The Doppler principle as we know it in physics is a wave theory that describes the relationship between velocity of objects and transmitted or received wave frequencies. This theory was first described in 1842 by the Austrian physicist Christian Doppler at the Royal Bohemian Society of Science in Prague. His theory can be applied to measure velocity of moving objects. In medical applications we use ultrasound of frequencies between 1 and 20 MHz, which are transmitted from a transducer. The reflected frequency shifted waves are received by the same transducer.
Doppler ultrasound has been in use in medicine for many years. The primary long-standing applications included monitoring of the fetal heart rate during labor and delivery and evaluating blood flow in the carotid artery. Applications that have developed largely in the last two decades have extended its use to virtually all medical specialities including cardiology, neurology, radiology, obstetrics, pediatrics, and surgery. Doppler technology today allows detection of flow even in vessels that are too small to imagine.
Although Doppler techniques have been applied to peripheral limb arteries for many years, investigators have only recently measured blood flow velocity in intracranial arteries. Since the bones of the skull both absorb and reflect ultrasound in the 4-9 MHz frequency range, ultrasonography of the intracerebral vessels has been limited to infants with open fontanelles. In order to facilitate ultrasound studies in older children with closed fontanelles, the Doppler technique has been modified. Investigators restricted the site of application of the ultrasound probe to those areas of the skull where the bone is relatively thin, i.e., the temporal window. To augment further the transmission of ultrasound waves through the skull, the frequency of the Doppler signal was decreased from 4-9 MHz to 1-2 MHz. An additional modification was the use of pulsed wave rather than continuous wave ultrasound signals. Pulsed Doppler systems provide depth information and the ability to select depth from which Doppler information is received. All of these modifications greatly improved the quality of Doppler measurements of the velocity of blood in the large basal intracranial arteries particularly those in the circle of Willis.
Doppler shifted ultrasound has been used to monitor blood flow velocity in the extracranial cerebral arteries since 1965. The barrier to monitor blood flow velocity in the intracranial cerebral arteries presented by the skull was, in fact, more psychological than physical, and it delayed the development of TCD ultrasound more than 20 years. It was Rune Aaslid and coworkers that made the first TCD recordings in summer 1981 after discussion with Helge Nornes, who pioneered intraoperative Doppler in cerebral circulation. It was originally introduced for assessment of intracranial vasospasm following SAH. The TCD methodology was thereafter developed at the Department of Neurosurgery in Berne. The further development of a simple and small, microprocessor-controlled instrument (TCD-64, produced by Medical Electronics, Überlingen, Germany) with analog/digital output of the velocity wave form facilitated the use of this technique for monitoring of the cerebral hemodynamics.
During the last two decades, the effects of anesthetics on CBF have been studied clinically, and several reviews have been published. However, deductions concerning the effects of anesthetics on cerebral circulation, and the clinical applications of anesthetics in neurosurgical practice are primarily based on experimental studies. The reasons therefore are multifactorial. Traditional methods used to measure CBF are limited by various difficulties including bulky equipment, placement of flow detectors within the operative field, discontinuous measurements and concern about potential risks of employing radioactive material. In addition, the equipment and expertise required to operate these techniques are still to be found in only a minority of anesthetic departments.
TCD ultrasonography has only recently been applied to perioperative CBF assessment. The interest of anesthetists in TCD technology derives from the capacity of this technique to provide cerebral hemodynamic data in acute care settings. In contrast to traditional techniques TCD ultrasonography provides an easy-to-use, non-invasive, non-radioactive, and relatively inexpensive method to assess intracerebral hemodynamics in connection with a time resolution detecting cerebral perfusion changes in the range of seconds. Cerebrovascular responsiveness to various physiological and pharmacological challenges during different anesthetic regimens can be assessed on-line i.e. instantaneously. Moreover, it is today well recognized that an isolated measurement of CBF - static measurement - provides little useful information about the state of the cerebral circulation. Instead, the cerebrovascular circulation should be assessed in stress-situations - dynamic measurement - provided by surgery and anesthesia, in order to measure its ability to respond to changing physiological conditions. Using TCD ultrasonography various cerebral circulatory tests can be repeated often and safely. Rapid changes of cerebral perfusion over time can be easily followed, documented and analyzed.