Allen`s Test: Does it Have Any Significance in Current Practice?

Abstract: Percutaneous coronary intervention (PCI) performed through transradial access (TRA) has been consistently demonstrated to be safer than transfemoral access (TFA), mainly due to significantly reduced bleeding and vascular complications. Despite demonstrating better patient safety, acceptance of TRA as an access site of choice has remained low, with geographical variations. One of the relative contraindications to performing PCI through TRA is an abnormal Allen’s test (AT), in which case the access site is usually switched to TFA. Bleeding and vascular complications are the “Achilles heel” for interventions through the TFA. The use of the AT is controversial given its low sensitivity in assessing both the collateral circulation of the hand and predicting ischemic complications. In this article, we present interesting data from the congenital heart and trauma literature that lead us to suggest the AT is an inadequate test to assess the collateral circulation of the hand. We think the case for its routine guideline-driven use is flawed.  

J INVASIVE CARDIOL 2015;27(5):E70-E73

Key words: radial, Allen’s test, coronary intervention

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Coronary angiography (CAG) and percutaneous coronary intervention (PCI) are routinely performed procedures. Radner demonstrated the feasibility of accessing thoracic aorta from the radial artery (RA) in 1948.¹ However, transradial access (TRA) was not used for performing CAG until 1989 and the first coronary stent was deployed in 1992.^2,3 Compared with transfemoral (TF)-PCI, TRA-PCI performed by experienced operators is safer, has a lower risk of access-site complications and bleeding, and allows more rapid ambulation post procedure. It is preferred by patients over TFA with higher quality of life scores, lower length of hospital stay, cost savings,^4-6 and a mortality benefit in patients undergoing primary PCI for ST-elevation MI (STEMI) with or without cardiogenic shock.^7-9 

Normal circulation of the hand. Arising from the brachial artery, the hand receives blood supply from the radial and ulnar arteries, both giving rise to superficial and deep palmar arches, from which digital branches perfuse the digits. Branches of the common interosseous artery that originate from the ulnar artery after the radial tuberosity play a crucial role in providing blood supply to the hand. The ulno-palmar arches in the hand are perfused from both the UA and RA, with a watershed point where flow from both sources meets under equal pressure. This watershed point is not static, implying that a dynamic flow volume and rate is required by the different parts of the hand at rest and upon exertion.¹⁰ This suggests if either the RA or UA are compromised, there is a rich network of vessels that maintains hand perfusion. 

Tests used in current practice to check collateral circulation in the hand. Potential ischemic complications are a major concern prior to performing any procedure through the RA. The UA supplies a greater proportion of blood to the hand than the RA. The modified AT, originally described in 1929, is used to test adequacy of collateral circulation to the hand prior to TR cardiac catheterization.^11,12 Originally, the AT had no time limit to assess patency of collateral circulation to the hand, seen as return of palmar blush. The AT, however, has a low sensitivity (73.2%) with higher specificity (97.1%) for testing the adequacy of collateral blood supply from the UA.¹³ In one study comparing AT with Doppler ultrasonography and digital plethysmography, AT was abnormal in 23% of patients, with cut-off set at 6 seconds for return of palmar blush.¹⁴ 

Some centers use digital plethysmography and pulse oximetry along with AT to increase sensitivity and specificity, which is also known as the Barbeau test.¹⁵ The response to this test is divided into four types from A to D, based upon the time required for restoration of the pulse wave signal recorded from the ipsilateral thumb. Responses A and B suggest patent palmar arch, whereas C suggests slow response, likely suggestive of vessel recruitment. Response D suggests lack of restoration of blood flow after 2 minutes, and it is recommended that these patients should not undergo TRA intervention. In a study of 1010 patients, only 1.5% of patients demonstrated type-D response.¹⁵ Duplex ultrasonography is the most accurate test to assess dual hand circulation, but it is the least commonly used test prior to TRA. Similarly, reverse AT or reverse Barbeau test can be used to assess the impact of radial artery occlusion (RAO) on hand circulation.

A recent international transradial practice survey found that 58% of operators used modified AT and 16% used Barbeau’s test, whereas 23% of operators did not perform any test prior to performing TRA-PCI.¹⁶ 

Challenges with forearm arterial access for PCI. The use of TRA in performing CAG has progressively increased over the last two decades. A learning curve, lack of adequate training, concern with prolonged procedure time, and excessive radiation exposure are all challenges in increasing uptake of the TRA as a default access route. In addition to these issues, an abnormal AT is considered to be a contraindication to TRA procedures. Although RAO occurs in 1%-10% of patients after TRA-PCI, ischemic complications are rarely reported.

We present observations and data, mainly from congenital heart disease and trauma literature, that strongly suggest that the AT as a prerequisite prior to TRA-PCI/catheterization is unnecessary.  

The case against the use of the Allen’s test. The original Blalock-Taussig shunt is an operative procedure performed to increase pulmonary blood flow by ligating the subclavian artery and connecting it to the pulmonary artery (end to side). This diversion of the arterial blood supply from the upper limb is very well tolerated.¹⁷ Follow-up of 28 such patients demonstrated that this procedure resulted in minimally diminished arm growth (circumference, volume, and length), with no impediment of motor activities or neurovascular symptoms. Furthermore, despite no palpable pulse in the affected hand, resting blood flow in the operated arm was only 2.5% lower than the unaffected hand. Interestingly, although peak blood flow in response to exercise and ischemia in the operated limb was nearly one-third less than the opposite limb, this was not associated with any significant symptoms. This difference in peak blood flow became even less marked 5 years after surgery.¹⁸ 

Left subclavian flap aortoplasty is performed for surgical correction of aortic coarctation. Patients who have undergone this procedure exhibit normal blood flow in the affected arm as a result of markedly reduced vascular resistance. In a study of 9 children who had previously undergone this procedure (mean duration, 9 years), blood flow at both rest and peak hyperemia was not significantly different compared with the unaffected arm.¹⁹ The rise in mean arterial pressure in response to exercise was similar in both arms, suggesting that vasculature in the surgically affected arm was able to mount similar vasodilatory response to exercise.¹⁹ These findings are in contrast with the previously discussed study. The variance in findings is likely explained by the length of time elapsed between the operation and assessment of hand blood supply, since  differences in blood flow between the treated and untreated arms improve over time. 

The brachial artery (BA), an extension from the axillary artery, has previously been used extensively for coronary and peripheral interventions. This route is currently reserved for patients with limited vascular access. BA thrombosis after cardiac catheterization was reported to be as high as 10%, sometimes requiring surgical repair. The probability of this complication rose if adjunctive heparin was not administered at the time of catheterization.^20,21 In one series of 323 BA access cases, the rate of vascular complication was 6.5% (21 patients), with 13 of these patients requiring surgical intervention.²² However, given the BA’s status as an end artery, it is interesting that the rate of complications was not higher. Observations on children who underwent Scribner shunt insertion for hemodialysis and required ligation of the BA in the distal one-third of the vessel are thought provoking. In a follow-up study of 27 patients 15 years after Scribner shunt insertion (mean age at the time of operation, 8.4 years; range, 3-15 years), none had limb ischemia or limb loss per-operatively, with neither motor nor neurological symptoms. Lower blood pressure in the arm with ligated BA (123 mm Hg in arm with the unligated artery vs 106 mm Hg in arm with the ligated BA) did not affect growth of the hand. Ligation of the BA in growing children demonstrated lower blood pressure in that arm, with some patients complaining of exertional discomfort, but no limb loss or any significant growth abnormalities.²³

The trauma literature provides similarly unexpected findings. In a series of 32 patients who sustained either radial or ulnar artery laceration, surgical ligation of the artery was shown to be non-inferior to vascular repair. No patients in either treatment group developed ischemic, motor, or neurological symptoms, thus appearing to confirm the concept of availability of alternative sources of collateral blood supply to the hand.²⁴ 

Possible mechanisms that may explain maintenance of adequate blood flow to the arm despite these surgical interventions are: (1) the vast number of capillaries that are non-functional may get recruited upon stimulation by distal tissue hypoxia; and (2) the affected arm may adapt to reduced perfusion pressure by increasing capillary density in response to tissue hypoxia, a powerful stimulus for capillary proliferation. The latter is unlikely to be the sole mechanism responsible for maintaining the arm’s blood flow, since there is little time for neo-capillary formation in circumstances of acute arm ischemia, eg, Blalock-Taussig shunt formation. Capillary recruitment, therefore, most likely explains the lack of ischemia with abrupt vessel closure, whereas the elimination of blood pressure differences in both arms (treated vs untreated) over time may be due to both mechanisms of capillary recruitment and neo-capillary formation.

Intraoperative fluorescein angiography has yielded unexpected findings in patients who were considered to have non-patent radial arteries as judged by preoperative and intraoperative AT under anesthesia. Although in a small series of 6 such patients, fluorescein angiography demonstrated the RA was not only patent and supplied blood to a small part of the hand (mainly the thenar eminence in the presence of patent UA), but supplied nearly the entire hand upon manual occlusion of the UA. These findings question the validity of the AT.¹⁰

The final piece of data to suggest that the AT does not accurately assess hand perfusion arises from an elegant work by Valgimigli et al, where 942 patients undergoing transradial cardiac catheterization were screened, and 203 patients were recruited (83 with normal AT, 60 with intermediate AT, and 60 with abnormal AT). There was no rise in thumb lactate or hemodynamic compromise of the hand in patients who underwent transradial PCI despite an abnormal AT. Interestingly, ulnar flow was observed to be enhanced and ulno-palmar collateralization improved with time, suggesting recruitment of collateral circulation.²⁵ In contrast, a smaller study with 55 patients (20 normal AT, 15 intermediate AT, and 20 abnormal AT) detected a rise in lactate and reduced blood flow 30 minutes after RA occlusion in the abnormal AT group. The authors concluded that the TR approach should not be used in patients with abnormal AT. Crucially, however, it should be noted that this study did not present long-term follow-up data.²⁶ 

The external diameter of the 6 Fr sheath, the most common size used in TRA, is 2.52 mm, which is larger than the average radial artery internal diameter. Thus, while the sheath is in situ, it is frequently occluding the RA. This should conceivably result in hand ischemia in those with abnormal AT during a TRA procedure, yet this is rarely if ever reported. Combined with the observational data from the congenital heart disease and surgical literature, this lends considerable support to the notion that the hand has a rich collateral supply and casts considerable doubt over the validity and accuracy of the AT.

How then can one explain the documented small incidence of hand ischemia after prolonged radial arterial line placement for blood pressure monitoring in intensive care units^27,28 or the extremely rare case report after cardiac catheterization via the radial artery in patients with normal AT? Patients described in these articles presented days and weeks after their intervention through TRA.^29,30 We think this is attributable to thrombus formation at the site of cannulation in these cases, with distal embolization jeopardizing the watershed perfusion zone and digital branches in the hand beyond the palmar arch. This is in sharp contrast to the setting of TRA cardiac catheterization, where although the sheath sizes are much larger, the indwelling time is short (especially for diagnostic cases). This factor, combined with the routine use of low-dose anticoagulation for diagnostic cases and higher doses during PCI, means the probability of significant thrombus formation and distal embolization is much lower than in the ICU setting described. These rare case reports do, however, serve to emphasize the importance of adequate anticoagulation and patent hemostasis practices.^31,32

Conclusion

Randomized trials of TRA-PCI have excluded patients with abnormal Allen’s tests, whereas routine practice includes a large number of such patients. High-volume transradial centers in the United Kingdom and other countries document the extremely low, almost unheard of, incidence of hand ischemia after TRA cardiac catheterization^33-35 without prior AT. Observations from the congenital heart disease and vascular surgery spheres lead us to conclude that AT is a misleading and inaccurate test of collateral blood supply to the hand. In our opinion, the case for its routine use is flawed. 

Study limitations. Our hypothesis is based on a large body of observational and registry data. To be strictly scientifically accurate, a randomized trial of TR cardiac catheterization with and without prior Allen’s test would be required. 

References

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From ¹Toronto General Hospital, University Health Network, Toronto, Canada; ²The Commonwealth Hospitals, Scranton, Pennsylvania; ³Apex Heart institute and Smt. NHL Medical College, Ahmedabad, India; and ⁴East Carolina Heart Institute, East Carolina University, Greenville, North Carolina.

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 remaining authors report no conflicts of interest regarding the content herein. 

Manuscript submitted August 25, 2014, provisional acceptance given November 7, 2014, final version accepted December 16, 2014.

Address for correspondence: Dr Ashish H. Shah, Toronto General Hospital, Toronto, ON M5G 0B2, Canada. Email: aashish.h.shah@gmail.com