Two-Depth Transcranial Doppler for measurements of ICP through the Opthalmic Artery
Innovative method using Two-Depth Transcranial Doppler (TDTD) of monitoring intracranial pressure (ICP) relies on the same fundamental principle used to measure blood pressure with a sphygmomanometer. A sphygmomanometer works using a balance principle - an air-filled pressure cuff wrapped around the arm compresses the brachial artery to a point where blood can no longer flow. The examiner slowly releases the air from the cuff and uses a stethoscope to listen for the return of blood flow. At the balance point, where pressure in the cuff equals systolic artery pressure, a ‘whooshing’ noise can be heard as blood flows through the artery again.
The TDTD method uses Doppler ultrasound to translate principle of blood pressure measurement with a sphygmomanometer to the measurement of ICP. Ophthalmic artery (OA) - a unique vessel with intracranial and extracranial segments is used as a natural pair of scales for absolute ICP measurement. Blood flow in the intracranial segment is affected by intracranial pressure, while flow in the extracranial segment is influenced by the externally applied pressure to the orbital tissues.
As with a sphygmomanometer, a pressure cuff is used - in this case to compress the tissues surrounding the eye and change the characteristics of blood flowing from inside the skull cavity into the eye socket. In place of the stethoscope, a Doppler ultrasound beam measures the blood flow in intracranial and extracranial segments of the Ophthalmic Artery. The aICP meter based on this method gradually increases the pressure over the eye so that the blood flow parameters in two sections of artery are equal. At this point, the applied external pressure equals the intracranial pressure.
This measurement method eliminates the main limiting problem- the individual patient calibration problem by direct comparison of aICP and externaly applied pressure – same fundamental principle used to measure blood pressure with a sphygmomanometer.
The mean value of OA blood flow, it’s systolic and diastolic values, pulsatility indexes are almost the same in both OA segments in the point of balance aICP=aPe. As a result of that all individual influential factors (ABP, cerebrovascular autoregulation impairment, individual pathophysiological state of patient, individual diameter and anatomy of OA, hydrodynamic resistance of eye ball vessels, etc.) do not influence the balance aICP=aPe and, as a consequence, such natural “scales” do not need calibration.
This method is further developed by Company Vittamed (Kaunas, Lithuania) together with consortium partners in EU FP7 project Brainsafe http://www.fp7brainsafe.com/)