Relationship between Flow-mediated Endothelial Vasodilation and the Pulse Wave Velocity, and Cervical Carotid Artery Stenosis.

Carotid artery stenosis is elicited by atherosclerosis and is the main cause of cerebral thrombosis. Flow-mediated endothelial vasodilation (FMD) can be measured noninvasively to assess vascular endothelial function related to atherosclerosis. The pulse wave velocity (PWV) is used to evaluate the vascular media involved in atherosclerosis. We investigated the relationship between these measurements in 75 consecutive patients with atherosclerotic cerebral thrombosis. They were assigned to three equal groups based on the severity of carotid artery stenosis on ultrasonograms. Group 1 had no stenosis, group 2 manifested moderate stenosis (<60%), and group 3 presented with severe stenosis (≥60%). We compared the FMD and PWV among the three groups. The PWV was significantly lower in group 1 than the other two groups. The FMD was significantly lower in group 3; it was significantly lower in group 2 than group 1. There was an inverse correlation between the FMD and the severity of carotid artery stenosis. Our findings show that for assessing the severity of carotid artery stenosis, the FMD is more useful than the PWV.

Introduction

Atherosclerosis of the cervical carotid artery plays an important role in ischemic cerebrovascular disease; it is also a reliable marker of systemic atherosclerosis.[1–3)] The pulse wave velocity (PWV) is associated with major cardiovascular risk factors; it is used to assess the function of the medial membrane. Although evaluation of the PWV is a noninvasive method to diagnose systemic atherosclerotic changes, it does not yield information on the severity and progression of cervical carotid artery stenosis.[4)]

Endothelial cells play an important role in the protection from atherosclerotic progression. Impairment of endothelial function may result in the promotion of atherosclerotic changes and in plaque formation.[5)] Measurement of flow-mediated endothelial vasodilation (FMD) in the brachial artery is a commonly used, noninvasive method to assess endothelial cell function (Fig. 1)[6–11)] related to atherosclerotic changes such as carotid elongation,[12)] cardiovascular risk,[5–9)] and cortical cerebral infarction.[11)] A meta-analysis performed by Inaba et al.[5)] showed that impairment of the brachial FMD was not only associated with known cardiovascular risk factors but also significantly associated with future cardiovascular events.

Fig. 1.

Measurement of flow-mediated endothelial vasodilation in the brachial artery.

We investigated the relationship between PWV and FMD measurements and the degree of cervical carotid artery stenosis ranging from mild to severe.

Materials and Methods

From among 186 consecutive patients with cerebral thrombotic infarction who were at least 60 years old at the time of hospitalization in Tama Nagayama Hospital between April 2009 and March 2012, we selected 75 for inclusion in this study. They were 52 males and 23 females; their mean age was 65 years (range 60–79 years) (Table 1). Patients unable to cooperate, patients with cerebral hemorrhage, transient ischemic attacks, subarachnoid hemorrhage, embolic stroke secondary to heart disease, brain tumors, dementia, occlusion of the cervical artery, and in postoperative status were excluded from our study.

Table 1

Patient characteristics

Degree of cervical carotid artery stenosis	Carotid artery stenosis			Difference
	None (group 1)	<60% (group 2)	≥60% (group 3)
Patients	25	25	25	NS
Age (years)	67 ± 7	69 ± 6	72 ± 6	NS
Gender (male/female)	16/9	17/8	19/6	NS
Medication
HT	21	23	24	NS
HL	17	15	19	NS
DM	8	16	17	P <0.05
Smoking	19	22	24	NS
Stroke types
Cortex CI/subcortex CI	2/23	10/15	25/0	P <0.05
Cervical carotid artery stenosis (%)		36 ± 27	85 ± 9

CI: cerebral infarction, DM: diabetes mellitus, HL: hyperlipidemia, HT: hypertension, NS: not significant.

Cerebral thrombosis was diagnosed in patients with rapid-onset brain infarction whose symptoms were present for more than 24 h and who manifested no other vascular occlusion.[4)] Based on their clinical characteristics and brain computed tomography (CT)- and/or findings, the infarctions were classified as cortical or subcortical.[4)] Cortical infarction was recorded when there was clinical evidence of cortical deficits and when CT- and/or magnetic resonance imaging (MRI) scans showed an infarct involving the cerebral cortex in the carotid artery territory without evidence of cardiogenic embolism. Subcortical infarction was recorded when CT- and/or MRI studies showed that a small infarct in the subcortical carotid artery territory was responsible for the symptoms. Patients with combined cortical and subcortical involvement on imaging studies were assigned to the cortical infarction group.

The 75 included patients were assigned to three equal groups. Group 1 manifested no-, group 2 moderate (<60%)-, and group 3 severe (≥60%) carotid artery stenosis. We compared the FMD and PWV among the three groups.

To evaluate maximum cervical carotid artery stenosis, we applied the area method using cervical echo findings.[1)] We performed high-resolution B-mode ultrasonography on a 7.5-MHz duplex scanner (Hitachi EUB-555, Hitachi, Japan). We defined carotid atheromatous plaques as lesions with a carotid [intima media thickness (IMT) >1.1 mm]. To determine the degree of right- and left-sided cervical carotid artery stenosis we recorded the area involved by stenosis.

To assess endothelial function we determined the FMD on a high-resolution ultrasound apparatus featuring a 7.5-MHz linear array transducer and on-line computer-assisted semi-automatic analysis software (EF, Unex Co. Ltd, Nagoya, Japan). We applied the guidelines of the International Brachial Artery Reactivity Task Force.[13)] Since the FMD is affected by factors such the body temperature, food, drugs, and sympathetic stimuli, we measured it in the early morning, before a meal, and after a 10-min rest when the systolic blood pressure was 160 mmHg and the pulse 100 beats per min or less. The subjects were instructed to fast for at least 4 h and to abstain from smoking and from ingesting alcohol, caffeine, and antioxidant vitamins for at least 12 h. They sat in a quiet, dark, air-conditioned room (22–25°C) for 15 min before FMD measurements were performed by a single examiner. The two echo probes were placed in the brachial artery and the vessel diameter between the two probes was recorded. First, the brachial artery diameter 5–10 cm above the elbow at rest was measured (Fig. 1). For blood-flow blockage we placed a pneumatic cuff and inflated it for 5 min to 50 mmHg above the systolic pressure. After deflating the cuff to elicit reactive hyperemia, the brachial artery diameter was recorded continuously for 2 min. We also measured the maximum diameter of the radial artery after blood-flow blockage. The FMD was calculated with the formula

We evaluated the PWV/ankle brachial pressure index (ABI) using the AT-form PWV/ABI (Nippon Colin Co., Ltd., Komaki, Japan)[4)] and simultaneously measured the bilateral brachial and tibial artery pressure waveforms using a volume-plethysmographic apparatus. The brachial-ankle PWV was calculated based on the time required for the waveform to travel between the right arm and both ankles. The distance between the right arm and the ankle was estimated based on the patient’s height. We used the mean value of our bilateral measurement data. None of our 75 patients had an ABI below 0.8.

Our findings are expressed as the mean ± SD. All statistical analyses were performed using StatView version 4.0 software (Abacus Concepts Inc., Berkeley, CA, USA). For parametric data analysis we used the unpaired t-test for 2-group comparisons. For 3-group comparisons we applied one-way analysis of variance with Fisher’s least-significant difference post-hoc test. For non-parametric data analysis, we performed the Mann–Whitney U-test for comparisons. Differences of P <0.05 were considered statistically significant.

Results

There was no statistically significant difference in our patients’ age, gender, and smoking habit. The number of patients with diabetes mellitus (DM) was significant lower in group 1 than the other groups. Of the 75 patients 37 (49.3%) presented with cortical- and 38 (50.7%) with subcortical infarction. The number of patients with cortical infarction was significantly higher in group 3; subcortical infarction was significantly lower in group 1.

As shown in Fig. 2, the PWV was significantly lower in group 1 (1702 ± 349 cm/s) than group 2 (2103 ± 427 cm/s) and group 3 (2225 ± 384 cm/s) (P <0.05). The FMD was significantly greater in group 1 (5.9 ± 2.3%) than group 2 (3.8 ± 1.8%) and group 3 (1.9 ± 1.3%) (all, P <0.05); it was significantly greater in group 2 than group 3 (Fig. 3).

Fig. 2.

Pulse wave velocity (PWV) recorded in patients with cervical carotid artery stenosis. The PWV in patients without- was significantly lower than in patients with stenosis. *P <0.05.

Fig. 3.

Flow-mediated endothelial vasodilatation (FMD) recorded in patients with cervical carotid artery stenosis. The FMD was significantly larger in patients without- than with moderate to severe stenosis (group 1 vs groups 2 and 3).

Of the 25 group 3 patients, 24 subsequently underwent carotid endarterectomy (CEA). There were no complications related to systemic arteriosclerosis such as ischemic heart disease during the perioperative period.

Discussion

The PWV is associated with major cardiovascular risk factors and a predictor of the prognosis of patients with end-stage renal failure and hypertension.[7)] According to Tamaki et al.,[4)] it is associated with carotid atherosclerosis but not with the severity of cervical carotid artery stenosis. However, it is correlated with the carotid IMT in patients with type 2 DM.[14)] We found a correlation between the PWV and carotid atherosclerosis but not with the severity of cervical carotid artery stenosis, although 24 of our group 3 patients manifested severe cervical carotid artery stenosis requiring CEA. Assessment of the PWV is useful for the noninvasive evaluation of systemic atherosclerotic changes. However, it does not shed light on the severity and progression of cervical carotid artery stenosis, factors needed to predict the risk for a cerebral stroke.[1,2)]

Whether the FMD of the cervical carotid artery is a predictor of IMT worsening remains controversial.[11–13)] A meta-analysis reported by Lorenz et al.[15)] failed to prove the association between IMT progression and cardiovascular risk in the general population. Juonala et al.[16)] reported that in young healthy adults, the FMD was inversely associated with the cervical carotid IMT; the number of risk factors was correlated with increased IMT in subjects with an anomalous FMD. These studies suggest that systemic endothelial function may affect the association between risk factors and atherosclerosis. The FMD also predicted changes in the number of cervical carotid artery plaques.[17)] Such results suggest that cervical carotid artery stenosis, indicative of an advanced stage of epithelial dysfunction, may be related with the FMD and that the FMD may alert to the progression of cervical carotid artery stenosis.

We enrolled patients with atherosclerotic cerebral thrombosis whose atherosclerosis seemed to be advanced; patients with severe carotid artery stenosis were also included. We assessed the relationship between the FMD and the degree of cervical carotid artery stenosis requiring CEA rather than the carotid IMT. We found that the FMD was significantly smaller in patients with severe cervical carotid artery stenosis (group 3) than in the other groups. Therefore, we think that FMD assessment is informative because it yields indirect qualitative information on not only the coronary- but also the cervical carotid artery.

The degree of stenosis and a cerebral infarct due to cervical carotid artery stenosis rendered some of our patients eligible for surgery. If the FMD can predict cerebral infarction related cervical carotid artery stenosis and alert to its progression, it can be used to identify patients needing CEA for the prevention of later cerebral infarction. Our cross-sectional study revealed that the severity of cervical carotid artery stenosis was correlated with the FMD. However, longitudinal prospective studies are needed to confirm that the FMD reflects the progression of cervical carotid artery stenosis and that it alerts to the danger of cerebral infarction.

We document that there is a stronger relationship between cervical carotid artery stenosis and the FMD than the PWV. While the PWV is useful for differentiating between patients with- and without stenosis, it is not a suitable parameter for the quantitative evaluation of stenosis. Because the PWV reflects the compliance of the blood vessel wall, it can be used to detect deterioration in the function of the media. The FMD, on the other hand, reflects the release of nitric oxide in the vascular endothelium; it can be used to check for deterioration of the inner membrane. As cervical carotid artery stenosis results in thickening of the intima, it makes sense to suggest that the FMD is a better parameter than the PWV for the evaluation of cervical carotid artery stenosis.

Our study has some limitations. The number of patients was relatively small. We encountered no perioperative complications related to systemic arteriosclerosis although 24 of our group 3 patients required CEA. While the relationship between the FMD, an index of arteriosclerosis, and systemic arteriosclerotic complications during the perioperative period is very interesting, it was not examined in this study. Although measurement of the FMD is noninvasive, highly versatile, and useful for managing lifestyle-related diseases, it requires pre-procedural fasting and patient medications must be taken into account. Environmental factors also affect FMD measurements and patient cooperation is required. Cervical carotid artery plaques may harbor unstable components such as a lipid core and intraplaque hemorrhage[18–20)] that may increase the risk for cerebral infarction. We did not evaluate the relationship between the plaque makeup and the FMD. Others[6,8,21)] reported that in patients whose FMD measurements indicated late, albeit maximal dilation, the cardiovascular risk profile was worse and the incidence of carotid atherosclerosis was higher than in patients with early dilation. Therefore, not only the magnitude of dilation but also its timing is predictive of cardiovascular risk. Studies are underway to address these issues. Our cross-sectional study was performed to investigate the relationship between the FMD and the severity of cervical carotid stenosis. Longitudinal studies are needed to determine whether stenotic progression can be predicted by the FMD.

Conclusion

The FMD is inversely correlated with the severity of carotid artery stenosis in patients with atherosclerotic cerebral thrombosis. The PWV, on the other hand, reflects only the absence/presence of carotid stenosis. We found that for assessing the severity of carotid stenosis, FMD- were superior to PWV measurements.