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

Influence of angiotensin II and telmisartan on in vivo high-resolution renal arterial impedance in rats

Angiotensin II (ANG II) is known to play an important role in regulating renal hemodynamics. We sought to quantify this effect in an in vivo rat model with high-resolution renal arterial (RA) impedance. This study examines the effects of ANG II and its type 1 receptor blocker telmisartan (TELM) on RA impedance. In baroreflex-deactivated rats, we measured RA pressure (Pr) and blood flow (Fr) during random ventricular pacing to induce pressure fluctuation at three different mean Pr (60, 80, and 100 mmHg). We then estimated RA impedance as the transfer function from Fr to Pr. The RA impedance was found to align with a three-element Windkessel model consisting of proximal (Rp) and distal (Rd) resistance and compliance (C). Our study showed Rd reflected the composite characteristics of afferent and efferent arterioles. Rd increased with increasing Pr under the baseline condition with a slope of 1.03 ± 0.21 (× 10-1) min·mL-1. ANG II significantly increased the slope by 0.72 ± 0.29 (× 10-1) min·mL-1 (P < 0.05) without affecting the intercept. TELM significantly reduced the intercept by 34.49 ± 4.86 (× 10-1) mmHg·min·mL-1 (P < 0.001) from the baseline value of 37.93 ± 13.36 (× 10-1) mmHg·min·mL-1, whereas it did not affect the slope. In contrast, Rp was less sensitive than Rd to ANG II or TELM, suggesting Rp may represent the characteristics of elastic large arteries. Our findings provide valuable insights into the influence of ANG II on the dynamics of the renal vasculature.NEW & NOTEWORTHY This present method of quantifying high-resolution renal arterial impedance could contribute to elucidating the characteristics of renal vasculature influenced by physiological mechanisms, renal diseases, or pharmacological effects. The present findings help construct a lumped-parameter renal hemodynamic model that reflects the influence of angiotensin II.

Determinants of the dynamic cerebral critical closing pressure response to changes in mean arterial pressure

Objective. Cerebral critical closing pressure (CrCP) represents the value of arterial blood pressure (BP) where cerebral blood flow (CBF) becomes zero. Its dynamic response to a step change in mean BP (MAP) has been shown to reflect CBF autoregulation, but robust methods for its estimation are lacking. We aim to improve the quality of estimates of the CrCP dynamic response.Approach. Retrospective analysis of 437 healthy subjects (aged 18-87 years, 218 males) baseline recordings with measurements of cerebral blood velocity in the middle cerebral artery (MCAv, transcranial Doppler), non-invasive arterial BP (Finometer) and end-tidal CO2(EtCO2, capnography). For each cardiac cycle CrCP was estimated from the instantaneous MCAv-BP relationship. Transfer function analysis of the MAP and MCAv (MAP-MCAv) and CrCP (MAP-CrCP) allowed estimation of the corresponding step responses (SR) to changes in MAP, with the output in MCAv (SRVMCAv) representing the autoregulation index (ARI), ranging from 0 to 9. Four main parameters were considered as potential determinants of the SRVCrCPtemporal pattern, including the coherence function, MAP spectral power and the reconstruction error for SRVMAP, from the other three separate SRs.Main results. The reconstruction error for SRVMAPwas the main determinant of SRVCrCPsignal quality, by removing the largest number of outliers (Grubbs test) compared to the other three parameters. SRVCrCPshowed highly significant (p< 0.001) changes with time, but its amplitude or temporal pattern was not influenced by sex or age. The main physiological determinants of SRVCrCPwere the ARI and the mean CrCP for the entire 5 min baseline period. The early phase (2-3 s) of SRVCrCPresponse was influenced by heart rate whereas the late phase (10-14 s) was influenced by diastolic BP.Significance. These results should allow better planning and quality of future research and clinical trials of novel metrics of CBF regulation.

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.

Pathophysiological and clinical considerations in the perioperative care of patients with a previous ischaemic stroke: a multidisciplinary narrative review

With an ageing population and increasing incidence of cerebrovascular disease, an increasing number of patients presenting for routine and emergency surgery have a prior history of stroke. This presents a challenge for pre-, intra-, and postoperative management as the neurological risk is considerably higher. Evidence is lacking around anaesthetic practice for patients with vascular neurological vulnerability. Through understanding the pathophysiological changes that occur after stroke, insight into the susceptibilities of the cerebral vasculature to intrinsic and extrinsic factors can be developed. Increasing understanding of post-stroke systemic and cerebral haemodynamics has provided improved outcomes from stroke and more robust secondary prevention, although this knowledge has yet to be applied to our delivery of anaesthesia in those with prior stroke. This review describes the key pathophysiological and clinical considerations that inform clinicians providing perioperative care for patients with a prior diagnosis of stroke.

Interplay of Proximal Flow Confluence and Distal Flow Divergence in Patient-Specific Vertebrobasilar System

Approximately one-quarter of ischemic strokes involve the vertebrobasilar arterial system that includes the upstream flow confluence and downstream flow divergence. A patient-specific hemodynamic analysis is needed to understand the posterior circulation. The objective of this study is to determine the distribution of hemodynamic parameters in the vertebrobasilar system, based on computer tomography angiography images. Here, the interplay of upstream flow confluence and downstream flow divergence was hypothesized to be a determinant factor for the hemodynamic distribution in the vertebrobasilar system. A computational fluid dynamics model was used to compute the flow fields in patient-specific vertebrobasilar models (n = 6). The inlet and outlet boundary conditions were the aortic pressure waveform and flow resistances, respectively. A 50% reduction of total outlet area was found to induce a ten-fold increase in surface area ratio of low time-averaged wall shear stress (i.e., TAWSS ≤ 4 dynes/cm²). This study enhances our understanding of the posterior circulation associated with the incidence of atherosclerotic plaques.

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.

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.

Impact of anesthetic agents on cerebrovascular physiology in children

The role of the pediatric neuroanesthetist is to provide comprehensive care to children with neurologic pathologies. The cerebral physiology is influenced by the developmental stage of the child. The understanding of the effects of anesthetic agents on the physiology of cerebral vasculature in the pediatric population has significantly increased in the past decade allowing a more rationale decision making in anesthesia management. Although no single anesthetic technique can be recommended, sound knowledge of the principles of cerebral physiology and anesthetic neuropharmacology will facilitate the care of pediatric neurosurgical patients.

Noninvasive monitoring of cerebral perfusion pressure in patients with acute liver failure using transcranial doppler ultrasonography

Elevated intracranial pressure (ICP) leads to loss of cerebral perfusion, cerebral herniation, and irreversible brain damage in patients with acute liver failure (ALF). Conventional techniques for monitoring ICP can be complicated by hemorrhage and infection. Transcranial doppler ultrasonography (TCD) is a noninvasive device which can continuously measure cerebral blood flow velocity, producing a velocity-time waveform that indirectly monitors changes in cerebral hemodynamics, including ICP. The primary goal of this study was to determine whether TCD waveform features could be used to differentiate ALF patients with respect to ICP or, equally important, cerebral perfusion pressure (CPP) levels. A retrospective study of 16 ALF subjects with simultaneous TCD, ICP, and CPP measurements yielded a total of 209 coupled ICP-CPP-TCD observations. The TCD waveforms were digitally scanned and seven points corresponding to a simplified linear waveform were identified. TCD waveform features including velocity, pulsatility index, resistive index, fraction of the cycle in systole, slopes, and angles associated with changes in the slope in each region, were calculated from the simplified waveform data. Paired ICP-TCD observations were divided into three groups (ICP < 20 mmHg, n = 102; 20 < or = ICP < 30 mmHg, n = 74; and ICP > or = 30 mmHg, n = 33). Paired CPP-TCD observations were also divided into three groups (CPP > or = 80 mmHg, n = 42; 80 > CPP > or = 60 mmHg, n = 111; and CPP < 60 mmHg, n = 56). Stepwise linear discriminant analysis was used to identify TCD waveform features that discriminate between ICP groups and CPP groups. Four primary features were found to discriminate between ICP groups: the blood velocity at the start of the Windkessel effect, the slope of the Windkessel upstroke, the angle between the end systolic downstroke and start diastolic upstroke, and the fraction of time spent in systole. Likewise, 4 features were found to discriminate between the CPP groups: the slope of the Windkessel upstroke, the slope of the Windkessel downstroke, the slope of the diastolic downstroke, and the angle between the end systolic downstroke and start diastolic upstroke. The TCD waveform captures the cerebral hemodynamic state and can be used to predict dynamic changes in ICP or CPP in patients with ALF. The mean TCD waveforms for corresponding, correctly classified ICP and CPP groups are remarkably similar. However, this approach to predicting intracranial hypertension and CPP needs to be further refined and developed before clinical application is feasible.

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.

Cerebral blood flow velocity during mental activation: interpretation with different models of the passive pressure-velocity relationship

The passive relationship between arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV) has been expressed by a single parameter [cerebrovascular resistance (CVR)] or, alternatively, by a two-parameter model, comprising a resistance element [resistance-area product (RAP)] and a critical closing pressure (CrCP). We tested the hypothesis that the RAP+CrCP model can provide a more consistent interpretation to CBFV responses induced by mental activation tasks than the CVR model. Continuous recordings of CBFV [bilateral, middle cerebral artery (MCA)], ABP, ECG, and end-tidal CO(2) (EtCO(2)) were performed in 13 right-handed healthy subjects (aged 21-43 yr), in the seated position, at rest and during 10 repeated presentations of a word generation and a constructional puzzle paradigm that are known to induce differential cortical activation. Due to its small relative change, the CBFV response can be broken down into standardized subcomponents describing the relative contributions of ABP, CVR, RAP, and CrCP. At rest and during activation, the RAP+CrCP model suggested that RAP might reflect myogenic activity in response to the ABP transient, whereas CrCP was more indicative of metabolic control. These different influences were not reflected by the CVR model, which indicated a predominantly metabolic response. Repeated-measures multi-way ANOVA showed that CrCP (P = 0.025), RAP (P = 0.046), and CVR (P = 0.002) changed significantly during activation. CrCP also had a significant effect of paradigm (P = 0.045) but not hemispheric dominance. Both RAP (P = 0.039) and CVR (P = 0.0008) had significant effects of hemispheric dominance but were not sensitive to the different paradigms. Subcomponent analysis can help with the interpretation of CBFV responses to mental activation, which were found to be dependent on the underlying model of the passive ABP-CBFV relationship.

The Effects of Propofol or Sevoflurane on the Estimated Cerebral Perfusion Pressure and Zero Flow Pressure

The zero flow pressure (ZFP) is the pressure at which blood flow ceases through a vascular bed. Using transcranial Doppler ultrasonography, we investigated the effects of propofol or sevoflurane on the estimated cerebral perfusion pressure (eCPP) and ZFP in the cerebral circulation. Twenty-three healthy patients undergoing nonneurosurgical procedures under general anesthesia were studied. After induction of anesthesia using propofol, the anesthesia was maintained with either propofol infusion (n = 13) or sevoflurane (n = 10). Middle cerebral artery flow velocity, noninvasive arterial blood pressure, and end-tidal carbon dioxide partial pressure were recorded awake as a baseline, and during steady-state anesthesia at normocapnia (baseline end-tidal carbon dioxide partial pressure) and hypocapnia (1 kPa below baseline). The eCPP and ZFP were calculated using an established formula. The mean arterial blood pressure decreased in both groups. The eCPP decreased significantly in the propofol group (median, from 58 to 41 mm Hg) but not in the sevoflurane group (from 60 to 62 mm Hg). Correspondingly, ZFP increased significantly in the propofol group (from 25 to 33 mm Hg) and it decreased significantly in the sevoflurane group (from 27 to 7 mm Hg). Hypocapnia did not change eCPP or ZFP in the propofol group, but it significantly decreased eCPP and increased ZFP in the sevoflurane group.

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.

Effects of inhaled nitrous oxide 50% on estimated cerebral perfusion pressure and zero flow pressure in healthy volunteers

The role of vascular tone in determining cerebral perfusion pressure is increasingly being appreciated. It has been suggested that zero flow pressure, the arterial pressure at which blood flow ceases, represents the effective downstream pressure of the cerebral circulation. Nitrous oxide is a cerebral vasodilator and may therefore decrease zero flow pressure and increase cerebral perfusion pressure. However, these effects may be opposed by the increase in intracranial blood volume produced by cerebral vasodilation. We studied eight healthy volunteers at normocapnia and studied the effects of the inhalation of nitrous oxide 50% on estimated cerebral perfusion pressure and zero flow pressure using transcranial Doppler ultrasonography. We found that nitrous oxide 50% significantly increased estimated cerebral perfusion pressure (p = 0.03), whilst decreasing zero flow pressure (p = 0.01). These results suggest that the vasomotor effects of nitrous oxide predominate in determining the effective downstream pressure of the cerebral circulation in healthy individuals.

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.

The relationship of intraocular pressure to intracranial pressure

STUDY OBJECTIVE

The early detection of intracranial hypertension can lead to timely medical and neurosurgical intervention, preventing brain herniation and death. In this investigation, we hypothesize that an increase in intracranial pressure can be detected by an increase in intraocular pressure using noninvasive existing technology, the handheld tonometer.

METHODS

This was a prospective observational pilot study conducted at a community hospital. Admitted patients with an invasive intracranial pressure monitor were solicited for participation. Patients were excluded if they had known glaucoma or had sufficient ocular or facial trauma that precluded intraocular pressure determination. Simultaneous measurements of intracranial and intraocular pressure were recorded.

RESULTS

Twenty-seven patients were enrolled, and 76 individual measurements were performed. All patients with an abnormal intracranial pressure had an abnormal intraocular pressure; similarly, all patients with a normal intracranial pressure had a normal intraocular pressure (sensitivity 1.00, 95% confidence interval 0.86 to 1.0; specificity 1.0, 95% confidence interval 0.93 to 1.0)

CONCLUSION

Abnormal intraocular pressure as measured with the handheld tonometer is an excellent indicator of abnormal intracranial pressure in patients with known intracranial pathology.