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Original Research |
1 Department of Radiology and Imaging, OLV Ziekenhuis, Moorselbaan 164, Aalst,
Oost Vlaanderen 9300, Belgium.
2 Cardiovascular Center Aalst, Aalst, Belgium.
3 Department of Radiology, University Hospital Antwerp, Edegem, Belgium.
Received May 30, 2007;
accepted after revision July 16, 2007.
Address correspondence to P. K. Vanhoenacker
(piet{at}vanhoenacker.be).
Abstract
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SUBJECTS AND METHODS. Forty-two patients were prospectively included in this study: 21 were examined without sublingual nitroglycerin (group A), and 21 were examined after the administration of sublingual nitroglycerin (group B). CT angiography was performed using a 64-MDCT scanner. Two blinded observers quantitatively assessed lumen diameter and volume in the left anterior descending artery (LAD) and the right coronary artery (RCA). The number of septal branches was counted. The SNR and CNR in the LAD and RCA were calculated in both groups. The number of clinical side effects was evaluated.
RESULTS. The lumen diameters and the average volumes were significantly larger in group B than in group A. The number of septal branches visualized in group B was significantly higher than in group A. No statistically significant difference in SNR and CNR between the groups was shown. The number of side effects in the two groups was not significantly different.
CONCLUSION. Sublingual nitroglycerin spray significantly dilates the coronary arteries and allows more septal branches to be visualized at coronary CT angiography without diminishing image quality or increasing the number of side effects.
Keywords: coronary angiography • coronary arteries • coronary vessels • MDCT coronary angiography • nitroglycerin
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Coronary volume and lumen diameter are thought to be significantly larger in patients who receive nitroglycerin than in patients who do not. We assessed the influence of sublingual nitroglycerin spray on lumen diameter, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and the number of visualized side branches on CTA. In addition, we assessed the number of patients with side effects after the administration of sublingual nitroglycerin.
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The first group of patients (group A) was the control group and did not receive nitroglycerin. The patients in the second group were given sublingual nitroglycerin (group B). The observers were blinded to the group from which each patient originated. Except for the addition of nitroglycerin, the scanning protocol of the two groups was similar. Our institutional review committee approved this study. Informed consent was obtained from all patients.
MDCTA Image Acquisition
After determination of the CAC score (24 x 1.2 collimation,
330-millisecond gantry rotation time, 0.2 pitch, 3-mm slice thickness, 120-kV
tube voltage, 145-mAs tube current), CTA was performed using an MDCT scanner
(Somatom 64, Siemens Medical Solutions) with a 64 x 0.6 collimation and
330-millisecond gantry rotation time, 3.8-mm per rotation table feed, 0.4-mm
reconstruction increment, 0.75-mm slice thickness, 120-kV tube voltage, and
750-mAs tube current. A bolus (average, 90 mL; 5 mL/s) of contrast material
(iomeprol, 816.5 mg/mL [Iomeron 400, Bracco Altana Pharma]) was administered
before examination followed by a 50-mL saline flush in all patients. Patients
with a baseline heart rate of > 65 bpm were given a β-blocker (25-100
mg of metoprolol, 1 mg/mL) orally before examination and this dose was
supplemented by IV administration as required. In group B, one dose of
nitroglycerin was administered as a sublingual spray (0.4 mg/L) 5 minutes
before initiation of scanning.
Analysis of MDCT Angiograms
MDCT data were transferred from the scanner to an offline workstation
(Aquarius, TeraRecon) for postprocessing and analysis.
Visibility of side branches—Each patient was evaluated by two observers with experience in the interpretation of more than 1,000 coronary scans, and a consensus decision was made.
In each patient, the number of septal branches originating from the proximal and mid left anterior descending artery (LAD), segments 6 and 7 according to the American Heart Association (AHA) [8], was counted. The sinoatrial branch and the conus branch, which are two side branches originating from the proximal part of the right coronary artery (RCA), were assessed and denoted as present or absent. Although the presence or absence of septal branches, the sinoatrial branch, and the conus branch is probably not clinically significant, we wanted to determine whether nitroglycerin improves visualization of these small vessels as a surrogate marker for improved vessel visualization. To obtain the exact number of septal branches, counting was done in the axial plane on source images, on a curved multiplanar reconstruction (MPR), and on different maximal intensity projections (MIPs). The sinoatrial branch and conus branch were assessed in the axial plane on source images, when necessary, and MIP images were reconstructed to confirm their presence or absence.
Image quality: SNR and CNR—All patients were evaluated by one observer. The SNR was measured in the left ventricular myocardium to assess the potential influence of nitroglycerin on the enhancement of myocardium in groups A and B. The SNR of the myocardium was assessed on a short-axis view in the lateral, septal, anterior, and posterior walls with a fixed region of interest. The SNR was defined as the ratio of attenuation in myocardium and image noise.
The SNR and CNR were measured in the left main coronary artery, proximal RCA, crux (the bifurcation of the RCA in the right posterior descending artery and right posterolateral artery), right posterior descending artery, distal part of the circumflex artery, distal part of the LAD, and distal part of the third diagonal branch (whenever visualized) in groups A and B. The SNR was defined as the ratio of attenuation in Hounsfield units (H) in the coronary lumen to image noise. Image noise was defined as the SD of the attenuation (H) in the coronary artery. The CNR was defined as the difference between the attenuation in the coronary artery lumen and the attenuation in the fat surrounding the coronary artery divided by image noise.
Volume determination and diameter of vessels— The same two independent observers evaluated all patients.
Vessel volume and average vessel cross-sectional diameter were evaluated in the LAD and RCA. Measurements of absolute coronary artery diameters were performed in a plane perpendicular to a curved MPR reconstruction along the long axis of the vessel. A 2D profile measuring attenuation along a line crossing through the lumen was made (Fig. 1). The pixels with attenuation values higher than the average attenuation of contrast material in opacified lumen minus 3 SD were supposed to be part of the coronary lumen. We hypothesized that by including values minus 3 SD, 99% of the possible values would be included [9]. The lumen diameter obtained in this way was used for calculation. The same measurement was repeated along a line perpendicular to the first measurement, and the average of the two values was calculated as the final cross-sectional diameter. The measurements were obtained halfway in the proximal RCA (segment 1, AHA) and halfway in the proximal LAD (segment 6, AHA).
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Side effects and clinical parameters—The occurrence of three potential well-known side effects of nitrates—headache, reflex tachycardia, and drop in blood pressure—were registered by the technicians. In all patients, heart rate was measured during scanning and blood pressure was measured before and after scanning. The number of patients who had significant stenosis (> 50% lumen diameter) was registered.
Statistical Analysis
Continuous data are reported as means with 95% CIs.
The sample size was set at 21 patients in each group to reach statistical significance. The unpaired two-sided Student's t test was used for comparison of SNR, CNR, diameter measurements, and volume measurements between group A and group B.
The number and proportion of side effects, the presence of artifacts, the patient's heart rate during scanning, the number of side branches in groups A and B, and the prevalence of significant disease were compared using the Fisher's exact test, unpaired two-sided Student's t test, or both.
Interobserver agreement between measurements of volume and diameter were
analyzed with the concordance coefficient correlation
[10-12].
The concordance correlation coefficient, 
c, contains
a measurement of precision, , and a measurement of accuracy,
Cb:
c = x Cb, where
is the Pearson's correlation coefficient, which measures how far each
observation deviates from the best-fit line, and is a measure of precision;
and Cb is a bias correction factor, which measures how far
the best-fit line deviates from the 45° line through the origin, and is a
measure of accuracy. Interobserver variability between measurements of volume
in the RCA and LAD were also analyzed with Bland-Altman plots. Agreement
between observers for the presence of the sinoatrial branch and conus branch,
the number of septal branches, and the presence of artifacts was analyzed with
Cohen's kappa.
A p value of > 0.05 was considered statistically nonsignificant. MedCalc software (version 7, MedCalc) was used.
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In all patients, there was a right dominant anatomy, meaning that the posterior descending artery originated from the RCA and was therefore labeled as the distal right posterior descending artery.
Image Analysis
Visibility of side branches—There was a statistically
significant difference in the number of visible septal branches between groups
A and B (Student's t test, p < 0.0001)
(Fig. 3). The interobserver
agreement for the number of septal branches was excellent to good (Cohen's
= 0.82). Table 2
illustrates the number of patients with a discernible sinoatrial branch and
conus branch in groups A and B. The sinoatrial branch was present in 67% of
group A compared with 95% of group B (Fisher's exact test, p =
0.069), and the conus branch was present in 33% of group A compared with 62%
of group B (Fisher's exact test, p = 0.129). The difference was not
statistically significant. The interobserver variability for detecting the
sinoatrial branch and conus branch in groups A and B was good (Cohen's
= 0.70 and 0.77, respectively).
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Volume determination and diameter of vessels—The differences in lumen diameter between group A and group B were statistically significant.
The average lumen diameter of the RCA for patients in group A and group B was 2.63 mm (95% CI, 2.47-2.80 mm) and 3.48 mm (3.27-3.69 mm) (p < 0.0001), respectively, whereas the average diameter in the LAD in group A and group B was 2.67 mm (2.50-2.85 mm) and 3.42 mm (3.20-3.65 mm) (p < 0.0001), respectively (Fig. 6).
The differences in volume of the lumen between group A and group B were also statistically significant. The average volume of the RCA for patients in group A and group B, respectively, was 0.58 cm3 (95% CI, 0.45-0.71 cm3) and 1.05 cm3 (0.92-1.2 cm3) (p < 0.0001), whereas the average volume of the LAD for patients in group A and group B was 0.35 cm3 (0.28-0.42 cm3) and 0.72 cm3 (0.55-0.88 cm3) (p = 0.0001) (Fig. 7). The mean difference in RCA volume and LAD volume between group A and group B was, respectively, 0.48 cm3 (0.29-0.66 cm3) and 0.37 cm3 (0.19-0.55 cm3).
Bland-Altman plots for agreement of volume measurements are shown in Figure 8A, 8B, 8C, 8D.
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for
precision), and bias correction factors (Cb for accuracy)
are displayed in Table 3.
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Artifacts—Table
4 reveals that there was no difference between the groups in the
number of scans with substantial artifacts (p > 0.99).
Interobserver agreement was excellent (Cohen's = 0.98).
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Side effects and clinical parameters— Table 4 reveals that there was no statistically significant difference in the number of side effects (p > 0.99), prevalence of significant disease (p = 0.71), and heart rate (p = 0.43) between groups A and B. The mean blood pressure was lower in group B than in group A (mean blood pressure, 120 over 70 vs 137 over 76 mm Hg, respectively) after scanning (see Table 1 for detailed results). However, this difference was not statistically or clinically significant, suggesting that nitroglycerin lowered blood pressure only slightly.
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Nitrates are widely used to treat angina and alleviate symptoms of ischemia through a variety of mechanisms [15]. Among their other effects, nitrates increase the coronary blood flow in the epicardial coronary arteries by vasodilation [1, 2]. They have been used widely in invasive coronary angiography to optimize the enhancement and size of the lumen and visualization of small branches and are administrated through an IV or intraarterial route during the procedure [3]. Nitrates are known to have an important effect on the diameter of the coronary arteries, especially on the size of nonstenotic segments, and to have less effect on stenotic segments; thus, nitrates can enhance the detection of obstructive lesions [3, 16]. Nitroglycerin has been used in the assessment of the microcirculation of myocardium on CT and in imaging the coronary tree with MRI, but the quantification of its effects on the size of the epicardial arteries and the clinical side effects associated with its use were not fully explored [17, 18].
In imaging the coronary tree with CTA, tiny branches (i.e., < 1-2 mm) were usually excluded from analysis, at least in earlier studies [4, 5, 19-21]. Knowing that the overall size of the coronary arteries is small on CTA, every attempt to enhance size and increase enhancement should be made [5]. Sublingual nitroglycerin has been used in one study [22], which showed results comparable to ours. However, the way the drug was administered in that study was different and the number of patients was rather small. In addition, only diameters were measured; neither image quality nor side effects were evaluated.
The use of sublingual nitroglycerin spray instead of another nitrate preparation is based on literature stating that sublingual nitroglycerin spray may be more efficacious than other nitrates and may cause fewer side effects [23], which are often seen with the other ways of administration [24]. This was also suggested by our experience because we did not see more clinical side effects, such as tachycardia, flushing, or headache, in group B than in group A.
A few limitations of this study must be acknowledged. We did not investigate the influence of sublingual nitroglycerin spray on diagnostic accuracy for the detection of significant lesions (stenosis > 50% lumen diameter) or calcified lesions because this study explored only the capacity of sublingual nitroglycerin spray to enlarge lumen size. The choice not to investigate diagnostic accuracy was made on the basis of the fact that the sample size for such a study, with the population that was included (i.e., low to intermediate pretest probability), would be very high and make the study difficult. Also, this increased sample size might have resulted in selection bias because in our institution only patients who have positive or equivocal scanning results undergo conventional angiography. A selected population would therefore have been investigated, and the influence of sublingual nitroglycerin spray on coronary lumen would have been investigated only in patients with diseased vessels. Another limitation is that we used two different patient samples with our study design; therefore, selection bias could have occurred. We think, however, that scanning patients twice for this observational study was not feasible for ethical reasons. This limitation was counterbalanced to some extent by the randomization procedure.
The conclusion of this study is that routine administration of sublingual nitroglycerin spray should be recommended. However, a largescale, randomized study with conventional angiographic correlation should be performed to investigate the influence of sublingual nitroglycerin spray on diagnostic accuracy.
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