Table 6.
Comparison of clinically utilized techniques to measure cerebral autoregulation in subarachnoid hemorrhage patients.
| CBF Measurement Properties | ||||||
|---|---|---|---|---|---|---|
| Technique | Invasiveness | Spatial resolution | Relative/Absolute | Snapshot/Continuous | Significant limitations | References |
| Thermal Conductivity Probe | Invasive | Regional level | Absolute | Continuous | Measurements limited to cranial surgeries. Measurement are not completed at bedside. | (20, 117) |
| Carotid Artery Flow Probe | Invasive | Low/Global level | Absolute | Continuous | Measurements limited to cranial surgeries. Requires surgical implantation of flow probes. | (119) |
| Measurements are not completed at bedside. | ||||||
| 133Xe Computed Tomography | Minimally or Non-Invasive | High/Local level | Absolute | Snapshot | Rapid washout limits the number of views/projections per trial. Soft tissue may attenuate signals, especially in anterior images. Method requires specialized equipment. | (101, 120, 122, 123) |
| Measurements are not completed at bedside. | ||||||
| Magnetic Resonance Imaging (MRI) | Minimally Invasive | High/Local level | Absolute | Snapshot | Long scan times required to obtain measurements. Very expensive equipment required. | (124) |
| Measurements are not completed at bedside. | ||||||
| 15O Positron Emission Tomography | Minimally Invasive | High/Local level | Absolute | Snapshot | Long scan times required to obtain measurements. Very expensive equipment required. | (125) |
| Measurements are not completed at bedside. | ||||||
| Transcranial Doppler | Minimally Invasive | Regional level | Relative | Continuous | Assumes that insonified artery diameter remains constant. Not a reliable measure of CBF. | (118) |
| Transient Hyperemic Response Test | Minimally Invasive | Regional level | Relative | Snapshot | Highly prone to inconsistency. Assumes that MCA diameter remains constant. | (126–131) |
| Pressure Reactivity Index (PRx) | Invasive | Low/Global level | Relative | Continuous | ICP measurements are invasive, but may already be included in standard of care. | (46, 143, 147–151) |
| Decompressive craniotomy or ventricular drains may compromise ICP measurements. | ||||||
| Measurements when ICP is high are likely unreliable | ||||||
| Systolic Flow Velocity Index (Sx) or Mean Flow Velocity Index (Mx) | Non-invasive | Regional level | Relative | Continuous | Assumes that insonified artery diameter remains constant. Not a reliable measure of CBF. Requires a highly-qualified operator. Cumbersome to complete at bedside. |
(21, 154–156) |
In this table, invasive techniques require surgical access, while minimally invasive techniques require injections (intravenous or intraarterial). CBF, cerebral blood flow; ICP, intracranial pressure; MCA, middle cerebral artery.