Should we measure cerebral blood flow in head-injured patients?

Abstract

Inadequate cerebral blood flow (CBF) after head injury is an important cause of secondary ischaemic damage. Rapid identification of episodes of hypo- or hyperperfusion would allow timely intervention and would possibly improve outcome. Despite a large number of methods to estimate CBF, this concept is only marginally implemented in clinical practice. The methods to detect such episodes are limited for technical reasons, but also because the thresholds of ischaemia and hyperaemia are variable after head injury. Furthermore, we are not always able to manipulate CBF in a controlled manner. Accordingly, it is not surprising that attempts to compare a CBF-targeted strategy with another management option have failed to demonstrate a clear benefit. Methods need to be developed that allow either identification of thresholds for critically low or high CBF in individual patients, allow monitoring oxygen extraction fraction, representing circulatory reserve, or alternatively provide a measure of the volume of ischaemic or hyperaemic brain.

Changes in the intracranial rheoencephalogram at lower limit of cerebral blood flow autoregulation

“…Specifically, the continuous monitoring of CBF autoregulation has a lifesaving importance (Czosnyka et al 2000, Steiner andCzosnyka 2002). The brain, which is the most sensitive sensor of hypoxia and ischemia, is the optimal organ for use as a feedback modality for life-sign monitoring and resuscitation.…”

“…Monitoring of CBF or cerebrovascular reactivity is performed in neurosurgical clinical practice in order to evaluate the status of the patient after brain injury or operation. The clinical background was detailed elsewhere (Schmidt et al 2002, Lang et al 2003ba, 2003b, Steiner and Czosnyka 2002, Beaumont and Marmarou 2002, Mayberg 1998, Rosner 1995, Wood 1987, Vespa 2003. In other words, reactive brain vessels offer a positive prognosis; non-reactive vessels are a bad prognosis and call for medical intervention.…”

Assessment of Cerebrovascular Autoregulation in Head-Injured Patients

“…There are many methods available to measure CBF in head-injured patients, but many bedside methods do not measure CBF but instead monitor a surrogate marker considered to be proportional to CBF. 7 In clinical practice, transcranial Doppler (TCD) is commonly used for dynamic 8 and static 9 measurements of autoregulation, although some investigators have used arterio-jugular oxygen content difference (AJDO 2 ) 10 or methods based on waveform analysis. 4,11 When autoregulation is determined at the bedside, the assumption is made that the cerebral metabolic rate for oxygen (CMRO 2 ) does not change during autoregulation testing.…”

Transcranial Doppler ultrasonography as an early outcome forecaster following severe brain injury

“…18 Accordingly, an inadequate level of CBF is an important cause of secondary ischaemic brain damage. 19,20 Therefore, early ischaemia after such an injury may be an important factor in determining outcome 6 since, instantly after the injury, patients are more vulnerable to secondary insults. Consequently, no less than 30% of severely brain-injured patients sustain episodes of early ischaemia.…”