Chapter 13 - Duplex of lower extremity Flashcards
Doppler equation
Delta f = 2 Fo V Cos theta / c
Delta f = doppler frequency shift
Fo = carrier frequency (transducer)
c = speed of ultrasound in tissue = 1540m/s
V = velocity = calculated velocity of the flow
Cos theta = cosine of angle between ultrasound beam and the blood flow
Optimal doppler angle
60 degrees
< 60 degrees = minuscule error
> 60 degrees = large error
Things to note in LE duplex
1) grey scale abnormalities
2) PSV
3) quality = triphasic, biphasic, monophasic
4) spectral broadening
Duplex criteria for stenosis in native LE arteries
PSV ratio > 2 = 50% stenosis
PSV ratio > 4 = 75% stenosis
PSV > 400 cm/sec = 75% stenosis
presence of plaque, disease and post-stenotic turbulence
Never rely on velocity alone
Duplex criteria for follow up vein bypass grafts
PSV ratio > 2 = 50% stenosis
PSV ratio > 4 = 75% stenosis
PSV > 400 cm/sec = 75% stenosis
Historic change of PSV > 30 cm/s
Historic change of ABI > 0.15
Duplex criteria for follow up PTFE bypass grafts
PSV ratio > 2 = 50% stenosis
PSV ratio > 4 = 75% stenosis
PSV > 400 cm/sec = 75% stenosis
NOT ESTABLISHED WELL but extrapolated
Intraoperative bypass assessment
1) high frequency transducer 12-15 MHz
2) PSV > 45 cm/s in outflow artery
3) low resistance flow in outflow artery
only established for VEIN bypassess
Abnormal intraop duplex findings with graft
Consider revision if PSV > 180/s and Velocity ratio > 2.5 PSV > 300 cm/s
Low PSV < 45 cm/s
Post bypass surveillance criteria for revision
1) Focal PSV > 200 cm/s
2) SVR > 3
3) high outflow resistance without diastolic flow
4) ABI drop > 0.2
5) clinical status change
Post stenting duplex criteria for < 50%, 50-70% and > 70%
<50% = PSV < 180, VR < 2, normal wave
50-70% = PSV 180-300, VR 2-3.5, EDV > 0, monophasic
> 70% = PSV > 300, VR > 3.5, EDV > 45, dampened monophasic
occluded = no flow
