New Material - Arterial Bypass Grafts, EVAR, Hemodialysis AV Fistulas, Transcranial Doppler (TCD)

Arterial Bypass Graft

Redirects flow around occluded artery via a synthetic graft or vein

Role of Sono. in Graft Mapping & Post-Graft Surveillance

Pre-surgical planning

Patency after surgery

Prevent failure

Monitoring

Synthetic Bypass Graft

Made of PTFE (polytetrafluoroethylene) or Dacron

Lower rates of early failure

Worse long term patency

Readily available with many sizes

Higher thrombogenicity

Axilla-Femoral Graft with Femoral-Femoral Bypass

Longest graft type

Femoral-femoral bypass supplies opposite leg

Aorto-Iliac Graft

Scars in abdomen & each groin

Femoral-Femoral Graft

Bypasses single iliac artery

Scar in each groin

Autogenous Vein Graft

GSV, SSV, cephalic, or basilic veins used

Higher rates of early failure

Better long term patency

In-Situ Vein Graft

Uses GSV in its native position

Venous valves removed to allow downward bloodflow

Perforators & tributaries removed

Larger proximal end connected to inflow artery

Smaller distal end connected to outflow artery

Preferred when vein diameter matches artery size at anastomosis

Reversed Vein Graft

GSV is removed & reversed -> smaller end connected to inflow, larger end connected to outflow

Perforators & tributaries removed

Venous valves do not need to be removed

Most Common Proximal Anastomosis

CFA or SFA

Distal Anastomosis

Placed below the lowest level of disease

Early Graft Failure

Within 30 days

25% of all graft failures

Technical defects - retained valve or anastomotic site issues

Compression/twisting of graft

Hypercoagulability

Thrombosis

Intermediate Graft Failure

Within 30 days to 2 years

75% of revisions are during this period

Due to myointimal hyperplasia - causes stenosis, typically at valve site

Leads to increased peak velocities at areas of stenosis

Late Graft Failure

> 2 years

Progression of atherosclerotic disease in native vessels

Changes in spectral waveform - increase rise time & decrease peak velocities

Aneurysms

Symptoms of Graft Failure

Return of pre-surgical claudication

Rest pain

Ulcers

Absent pulses

ABI < 0.15

Pre & Post-Op Graft Mapping/Surveillance Protocol

Grayscale, PSV (with angle correct), and Color of:

Inflow artery

Proximal anastomosis

Mid-graft

Distal anastomosis

Outflow artery

Normal Graft Diameter

Minimum for any bypass = 2 mm

Preferred = 2.5-3 mm

Normal Graft Spectral Appearance

Multiphasic arterial waveform

Sharp upstroke

Narrow systolic peak

Reversal of flow may not be seen early post-op

Normal Graft Velocity Ratio

< 2

Normal GFV Velocity

> 45 cm/sec

GFV Graft Failure Velocity

< 30 cm/sec

Graft Stenosis

Continuous diastolic flow with loss of diastolic reversal

Peak velocity > 180 cm/sec or < 45 cm/sec

Stenotic profile

50% Graft Stenosis

PSV > 180 cm/sec

PSV ratio of 2

75% Graft Stenosis

PSV > 300 cm/sec

PSV ratio of 3.5

Walk Through Technique

Drag PW cursor through entire stenotic area

Record pre-stenotic, stenotic, post-stenotic, and distal to stenosis zones

Peri Graft Fluid Collection

Fluid collections around graft

May be hematoma

Can compress graft & impact function

Can be infection

Fistula

Patent venous tributary that may communicate with deep venous system

Color flow is turbulent with aliasing

Spectral may be pulsatile or continuous

Proximal antegrade diastolic flow evident in graft

Aneurysm

Focal increase of diameter 1.5x adjacent normal vessel

Swirling color (yin/yang) & low velocity turbulent waveform

Pseudoaneurysm

Common in CFA grafts

Look for neck

To and fro flow

Dissection

Intimal flap parallel to vessel walls

Turbulent color in either lumen

Bidirectional spectral

Increased resistance

Neointimal/Myointimal Hyperplasia

Overgrowth of cells into the intimal layer

Creates narrowing and stenosis

Retained Venous Valves

Increased velocities through stenotic area at level of retained materials

Turbulent post-stenotic zone

EVAR

Endovascular Aortic Aneurysm Repair

Minimally invasive procedure

Treats AAA's using a stent graft

Excludes aneurysmal sac from circulation to reduce rupture risk

Other Imaging Modalities for EAVR's

Digital subtraction angiography (DSA)

Computed tomographic angiography (CTA)

Placing an EVAR

Catheter access through CFA in groin

Device is anchored with stents in the normal aorta proximal & distal to AAA

Sonographic Appearance of Residual Aneurysm

Homogenous

Heterogeneous/spongy = endoleak

Sonographic Appearance of EVAR

Reflective (metal)

Hyperechoic proximal end

PW Waveform of EVAR

Multiphasic

Evaluate prox to, at entrance, through, at exit, and distal to graft

Perigraft Leaks

Arterial waveforms are different from flow within endograft

Endoleak Type 1

Inadequate seal at attachment sites

Waveforms look like that of the graft

High flow

Endoleak Type 2

Leaks from branching vessel of aorta (IMA, renal A, lumbar A)

Spectral can be bidirectional, monophasic, multiphasic, slow or high flow

Endoleak Type 3

Modular disconnection of graft segments or fabric disruption (fracture)

Waveform similar to regular graft waveform

High flow

Endoleak Type 4

Blood flows through EVAR graft into aneurysm sac

Endotension (endoleak type 5)

Aneurysm continues to expand in the absence of an endoleak

Hemodialysis AV Fistula

Anastomosis of artery & vein

Reliable, repeatable hemodialysis access with minimal complications

Types of Anastomoses

Site Preferences for AV Fistula

Non dominant forearm

Dominant forearm

Non dominant upper arm

Dominant upper arm

Lower extremity

Preferred Site for AV Fistula

As distal as possible in non-dominant arm

Why is the upper extremity preferred for AV fistulas?

Patient comfort & preference

Lower infection rates

Greater longevity

Easier to access

Patient Prep & Assessment

Keep room warm

Patient in supine/sitting position

Take bilateral blood pressures

Take pulses of brachial, radial, and ulnar arteries

Allen Test: assesses for intact palmar arch - clenched fist - reactive hyperemia after compression indicates patency

Assess superficial veins using tourniquets

Fistula Maturation Failure

Caused by obligatory use of small/suboptimal veins

Quality of Artery

Determines capacity to dilate & accommodate increased flow

Fistula/Graft Mapping

Find suitable artery before moving onto venous system

AV Fistula & Hemodialysis Graft Arterial Mapping

Start with distal forearm of non-dominant arm

Assess for plaque, thickening, stenosis, compliance

Evaluate waveform & note PSV - high resistant (rapid upstroke, sharp peak, low diastolic flow)

Arterial Diameter for Fistulas/Grafts

> 2.5 mm

AV Fistula & Hemodialysis Graft Venous Mapping

Start with superficial system of non-dominant forearm

Begin with cephalic, basilic, and median cubical veins at wrist and move proximally to axilla

Assess for thrombus, compressibility, narrowing, tributaries , scarring- central veins have respirophasicity & cardiac pulsatility

Compress & record diameter every 2 cm

Doppler with augmentation (include subclavian & IJV)

Measure vein diameter with & without tourniquet (2 tourniquets-at axillary & forearm-for 3 minutes)

Assess depth from skin surface to anterior wall of vein

Vein Diameter for Fistulas/Grafts

> 2.5 mm

Vein Diameter for Synthetic Fistulas/Grafts

≥ 4 mm

Basilic Vein Length for Fistulas/Grafts

≥ 10 cm

Pre-Mapping Contraindications for AV Fistula Placement

Thrombus

Calcifications

Thickened vessels

Local infection

Dressings that can't be removed

Open wounds

AV Fistula/Graft Maturity

Occurs 6 weeks - 6 months after placement

Assess 10-12 weeks after placement before hemodialysis begins

Mature AV Fistula/Graft

Drop in peripheral resistance

Can handle 6 cycles a month

Audible swishing bruit

Palpable thrill/vibration - turbulent flow at anastomosis

Large enough for two 15-gauge needles

Normal PSV for Fistula

100-400 cm/sec

Normal EDV for Fistula

60-200 cm/sec

Fistula Surveillance

Fistula diameters

Depth from skin surface

PSV's of:

- Native artery prox to anastomosis/arterial inflow

- Arterial anastomosis

- Throughout fistula - walk-through technique

- Venous outflow

Assess patency of all inflow arts/outflow veins

Arterial Side of Fistula

Low-resistant waveform

Venous Side of Fistula

High flow volume

Pulsatile prox to anastomosis

Normal Fistula Inflow Artery

Proximal to anastomosis

Low resistant - forward diastolic flow & spectral broadening

Normal Fistula Flow

1 cm below skin

Low resistant - forward diastolic flow & spectral broadening

Elevated PSV & EDV velocities

Normal Venous Outflow

Pulsatile flow

Acquiring Flow Volume of a Mature Fistula

Evaluate function at mid-fistula

Large sample volume (wide as vessel)

Measure diameter on grayscale

Use auto-tracing or trace 3-4 waveforms for mean velocity

Take at least 3 times

Normal Flow Volume of a Mature Fistula

> 800 ml/min

AV Fistula & Graft Complications

Immaturity

Stenosis

Occlusion

Thrombosis

Aneurysm and Pseudoaneurysm

Fluid collections

CHF

Arterial Steal Syndrome

Immature AV Fistula

Proximal hammer pulse

Minimal thrill

Lack of venous distention

Lack of high-pitched bruit

Signs of stenosis & palpable distal thickening

AV Fistula Stenosis

Most common in venous anastomosis & outflow vein

Echogenic intraluminal lesion

Flow reduction

Mild/Moderate Stenosis Flow Volume

500-800 ml/min

Severe Stenosis Flow Volume

< 500 ml/min

PSV of AV Fistula Stenosis

> 375 cm/sec

PSV Ratio for > 50% Stenosis on Arterial Side

> 3:1

PSV Ratio for > 50% Stenosis on Venous Side

> 2:1

AV Fistula Occlusion

Absent flow in lumen

Echogenic thrombus in lumen

Prox high-resistant flow

AV Fistula Thrombosis

To and Fro flow- inflow

Low PSV

Absence of color flow & no outflow

Echogenic material

AV Fistula Aneurysm

AV Fistula Pseudoaneurysm

AV Fistula Fluid collections

Arterial Steal Syndrome

Occurs in 75-90% of patients

Most patients are asymptomatic

Due to poor distal collateral circulation & high flow through fistula

Low-resistant outflow vein draws antegrade flow from inflow artery & steals retrograde flow from distal artery

Failing AV Fistula/Graft Interventions

Percutaneous transluminal angioplasty

Percutaneous recanalization

Interoperative branch ligation

Interoperative revision and vein interposition

DRIL procedure

DRIL Procedure

Ligation of native artery distal to dialysis access

Bypass from native artery to artery distal to ligation

Complimentary Imaging for AV Fistula Mapping/Monitoring

Venography

Enhanced MR Venogram

Anterior Circulation of the Circle of Willis

Carotid siphon - parasellar, genu, and supraclinoid segments

ICA branches - ophthalmic & posterior communicating

MCA

Anterior cerebral - pre & post-communicating

Anterior communitating

Posterior Circulation of the Circle of Willis

Vertebral

Basilar

Posterior Cerebral - pre & post-communicating

Vertebrobasilar Circulation of the Circle of Willis

Vertebral - posterior inferior cerebellar

Basilar - anterior inferior cerebellar & superior cerebellar

Common Anatomical Variants in Cranial Vessels

Hypoplasia of posterior communicating A

Hypoplasia of pre-communicating posterior cerebral A

Hypoplasia &/or Duplication of anterior communicating A

Hypoplasia of pre-communicating anterior cerebral A

Collateral Flow Patterns of Cranial Vessels

ECA to ICA via reversed ophthalmic A

Crossover collateral via anterior communicating A

Posterior-to-anterior collateral via posterior communicating A

Disadvantages to TCD

Extracranial status

CO2 status

Intracranial pressure

Limited windows

Blind spots

Pilot error (6 month learning curve)

TCD Indications

Vasospasm - trauma or aneurysm

Vertebral basilar insufficiency

Intraoperative monitoring

Intracranial pressure

Brain death

PFO - right to left shunts

Sickle Cell

Sub arachnoid hemorrhage - head trauma, aneurysms, AVM

TCD Risk Factors

Diabetes

Hypertension

High cholesterol

Tobacco abuse

Cardiac disease or hx of heart attack

Claudication

Stroke or TIA

Previous vascular procedures - stents or bypass

TCD Relevant Patient History

Face, leg, or arm numbness, tingling, weakness

Problems speaking/understanding speech

Vision problems - blindness or double vision

Gait disturbance or leg weakness

Vertigo or fainting

Severe headaches

Equipment for Non-Imaging TDC

Dedicated non-imaging System

1-2 MHz transducer

Equipment for TDC Duplex Imaging

Ultrasound system

1-5 MHz sector transducer