TCD Flashcards
What is the formula for the Doppler shift (f)
f=2 fo v/c v=velocity of blood fo = frequency of emitted US wave c = velocity of emitted US wave
What is the frequency of TCD probe
2 MHz
What is the avg error of MCA FV due to changes in angle of insonation?
Small, ~3%. The observed velocity varies between 0.87 and 1.0 of the true velocity
TCD recorded velocity depends on what?
Cosine of the angle of insonation.
What is the typical depth of insonation for MCA
5 - 6 cm
What are the units for FV and CBF?
FV = cm/s CBF = ml/100g/min
How is the correlation between FV and CBF? What assumptions are required?
Poor in absolute terms, however, they should vary in the same direction. Assumption: MCA diameter is constant
What is the ‘normal’ range of MCA FV?
35 - 90 cm/sec
What percentage of patients have poor temporal windows?
~5%
What arteries can be insonated through a transtemporal approach?
MCA, ACA, ICA bifurcation and PCA
What is the typical depth of the ICA bifurcation
60 - 65 mm
What is the approximate insonation depth of M1?
45 - 55 mm
When insonating, what anatomic structure can be confused with MCA FV?
PCA, but this cannot be insonated at depths less than 55 mm
What are the measured velocity parameters?
Peak systolic velocity (S), end-diastolic velocity (D), mean velocity (M) and pulsatility index (PI)
What is the pulsatility index (PI)?
PI = (S-D)/M
How is FVmean calculated?
AUC
What velocity parameter is preferred?
FVmean.
What leads to increased PI?
- Cerebral vasoconstriction
- Increased ICP
Draw the relationship between FVmean, FVs and FVd over the range of CPP
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What will you observe with FV in the presence of intact and abnomal autoregulation?
Assessment of autoregulation using TCD depends on
the assumption that relative changes in FV correlate
with relative changes in CBF. Thus, if autoregulation
is intact, FV should remain constant with a changing
CPP. Conversely, if autoregulation has failed, then the
relation between CPP and FV becomes linear
Draw the relationship between PI and CPP
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What is a better correlated with CPP, FV or PI? Why?
PI, because it is independent of the mechanism of CPP (ICP or decreased MAP)
What is the relationship between FV blood flow volume and vessel diameter?
FV = blood flor volume / diameter
What is the Lindegaard index or ratio?
MCA FV / ICA FV
1) < 3: normal
2) 3 - 6 mild - moderate
3) >6 severe