Plateau waves: changes of cerebrovascular pressure transmission

OBJECTIVE

To test the validity of the hypothesis that active vasodilatation and vasoconstriction underlie the occurrence of intracranial pressure (ICP) plateau waves by evaluating corresponding changes of cerebrovascular pressure transmission of arterial blood pressure (ABP) to ICP.

METHODS

Digitized recordings of ICP and ABP sampled at 30 Hz were obtained from nine patients with traumatic brain injury. For each 16.5 s recording interval mean values of ICP, ABP, cerebral perfusion pressure (CPP), and the corresponding highest modal frequency (HMF) of cerebrovascular pressure transmission were calculated.

RESULTS

Mean ICP and HMF significantly increased (P < 0.003) and mean CPP decreased significantly (P < 0.00036) at onset of the wave. Conversely at termination, mean ICP and HMF significantly decreased (P < 0.026) and mean CPP significantly increased (P < 0.028). In addition, the strong negative correlations between mean ICP and mean CPP (r = -0.87) and mean HMF and CPP (r = -0.87) were demonstrated.

CONCLUSION

The findings that HMF increased at onset and decreased at the termination of plateau wave support the validity of the vasodilatatory/constriction cascade model that postulates active vasodilation at the onset and active vasoconstriction of the cerebrovascular bed at the termination of a plateau wave.

Clinical testing of CSF circulation in hydrocephalus

INTRODUCTION

Recent 'NPH Dutch trial' has re-emphasised the importance of the resistance to cerebrospinal fluid (CSF) outflow (Rcsf) in the diagnosis of hydrocephalus. We re-evaluated the clinical utility of the physiological measurements revealing CSF dynamics. The results were summarized from our previous publications. The Computerised Infusion Test was designed to perform quick and low-invasive assessment of CSF dynamics described by parameters as Rcsf, brain compliance, elasticity coefficient, estimated sagittal sinus pressure, CSF formation rate and other variables. Overnight ICP monitoring with quantitative analysis of CSF dynamics was used in those cases where infusion study was unreliable or producing results close to the borderline. We found that the threshold of normal and increased Rcsf should be age-matched because in patients older than 55 Rcsf increases 0.2 mm Hg/(ml/min) per year (p < 0.04: N = 56). Rcsf was positively correlated with cerebral autoregulation (R = 0.41; p < 0.03; N = 36) indicating that in patients with symptoms of NPH but normal Rcsf underlying cerebrovascular disease is more frequent. Computerized infusion tests and overnight ICP monitoring are useful diagnostic technique alone or in conjunction with other forms of physiological measurement.

Effects of moderate hyperventilation on cerebrovascular pressure-reactivity after head injury

In volunteers, hyperventilation improves autoregulation. However, in head-injured patients, hyperventilation-induced deterioration and improvement of autoregulation have been reported. We have re-examined this question using an index of pressure reactivity. Thirty patients with severe or moderate head-injury were studied. Arterial blood pressure, cerebral perfusion pressure (CPP), and intracranial pressure (ICP) were recorded over 20 minute epochs separated by ten minutes of equilibration at baseline and during moderate (>3.5 kPa) hyperventilation. End-tidal CO2 was constant during each phase of data acquisition. Pressure reactivity was assessed using an index 'PRx' based on the response of ICP to spontaneous blood pressure changes. Hyperventilation decreased PaCO2 from 5.1 +/- 0.4 to 4.4 +/- 0.4 kPa (p < 0.0001). ICP decreased by 3.7 +/- 2.2 mmHg (p < 0.001). CPP increased by 5.9 +/- 8.2 mmHg (p < 0.001). Overall, PRx did not change significantly with hyperventilation. However, there was a significant negative correlation between baseline PRx and the change in PRx (r = -0.71, p < 0.0001). This suggests that patients with disturbed pressure-reactivity may improve, whereas patients with intact pressure reactivity remain largely unchanged. Our data suggest that the response of pressure reactivity to hyperventilation is heterogeneous. This could be due to hyperventilation-induced changes in cerebral metabolism, or the change in CPP.

Evaluation of three new models of hydrocephalus shunts

OBJECTIVE

To assess the hydrodynamic properties of three new types of hydrocephalus valve.

METHODS

Three new constructions have been recently tested in the UK shunt Evaluation Laboratory: the magnetically adjustable Strata Valve (Medtronic PS Medical), the gravitational Miethke Dual-Switch Valve (Aesculap) and the ventriculo-sinus SinuShunt (CSF Dynamics). Pressure-flow performance curves were assessed in a minimum of three samples of each valve to study their longterm variability, influence of temperature, negative outlet pressure, external pressure, presence of pressure pulsations, etc.

RESULTS

The operating pressure of the Strata Valve can be adjusted magnetically in five steps. This Shunt prevents 'siphoning' but is sensitive to external pressure. The Dual Switch Miethke Valve is a system of two fixed-pressure ball-on-spring valves with a lower opening pressure operating in a horizontal body position and higher when vertical. This function is designed to cancel the effect of siphoning related to body posture. Both Strata and DSV valves have a low hydrodynamic resistance (less than 3 mm Hg/ml/min), and hence they cannot prevent overdrainage related to nocturnal vasomotor waves. The SinuShunt has a higher resistance (9 mm Hg/(ml/min)) and a lower opening pressure. The valve is intended to drain CSF from ventricles to the transverse sinus.

CONCLUSION

New shunt technology continues to evolve. Laboratory evaluation independent of the manufacturer forms an important link between R&D laboratories and clinical practice.

Fuzzy pattern classification of hemodynamic data can be used to determine noninvasive intracranial pressure

OBJECTIVE

The authors previously introduced a method in which intracranial pressure (ICP) was estimated using parameters (TCD characteristics) derived from cerebral blood flow velocity (FV) and arterial blood pressure (ABP). Some results suggested that this model might be influenced by the patient's state of cerebral autoregulation and other clinical parameters. Hence, it was the aim of the present study to improve the method by modifying the previously used global procedure in certain subgroups of patients.

METHODS

In 103 traumatic brain injured patients (3-76 years, mean: 31 +/- 16 years) signal data of FV, ABP and ICP were used to generate samples of TCD characteristics together with time corresponding ICP. Fuzzy Pattern Classification was used to identify cluster subsets (classes) of the sample space. On each class a local estimator of ICP was defined. This approach provides a non-invasive assessment of ICP (nICP) as follows: Using FV and ABP the TCD characteristics were computed and related to the matching classes. nICP was calculated as a weighted sum of local ICP estimations.

RESULTS

ICP A and B waves and long-term trends could be visibly assessed. The median absolute difference between ICP and nICP was 5.7 mmHg.

CONCLUSIONS

The class structure of the model facilitates nICP assessment in heterogeneous patient groups and supports a stepwise extension of the target patient group without affecting the former validity.

Monitoring and interpretation of intracranial pressure

Intracranial pressure (ICP) is derived from cerebral blood and cerebrospinal fluid (CSF) circulatory dynamics and can be affected in the course of many diseases of the central nervous system. Monitoring of ICP requires an invasive transducer, although some attempts have been made to measure it non-invasively. Because of its dynamic nature, instant CSF pressure measurement using the height of a fluid column via lumbar puncture may be misleading. An averaging over 30 minutes should be the minimum, with a period of overnight monitoring in conscious patients providing the optimal standard. Computer-aided recording with online waveform analysis of ICP is very helpful. Although there is no "Class I" evidence, ICP monitoring is useful, if not essential, in head injury, poor grade subarachnoid haemorrhage, stroke, intracerebral haematoma, meningitis, acute liver failure, hydrocephalus, benign intracranial hypertension, craniosynostosis etc. Information which can be derived from ICP and its waveforms includes cerebral perfusion pressure (CPP), regulation of cerebral blood flow and volume, CSF absorption capacity, brain compensatory reserve, and content of vasogenic events. Some of these parameters allow prediction of prognosis of survival following head injury and optimisation of "CPP-guided therapy". In hydrocephalus CSF dynamic tests aid diagnosis and subsequent monitoring of shunt function.

Effect of Carotid Endarterectomy or Stenting on Impairment of Dynamic Cerebral Autoregulation

Background and Purpose— Analysis of dynamic cerebral autoregulation (DCA) from spontaneous blood pressure fluctuations might contribute to prognosis of severe internal carotid artery stenosis, but its response to carotid recanalization has not been investigated so far. This study investigates the effect of carotid endarterectomy or stenting on various DCA parameters.

Methods— In 58 patients with severe unilateral stenosis undergoing carotid endarterectomy (n=41) or stenting (n=17), cerebral blood flow velocity (CBFV, transcranial Doppler) and arterial blood pressure (ABP, Finapres method) were recorded over 10 minutes before and on average 3 days after carotid recanalization. Nineteen patients were additionally examined after 7 months. Correlations between diastolic and mean ABP and CBFV fluctuations were averaged to form the correlation coefficient indices (diastolic [Dx] and mean values [Mx]). Transfer function parameters (low-frequency phase and high-frequency gain between ABP and CBFV oscillations) were calculated over the same 10 minutes. CO 2 reactivity was assessed via inhalation of 7% CO 2 .

Results— Before recanalization, all DCA parameters were clearly impaired ipsilaterally compared with contralateral sides. Phase, Dx, and Mx indicated early normalization of DCA after both endarterectomy and stenting. By multiple regression, the degree of DCA improvement was highly significantly related to the extent of impairment before recanalization. No significant change in DCA was found at follow-up. Ipsilateral gain and CO 2 reactivity increased significantly less after endarterectomy than after stenting ( P <0.05).

Conclusions— Dynamic cerebral dysautoregulation in patients with severe carotid obstruction is readily and completely remedied by carotid recanalization.

Intracranial hypertension: what additional information can be derived from ICP waveform after head injury?

OBJECTIVE

Although intracranial hypertension is one of the important prognostic factors after head injury, increased intracranial pressure (ICP) may also be observed in patients with favourable outcome. We have studied whether the value of ICP monitoring can be augmented by indices describing cerebrovascular pressure-reactivity and pressure-volume compensatory reserve derived from ICP and arterial blood pressure (ABP) waveforms.

METHOD

96 patients with intracranial hypertension were studied retrospectively: 57 with fatal outcome and 39 with favourable outcome. ABP and ICP waveforms were recorded. Indices of cerebrovascular reactivity (PRx) and cerebrospinal compensatory reserve (RAP) were calculated as moving correlation coefficients between slow waves of ABP and ICP, and between slow waves of ICP pulse amplitude and mean ICP, respectively. The magnitude of 'slow waves' was derived using ICP low-pass spectral filtration.

RESULTS

The most significant difference was found in the magnitude of slow waves that was persistently higher in patients with a favourable outcome (p<0.00004). In patients who died ICP was significantly higher (p<0.0001) and cerebrovascular pressure-reactivity (described by PRx) was compromised (p<0.024). In the same patients, pressure-volume compensatory reserve showed a gradual deterioration over time with a sudden drop of RAP when ICP started to rise, suggesting an overlapping disruption of the vasomotor response.

CONCLUSION

Indices derived from ICP waveform analysis can be helpful for the interpretation of progressive intracranial hypertension in patients after brain trauma.

Predictive value of Glasgow Coma Scale after brain trauma: change in trend over the past ten years

BACKGROUND

Age and the Glasgow Coma Scale (GCS) score on admission are considered important predictors of outcome after traumatic brain injury. We investigated the predictive value of the GCS in a large group of patients whose computerised multimodal bedside monitoring data had been collected over the previous 10 years.

METHODS

Data from 358 subjects with head injury, collected between 1992 and 2001, were analysed retrospectively. Patients were grouped according to year of admission. Glasgow Outcome Scores (GOS) were determined at six months. Spearman's correlation coefficients between GCS and GOS scores were calculated for each year.

RESULTS

On average 34 (SD: 7) patients were monitored every year. We found a significant correlation between the GCS and GOS for the first five years (overall 1992-1996: r = 0.41; p<0.00001; n = 183) and consistent lack of correlations from 1997 onwards (overall 1997-2001: r = 0.091; p = 0.226; n = 175). In contrast, correlations between age and GOS were in both time periods significant and similar (r = -0.24 v r = -0.24; p<0.002).

CONCLUSIONS

The admission GCS lost its predictive value for outcome in this group of patients from 1997 onwards. The predictive value of the GCS should be carefully reconsidered when building prognostic models incorporating multimodality monitoring after head injury.

Cerebral autoregulation in carotid artery occlusive disease assessed from spontaneous blood pressure fluctuations by the correlation coefficient index

BACKGROUND AND PURPOSE

Estimation of dynamic cerebral autoregulation from spontaneous fluctuations of arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV) is an attractive monitoring option for cerebral hemodynamic impairment. We evaluated the correlation coefficient index method in patients with severe obstructive carotid disease and compared it with transfer function analysis (frequency domain approach to cerebral autoregulation) and CO2 vasomotor reactivity.

METHODS

In 139 patients with severe unilateral carotid stenosis (>or=70%) or occlusion, CBFV (transcranial Doppler) and ABP (Finapres method) were recorded over 10 minutes. Correlations between systolic pressure, diastolic pressure, and mean ABP and CBFV oscillations over 1-minute epochs were averaged over 10 minutes to form the correlation coefficient indexes (Sx, Dx, Mx, respectively). Transfer function parameters (phase shift and gain between ABP and CBFV oscillations) were determined from the entire 10-minute period. CO2 reactivity was assessed by inhalation of 7% CO2.

RESULTS

The correlation indexes Dx and Mx were significantly higher ipsilateral to stenosis and increased with degree of stenosis, indicating increasing dependence of CBFV on ABP and thus impairment of cerebral autoregulation. Dx and Mx correlated moderately but highly significantly with transfer function parameters and CO2 reactivity and showed a good level of agreement in detecting pathological values. Patients with a small variance of the 1-minute source correlations of Dx and Mx showed clearly better correlation values. Transfer function parameters and CO2 reactivity but not Dx and Mx were significantly poorer in patients with symptomatic stenosis or occlusion.

CONCLUSIONS

The potential of the correlation coefficient indexes Dx and Mx in detecting hemodynamic impairment in patients with carotid stenosis is comparable to that of transfer function analysis and CO2 reactivity testing. In future, a combination of various hemodynamic tests might help to identify patients at risk for ischemic events.

Pressure-autoregulation, CO2 reactivity and asymmetry of haemodynamic parameters in patients with carotid artery stenotic disease. A clinical appraisal

OBJECTIVES

Patients with carotid artery stenotic disease and poor cerebral haemodynamic reserve are in increased risk of stroke. Haemodynamic reserve can be estimated by measuring cerebrovascular reactivity induced by breathing CO2 and pressure-autoregulation by analyzing spontaneous slow fluctuation in arterial pressure and MCA blood flow velocity. We evaluated the relationship between clinical status, CO2 reactivity, pressure-autoregulation and other haemodynamic variables derived from Transcranial Doppler ultrasonography in patients with carotid artery disease.

METHODS

38 patients were investigated. Arterial pressure and blood flow velocity were monitored during CO2 reactivity tests. Arterial pressure-corrected and non-corrected indices of CO2 reactivity were calculated to compare an influence of rise in arterial pressure during the test. The pressure-autoregulation index was calculated as correlation coefficient between slow waves in blood flow velocity and arterial pressure.

RESULTS

The increase in CO2 produced a consistent rise in arterial pressure and blood flow velocity and weakened the pressure autoregulation. The value of pressure-corrected CO2 reactivity was lower (p<0.0001) than the non-corrected one, indicating that the rise in arterial pressure during the test tends to over-estimate CO2 reactivity. The pressure-corrected reactivity was correlated with pressure autoregulation (r=-0.46; p<0.005). Both CO2 reactivity and pressure-autoregulation index correlated with a degree of carotid artery stenosis. Side-to-side difference of TCD pulsatility index demonstrated a close relationship with the asymmetry of stenosis (r=-0.61; p<0.0002) and symptoms (r=-0.49; p<0.003).

CONCLUSIONS

When calculating CO2 reactivity in patients with carotid artery disease, changes in arterial pressure should be considered. Both CO2 reactivity and pressure-autoregulation describe the magnitude of haemodynamic deficit caused by stenosis, pulsatility index expresses the asymmetry of stenosis.

Cerebrovascular pressure reactivity is related to global cerebral oxygen metabolism after head injury

BACKGROUND

After head injury, impaired cerebrovascular autoregulation has been associated with abnormally high or low cerebral blood flow. The physiological relevance of cerebral blood flow levels is difficult to assess in these patients, whose cerebral metabolic rate for oxygen (CMRO(2)) is known to be abnormal. Investigation of these relations requires quantitative measures of cerebral blood flow and CMRO(2), to allow assessment of oxygen supply and demand relations.

OBJECTIVES

To investigate the relation between dysautoregulation and global cerebral oxygen metabolism following head injury.

METHODS

Using positron emission tomography, global cerebral blood flow, CMRO(2), and oxygen extraction fraction were determined in 22 patients who were investigated in 26 examinations on days 1 to 11 (mean (SD), 3.5 (2.3)) after head injury. Cerebrovascular pressure reactivity was assessed using a pressure reactivity index, calculated as the moving linear correlation coefficient between mean arterial blood pressure and intracranial pressure. Outcome was assessed six months after injury using the Glasgow outcome scale.

RESULTS

Low CMRO(2) was associated with disturbed pressure reactivity (inverse function, R(2) = 0.21, p = 0.018) and there was a correlation between disturbed pressure reactivity and oxygen extraction fraction (quadratic function, R(2) = 0.55, p = 0.0001). There was no significant relation between pressure reactivity and cerebral blood flow. An unfavourable outcome was associated with disturbed pressure reactivity. There was no significant relation between outcome and CMRO(2) or oxygen extraction fraction.

CONCLUSIONS

There is a close relation between dysautoregulation and abnormal cerebral metabolism but not blood flow. Further studies are needed to determine whether metabolic dysfunction is a result of or a cause of disturbed pressure reactivity, and to establish if there is a relation between cerebral oxygen metabolism and outcome.

Validation of a tonometric noninvasive arterial blood pressure monitor in the intensive care setting

Intra-arterial measurement is considered the gold standard for continuous, beat-to-beat arterial blood pressure monitoring. However, arterial cannulation can be difficult and may cause complications such as thrombosis and ischaemia. Recently, a tonometric system, the Colin CBM-7000 has been developed for noninvasive beat-to-beat measurement of arterial blood pressure from the radial artery. We assessed the level of agreement between the CBM-7000 and invasive radial artery measurements in 15 patients on a neuro-intensive care unit. Agreement of systolic, diastolic and mean arterial pressure values was limited, with approximately 34% of mean arterial pressures differing by over 10 mmHg. In many cases, this was due to a downward drift of the noninvasive measurements over time. Furthermore, there was a tendency to underestimate low pressures and overestimate high pressures. In our opinion, the Colin CBM-7000 cannot be recommended for continuous blood pressure monitoring in the intensive care setting.

Communicating hydrocephalus: the biomechanics of progressive ventricular enlargement revisited

BACKGROUND

This article investigates the physical mechanisms involved in the chronic ventricular enlargement that accompanies communicating hydrocephalus (CH)--including its normal and low-pressure forms. In particular, it proposes that this phenomenon can be explained by the combined effect of: (a) a reversal of interstitial fluid flow in the parenchyma, and (b) a reduction in the elastic modulus of the cerebral mantle.

METHOD

To investigate this hypothesis, these changes have been incorporated into a finite element computer simulation of CH, in which brain tissue is idealized as a sponge-like material. The fluid pressure in the lateral ventricles and the subarachnoid space has been set to 10 mmHg, while the fluid pressure inside the parenchyma has been set to 7.5 mmHg. The elastic moduli of white and gray matter have been set to the reduced values of 1 and 5 kPa, respectively.

FINDINGS

The simulation revealed a substantial ventricular distension (6.5 mm mean outward displacement), which was accompanied by the appearance of stress concentrations in the cerebral mantle.

INTERPRETATION

These results support the notion that a relative reduction in intraparenchymal fluid pressure coupled with low tissue elasticity can produce both a significant ventricular enlargement and periventricular solid stress concentrations.

Effects of variation in cerebral haemodynamics during aneurysm surgery on brain tissue oxygen and metabolism

OBJECTIVES

This study explores the sensitivities of multiparameter tissue gas sensors and microdialysis to variations in blood pressure, CSF drainage and to well-defined periods of ischaemia accompanying aneurysm surgery, and their predictive value for infarction.

METHODS

A Neurotrend sensor [brain tissue partial pressure of oxygen (PBO2), carbon dioxide (PBCO2), brain pH (pHB) and temperature] and microdialysis catheter were inserted into the appropriate vascular territory prior to craniotomy.

RESULTS

Baseline data showed a clear correlation between PBO2 and mean arterial pressure (MAP) below a threshold of 80 mmHg. PBO2 improved with CSF drainage in 20 out of 28 (Wilcoxon: P < 0.05) cases where data was available. In 26 patients the effects of temporary vascular clipping (TC) (mean duration 16 minutes) were assessed. 2 patients subsequently declared infarction in the region of the probes. PBO2 fell from a mean 3.2 (95% CI 2.4-4.1) kPa to a minimum of 1.5 (95% CI 1.0-2.0) kPa in the non-infarct group. There was a lower baseline PBO2 (mean 0.8 kPa) in the patients who infarcted. PBCO2 mirrored PBO2 changes, whereas pHB did not change significantly in either group. Microdialysis changes associated with decreased PBO2 included a delayed increase in lactate, a raised lactate/pyruvate ratio and more rarely an increased glutamate. These changes were seen in 11 patients but were not predictive of infarction.

CONCLUSION

Hypotension during aneurysm surgery is associated with a low PBO2. Multiparameter sensors can be sensitive to acute ischaemia. Microdialysis shows potential in the detection of metabolic changes during tissue hypoxia.

Shunt testing in-vivo: a method based on the data from the UK shunt evaluation laboratory

OBJECTIVES

The objective of the UK Shunt Evaluation Laboratory was to perform an independent testing of hydrodynamic performance of hydrocephalus shunts and provide systematic reviewing for neurosurgeons and patients.

METHODS

Valves were tested long-term in a computer-controlled rig to evaluate their pressure-flow performance both at baseline and under conditions mimicking phenomena, which may alter CSF drainage in vivo. The operating pressures (Poperating) and hydrodynamic resistances (R) of all types of valves, currently in use in the UK, have been evaluated (Codman: Hakim-Precision, Hakim-Programmable, Uni-Shunt, Accu-flo, Holter. Medtronic PS Medical: Delta, Flow Control, Lumbo-peritoneal. NMT: Orbis-Sigma, Omni-shunt, Hakim Valve. Heyer-Schulte: In-line, Pudenz-Flushing, LowProfile. Radionics ContourFlex. Sophy Programmable). 67 patients, who had improved in the past following shunting but had recent recurrence of their clinical symptoms, were admitted to undergo a computerized infusion test through the shunt pre-chamber or Ommaya reservoir implanted prior to shunting. The criterion used to detect shunt underdrainage was an increase in ICP during constant infusion above (Poperating) + R * Infusion rate + 5 mm Hg. The validity of this formula has been confirmed in a laboratory study.

RESULTS

35 patients met the criteria for shunt underdrainage while in 24 normal drainage was demonstrated. Mean ICP achieved during the test was 24 mm Hg in patients with underdrainage versus 14 mm Hg with normally functioning shunts (p < 0.01). Out of 35 patients 25 improved and 10 were seen again to have the test repeated. In 7 patients shunt was blocked again. Only 3 patients with primarily confirmed shunt underdrainage did not improve following revisions. In 8 patients overdrainage related to body posture was confirmed using tilt-test.

CONCLUSIONS

Shunt testing in-vivo is easy, clinically useful and has good prediction power (90%).

Asymmetry of cerebral autoregulation following head injury

OBJECTIVES

To investigate asymmetry of cerebra autoregulation in head-injured patients with lateral brain contusions.

METHODS

Sixty five patients were admitted to Addenbrooke's Hospital suffering from head injuries with mean Glasgow Coma Score 6 (range 3 to 10). The patients were paralyzed, sedated and ventilated to achieve mild hypocapnia. Intracranial pressure (ICP), arterial pressure (ABP) were monitored directly. The left and right Middle Cerebral Arteries were insonated daily and flow velocity (FV) was recorded. Correlation coefficients between slow waves in cerebral perfusion pressure (CPP) and FV were calculated for every 3 minute period. Positive value of Mx denotes a positive association between waves in FV and CPP, therefore deranged autoregulation. Zero or slightly negative value of Mx denotes a good autoregulatory capacity. In each patient all CT scans were reviewed to assess a dominant side of brain contusion and a level of brain compression.

RESULTS

The side-to-side difference in FV, pulsatility indices or critical closing pressures, did not correlate with the side of contusion or midline shift. In contrary, the side-to-side difference in Mx indices were significantly (p < 0.05) worse at a side of contusion and at the side of brain expansion in patients presenting with a midline shift (p < 0.05). Of those patients who died in hospital, significantly more presented within meaningful (ABS(Mx) > 0.2) asymmetry in cerebral autoregulation (40% versus 12%; p < 0.05).

CONCLUSIONS

Side-to-side difference in cerebral hemodynamic reserve of injured brain is a predictor of fatal outcome following head injury and correlates with the side of contusion or brain expansion.

Clinical significance of cerebral autoregulation

OBJECTIVES

Disturbed cerebral autoregulation is believed to be associated with an unfavourable outcome following head injury. Previously, using ICP monitoring and transcranial Doppler ultrasonography, we investigated whether cerebral response to spontaneous variations in arterial pressure (ABP) or cerebral perfusion pressure (CPP) provide reliable information on cerebral autoregulatory reserve. In the present study we have correlated these methods with clinical findings.

METHODS

188 head injured sedated and ventilated patients were studied daily. Waveforms of intracranial pressure (ICP), arterial pressure and transcranial Doppler flow velocity (FV) were captured over a half to two hour periods. Time averaged mean flow velocity (FV) and CPP were resolved. The correlation coefficient indices between FV and CPP (Mx) and between ICP and ABP (PRx) were calculated over 3 minutes epochs, and averaged for each investigation.

RESULTS

The relationship between indices of autoregulation and outcome (favourable-unfavourable) was significant and stronger than the association between admission GCS and outcome. With rigorously maintained CPP-oriented therapy relationship between CPP and outcome became non-significant. Mortality in patients with consistently disturbed autoregulation ranged 47%, while in patients with good autoregulation mortality was 11% (difference: p < 0.0001).

CONCLUSIONS

Positive values of indices of autoregulation, expressing positive association between slow waves of CPP and blood flow velocity or ABP and ICP, indicate disturbed autoregulation. These indices correlate with unfavourable outcome following head injury and should be used to guide intensive therapy.

Factors determining mean ICP in hydrocephalic patients with Hakim-programmable valve: implications of the parallel arrangement of the CSF outflow resistance and shunt

OBJECTIVE

Measurement of CSF pressure is used clinically to test shunt function in vivo in hydrocephalic patients. Criteria for appropriate shunt function have never been validated.

METHOD

Hakim-Programmable valve was tested in a model of CSF circulation with variable resistance to CSF outflow (from 12 to 50 mm Hg/ml/min), increased hydrodynamic compliance (> 1.5 ml/mm Hg), and constant perfusion of a rate of 0.4 ml/min, i.e. conditions typical for hydrocephalus. The main question was how the simulated CSF pressure was influenced by the shunt setting and the residual resistance to CSF outflow.

RESULTS

Measured baseline CSF pressure correlated well with shunt operating pressure only when high resistance to CSF outflow (50 mm Hg/(ml/min)) was used. For the medium resistance (20 mm Hg/(ml/min)) operating pressure was strongly affected by system's absorption capacity. For low resistance (12 mm Hg/(ml/min)) operating pressure through the valve was independent on valve's settings and no fluid drainage through the valve was recorded.

CONCLUSION

Patients with moderately elevated resistance to CSF outflow (12-18 mm Hg/(ml/min)) cannot possibly react to changes of the valve's settings above 100 mm H2O. Mean CSF pressure results both from shunt setting and patient's own re-absorption capacity.

Hysteresis of the cerebrospinal pressure-volume curve in hydrocephalus

The objective was to study the displacement of the cerebrospinal fluid pressure-volume curve during the descent relative to the ascent of intracranial pressure recorded during the cerebrospinal fluid constant rate infusion test. This phenomenon can be interpreted as the hysteresis of the pressure-volume curve. The cerebrospinal fluid dynamics were tested in fifty-eight patients with clinical symptoms of hydrocephalus. After finished infusion, ICP was recorded until it returned to steady state level. Pressure-volume curves were plotted separately for ascending and descending phases of the test. The parameters of CSF compensation were estimated on the basis of mathematical mono-exponential model of CSF circulation. The pressure-volume curve post-infusion was visibly shifted upward in 69% of tests. Those who demonstrated the upward shift of the pressure-volume curve had greater an elastance coefficient of the cerebrospinal space (with shift: E1 = 0.26 +/- 0.14; without shift: E1 = 0.17 +/- 0.06; p < 0.05). Magnitude of the shift was positively correlated with pulse amplitude of ICP (r = -0.763; p < 0.0001). The accuracy of clinical examination of the pressure-volume compensatory reserve, which take into account both compression and decompression phase of the study, may be affected by this phenomenon.