Department of Cardiovascular Sciences, Integrative Human Cardiovascular Physiology and Cardiac Surgery Unit, University of Leicester, The Glenfield Hospital, Groby Road, Leicester LE3 9QP, UK
Received March 17, 2003;
revised January 1, 2004;
accepted January 22, 2004
* Corresponding author. Tel.: +44-116-250-2450; fax: +44-116-250-2449
E-mail address: lh63{at}le.ac.uk
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Abstract |
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Methods The compliance, distensibility, and incremental elastic modulus (iEmod) of the internal thoracic arteries (), long saphenous veins (
), and radial arteries (
) from 74 patients undergoing coronary surgery were studied in organ baths. Twenty-four patients had left main stem (LMS) disease and 50 non-LMS coronary disease.
Results Internal thoracic arteries from patients with LMS presented significantly lower compliance (17%) and distensibility (18%) and significantly higher iEmod (19%) than internal thoracic arteries from patients with non-LMS disease. Radial arteries from patients with LMS presented higher iEmod (50%) than radial arteries from patients with non-LMS disease. Furthermore, long saphenous veins from patients with LMS had reduced compliance (45%), reduced distensibility (40%) and increased iEmod (34%) compared to those from patients with non-LMS disease.
Conclusions LMS coronary disease is associated with a significantly reduced elasticity of extracardiac arteries and veins compared to non-LMS coronary disease. This finding suggests that widespread vascular elasticity defects may play a role in the development of LMS disease and be responsible for the higher incidence of early and late cardiovascular morbidity observed in this condition.
Key Words: Elasticity Coronary disease Extracardiac vessels
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Introduction |
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Recently, Trip et al.6 demonstrated that pseudoxanthoma elasticum, an inborn disorder of the elastic connective tissue, is associated with a four-times higher risk of coronary disease, implying that elasticity defects might, in fact, be a primary event in the pathogenesis of coronary atherosclerosis.7
Patients with left main stem (LMS) coronary disease have a worse prognosis than those with distal coronary disease, with 5-year mortality rate of 50% without surgery.8,9 In preliminary studies in our laboratory, peripheral arteries and veins from patients undergoing coronary artery bypass grafting showed significant variability and a tendency to more stiffness in patients with advanced coronary disease. We hypothesised that if an elasticity defect is a primary event in coronary disease it may be more pronounced in disease of the LMS. Therefore, the aim of this study was to investigate the elastic properties of medium-size arteries and veins in patients with ischaemic heart disease and to examine whether LMS coronary disease was associated with greater elasticity defects compared to non-LMS disease.
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Materials and methods |
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A single researcher who followed exactly the same protocol performed all the experiments. The vessel mounted on the wire support was allowed to equilibrate unstretched at 37 °C for 30 min before undergoing a graded passive distension procedure. The two wires were brought up to where they touched, leaving no space between the two wires, and the micrometer was zeroed. The wires were then separated stepwise every 60 seconds in 4-g steps. The wall force and micrometer reading were recorded at each step. The maximum wall force allowed was 25 g and the procedure was repeated four times for each vessel ring. The internal circumference, wall tension, and transmural pressure were calculated at each step according to the method described by Angus et al.10 and shown in the Appendix A. Stressstrain relationships and pressurediameter relationships were determined for each set of measurements using exponential and third-order polynomial regression analysis,11,12 respectively. In preliminary studies in our laboratory, it was demonstrated that the first length-tension curve differed significantly from the second, third, and fourth measurements. Because of this, the first measurement was interpreted as an accommodation of the vessel to the experimental conditions and, therefore, excluded. The results obtained from each vessel were the mean of the second, third, and fourth measurements to reduce the intraobserver variability. The average R2 for the stress-strain relationships were 0.92±0.04, 0.92±0.04, and 0.91±0.05 for the internal thoracic arteries, radial arteries, and long saphenous veins, respectively. The average R2 for the pressurediameter curves were 0.96±0.04, 0.94±0.04, and 0.92±0.05 for the internal thoracic arteries, radial arteries, and long saphenous veins, respectively.
The compliance, distensibility, and incremental elastic modulus (iEmod) were calculated for transmural pressures of 40 and 80 mmHg for arteries and at a pressure of 40 mmHg for veins. These pressures were chosen because they fell within the range of reliability of the organ-bath experimental setting. At very small forces, corresponding to pressures of less than 30 mmHg, the vessel may not be properly stretched, and at very high forces, corresponding to pressures of more than 90 mmHg, the wires may bend. In both situations the circumference calculations may not be reliable. The latter was particularly obvious in large-diameter vessels like saphenous veins. Therefore, in saphenous veins calculations were made only for 40-mmHg pressure.
The first derivative of the pressurediameter relationship was used to calculate compliance and distensibility. The first derivative of the stressstrain curve was used to calculate iEmod. Compliance and distensibility are directly related while iEmod is inversely related to elasticity.
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Statistical analysis |
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At first, the sample size was calculated to detect one standardised difference with a power of 90% at a significance level of 0.05. Since the ratio between LMS and non-LMS coronary patients in our centre is 0.20.5, the modified total sample size was calculated as 4776 subjects. The final power of the study to detect one standardised difference at a significance level of 0.05 was more than 95%.
Multivariate analysis was performed with multiple linear regression. The covariates used in the model were selected on clinical grounds and are presented in Table 1 (clinical data). Forward stepwise regression with an entry probability of 0.05 was used to evaluate the model's best fit. The final model was chosen on the basis of the adjusted R2 and the model's ANOVA probability value. Results are expressed as the 25th, 50th, and 75th percentiles or mean±standard error. Differences were considered significant at a probability level of less than 0.05.
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Results |
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Table 2 shows a multiple regression analysis of the iEmod on clinical variables for internal thoracic arteries, radial arteries, and long saphenous veins. This analysis confirmed the above relationships between LMS disease and iEmod and provides evidence of a higher iEmod for radial arteries at 80 mmHg, which was not obvious in the univariate analysis. Furthermore, it revealed that additional factors might affect vascular elasticity. Thus, the internal thoracic arteries from patients with a small body surface area and the radial arteries from patients with reduced left ventricular ejection fraction were stiffer. The iEmod of internal thoracic arteries also increased when the patient used calcium antagonists.
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Discussion |
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An intriguing aspect of the present study was the increased stiffness of the saphenous veins of patients with LMS disease, showing that the defect is global and not limited to the arterial system. It has been reported that factors such as ageing,16,17 hypertension,18 and heart failure19 may affect venous stiffness; however, in the present study none of these factors appeared to influence venous stiffness. Our finding that arteries and veins are stiffer in patients with LMS disease supports the concept that there is a global deficit in the elasticity of vessels in this group of patients.
In univariate analysis there was a marginal difference in age between the two groups that did not reach statistical significance (). In order to exclude the influence of age on the variability of the elastic properties of extracardiac vessels, a multivariate model was used that showed that age was not a predictor of the variability observed.
Patients with LMS disease present earlier graft failure and have a worse long-term survival following coronary surgery than patients with distal coronary disease. As early as 1989, Rowe et al.20 demonstrated that LMS disease was an independent risk factor for late mortality following coronary surgery. In spite of the extensive use of arterial grafts in the 1990s, LMS disease has continued to be an independent risk factor for late cardiac mortality and for the need for earlier repeat myocardial revascularisation.21 The reason for a worse clinical outcome in patients with LMS is unclear; however, reduced elasticity of arteries and veins used as coronary grafts in these patients may explain premature graft failure and cardiac deaths.
Our finding that radial arteries are stiffer in patients with left ventricular dysfunction contrasts with the findings of Kaiser et al.,22 who demonstrated decreased iEmod in patients with heart failure. A possible explanation for this discrepancy is that their study was performed in vivo and in the presence of pharmacological agents, while the assessment of vascular elasticity in our study was carried out in vitro and without the use of drugs. In this connection it is worth noting that previous studies have demonstrated that acetylcholine and catecholamines may have an effect on vascular elasticity.19 Our observation that calcium antagonists decrease the elasticity of the internal thoracic artery is difficult to interpret since the disparate effects of these medications on arterial compliance are well known.2326 The significance of these findings remains to be elucidated.
A potential limitation of the present study is that vessels from normal subjects could not be obtained to serve as controls because of obvious ethical constraints. For that reason, the hypothesis that patients without left main stem coronary disease may also have a degree of vascular elasticity deficit compared to normal individuals cannot be excluded. It may also be argued that our use of an in vitro preparation removes the potential influence of circadian circulating hormones and local metabolites that may influence vascular elastic properties and, in fact, its use may be considered advantageous. It must be conceded that in vitro experiments may carry a degree of inaccuracy regarding the calculation of `transmural pressure', with a relative uncertainty about its correspondence to real arterial pressure. However, in compensation, they may be more accurate in the calculation of iEmod since stressstrain relationships are under direct control.
In conclusion, LMS disease is associated with significantly reduced vascular elasticity when compared to non-LMS coronary disease. This finding suggests that a defect in vascular elasticity may play a role in the development of LMS disease and can explain the less satisfactory long-term clinical results seen in this group of patients. These results indicate that pharmacological modifications of vascular elasticity could be a therapeutic target with significant prognostic implications.
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Appendix A |
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Formula 2
Diameter (mm)=Internal circumference/.
Formula 3
Wall tension (mN/mm)=9.807 (mN/g)xforce (g)/2xlength (mm).
Formula 4
Transmural pressure (mmHg)=7.403 (mmHg/mN)x2xxWall tension (mN)/diameter (mm).
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References |
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