique, evaluating the CFR, which represents an integrated measure of CBF in both the macro-and microcirculation and consists in the ratio of hyperemic to baseline blood flow. 6 In the absence of obstructive stenosis of the epicardial arteries, reduced CFR is a marker of CMD, but because obstructive disease of the epicardial arteries and CMD often coexist, discrimination between the effects of these two conditions on myocardial perfusion can be difficult. CFR is influenced by several factors: metabolic demand, diastolic time, systolic arterial blood pressure, heart rate, age, and sex. 7 Pharmacological stressors are usually used to produce hyperemic response for CFR calculation. In clinical practice, the most used agents are dipyridamole and adenosine. 6 Both inhibit phosphodiesterase, increasing cellular cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), with consequent coronary dilation. 8 Coronary vasodilators are usually administered intravenously in noninvasive techniques, but adenosine is generally given intracoronary when measuring CFR in the catheterization laboratory. Cold pressor test is a nonpharmacologic stressor. 6 It induces hyperemic vasodilation that depends totally on the endothelial release of nitric oxide, whereas both adenosine and dipyridamole stimuli are partially dependent on endothelial function. 9 Cold pressor test is performed according to a standardized protocol, by placing the subject's hand and distal part of the forearm in ice-water slurry for 3 minutes. 9

| CMD: PATHOG ENE S IS AND MECHANIS MS
Multiple mechanisms are potentially responsible for CMD and can be classified in structural, functional, and extravascular changes.

| CRF A SS E SS MENT BY ECHOC ARDIOG R APHY: TECHNIC AL A S PEC TS
The left anterior descending (LAD) coronary CBF profile can be recorded with pulsed-wave Doppler, either with transesophageal echocardiography (TEE), sampling the proximal tract, or TTE exploring the mid-distal tract. Recently, the success rates to measure CBF of right coronary artery (RCA) and left coronary artery (LCA) have been increasing owing to the advancement in ultrasonic technology. 12,13 CBF velocity evaluated with Doppler is represented by a biphasic wave, with a lower peak during systole and a higher peak during diastole, for the effect of myocardial contraction. 14 Several parameters might be measured from Doppler tracings of LAD artery flow, including systolic flows, time-velocity integrals, and mean flows, but the most used parameter is the peak diastolic flow: It is easy to measure and reproducible, and it has a closest correlation with CFR measured with positron emission tomography (PET). 15 Second harmonic imaging and high frequency transducers (up to 8 MHz) provide better definition of coronary artery and improved resolution. Contrast agents also improved the signal-to-noise ratio, increasing the feasibility of transthoracic imaging of the LAD artery. 16

| Detection of coronary arteries
The principal coronary artery investigated is the LAD branch. Feasibility of TTE in assessing coronary arteries with the addition of harmonics and contrast agents has been reported to be as high as 100% for distal LAD and 33%-97% for the PDA. 17,18

| Pitfalls in detecting coronary arteries
There are many pitfalls regarding the CFR execution by TTE. First, regarding the assessment of coronary arteries, some branches of coro-

| CBF velocity and CFR assessment with pulsedwave color Doppler
To obtain a correct CBF velocity and CFR assessment, it is necessary that the correct Doppler angle and the sample volume are maintained in the correct position throughout the infusion of the vasodilator agent. 19 The sample volume should be sized (~ 3.0 mm) and placed within the coronary artery. 21 Time velocity integral, peak velocity, and mean velocity in systole and diastole should assessed at rest and during the pharmacological stimuli. The diastolic blood flow velocity is the most used parameter. Furthermore, it is possible to assess the diastolic and systolic flow length of time 22 (Figure 2). The normal value of the peak diastolic velocity in distal LAD was 21.2 ± 7.9 cm/s, and the duration of diastolic coronary artery flow was 58.5 ± 6.4% of the R-R interval at rest and with normal heart rates (60-100 b/m), and also coronary flow velocities showed a nonsignificant decrease from proximal segments to distal segments. 23 Adenosine protocol usually consists in intravenously infusion at the rate of 140 mcg/kg/min for 6 minutes, measuring CBF velocity before and after of adenosine infusion. Adenosine has a short half-life, compared with dipyridamole (up to 30 minutes) and CFR achieved with adenosine can be obtained with dipyridamole infusion rate of 0.84 mg/kg for 6 minutes or in two separated doses, as suggested by the 2008 recommendations of the European Association of Echocardiography. 24 Table 1 summarized the stress echocardiography protocols for CFR assessment. F I G U R E 2 an example of normal coronary flow reserve in a healthy subject. Velocity patterns are registered by pulsed-wave Doppler. Resting laminar peak diastolic velocity in normal coronary artery is from 0.21 ± 0.08 m/s to 0.28 ± 0.09 m/s, and the velocity will not exceed 1 m/s even in case of its threefourfold increase in the stenosis site. CFR is expressed as the ratio of coronary flow velocity under maximal vasodilatation to coronary flow velocity at rest. In this case, the basal peak diastolic flow velocity is normal, and it increases more than 2 times after dipyridamole, and therefore, CFR is normal. LV = left ventricle; Vp(d) = velocity peak (diastolic); CFVR = coronary flow velocity ratio After measuring the baseline and the hyperemic coronary flow velocities, CFR should be calculated as the ratio of CBF velocity under maximal vasodilatation to the same parameter at rest. 25 Many authors assume that the peak velocity (VpD) can be used for CFR evaluation and the cutoff <2 for predicts significant LAD stenosis in distal LAD and in PDA, 26,27 whereas CFR <1 denotes LAD or PDA occlusion. 28,29 In patient with CAD, CFR is related to the severity of coronary disease, while in case of angiographically normal arteries it is a marker of microvascular dysfunction. 30 Recently, the ABCDE protocol, validated by the Stress Echo 2020 study group of the Italian Society of Echocardiography and Cardiovascular Imaging, has been proposed during stress echocardiography to investigate dyspnea of cardiac origin, analyzing 5 parameters: regional wall motion (step A); pulmonary congestion with B-lines (step B); LV contractile reserve (step C); coronary microvascular dysfunction with CFR (step D); and EKG-based heart rate reserve during exercise (step E). 31 Reduced CFR is often accompanied by regional wall motion anomalies, abnormal left ventricle contractile reserve, and pulmonary congestion during stress and shows independent value over segmental motion anomalies in predicting an adverse outcome. 32

| CFR A SS E SS ED BY TEE
CBF velocity can be assessed by TEE with Doppler examination of the proximal LAD coronary artery. This is a simple, reproducible, safe, and reliable method for coronary flow reserve assessment and is correlated with CBF velocity derived from intracoronary Doppler guide wire studies. 33 Coronary artery imaging was performed after routine cardiac examination. The LMCA was visualized, with the probe approximately 28 cm from the mouth, at a level above the aortic leaflets. 34 The LMCA, arising from the corresponding sinus, is seen as an echo-free structure and small adjustments in transducer orientation are necessary to visualize the artery, its bifurcation, and LAD. CBF velocity was evaluated by pulsed-wave Doppler of the ostial part of the LAD. 34

| MYOC ARDIAL CONTR A S T ECHOC ARDIOG R APHY (MCE )
Quantitative contrast echocardiography has been shown to be useful for assessing global and regional CFR using different pharmacologic stimuli. This technique seems feasible for the assessment of mechanistic insights at coronary microcirculation. 35 MCE utilizes gas-filled microbubbles to produce myocardial opacification on ultrasound examination. After complete destruction of microbubbles using a beam of high-intensity ultrasounds, the replenishment of myocardial microcirculation may be assessed as a time-intensity curve, with a mathematical function. 36 In patients without significant stenosis of coronary arteries, contrast infusion determines homogeneous opacification of the myocardial wall, because microbubbles continuously replaced those that are destroyed by ultrasounds. In case of microvascular defects, CBF increases during pharmacological/physical stress and determines regional differences of opacification. An excellent correlation was TA B L E 1 Dipyridamole and adenosine stress echocardiography protocol General methodology during stress echo Perform 12 lead ECG in resting condition and each minute throughout examination. An ECG is continuously displayed on echo monitor. Cuff blood pression is measured in resting condition and at each stage.

Contraindications
Active bronchospasm. > or equal 2nd degree AV block. SBP <90 mm Hg. Use of methylxanthine. Remote history of reactive airway disease Chronic dipyridamole therapy, recent (<12 h) coffee, tea, chocolate ingestion. Avoid dipyridamole stress echocardiography in patients presenting severe bilateral carotid disease with unknown Willis polygon circulation, due to the potential risk of brain ischemia.

Echocardiography imaging acquisition
Assess the distal LAD artery and the CBF velocity (peak velocity) at rest and after starting dipyridamole/ adenosine infusion. If possible, RCA and LCA can be visualized. Perform regional wall motion analysis at rest and at peak. Echocardiography is continued monitored and intermittently stored. found with PET. 36 MCE did not receive regulatory authorities approval for clinical use, and the technique remains an option for research. 36

| THE PROG NOS TIC ROLE OF CFR IN DIFFERENT CLINIC AL S E T TING S
Cardiac microvascular function may be altered in many clinical conditions. CFR assessment has an important prognostic value and adds incremental information over the value of inducible wall motion abnormalities. Indeed, CMD evaluated by TTE is independent predictor of cardiovascular events in patients with suspected CAD. In a study by Li-Ming Gan, 371 patients underwent to myocardial perfusion scintigraphy for suspected myocardial ischemia.
The major cardiovascular events rate was 7.5% in patients without myocardial ischemia and normal CFR, whereas event rate was 24.2% in patients without ischemia but with reduced CFR, and 46.5% in patients with both myocardial perfusion scintigraphydetected myocardial ischemia and a reduced CFR (P < .001). 37 To  CFR is also impaired in patients with severe mitral regurgitation and improves after successful mitral surgery for the reduction of left ventricular preload, volume, and hypertrophy. 50

| CMD in cardiomyopathies
Reduction in CFR is a recognized feature in HCM and is a strong predictor for future cardiovascular events. 51 Patients with HCM show important wall thickening of coronary arterioles, due to intimal hyperplasia and interstitial fibrosis reduces capillary density, with angina, progressive deterioration of cardiac function 52 (Figure 3). Severe CMD is situated in the hypertrophied septum, but also in the nonhypertrophied left ventricle free wall using PET 53 and CMR. 54 Severe impairment of microvascular function is more frequent in HCM patient with sarcomere mutations. 55 CFR was lower in patients with obstructive HCM compared with nonobstructive HCM, 56 and alcohol septal ablation results in improvement of CFR. 57 CFR is decreased in patients with nonischemic dilated cardiomyopathy. 58 An impaired CFR identifies patients at higher risk of, such as death and worsening of clinical conditions. 59

| CMD in heart transplant
Cardiac allograft vasculopathy (CAV) is characterized by diffuse concentric myo-intimal thickening of the distal epicardial artery and the small endocardial vessels. 60 Annual coronary angiography is the most common approach to evaluate the progression of CAV.
Recently, the assessment of CFR on TTE has been proposed as an alternative method to diagnosis CAV. A recent study has shown that CFR is sensitive to assess microvascular and macrovascular dysfunction, and when CFR is combined with dobutamine stress echocardiography, CAV can be accurately diagnosed with high specificity. 61 This method correlates well with flow acquired from an intracoronary Doppler wire but is poorly correlated with myocardial perfusion reserve calculated by PET. 3 Some side effects (hypotension and/or bradycardia, headache, dizziness, and/or nausea) preclude maximal pharmacological stress in less than 5% of patients. 67

| CON CLUS IONS
TTE, a noninvasive and widely used method, can be used for the diagnosis of CMD.
After an adequate period of training, detection and measure-

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing not applicable to this article as no datasets were generated or analyzed during the current study. The data that support the findings of this study are openly available.