What are the physiological effects and potential complications of arteriovenous (AV) shunts?

Physiological Effects and Complications of Arteriovenous Shunts

Arteriovenous shunts create significant hemodynamic alterations by diverting arterial blood directly into the venous system, resulting in increased cardiac output, volume overload, and potential distal ischemia—with complications ranging from thrombosis and infection to high-output heart failure and limb-threatening steal syndrome. 1

Hemodynamic Effects

Cardiovascular Impact

• Increased cardiac output and heart rate occur immediately after AV shunt creation, with cardiac index rising significantly (from normal 3.89 l/min/m² to 4.31 l/min/m²) due to the low-resistance pathway created by the shunt 2
• Volume overload of the right ventricle develops from the continuous high-flow state, potentially leading to right-sided heart failure over time 1
• High-output cardiac failure represents a common consequence, particularly in patients with pre-existing cardiac dysfunction or large-flow shunts 3, 4
• The physiologic compensatory response includes augmentation in cardiac output, arterial vasodilation, and formation of arterial collaterals 1

Pulmonary Circulation Changes

• Pulmonary overcirculation results from left-to-right shunting in congenital heart disease contexts, leading to flow-related pulmonary arterial hypertension 1
• RV volume overload from excessive pulmonary blood flow causes fatigue, exercise intolerance, and frequent pulmonary infections 1

Major Complications

Thrombotic Flow-Related Complications

• Stenosis from neointimal hyperplasia is the primary precipitator of vascular access failure, occurring most commonly at the vein-graft anastomosis or juxta-anastomotic region 1
• These stenoses augment pressure within the access, decrease blood flow, and if hemodynamically significant and untreated, result in thrombosis 1
• Access thrombosis is the primary cause of loss of vascular access patency and is associated with increased healthcare expenditure and compromised quality of life 1
• Thrombosis risk is higher with arteriovenous grafts compared to native arteriovenous fistulas 1

Steal Syndrome (Digital Hypoperfusion Ischemic Syndrome)

• Prevalence ranges from 1% to 20% and occurs secondary to shunting of arterial blood flow through the AV access and away from the peripheral system 1
• Presents with a constellation of signs and symptoms ranging from mild digital numbness to severe motor impairment, potentially progressing to skin ulceration or gangrene necessitating digit or limb amputation 1
• More frequent in proximal accesses supported by brachial artery inflow compared to distal radial artery accesses 1
• Usually manifests as hand pain during and off dialysis, less commonly as loss of function or tissue death 1
• The presence of arterial inflow stenosis (e.g., subclavian artery stenosis) or outflow stenosis exacerbates the condition 1
• Diagnostic arteriography of the extremity and its entire inflow is foundational before determining management, as concomitant arterial stenoses may have deleterious effects on surgical procedures 1

Aneurysm and Pseudoaneurysm Formation

• Pseudoaneurysm formation represents a surgical emergency when associated with skin erosion or active hemorrhage, necessitating prompt recognition and definitive management 1
• Risk factors for rupture include poor eschar formation, evidence of spontaneous bleeding, rapid expansion, and severe degenerative changes in graft material 1
• Treatment routinely involves placement of an interposition prosthetic graft tunneled in situ or extra-anatomically depending on infection presence 1

Infectious Complications

• Infections are rare in native fistulae but potentially lethal given the impaired immunologic status of long-term dialysis patients 1, 4
• Access infections at the AV anastomosis require immediate surgery with resection of infected tissue 1
• Cannulation site infections necessitate cessation of cannulation at that site and arm rest 1
• Antibiotic therapy is mandatory, initiated with broad-spectrum vancomycin plus an aminoglycoside, with infections requiring 6 weeks of treatment analogous to subacute bacterial endocarditis 1
• Risk of metastatic complications including bacteremia exists 1

Extremity Edema and Central Venous Complications

• Persistent extremity edema beyond 2 weeks after graft placement requires imaging studies to evaluate central vein patency 1
• Central vein stenosis is preferentially treated with percutaneous transluminal angioplasty, with stent placement considered for acute elastic recoil >50% or recurrence within 3 months 1

Monitoring and Prevention

Clinical Monitoring Strategies

• Physical examination (inspection, palpation, auscultation) should detect abnormal clinical signs including changes in access thrill or bruit, prolonged bleeding after decannulation, or arm swelling 1
• As high as 90% of accesses with abnormal physical examinations will have an underlying clinically significant finding on imaging 1
• The "One-Minute Access Check" provides rapid, effective monitoring that can be conducted by patients and providers before dialysis 1

Surveillance Approaches

• KDOQI does not suggest routine surveillance by measuring access blood flow, pressure monitoring, or imaging for stenosis additional to regular clinical monitoring 1
• Pre-emptive angioplasty of stenosis not associated with clinical indicators is not recommended to improve patency 1
• However, patients with consistently persistent clinical indicators and underlying stenosis should reasonably undergo pre-emptive angioplasty to reduce thrombosis risk 1

Common Pitfalls

• Failure to recognize arterial stenoses before surgical procedures for steal syndrome may prove detrimental post-surgically 1
• Delayed diagnosis occurs in 43.4% of thrombotic complications, with 17% not diagnosed until autopsy 1
• Cannulation through a pseudoaneurysm must be avoided, particularly if the pseudoaneurysm is increasing in size 1
• Central venous catheters should be avoided when possible given associated increased risks for infection, higher mortality, and lower patency compared to AVF and AVG 1