Detection of diminished response to cold pressor test in smokers: Assessment using phase-contrast cine magnetic resonance imaging of the coronary sinus

Magnetic Resonance Imaging 32 (2014) 217–223

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Detection of diminished response to cold pressor test in smokers: Assessment using phase-contrast cine magnetic resonance imaging of the coronary sinus☆ Shingo Kato a, Kakuya Kitagawa a,⁎, Yeonyee E. Yoon a, Hiroshi Nakajima b, Motonori Nagata a, Shinichi Takase a, Shiro Nakamori b, Masaaki Ito b, Hajime Sakuma a a b

Department of Radiology, Mie University Hospital, 2-174 Edobashi, Tsu, Mie 514-8507, Japan Department of Cardiology, Mie University Hospital, 2-174 Edobashi, Tsu, Mie 514-8507, Japan

a r t i c l e

i n f o

Article history: Received 30 January 2013 Revised 1 November 2013 Accepted 23 December 2013 Keywords: Phase-Contrast Cine Magnetic Resonance Imaging Cold Pressor Test Coronary endothelial function Smoker 3 T MR imager Nitric oxide

a b s t r a c t Purpose: The purposes of this study were to evaluate the reproducibility for measuring the cold pressor test (CPT)-induced myocardial blood flow (MBF) alteration using phase-contrast (PC) cine MRI, and to determine if this approach could detect altered MBF response to CPT in smokers. Materials and methods: After obtaining informed consent, ten healthy male non-smokers (mean age: 28 ± 5 years) and ten age-matched male smokers (smoking duration ≥5 years, mean age: 28 ± 3 years) were examined in this institutional review board approved study. Breath-hold PC cine MR images of the coronary sinus were obtained with a 3 T MR imager with 32 channel coils at rest and during a CPT performed after immersing one foot in ice water. MBF was calculated as coronary sinus flow divided by the left ventricular (LV) mass which was given as a total LV myocardial volume measured on cine MRI multiplied by the specific gravity (1.05 g/mL). Results: In non-smokers, MBF was 0.86 ± 0.25 mL/min/g at rest, with a significant increase to 1.20 ± 0.36 mL/min/g seen during CPT (percentage change of MBF (ΔMBF (%)); 39.2% ± 14.4%, p b 0.001). Inter-study reproducibility for ΔMBF (%) measurements by different MR technologist was good, as indicated by the intraclass correlation coefficient of 0.93 and reproducibility coefficient of 10.5%. There was no significant difference between smokers and non-smokers for resting MBF (0.85 ± 0.32 mL/min/g, p = 0.91). However, ΔMBF (%) in smokers was significantly reduced (-4.0 ± 32.2% vs. 39.2 ± 14.4%, p = 0.011). Conclusion: PC cine MRI can be used to reproducibly quantify MBF response to CPT and to detect impaired flow response in smokers. This MR approach may be useful for monitoring the sequential change of coronary blood flow in various potentially pathologic conditions and for investigating its relationship with cardiovascular risk. © 2014 Elsevier Inc. All rights reserved.

1. Introduction The endothelium is a monolayer of endothelial cells lining the lumen of the vascular beds and plays a central role in regulating vasomotor tone, thrombosis, and platelet adhesion [1]. Endothelial dysfunction is considered to be the first stage of atherosclerosis [2], and a number of previous studies have demonstrated the prognostic significance of assessing endothelial function in predicting future Abbreviations: ATP, adenosine triphosphate; CAD, coronary artery disease; CPT, cold pressor test; LV, left ventricle; MBF, myocardial blood flow; ΔMBF, change of MBF by CPT as compared with resting MBF; ΔMBF (%), percentage change of MBF by CPT as compared with resting MBF; MR, magnetic resonance; NO, nitric oxide; PC, phasecontrast; PET, positron emission tomography; RPP, rate pressure product; VCG, vector-electrocardiographic. ☆ No conflicts of interest. ⁎ Corresponding author. Tel.: +81 59 231 5029; fax: +81 59 232 8066. E-mail address: [email protected] (K. Kitagawa). 0730-725X/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.mri.2013.12.015

cardiovascular events [2–4]. Recent studies indicated that cigarette smoking is associated with an increased risk of mortality from coronary heart disease [5], even in young smokers [6, 7]. The assessment of altered endothelial function in young smokers may offer an important clue in predicting future cardiovascular risk. The cold pressor test (CPT) is a non-pharmacological stress test that induces endothelium dependent coronary vasodilatation [8, 9]. In healthy subjects, the vasodilatory effect of CPT is mediated through a central sympathetic response that stimulates the release of nitric oxide (NO) from the coronary endothelium via an alpha-2 adrenergic receptor [8]. A previous study used positron emission tomography (PET) and N-13 ammonium to measure the CPT-caused alterations in the myocardial blood flow (MBF) and demonstrated there was impairment in patients with coronary risk factors [10], along with an abnormal CPT-caused MBF alteration that was associated with the risk of developing cardiovascular events [11]. Therefore, PET assessment of the MBF response to CPT can be used as a noninvasive

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measure of coronary endothelial dysfunction in the early stages of atherosclerosis. However, PET is relatively expensive and thus, cannot be widely used to assess coronary endothelial function. Phase-contrast (PC) cine magnetic resonance (MR) imaging is another approach that makes it possible to noninvasively quantify the global MBF of the left ventricle (LV) myocardium [12–16]. This method can be used to noninvasively quantify MBF responses to CPT without exposing patients to radiation or the need to use venous injections of contrast medium or radioactive tracers [17, 18]. However, the amount of endothelium dependent flow augmentation caused by CPT is smaller (approximately 30%–40%) than the flow augmentation that can be achieved by pharmacological vasodilators such as adenosine or dipyridamole, which typically ranges from 3 to 4 fold in normal subjects [19, 20]. Therefore, before this technique can be employed as a clinical tool for noninvasive assessment of coronary endothelial function, the reproducibility of the MR CPT method needs to be fully evaluated. In addition, no previous study investigated the feasibility of MR CPT method for the detection of impaired MBF response to CPT in young healthy smokers. In the present study, we determined the reproducibility of the MBF measurement by including the inter-study reproducibility for the different MR operators as well as intra- and inter-observer reproducibility. Then, we examined whether the MR CPT method could detect altered coronary endothelial function in young smokers. 2. Materials and methods 2.1. Subjects Ten male non-smokers (mean age: 28 ± 5 years, range 23-36 years) and 10 male smokers (mean age: 28 ± 3 years, range 24–32 years; smoking duration ≥ 5 years; mean pack-years: 9.4 ± 4.8, range 2–18 years) without cardiovascular risk factors, except for cigarette smoking, were studied. Mean body mass index was 22 ± 2 kg/m 2 for non-smokers and 22 ± 3 kg/m 2 for smokers (Table 1). All participants in this study demonstrated normal electrocardiogram. All subjects refrained from caffeine-containing beverages for at least 24 h before the MR scan. The smokers refrained from smoking for at least 4 h before the MR study [20]. This study was approved by the local institutional review board, and all subjects gave written informed consent prior to participating in this study. 2.2. MR image acquisition MR images were acquired with a 3 T MR system equipped with 32 channel cardiac coils (Achieva, Philips Healthcare, Best, The Netherlands). After placement of vector-electrocardiographic (VCG) monitoring leads, subjects underwent imaging in the supine position. For cardiac orientation, scout images were acquired in

Table 1 Characteristics of the study groups.

Age, years Range Body weight, kg Range Height, cm Range Body mass index, kg/m2 Range

Non-smokers (N = 10)

Smokers (N = 10)

⁎ P-value

28 ± 5 23–36 63 ± 7 53–80 170 ± 4 165–170 22 ± 2 19–27

28 ± 3 24–32 64 ± 8 52–80 170 ± 7 159–184 22 ± 3 18–29

0.91 0.64 0.68 0.80

Data are expressed as mean ± SD. ⁎ P-value represents significance of the difference between non-smokers and smokers.

three orthogonal planes. Vertical and horizontal long axis cine MR images of the LV were obtained using a steady state free precession (SSFP) sequence. To calculate the LV volume and mass, short-axis cine MR images of the LV were acquired from the apex to the base (TR/TE, 4.1/1.7 ms; flip angle, 55°; FOV, 350 × 350 mm; acquisition matrix, 128 × 256; reconstruction matrix, 512 × 512; slice thickness, 10 mm; number of phases per cardiac cycle, 20). To identify the location of the coronary sinus, cine MR images on the axial planes were obtained through the atrioventricular groove (Fig. 1). The imaging plane for blood flow measurement by the PC cine MRI was positioned so that it was perpendicular to the coronary sinus at a distance of 2 cm from the ostium of the coronary sinus. PC cine MR images of the coronary sinus were acquired during a suspended shallow breath-hold using a VCG triggered gradient echo sequence (Venc, ± 50 cm/s; TR/TE, 7.3/4.4 ms; flip angle, 10°; slice thickness, 5 mm; FOV, 240 × 194 mm; acquisition matrix, 128 × 128; reconstruction matrix, 256 × 256; number of phases per cardiac cycle, 20). 2.3. Cold pressor test protocol After acquisition of cine MR images of the LV and PC cine MR images of the coronary sinus in the resting state, the subject’s foot was immersed in a magnet compatible ice-water box for 2 min [21]. At 1 min after initiation of the foot immersion, we once again performed coronary sinus flow measurements. Since different MR technologists performed the measurements, we evaluated interstudy reproducibility by repeating the resting and CPT studies. This included scout imaging, which was done after a 40-min deconditioning period in the non-smokers [21]. Two different MR technologists who were blinded to all of the images obtained by another operator, including scout MRIs, performed the first set for the resting (Rest-1) and CPT (CPT-1) MR images and the second set for the resting (Rest-2) and CPT (CPT-2) MR images. 2.4. Data analysis Two observers (S.K. and Y.E.Y., with 6 and 5 years of experience in cardiac MRI, respectively) used a workstation (Extend MR WorkSpace, Philips Healthcare, Best, The Netherlands) to analyze the cine and PC cine MR images. All of the cine and PC cine MR images were anonymized and reviewed in a random order. For the LV cardiac mass measurements, epi- and endocardial borders of the LV on the short axis cine MR images were manually traced, with the exclusion of the papillary muscles at each anatomic level that encompassed the LV. The LV mass was calculated as the sum of the myocardial volume areas multiplied by the specific gravity (1.05 g/ mL) of the myocardial tissue [22]. Two observers performed the LV mass measurement by consensus. For the quantification of blood flow in the coronary sinus, the contour of the coronary sinus was manually traced on the magnitude images of the PC cine MR images for each cine frame. To evaluate inter-observer reproducibility, blood flow measurements were independently taken by two observers, with one of the two observers measuring the blood flow twice in order to assess the intra-observer reproducibility. To perform the phase-offset correction, the velocity in the adjacent tissue was also measured. Coronary sinus blood flow was calculated by integrating the product of the cross-sectional area and the mean velocity in the coronary sinus. This value was corrected using the mean velocity in the adjacent tissue for all cardiac phases in the cardiac cycle. MBF (mL/min/g) was calculated as coronary sinus blood flow divided by the LV mass that drains into the coronary sinus [13]. We measured blood pressures and heart rates in the MR exam room by using MRI compatible automated sphygmomanometer. In line with other studies, we also corrected the MBF using the rate pressure products (RPP) [13, 16, 21, 23–25].

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Fig. 1. 3 T phase contrast cine magnetic resonance imaging of the coronary sinus. (A) Axial scout image of the coronary sinus obtained using the steady state free precession with slice orientation for flow imaging (white solid line). (B) Magnitude image. (C) Phase difference image. The blood flow in the coronary sinus is clearly depicted as a low signal intensity area in (C), (black arrow).

MBF (mL/min/g) = coronary sinus blood flow (mL/min)/LV mass (g)/RPP (mmHg/min) × 7500 RPP (mmHg/min) = systolic blood pressure (mmHg)×heart rate (beats/min) The changes of MBF by CPT and the percentage change of MBF by CPT were calculated as follows. ΔMBF = MBF during CPT − MBF at rest ΔMBF (%) = ΔMBF/MBF at rest × 100

All CPT procedures were well tolerated and no subject withdrew from the testing. PC cine MR images were successfully acquired in the resting state and during CPT in all subjects (Fig. 1). Blood flow curves at rest and during CPT in a representative non-smoking subject are shown in Fig. 2. Coronary sinus blood flow demonstrated a biphasic pattern with the first peak occurring during systole and the other during diastole. Hemodynamic data of non-smokers and smokers are shown in Table 3. RPP exhibited a significant increase during CPT in both non-smokers and smokers.

2.5. Statistical analysis 3.2. Reproducibility of the hemodynamic responses to CPT Data were statistically analyzed using SPSS software, version 17.0 (SPSS, Inc., Chicago, IL, USA). Continuous values are presented as mean ± standard deviation (SD). Normality was determined by the Shapiro–Wilk test. To determine if there were significant differences between the non-smokers and smokers, we evaluated patient characteristics (age, body weight, height, body mass index), LV volumes and mass (end diastolic volumes, end systolic volumes, stroke volumes, ejection fraction, LV mass), MBF values (MBF at rest, MBF during CPT, ΔMBF, ΔMBF (%)), using an unpaired t-test for normally distributed variables, and the Mann–Whitney U test for skewed variables. A paired t-test was used to assess the significance of difference between the resting status and during CPT for the hemodynamic parameters (systolic blood pressure, diastolic blood pressure, heart rate, and rate pressure product) and the MBF, as both the hemodynamic parameters and the MBF showed normal distributions. Pearson's correlation coefficient was used to investigate the relationship between the RPP values and between the MBF values. Inter-study, and the inter- and intra-observer reproducibilities were evaluated using the intraclass correlation coefficient (ICC). The repeatability coefficients for the inter-study, and the inter- and intra-observer reproducibilities were calculated as 1.96 times the SD of the Bland–Altman plot differences [26]. P values b 0.05 were considered statistically significant.

The results of the linear regression analyses and the Bland– Altman plots for the RPP values that were observed during the two repeated MR acquisitions are shown in Fig. 3. A good correlation of the RPP values was found between the Rest-1 and Rest-2 acquisitions (R = 0.96, P b 0.01, Fig. 3A), and the CPT1 and CPT2 acquisitions (R = 0.97, P b 0.01, Fig. 3B). The Bland–Altman plot demonstrated a repeatability coefficient of 560 mmHg/min for the RPP in the resting state (7.5% of the mean RPP at rest, Fig. 3C) and 704 mmHg/min for the RPP during CPT (8.1% of the mean RPP during CPT, Fig. 3D). The mean ΔRPP was 1310 ± 535 mmHg/min for CPT1 and 1193 ± 597 mmHg/min for CPT2, respectively. 3.3. Reproducibility of MBF measurements by MRI Inter-study reproducibility seen for the ΔMBF (%) measurements by the different MR operators was good, with an ICC of 0.93. In addition, ICCs for the ΔMBF (%) measurements were 0.93 for interobserver reproducibility and 0.95 for intra-observer reproducibility (Table 4). The linear regression analyses and the Bland–Altman plots of the ΔMBF (%) for the repeated CPT studies are shown in Fig. 4. InterTable 2 Left ventricular volumes and mass measured by cine MRI.

3. Results 3.1. LV function at rest and the hemodynamic responses to CPT LV ejection fraction was normal (N55%) in all non-smokers and smokers, and there was no significant difference between the two groups (60% ± 4% vs. 60% ± 6%, p = 0.93). There was also no significant difference found between non-smokers and smokers for end diastolic volume, end systolic volume, stroke volume and LV mass (Table 2).

End diastolic volume, mL End systolic volume, mL Stroke volume, mL Ejection fraction, % LV mass, g

Non-smokers (N = 10)

Smokers (N = 10)

⁎P-value

129 52 78 60 108

135 55 80 60 113

0.22 0.53 0.32 0.93 0.33

± ± ± ± ±

21 9 15 4 9

± ± ± ± ±

14 12 9 6 13

Data are expressed as mean ± SD. LV, left ventricle. ⁎ P-value represents significance of the difference between non-smokers and smokers.

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Fig. 2. Curves of the flow volume in the coronary sinus of a non-smoker. Measurements were made using phase-contrast cine MR imaging. The thin line indicates the blood flow pattern in the resting state, while the thick line indicates the blood flow pattern during CPT. In this non-smoker, CPT resulted in a 50% increase of the coronary sinus blood flow (coronary sinus blood flow at rest = 120 mL/min, coronary sinus blood flow during CPT = 184 mL/min). CPT, cold pressor test.

study, inter- and intra-observer variabilities for the ΔMBF (%) measurements were low, with a mean difference of −0.8% (95% limit of agreement; −11.3% to 9.6%), 0.2% (−11.6% to 11.9%), and 1.4% (− 7.0% to 9.7%), respectively (Fig. 4). Inter-study repeatability coefficients were 0.18 mL/min/g (15% of the mean MBF during CPT) for MBF during CPT, and 10.5% for ΔMBF (%). Repeatability coefficients for ΔMBF (%) were 11.7% for the inter-observer reproducibility, and 8.4% for the intra-observer reproducibility (Table 4). 3.4. MBF responses to CPT in non-smokers and smokers MBF values measured by coronary sinus blood flow in the resting state and during CPT are shown in Table 5. In non-smokers, MBF was 0.86 ± 0.25 ml/min/g in the resting state, with a significant increase to 1.20 ± 0.36 ml/min/g noted for CPT (p b 0.001). In smokers, MBF was 0.85 ± 0.32 ml/min/g in the resting state and 0.77 ± 0.24 ml/min/g during CPT (p = 0.39). The mean ΔMBF (%) in smokers was significantly lower than the mean ΔMBF (%) in nonsmokers (− 4.0% ± 32.2% vs. 39.2% ± 14.4%, p = 0.011). 4. Discussion To the best of our knowledge, this is the first study demonstrating the feasibility of MR CPT method for the detection of impaired MBF response to CPT in young healthy smokers.

Table 3 Hemodynamic responses to cold pressor test. At Rest Non-smokers (N = 10) SBP, mmHg DBP, mmHg HR, bpm RPP, mmHg/min Smokers (N = 10) SBP, mmHg DBP, mmHg HR, bpm RPP, mmHg/min

⁎ P-value

CPT

116 64 63 7374

± ± ± ±

6 8 7 986

127 68 69 8684

± ± ± ±

9 9 8 1238

0.0012 0.040 0.013 b0.001

112 63 58 6509

± ± ± ±

8 6 5 715

122 77 65 7932

± ± ± ±

12 13 6 885

0.0033 0.0014 b0.001 b0.001

Data are expressed as mean ± SD. CPT, cold pressor test; SBP, systolic blood pressure; DBP, diastolic blood pressure; HR, heart rate; RPP, rate pressure product. ⁎ P-value represents significance of the difference between values at rest and values during CPT.

4.1. MR blood flow measurement in the coronary sinus Recent studies demonstrated that MR coronary sinus flow measurement can be used to reveal flow increase in response to CPT in asymptomatic women with cardiovascular risk factors [17] and healthy volunteers [18]. In the present study, we used a 3 T MR imager and 32 channel cardiac coils to quantify the blood flow in the coronary sinus and extensively evaluated the reproducibility of MR measurements of MBF alteration in response to CPT. Moreover, diminished response to CPT in smokers was successfully detected by using MR flow measurement. A potential advantage of 3.0 T over 1.5 T in MR flow measurement is better signal to noise ratio, which may lead to better reproducibility. 4.2. Reproducibility of MBF measurements As the CPT-caused blood flow increase is usually 30%–40% [27], it was important to investigate if the reproducibility of the MR CPT method is high enough to allow for differentiation between the CPTinduced blood flow increase in normal subjects and in those with impaired endothelial function, such as that seen in smokers. In our study, the inter-study reproducibility coefficient for the MR CPT method when measuring MBF during CPT in non-smokers was 0.18 mL/min/g (15% of the mean MBF), which was substantially better than inter-study repeatability coefficient that was reported for [13] N-ammonium myocardial perfusion PET (0.46 mL/min/g, 27% of the mean MBF) in a previous study [21]. The limited spatial resolution of the MR image remains a major concern for the accuracy of measuring volume flow with a phasecontrast MR technique. However, the coronary sinus is larger than the coronary artery; the coronary sinus measures approximately 7– 10 mm in diameter and occupies more than 6 pixels. Therefore, errors caused by the limited spatial resolution and variation of ROI placement are less important in quantifying flow volume in the coronary sinus than in quantifying flow volume in the coronary artery [12]. 4.3. MBF values before and after CPT in smokers In our study, MBF in male non-smokers was 0.86 ± 0.25 mL/ min/g in the resting state and 1.38 ± 0.46 mL/min/g during CPT. These values were consistent with the MBF values reported in the previous PET and MR studies, with the resting MBF ranging from 0.57 to 0.91 mL/min/g and the MBF during CPT ranging from 0.88 to

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Fig. 3. Linear regression analysis (A, B) and Bland–Altman plots (C, D) for RPP values at repeated rest (Rest-1 and Rest-2) and that during repeated CPT (CPT-1 and CPT-2) in nonsmokers. (C, D) The horizontal solid line indicates the mean difference between the two RPPs, and the dashed lines indicate the limits of agreement (mean difference ± 1.96 times the SD of the difference). CPT, cold pressor test; SD, standard deviation.

1.41 mL/min/g [10, 11, 17, 18, 21, 25]. For the smokers in the present study, the mean MBF was 0.85 ± 0.32 mL/min/g at rest while it was 0.77 ± 0.24 ml/min/g during CPT. This is also similar to the PET study results, which found the resting MBF ranged from 0.73 to 0.79 mL/min/g and the MBF during CPT ranged from 0.70 to 0.90 mL/min/g [10, 20]. 4.4. Clinical implications Cigarette smoking is associated with an increased risk of mortality from coronary heart disease. In the United States, 30.3% of male students and 21% of female students reported current tobacco use [5]. Even in young smokers, cigarette smoking has a substantial influence on ischemic heart disease mortality [6, 7], but cessation before the age of 40 years reduces the risk of death associated with continued smoking by about 90% [28]. As demonstrated by our study, the effect of cigarette smoking on coronary

circulation can be monitored non-invasively and quantitatively with high reproducibility by using PC cine MRI. This method can be repeated in young healthy subjects for longitudinal follow-up as MR CPT doesn’t necessitate radiation exposure or injection of contrast material. Therefore, MR CPT method may offer new insights into the sequential change of coronary blood flow in various potentially pathologic conditions not limited to cigarette smoking. Moreover, PC cine MRI may be combined with other stress tests such as hyperventilation, mental, and exercise stress tests. Further study is necessary to explore the clinical utility of measurement of coronary flow responses to CPT and other stress tests. 4.5. Limitations Several limitations in this study need to be acknowledged. First, the sample size of this study was small and the study population was limited to healthy young male subjects. Second, MR assessment of

Table 4 Intraclass correlation coefficients and repeatability coefficients of the 3 T MR CPT method for measuring myocardial blood flow in non-smokers. Intraclass correlation coefficient

Repeatability coefficient

Inter-study reproducibility

Inter-observer reproducibility

Intra-observer reproducibility

Inter-study reproducibility

Inter-observer reproducibility

Intra-observer reproducibility

MBF at rest

0.97

0.98

0.98

MBF during CPT

0.97

0.97

0.99

ΔMBF (%)

0.93

0.93

0.95

0.14 mL/min/g (16% of mean MBF at rest) 0.18 mL/min/g (15% of mean MBF during CPT) 10.5 %

0.10 mL/min/g (11% of mean MBF at rest) 0.11 mL/min/g (9% of mean MBF during CPT) 11.7 %

0.09 mL/min/g (11% of mean MBF at rest) 0.11 mL/min/g (9% of mean MBF during CPT) 8.4 %

ΔMBF (%) = (MBF during CPT − MBF at rest)/MBF at rest × 100. Repeatability coefficients were calculated as 1.96 times the standard deviation of the differences on the Bland–Altman plots. MR, magnetic resonance; CPT, cold pressor test; MBF, myocardial blood flow.

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Fig. 4. Linear regression analysis and Bland–Altman plot of ΔMBF (%) by CPT in non-smokers (A: inter-study ΔMBF (%), B: inter-observer ΔMBF (%), C: intra-observer ΔMBF (%)). Bland–Altman plot of ΔMBF (%) in non-smokers (D: inter-study ΔMBF (%), E: inter-observer ΔMBF (%), F: intra-observer ΔMBF (%)). (D–F) The horizontal solid line indicates the mean difference, and the dashed lines indicate the limits of agreement (mean difference ± 1.96 times the SD of the difference). ΔMBF (%): percentage change of myocardial blood flow by CPT. CPT, cold pressor test; SD, standard deviation.

endothelial function by CPT was not compared with the results by reference standard method in this study. The third limitation of the present study is that in smokers, the MBF responses to CPT can vary from subject to subject. For example, there were two smokers in the present study that showed relatively normal MBF responses to CPT. It is possible that smoking duration, packs per year and time from the last cigarette smoked prior to the MR scan, along with dietary habits such as Vitamin C intake may considerably influence the degree of endothelial dysfunction [10]. Thus, the variations of the MBF responses to CPT in smokers may be related to the heterogeneity of the endothelial function in each smoker, as well as the measurement errors. Fourth, cardiac MRI has negative aspects such as expensive cost and long examination time. However, MR CPT can be performed in a relatively short examination time (about 20 min) without administration of contrast medium.

Table 5 Myocardial blood flow at rest and during the cold pressor test in non-smokers and smokers.

MBF at rest (mL/min/g) MBF during CPT (mL/min/g) ΔMBF (mL/min/g) ΔMBF (%)

Non-smokers (N = 10)

Smokers (N = 10)

0.86 ⁎⁎1.20 0.33 39.2

0.85 0.77 −0.08 - 4.0

± ± ± ±

0.25 0.36 0.14 14.4

± ± ± ±

⁎ P-value 0.32 0.24 0.28 32.2

0.91 0.0056 b0.001 0.011

Data are expressed as mean ± SD. ΔMBF = MBF during CPT − MBF at rest. ΔMBF (%) = ΔMBF/MBF at rest × 100. CPT, cold pressor test; MBF, myocardial blood flow. ⁎ P-value represents significance of the difference between non-smokers and smokers. ⁎⁎ P b 0.05 vs. MBF at rest.

In conclusion, PC cine MR imaging can be used to quantify the MBF response to CPT with high reproducibility. This MR approach successfully detected impaired MBF response to CPT in young smokers and may be useful for monitoring the sequential change of coronary blood flow in various potentially pathologic conditions and for investigating its relationship with cardiovascular risk.

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