First transcranial Doppler Holter

Transcranial Doppler (TCD) evaluates both the direction and speed of blood flow in the major cerebral arteries of the brain. Physicians also use this test during surgery to monitor blood flow in the brain. Physicians may use TCD alone or with magnetic resonance imaging (MRI), magnetic resonance angiography (MRA) or computed tomography (CT).

Ultrasound is a noninvasive medical test that helps physicians diagnose and treat medical conditions. It is safe and painless. It produces images of the inside of the body using sound waves. Ultrasound imaging is also known as sonography. It uses a small probe called a transducer and a gel that is placed directly on the skin. High-frequency sound waves travel from the probe through the gel and into the body. The probe picks up the sounds that bounce back. A computer uses those sound waves to create an image. Ultrasound examinations do not use radiation (x-rays). Because ultrasound captures images in real time, it can show the structure and movement of the body's internal organs. It can also show blood flowing through blood vessels.

Ultrasonography of supra-aortic trunks is performed with the patient lying down, placing the transducer over the neck in different positions until the carotid and vertebral arteries are identified. The examination is completed with the study of blood flow in the orbital circuit (ophthalmic artery).

Transcranial Doppler is also performed with the patient lying down, placing the Doppler probe in the skull over the so-called acoustic windows (transtemporal, occipital, ophthalmic and submandibular windows), through which the ultrasound can penetrate.

In this way, the main intracranial arteries can be identified, depending mainly on the acoustic window used and the depth of insonation.

Occasionally Doppler signal enhancers are used, which are administered intravenously and have no significant side effects. Both techniques can be performed on any person because, like any ultrasound, they are free of irradiation risks.

Transcranial Doppler allows demonstrating the cerebral circulatory arrest that accompanies brain death, being especially useful in sedated patients, or in those who cannot undergo a complete neurological examination.

Transcranial Doppler is a portable, noninvasive and highly available technique. Among its limitations are the absence of sonic window and false negative cases. In patients with a clinical diagnosis of brain death, who have open skulls or anoxia as the cause of death, cerebral blood flow can be sonicated, since cerebral circulatory arrest is not always synchronous with the clinical diagnosis. Its diagnostic yield is therefore time-dependent, a fact that must be recognized in order not to delay the declaration of death.

Despite its limitations, transcranial Doppler helps to solve frequent diagnostic problems, avoids unnecessary consumption of resources and can optimize organ procurement for transplantation.

With transcranial Doppler it is possible to accurately evaluate blood flow in the brain, identify fistulas and vascular malformations, and measure the volume of cerebral hematomas.

As it is a neurological emergency, stroke must be diagnosed and treated immediately, since the longer the waiting period, the greater the brain damage. Once the diagnosis is confirmed, the application of specific treatment significantly improves the patient's prognosis.

Within this highly accurate and reliable ultrasonographic technique there are two variants: Doppler and color Doppler ultrasound.

The second is of more recent creation, and although similar to the first, it introduces the benefit of providing anatomical images, which allows a clear view of the vessels of the brain, important for the identification of intercranial arterial stenosis.

TCD is an ideal functional test to detect rapid changes in cerebral perfusion, and to evaluate cerebral vascular reserve mechanisms, using different stimuli (hypo- or hypercapnia, hypo- or hypertension, and hypoxia). With varying CO2 concentrations the relationship between blood velocity and flow volume in the large cerebral arteries is linear, since the CO2 level does not directly affect the diameter of these arteries. Therefore, changes that occur in blood velocity reflect changes in flow volume.

Proper functioning of this vasomotor reserve implies that a drop in perfusion pressure can be compensated for by vasodilation of the cortical arterioles and thus maintain a sufficient supply to the cerebral cortex. If the reserve is depleted, there can be no further vasodilation with CO2, a critical condition because cerebral ischemia will occur as a consequence of a further fall in perfusion pressure. The COZ reactivity test allows to study the reaction of cerebral arteries in different pathological situations and to measure cerebral vascular reserve.

These tests are useful to assess the hemodynamic impact of extracranial carotid occlusive disease and other conditions with reduced cerebral perfusion such as hypoxic migraine, high-altitude exposure, brain trauma and embolic stroke.