Monitoring Cerebral Blood Flow in Neurosurgical Intensive Care

Abstract

Therapeutic concepts in neurosurgical intensive care require sophisticated and refined neuromonitoring applications, because the individualised approach to the treatment of head-injured patients relies on the assessment and interpretation of the key parameters of brain tissue viability and function. Monitoring of regional cerebral blood flow (rCBF) in particular has been a long-term problem due to a lack of devices available for continuous bedside online monitoring. A novel thermal diffusion (TD) microprobe was introduced recently for continuous bedside monitoring of rCBF (TD-rCBF). The following article provides a description of the technique and describes the clinical application and the potential of this novel microprobe to assess CBF in patients suffering from subarachnoid haemorrhage (SAH) or traumatic brain injury (TBI). In neurosurgical practice, monitoring of CBF plays an important role, as the brain depends on continuous blood supply due to its inability to store glucose or oxygen. During pathological conditions such as SAH or TBI, the patient – and consequently the functions of the brain – cannot be supervised clinically. Furthermore, secondary insults in SAH or TBI represent dangerous and often lethal complications, making continuous neuromonitoring essential for neurosurgical intensive care. Under these conditions, rCBF is considered an important upstream monitoring parameter that is indicative of tissue viability. In order to be able to establish new therapeutic approaches that focus on the pathophysiological basis of the secondary insult, neuromonitoring strategies need further refinement, as continuous monitoring of intracranial pressure (ICP) and cerebral perfusion pressure (CPP) has failed to adequately identify malperfusion in the brain-injured patient. 1 In this scenario, continuous monitoring of rCBF could provide the opportunity to diagnose and to correct insufficient rCBF before deficits in tissue oxygenation and metabolism are recognised. 2 There are a variety of CBF measurement techniques available, such as stable xenon-enhanced computed tomography (sXe-CT), single-photon-emission computed tomography (SPECT), magnetic resonance imaging (MRI) and positron emission tomography (PET); however, these methods are hampered by several clinical and practical drawbacks. Monitoring of rCBF in neurosurgical intensive care should ideally be performed in a continuous way at the bedside, providing quantitative rCBF values with high temporal and spatial resolution. Although laser-Doppler flowmetry (LDF)-and thermal diffusion flowmetry (TDF)-based measurement techniques provide continuous bedside monitoring of CBF, their clinical acceptance has been very low due to enduring technical drawbacks. Since LDF detects and measures erythrocyte flux, definite conclusions about nutritive perfusion and quantitative CBF cannot be drawn by means of this method. 3 Furthermore, TDF-based measurement techniques, such as cortically placed probes, were hampered by difficulties regarding the reliability and validity of the readings obtained. 3 Recently, however, the TD-rCBF microprobe, which is implanted intra-parenchymally and therefore circumvents the major drawbacks of the old systems that have been in use so far, has been introduced in clinical practice. It enables the quantitative, continuous bedside assessment of rCBF, which guarantees high reliability due to advanced mathematical modelling systems. 4 This article illustrates the technique of the TD-rCBF microprobe and introduces its clinical application in patients with SAH and TBI. Intra-parenchymal Thermal Diffusion Flowmetry of Regional Cerebral Blood Flow Thermal Diffusion Flowmetry – Probe Design The TD-microprobe consists of a flexible, medical-grade polyurethane catheter of 0.9mm diameter with two thermistors (a proximal and a distal thermistor, 5mm apart) embedded within the catheter. The distal thermistor is heated to approximately 2°C above the tissue temperature, thereby generating a constant spherical temperature field with a diameter of approximately 4mm. By positioning the thermistors 5mm apart, the proximal thermosensor is located outside the thermal field, thereby allowing continuous monitoring of tissue temperature and compensation of baseline fluctuations. The power dissipated by the heated thermistor (0.005–0.01W) provides a direct measure of the tissue's ability to transport heat. However, thermal transfer includes both intrinsic conductive properties of the tissue and convective effects induced by blood perfusion. Therefore, it is necessary to separate thermal conduction and convection components in order to achieve an adequate and reliable measurement of CBF in quantitative and absolute Neurosurgery 6 Monitoring of regional cerebral blood flow in particular has been a long-term problem due to a lack of devices available for continuous bedside online monitoring. physiological data. This was usually achieved by no-flow calibration with earlier heated probes, making heated probes impractical for daily clinical use in neurosurgical intensive care. The TD-rCBF microprobe discussed in this article permits reliable quantification of tissue perfusion by determining the conductive properties of the tissue from the initial rate of propagation of the thermal field and by separating this component from the total heat transfer as the determinant of the thermal convection component. Using a series of data-reduction algorithms, convection and conduction components are acquired separately, making no-flow calibration unnecessary. The volume of the generated temperature field is approximately 27mm 3 and therefore allows analysis of rCBF in this area. Usually, CBF values are recorded at a rate of 1Hz during clinical application. Illustrations of the TD-rCBF microprobe usage and technical settings are depicted in Figure 1. Thermal Diffusion Flowmetry – Probe Placement and Use According to evidence obtained in experimental and clinical validation studies of the TD-rCBF microprobe, the TDF probe is best placed 20–25mm below the cortical surface via a small burr hole and secured by tightening a metal bolt. Under these conditions, white-matter perfusion is obtained. As a general rule, mean TD-rCBF values range between 18 and 25ml/100g/min using this application form. At this point, it is important to stress that CBF values measured by TD-rCBF vary depending on the placement of the probe. Therefore, control of microprobe placement is important for interpreting CBF data acquired by TD-rCBF. Clinical Application Thermal Diffusion Flowmetry of Regional Cerebral Blood-flow Monitoring in the Management of Subarachnoid Haemorrhage Among the major goals in the management of patients with SAH are the reliable and early detection of symptomatic vasospasm and the early prevention of vasospasm-induced ischaemia, which carries a considerable risk for secondary morbidity. As many as 50–70% of SAH patients develop angiographic vasospasm and 19–46% of SAH patients develop clinically symptomatic vasospasm leading to a delayed ischaemic neurological deficit (DIND). 5–7 Among the latter, 64% develop cerebral infarction. 8,9 These data emphasise the importance of the early and reliable detection of haemodynamically relevant vasospasm in order to start specific therapeutic interventions such as triple-H therapy. In awake patients, clinical examination remains the gold standard and best surveillance tool for detecting neurological deterioration. However, comatose and ventilated patients are not amenable to neurological examination. Therefore, clinicians have to rely on various direct and indirect techniques aiming to detect critical changes in rCBF in order to diagnose haemodynamically relevant vasospasm. Cerebral angiography remains the gold standard in the diagnosis of cerebral vasospasm. However, the specificity of angiography in the diagnosis of symptomatic or haemodynamically relevant vasospasm has recently been calculated to be only 50%, indicating that the haemodynamic relevance of angiographic vasospasm cannot be assessed adequately. 10 Based on this significant disadvantage, transcranial Doppler (TCD) ultrasonography is often used in combination with cerebral angiography to detect haemodynamic changes in major cerebral arteries. Since its introduction, TCD has found widespread acceptance as an elegant,