Relationship between intracranial pressure and critical closing pressure in patients with neurotrauma

Critical Closing Pressure and Cerebrovascular Resistance Responses to Intracranial Pressure Variations in Neurocritical Patients

BACKGROUND

Critical closing pressure (CrCP) and resistance-area product (RAP) have been conceived as compasses to optimize cerebral perfusion pressure (CPP) and monitor cerebrovascular resistance, respectively. However, for patients with acute brain injury (ABI), the impact of intracranial pressure (ICP) variability on these variables is poorly understood. The present study evaluates the effects of a controlled ICP variation on CrCP and RAP among patients with ABI.

METHODS

Consecutive neurocritical patients with ICP monitoring were included along with transcranial Doppler and invasive arterial blood pressure monitoring. Internal jugular veins compression was performed for 60 s for the elevation of intracranial blood volume and ICP. Patients were separated in groups according to previous intracranial hypertension severity, with either no skull opening (Sk1), neurosurgical mass lesions evacuation, or decompressive craniectomy (DC) (patients with DC [Sk3]).

RESULTS

Among 98 included patients, the correlation between change (Δ) in ICP and the corresponding ΔCrCP was strong (group Sk1 r = 0.643 [p = 0.0007], group with neurosurgical mass lesions evacuation r = 0.732 [p < 0.0001], and group Sk3 r = 0.580 [p = 0.003], respectively). Patients from group Sk3 presented a significantly higher ΔRAP (p = 0.005); however, for this group, a higher response in mean arterial pressure (change in mean arterial pressure p = 0.034) was observed. Exclusively, group Sk1 disclosed reduction in ICP before internal jugular veins compression withholding.

CONCLUSIONS

This study elucidates that CrCP reliably changes in accordance with ICP, being useful to indicate ideal CPP in neurocritical settings. In the early days after DC, cerebrovascular resistance seems to remain elevated, despite exacerbated arterial blood pressure responses in efforts to maintain CPP stable. Patients with ABI with no need of surgical procedures appear to remain with more effective ICP compensatory mechanisms when compared with those who underwent neurosurgical interventions.

Point/counterpoint: Cerebrovascular resistance is a flawed concept

The relationship between cerebral blood flow and blood pressure is a critical part of investigation of cerebral autoregulation. Conventionally, cerebrovascular resistance (CVR) has been used to describe this relationship, but the underlying principles used for this method is flawed in real-world application for several reasons. Despite this, the use of CVR remains entrenched within current literature. This 'Point/Counterpoint' review provides a summary of the flaws in using CVR and explains the benefits of calculating the more accurate critical closing pressure (CrCP) and resistance-area product (RAP) parameters, with support of real-world data.

Critical closing pressure as a new hemodynamic marker of cerebral small vessel diseases burden

Purpose

To investigate cerebrovascular hemodynamics, including critical closing pressure (CrCP) and pulsatility index (PI), and their independent relationship with cerebral small vessel disease (CSVD) burden in patients with small-vessel occlusion (SVO).

Methods

We recruited consecutive patients with SVO of acute cerebral infarction who underwent brain magnetic resonance imaging (MRI), transcranial Doppler (TCD) and CrCP during admission. Cerebrovascular hemodynamics were assessed using TCD. We used the CSVD score to rate the total MRI burden of CSVD. Multiple regression analysis was used to determine parameters related to CSVD burden or CrCP.

Results

Ninety-seven of 120 patients (mean age, 64.51 ± 9.99 years; 76% male) completed the full evaluations in this study. We observed that CrCP was an independent determinant of CSVD burden in four models [odds ratio, 1.41; 95% confidence interval (CI), 1.17-1.71; P < 0.001] and correlated with CSVD burden [β (95% CI): 0.05 (0.04-0.06); P < 0.001]. In ROC analysis, CrCP was considered as a predictor of CSVD burden, and AUC was 86.2% (95% CI, 78.6-93.9%; P < 0.001). Multiple linear regression analysis showed that CrCP was significantly correlated with age [β (95% CI): 0.27 (0.06 to 0.47); P = 0.012], BMI [β (95% CI): 0.61 (0.00-1.22)] and systolic BP [β (95% CI): 0.16 (0.09-0.23); P < 0.001].

Conclusions

CrCP representing cerebrovascular tension is an independent determinant and predictor of CSVD burden. It was significantly correlated with age, BMI and systolic blood pressure. These results provide new insights in the mechanism of CSVD development.

Comparison of optical measurements of critical closing pressure acquired before and during induced ventricular arrhythmia in adults

Significance: The critical closing pressure (CrCP) of cerebral circulation, as measured by diffuse correlation spectroscopy (DCS), is a promising biomarker of intracranial hypertension. However, CrCP techniques using DCS have not been assessed in gold standard experiments. Aim: CrCP is typically calculated by examining the variation of cerebral blood flow (CBF) during the cardiac cycle (with normal sinus rhythm). We compare this typical CrCP measurement with a gold standard obtained during the drops in arterial blood pressure (ABP) caused by rapid ventricular pacing (RVP) in patients undergoing invasive electrophysiologic procedures. Approach: Adults receiving electrophysiology procedures with planned ablation were enrolled for DCS CBF monitoring. CrCP was calculated from CBF and ABP data by three methods: (1) linear extrapolation of data during RVP ( CrCP RVP ; the gold standard); (2) linear extrapolation of data during regular heartbeats ( CrCP Linear ); and (3) fundamental harmonic Fourier filtering of data during regular heartbeats ( CrCP Fourier ). Results: CBF monitoring was performed prior to and during 55 episodes of RVP in five adults. CrCP RVP and CrCP Fourier demonstrated agreement ( R = 0.66 , slope = 1.05 (95%CI, 0.72 to 1.38). Agreement between CrCP RVP and CrCP Linear was worse; CrCP Linear was 8.2 ± 5.9    mmHg higher than CrCP RVP (mean ± SD; p < 0.001 ). Conclusions: Our results suggest that DCS-measured CrCP can be accurately acquired during normal sinus rhythm.

Cerebrovascular tone and resistance measures differ between healthy control and patients with acute intracerebral haemorrhage: exploratory analyses from the BREATHE-ICH study

Objective.Cerebral autoregulation impairment in acute neurovascular disease is well described. The recent BREATHE-ICH study demonstrated improvements in dynamic cerebral autoregulation, by hypocapnia generated by hyperventilation, in the acute period following intracranial haemorrhage (ICH). This exploratory analysis of the BREATHE-ICH dataset aims to examine the differences in hypocapnic responses between healthy controls and patients with ICH, and determine whether haemodynamic indices differ between baseline and hypocapnic states.Approach.Acute ICH patients were recruited within 48 h of onset and healthy volunteers were recruited from a university setting. Transcranial Doppler measurements of the middle cerebral artery were obtained at baseline and then a hyperventilation intervention was used to induce hypocapnia. Patients with ICH were then followed up at 10-14 D post-event for repeated measurements.Main results.Data from 43 healthy controls and 12 patients with acute ICH met the criteria for statistical analysis. In both normocapnic and hypocapnic conditions, significantly higher critical closing pressure and resistance area product were observed in patients with ICH. Furthermore, critical closing pressure changes were observed to be sustained at 10-14 D follow up. During both the normocapnic and hypocapnic states, reduced autoregulation index was observed bilaterally in patients with ICH, compared to healthy controls.Significance.Whilst this exploratory analysis was limited by a small, non-age matched sample, significant differences between ICH patients and healthy controls were observed in factors associated with cerebrovascular tone and resistance. These differences suggest underlying cerebral autoregulation changes in ICH, which may play a pivotal role in the morbidity and mortality associated with ICH.

Impacts of a Pressure Challenge on Cerebral Critical Closing Pressure and Effective Cerebral Perfusion Pressure in Patients with Traumatic Brain Injury

INTRODUCTION

Cerebral critical closing pressure (CrCP) comprises intracranial pressure (ICP) and arteriolar wall tension (WT). It is the arterial blood pressure (ABP) at which small vessels close and circulation stops. We hypothesized that the increase in WT secondary to a systemic hypertensive challenge would lead to an increase in CrCP and that the "effective" cerebral perfusion pressure (CPPeff; calculated as ABP - CrCP) would give more complete information than the "conventional" cerebral perfusion pressure (CPP; calculated as ABP - ICP).

OBJECTIVE

This study aimed to compare CrCP, CPP, and CPPeff changes during a hypertensive challenge in patients with a severe traumatic brain injury.

PATIENTS AND METHODS

Data on ABP, ICP, and cerebral blood flow velocity, measured by transcranial Doppler ultrasound, were acquired simultaneously for 30 min both basally and during a hypertensive challenge. An impedance-based CrCP model was used.

RESULTS

The following values are expressed as median (interquartile range). There were 11 patients, aged 29 (14) years. CPP increased from 73 (17) to 102 (26) mmHg (P ≤ 0.001). ICP did not change. CrCP changed from 23 (11) to 27 (10) mmHg (P ≤ 0.001). WT increased from 7 (5) to 11 (7) mmHg (P ˂ 0.005). CPPeff changed less than CPP.

CONCLUSION

The CPP change was greater than the CPPeff change, mainly because CrCP increased simultaneously with the WT increase as a result of the autoregulatory response. CPPeff provides information about the real driving force generating blood movement.

Non-invasive estimation of cerebral perfusion pressure using transcranial Doppler ultrasonography in children with severe traumatic brain injury

OBJECTIVE

To identify if cerebral perfusion pressure (CPP) can be non-invasively estimated by either of two methods calculated using transcranial Doppler ultrasound (TCD) parameters.

DESIGN

Retrospective review of previously prospectively gathered data.

SETTING

Pediatric intensive care unit in a tertiary care referral hospital.

PATIENTS

Twenty-three children with severe traumatic brain injury (TBI) and invasive intracranial pressure (ICP) monitoring in place.

INTERVENTIONS

TCD evaluation of the middle cerebral arteries was performed daily. CPP at the time of the TCD examination was recorded. For method 1, estimated cerebral perfusion pressure (CPPe) was calculated as: CPPe = MAP × (diastolic flow (Vd)/mean flow (Vm)) + 14. For method 2, critical closing pressure (CrCP) was identified as the intercept point on the x-axis of the linear regression line of blood pressure and flow velocity parameters. CrCP/CPPe was then calculated as MAP-CrCP.

MEASUREMENTS AND MAIN RESULTS

One hundred eight paired measurements were available. Using patient averaged data, correlation between CPP and CPPe was significant (r = 0.78, p = < 0.001). However, on Bland-Altman plots, bias was 3.7 mmHg with 95% limits of agreement of - 17 to + 25 for CPPe. Using patient averaged data, correlation between CPP and CrCP/CPPe was significant (r = 0.59, p = < 0.001), but again bias was high at 11 mmHg with wide 95% limits of agreement of - 15 to + 38 mmHg.

CONCLUSIONS

CPPe and CrCP/CPPe do not have clinical value to estimate the absolute CPP in pediatric patients with TBI.

Cerebral Critical Closing Pressure: Is the Multiparameter Model Better Suited to Estimate Physiology of Cerebral Hemodynamics?

BACKGROUND

Cerebral critical closing pressure (CrCP) is the level of arterial blood pressure (ABP) at which small brain vessels close and blood flow stops. This value is always greater than intracranial pressure (ICP). The difference between CrCP and ICP is explained by the tone of the small cerebral vessels (wall tension). CrCP value is used in several dynamic cerebral autoregulation models. However, the different methods for calculation of CrCP show frequent negative values. These findings are viewed as a methodological limitation. We intended to evaluate CrCP in patients with severe traumatic brain injury (TBI) with a new multiparameter impedance-based model and compare it with results found earlier using a transcranial Doppler (TCD)-ABP pulse waveform-based method.

METHODS

Twelve severe TBI patients hospitalized during September 2005-May 2007. Ten men, mean age 32 years (16-61). Four had decompressive craniectomies (DC); three presented anisocoria. Patients were monitored with TCD cerebral blood flow velocity (FV), invasive ABP, and ICP. Data were acquired at 50 Hz with an in-house developed data acquisition system. We compared the earlier studied "first harmonic" method (M1) results with results from a new recently developed (M2) "multiparameter method."

RESULTS

M1: In seven patients CrCP values were negative, reaching -150 mmHg. M2: All positive values; only one lower than ICP (ICP 60 mmHg/ CrCP 57 mmHg). There was a significant difference between M1 and M2 values (M1 < M2) and between ICP and M2 (M2 > ICP).

CONCLUSION

M2 results in positive values of CrCP, higher than ICP, and are physiologically interpretable.

The cerebrovascular response to lower-body negative pressure vs. head-up tilt

Lower-body negative pressure (LBNP) has been proposed as a MRI-compatible surrogate for orthostatic stress. Although the effects of LBNP on cerebral hemodynamic behavior have been considered to reflect those of orthostatic stress, a direct comparison with actual orthostasis is lacking. We assessed the effects of LBNP (-50 mmHg) vs. head-up tilt (HUT; at 70°) in 10 healthy subjects (5 female) on transcranial Doppler-determined cerebral blood flow velocity (CBFv) in the middle cerebral artery and cerebral perfusion pressure (CPP) as estimated from the blood pressure signal (finger plethysmography). CPP was maintained during LBNP but decreased after 2 min in response to HUT, leading to an ~15% difference in CPP between LBNP and HUT (P ≤ 0.020). Mean CBFv initially decreased similarly in response to LBNP and for HUT, but, from minute 3 on, the decline became ~50% smaller (P ≤ 0.029) during LBNP. The reduction in end-tidal Pco₂ partial pressure (Pet_CO2 ) was comparable but with an earlier return toward baseline values in response to LBNP but not during HUT (P = 0.008). We consider the larger decrease in CBFv during HUT vs. LBNP attributable to the pronounced reduction in Pet_CO2 and to gravitational influences on CPP, and this should be taken into account when applying LBNP as an MRI-compatible orthostatic stress modality.NEW & NOTEWORTHY Lower-body negative pressure (LBNP) has the potential to serve as a MRI-compatible surrogate of orthostatic stress but a comparison with actual orthostasis was lacking. This study showed that the pronounced reduction in end-tidal Pco₂ together with gravitational effects on the brain circulation lead to a larger decline in cerebral blood flow velocity in response to head-up tilt than during lower-body negative pressure. This should be taken into account when employing lower-body negative pressure as MRI-compatible alternative to orthostatic stress.

An effective model of blood flow in capillary beds

In this article we derive applicable expressions for the macroscopic compliance and resistance of microvascular networks. This work yields a lumped-parameter model to describe the hemodynamics of capillary beds. Our derivation takes into account the multiscale nature of capillary networks, the influence of blood volume and pressure on the effective resistance and compliance, as well as, the nonlinear interdependence between these two properties. As a result, we obtain a simple and useful model to study hypotensive and hypertensive phenomena. We include two implementations of our theory: (i) pulmonary hypertension where the flow resistance is predicted as a function of pulmonary vascular tone. We derive from first-principles the inverse proportional relation between resistance and compliance of the pulmonary tree, which explains why the RC factor remains nearly constant across a population with increasing severity of pulmonary hypertension. (ii) The critical closing pressure in pulmonary hypotension where the flow rate dramatically decreases due to the partial collapse of the capillary bed. In both cases, the results from our proposed model compare accurately with experimental data.

Current concepts of optimal cerebral perfusion pressure in traumatic brain injury

Traumatic brain injury (TBI) consists of varied pathophysiological consequences and alteration of intracranial dynamics, reduction of the cerebral blood flow and oxygenation. In the past decade more emphasis has been directed towards optimizing cerebral perfusion pressure (CPP) in patients who have suffered TBI. Injured brain may show signs of ischemia if CPP remains below 50 mmHg and raising the CPP above 60 mmHg may avoid cerebral oxygen desaturation. Though CPP above 70 mmHg is influential in achieving an improved patient outcome, maintenance of CPP higher than 70 mmHg was associated with greater risk of acute respiratory distress syndrome (ARDS). The target CPP has been laid within 50-70 mmHg. Cerebral blood flow and metabolism are heterogeneous after TBI and with regional temporal differences in the requirement for CPP. Brain monitoring techniques such as jugular venous oximetry, monitoring of brain tissue oxygen tension (PbrO₂), and cerebral microdialysis provide complementary and specific information that permits the selection of the optimal CPP. This review highlights the rationale for use CPP directed therapies and neuromonitoring to identify optimal CPP of head injured patients. The article also reviews the evidence provided by various clinical trials regarding optimal CPP and their application in the management of head injured patients.

Moderate hyperventilation during intravenous anesthesia increases net cerebral lactate efflux

BACKGROUND

Hyperventilation is known to decrease cerebral blood flow (CBF) and to impair cerebral metabolism, but the threshold in patients undergoing intravenous anesthesia is unknown. The authors hypothesized that reduced CBF associated with moderate hyperventilation might impair cerebral aerobic metabolism in patients undergoing intravenous anesthesia.

METHODS

Thirty male patients scheduled for coronary surgery were included in a prospective, controlled crossover trial. Measurements were performed under fentanyl-midazolam anesthesia in a randomized sequence aiming at partial pressures of carbon dioxide of 30 and 50 mmHg. Endpoints were CBF, blood flow velocity in the middle cerebral artery, and cerebral metabolic rates for oxygen, glucose, and lactate. Global CBF was measured using a modified Kety-Schmidt technique with argon as inert gas tracer. CBF velocity of the middle cerebral artery was recorded by transcranial Doppler sonography. Data were presented as mean (SD). Two-sided paired t tests and one-way ANOVA for repeated measures were used for statistical analysis.

RESULTS

Moderate hyperventilation significantly decreased CBF by 60%, blood flow velocity by 41%, cerebral oxygen delivery by 58%, and partial pressure of oxygen of the jugular venous bulb by 45%. Cerebral metabolic rates for oxygen and glucose remained unchanged; however, net cerebral lactate efflux significantly increased from -0.38 (2.18) to -2.41(2.43) µmol min 100 g.

CONCLUSIONS

Moderate hyperventilation, when compared with moderate hypoventilation, in patients with cardiovascular disease undergoing intravenous anesthesia increased net cerebral lactate efflux and markedly reduced CBF and partial pressure of oxygen of the jugular venous bulb, suggesting partial impairment of cerebral aerobic metabolism at clinically relevant levels of hypocapnia.

Experimental model of intracranial hypertension with continuous multiparametric monitoring in swine

Objective Intracranial hypertension (IH) develops in approximately 50% of all patients with severe traumatic brain injury (TBI). Therefore, it is very important to identify a suitable animal model to study and understand the pathophysiology of refractory IH to develop effective treatments. Methods We describe a new experimental porcine model designed to simulate expansive brain hematoma causing IH. Under anesthesia, IH was simulated with a balloon insufflation. The IH variables were measured with intracranial pressure (ICP) parenchymal monitoring, epidural, cerebral oximetry, and transcranial Doppler (TCD). Results None of the animals died during the experiment. The ICP epidural showed a slower rise compared with parenchymal ICP. We found a correlation between ICP and cerebral oximetry. Conclusion The model described here seems useful to understand some of the pathophysiological characteristics of acute IH.

A method for estimating zero-flow pressure and intracranial pressure

BACKGROUND

It has been hypothesized that the critical closing pressure of cerebral circulation, or zero-flow pressure (ZFP), can estimate intracranial pressure (ICP). One ZFP estimation method used extrapolation of arterial blood pressure as against blood-flow velocity. The aim of this study was to improve ICP predictions.

METHODS

Two revisions have been considered: (1) the linear model used for extrapolation is extended to a nonlinear equation; and (2) the parameters of the model are estimated by an alternative criterion (not least squares). The method is applied to data on transcranial Doppler measurements of blood-flow velocity, arterial blood pressure, and ICP from 104 patients suffering from closed traumatic brain injury, sampled across the United States and England.

RESULTS

The revisions lead to qualitative (eg, precluding negative ICP) and quantitative improvements in ICP prediction. While moving from the original to the revised method, the ±2 SD of the error is reduced from 33 to 24 mm Hg, and the root-mean-squared error is reduced from 11 to 8.2 mm Hg. The distribution of root-mean-squared error is tighter as well; for the revised method the 25th and 75th percentiles are 4.1 and 13.7 mm Hg, respectively, as compared with 5.1 and 18.8 mm Hg for the original method.

CONCLUSIONS

Proposed alterations to a procedure for estimating ZFP lead to more accurate and more precise estimates of ICP, thereby offering improved means of estimating it noninvasively. The quality of the estimates is inadequate for many applications, but further work is proposed, which may lead to clinically useful results.

Cerebral haemodynamic physiology during steep Trendelenburg position and CO(2) pneumoperitoneum

BACKGROUND

The steep (40°) Trendelenburg position optimizes surgical exposure during robotic prostatectomy. The goal of the current study was to elucidate the influence of this patient positioning on cerebral blood flow and zero flow pressure (ZFP), and to assess the validity of different methods of evaluating ZFP.

METHODS

In 21 consecutive patients who underwent robotic endoscopic radical prostatectomy under general anaesthesia, transcranial Doppler flow velocity waveforms and invasive arterial and central venous pressure (CVP) waveforms suitable for analysis were recorded throughout the whole operative procedure in 14. The ZFP was determined by regression analysis of the pressure-flow plot and by different simplified formulas. The effective cerebral perfusion pressure (eCPP), pulsatility index (PI), and resistance index (RI) were determined.

RESULTS

While patients were in the Trendelenburg position, the ZFP increased in parallel with the CVP. The PI, RI, gradient between the ZFP and CVP, and the gradient between the CPP and the eCPP did not increase significantly (P<0.05) after 3 h of the steep Trendelenburg position. Using the formula described by Czosnyka and colleagues, the ZFP correlated closely with that calculated by linear regression throughout the course of the operation.

CONCLUSIONS

Prolonged steep Trendelenburg positioning and CO(2) pneumoperitoneum does not compromise cerebral perfusion. ZFP and eCPP are reliable variables for assessing brain perfusion during prolonged steep Trendelenburg positioning.

Arterial transducer placement and cerebral perfusion pressure monitoring: a discussion

AIM

To discuss existing disparity of practice and clinical implications of measuring cerebral perfusion pressure (CPP) from differing reference points thus highlighting the need for standardized benchmarks.

BACKGROUND

When managing traumatic brain injury (TBI), the arterial transducer level is a key to an accurate CPP reading; however, there is a lack of national standards about where to zero arterial transducers when monitoring CPP.

METHODS

A systematized search using the Cochrane library database, Pubmed database, Medline, British Library on line, CINAHL and PROQUEST using key search terms was used to identify articles that could form a basis for a discussion. Papers published between 2000 and 2008 were included. Papers that did not discuss arterial transducer level placement and CPP were excluded. The Brian Trauma Guidelines 2007 were scrutinized for recommendations.

RESULTS

Of 57 empirical studies accessed, none reported or explored the placement of the arterial transducer during CPP measurement. Conflicting opinions were identified within the literature and there were no recommendations made for practice within the Brain Trauma Foundation Guidelines 2007.

DISCUSSION

At the present time, there is insufficient evidence for recommending standard placement for mean arterial pressure (MAP) measurements for patients with TBI. There are implications to consider as the treatment prescribed will differ depending on where the arterial transducer is placed because the MAP and CPP displayed will fall by 15 mm Hg at a head elevation of 30 degrees. This poses a number of questions: is the CPP underestimated with the arterial transducer placed at head level? Is the CPP overestimated if the transducer is placed at mid axilla level?

RECOMMENDATIONS

Further research is recommended. However, studies would be difficult to power as head-injured patients constitute a heterogeneous population. Professional consensus should be applied and standardized benchmarks agreed.

Simultaneous bedside assessment of global cerebral blood flow and effective cerebral perfusion pressure in patients with intracranial hypertension

BACKGROUND

We examined a bedside technique transcerebral double-indicator dilution (TCID) for global cerebral blood flow (CBF) as well as the concept of effective cerebral perfusion pressure (CPP(eff)) during different treatment options for intracranial hypertension, and compared global CBF and CPP(eff) with simultaneously obtained conventional parameters.

METHODS

Twenty-six patients developing intracranial hypertension in the course of traumatic brain injury or subarachnoid hemorrhage were prospectively analyzed using a combined assessment during elevated ventilation (n = 15) or osmotherapy (hypertonic saline or mannitol). For calculation of global CBF, injections of ice-cold indocyanine green boluses were performed and temperature and dye concentration changes were monitored in the thoracic aorta and the jugular bulb. CBF was then calculated according to the mean transit time principle. Estimation of CCP, the arterial pressure at which cerebral blood flow becomes zero, was performed by synchronized registration of corresponding values of blood flow velocity in the middle cerebral artery and arterial pressure and extrapolation to zero-flow velocity. CPP(eff) was calculated as mean arterial pressure minus critical closing pressure (CPP(eff) = MAP(c) - CCP).

RESULTS

Elevated ventilation causes a decrease in both ICP (P < 0.001) and CBF (P < 0.001). While CPP(conv) increased (P < 0.001), CPP(eff) decreased during this observation (P = 0.002). Administration of osmotherapeutic agents resulted in a decrease of ICP (P < 0.001) and a temporary increase of CBF (P = 0.052). CPP(conv) and CPP(eff) showed no striking difference under osmotherapy.

CONCLUSION

TCID allows repeated measurements of global CBF at the bedside. Elevated ventilation lowered and osmotherapy temporarily raised global CBF. In situations of increased vasotonus, CPP(eff) is a better indicator of blood flow changes than conventional CPP.

Cerebral haemodynamics and carbon dioxide reactivity during sepsis syndrome

Background

Most patients with sepsis develop potentially irreversible cerebral dysfunctions. It is yet not clear whether cerebral haemodynamics are altered in these sepsis patients at all, and to what extent. We hypothesized that cerebral haemodynamics and carbon dioxide reactivity would be impaired in patients with sepsis syndrome and pathological electroencephalogram patterns.

Methods

After approval of the institutional ethics committee, 10 mechanically ventilated patients with sepsis syndrome and pathological electroencephalogram patterns underwent measurements of cerebral blood flow and jugular venous oxygen saturation before and after reduction of the arterial carbon dioxide partial pressure by 0.93 ± 0.7 kPa iu by ypervent ilation. The cerebral capillary closing pressure was determined from transcranial Doppler measurements of the arterial blood flow of the middle cerebral artery and the arterial pressure curve. A t test for matched pairs was used for statistical analysis (P < 0.05).

Results

During stable mean arterial pressure and cardiac index, reduction of the arterial carbon dioxide partial pressure led to a significant increase of the capillary closing pressure from 25 ± 11 mmHg to 39 ± 15 mmHg (P < 0.001), with a consecutive decrease of blood flow velocity in the middle cerebral artery of 21.8 ± 4.8%/kPa (P < 0.001), of cerebral blood flow from 64 ± 29 ml/100 g/min to 39 ± 15 ml/100 g/min (P < 0.001) and of jugular venous oxygen saturation from 75 ± 8% to 67 ± 14% (P < 0.01).

Conclusion

In contrast to other experimental and clinical data, we observed no pathological findings in the investigated parameters of cerebral perfusion and oxygenation.

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.

Neuromonitoring in the intensive care unit. I. Intracranial pressure and cerebral blood flow monitoring

BACKGROUND

Monitoring the injured brain is an integral part of the management of severely brain injured patients in intensive care. Brain-specific monitoring techniques enable focused assessment of secondary insults to the brain and may help the intensivist in making appropriate interventions guided by the various monitoring techniques, thereby reducing secondary brain damage following acute brain injury.

DISCUSSION

This review explores methods of monitoring the injured brain in an intensive care unit, including measurement of intracranial pressure and analysis of its waveform, and techniques of cerebral blood flow assessment, including transcranial Doppler ultrasonography, laser Doppler and thermal diffusion flowmetry.

CONCLUSIONS

Various modalities are available to monitor the intracranial pressure and assess cerebral blood flow in the injured brain in intensive care unit. Knowledge of advantages and limitations of the different techniques can improve outcome of patients with acute brain injury.

Neuromonitoring in the intensive care unit. I. Intracranial pressure and cerebral blood flow monitoring

BACKGROUND

Monitoring the injured brain is an integral part of the management of severely brain injured patients in intensive care. Brain-specific monitoring techniques enable focused assessment of secondary insults to the brain and may help the intensivist in making appropriate interventions guided by the various monitoring techniques, thereby reducing secondary brain damage following acute brain injury.

DISCUSSION

This review explores methods of monitoring the injured brain in an intensive care unit, including measurement of intracranial pressure and analysis of its waveform, and techniques of cerebral blood flow assessment, including transcranial Doppler ultrasonography, laser Doppler and thermal diffusion flowmetry.

CONCLUSIONS

Various modalities are available to monitor the intracranial pressure and assess cerebral blood flow in the injured brain in intensive care unit. Knowledge of advantages and limitations of the different techniques can improve outcome of patients with acute brain injury.

Noninvasive measurement of intracranial pressure: is it possible?

BACKGROUND

Some publications suggest a strong correlation between the intracranial pressure and the intraocular pressure. Other studies claim no correlation between these two physiologic variables. Our aim was to study whether the tonometry could be a useful method to evaluate intracranial pressure in patients with suspected intracranial abnormality.

METHODS

We evaluated the correlation between the intracranial pressure and the intraocular pressure, the intracranial pressure and the mean arterial pressure, and the intraocular pressure and the mean arterial pressure in 22 patients, initially comatose, who were admitted to our hospital. All patients required the intracranial pressure monitoring on clinical grounds. Simultaneous measurements were performed and recorded.

RESULTS

We calculated both the linear correlation coefficient and the Spearman rank-order correlation coefficient. We found significant correlation between the intraocular pressure and the mean arterial pressure in 12 patients; however, significant correlation between the intraocular pressure and the intracranial pressure was found in only 2 patients.

CONCLUSION

Tonometry is not an appropriate method for the assessment of intracranial pressure increases.

Comparison of Critical Closing Pressures Extracted from Carotid Tonometry and Finger Plethysmography

BACKGROUND

The reliability of critical closing pressure (CrCP) estimates derived from peripheral blood pressure (BP) measurements is unclear. We attempted to evaluate the influences of peripheral circulation on determining CrCP.

METHODS

Twenty-five young healthy volunteers were studied. BP waves were obtained with plethysmography (Portapres) and carotid applanatory tonometry, respectively, for analysis. Transcranial Doppler was used to monitor cerebral flow velocity. Using linear regression analysis, beat-to-beat CrCP was calculated at rest, during voluntary hyperventilation and during 5% CO2 inhalation.

RESULTS

Twenty of 25 participants demonstrating satisfactory tonometric tracings for both tests were included in the analysis. The systolic BP measured using plethysmography was higher than that derived from tonometry (139.4 +/- 24.7 vs. 105.5 +/- 29.6, p < 0.001). CrCP values derived from tonometry were all positive and higher than CrCP values derived from plethysmography (62.9 +/- 19.9 vs. 11.1 +/- 17.8, p < 0.001). The changes in CrCP induced by 5% CO2 inhalation and hyperventilation had a correlation between two BP monitoring methods (r = 0.52, p = 0.001).

CONCLUSIONS

Pressure waveform is an important determinant in calculating CrCP by linear regression analysis. The relative changes in CrCP induced by hemodynamic challenges remained a relevant indicator of cerebrovascular regulation regardless of the methods used for non-invasive BP recording.

Doppler estimation of zero flow pressure during changes in downstream pressure in a bench model of a circulation using pulsatile flow

Zero flow pressure is the arterial pressure at which blood flow ceases in the cerebral circulation and may represent the effective downstream pressure of this system. We used a bench model of pulsatile fluid flow to determine whether simulated changes in downstream pressure may be detected by estimation of zero flow pressure. A Doppler probe was used to record flow velocity and a pressure transducer was used to measure driving pressure. Eight different configurations of the circuit were produced, and at each configuration the external pressure around a collapsible segment of the circuit was changed in order to simulate intracranial pressure. Perfusion pressure and zero flow pressure were estimated for each configuration and each level of external pressure. The sensitivity of the model in predicting the change in external pressure from the change in zero flow pressure was 94%. This indicates that estimation of zero flow pressure by this method is a sensitive way of monitoring trends in changes in downstream pressure.

Non-invasive assessment of cerebral perfusion pressure in brain injured patients with moderate intracranial hypertension

BACKGROUND

A non-invasive estimation of cerebral perfusion pressure (CPP) using transcranial Doppler sonography was assessed in brain-injured patients by comparing conventional measurements of CPP (difference between mean arterial pressure and intracranial pressure) (CPPm) with the difference between AP(mean) and the critical closing pressure of the cerebral circulation (CPPe).

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 CO(2) 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 Pe'(co(2))) 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.

Correlations among critical closing pressure, pulsatility index and cerebrovascular resistance

We attempted to explore the relationships among critical closing pressure (CrCP), resistance-area product (RAP) and traditional resistance indices of cerebral hemodynamics. Twenty healthy volunteers were studied. Blood pressure was obtained with servo-controlled plethysmography. Cerebral blood flow velocity (CBFV) was monitored by transcranial Doppler. Hemodynamic changes were induced by hyperventilation and by 5% CO(2) inhalation. Beat-to-beat CrCP and RAP values were extracted by linear regression analysis of instantaneous arterial blood pressure (ABP) and CBFV tracings. Gosling's pulsatility index (PI) and cerebrovascular resistance (CVR) were calculated. RAP correlated well with CVR at rest and during provocative tests (p = 0.006 approximately <0.001). There was no correlation among CrCP, CVR and PI. The changes in CVR correlated with those in RAP (p = 0.008 for the 5% CO(2) test and p = 0.014 for the hyperventilation test). The changes in PI and CrCP showed significant correlation (p = 0.004 for the 5% CO(2) test and p = 0.003 for the hyperventilation test). RAP reliably reflected cerebrovascular resistance. The changes in CrCP were valuable in assessing cerebrovascular regulation. Estimating changes in CrCP and RAP provided better understanding of the nature of cerebrovascular regulation.

Critical Closing Pressure in Subarachnoid Hemorrhage

BACKGROUND AND PURPOSE

Critical closing pressure (CCP) is thought to be jointly influenced by intracranial pressure and cerebrovascular tone. We examined how CCP is affected by cerebral vasospasm after subarachnoid hemorrhage (SAH).

METHODS

In 15 patients with vasospasm of the middle cerebral artery, CCP was calculated using 2 methods previously reported (ad modem Aaslid and Michel, indexed CCP(Aaslid) and CCP(Michel), respectively) based on data of arterial blood pressure and flow velocity (FV) as assessed by transcranial Doppler.

RESULTS

CCP decreased significantly (P<0.05) during vasospasm (CCP(Aaslid)=6.3+/-22.9 mm Hg, CCP(Michel)=14.9+/-16.5 mm Hg, mean+/-SD) as compared with baseline (CCP(Aaslid)=24.4+/-20.3 mm Hg, CCP(Michel)=27.8+/-19.4 mm Hg). This was not attributable to ICP, which remained unaffected by vasospasm. In addition, CCP was significantly lower on the side of vasospasm (CCP(Aaslid)=11.9+/-24.2 mm Hg, CCP(Michel)=18.4+/-19.6 mm Hg) as compared with the contralateral nonvasospastic side (CCP(Aaslid)=24.7+/-22.3 mm Hg, CCP(Michel)=28.2+/-18.0 mm Hg).

CONCLUSIONS

Assuming that autoregulation-related distal vasodilatation outweighs proximal vasospasm, CCP should decrease. Alternatively, CCP might have increased during vasospasm as the tension of big vessels increase, but the turbulence occurring during vasospasm may have impaired the linear relationship between pressure and FV, thus leading to a marked underestimation of CCP. In conclusion, interpretation of CCP in vasospasm is difficult and may be overshadowed by nonlinear hemodynamic effects.

The critical closing pressure of the cerebral circulation

The critical closing pressure (CrCP) of the cerebral circulation indicates the value of arterial blood pressure (ABP) at which cerebral blood flow (CBF) approaches zero. Measurements in animals and in humans, have shown that the CrCP is significantly greater than zero. A simple mathematical model, incorporating the effects of arterial elasticity and active wall tension, shows that CrCP can be influenced by several structural and physiological parameters, notably intracranial pressure (ICP) and active wall tension. Due to the non-linear shape of the complete ABP-CBF curve, most methods proposed for estimation of CrCP can only represent the linear range of the pressure-flow (or velocity) relationship. As a consequence, only estimates of apparent CrCP can be obtained, and these tend to be significantly higher than the true CrCP. Estimates of apparent CrCP have been shown to be influenced by arterial PCO2, ICP, cerebral autoregulation, intra-thoracic pressure, and mean ABP. There is a lack of investigation, under well-controlled conditions, to assess whether CrCP is altered in disease states. Studies of the cerebral circulation need to take CrCP into account, to obtain more accurate estimates of cerebrovascular resistance changes, and to reflect the correct dynamic relationship between instantaneous ABP and CBF.

Noninvasive estimation of cerebral perfusion pressure and zero flow pressure in healthy volunteers: the effects of changes in end-tidal carbon dioxide

Zero flow pressure (ZFP) in the cerebral circulation is defined as the arterial pressure at which flow ceases. Noninvasive methods of estimating cerebral perfusion pressure (CPP) and ZFP using transcranial Doppler ultrasonography have been described. There is a paucity of normal physiological data related to changes in estimated CPP (eCPP) and ZFP induced by changes in carbon dioxide (CO(2)). We studied the effects of CO(2) on eCPP and ZFP in 17 healthy volunteers. After baseline measurements of middle cerebral artery blood-flow velocity and blood pressure, subjects voluntarily hyperventilated to decrease their end-tidal CO(2) (PE'CO(2)) by approximately 7.5 mm Hg, and then they increased their PE'CO(2) by approximately 7.5 mm Hg by breathing through a Mapleson D circuit. Blood-flow velocity and blood pressure were recorded at each stage. The eCPP and ZFP were calculated by using established formulas, and the results were analyzed with analysis of variance. With increasing PE'CO(2), eCPP increased from 50.67 mm Hg (8.33 mm Hg) (mean [SD]) to 60.87 mm Hg (9.28 mm Hg) (20% increase; P < 0.001), with a corresponding decrease in ZFP (P = 0.017); hypocapnia resulted in the opposite effects on eCPP and ZFP. These results indicate physiological changes in eCPP and ZFP that can be expected from changes in CO(2) in subjects without any neurological disorder.

IMPLICATIONS

Increasing end-tidal CO(2) increases the estimated cerebral perfusion pressure and vice versa. These results are opposite to those expected from the known effects of CO(2) on intracranial pressure. Thus, we support the suggestion that, in the absence of intracranial hypertension, vascular tone remains a major determinant of effective downstream pressure and cerebral perfusion.

Monitoring of cerebral perfusion pressure during intracranial hypertension: a sufficient parameter of adequate cerebral perfusion and oxygenation?

OBJECTIVE

A cerebral perfusion pressure (CPP) oriented treatment is a widely accepted standard for patients with intracranial hypertension. In an animal model of controlled intracranial hypertension we investigated whether CPP is a reliable parameter of sufficient cerebral perfusion and oxygenation. Using near-infrared reflexion spectroscopy the effect of decreasing CPP due to increasing intracranial pressure (ICP) on cerebral tissue oxygenation was studied.

METHODS

Ten rabbits were subjected to artificially elevated ICP using the cisterna-magna infusion technique. Regional cerebral O(2) saturation of hemoglobin (tiSO(2)), regional tissue concentration of hemoglobin (tiHb), and CPP were recorded continuously. CPP was investigated with respect to tiSO(2). Electrocortical activity was simultaneously recorded by two-channel EEG to determine the onset of ischemia.

RESULTS

Reduced CPP due to increased ICP led to a continuous decrease in tiSO(2.) There was progressive suppression of EEG frequency and amplitude with decreasing CPP in all animals. Onset of EEG-silence due to elevated ICP was observed in a wide range of CPP-values between 9 and 42 mmHg. At the same time tiSO(2) varied merely between 0 and 5%.

CONCLUSIONS

Regarding the EEG effects due to increased ICP (EEG silence), CPP values showed a wide interindividual variability, in contrast to tiSO(2). In our animal model the sole calculation of CPP did not reflect adequate cerebral perfusion.

Extrapolation to zero-flow pressure in cerebral arteries to estimate intracranial pressure

BACKGROUND

Cerebral perfusion pressure (CPP) is commonly calculated from the difference between arterial blood pressure (AP) and intracranial pressure (ICP). ICP can be considered the effective downstream pressure of the cerebral circulation. Consequently, cerebral circulatory arrest would occur when AP equals ICP. Estimation of AP for zero-flow pressure (ZFP) may thus allow estimation of ICP. We estimated ZFP from cerebral pressure-flow velocity relationships so that ICP could be measured by transcranial Doppler sonography.

METHODS

We studied 20 mechanically ventilated patients with severe head injury, in whom ICP was monitored by epidural pressure transducers. AP was measured with a radial artery cannula. Blood flow velocity in the middle cerebral artery (V(MCA)) ipsilateral to the site of ICP measurement was measured with a 2 MHz transcranial Doppler probe. All data were recorded by a microcomputer from analogue-digital converters. ZFP was extrapolated by regression analysis of AP-V(MCA) plots and compared with simultaneous measurements of ICP.

RESULTS

ZFP estimated from AP-V(MCA) plots was linearly related to ICP over a wide range of values (r=0.93). There was no systematic difference between ZFP and ICP. Limit of agreement (2 SD) was 15.2 mm Hg. Short-term variations in ICP were closely followed by changes in ZFP.

CONCLUSION

Extrapolation of cerebral ZFP from instantaneous AP-V(MCA) relationships enables detection of severely elevated ICP and may be a useful and less invasive method for CPP monitoring than other methods.