The role of impaired sympathetic nerve function in enhancing coronary vasoconstriction in patients with hypertrophic cardiomyopathy

Abstract

Coronary vasospasm and diminished coronary blood flow reserve have often been reported in patients with hypertrophic cardiomyopathy (HCM). However, the mechanism of coronary spasm in HCM is unknown. Thus, coronary endothelial function and sympathetic nerve function in 11 patients with HCM and 11 control patients matched for age and sex were examined. The diameter of the left anterior descending coronary artery was assessed by quantitative coronary angiography, and the change in coronary blood flow was estimated using an intracoronary Doppler flow wire. To assess myocardial sympathetic nerve function, metaiodobenzylguanidine images – 15 min and 180 min after the injection of ¹²³I-metaiodoben-zylguanidine at a dosage of 111 MBq – were obtained, and the heart to mediastinum (H/M) count ratio and the washout rate (WR) were calculated. The H/M ratio was significantly lower in patients with HCM (2.1±0.3) than in control patients (2.6±0.4) (P<0.01). In addition, the WR was higher in patients with HCM (35±6%) than in control patients (28±3%) (P<0.01). The HCM subjects with coronary spasm had lower H/M ratios and higher WRs than HCM subjects without coronary spasm (P<0.05, respectively). In conclusion, impaired sympathetic nerve function may be associated with coronary vasospasm and diminished coronary blood flow reserve in HCM.

Coronary vasospasm and decreased coronary blood flow reserve have been reported in patients with hypertrophic cardiomyopathy (HCM) (1–3). Coronary vasospasm reportedly plays a significant role in the etiology of myocardial ischemia in Japanese patients with HCM (3). Intracoronary injection of acetylcholine (ACh) induces coronary vasospasm in patients with coronary spastic angina. However, the mechanism of coronary spasm in HCM is unknown. On the other hand, HCM is characterized by the presence of many alterations of adrenergic nerves such as decreased cardiac uptake of noradrenaline (NA), increased NA release, decreased NA cardiac content and partial denervation (4). ¹²³I-metaiodobenzyl-guanidine (MIBG) scintigraphy has been established as an important technique for studying cardiac neuronal function (5–12). MIBG, an analogue of guanethidine, is taken up and stored similarly to NA. Abnormalities in myocardial ¹²³I-MIBG uptake were reported in patients with various types of heart disease (5–12). Previous HCM studies (7,8) have reported cardiac MIBG abnormalities, shown by a decreased heart to mediastinum (H/M) ratio or increased washout rate. In addition, ¹²³I-MIBG cardiac scintigraphy has been considered a useful technique to assess the severity or prognosis of patients with HCM or dilated cardiomyopathy (13–15). However, the relation between cardiac sympathetic nerve function and coronary vasospasm remains to be investigated.

The present study was performed to examine endothelium-dependent and -independent coronary vasomotor responses and cardiac sympathetic nerve function in patients with HCM.

METHODS

Patient characteristics

Eleven patients (eight men and three women) with HCM were eligible for enrolment in the study at the University Hospital (Japan). No patient had significant coronary artery stenosis, hypertension or diabetes mellitus. The ¹²³I-MIBG study was performed during the same hospitalization period on all patients. The diagnosis of HCM was made principally according to the definition and classification proposed by the World Health Organization/International Society and Federation of Cardiology Task Force (16). In seven patients, the diagnosis was also supported by endomyocardial biopsy findings, including marked derangement of normal myocardial architecture and fibrosis. At enrolment, complete medical histories were taken and physical examinations were performed in all patients. Eleven patients with atypical chest pain or myocardial ischemia on electrocardiogram (ECG) were studied. All patients had nonobstructive HCM.

Smokers refrained from smoking for at least seven days before the study to rule out the direct effects of oxidants contained in cigarette smoke. All vasoactive medications, including calcium channel blockers, nitrates, α-blockers and angiotensin-converting enzyme inhibitors, as well as antioxidants, including statins and probucol, were discontinued for at least 48 h before the study.

Echocardiographic studies

Conventional two-dimensional echocardiographic studies (SSH-160A, Toshiba, Japan) were performed within one week of the radionuclide studies in all patients. The M-mode echocardio-graphic measurement was performed according to the criteria recommended by the American Society of Echocardiography (17), and the results are shown in Table 1. Standard echocardiographic images, including the parasternal long-axis, parasternal short-axis, apical four-chamber and apical two-chamber images, were acquired.

TABLE 1.

Baseline clinical and procedural variables.

Clinical data	HCM (n=11)	Control (n=11)
Age (years)*	48±4	53±3
Sex ratio (men:women)	8:3	8:3
Systolic blood pressure (mmHg)*	123±4	128±7
Diastolic blood pressure (mmHg)*	71±3	72±4
Myocardial blood pressure (mmHg)*	88±3	91±4
Intraventricular septum thickness (mm)*	16±1†	8±1
LVEDP (mmHg)*	15±1†	8±1
LVMI (g/m2)*	142±10†	86±5
Smoking status, n (%)	3 (27)	3 (27)

^*Values are expressed as mean ± SD;

^†P<0.05 versus control (ANOVA). HCM Hypertrophic cardiomyopathy; LVEDP Left ventricular end-diastolic pressure; LVMI Left ventricular mass index

¹²³I-MIBG imaging

¹²³I-MIBG (Daiichi Radioisotope Laboratories, Japan), at a dose of 111 MBq, was injected slowly through the antecubital cannula and flushed with 10 mL saline at rest after a 3 h fast. The planar and single photon emission computed tomography views were obtained approximately 15 min after injection. A wide field-of-view gamma camera (GCA-901, Toshiba Medical, Japan), equipped with a low-energy, general-purpose collimator, was rotated over 180° from the right anterior oblique 45° view to the left posterior oblique 45° view. Thirty-two images were obtained for 40 s in each 6° interval. Image reconstruction and washout analysis were performed with a nuclear medicine computer system (Toshiba, Japan) by means of a back-projection algorithm, with filtering and without attenuation correction. Oblique, orthogonal tomographic slices, each 6 mm thick, were reconstructed parallel to the short-axis, the vertical long-axis, and the horizontal long-axis of the left ventricle. Delayed imaging was obtained 3 h after initial imaging. During the period between the initial and delayed scans, the subjects remained in the fasted state (8–11).

Intraobserver variability of H/M ratio and washout rate was less than 2%, and interobserver variability (two observers) was less than 3%.

The Ethical Committee on Human Research at Shiga University of Medical Science (Japan) approved the study protocols, and written informed consent was obtained from all the patients.

Coronary endothelial function study

Cardiac catheterization was performed between 09:00 and 11:00 in the fasted state. A 0.014-inch, Doppler-tipped guidewire (FloWire, Cardiometrics Inc, USA) was advanced to the proximal segment of the left anterior descending (LAD) coronary artery to measure coronary diameter. All drugs were infused directly into the left main coronary artery via the guide catheter at infusion rates ranging from 0.5 mL/min to 1 mL/min. A 6F multipurpose catheter (GCS6, Goodtec, Japan) was inserted via the right femoral vein into the coronary sinus for blood sampling. Baseline measurement of coronary diameter and coronary angiography were performed and confirmed to be unchanged by a 2 min infusion of saline at 1 mL/min (18).

ACh chloride (Daiichi Pharmaceutical Co, Japan) was started at 3 μg/min and then increased to 10 μg/min and 30 μg/min for 2 min each. Thus, all patients received doses up to 100 μg of ACh chloride (13).

Quantitative coronary angiography and measurement of coronary blood flow

Coronary cineangiograms were recorded using a Philips cineangiographic system (Philips Medical Systems, Japan). The change in diameter of the LAD coronary artery was measured in a vessel segment 5 mm beyond the tip of the Doppler wire. Coronary angiography was performed using the Judkins technique with contrast material (Omnipaque, Daiichi Pharmaceutical Co, Japan). Coronary angiograms were analyzed by quantitative coronary angiography using the Cardiovascular Measurement System (CMS-MEDICS Medical Imaging Systems, Netherlands). Peak coronary blood flow velocity was continuously monitored using a fast Fourier transform-based spectral analyzer (FloMap, Cardiometrics Inc, USA). Coronary blood flow was derived from coronary blood flow velocity and diameter measurements by the formula (18):

$$\pi \times \text{mean peak coronary blood flow velocity}\times 0.125\times {(\text{arterial diameter})}^2$$

Data analysis

Cardiac uptake was quantified in planar anterior views at 15 min and 3 h after injection of MIBG. As shown in Figure 1, the regions of interest on planar imaging were set at the M and the H to quantify cardiac MIBG uptake in terms of the H/M activity ratio (5–12). The clearance rate from the myocardium (washout rate) was calculated as follows:

Figure 1.

Anterior planar images were obtained 15 min and 3 h after intravenous injection of ¹²³I-metaiodobenzylguanidine. Cardiac uptake of ¹²³I-metaiodobenzylguanidine was measured as heart to mediastinum (H/M) count ratio, using regions of interest positioned over H and upper M

$$\begin{array}{l}\text{(initial myocardial MIBG uptake}-\text{delayed myocardial}\\ \text{MIBG uptake})/\text{initial myocardial MIBG uptake }\times \text{100}\text{.}\end{array}$$

Statistical analysis

Scintigraphic values are presented as mean ± SD. Scheffé’s F test for multiple comparisons was applied to detect significant differences as defined by ANOVA. Linear regression analysis was used to determine the correlation between the H/M ratio and neurohumoral factors, and between the washout rate and neurohumoral factors. Stepwise multivariate regression analysis was performed to examine the potential interactions among the entered covariates. Student’s t test was used for comparison of paired data, and P<0.05 was considered to be significant.

RESULTS

Clinical characteristics (Table 1) and changes in hemodynamic variables during the infusion of ACh are shown in Table 2. Age, sex, systemic blood pressure and heart rate did not differ between the two groups. Intraventricular septum thickness, left ventricular end-diastolic pressure and left ventricular mass index were higher in the HCM group than in the control group (P<0.05).

TABLE 2.

Systemic hemodynamic variables

		Acetylcholine dose (μg/min)
	Baseline	3	10	30
Heart rate (beats/min)
Hypertrophic cardiomyopathy	64±3	64±3	64±3	65±4
Control	67±2	69±2	68±2	65±4
Mean arterial pressure (mmHg)
Hypertrophic cardiomyopathy	85±5	84±5	83±4	86±5
Control	93±5	90±6	90±5	89±5

Values are expressed as mean ± SE.

Coronary vasomotor responses induced by ACh

Baseline mean arterial pressure and heart rate did not differ between the two groups. Intracoronary infusion of ACh did not cause significant changes in arterial pressure or heart rate in either group (Table 2). ACh at doses of 3 μg/min, 10 μg/min and 30 μg/min caused dose-dependent constriction of the LAD arteries in the HCM group (0±0%, –9±5% and –20±8%, respectively), but no significant changes induced by ACh in the LAD arteries were observed in the control group. Nitroglycerin at 0.25 mg caused similar vasodilations of the LAD coronary arteries in both groups. Three of the 11 HCM patients experienced 100 μg ACh-induced diffuse coronary spasms of the LAD coronary arteries associated with chest pain or ischemic ECG changes. Intracoronary infusion of ACh did not induce coronary vasospasm, chest pain or ischemic ECG changes in eight HCM patients or in any of the control subjects.

A representative patient with coronary spasm through intracoronary infusion of ACh is illustrated in Figure 2. Representative MIBG images are illustrated in Figure 3.

Figure 2.

Coronary angiography of a 71-year-old man with hypertrophic cardiomyopathy. Coronary spasm was provoked by the infusion of acetylcholine (ACh). Coronary vasospasm was observed in the proximal left anterior descending coronary artery (#6) and diagonal branch (#9). Left coronary artery from the right anterior oblique view

Figure 3.

Resting ¹²³I-metaiodobenzylguanidine anterior planar images in a 71-year-old man with hypertrophic cardiomyopathy. Top Early image (15 min) showing regions of interest for the heart (round outline, number 1), mediastinum (square outline, number 2) and lung (square outline, number 3). Bottom Delayed image (3 h)

¹²³I-MIBG distribution and washout from the myocardium

Cardiac MIBG uptake, measured by the H/M count ratio and the washout rate, is shown in Figure 4. There was a significant difference in the H/M ratio between HCM and control subjects (2.1±0.3 versus 2.6±0.3, P<0.01, Scheffé’s test).

Figure 4.

Comparison of ¹²³I-metaiodobenzylguanidine parameters (heart [H] to mediastinum [M] ratio and washout rate [WR]) between hypertrophic cardiomyopathy (HCM) subjects and control (C) subjects. Values are mean ± SD

An increased washout rate of ¹²³I-MIBG was observed in the HCM subjects compared with the control subjects (35±6% versus 28±3%, P<0.01). There was a significant difference in the H/M ratio between HCM subjects with cornonary spasm and HCM subjects without spasm (1.8±0.1 versus 2.2±0.2, P<0.05). The HCM subjects with coronary spasm had a higher washout rate than HCM subjects without coronary spasm (41.3±3.1% versus 33.1±4.5%, P<0.05) (Figure 5).

Figure 5.

Comparison in ¹²³I-metaiodobenzylguanidine parameters (heart to mediastinum [H/M] ratio and washout rate [WR]) between hypertrophic cardiomyopathy subjects with spasm (Spasm [+]) and hypertrophic cardiomyopathy subjects without spasm (Spasm [–]). Values are mean ± SD

DISCUSSION

Coronary endothelial function and ischemia in HCM

Several possible mechanisms have been proposed to explain myocardial ischemia in HCM, including small coronary artery disease (19), coronary artery spasm (3), inadequate capillary density in relation to the increased myocardial mass (20) and impaired coronary blood flow reserve (21). Recent studies (3) have reported that coronary spasm is occasionally observed in HCM patients, which is consistent with our results. ACh has been used as a pharmacological tool to induce coronary vasospasm and to stimulate the endothelial production of nitric oxide. Therefore, ACh may not only stimulate the release of endothelium-derived relaxing factors, but may also cause the direct smooth muscle contraction (ie, the net effect of ACh on coronary circulation is due to a combination of both effects). In the present study, three of 11 patients with HCM had ACh-induced coronary spasm. Therefore, it is plausible that ACh-induced coronary vasoconstrictions were caused by the impaired release of endothelium-derived nitric oxide or enhanced direct smooth muscle constriction, or a combination of both.

Myocardial uptake of MIBG

The present study clearly shows that there is a decrease of ¹²³I-MIBG uptake in HCM patients compared with control patients. ¹²³I-MIBG shares the same uptake and storage mechanisms as NA. It has been reported (22) that the uptake-1 system is mediated by the NA transporter and the uptake-2 system is an extraneuronal system. Planar imaging of the H/M ratio is a simple method that allows comparison of inter-individual and inter-institutional results by correcting for differences in body geometry and attenuation between individual subjects. The reason for using the H/M ratio to quantify myocardial MIBG activity is based on the close relation between the H/M ratio and cardiac NA content (23). The lack of uptake in the early time period after cardiac allograft suggests that the uptake-2 system (extraneuronal uptake) is low in humans. This finding suggests that the decreased H/M ratio observed in the present study is likely due to an impaired uptake-1 system.

Myocardial washout of MIBG

The present study shows that the washout rate of ¹²³I-MIBG is impaired in the hearts of patients with HCM, in accordance with a previous report (7). Cardiac hypertrophy with hypertension has been reportedly related with sympathetic activity (24). In hypertensive patients with left ventricular hypertrophy, the washout rate becomes higher with the advance of hypertension and with the development of left ventricular hypertrophy (24). A study by Takatsu et al (25) reported that reduced MIBG accumulation in cardiomyopathic hamsters at an early stage of heart failure was mainly due to increased washout from the neural component in the myocardium, indicating reduced myocardial MIBG retention (25). The accelerated release of NA, possibly resulting from impaired vesicular storage or increased spillover at the nerve endings, could be responsible for MIBG activity in HCM (26). In the inferior segment with reduced sympathetic nerve terminals, the remaining nerves may be activated to compensate for the loss of quantity. Another possible explanation for increased washout of MIBG is a larger extraneuronal accumulation of MIBG in patients with HCM. When the myocardium tends to have a reduced number of sympathetic neurons, it follows that a larger proportion of the MIBG remains extraneuronal and demonstrates faster clearance from the myocardium. Considering that the uptake-2 system is low in humans, we can assume that rapid MIBG washout from the myocardium reflects increased sympathetic nerve activity, including reduced myocardial MIBG retension.

Possible mechanisms underlying abnormal cardiac sympathetic function in patients with HCM

Several mechanisms are involved in abnormal uptake of ¹²³I-MIBG in patients with HCM. The increased wall thickness of the left ventricule in HCM subjects is closely related to perfusion defects of thallium (1), suggesting myocardial damage, including myocardial hypertrophy, ischemia, disarray or fibrosis (8). Furthermore, in patients with left ventricular dysfunction, decreased uptake-1 function has been found to be related to both myocardial overexposure to NA and decreased myocardial beta-receptors (7,8,27–29). NA levels required to inhibit MIBG uptake in these conditions seem to be much higher than those in patients with HCM. It has been reported that abnormal sympathetic abnormality is related to the activity of vasospasm in patients with vasospastic angina (30). The present study showed, for the first time, that cardiac sympathetic abnormalities can be observed in HCM patients with coronary vasospasm. These results suggest that impaired sympathetic nerve function may be related to increased vasomotor activity and diminished coronary blood flow reserve in patients with HCM.

The present study was limited by the small number of patients. Further studies are needed to clarify the exact principal mechanisms that are responsible for MIBG abnormalities in patients with HCM. We evaluated sympathetic nerve function in HCM patients by the use of global indexes of MIBG. Regional quantitative assessment of MIBG activity could be compared with a large number of patients.

In conclusion, impaired sympathetic nerve function may be associated with coronary vasospasm and diminished coronary blood flow reserve in patients with HCM.