Single Coronary Artery Demonstrating Slightly Decreased ¹³NH₃ Stress Flows in its Distal Flow Territories

Introduction

A single coronary artery (SCA) is an uncommon coronary anomaly that may go unnoticed, but could result in major cardiac complications when the artery has a malignant course. Little is known about myocardial perfusion in patients without or with only minor coronary plaques of such SCA. The present case demonstrates a patient with a single coronary artery showing a benign coarse and without significant plaques, yet with slightly decreased 13NH3 stress flows in its distal territories.

Case Report

A 54-year-old Caucasian male patient presented with angina-like chest pain to the cardiology outpatient clinic of our hospital. His cardiovascular risk profile included smoking, hypercholesterolemia, a body mass index of 29 kg/cm², and a family history of cardiovascular disease. The patient had no other relevant medical history and did not use any medication at presentation. Results of his physical examination, electrocardiogram, echocardiogram, and cycle ergometer test were normal. On coronary computed tomography angiography (CCTA), a single coronary artery (SCA) with benign course originating from the right sinus of Valsalva was visualized [Figure 1, left panel], whereas no left main coronary artery was originating from the left sinus of Valsalva [Figure 1, right panel]. The path of the SCA ran along the usual domain of the left circumflex artery (LCX) and subsequently along the left anterior descending artery (LAD) domain providing all regular side branches [Figure 2a]. At calcium scoring, an Agatston score of 59.9 was calculated consistent with only a mild grade of coronary artery disease. The CCTA revealed two calcified plaques with minimal stenosis in both the proximal SCA (corresponding with the right coronary artery [RCA] domain) and the mid part of the vessel corresponding with the LCX domain. The successively performed coronary angiography (CAG) [Figure 2b] confirmed the presence of only minimal luminal stenosis. The patient received medication to treat his hypercholesterolemia, comprising simvastatin. Due to concerns expressed by the patient, 35 months later, a ¹³NH₃ myocardial perfusion positron emission tomography (PET) was performed with low-dose CT attenuation correction with and without adenosine stress (0.14 mg/kg/min during 6 min) to further rule out flow limitations. Myocardial perfusion was evaluated by means of the Syngo MBF software package (Siemens Healthcare, Knoxville, Tennessee, USA) using the conventional three-vessel model [territories superimposed on the polar plots in Figure 3], which was feasible for this purpose in the present case since all regular side branches are present and appeared to be positioned normally. The ¹³NH₃ myocardial perfusion PET/CT demonstrated homogeneous tracer uptake throughout the myocardium of the left ventricle [Figure 3], with normal stress and rest left ventricular ejection fractions. Remarkably, the regional myocardial stress blood flow [Table 1] demonstrated a relative perfusion reduction of 26% and 23%, respectively, in the typical LAD and LCX flow territories compared to myocardial stress blood flow in the RCA territory. Follow-up during 20 months revealed no new findings, major cardiac events, or resubmission to the outpatient clinical heart unit.

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Figure 1 Left panel: Detailed image of the origin of the single coronary artery from the right sinus of Valsalva. Right panel: Detailed image demonstrating the absence of the left main coronary artery at the level of the left sinus of Valsalva

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Figure 2 (a) The path of the SCA runs along the usual domain of the LCX and subsequently along the LAD domain providing all regular side branches. (b) Coronary angiography images of the path of the SCA. *Point of minimal stenosis in the proximal RCA and distal in the LCX. SCA: Single coronary artery, LCX: Left circumflex artery, LAD: Left anterior descending artery, RCA: Right coronary artery

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Figure 3 ¹³NH₃ myocardial perfusion PET/CT polar plots demonstrating a visually near homogeneous tracer distribution in the myocardium of the left ventricle during stress versus rest. PET/CT: Positron emission tomography/computed tomography

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Table 1 Regional myocardial stress blood flow in coronary vessel territories

Discussion

SCA refers to the origin of both the RCA and the left coronary artery from a single ostium originating from the aortic trunk, supplying the entire heart. This anomaly is rare with a reported prevalence of 0.014%–0.066%.[12] Different types of anomalies of the SCA have been described in literature such as a single coronary arising from the left cusp and dividing into the RCA and LD with the RCA traveling posterior to the aorta, or the LCX can arise from the right aortic cusp and then progress posterior to the aorta, or the RCA can arise from the root of the aorta.[3] Generally, this variant goes unnoticed and is usually encountered during CCTA, CAG, or at autopsy. A higher incidence has been recorded in association with other types of congenital heart anomalies, for example, in patients with tetralogy of Fallot, an SCA was found in 2.4% of the patients.[1]

The prognostic significance is dependent on the course of the SCA. This translates to the majority of patients with a benign course usually being asymptomatic and patients with a malignant course being at risk of sudden death. Maron et al. reported a malignant course to be the second most common cause of sudden death in young athletes following hypertrophic cardiomyopathy.[4]

Some subgroups of SCA are more likely to lead to angina pectoris, ischemia, or even acute myocardial infarction. Fifteen percent of the patients with an SCA may have myocardial ischemia due to a malignant anatomy without atherosclerotic disease.[5] Factors such as the presence of a slit-like ostium, an ostial ridge, and an acute-angle takeoff course of the coronary artery can contribute to the possible presence of ischemia. The course of the SCA in our patient was benign and did not have a slit-like ostium or acute-angle take off.

Previous investigations of myocardial perfusion have demonstrated the ability of PET imaging to reveal coronary endothelial and microcirculatory dysfunction occurring in patients with coronary risk factors such as hypercholesterolemia, smoking, diabetes mellitus, and essential hypertension.[6] The patient was diagnosed with hypercholesterolemia at the time of presentation and was also a smoker. The Agatston score showed only a mild grade of coronary artery disease, which was confirmed by both the CCTA and CAG, showing no significant stenosis. The ¹³NH₃-PET revealed relative and gradual stress blood flow reduction in the distal flow territories along the SCA, which was not perceived in the proximal territory. Quantitative analysis is more accurate in detecting such subtle perfusion differences, especially when overall perfusion is within normal limits. By visual appraisal only, this can easily be overlooked. If, in our patient, coronary endothelial damage and microcirculatory dysfunction were present due to hyperlipidemia or history of smoking, it is expected to result in more diffuse perfusion reduction in all territories and not merely in the distal segments.

The coronary vasculature has an auto-regulation system that allows it to maintain basal resting flow over a range of pressures along the coronary tract. The coronary blood flow is then determined by the pressure gradient across the myocardial vasculature and vascular resistance.[7] During pharmacological stress, this pressure gradient changes due to vasodilation, resulting in a change of myocardial blood flow. One possible explanation for the perfusion pattern in the present case may be a relatively more pronounced drop in pressure and vascular resistance during vasodilation in the proximal part as compared to the distal part of the SCA as compared to a situation with normal coronary anatomy. This decrease in pressure gradient over the full length of this vessel could result in a relative inability to increase flow in distal segments in the same degree as the proximal segments. The relative difference in myocardial stress flow as demonstrated in this patient could be more pronounced due to the presence of risk factors.

Unfortunately, there are insufficient data about flow characteristics in SCAs. Expectantly, with the increase in diagnostic cardiovascular imaging, more coronary anomalies will be encountered resulting in more data on this rare coronary anomaly.

To the best of our knowledge, this is the first case of PET-CT cardiac perfusion imaging evaluation in a patient with a benign SCA in which reduced flow in the distal path of the monocoronary is documented, in the absence of significant coronary disease.