22. Cerebrovascular

Impedance and Critical

Closing Pressure.

The Autoregulation, Stupid!

Drawing of her daughter (age 7)

AUTOREGULATION IMPEDANCE, PULSATILITY,

CRITICAL CLOSING PRESSURE

3 cycles/min

Pulsatility of arterial pressure is lower than pulsatility of blood flow

F1/FVm > A1/ABPm

FV

ABP

PI

PIabp

Critical Closing Pressure: arterial pressure at which CBF stops

Burton AC. On the physical equilibrium of the small

blood vessels. Am J Physiol 1951; 164:319-329

[mm Hg]

Only small vessels flow, or larger vessels are useful as well?

[cm/s]

[mm Hg] [mm Hg]

[cm/s]

Existing methods for calculating CCP

Non-invasively, by using the pulse waveforms of ABP and blood flow

velocity (FV) acquired with Transcranial Doppler

Ultrasonography (TCD) [A,B,C].

CCP may be estimated by the intercept point of a regression line between arterial systolic and diastolic pressure plotted along the x-axis and the systolic and diastolic values of FV plotted along the y- axis [D].

Zero value of FV corresponds to a min value of ABP: CCP

CCP waveform is not that well correlated with CCP assessed in cardiac arrest

Can CCP be calculated from the single pulse

waveform?

Two methods: regression of single

waveform and ratio of first

harmonics

Relatively good correlation between ‘golden standard’ CCP (CCPd) and

‘regression ‘ CCP (CCPr) and between CPPd and ‘first harmonic CCP (CCPf)

R=0.924 R=0.935

What do we need? TCD, ABP

[mm Hg]

[mm Hg]

[cm/s]

CCP= ICP + tension of arterial walls

Dewey RC, Pieper HP, Hunt WE. Experimental

cerebral hemodynamics. Vasomotor tone, critical

closing pressure, and vascular bed resistance.

Neurosurgery 1974; 41:597-606,

No diastolic flow my signify reaching by

Diastolic ABP level of critical closing?

[mm Hg]

[mm Hg]

[mm Hg]

[cm/s]

[cm/s]

Existing model for calculating CCP

According to Michel et al [E] a

model describing CCP can be

based on pulse waveforms of ABP

& FV.

Fraction of first

harmonics

amplitudes of ABP &

FV waveforms.

[A] Czosnyka M, Smielewski P, Piechnik S, Al-Rawi PG, Kirkpatrick PJ, Matta BF, Pickard JD. Critical closing pressure

in cerebrovascular circulation. J. Neurol. Neurosurg. Psychiatry 1999; 66;606-611.

[B] Panerai RB et al. The critical closing pressure of the cerebral circulation. Medical Engineering & Physics 25.

2003; 621–632.

[C] Aaslid R, Lash SR, Bardy GH, Gild WH, Newell DW. Dynamic pressure-flow velocity relationships in the human

cerebral circulation. Stroke. 2003 Jul; 34(7):1645-9.

[D] Aaslid R. Cerebral hemodynamics. In: Newell DW, Aaslid R, eds. Transcranial Doppler. New York: Raven Press,

1992.

[E] Michel E, Zernikow B. Goslig’s Doppler pulsatility index revisited. Ultrasound Med Biol. 1998;24(4):597-9.

Model Methodology Model of Cerebrovascular Bed

and CSF dynamics:

Input circuit of electrical model of the cerebrovascular bed &

diagram shoing the resulting module of impedance.

ω=2πf

circular frequency

Module of

Impedance

CCPm & WTm

[mm Hg]

Thanks to G.Varsos

Discussion Point – Negative? CrCP

After introduction of deep hypercapnia,

CCP becomes negative.

Similar effect is illustrated by gradual increase in PaCO2.

High PaCO2

Increase in mean FV

CCP1<0

CCPm > 0 under all circumstances

Zero Line

Thanks to G.Varsos

Are CCP1 and CCPm similar?

Significant correlation between CCPm and CCP1, reaching a Pearson

coefficient: R=0.929

OVERAL GOOD MATCH

The same high correlation was observed between values of wall tension WTm and WT1.

Thanks to G.Varsos

Head injury

Refractory intracranial hypertension Plateau wave

Arterial hypotension B waves

CCP is not a reliable estimator of ICP

CCP can be helpful in estimation of

CPP but the accuracy is poor

Figure 3 a

R L R L

Mxl–Mxr >0

i.e. Mxl > Mxr

Worse autoregulation

on the Left

Mxl–Mxr <0

i.e. Mxr > Mxl

Shift from

the

Left

to

the

Right

Left

side

expansion

Right side

expansion

Shift from

the

Right

to

the

Left

Worse autoregulation

on the Right

R L

Asymmetry of cerebral hemodynamics after

head injury

Mild hyperventilation in head injury

Critical Closing Pressure generally increased in hypocapnia

Carotid artery stenotic disease

[mm Hg]

[mm Hg]

[kHz]

[mm Hg]

Most significant difference between ipsilateral (i) and contralateral sides are

in pulsatility indices and critical closing pressure.

Thanks to G.Varsos

Thanks to G.Varsos

For DCM (closing

margin) of 0 and below-

ischaemia accelerates

Thanks to G.Varsos

What to do if you see no diastolic flow at bedside?

Increase DCM!

-you may increase ABPd

-or lower CrCP by:

• lowering ICP

• or decreasing wall tension (WT): like by

increase PaCO2

Cerebral vasospasm

[mm Hg]

[mm Hg]

[cm/s]

[cm/s]

ABP [mmHg]

0 20 40 60 80 100 120 140

FV

MC

A [cm

/s]

0

40

80

120

160

200

Critical closingpressure

vaso

spasm

baseline

ABP [mmHg]

0 20 40 60 80 100 120 140

FV

MC

A [cm

/s]

0

40

80

120

160

200

contra

latera

l side

side o

f vaso

spasm

Critical closingpressure

a)

b)

Temporal comparison

Spatial comparison

Contralateral versus ipsilateral side of vasospasm

Method of calculation

Aaslid Michel

Critical clo

sin

g p

ressure

[m

mH

g]

0

10

20

30

40

50

60p = 0.011 p = 0.022

contralateral side

ipsilateral side

severe (mRS: 4-6) vs non-severe (mRS: 0-3)

CrCP in SAH- analysis of outcome,

N=98

Thanks to G.Varsos

Conclusion

Cerebrovascular impedance has low-pass character

CrCP is dependent both on ICP and CVR

CrCP decreases with vasodilatation and increases with vasoconstriction

CrCP may describe asymmetry of cerebral circulation in head injury

CrCP also reflects asymmetry in CCA stenotic disease

CrCP decreases ‘artificially’ during vasospasm

Model CrCP should be recalculated with PFF model