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R B Panerai

Publications and source records attributed to R B Panerai.

At least 19 recordsLinked to original sources
PubMed2006

Cerebral critical closing pressure estimation from Finapres and arterial blood pressure measurements in the aorta.

Estimates of cerebral critical closing pressure (CrCP) and resistance-area product (RAP) are often derived using noninvasive measurements of arterial blood pressure (ABP) in the finger, but the errors introduced by this approach, in relation to intra-vascular measurements of ABP, are not known. Continuous recordings of ABP (Finapres and solid-state catheter-tip transducer in the ascending aorta), cerebral blood flow velocity (CBFV, bilateral Doppler), ECG and transcutaneous CO(2) were performed following coronary catheterization. CrCP and RAP were calculated for each of 12,784 cardiac cycles from 27 subjects using the classical linear regression (LR) of the instantaneous CBFV-ABP relationship and also the first harmonic (H(1)) of the Fourier transform. There was a better agreement between LR and H(1) for the aortic measurements than for the Finapres (p < 0.000,01). For LR there were no significant differences for either CrCP or RAP due to the source of ABP measurement, but for H(1) the differences were highly significant (p < 0.000,03). The coherence functions between either CrCP or RAP values calculated with aortic pressure (input) or the Finapres (output) were significantly higher for H(1) than for LR for most harmonics below 0.2 Hz. When using the Finapres to estimate CrCP and RAP values, the LR method produces similar results to intra-arterial measurements of ABP for time-averaged values, but H(1) should be preferred in applications analysing beat-to-beat changes in these parameters.

Airway Obstruction↗
PubMed2005

Objective selection of signals for assessment of cerebral blood flow autoregulation in neonates.

A number of different system identification techniques have been proposed to assess dynamic cerebral autoregulation in critically ill patients. From these methods, the response to a standard stepwise change in blood pressure can be estimated. Responses lacking physiological consistency are a common occurrence and could be the consequence of particular system identification procedures or, alternatively, caused by measurements with a poor signal-to-noise ratio. A multi-observer approach was adopted in this paper to classify cerebral blood flow velocity (CBFV) step responses to spontaneous changes in arterial blood pressure in a group of 43 neonates with a mean gestational age of 33.7 weeks (range 24-42 weeks) and a mean birthweight of 1,980 g (range 570-3,910 g). Three experienced observers independently analysed the estimated step responses in 191 recordings each lasting 100 s; for an autoregressive (ARX) model, 124 (65%) of the step responses were accepted by at least two of the three observers. Two other system identification methods, transfer function analysis and the moving average Wiener-Laguerre model, gave 90 (45%) and 98 (51%) acceptable responses, respectively. Only 54 epochs (28%) were accepted with all three methods. With 88 (46%) responses rejected by at least two methods, it can be concluded that signal quality was the main reason for nonphysiological step responses. To avoid the need for subjective visual selection, an automatic procedure for classifying step responses was implemented leading to sensitivities and specificities in the range 85-90%, with respect to the agreement with subjective evaluations. Objective selection of CBFV step responses is thus feasible and could also be adapted for other physiological measurement techniques relying on system identification methods.

Blood Flow Velocity↗
PubMed2004

Neural network modelling of dynamic cerebral autoregulation: assessment and comparison with established methods.

A time lagged recurrent neural network (TLRN) was implemented to model the dynamic relationship between arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV) and its performance was compared to classical linear model such as transfer function analysis, Aaslid's dynamic autoregulation model, and the Wiener-Laguerre moving average filter. A simple linear regression was also tested as a naive estimator. In 16 normal subjects, CBFV was continuously recorded with Doppler ultrasound and ABP with the Finapres device during six repeated thigh cuff manoeuvres. Using mean beat-to-beat values of ABP as input and CBFV as output, the performance of each method was assessed by the model's predicted velocity correlation coefficient and normalized mean square error (MSE). Cross-validation was performed using three thigh cuff manoeuvres for the training data set and the other three for the validation set. The four methods studied performed significantly better than the zero-order naive estimator. The TLRN performed better than transfer function analysis, but was not significantly different from the time-domain techniques, despite showing the minimum predictive MSE. CBFV step responses could be extracted from the TLRN showing the presence of non-linear behaviour both in terms of amplitude and directionality.

Adaptation, Physiological↗
PubMed2004

Association between dynamic cerebral autoregulation and mortality in severe head injury.

The objective of the study was to test the hypothesis that dynamic cerebral pressure-autoregulation is associated with the outcome of patients with severe head injury and to derive optimal criteria for future studies on the predictive value of autoregulation indices. Repeated measurements were performed on 32 patients with severe head injury. Arterial blood pressure (ABP) was measured continuously with an intravascular catheter, intracranial pressure (ICP) was recorded with a subdural semiconductor transducer and cerebral blood flow velocity (CBFV) was measured with Doppler ultrasound in the middle cerebral artery. Transfer function analysis was performed on mean beat-to-beat values, using ABP or CBFV as input variables and CBFV or ICP as the output variables. A dynamic index of autoregulation (ARI) ranging between 0 and 9 was extracted from the CBFV step response for a change in ABP. No significant differences between survivors and non-survivors were found due to mean values of ICP, ABP, CPP, CBFV, pCO2, GCS, age or heart rate. The transfer functions between ABP-ICP and CBFV-ICP did not show any significant differences either. The median [lower, upper quartiles] ARI was significantly lower for non-survivors compared with survivors [4.8 (0.0, 5.9) v. 6.9 (5.9, 7.4), p= 0.004]. The correlation between ARI and GOS was also significant (r=0.464, p=0.011). Cohen's coefficient was optimal for a threshold of ARI= 5.86 (kappa 0.51, p=0.0036), leading to a sensitivity for death of 75%, specificity=76.5%, odds ratio =9.75 and overall precision = 75.8%. The difference in ARI values between survivors and non-survivors persisted when results were adjusted for GCS (p = 0.028). A similar analysis for the Marshall CT scale did not reach significance (p = 0.072). A logistic regression analysis confirmed that apart from the ARI, no other variables had a significant contribution to predict outcome. In this group of patients, death following severe head injury could not be explained by traditional indices of risk, but was strongly correlated to indices of dynamic cerebral pressure-autoregulation extracted by means of transfer function analysis. Future studies using a prospective design are needed to validate the predictive value of the ARI index, as estimated by transfer function analysis, in relation to death and other unfavourable outcomes.

Adolescent↗
PubMed2003

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.

Animals↗
PubMed2003

Variability of time-domain indices of dynamic cerebral autoregulation.

The intra- and inter-subject variabilities of the cerebral dynamic autoregulatory index (ARI) were studied in a group of 14 healthy subjects aged 23-51 years. An alternative index, derived from autoregressive-moving average (ARMA) modelling of the arterial blood pressure (ABP)-cerebral blood flow velocity (CBFV) dynamic relationship, named ARMA-ARI, is also proposed. The susceptibility of both indices to physiological sources of variability was studied by performing measurements during spontaneous respiration (SR), and controlled breathing at 6, 10 and 15 breaths min(-1). ABP was measured non-invasively (Finapres), CBFV was recorded with Doppler ultrasound in both middle cerebral arteries and end-tidal CO2 (EtCO2) was estimated with an infrared capnograph. ARI and ARMA-ARI were calculated as a summary measure for the whole of each recording period, and also continuously, using a 60 s moving data window. Respiration did not have an effect on either of these indices, despite significant, but relatively small, reductions in EtCO2 at 10 and 15 bpm, compared to SR. Very significant differences were observed between ARI and ARMA-ARI in relation to their stability, variability and sensitivity to discriminate between subjects. For continuous estimates the coefficient of variation of ARI was 30 +/- 21% compared to 15 +/- 8% for ARMA-ARI (p < 0.000). The cumulative probability distributions were also significantly different for the two indices for each of the respiratory manoeuvres. The greater stability and reduced variability of ARMA-ARI, in relation to the classic ARI, suggest that the former should be used in future studies of dynamic autoregulation, mainly in situations where an improved temporal resolution might be required, such as the investigation of vaso-vagal syncope or the physiology of exercise.

Adult↗
PubMed2002

Continuous cardiac baroreceptor measurement during tilt in healthy elderly subjects.

INTRODUCTION: Abnormalities of cardiac baroreceptor sensitivity (BRS) may contribute towards the high prevalence of orthostatic hypotension and falls in the elderly. Most mathematical analyses used to determine BRS in the time or frequency domains assume a stationary physiological state and cannot be performed under dynamic change and therefore are not valid during tilt. We describe a new method of estimating BRS during tilt. METHODS: Twenty-five healthy elderly volunteers with a mean age of 69 +/- 3 years underwent head-up tilt to 70 degrees, within 5 s, 3 times on 2 separate visits. Blood pressure (BP) and heart rate were recorded continuously using a Finapres beat-to-beat BP monitor and surface ECG. A continuous estimate of BRS was obtained by combining beat-to-beat linear regression with Legendre polynomial interpolation. RESULTS: The values for supine BRS prior to tilt on the two visits, calculated using the new regression method (10.4 +/- 8.2 ms/mmHg and 12.5 +/- 9.7 ms/mmHg) were similar to those using fast Fourier analysis (10.7 +/- 6.7 ms/mmHg and 12.4 +/- 7.1 ms/mmHg). A rapid fall in BP and pulse interval along with cardiac BRS values occurred with tilt within the first 20 s of onset on both visits and remained reduced up to 90 s post tilt (p < 0.01). CONCLUSIONS: The values for cardiac BRS obtained using continuous time domain analysis agree well with those calculated using spectral methods and can be used to assess the dynamic changes in BRS to rapid perturbations in BP such as that occur with tilt.

Aged↗
PubMed2002

Assessment of dynamic cerebral autoregulation based on spontaneous fluctuations in arterial blood pressure and intracranial pressure.

Assessments of dynamic cerebral autoregulation usually measure the cerebral blood flow velocity (CBFV) response to changes in arterial blood pressure (ABP). We studied the effect of substituting ABP by cerebral perfusion pressure (CPP), expressed as the difference between ABP and intracranial pressure (ICP), in estimates of dynamic autoregulation obtained by transfer function analysis. CBFV, ABP and ICP were recorded during periods of physiological stability in 30 patients with severe head injury. Transfer function analysis was performed using the following combinations of input-output variables: ABP-CBFV, CPP-CBFV and CBFV-ICP. Frequency and time-domain (step response) functions were averaged for recordings with mean ICP < 20 mmHg (group A) and mean ICP > or = 20 mmHg (group B). The ABP-CBFV transfer function parameters and step response for group A were similar to previous studies in normal subjects, but group B showed deterioration of dynamic autoregulation. Radically different step responses were obtained from both groups for the CPP-CBFV transfer function and the coherence was not significantly improved. The CBFV-ICP transfer function had the highest values of coherence and indicates that changes in CBFV are the cause of spontaneous fluctuations in ICP. Furthermore, the ICP step response plateau was significantly higher for group B than for group A. An alternative calculation of the CBFV step response to changes in CPP resembled the corresponding responses for the ABP input. For spontaneous fluctuations in ABP, ICP and CBFV, it is not possible to calculate the CPP-CBFV transfer function directly due to the high positive correlation between ICP and CBFV, but an alternative estimate can be obtained by using the CBFV-ICP transfer function. The latter could also be useful as a method to assess intracranial compliance in head injury patients.

Adult↗
PubMed2002

Dynamic cerebral autoregulation and beat to beat blood pressure control are impaired in acute ischaemic stroke.

OBJECTIVES: Hypertension and chronic cerebrovascular disease are known to alter static cerebral autoregulation (CA) but the effects of acute stroke on dynamic CA (dCA) have not been studied in detail. Those studies to date measuring dCA have used sympathetically induced blood pressure (BP) changes, which may themselves directly affect dCA. This study assessed whether dCA is compromised after acute stroke using spontaneous blood pressure (BP) changes as the stimulus for the dCA response. METHODS: 56 patients with ischaemic stroke (aged 70 (SD 9) years), studied within 72 hours of ictus were compared with 56 age, sex, and BP matched normal controls. Cerebral blood flow velocity was measured using transcranial Doppler ultrasound (TCD) with non-invasive beat to beat arterial BP levels, surface ECG, and transcutaneous CO(2) levels and a dynamic autoregulatory index (dARI) calculated. RESULTS: Beat to beat BP, but not pulse interval variability was significantly increased and cardiac baroreceptor sensitivity (BRS) decreased in the patients with stroke. Dynamic CA was significantly reduced in patients with stroke compared with controls (strokes: ARI 3.8 (SD 2.2) and 3.2 (SD 2.0) for pressor and depressor stimuli respectively v controls: ARI 4.7 (SD 2.2) and 4.5 (SD 2.0) respectively (p<0.05 in all cases)). There was no difference between stroke and non-stroke hemispheres in ARI, which was also independent of severity of stroke, BP, BP variability, BRS, sex, and age. CONCLUSION: Dynamic cerebral autoregulation, as assessed using spontaneous transient pressor and depressor BP stimuli, is globally impaired after acute ischaemic stroke and may prove to be an important factor in predicting outcome.

Aged↗
PubMed2002

Deterioration in carotid baroreflex during carotid endarterectomy.

OBJECTIVE: Blood pressure instability after carotid endarterectomy (CEA) has been associated with a disturbance of the baroreflex control mechanism caused by the surgery in the carotid sinus region. The purpose of this study was to determine if a deterioration in carotid baroreceptors occurs during the surgery. METHOD: Heart rate (HR) and blood pressure (BP) were recorded continuously in 60 patients undergoing CEA as well as preoperatively and postoperatively at 2 days and 6 weeks. The baroreflex sensitivity was determined by cross-spectral analysis of HR and systolic blood pressure (SBP). During the surgery, three tests were used to assess the baroreflex response. The first test simulated a sudden fall in systemic blood pressure by clamping the common carotid artery. The second test simulated a rise in systemic blood pressure by applying pressure by using a rubbing action on the luminal surface of the carotid sinus region. The rub test was performed twice, once with the atheromatous plaque in situ and once when the plaque had been removed. The third test is clamp removal and restoration of blood flow through the carotid sinus. RESULTS: Carotid cross-clamping increased mean +/- standard error of the mean SBP from 117 +/- 3 mm Hg before clamping to 125 +/- 3 mm Hg (P <.05) at 30 beats after clamping. The first rub test with the plaque in situ decreased SBP from 121 +/- 3 mm Hg to 117 +/- 3 mm Hg (P <.01) at 10 beats after the rub test, indicating a functioning baroreceptor reflex. The second rub test increased SBP from 126 +/- 3 mm Hg to 128 +/- 3 mm Hg (P <.05). SBP dropped (P <.01) when unclamping suggesting a selective alteration of the baroreflex sensitivity. The baroreflex sensitivity was significantly reduced 2 days postoperatively when compared to preoperative values (P <.05). CONCLUSIONS: These findings suggest that the act of plaque removal could be associated with a partial disruption of baroreceptor mechanism in the carotid artery. This could affect type I baroreceptors.

Aged↗
PubMed2001

Cerebral autoregulatory responses to head-up tilt in normal subjects and patients with recurrent vasovagal syncope.

BACKGROUND: The effect of orthostatic stress on dynamic cerebral autoregulation (CA) in normal subjects and patients with recurrent vasovagal syncope (VVS) is unclear. This study assessed the dynamic CA responses of both groups to head-up tilt. METHODS AND RESULTS: Seventeen patients with recurrent VVS and 17 pair-matched control subjects underwent 70 degrees head-up tilt for up to 30 minutes. Bilateral middle cerebral artery blood flow velocities (CBFV) were measured with transcranial Doppler ultrasound along with noninvasive beat-to-beat blood pressure (BP), heart rate, and transcutaneous and end-tidal CO(2) concentrations. Indices of dynamic CA were derived for periods before, during, and after tilt. Eight normal subjects who developed VVS in an identical protocol but who had no previous clinical history of syncope were also studied. CBFV and transcutaneous and end-tidal CO(2) levels declined significantly during head-up tilt in all groups (P<0.0001). Dynamic CA indices were unchanged throughout tilt in nonsyncopal control subjects and were initially unchanged in patients but deteriorated significantly in patients and syncopal control subjects in the minutes before (P=0.027 and P=0.012, respectively) and after (P=0.002 and P=0.007, respectively) syncope. CONCLUSIONS: Dynamic CA is preserved in patients and control subjects initially after head-up tilt. Autoregulatory function remains intact in nonsyncopal control subjects during prolonged orthostasis but deteriorates in patients and syncopal control subjects immediately before and after VVS.

Blood Flow Velocity↗
PubMed2001

A parametric approach to measuring cerebral blood flow autoregulation from spontaneous variations in blood pressure.

Autoregulation maintains cerebral blood flow (CBF) almost constant in the face of changes in arterial blood pressure (ABP). Tests for impairment of this process using only spontaneous fluctuations in ABP, without provoking large variations, are of great clinical interest, and a range of different approaches have previously been applied. Extending earlier work based on linear filters, we propose a simple parametric method using a first order finite impulse response filter. We evaluate the method on ABP and CBF velocity [(CBFV), from trancranial Doppler ultrasound] signals collected in 60 patients with stenosis or occlusion of the carotid arteries. Data were collected during the inspiration of ambient air, a 5% CO2/air mixture, and finally the return to ambient air. Equivalent data were collected in 15 normal subjects. The filters estimated from the data segments with constant inspiratory pCO2 showed the expected high-pass characteristic, which was reduced during hypercapnia and also in patients. Highly significant correlation between the filter parameters and cerebrovascular reactivity (percent increase in CBFV per unit change in end-tidal pCO2) gives further evidence that the filters reflect autoregulation. The method allows simple parametrization of the dynamic autoregulatory responses in CBFV, and the analysis of short (1 min) data segments.

Aged↗
PubMed2001

Cerebral blood flow velocity response to induced and spontaneous sudden changes in arterial blood pressure.

The influence of different types of maneuvers that can induce sudden changes of arterial blood pressure (ABP) on the cerebral blood flow velocity (CBFV) response was studied in 56 normal subjects (mean age 62 yr, range 23-80). ABP was recorded in the finger with a Finapres device, and bilateral recordings of CBFV were performed with Doppler ultrasound of the middle cerebral arteries. Recordings were performed at rest (baseline) and during the thigh cuff test, lower body negative pressure, cold pressor test, hand grip, and Valsalva maneuver. From baseline recordings, positive and negative spontaneous transients were also selected. Stability of PCO2 was monitored with transcutaneous measurements. Dynamic autoregulatory index (ARI), impulse, and step responses were obtained for 1-min segments of data for the eight conditions by fitting a mathematical model to the ABP-CBFV baseline and transient data (Aaslid's model) and by the Wiener-Laguerre moving-average method. Impulse responses were similar for the right- and left-side recordings, and their temporal pattern was not influenced by type of maneuver. Step responses showed a sudden rise at time 0 and then started to fall back to their original level, indicating an active autoregulation. ARI was also independent of the type of maneuver, giving an overall mean of 4.7 +/- 2.9 (n = 602 recordings). Amplitudes of the impulse and step responses, however, were significantly influenced by type of maneuver and were highly correlated with the resistance-area product before the sudden change in ABP (r = -0.93, P < 0.0004). These results suggest that amplitude of the CBFV step response is sensitive to the point of operation of the instantaneous ABP-CBFV relationship, which can be shifted by different maneuvers. Various degrees of sympathetic nervous system activation resulting from different ABP-stimulating maneuvers were not reflected by CBFV dynamic autoregulatory responses within the physiological range of ABP.

Adult↗
PubMed2001

Carbon dioxide, critical closing pressure and cerebral haemodynamics prior to vasovagal syncope in humans.

The cerebrovascular changes that occur prior to vasovagal syncope (VVS) are unclear, with both increases and decreases in cerebrovascular resistance being reported during pre-syncope. This study assessed the cerebrovascular responses, and their potential underlying mechanisms, that occurred before VVS induced by head-up tilt (HUT). Groups of 65 normal subjects with no previous history of syncope and of 16 patients with recurrent VVS were subjected to 70 degrees HUT for up to 30 min. Bilateral middle cerebral artery (MCA) cerebral blood flow velocities (CBFVs) were measured using transcranial Doppler ultrasound, along with simultaneous measures of MCA blood pressure, heart rate, and end-tidal and transcutaneous carbon dioxide concentrations. All 16 patients and 14 of the control subjects developed VVS during HUT. During pre-syncope, mean CBFV declined, due predominantly to a decrease in diastolic rather than systolic CBFV (decreases of 44.5+/-19.8% and 6.3+/-12.9% respectively; P<0.0001). CO(2) levels and indices of cerebrovascular resistance decreased during pre-syncope, while critical closing pressure (CrCP) increased to levels approaching MCA diastolic blood pressure before decreasing precipitously on syncope. Pre-syncopal changes were similar in syncopal patients and syncopal controls. CrCP, therefore, rises during pre-syncope, possibly related to progressive hypocapnia, and may account for the relatively greater fall in diastolic CBFV. Falls in cerebrovascular resistance, therefore, may be offset by rises in CrCP due to hypocapnia, leading to diminished cerebral blood flow during pre-syncope.

Adolescent↗
PubMed2000

Estimating normal and pathological dynamic responses in cerebral blood flow velocity to step changes in end-tidal pCO2.

The regulation of cerebral blood flow (CBF) following changes in arterial blood pressure (ABP) and end-tidal pCO2 (EtCO2) are of clinical interest in assessing cerebrovascular reserve capacity. Linear finite-impulse-response modelling is applied to ABP, EtCO2 and CBF velocity (CBFV, from transcranial Doppler measurements), which allows the CBFV response to ideal step changes in EtCO2 to be estimated from clinical data showing more sluggish, and additional random variations. The confounding effects of ABP changes provoked by hypercapnia on the CBFV are also corrected for. Data from 56 patients suffering from stenosis of the carotid arteries (with normal or diminished cerebrovascular reactivity to EtCO2 changes--CVRCO2) were analysed. The results show the expected significant differences (p < 0.05) between EtCO2 steps up and down, the significant contribution from ABP variation, and also differences in the dynamic responses of patients with reduced CVRCO2 (p < 0.01 after 10 s). For the latter the CBFV response appears exhausted after about 15 s, whereas for normals CBFV continues to increase. While dispersion of individual step responses remains large, the method gives encouraging results for the non-invasive study of compromised haemodynamics in different patient groups.

Aged↗
PubMed2000

Grading of cerebral autoregulation in preterm and term neonates.

The response of cerebral blood flow velocity to a single spontaneous transient rise in blood pressure was studied to grade the cerebral autoregulatory response of newborns. Blood pressure was measured continuously through an umbilical or peripheral arterial catheter; continuous flow velocity recordings were taken from the middle cerebral artery using continuous wave Doppler ultrasound. From a cohort of 62 healthy term and preterm neonates, 325 transients in mean arterial blood pressure and mean cerebral blood flow velocity were identified for analysis using a foot-seeking algorithm. An initial classification of active or impaired autoregulation was given to each transient using a self-clustering technique. The grading of the transients was studied by examining the slope of the return of the cerebral blood flow velocity to baseline. Negative slopes indicate a normal autoregulation; slopes of 0 or greater indicate an absence of autoregulation. This classification was in agreement with the self-clustering method (Cohen's kappa = 0.94, P<0.0001). The relationship between the autoregulatory response assessed by the grading method and gestational age, postnatal age, and PCO(2) was examined using linear regression analysis. A significant relationship with gestational age (P = 0.002) but not PCO(2) (P = 0.06) or postnatal age (P = 0.14) was evident.

Blood Flow Velocity↗
PubMed2000

Multivariate dynamic analysis of cerebral blood flow regulation in humans.

The contributions of beat-to-beat changes in mean arterial blood pressure (MABP) and breath-by-breath fluctuations in end-tidal CO2 (EtCO2) as determinants of the spontaneous variability of cerebral blood flow velocity (CBFV) were studied in 16 normal subjects at rest. The two input variables (MABP and EtCO2) had significant cross-correlations with CBFV but not between them. Transfer functions were estimated as the multivariate least mean square finite impulse response causal filters. MABP showed a very significant effect in explaining CBFV variability (p < 10(-11), Fisher's aggregated-p test) and the model mean square error was significantly reduced (p < 0.001) by also including the contribution of EtCO2. The estimated mean CBFV step response to MABP displayed the characteristic return to baseline caused by the cerebral autoregulatory response. The corresponding response to EtCO2 showed a gradual rise taking approximately 10 s to reach a plateau of 2.5%/mmHg. This study demonstrated that spontaneous fluctuations in EtCO2 can help to explain the CBFV variability at rest if appropriate signal processing techniques are employed to address the limited power and bandwith of the breath-by-breath EtCO2 signal.

Adult↗