1. Correct Answer: C. Depth 45 to 55 mm, positive velocities

Rationale: The majority of the transcranial Doppler examination in the intensive care unit is done through the temporal acoustic window using a lower frequency probe (1-2 MHz) like that used for echocardiography. The depth, direction of blood flow, and cerebral blood flow velocity range are used to identify the insonated vessel through the temporal acoustic window (Table 81.1).

1. Alexandrov AV, Sloan MA, Wong LK, et al. Practice standards for transcranial Doppler ultrasound: part I—test performance. J Neuroimaging. 2007;17(1):11-18.

2. Couture EJ, Desjardins G, Denault AY. Transcranial Doppler monitoring guided by cranial two-dimensional ultrasonography. Can J Anaesth. 2017;64(8):885-887.

3. Rigamonti A, Ackery A, Baker AJ. Transcranial Doppler monitoring in subarachnoid hemorrhage: a critical tool in critical care. Can J Anaesth. 2008;55(2):112-123.

2. Correct Answer: C. Normal tracing, normal systolic flow and reduced diastolic flow, biphasic flow, systolic spikes, no flow

Rationale: A normal transcranial Doppler tracing of the middle cerebral artery, found from a temporal acoustic window at a depth around 4 to 6 cm with a flow direction toward the probe, is characterized by a positive Doppler signal of approximately 100 cm/s in systole and 50 cm/s at end diastole. Increases in intracranial pressure will first show a reduction of diastolic flow velocity, followed by a slightly decreased duration of systolic flow, and then the absence of diastolic flow. If the intracranial pressure is above the diastolic blood pressure, it will preclude diastolic cerebral blood flow.

The progression of intracranial pressure will then lead to the three stages of intracranial circulatory arrest, characterized by distinct flow velocity patterns that succeed one another in the following order: oscillating flow, systolic spike flow, and absence of flow. Oscillating flow is characterized by an antegrade systolic flow with a retrograde diastolic flow in one cardiac cycle. As intracranial hypertension progresses, the ratio of the area under the curve of the antegrade systolic and the retrograde diastolic flow will decrease from highly positive to 1 (equal flow forward in systole and backward in diastole). Further progression of intracranial hypertension will produce only brief systolic spikes of less than 200 ms, without any other flow during the rest of the cardiac cycle. When intracranial pressure reaches mean arterial blood pressure, there will be no blood flow in the major intracranial arteries (Figure 81.1).