Study population
This was a case-control study performed in the cardiology department of our hospital between December 2017 and November 2018. Subjects were consecutively included in the study and had normal or near-normal (<40% stenosis) coronary arteries on coronary angiography, which was performed because of angina, coronary risk factors or abnormal electrocardiography changes. The CSFP group consisted of individuals with a corrected thrombolysis in myocardial infarction (TIMI) frame count (TFC) exceeding 27 in one or more vessels [14]. The control group consisted of individuals with a corrected TFC not more than 27 in all vessels.
Patients having the following features were excluded: incalculable TFC; coronary artery spasm or ectasia; LV ejection fraction <52% in males or <54% in females [15]; any arrhythmia (atrioventricular conduction abnormalities, bundle branch block, ventricular preexcitation, atrial fibrillation, or paced rhythm); abnormal heart structure (congenital heart disease, cardiomyopathies, or valvular dysfunction); pericardial disease (pericardial effusion or constrictive pericarditis); previous history of myocardial infarction; uncontrolled hypertension (systolic blood pressure >160 mmHg or diastolic blood pressure >105 mmHg); hyperthyroidism; hypothyroidism; malignancy; autoimmune disease; infection; pulmonary, hepatic, and renal disorders; hematological disorders (anemia, bone marrow involved by neoplastic disease, or red blood cell transfusions); and a recent major operation (within 90 days).
All examinations were performed by investigators who had no information about the clinical status of the participants. All concomitant medications were stopped ≥12 hours prior to the procedure. Written informed consent was obtained from all participants, and the study protocol was approved by China Medical University Ethics Committee and complied with the ethical guidelines of the 1975 Declaration of Helsinki.
Coronary angiography and TFC calculation
Coronary angiography was performed using the General Electric Innova 3100 (Milwaukee, WI, USA) by the femoral approach in multiple angulated views. A standard Judkins technique was used in all the studied individuals with 5F Judkins catheters, and iohexol (350/100 mL) was used as a contrast agent and manually injected intravenously at the same rate of 3–4 mL/s for the left coronary artery and 2–3 mL/s for the RCA. TFC was used to quantitatively evaluate flow rates of each major coronary artery, including the left anterior descending artery (LAD), the left circumflex coronary artery (LCX), and the right coronary artery (RCA), according to the method first described by Gibson et al [14]. TFC, recorded at 30 frames per second, was the number of frames from the second the contrast medium first appeared in the ostium of the coronary artery to the second it reached a distal coronary landmark. Because the LAD is usually longer than the LCX and RCA, the TFC of LAD is divided by 1.7 to obtain the corrected TFC of LAD (cLAD). The mean TFC for each subject was the average of TFC of RCA, LCX, and cLAD. The TFC was undertaken by two separate cardiologists and a third observer resolved any disagreement.
Seattle Angina Questionnaire
Seattle Angina Questionnaire (SAQ) was collected at the time of study enrollment under the supervision of a trained cardiologist to assess symptoms of angina and their impact on quality of life. SAQ is a validated 19-item questionnaire that measures five key domains related to coronary artery disease: physical limitations, angina stability, angina frequency, treatment satisfaction, and quality of life. Scores range from 0 through 100 for all domains. Higher scores indicate fewer physical limitations due to angina, less angina, and better quality of life [16, 17].
Exercise stress electrocardiography
Exercise testing was performed within 72 hours after coronary angiography using standard Bruce protocol according to standard clinical practice. Heart rate and blood pressure were measured, and a 12-lead ECG was taken at rest, at each stage of the exercise protocol, and during recovery (≥6 min after exercise). Patients were motivated and encouraged to reach 85% of maximal predicted heart rate, until they reached an endpoint. Exercise endpoints included physical exhaustion, severe ischemia (severe chest pain, >2 mm horizontal or downsloping ST depression), severe hypertension (systolic blood pressure >240 mmHg or diastolic blood pressure >110 mmHg), severe hypotension (decrease >20 mmHg in systolic blood pressure from baseline), significant arrhythmia, or pre-syncope. Rate-pressure product and metabolic equivalents (METs) were recorded. Positive exercise stress ECG was defined as significant chest pain, hypotension, or ≥1 mm planar or downsloping ST depression in two or more leads of the same territory, during exercise or recovery. The results of ExECG were interpreted by two separate experienced cardiologists and a third observer resolved any disagreement.
Resting and exercise stress echocardiography
According to the recommendations of the American Society of Echocardiography [15], standard echocardiographic examination was performed in the lateral decubitus position using a Vivid E9 ultrasound system (GE Healthcare, Waukesha, WI, USA) equipped with M5S phased-array probe. The two-dimensional cine loops were recorded for offline analysis using an EchoPAC work station (GE Healthcare).
All patients underwent a comprehensive echocardiography at rest. LV ejection fraction was measured by biplane Simpson method. In order to assess LV diastolic function, we measured left atrial (LA) volume index, mitral E, mitral A, mitral septal e’, mitral lateral e’, and tricuspid regurgitation velocity, and calculated mitral E/A, mitral average E’, and mitral average E/e’ [18].
Exercise stress echocardiography images were acquired immediately (within 90 s) after peak exercise from the second the patients lay in the bed and during recovery (≥6 min after exercise). The images included two-dimensional images from parasternal long-axis and three apical views (long-axis, four-chambers, and two-chambers), mitral valve flow Doppler spectrum, and mitral annular tissue Doppler spectrum. Immediate post-exercise two-dimensional images were obtained using a continuous imaging capture system and the images with best quality were chosen for analysis. Patients with poor imaging quality due to significant respiratory movements immediately after exercise were excluded. LV ejection fraction, LA volume index, mitral E, mitral average e’, and mitral average E/ E’ were assessed.
Two-dimensional speckle-tracking analysis was performed at rest, in the immediate post-exercise period, and in the recovery phase according to the common standard from the consensus document of the EACVI/ASE/Industry Task Force [19]. After manual delineation of the LV endocardial boundary, the software automatically drew the epicardial boundary. Then the widths of the interesting regions were adjusted manually to match the boundaries of the myocardium. The software automatically tracked speckle patterns during the cardiac cycle and yielded a strain curve of the 18 segments of LV. Patients with inadequate tracking of more than one segment in at least one apical view were excluded from the study. LV global longitudinal strain (GLS) was calculated by averaging end-systolic strain of all LV myocardial segments.
Reproducibility
Intraobserver and interobserver variabilities for LV GLS in the immediate post-exercise period were examined in 10 randomly selected patients. The same observer who was blinded to the initial measurements repeated the measurements after more than 4 weeks to assess intraobserver variability. A second independent observer repeated the measurements twice to assess interobserver variability.
Statistical Analysis
Normality plots with tests were performed using the Shapiro–Wilk test. Continuous variables were presented as the mean±standard deviation (SD) or median (interquartile range) and categorical variables as percentages. Continuous variables were compared using the independent t-test or Mann–Whitney U test, where appropriate. To compare the proportion of categorical variables, chi-square or Fisher exact test was used. Comparisons among ≥3 independent groups were assessed using one-way analysis of variance (ANOVA), and comparisons between groups were performed by post-hoc pairwise comparisons (Scheffe's). Comparisons among ≥3 matching groups were assessed using one-way repeated measures ANOVA, and post-hoc pairwise comparisons (Tukey's) were used to probe significant differences between groups. Intraobserver and interobserver variabilities were evaluated by Bland-Altman analysis. For all parameters, P<0.05 (two-tailed) was considered statistically significant. All statistical analyses were performed using SPSS 17.0 software package (SPSS version 17, Chicago, IL, USA).