The relationship between the intracranial pressure-volume index and cerebral autoregulation.

OBJECTIVE: The pressure-volume index (PVI) can be used to assess the cerebrospinal fluid dynamics and intracranial elastance in critically ill brain injured patients. The dependency of PVI on the state of cerebral autoregulation within the physiologic range of cerebral perfusion pressure (CPP) can be described by mathematical models that account for changes in cerebral blood volume during PVI testing. This relationship has never been verified clinically using direct PVI measurement and independent cerebral autoregulation assessment. DESIGN, SETTING, AND PATIENTS: PVI and cerebral autoregulation were prospectively assessed in a cohort of 19 comatose patients admitted to an academic intensive care unit in Brescia, Italy. INTERVENTION: None. METHODS: PVI was measured injecting a fixed volume of 2 ml of 0.9% sodium chloride solution into the cerebral ventricles through an intraventricular catheter. Cerebral autoregulation was assessed using transcranial Doppler transient hyperaemic response (THR) test. MEASUREMENTS AND RESULTS: Fifty-nine PVI assessments and 59 THR tests were performed. Mean PVI was 20.0 (SD 10.2) millilitres in sessions when autoregulation was intact (THR test >or=1.1) and 31.6 (8.8) millilitres in sessions with defective autoregulation (THR test <1.1) (DeltaPVI = 11.7 ml, 95% CI = 4.7-19.3 ml; P = 0.002). Intracranial pressure, CPP and brain CT findings were not significantly different between the measurements with intact and disturbed autoregulation. CONCLUSIONS: Cerebral autoregulation status can affect PVI estimation despite a normal CPP. PVI measurement may overestimate the tolerance of the intracranial system to volume loads in patients with disturbed cerebral autoregulation.

Transcranial Doppler ultrasonography in intensive care.

Transcranial Doppler is an innovative, flexible, accessible tool for the bedside monitoring of static and dynamic cerebral flow and treatment response. Introduced by Rune Aaslid in 1982, it has become indispensable in clinical practice. The main obstacle to ultrasound penetration of the skull is bone. Low frequencies, 1-2 MHz, reduce the attenuation of the ultrasound wave caused by bone. Transcranial Doppler also provides the advantage of acoustic windows representing specific points of the skull where the bone is thin enough to allow ultrasounds to penetrate. There are four acoustic windows: transtemporal, transorbital, suboccipital and retromandibular. The identification of each intracranial vessel is based on the following elements: (a) velocity and direction; (b) depth of signal capture; (c) possibility of following the vessel its whole length; (d) spatial relationship with other vessels; and (e) response to homolateral and contralateral carotid compression. The main fields of clinical application of transcranial Doppler are assessment of vasospasm, detection of stenosis of the intracranial arteries, evaluation of cerebrovascular autoregulation, non-invasive estimation of intracranial pressure, measure of effective downstream pressure and assessment of brain death. Mean flow velocity is directly proportional to flow and inversely proportional to the section of the vessel. Any circumstance that leads to a variation of one of these factors can thus affect mean velocity. The main pathological condition affecting flow velocity is the vasospasm. Vasospasm is a frequent complication of subarachnoid haemorrhage, it often remains clinically silent and the factors that make it symptomatic are largely unknown. Threshold velocities above which vasospasm comes into place are well defined as regards the median cerebral artery, while there is no consensus for the other vessels. Nevertheless, an increase in velocity alone is not sufficient to arrive at a diagnosis of vasospasm; a condition of hyperaemia also presents with an increase in flow velocity. The Lindegaard Index has therefore been introduced, which is defined by the ratio between the mean flow velocity in the median cerebral artery and the mean flow velocity in the internal carotid artery. Criteria for diagnosis of a stenosis >50% of an intracranial vessel with transcranial Doppler include: (a) segmentary acceleration of flow velocity; (b) drop in velocity below the stenotic segment; (c) asymmetry; and (d) circumscribed flow disturbances (turbulence and musical murmur). The transcranial Doppler enables us to assess both components of self-regulation. The static component is measured by observing changes in flow velocity caused by pharmacologically induced episodes of hypertension and hypotension. The dynamic component of autoregulation can be measured using a method devised by Aaslid known as the 'cuff test'. A very effective and safe device for measuring cerebral autoregulation is the transient hyperaemic response test. This test is based on the compensatory vasodilatation of the arterioles, which occurs after brief compression of the common carotid. Csonyka proposed the following formula based on clinical observation for the calculation of cerebral perfusion pressure: CPP = MAP x FVd/FVm + 14. Brain death is defined as the irreversible cessation of all functions of the whole brain. The clinical criteria are usually considered sufficient to establish a diagnosis of brain death; however, they might not be sufficient in patients who have been on sedatives or when there are ethical or legal controversies. Many authors have demonstrated the existence of a transcranial Doppler pattern, which is typical of brain death.

Intracranial pressure monitoring.

Recent studies have demonstrated that bedside cranial burr hole and insertion of intraparenchymal catheters for intracranial pressure monitoring performed by intensive care physicians is a safe procedure, with a complication rate comparable to other series published by neurosurgeons. The overall morbidity rate is comparable to, or even lower than, that caused by central vein catheterization. The procedure is also quite simple and modern disposable intracranial procedural kits are available. After the skin is prepped the landmark for skin incision, called the 'Kocher's point', located about 2-4 cm lateral to the midline (mid-pupillary line) and 2-3 cm anterior to the coronal suture, is found. Then the surgical field is prepared with the sterile drapes and the skin infiltrated with local anaesthetic (0.5% lidocaine with 1 : 200000 epinephrine). After skin incision and retraction of the skin and subcutaneous tissue, the periosteum should be scraped off in order expose the skull. The skin is then divaricated, exposing the underlying bone. The hole is drilled with either an electric drill or a twist drill (the drilling procedure must be performed with the drill held within 10 degrees of the perpendicular position to the incision site). The hole is then irrigated with sterile saline and an 18-G spinal needle may be used to open the dura (exercise caution when perforating the dura so as to avoid damage to the underlying structures). Following opening of the dura, the Bolt, containing a stylet, is screwed manually into the skull at approximately 5 mm to 1 cm for adults. The stylet is then removed after the bolt has been screwed in, after which the bolt should be filled with saline. Finally, the zeroing of the transducer is performed by simply holding the tip in air while zeroing on the monitor. The transducer is inserted inside the bolt and the screw tightened. The intracranial pressure value can then be read.

Bedside burr hole for intracranial pressure monitoring performed by anaesthetist-intensive care physicians. Extending the practice to the entire ICU team.

BACKGROUND: To evaluate the effects of extending the practice of bedside burr hole for intracranial pressure (ICP) monitoring to the entire ICU team. DESIGN: a 10-year observational cohort study. SETTING: A general-neurologic 12-bed ICU of a University Hospital with 10 full-time specialists in anaesthesia-intensive care. PATIENTS: patients admitted for acute neurologic lesions requiring ICP monitoring. MEASUREMENTS: ICP monitoring was performed by means of intraparenchymal fiberoptic cathe-ters. Rates of successfully performed procedures and complications were compared between 2 study periods: January 1990 - August 1997, in which the procedure was performed by 2 experienced physicians (group 1), and September 1997-July 2000 in which the procedure was extended to the entire team (group 2). RESULTS: Two hundred and seventy patients had 293 catheters positioned (group 1=180; 2=113). The procedure was successfully performed in all cases. Occurrence of complications was similar in the 2 groups: minor surgical wound infections (3.3%; 2.6%); meningitis (0.5%; 0); scalp (3.9%; 2.6%) and dural (5.0%; 6.2%) bleeding; intracranial haematoma (1.1%; 1.8%). Two of these latter (1 for each group) required surgical evacuation. Twelve anaesthetist-intensive care physicians were trained, and they were able to perform burr hole for ICP monitoring without help after 2-3 assisted procedures. CONCLUSIONS: Extending the practice of ICP monitoring to the entire ICU team is safe and feasible. To decide whether or not to implement this technique, one should consider the high costs on one side and prompt availability of ICP monitoring on the other. Haemorrhagic and infectious complications are comparable to those of neurosurgical series.

Limits of intermittent jugular bulb oxygen saturation monitoring in the management of severe head trauma patients.

OBJECTIVE: To evaluate, in a prospective, observational study, whether bilateral monitoring of jugular bulb oxyhemoglobin saturation (SjO2), in addition to standard monitoring, results in modification of the management of severe head trauma. METHODS: The patients underwent bilateral jugular bulb cannulation and observation at 8-hour intervals, during which SjO2 was measured and the neurological condition and physiological variables were assessed. The study group was responsible for evaluating whether the physician's decision-making process was influenced by the detection of SjO2 abnormalities. The SjO2 discrepancy in simultaneous bilateral samples was also evaluated to determine whether it interfered with the interpretation of data and with clinical decision-making. The SjO2-related complications were monitored. RESULTS: Thirty patients underwent 319 observations. In 96% of patients, SjO2 was normal or high and had no influence on the diagnostic or therapeutic strategies. Treatment decisions were dictated by changes in clinical status and in intracranial and cerebral perfusion pressure. When these parameters were abnormal, treatment was administered, even if SjO2 was normal (101 observations). Conversely, when SjO2 was the only detected abnormality (34 observations), no treatment was administered. Abnormally low SjO2 values, caused by hypovolemia and hypocapnia, were detected in 3.4% of observations and actually modified the management. The discrepancies in simultaneous bilateral samples were substantial and gave rise to relevant interpretation problems. Fifteen percent of jugular catheters showed evidence of bacterial colonization. CONCLUSION: Intermittent SjO2 monitoring did not substantially influence the management of severe head trauma. Therefore, recommendation for its routine use in all patients seems inadvisable, and indications for this invasive method should no longer be defined on the basis of experts' opinions, but rather on randomized, prospective studies.

Work of breathing-tidal volume relationship: analysis on an in vitro model and clinical implications.

OBJECTIVE: Work of breathing (WoB) is currently employed to assess the afterload on the respiratory muscles and to estimate the energy expenditure for breathing. Since WoB depends on the ventilated tidal volume (TV), WoB*L(-1), the indicized form of WoB has been employed as a measure of WoB which is independent of TV. Actually, the independence of WoB*L(-1) from the ventilated TV has never been demonstrated. The aim of this study was to verify the predicted TV-independence of WoB*L(-1) on an in vitro model. METHODS: Our experimental model was constituted as follows: two endotracheal tubes, with internal diameter measuring respectively 6.5 and 8.5 mm, were alternatively connected with two rubber balloons whose compliance was respectively 0.02 and 0.06 L/hPa; the system was mechanically ventilated at ten different tidal volumes, ranging from 0.3-1 l. Flow rate was kept constant (35 l/m) during the whole experiment. RESULTS: Both elastic components of the model showed a static volume-pressure relationship which was linear in the experimental range of TV. In all combinations of resistance and compliance WoB increased quadratically whereas WoB*L(-1) increased linearly with the growing TV (p < 0.001). CONCLUSIONS: These results demonstrate the TV-dependence of WoB*L(-1) and suggest that WoB*L(-1), if TV changes, cannot be considered as an index of respiratory muscle afterload and should not be used as a guide for weaning patients from the mechanical ventilation. Finally, we introduced a new parameter (WoB1L) which seems to be a more TV-independent measure of respiratory work.

Acute quadriplegia with delayed onset and rapid recovery. Case report.

The authors describe a patient with severe head injury and sepsis who became acutely quadriplegic 3 days postinjury because of a critical illness polyneuropathy (CIP) and critical illness myopathy (CIM), which resolved rapidly after treatment of the underlying infection. In only 3 days the patient developed septic shock together with flaccid quadriplegia and absent deep tendon reflexes with no clinical or radiological evidence of central nervous system deterioration. Neurophysiological studies showed an acute axonal sensorimotor polyneuropathy, whereas the clinical course strongly suggested a concurrent myopathy. A severe Staphylococcus epidermidis infection accompanied by bacteremia was treated and the patient recovered fully within a few days. Although the case described here is unique because of its very early onset and rapid resolution, CIP and CIM are frequent complications of sepsis and multiple organ failure. The authors suggest that severely head injured patients with sepsis should be evaluated for CIP and CIM when presenting with unexplained muscle weakness or paralysis.