Bedise invasive monitoring techniques in severe brain-injured patients
Vinas, Federico C
Of all injuries sustained in traumatic events, head injury is frequently associated with the most devastating outcome. Often, patients survive the initial insult, but progressively deteriorate, ending up with major neurological deficits. Improvements in pre-hospital care, better patient transport, and the ready availability of multidisciplinary teams have decreased the initial mortality previously associated with severe brain injury. Still, the functional outcome of severely brain-injured patients remains poor, due to ‘secondary mechanisms of injury’1,2 such as release of free oxygen radicals and excitatory amino acids in toxic concentrations, lipid peroxidation of neuronal membranes, etc. A disruption in the autoregulation of cerebral blood flow can lead to brain edema, leading to uncontrolled intracranial pressure (ICP) elevation. In addition, local cerebral ischemia has important implications for the clinical management of brain-injured patients because it can be prevented and/or reversed, sometimes just by blood pressure elevation3.
Several monitoring modalities have been developed in an attempt to diagnose and reverse the underlying secondary mechanisms that cause delayed neuronal damage. The present article provides a review of recent advances in the invasive monitoring in patients with severe brain injury.
INTRACRANIAL PRESSURE (ICP) MONITORING
Interest in the pathophysiology of raised ICP extends back over 200 years. Since its introduction by Guillaume in 1951 and the pioneering work of Lundberg, measurement of intracranial pressure has become an important part of the management of comatose head– injured patients4,5. Uncontrolled high ICP is probably the single most important factor in mortality from severe head injury, and monitoring ICP in the neurointensive care unit has become routine in the care of patients with disorders complicated with intracranial hypertension. However, while there is considerable indirect and retrospective evidence supporting the role of ICP– directed therapy, monitoring ICP has not unequivocally demonstrated to improve patient outcome6.
There is still some controversy regardng the indications and ideal type of monitoring device to be used; nevertheless, ICP monitoring is generally considered appropriate for patients with severe brain injury, defined as a Glasgow Come Score of 8 or below after cardiopulmonary resuscitation6.
Several devices can be used to monitor ICP. The first ICP monitor used in patients, and the ‘gold standard’ against which new methods for monitoring of intracranial pressure have to be judged, is a fluid-filled ventricular catheter connected to an external ly-mounted transducer that can be zeroed and calibrated against a known pressure. This method has been proven to be reliable and allows for periodic re-zeroing; in addition it allows one to withdraw CSF for immediate reduction of ICP and for chemical and bacteriological analysis. The catheter may also be used to instill therapeutic medications into the CSF such as thrombolytic agents 7 or antibiotics in certain cases of ventriculitis8-10. It is still probably the most cost-effective method of monitoring ICP. The use of ventricular catheters does however offer a series of limitations, including blockage by blood clots or fibrin debris, the potential for leaks in the fluid lines, the need for periodic re-zeroing, and the inability to accurately measure pressure in cases of slit ventricles. In this system, the zeroing is crucial; a hydrostatic error of 1.86 mmHg occurs for every 2.54 cm above or below the true anatomic zero point that the transducer is placed11,12.
An alternative method of monitoring ICP is to use an intraparenchymal transducer. ICP measurements obtained with intraparenchymal transducers correlate well with the values obtained with an intraventricular catheter 13 . Apart from early laboratory prototypes, the first commercial design for intraparenchymal ICP monitoring was the Honeywell Microtransducer catheter MTC-P5F . Contemporary transducers may be classified as solid state (Codman, Johnson & Johnson Professional, Inc., Randolph, MA, USA) (Figure 1) and fiberoptic design (Camino ICP Monitor V420, Camino Laboratories, San Diego, CA, USA) (Figure 2). Solid state transducers are based on silicon chips, with diffused pressure-sensitive resistors forming a bridge . In contrast to the fiberoptic transducers, they do not require a specific digital pre-amplifier and can be connected to the patient through a relatively simple, inexpensive interfacing unit16. Although both systems are very accurate at the time of placement, they have been reported to produce a drift over time on the ICP readings, which can result in an error after three or four days. Using a microsensor fiberoptic device (Camino ICP monitor, Camino Laboratories), Crutchfield et al.’s found an average daily drift of 0.6 mmHg, Czosnyka et al.14 reported a drift
More recently, some clinicians have advocated the use of a combined or hybrid system26, which combines the advantages of the ventriculostomy with a microimplantable transducer, either a strain-gauge type (Codman Microsensor TM) or a fiber optic type (Camino Ventrix, NeuroCare Group, Pleasant Prairie, WI, USA)22. These systems appear to be comparable in utility and convenience to a standard ventriculostomy, with the advantage that the microsensor should provide ICP measurements even when the catheter is occluded. These hybrid systems are more expensive than the standard ventriculostomy; studies are currently in progress to determine whether increased utility over ventriculostomy may justify their increased cost.
In a series of 536 indwelling cerebral monitoring devices from Detroit Receiving Hospital, ventriculostomies were associated with a 7.29% infection rate and 3.28% hemorrhage, whereas the use of fiberoptic devices was associated with 0.87% hemorrhage rate and no apparent infections27. The incidence of hemorrhage associated with ventriculostomies and intraparenchymal ICP monitors from the literature is shown in Table 1. Infection of the insertion site leading to cerebritis or ventriculitis is a well known complication associated with the use of both intraparenchymal pressure monitors and ventricular catheters. Several studies have demonstrated that ventriculostomies are associated with a higher risk of ventriculitis than other types of ICP monitor, and that the risk of infection increases with the duration of monitoring. Although incidences of ventricular catheter infections ranging from 0% to 40% have been reported, they lie more commonly in the 10%-17% range 28–36. Most investigators agree that the risk of infection increases with the length of monitoring, and some centers routinely change ventricular catheters every 3-5 days. In a series of 584 severely head-injured patients with ventriculostomies, however, Holloway did not find a lower infection rate when the intraventricular catheters were replaced prophylactically prior to 5 days, compared with those patients in whom catheters were exchanged at intervals of 5 days or more37. Infection rates in patients with intraparenchymal ICP monitors are considerably lower than in patients with ventriculostomies (Table 2).
JUGULAR BULB CATHETERS OXIMETRY
Monitoring of the jugular venous oxygen saturation allows one to continuously evaluate cerebral oxygenation and to estimate the adequacy of cerebral blood flow (CBF) in brain-injured patients. This technique is based on the theory that both oxygen delivery (reflected on CBF and AVDO^sub 2^) and oxygen consumption (reflected in consumption metabolic rate of oxygen or CMRO^sub 2^) can be affected following traumatic brain injury 38,39. However, since CMRO^sub 2^ remains relatively constant at its new level, SjvO^sub 2^ gives an estimation of the degree of adequacy of the global CBF to meet the oxygen metabolic needs404.
With this method, an oximetric catheter is inserted percutaneously into the internal jugular vein. The tip of the catheter is located in the jugular bulb, where blood is approximately 98% of intracerebral origin, allowing the continuous measurement of the oxygen saturation 45,46. The catheter patency is secured by either continuous or intermittent pressurized heparinized saline flush through the catheter.
Several reports have demonstrated the potential utility of continuously monitoring jugular bulb oxygen saturation in the intensive care unit after traumatic brain injury44,45,47-51. However, a drawback of this method is that it assesses the global oxygen saturation only and focal, small ischemic events might be undetected. SjvO^sub 2^ values below 50%-55% are generally considered dangerously low. Robertson et al. 3 have demonstrated in 177 patients with severe head injury (GCS 8 or less) that episodes of confirmed jugular venous oxygen desaturation ( 50% for at least 10 min) occur in 39% of these patients, even in the intensive care unit setting with advanced cardiovascular and intracranial monitoring53. Gopinath et a1.49 demonstrated a strong correlation between the number of incidences of jugular venous desaturation and poor neurological outcome.
There are currently two commercially available systems for the continuous monitoring of SjvO^sub 2^ (Figure 3), a two-light wavelength system (Baxter– Edwards Explorer, Baxter Healthcare, Santa Anna, CA, USA) and a three light wavelength system (Abbott Opticath, Abbott Laboratories, North Chicago, IL, USA). Both systems have been reported to provide acceptable accuracy in brain injured patients in the ICU44,45,48,54. The three-light wavelength system can be calibrated in vitro (pre-insertion calibration) or in vivo; the two-light wavelength system uses only in vivo calibration from a simultaneously drawn blood sample.
INVASIVE CEREBRAL TISSUE OXYGEN MONITORING
The availability of oxygen to brain tissue results from the balance between the local delivery rate of oxygen to tissue (blood flow) and its local uptake (metabolism). Direct bedside monitoring of cerebral white matter oxygen is a promising technique which has attracted considerable interests54-60. This technique allows one to study both focal and regional changes in cerebral oxygenation. There are two available systems containing miniaturized Clark electrodes (Figure 4). The Licox (Locix GMS, Mielkendorf, Germany) allows the measurement of tissue PO^sub 2^ and temperature. The paratrend (Paratrend 7 Multiparameter Intravascular Sensor, Biomedical Sensors, Malvern, PA, USA) allows the measaurement of PO^sub 2^, pH, and PCO^sub 2^. Of these three parameters, PO^sub 2^ seems to be the most sensitive . In order to measure PO^sub 2^ and temperature, the Licox requires the placement of two different probes, which can be placed at the same depth. The various sensors in the Paratrend are separated by as much as 30 mm along the length of the probe-5. When placing either probe, it is important to carefully assess the depth of insertion; in the Licox, the oxygen sensor lies
In the literature, few data exists on normal brain PO^sub 2^ values in humans. The critical threshold of brain tissue PO^sub 2^ depends on the type of monitor used. In a preliminary series of severely head-injured patients, Maas et al.57,511 used Licox in the frontal white matter and obtained baseline PO^sub 2^ values of 25-30 torn which correlated well with their animal data. In a subsequent report , they reported death in four of five patients who had a PO^sub 2^ of
Few complications related to the use of brain tissue PO^sub 2^ have been reported in the literature. In a study of 73 patients Dings et al.65 reported a 2.7% incidence of small, self-limited intracerebral hematomas, and no infections65. These complications were related to the use of a three-way bolt that included within the same burr hole a fiberoptic ICP monitor and a laser Doppler probe.
Although a few centers have implemented this technique for patient monitoring, further work is needed to evaluate the method in comparison with established techniques to establish whether its practical usefulness measures up to its theoretical possibilities.
Microdialysis is a well-established neurochemistry technique capable of both recovering and administering substances into target tissues. In vivo microdialysis was introduced in 1982 as a technique to study cerebral neurochemistry in awake, freely moving animals. The principle application of microdialysis is the estimation of the extracellular fluid concentrations of both endogenously and exogenously administered substances66. Intracerebral microdialysis coupled with a sensitive radioenzymatic assay provides information on the cerebral metabolic state by measuring the concentrations of metabolites, neurotransmitters, and extracellular pH 67,68. This method also allows in vivo measurement of drug levels in the cerebral extracellular space and investigation of their effect on metabolism.
Persson et al.69 performed the first study of long-term chemical monitoring of the brain using microdialysis in four ICU patients (three of them with severe brain injury) demonstrating the feasibility of long-term microdialysis in humans. Kanthan et al.70 reported the use of microdialysis over a 4 h period in one patient with severe brain injury by means of a bolt placed in the right frontal horn. High glutamate concentrations and glycine were found through the study with high glutamate: GABA concentrations in all samples. Bullock et al.2 measured sodium, potassium, and excitatory amino acids in 17 patients with severe brain injury and correlated these findings with the clinical type of injury, ICP, and CPP. Patients with focal cerebral contusions displayed excitatory amino acids levels 6-20 times higher than the baseline observed by other investigators. In patients who had diffuse brain injury and prior ischemic events, the concentration of excitatory amino acids was 20-50 times higher than the baseline levels previously reported. In patients with diffuse injuries and no secondary ischemic complications the rise in excitatory amino acids levels appeared to be a transient phenomenon. In patients with secondary ischemic events, the elevated levels persisted throughout the study.
Current microdialysis techniques are expensive and labor intensive; The number of clinical publications involving microdialysis in head-injured patients have been small. Since no large clinical studies have been performed, the true utility of microdialysis as a diagnostic or therapeutic tool in head-injured patients is yet to be defined. Further work is needed in order to establish the clinical usefulness of microdialysis.
Newer monitors assess simultaneously different parameters. There are currently two devices available. The Neurotrend (Codman, Johnson & Johnson Professional, Inc., Randolph, MA, USA) allows measurement of intracranial pressure, partial oxygen pressure (PO^sub 2^), partial CO^sub 2^ pressure (PCO^sub 2^), and tissue pH. The MPM 1 monitor (Integra NeuroSciences Neurocare, Plainsboro, NJ, USA) is a fiber optic-based transducer that allows the monitoring of ICP, cerebral perfusion pressure, and tissue temperature.
There is evidence that intracranial pressure and cerebral perfusion pressure correlate with outcome in head injured patients. Intracranial pressure greater than 20 mmHg, and cerebral perfusion pressure below 60 mmHg have been associated with poor outcomes. However, monitoring of intracranial pressure alone might not suffice in the management of acute brain injury patients. Although monitoring of SjvO^sub 2^ has been correlated with patient outcome, this method is technically demanding and numerous artifacts influence the measurements. In a series of patients with severe brain injury, van Santbrink et a1.56 found no clear correlation between brain PO^sub 2^ and ICP or CPP. It is possible that brain PO^sub 2^ might afford additional information to be used in conjunction with ICP and CPP. Further research needs to be conducted in order to analyze whether therapy aimed at increasing brain PO^sub 2^ can yield better treatment results.
Copyright Forefront Publishing Group Mar/Apr 2001
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