Point-of-care transcranial Doppler by intensivists

Abstract

In the unconscious patient, there is a diagnostic void between the neurologic physical exam, and more invasive, costly and potentially harmful investigations. Transcranial color-coded sonography and two-dimensional transcranial Doppler imaging of the brain have the potential to be a middle ground to bridge this gap for certain diagnoses. With the increasing availability of point-of-care ultrasound devices, coupled with the need for rapid diagnosis of deteriorating neurologic patients, intensivists may be trained to perform point-of-care transcranial Doppler at the bedside. The feasibility and value of this technique in the intensive care unit to help rule-in specific intra-cranial pathologies will form the focus of this article. The proposed scope for point-of-care transcranial Doppler for the intensivist will be put forth and illustrated using four representative cases: presence of midline shift, vasospasm, raised intra-cranial pressure, and progression of cerebral circulatory arrest. We will review the technical details, including methods of image acquisition and interpretation. Common pitfalls and limitations of point-of-care transcranial Doppler will also be reviewed, as they must be understood for accurate diagnoses during interpretation, as well as the drawbacks and inadequacies of the modality in general.

Keywords: Cerebral circulatory arrest; Intra-cranial pressure; Midline shift; Neuro-critical care; Point-of-care ultrasound; Vasospasm.

Fig. 1

Transcranial imaging for midline shift. a Insonation from right temporal bone to third ventricle, representing distance A (7.41 cm). b Insonation from left temporal bone to third ventricle, representing distance B (6.11 cm). c Follow-up CT scan post TCD which reveals midline shift to be 7 mm

Fig. 2

Transcranial Doppler for vasospasm following subarachnoid hemorrhage. a CT Head demonstrating diffuse subarachnoid hemorrhage. b TCD measurement of left MCA flows, demonstrating a mean MCA velocity of 123 cm/s (in-keeping with mild vasospasm). c Measurement of ipsilateral left ICA flows for calculation of Lindegaard ratio of 3.8 (Mean MCA/ICA velocity = 123/32.5 cm/s), which confirmed mild MCA vasospasm

Fig. 3

Raised intra-cranial pressure by spectral Doppler on TCD, as calculated by pulsatility index. a Diffuse subarachnoid hemorrhage on CT head. b Spectral Doppler of MCA, demonstrating diastolic blunting secondary to raised ICP (pulsatility index = 4.31, ICP = 46 mmHg). c Following interventions to reduce ICP, there was normalization of diastolic flow in the MCA, and resolution of high ICP (pulsatility index = 1.65, ICP = 17 mmHg)

Fig. 4

Step-wise progression of cerebral circulatory arrest. a CT Head demonstrating diffuse subarachnoid hemorrhage. b Evidence of raised ICP causing decreased diastolic flow as evidenced by blunting of the spectral Doppler signal. c Further progression with diastolic flow reversal as raised ICP prevents forward flow in MCA, and even induces backwards flow. d Biphasic and oscillating flow as evidenced by net zero flow (where systolic and diastolic flow are near equal to each other), indicating the first TCD stage of cerebral circulatory arrest

Fig. 5

Method for acquisition of transcranial Doppler. a Left side shows location of the trans-temporal window and its various sections: frontal, anterior, medial, and posterior. Right side illustrates Circle of Willis, with MCA highlighted. b Probe index mark orientation towards the patient’s anterior/front. c Typical 2D image of TCD from trans-temporal window with index mark to screen left, with bright echogenic contralateral temporal bone, and anechoic space of midline third ventricle. d Typical spectral Doppler velocity waveform from MCA, with steep systolic upstroke and step-down diastolic flow (typical mean velocity of 80 cm/s)

Fig. 6

Progression of intra-cranial circulatory arrest via transcranial Doppler of middle cerebral artery flows. a Normal systolic upstroke with normal step-down of diastolic flow. b Increased peak systolic flow with decreasing diastolic flow and eventual blunting of diastolic flow. c Diastolic flow reversal. d Biphasic or oscillating flow—where diastolic flow reversal approaches equal size to systolic flow. e Isolated sharp systolic peak flows of < 200 ms and small systolic amplitude of < 50 cm/s. f Zero flow—where there was previously documented TCD flow. The red box denotes states (d, e, f) in which cerebral circulatory arrest can be diagnosed

Fig. 7

Disadvantage to transcranial Doppler when interpreting MCA flows for patient with non-pulsatile flow states (i.e., V–A ECMO, LVAD). a Normal pulsatile flow: systolic upstroke with normal step-down of diastolic flow. b Non-pulsatile flow: Loss of sharp systolic peak flows, with overall lowering of means flows—which would make interpretation of pulsatility index for ICP as well as mean flows for vasospasm more difficult