Increased intracranial pressure ICP

A normal low-resistance intracranial waveform is detected by TCD since the brain is a low-resistance vascular system with normal or low ICP values. When ICP increases up to the diastolic pressure of the resistance vessels, EDV decreases and flow deceleration occurs more rapidly, and the changes on TCD are noted with ICP values of 20 mmHg or higher [248-258]. The following changes can be observed on TCD with increasing ICP:

Figure 6.21 Microembolic signals. The consensus definition is provided in the text. A single-gate spectral transcranial Doppler recording of the middle cerebral artery flow shows multiple high-intensity transient and primarily unidirectional signals after intravenous injection of agitated saline in a patient with right-to-left cardiac shunt.

1 end-diastolic velocity decrease;

2 pulsatility index increase (PI > 1.2 for previously normotensive young individuals);

3 resistance index increase;

4 shortening of the trans-systolic time; and

5 decrease in peak and mean flow velocities.

When ICP becomes greater than diastolic blood pressure but less than systolic pressure, the result is either a triphasic waveform as seen in the peripheral arteries, or a sharply peaked systolic signal and an absent end-diastolic component. Further increase in ICP may lead to cerebral circulatory arrest (Figure 6.22).

Increased ICP may result in high-resistance waveforms: PI > 1.2, decreased or absent EDV, and triphasic or reverberating flow. We use the following algorithm that may help to differentiate among the mechanisms of increased resistance to flow. If PI is > 1.2 and positive end-diastolic flow is present. A in all arteries: hyperventilation; increased cardiac output, hypertension; increased ICP; B unilaterally: compartmental ICP increase; stenoses distal to the site of insonation; C in one artery: distal obstruction (hypoplasia, spasm, stenosis, edema).

If PI is >2.0 and end-diastolic flow is absent A in all arteries: arterial occlusion; extremely high ICP; possible arrest of cerebral circulation; B unilaterally: compartmental ICP increase, occlusion distal to insonation site;

C in one artery: distal obstruction (occlusion, severe spasm, edema).

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