Forearm Arterial Anatomy and Flow Characteristics: A Prospective Observational Study

Abstract: Background. Morphometric data on Caucasian radial and ulnar arteries are limited, with no data on flow interdependence in the forearm arterial circuit. Methods. A total of 250 upper extremities in 125 patients were evaluated. Ultrasonography was performed and radial and ulnar artery lumen diameters were measured. Ulnar artery (UA) was compressed at the level of the wrist, and flow parameters in radial artery (RA) were recorded using duplex Doppler ultrasound. Results. Radial and ulnar artery diameters were comparable at the level of the distal forearm (RA = 2.03 ± 0.28 mm, UA = 2.07 ± 0.27 mm; P=.14). There was no significant difference in radial or ulnar artery diameter between the dominant upper extremity and the non-dominant upper extremity. Upon compression of the ulnar artery, radial artery velocity-time integral (VTI) increased from 8.4 ± 3.8 cm to 12.8 ± 5.5 cm, which was statistically significant (P<.001). There was a significant inverse correlation between radial artery diameter and the magnitude of increase in radial VTI observed with UA compression (r² = 0.106; P<.001). Conclusion. RA diameter at the level of the distal forearm is comparable to UA. RA-VTI and likely flow significantly increase by compression of the UA. The smaller the radial artery, the larger the increase in radial artery flow with ulnar compression.  

J INVASIVE CARDIOL 2015;27(4):218-221

Key words: access-site management, hemodynamics, radial artery intervention

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Forearm arterial circulation provides an ideal access site for invasive procedures as it has two medium-sized arteries that are not end arteries, and the anatomy (especially with the radial artery) allows easy hemostasis. Typically, the ulnar artery is thought to be the larger of the two vessels.¹ Macro- and micro-collateralization at the level of the palmar arches and interosseous membrane allows a closed-loop system with no interruption of flow to the tributaries, even after complete occlusion of a point in the circuit. The data regarding size of radial and ulnar arteries, although available in the literature, are mostly available from non-Caucasian populations.^2-5 The closed-circuit nature of the forearm circulation also allows the potential alteration of flow characteristics in one limb by altering flow in the other limb.⁶ Radial artery size has been found to be associated with radial artery occlusion (RAO) after transradial access (TRA).⁴ The size of the radial artery in the dominant upper extremity (UE) versus the non-dominant UE is not known. 

Radial artery patency after TRA is associated with maintenance of uninterrupted radial artery flow using patent hemostasis technique.⁷ Maneuvers to augment radial artery flow may have a potential role in lowering the incidence of RAO after TRA. We hypothesize that radial artery flow may increase by occlusive compression of the ulnar artery.

We sought to conduct an ultrasound evaluation of radial artery and ulnar artery lumen diameters, as well as change in the flow characteristics and diameter of the radial artery, with interruption of ulnar artery flow. 

Methods

Between May 2010 and January 2012, a total of 192 consecutive patients, presenting to an outpatient clinic in northeastern Pennsylvania for consultation prior to elective cardiac catheterization, were evaluated using forearm arterial ultrasonography to allow better access management as a part of a quality improvement project. All patients provided verbal informed consent. The study was approved by the local institutional review board. Patients with previous radial artery catheterization (n = 55), arteriovenous fistulae (n = 6), history of scleroderma (n = 1), proximal bypass grafting (axillofemoral bypass) (n = 3), as well as upper-extremity amputation (n = 2) were excluded from the study. The remaining 125 patients were prospectively enrolled in the evaluation. 

Demographic data, height, weight, arm dominance, and comorbidities including diabetes mellitus, hypertension, renal disease, wrist circumference, and gender were recorded. 

Ultrasound evaluation. B-mode ultrasound evaluation of both left and right radial and ulnar arteries was performed using GE Vivid-I ultrasonograph with an 8L-RS probe (4-13.3 MHz). Lumen diameter was calculated in a standard fashion. Duplex Doppler evaluation of both left and right radial arteries was performed at baseline. The ulnar artery was palpated at the level of the wrist joint, at Guyon’s canal, between the pisiform and hamate bones. It was then manually compressed with complete obliteration of the ulnar pulse for a total duration of 60 seconds. Radial artery duplex Doppler evaluation was performed continuously, and the effect of ulnar artery compression was recorded. Velocity-time integral (VTI) was calculated at baseline and after ulnar artery occlusion, from radial artery pulse Doppler envelope. Three sampled beats were analyzed and the average value was recorded. Peak velocity of the radial artery flow envelope before and during ulnar artery compression was recorded. 

Statistical analysis was performed using student’s t-test for continuous variables and chi-square test for categorical variables. Pearson’s test was used to evaluate correlation between numeric variables. Multivariable analysis was performed using linear regression. A P-value of <.05 was considered statistically significant. SPSS version 17.0 was used. 

Results

A total of 125 Caucasian patients with 250 upper extremities were studied. Thirty-one percent of patients were diabetic, 48% were female, 63% had a history of hypertension, and 26% were left handed. Mean age was 63.6 ± 13 years (range, 20-92 years), height was 173 ± 11 cm (range, 145-206 cm), and weight was 88 ± 20 kg (range, 48-176 kg). RA diameter was 2.03 + 0.28 mm and UA diameter was 2.07 ± 0.27 mm, with no statistically significant difference (P=.14) (Figure 1). Women had significantly smaller RA diameter (1.9 ± 0.29 mm vs 2.1 ± 0.24 mm, respectively; P<.001) and ulnar artery diameter (1.9 ± 0.17 mm vs 2.2 ± 0.28 mm, respectively; P=.01) compared with men (Figure 2A). Diabetics had significantly smaller RA diameter compared with non-diabetics (1.9 ± 0.3 mm for diabetics vs 2.1 ± 0.24 mm for non-diabetics; P<.001) and ulnar artery diameter (1.98 ± 0.21 mm for diabetics vs 2.1 ± 0.28 mm for non-diabetics; P<.001) (Figure 2B). Upper-extremity dominance was not found to be significantly associated with radial artery diameter (2.04 ± 0.28 mm in dominant UE vs 2.02 ± 0.28 mm in non-dominant UE; P=.41). Ulnar artery diameter was significantly larger on the dominant side compared with the non-dominant UE (2.03 ± 0.27 mm in non-dominant UE and 2.1 ± 0.26 mm in dominant UE; P=.02) (Figure 2C). A significant correlation was observed between height and RA diameter (r² = 0.10; P<.001) (Figure 2D). No significant association was observed between ulnar artery diameter and patient height (r = 0.04; P=.50). Multivariable linear regression analysis to identify independent predictors of radial artery lumen diameter was performed by entering gender, diabetes mellitus, and height as independent variables. Diabetes (t = -5.1; P<.001), height (t = 4.6; P<.001), and gender (t = -4.3; P<.001) were identified to be independent predictors of radial artery diameter. 

A significant increase in the peak radial artery flow velocity was noted with ipsilateral ulnar artery compression (1.7 ± 1.1 cm/s at baseline and 3.5 ± 2.2 cm/s after ulnar compression; P<.001) (Figure 3). A weak but significant inverse correlation was noted between the increase in radial artery peak flow velocity and radial artery diameter (r² = 0.032; P=.01) (Figure 4). Baseline radial artery VTI was 8.4 ± 3.9 cm. Radial artery VTI increased to 12.8 ± 5.5 cm immediately after compression of ulnar artery, with a mean increase (ΔVTI) of 4.3 ± 2.8 cm. A significant inverse association was observed between radial artery diameter and ΔVTI with ulnar compression (r² = 0.106; P<.001) (Figure 5). 

Discussion

Our study showed that radial and ulnar artery diameters in Caucasian patients at the level of the distal forearm are comparable. In >50% of patients, the radial artery was the larger vessel. The previous report¹ that the ulnar artery is a larger vessel probably resulted from observation of the proximal vessels in the forearm, after brachial bifurcation, where the ulnar artery is frequently the larger vessel. The ulnar artery probably behaves similarly to the left anterior descending coronary artery, with significant tapering after the muscular branches originate, resulting in the size at the level of distal forearm similar to the radial artery. 

We also observed that unlike the presence of superficial palmar arch, which has been found to be more frequent on the dominant side,⁸ the size of radial artery does not appear to have an association with UE dominance. The radial artery likely serves as a conduit to the palmar circulation, not affected by UE dominance. The ulnar artery, on the other hand, was found to be larger on the dominant side, likely in view of larger muscle mass on the dominant side. Height, diabetes, and gender had an association with radial artery lumen diameter, with shorter patients, diabetics, and women found to have smaller radial arteries. These patient subsets, in view of smaller radial artery size, may especially be at risk for RAO after TRA, and hence added attention to using lower-profile equipment, anticoagulation, and patent hemostasis is needed to prevent RAO. 

The radio-ulnar circuit is a closed-loop system with collateralization in the majority of patients,⁹ and provides an excellent opportunity for flow alteration in either limb by altering the resistance in the other limb. Although ulnar artery compression in its proximal portion, immediately after bifurcation, before the origins of the larger muscular branches, is likely to result in the largest increase in radial artery flow, it is deeply situated in the proximal forearm, making occlusive compression difficult without significantly altering the pressure in the forearm compartment, which in turn will compress the radial artery, altering its flow characteristics. The distal ulnar artery, on the other hand, surfaces in the Guyon’s canal, formed by the hamate bone medially and pisiform bone on its lateral aspect, making it easy to compress, without affecting the radial artery. 

Our observations indicate that ulnar compression in Guyon’s canal leads to a 41% increase in the radial artery VTI. The smaller the radial artery diameter, the larger the observed increase in radial artery VTI with ipsilateral ulnar compression. As a smaller-caliber radial artery is more susceptible to RAO after TRA, this relationship could be exploited to obtain a larger increase in peak velocity, VTI, and likely flow in these smaller radial arteries by ipsilateral ulnar compression, and hence lower the incidence of RAO.

Unlike the femoral artery, the intramural pressure to intraluminal pressure ratio in the radial artery favors lumen collapse when extrinsic pressure is applied to achieve hemostasis. Increase in VTI and peak flow velocity associated with ulnar artery compression likely alters this ratio in a favorable fashion, potentially helping maintain lumen patency when hemostatic compression is applied.

As shown previously, ipsilateral ulnar artery compression, in patients with RAO immediately following TRA, leads to frequent recanalization.¹⁰ The mechanism of this effect is likely a sudden increase in flow through the non-ulnar vessels, including radial artery, proximal to the site of RAO, likely facilitating recanalization. The hemodynamic alterations in the radial artery caused by ulnar artery compression may provide an opportunity to “prevent” RAO after TRA, with likely a larger impact compared with a “therapeutic recanalizing” effect after RAO has already occurred. The relationship between radial artery diameter and increase in radial flow provides an ideal circumstance, with larger flow increases in smaller diameter radial arteries more likely to develop RAO. 

Conclusion

At the level of the distal forearm, radial and ulnar artery lumen diameters are not significantly different. Ulnar artery compression at the level of the wrist leads to a significant and sustained increase in radial artery flow. 

References

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From the ¹Wright Center for Graduate Medical Education, The Commonwealth Medical College, Scranton, Pennsylvania; and ²Apex Heart Institute, Seth N.H.L. Municipal Medical College, Ahmedabad, India.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Pancholy is a consultant for Terumo Corporation. The authors report no conflicts of interest regarding the content herein.

Manuscript submitted February 11, 2014, provisional acceptance given June 4, 2014, final version accepted September 22, 2014.

Address for correspondence: Samir B. Pancholy, MD, FSCAI, 401 N. State Street, Clarks Summit, PA 18411. Email: pancholy8@gmail.com