Is it possible to monitor CBF using the c-FLOW™ Monitor on a brain-dead patient, and would it enable us to better determine brain death with an easy-to-use bedside tool?

When addressing this issue there are 2 important points to consider:

  • The c-FLOW™ System is a monitor and not a diagnostic tool.
  • The CFI (Cerebral Flow Index) is a relative parameter looking at changes in light patterns that are a result of CBF changes.

Clinical Discussion: When discussing Cerebral Blood Flow (CBF) in brain-dead patients while performing radio-imaging studies, there is an absence of intracerebral filling at the level of the carotid bifurcation or the Circle of Willis. The external carotid circulation is patent, and filling of the superior sagittal sinus may be delayed. Angiography is seldom performed to reassure clinicians and family members after clinical signs criteria are positive for determining brain death. Overall, brain death represents a long-term “zero” blood flow to the brain and does not represent an acute event of change. Therefore when monitoring patients who, for example, experience cardiac arrest or any other acute event that compromises the brain’s blood flow, the c-FLOW™ Monitor will be able to detect these changes and their magnitude. Yet when monitoring a stable or static situation, changes in readings will be insignificant.

It is important to understand that even though there is no CBF, the c-FLOW™ Monitor might get a reading and display a number (another important point is that the c-FLOW™ monitor numbers are relative and not absolute). The sensor is built to detect the Ultrasound-Tagged Near-Infrared (NIR) laser light that it emits. The light travels through media that produce changes in the light pattern – in brain dead people there is still liquid flow within the body (Brownian motion) which can still be detected by the sensor. In order to get a comprehensive reading, some change needs to be made that can distinguish between two diverse CBF states. A good example would be changing ventilation settings to produce hyper / hypoventilation. In this case, changing CO2 levels consequently changes CBF in a non-brain-dead patient and causes “zero” change in the brain-dead one. With liquids still exhibiting Brownian motion in the brain-dead patient, the CFI provided by the c-FLOW™ Monitor will become flat and stable (but not zero), and present none of the normal small perturbations that would be seen in a patient with intact flow.

Therefore, in conclusion, the c-FLOW™ Monitor was not designed to determine or diagnose brain death but rather continuous CBF. Taking this point one step further and if one so desires nonetheless to do so, the device specifications and capabilities must be taken into consideration in order to facilitate a favorable environment to answer the pending question, where a CBF challenge test needs to be performed.