Service d'Anesthésie-Réanimation et Unité Propre de Recherche de l'Enseignement Supérieur-Equipe d'Accueil (UPRES-EA 3540), Hôpital de Bicêtre, 94275 Le Kremlin Bicêtre, France
* Corresponding author. E-mail: alain.edouard{at}bct.ap-hop-paris.fr
Accepted for publication October 10, 2004.
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Abstract |
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Methods. Twenty adults with bilateral and diffuse brain injuries were included in the study. CPPe was estimated using a formula combining the phasic values of flow velocities and arterial pressure. In group A (n=10) the comparison was repeatedly performed under stable conditions. In group B (n=10) the comparison was performed during a CO2 reactivity test. Covariance analysis was used to assess the relationships.
Results. In group A, CPPe and CPPm were correlated (slope, 0.76; intercept, +10.9; 95% CI, 3.5 to +25.4). During the increase in intracranial pressure (group B) (+1.9 (SD 1.5) mm Hg per mm Hg of ) the relationship persisted (slope, 0.55; intercept, +32.6; 95% CI, +16.3 to +48.9) but the discrepancy between the two variables increased as reflected by the increase in bias and variability.
Conclusion. Non-invasive estimation of CPP can be used for brain monitoring of head-injured patients, but the accuracy of the method may depend on the level of intracranial hypertension.
Keywords: brain, cerebral perfusion pressure ; brain, intracranial pressure ; complications, traumatic brain injury ; monitoring, jugular venous oxygen saturation ; monitoring, transcranial Doppler ultrasonography
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Introduction |
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Thus the aim of the present prospective study was to assess the adequacy of the latter formula for the care of brain-injured patients, monitored by an intracerebral sensor, in both a stable state and during a rapid change in cerebrovascular tone following a deliberate change in arterial blood carbon dioxide pressure () as previously reported in healthy volunteers, pregnant women and brain-injured patients.7 14 15
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Methods |
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Clinical and haemodynamic data
The Injury Severity Score (ISS)18 and the Simplified Acute Physiological Score (SAPS II)19 were calculated to assess the magnitude of the trauma and its general consequences. An Abbreviated Injury Score for the cephalic region (AIShead) 3 corresponded to a significant brain injury.
The AP was measured through a radial catheter (Summit®, Baxter, 78310 Maurepas, France), and the ICP was measured through an intraparenchymal catheter (Neuromonitor-MicroSensor kit®, Codman, 92290 Chatenay-Malabry, France) inserted in the right cerebral hemisphere. The measured CPP (CPPm), calculated as the arithmetic difference between mean AP and mean ICP, was continuously displayed on a monitor (Merlin 1006A®, Hewlett Packard, 91400 Les Ulis, France). A 2 MHz pulsed transcranial Doppler probe (HP Sonos 5500®, Hewlett Packard, 91400 Les Ulis, France) was used to examine the M1 portion (first 2 cm after branching from the circle of Willis) of the MCA. On average, the Doppler examination was completed within 10 min. The systolic, diastolic and mean MCA velocities were recorded. Measurements were performed bilaterally and a minimum of five waveforms were averaged for each measurement. The estimated CPP (CPPe) was calculated using the following formula:
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Study procedure
In the first group of 10 patients (group A), the values of CPPe and CPPm were compared during the clinical course. Eight to ten pairs of values were obtained from each patient. Haemodynamic stability was defined by limited changes (±20%) in ICP, an unchanged dosage of norepinephrine infusion and no need for additional volume loading or mannitol therapy during the preceding 24 h period.
Following this first part of the study, the values of CPPe and CPPm were recorded in a second group of 10 patients (group B) before and after the routine assessment of cerebral carbon dioxide vascular reactivity.21 A deliberate increase in was induced by a 20% reduction in tidal volume while maintaining the ventilatory frequency. The test was completed when
reached a plateau or was interrupted if CPPm decreased <40 mm Hg through an increase in ICP. Arterial and jugular venous (if available) blood samples were obtained before and after the test for arterial and venous (
) blood gas analysis.
Data analysis
Results are expressed as mean (SD). A covariance analysis was used to assess the linear regression between CPPe and CPPm from each side of the head in group A patients. CPPm was considered as the independent variable. The analysis was performed using the routine lme from S-Plus.22 Two fixed effects (the slope and the intercept) were considered. A random effect with mean zero and variance SE2 was added to each of the two fixed effects to model inter-individual variability, and the significance of this random effect in the model was tested. The goodness of fit was assessed using the log-restricted likelihood. Fitted values of the two fixed-effect parameters are reported with a 95% CI calculated as ±2SE, where SE is the random effect error. The bias in prediction was calculated as the mean error
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The clinical and biological variables reported before and after the assessment of vascular reactivity in group B were compared using Student's t-test for paired data after a check for normality by a ShapiroWilk test. The same analytical method as in group A was used in group B to compare CPPe and CPPm before and after CO2 vascular reactivity assessment, despite the fact that only two values were available for each patient. The bias (ME) and precision (RMSE) of the prediction were also calculated before and after the vascular reactivity test for both sides. An analysis of residuals was performed and shown graphically.
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Results |
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Discussion |
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The transcranial Doppler probe, which is a major component of multimodal neurological monitoring, allows non-invasive assessment of cerebral perfusion.1 6 The determination of effective downstream pressure using the zero-flow pressure derived from the cerebral pressureflow velocity relationships,79 or the critical closing pressure derived from Fourier analysis of the first harmonics of velocity and pressure waveforms,10 11 requires a rather complex computerized method. A formula originally proposed by Aaslid in 1986 (see Belfort et al.13) was modified to use areas under pulsatile amplitudes rather than under the first harmonics. The mean velocity was defined as the area under the entire velocity waveform curve and the pulsatile component, directly related to the CPP, was the difference between the mean and diastolic areas of the velocity waveform. The equation was further simplified to allow the simple use of velocity measurements without requiring a fast Fourier transformation.33 This method was applied in pregnant women by estimating the ICP through a puncture in the epidural space13 and was used for physiological and pharmacological studies.14 15 The present results obtained in brain-injured patients using intraparenchymal measurement of ICP demonstrated a higher bias (7.1 versus 0.28 mm Hg) compared with a zero-flow pressure study.8 The increased difference between CPPe and CPPm, i.e. between the critical closing pressure and ICP in the absence of changes in AP, was previously demonstrated in patients with moderate intracranial hypertension.79 The difference between APmean and critical closing pressure is frequently considered as the real cerebral perfusion pressure, because it represents the sum of tissue pressure, vasomotor tone and backward venous pressure.8 The apparent discrepancy between CPPe and CPPm disappears in patients with severe intracranial hypertension when ICP exceeds the critical closing pressure at the arteriolar level and represents the outflow pressure of the proximal Starling resistor.7
Within the limits of these preliminary data, especially the small number of selected patients, a bilateral and continuous measurement of MCA velocities automatically linked to a continuous and invasive, or intermittent and non-invasive, measurement of AP may have potential benefit for the assessment of CPP when it is neither practical nor safe to measure ICP directly in the cerebral circulation of trauma patients. This is particularly true during the surgical treatment of extracranial injuries in patients with severe head trauma where adequate brain monitoring has been demonstrated to allow the prevention of the deleterious consequences of intraoperative hypotension.34
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Acknowledgments |
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References |
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