Dynamic behaviour of the common femoral artery: age and gender of minor importance

Normal tolerance limits for side-to-side differences in diameters of major lower limbs arteries of 228 healthy subjects

PURPOSE

To determine reference values and tolerance limits of between-side differences for the calibers of the common femoral artery (CFA), superficial femoral artery (SFA), popliteal artery (PA), dorsalis pedis artery (DPA), and posterior tibial artery (PTA).

MATERIALS AND METHODS

Calibers of arteries, defined as the largest distance between internal hyperechogenic lines of the intima-media complex of the arterial wall, were measured during the diastole phase determined from echo-tracking B mode ultrasound scanning and grey-scale ultrasound in 228 healthy volunteers aged 18-81 years (43.1 ± 16.7).

RESULTS

The mean, 95% confidence and tolerance limits covering 90% of population for left and right side of each artery were: CFA: 8.1 mm, 7.9-8.3 mm, 6.0-10.3 mm; 8.1 mm, 7.9-8.5 mm, 5.9-10.2 mm; SFA: 6.2 mm, 6.0-6.3 mm, 4.7-7.6 mm; 6.1 mm, 6.0-6.3 mm, 4.7-7.6 mm; PA: 6.1 mm, 6.0-6.2 mm, 4.6-7.6 mm; 6.1 mm, 5.9-6.2 mm, 4.5-7.6 mm; DPA: 2.0 mm, 1.9-2.0 mm, 1.2-2.7 mm; 2.0 mm, 1.9-2.0 mm, 1.2-2.8 mm; PTA: 2.1 mm, 2.0-2.1 mm, 1.4-2.8 mm; 2.1 mm, 2.1-2.2 mm, 1.4-2.8 mm, respectively. Tolerance limits for between-side differences and ratios were: CFA - 0.5-0.7 mm, 0.9-1.1; SFA - 0.5-0.6 mm, 0.9-1.1; PA - 0.5-0.5 mm, 0.9-1.1; DPA -0.4-0.4 mm, 0.8-1.2; PTA - 0.4-0.4 mm, 0.8-1.2. Regression analysis showed weight and age dependency of vessels diameters. There are no differences between men and woman in vessels size, except in DPA's, when body weight and age are taken into account in a regression analysis.

CONCLUSIONS

We estimated normal reference tolerance limits of side-to-side differences in diameters of lower limb arteries. The limits can inform an investigator what differences in diameters occur in healthy individuals, and hence can serve as cut-offs in diagnostic and screening strategies.

The status and challenges of replicating the mechanical properties of connective tissues using additive manufacturing

The ability to fabricate complex structures via precise and heterogeneous deposition of biomaterials makes additive manufacturing (AM) a leading technology in the creation of implants and tissue engineered scaffolds. Connective tissues (CTs) remain attractive targets for manufacturing due to their "simple" tissue compositions that, in theory, are replicable through choice of biomaterial(s) and implant microarchitecture. Nevertheless, characterisation of the mechanical and biological functions of 3D printed constructs with respect to their host tissues is often limited and remains a restriction towards their translation into clinical practice. This review aims to provide an update on the current status of AM to mimic the mechanical properties of CTs, with focus on arterial tissue, articular cartilage and bone, from the perspective of printing platforms, biomaterial properties, and topological design. Furthermore, the grand challenges associated with the AM of CT replacements and their subsequent regulatory requirements are discussed to aid further development of reliable and effective implants.

Increased cardiovascular risk without generalized arterial dilating diathesis in persons who do not have abdominal aortic aneurysm but who are first-degree relatives of abdominal aortic aneurysm patients

There is a strong genetic predisposition towards abdominal aortic aneurysm (AAA), but it is unknown whether persons without AAA but with first-degree relatives who are AAA patients have a generalized dilating diathesis, defect arterial wall mechanics, or increased cardiovascular risk. The aim of the study was to investigate arterial diameters and wall mechanics at multiple arterial sites in these subjects and compare them with controls without a family history of AAA. This study included 118 first-degree relatives of patients with AAA and 66 controls (age: 40-80 years). The abdominal aorta, common carotid artery, common femoral artery, and popliteal artery were investigated by echo-tracking ultrasound. The relatives had no arterial dilatation, but they did tend to have smaller diameters than controls. Relatives had a higher heart rate, diastolic blood pressure, and mean arterial pressure than controls. The distensibility coefficient and the compliance coefficient were decreased in all arteries in male relatives, adjusted for age and smoking; these coefficients were normalized after adjustment for mean arterial pressure and heart rate. Female relatives had a lower compliance coefficient in the abdominal aorta, adjusted for age and smoking. After adjustment for mean arterial pressure and heart rate, the difference disappeared. No general arterial dilatation in relatives without AAA was found, supporting the hypothesis that the dilating diathesis is linked to the aneurysmal manifestation in the abdominal aorta. Although the threat of aneurysmal dilatation and rupture seems to be lacking in these subjects, heart rate, blood pressure, and arterial wall stiffness were all increased, which may indicate a higher risk of developing cardiovascular morbidity and mortality.

Low wall stress in the popliteal artery: Other mechanisms responsible for the predilection of aneurysmal dilatation?

The popliteal artery (PA) is, after aorta, the most common site for aneurysm formation. Why the PA is more susceptible than other peripheral muscular arteries is unknown. We hypothesized that the wall composition, which in turn affects wall properties, as well as the circumferential wall stress (WS) imposed on the arterial wall, might differ compared to other muscular arteries. The aim was to study the WS of the PA in healthy subjects with the adjacent, muscular, common femoral artery (CFA) as a comparison. Ninety-four healthy subjects were included in this study (45 males, aged 10-78 years and 49 females, aged 10-83 years). The diameter and intima-media thickness (IMT) in the PA and CFA were investigated with ultrasound. Together with blood pressure the WS was defined according to the law of Laplace adjusted for IMT. The diameter increased with age in both PA and CFA (p<0.001), with males having a larger diameter than females (p<0.001). IMT increased with age in both PA and CFA (p<0.001), with higher IMT values in males only in PA (p<0.001). The calculated WS was unchanged with age in both arteries, but lower in PA than in CFA in both sexes (p<0.001). In conclusion, this study shows that the PA and CFA WS is maintained during aging, probably due to a compensatory remodelling response with an increase in arterial wall thickness. However, the stress imposed on the PA wall is quite low, indicating that mechanisms other than WS contribute to the process of pathological arterial dilatation in the PA.

Non-triggered quantification of central and peripheral pulse-wave velocity

PURPOSE

Stiffening of the arteries results in increased pulse-wave velocity (PWV), the propagation velocity of the blood. Elevated aortic PWV has been shown to correlate with aging and atherosclerotic alterations. We extended a previous non-triggered projection-based cardiovascular MR method and demonstrate its feasibility by mapping the PWV of the aortic arch, thoraco-abdominal aorta and iliofemoral arteries in a cohort of healthy adults.

MATERIALS AND METHODS

The proposed method "simultaneously" excites and collects a series of velocity-encoded projections at two arterial segments to estimate the wave-front velocity, which inherently probes the high-frequency component of the dynamic vessel wall modulus in response to oscillatory pressure waves. The regional PWVs were quantified in a small pilot study in healthy subjects (N = 10, age range 23 to 68 yrs) at 3T.

RESULTS

The projection-based method successfully time-resolved regional PWVs for 8-10 cardiac cycles without gating and demonstrated the feasibility of monitoring beat-to-beat changes in PWV resulting from heart rate irregularities. For dual-slice excitation the aliasing was negligible and did not interfere with PWV quantification. The aortic arch and thoracoabdominal aorta PWV were positively correlated with age (p < 0.05), consistent with previous reports. On the other hand, the PWV of the iliofemoral arteries showed decreasing trend with age, which has been associated with the weakening of muscular arteries, a natural aging process.

CONCLUSION

The PWV map of the arterial tree from ascending aorta to femoral arteries may provide additional insight into pathophysiology of vascular aging and atherosclerosis.

In vivo estimation of the contribution of elastin and collagen to the mechanical properties in the human abdominal aorta: effect of age and sex

The mechanical properties of the aorta affect cardiac function and are related to cardiovascular morbidity/mortality. This study was designed to evaluate the isotropic (mainly elastin, elastiniso) and anisotropic (mainly collagen, collagenani) material parameters within the human aorta in vivo. Thirty healthy men and women in three different age categories (23–30, 41–54, and 67–72 yr) were included. A novel mechanical model was used to identify the mechanical properties and the strain field with aid of simultaneously recorded pressure and radius in the abdominal aorta. The magnitudes of the material parameters relating to both the stiffness of elastiniso and collagenani were in agreement with earlier in vitro studies. The load-bearing fraction attributed to collagenani oscillated from 10 to 30% between diastolic and systolic pressures during the cardiac cycle. With age, stiffness of elastiniso increased in men, despite the decrease in elastin content that has been found due to elastolysis. Furthermore, an increase in stiffness of collagenani at high physiological pressure was found. This might be due to increased glycation, as well as changed isoforms of collagen in the aortic wall with age. A marked sex difference was observed, with a much less age-related effect, both on elastiniso and collagenani stiffness in women. Possible factors of importance could be the effect of sex hormones, as well as differing collagen isoforms, between the sexes.

Arterial properties along the upper arm in humans: age-related effects and the consequence of anatomical location

The normal aging process of the brachial artery (BA) wall is of specific interest since it is often selected as a model artery in studies of vascular function. With echo-tracking ultrasound, diameter, absolute diameter change, and intima-media thickness (IMT) were registered in 60 healthy subjects, 21-86 yr (30 men), at a proximal, upper third, and distal arterial site along the upper arm. Blood pressure was recorded noninvasively, and the distensibility coefficient (DC) was calculated. The diameter at the proximal site increased with age from 5.5 +/- 0.2 mm in the young subjects to 6.9 +/- 0.3 mm (P < 0.01) in the elderly subjects, concomitantly as IMT increased from 0.40 +/- 0.01 to 0.65 +/- 0.03 mm (P < 0.001). The diameter at the other sites was similar in the young and elderly subjects, whereas IMT increased slightly with age. At the proximal site, DC decreased dramatically from 40.7 +/- 2.2 to 10.1 +/- 0.8 10(-3)/kPa (P < 0.001) with age, whereas hardly no change was seen in the distal upper arm. The principal transit zone between elastic to predominantly muscular artery behavior seems to be located within the proximal part of the brachial artery, emphasizing the importance of carefully defining the arterial examination site.

Toward a noninvasive subject-specific estimation of abdominal aortic pressure

A method for estimation of central arterial pressure based on linear one-dimensional wave propagation theory is presented in this paper. The equations are applied to a distributed model of the arterial tree, truncated by three-element windkessels. To reflect individual differences in the properties of the arterial trees, we pose a minimization problem from which individual parameters are identified. The idea is to take a measured waveform in a peripheral artery and use it as input to the model. The model subsequently predicts the corresponding waveform in another peripheral artery in which a measurement has also been made, and the arterial tree model is then calibrated in such a way that the computed waveform matches its measured counterpart. For the purpose of validation, invasively recorded abdominal aortic, brachial, and femoral pressures in nine healthy subjects are used. The results show that the proposed method estimates the abdominal aortic pressure wave with good accuracy. The root mean square error (RMSE) of the estimated waveforms was 1.61 +/- 0.73 mmHg, whereas the errors in systolic and pulse pressure were 2.32 +/- 1.74 and 3.73 +/- 2.04 mmHg, respectively. These results are compared with another recently proposed method based on a signal processing technique, and it is shown that our method yields a significantly (P < 0.01) lower RMSE. With more extensive validation, the method may eventually be used in clinical practice to provide detailed, almost individual, specific information as a valuable basis for decision making.

Determination of human arterial wall parameters from clinical data

This study suggests a method to compute the material parameters for arteries in vivo from clinically registered pressure-radius signals. The artery is modelled as a hyperelastic, incompressible, thin-walled cylinder and the membrane stresses are computed using a strain energy. The material parameters are determined in a minimisation process by tuning the membrane stress to the stress obtained by enforcing global equilibrium. In addition to the mechanical model, the study also suggests a preconditioning of the pressure-radius signal. The preconditioning computes an average pressure-radius cycle from all consecutive cycles in the registration and removes, or reduces, undesirable disturbances. The effect is a robust parameter identification that gives a unique solution. The proposed method is tested on clinical data from three human abdominal aortas and the results show that the material parameters from the proposed method do not differ significantly (p < 0.01) from the corresponding parameters obtained by averaging the result from consecutive cycles.

One important area gradient force has been omitted in the Moens and Korteweg's Equation

Pulse wave velocity is mostly related to the young's modulus by the Moens and Korteweg's formula. In deriving this formula, the wall's effect on the blood was ignored at the first step and the pressure gradient was considered as the only driving force in the axial flow of the blood. The local area gradient was assumed to be zero. However, for a real arterial system with compliance, the area gradient is automatically accompanied with the pressure gradient and it will contribute an area gradient force. This force has been omitted so far without any justification because it will contribute a nonlinear term and also leads an obstacle for the analogy with the transmission line theory. We give a quantitative evaluation of the ratio of the area gradient force to the pressure gradient force and find that it is more than 50% in the main artery. This shows the inadequacy and the inaccuracy of the pulse wave velocity derived based on this axial flow equation. We propose that starting from the radial equation of motion might provide an alternate, feasible method to study hemodynamics by energy concern.

The effect of sympathetic stimulation on proximal brachial artery mechanics in humans - differential behaviour within the length of the brachial artery?

AIMS

The mechanical properties of arteries play a major role in the regulation of blood pressure and cardiac performance. The effect of sympathetic stimulation on the mechanical properties of the proximal brachial artery was analysed in 18 healthy volunteers, nine young (25 +/- 2 years) and nine elderly (69 +/- 2 years).

METHODS

A non-invasive ultrasonic echo-tracking system for measurement of systolic/diastolic variation of the proximal brachial artery diameter in combination with intra-arterial pressure measurements was used to determine wall mechanics. The pressure-diameter (P-D) relationship, distensibility coefficient (DC), compliance coefficient (CC) and stiffness(beta) were obtained at rest and during sympathetic stimulation induced by lower body negative pressure (LBNP).

RESULTS

The peripheral vascular resistance increased by 100 and 72%, respectively in the young and elderly during LBNP (P < 0.001). Simultaneously, the mechanical properties of the proximal brachial artery remained unaltered, as estimated from both P-D relationship and stiffness in young (beta-index rest: 5.2 +/- 0.9, LBNP: 5.5 +/- 1.3, NS) as well as elderly (beta-index rest: 13.6 +/- 4.6, LBNP: 16.1 +/- 4.7, NS).

CONCLUSIONS

LBNP-induced sympathetic activation does not change proximal brachial artery mechanics, in contrast to earlier reports on the muscular distal brachial artery. This may imply that the transition between elastic and muscular artery behaviour is within the length of the brachial artery, where the site of transition from elastic to muscular wall structure needs to be specified in future studies.

The popliteal artery, an unusual muscular artery with wall properties similar to the aorta: implications for susceptibility to aneurysm formation?

OBJECTIVE

The popliteal artery is, after the aorta, the most common site for aneurysm formation. Why the popliteal artery is more susceptible than other peripheral muscular arteries is unknown. An important factor may be differences in arterial wall composition as compared with other peripheral muscular arteries, which in turn affect wall properties. These are however unknown. We studied the mechanical wall properties of the popliteal artery in healthy subjects.

MATERIAL AND METHODS

An ultrasound echo-tracking system was used to measure pulsatile changes in popliteal diameter in 108 healthy subjects (56 female, 52 male; age range, 9-82 years). In combination with blood pressure, stiffness (beta), strain, cross-sectional artery wall compliance coefficient (CC), and distensibility coefficient (DC) were calculated. Intima-media thickness (IMT) was registered with a Philips P700 ultrasound scanner.

RESULTS

The popliteal diameter increased with age, and was larger in male subjects than in female subjects (P<.001). Fractional diameter change (strain) decreased with age (P<.001), and strain values were lower in male subjects than in female subjects (P<.01). Accordingly, stiffness increased with age (P<.001), with higher stiffness values in male subjects (P<.01). DC decreased with age (P<.001), with lower DC values in male subjects (P<.01). CC decreased with age, with no difference between genders (P<.001). IMT increased with age (P<.001), with higher IMT values in male subjects (P<.001). The increase in IMT did not affect distensibility.

CONCLUSION

The wall properties of the popliteal artery are affected by age and gender, not only with an increase in diameter, but also with an age-related decrease in distensibility, with male subjects having lower distensibility than in female subjects. This seems not to be the behavior of a true muscular artery, but of a central elastic artery, such as the aorta, and might have implications for susceptibility to arterial dilatation, as well as the association of aneurysm formation between the aorta and the popliteal artery.

CLINICAL RELEVANCE

The popliteal artery is, after the abdominal aorta, the most common location for aneurysm formation in the arterial system. Why it is more susceptible than other arteries is unknown. This study shows that the wall function of the popliteal artery differs from other peripheral arteries, and instead show striking similarities to the abdominal aorta, indicating that the functional arrangement of arterial wall components are similar in the two arteries. This may have implications for the susceptibility to aneurysm formation, as well as the association of dilating disease between the popliteal artery and the abdominal aorta.

Age affects proximal brachial artery stiffness; differential behavior within the length of the brachial artery?

With increasing age, the diameter of central elastic arteries increases, whereas their distensibility decreases. The purpose of this study was to investigate the mechanical properties of the proximal brachial artery in relation to age and gender. Distensibility coefficient (DC), stiffness and compliance coefficient (CC) were calculated in 136 healthy males and females (range 9-82 y) using echo-tracking sonography. CC decreased with age in both genders, but CC was higher in males. Stiffness increased and DC decreased with age in an exponential manner, without any differences between genders. In conclusion, as in central elastic arteries, the distensibility of the proximal brachial artery decreases with age, in contrast to earlier reports on the muscular distal brachial artery. This may imply that the transition between elastic and muscular artery behavior is within the length of the brachial artery. In future studies using the brachial artery, the examination site needs to be defined.

Noninvasive ultrasound measurements of aortic intima-media thickness: implications for in vivo study of aortic wall stress

OBJECT

The abdominal aorta (AA) has a predilection for aneurysm formation. An etiologic factor may be underlying aortic wall stress. The purpose of this study was to examine whether the intima-media thickness (IMT) of the AA, as a surrogate to arterial wall thickness, can be measured noninvasively with satisfactory results to calculate circumferential wall stress, and to evaluate regional and gender differences in wall stress.

METHODS

Sixty-five middle-aged healthy subjects were examined with B-mode ultrasound to determine the diameter and IMT in the infrarenal AA, common carotid artery (CCA), common femoral artery (CFA), and popliteal artery (PA). Blood pressure was measured noninvasively in the brachial artery. Wall stress was calculated according to the law of LaPlace.

RESULTS

Intraobserver variability for the IMT in the AA showed a coefficient of variation of 11%. IMT was thickest in the AA compared with the CCA, CFA, and PA (P <.001). There was a gender difference in IMT in the CFA (P <.05) and PA (P <.01) but not in the AA. Greater wall stress was found in the AA than in the CCA (P <.001) and PA (P <.001), with men having greater wall stress in all studied arterial regions.

CONCLUSIONS

Aortic IMT can be satisfactorily studied in vivo with noninvasive B-mode ultrasound. There are gender differences in IMT and wall stress, and the largest wall stress is found in the AA in men, which might be important in aneurysm development.

Female gender increases stiffness of elastic but not of muscular arteries in type I diabetic patients

The reason for the particularly increased risk for cardiovascular complications in diabetic women is still unclear. We have previously found decreased distensibility of elastic arteries in type I diabetic women, indicating increased cardiac load, not seen in type I diabetic men, which might be one contributing factor. Whether the effect of gender is different in muscular arteries in type I diabetic patients has not been assessed. As estimates of arterial distensibility we measured stiffness (beta) and pressure strain elastic modulus (Ep) in the muscular common femoral artery using echo-tracking sonography in 30 women (mean age 34 years, range 20-61) and 26 men (mean age 38 years, range 22-56) with type I diabetes. The results were compared with those of 89 healthy individuals of corresponding age and gender and with previously published results from elastic arteries in these patients obtained at the same occasion. The internal common femoral diameter was significantly decreased in both diabetic men and women. In sharp contrast to the highly significant decreased distensibility of the elastic abdominal aorta and common carotid artery in the type I diabetic women, the distensibility of the common femoral artery did not clearly differ between patients and controls, neither for women nor for men. Thus, the gender difference in changes of arterial distensibility found in elastic arteries was absent or far less obvious in the femoral artery. In conclusion, female gender seems to affect the mechanical properties of elastic, but not of large muscular arteries in type I diabetic patients. Thus, putative gender differences in arterial changes in type I diabetes are to be sought in elastic rather than muscular arteries.

Comparison of generalized and gender-specific transfer functions for the derivation of aortic waveforms

Arterial transfer functions have been promoted for the derivation of central aortic waveform characteristics not usually accessible noninvasively, but possibly of prognostic significance. The utility of generalized rather than gender-specific transfer functions has not been assessed. Invasive central aortic and noninvasive radial (Millar Mikro-tip tonometer) blood pressure waveforms were recorded simultaneously in 78 subjects (61 male and 17 female). Average transfer functions were obtained for the whole group and for each gender by two methods. Reverse transformation was performed with the use of each transfer function. Measured aortic waveform parameters were compared with those derived using average, gender-appropriate, and gender-inappropriate transfer functions. Differences in central waveform characteristics were demonstrated between men and women. Derived waveform parameters were significantly different from measured values [e.g., subendocardial viability index and augmentation index (P < 0.001)]. A gender-appropriate transfer function significantly improved the derivation of some parameters, including systolic pressure and systolic and diastolic pressure time integrals (P < 0.05). Generalized arterial transfer functions may not be universally applicable across all waveform parameters of potential interest, and gender-specific transfer functions may be more appropriate.