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

Normative cerebral microvascular blood flow waveform morphology assessed with diffuse correlation spectroscopy

Microvascular cerebral blood flow exhibits pulsatility at the cardiac frequency that carries valuable information about cerebrovascular health. This study used diffuse correlation spectroscopy to quantify normative features of these waveforms in a cohort of thirty healthy adults. We demonstrate they are sensitive to changes in vascular tone, as indicated by pronounced morphological changes with hypercapnia. Further, we observe significant sex-based differences in waveform morphology, with females exhibiting higher flow, greater area-under-the-curve, and lower pulsatility. Finally, we quantify normative values for cerebral critical closing pressure, i.e., the minimum pressure required to maintain flow in a given vascular region.

A comparison of the time constant of the cerebral arterial bed using invasive and non-invasive arterial blood pressure measurements

OBJECTIVE

The time constant of the cerebral arterial bed (τ), which is an index of brain haemodynamics, can be estimated in patients using continuous monitoring of arterial blood pressure (ABP), transcranial Doppler cerebral blood flow velocity (CBFV) and intracranial pressure (ICP) if these measures are available. But, in some clinical scenarios invasive measurement of ABP is not feasible. Therefore, in this study we aimed to investigate whether invasive ABP can be replaced with non-invasive ABP, monitored using the Finapres photoplethysmograph (fABP).

APPROACH

Forty-six recordings of ICP, ABP, fABP, and CBFV in the right and left middle cerebral arteries were performed daily for approximately 30 min in 10 head injury patients. Two modelling approaches (constant flow forward [CFF, pulsatile blood inflow and steady blood outflow] and pulsatile flow forward [PFF, where both blood inflow and outflow are pulsatile]) were applied to estimate τ using either invasive ABP (τ_CFF, τ_PFF) or non-invasive ABP (fτ_CFF, fτ_PFF).

MAIN RESULTS

Bland-Altman analysis showed quite poor agreement between the fτ and τ methods of estimation. The fτ method produced significantly higher values than the τ method when calculated using both the CFF and PFF models (p < .001 for both). The correlation between fτ_CFF and τ_CFF was moderately high (r _s = 0.63; p < .001), whereas that between fτ_PFF and τ_PFF was weaker (r _s = 0.40; p = .009).

SIGNIFICANCE

Our results suggest that using non-invasive ABP for estimation of τ is inaccurate in head injury patients.

A noninvasive estimation of cerebral perfusion pressure using critical closing pressure

OBJECT

Cerebral blood flow is associated with cerebral perfusion pressure (CPP), which is clinically monitored through arterial blood pressure (ABP) and invasive measurements of intracranial pressure (ICP). Based on critical closing pressure (CrCP), the authors introduce a novel method for a noninvasive estimator of CPP (eCPP).

METHODS

Data from 280 head-injured patients with ABP, ICP, and transcranial Doppler ultrasonography measurements were retrospectively examined. CrCP was calculated with a noninvasive version of the cerebrovascular impedance method. The eCPP was refined with a predictive regression model of CrCP-based estimation of ICP from known ICP using data from 232 patients, and validated with data from the remaining 48 patients.

RESULTS

Cohort analysis showed eCPP to be correlated with measured CPP (R = 0.851, p < 0.001), with a mean ± SD difference of 4.02 ± 6.01 mm Hg, and 83.3% of the cases with an estimation error below 10 mm Hg. eCPP accurately predicted low CPP (< 70 mm Hg) with an area under the curve of 0.913 (95% CI 0.883-0.944). When each recording session of a patient was assessed individually, eCPP could predict CPP with a 95% CI of the SD for estimating CPP between multiple recording sessions of 1.89-5.01 mm Hg.

CONCLUSIONS

Overall, CrCP-based eCPP was strongly correlated with invasive CPP, with sensitivity and specificity for detection of low CPP that show promise for clinical use.

Multivariate modeling of cognitive-motor stimulation on neurovascular coupling: transcranial Doppler used to characterize myogenic and metabolic influences

Neural activation induces changes in cerebral blood flow velocity (CBFV) with separate contributions from resistance-area product (VRAP) and critical closing pressure (VCrCP). We modeled the dependence of VRAP and VCrCP on arterial blood pressure (ABP), end-tidal CO2 (EtCO2), and cognitive stimulation to test the hypothesis that VRAP reflects myogenic activity while VCrCP reflects metabolic pathways. In 14 healthy subjects, CBFV was measured with transcranial Doppler ultrasound, ABP with the Finapres device and EtCO2 with infrared capnography. Two different paradigms (word or puzzle) were repeated 10 times (30 s on-off), and the corresponding square-wave signal was used, together with ABP and EtCO2, as inputs to autoregressive-moving average (ARMA) models, which allowed identification of the separate contributions of the three inputs to either VRAP or VCrCP. For both paradigms, the contribution of ABP was mainly manifested through VRAP ( P < 0.005 for word; P < 0.004 for puzzle), while stimulation mainly contributed to VCrCP ( P < 0.002 for word; P < 0.033, for puzzle). The contribution of EtCO2 was relatively small (<10%) with greater contribution to VCrCP ( P < 0.01 for puzzle; not significant for word). Separate step responses were also obtained for each of the three inputs. ARMA modeling of VRAP and VCrCP allows the separation of the effects of cerebral autoregulation and CO2 reactivity from the main effects of cognitive-motor stimulation and have the potential to improve the diagnostic value of neurovascular coupling testing in physiological and clinical studies.

The influence of calculation method on estimates of cerebral critical closing pressure

The critical closing pressure (CrCP) of cerebral circulation is normally estimated by extrapolation of instantaneous velocity-pressure curves. Different methods of estimation were analysed to assess their robustness and reproducibility in both static and dynamic applications. In ten healthy subjects (mean ± SD age 37.5 ± 9.2 years) continuous recordings of arterial blood pressure (BP, Finapres) and bilateral cerebral blood flow velocity (transcranial Doppler ultrasound, middle cerebral arteries) were obtained at rest. Each session consisted of three separate 5 min recordings. A total of four recording sessions for each subject took place over a 2 week period. A total of 117 recordings contained 34 014 cardiac cycles. For each cardiac cycle, CrCP and resistance-area product (RAP) were estimated using linear regression (LR), principal component analysis (PCA), first harmonic fitting (H1), 2-point systolic/diastolic values (2Ps) and 2-point mean/diastolic values (2Pm). LR and PCA were also applied using only the diastolic phase (LRd, PCAd). The mean values of CrCP and RAP for the entire 5 min recording ('static' condition) were not significantly different for LRd, PCAd, H1 and 2Pm, as opposed to the other methods. The same four methods provided the best results regarding the absence of negative values of CrCP and the coefficient of variation (CV) of the intra-subject standard error of the mean (SEM). On the other hand, 'dynamic' applications, such as the transfer function between mean BP and RAP (coherence and RAP step response) led to a different ranking of methods, but without significant differences in CV SEM coherence. For the CV of the RAP step response though, LRd and PCAd performed badly. These results suggest that H1 or 2Pm perform better than LR analysis and should be used for the estimation of CrCP and RAP for both static and dynamic applications.

Effects of acetazolamide and dexamethasone on cerebral hemodynamics in hypoxia

Previous attempts to detect global cerebral hemodynamic differences between those who develop headache, nausea, and fatigue following rapid exposure to hypoxia [acute mountain sickness (AMS)] and those who remain healthy have been inconclusive. In this study, we investigated the effects of two drugs known to reduce symptoms of AMS to determine if a common cerebral hemodynamic mechanism could explain the prophylactic effect within individuals. With the use of randomized, placebo-controlled, double-blind, crossover design, 20 healthy volunteers were given oral acetazolamide (250 mg), dexamethasone (4 mg), or placebo every 8 h for 24 h prior to and during a 10-h exposure to a simulated altitude of 4,875 m in a hypobaric chamber, which included 2 h of exercise at 50% of altitude-specific VO(2max). Cerebral hemodynamic parameters derived from ultrasound assessments of dynamic cerebral autoregulation and vasomotor reactivity were recorded 15 h prior to and after 9 h of hypoxia. AMS symptoms were scored using the Lake Louise Questionnaire (LLQ). It was found that both drugs prevented AMS in those who became ill on placebo (~70% decrease in LLQ), yet a common cerebral hemodynamic mechanism was not identified. Compared with placebo, acetazolamide reduced middle cerebral artery blood flow velocity (11%) and improved dynamic cerebral autoregulation after 9 h of hypoxia, but these effects appeared independent of AMS. Dexamethasone had no measureable cerebral hemodynamic effects in hypoxia. In conclusion, global cerebral hemodynamic changes resulting from hypoxia may not explain the development of AMS.

Parametric versus nonparametric transfer function estimation of cerebral autoregulation from spontaneous blood-pressure oscillations

Cerebral autoregulation (CAR) is a control mechanism of the brain keeping cerebral blood flow constant albeit the arterial blood pressure varies. Impaired CAR may be associated with an increased risk of cerebral ischemic events in patients with obstructive cerebrovascular disease. Spontaneous blood pressure oscillations are analyzed using a nonparametric and two parametric transfer function estimators, i.e. the autoregressive-moving-average model with exogenous inputs or the vector-autoregressive model. Performance of the methods was compared using data from patients with unilateral stenosis or occlusion. We also analyzed reproducibility by comparing partitions of the data an with data from other patients which have been measured twice. Results show that there is no significant difference between methods (ANOVA, p > 0.27), and that CAR measurements can be performed reproducibly (Kendall's tau, p < 0.0016) by all three methods. In conclusion, CAR measurements by means of spontaneous oscillations can be obtained stably and the presented parametric approaches can serve for future online application of CAR measurement.

Critical closing pressure: comparison of three methods

Critical closing pressure (CCP) is an arterial pressure threshold below which small arterial vessels collapse. Our aim was to compare different methods to estimate CCP in the cerebrovascular circulation using the relationships between transcranial Doppler flow velocity (FV), laser-Doppler flux (LDF), and arterial blood pressure (ABP). A total of 116 experiments in rabbits were analyzed retrospectively. At the end of each recording, cardiac arrest (CA) was induced. Arterial blood pressure in femoral artery, basilar artery FV, cortical blood LDF, intracranial pressure (ICP) was recorded. Critical closing pressure was estimated using linear regression between decreasing mean ABP values, FV, and LDF during CA. In addition, CCP was calculated from FV waveform just before CA. The correlation between CCP evaluated using LDF and FV during CA was 0.98 (P<0.0001). The correlation between CCP measured during CA and CCP estimated from the transcranial Doppler ultrasonography (TCD) waveform was weaker (R=0.39; P<0.001), with CCP calculated from waveform being significantly greater than CCP from CA (median difference 9 mm Hg; P<0.003). Critical closing pressures obtained from FV waveform and CA correlated with mean ICP before CA (R=0.40; P=0.001). In conclusion strong correlation exists between CCP values obtained by means of FV and LDF during cardiac arrest. However, predictions of CCP using TCD waveform analysis show substantial differences from values of CCP recorded during cardiac arrest.

The effects of hypocapnia and the cerebral autoregulatory response on cerebrovascular resistance and apparent zero flow pressure during isoflurane anesthesia

BACKGROUND

Simultaneous recordings of arterial blood pressure (ABP) and middle cerebral artery blood velocity can be used to calculate the apparent zero flow pressure (aZFP). The inverse of the slope of the pressure-velocity relationship is known as resistance area product (RAP) and is an index of cerebrovascular resistance. There is little information available regarding the effects of vasoactive drugs, arterial carbon dioxide (Paco(2)), and impaired cerebral autoregulation on aZFP and RAP during general anesthesia. During isoflurane anesthesia, we investigated the effects of hypocapnia and the effects of a phenylephrine infusion, on aZFP and RAP.

METHODS

Radial ABP and transcranial Doppler middle cerebral artery blood velocity signals were recorded in 11 adults undergoing isoflurane anesthesia. A phenylephrine infusion was used to increase ABP and ventilation was adjusted to control Paco(2). Cerebral hemodynamic variables were compared at two levels of mean ABP (approximately 80 and 100 mm Hg) and at two levels of Paco(2): normocapnia (Paco(2) 38-43 mm Hg) and hypocapnia (Paco(2) 27-34 mm Hg). Two aZFP analysis methods were compared: one based on linear regression and one based on Fourier analysis of the waveforms.

RESULTS

At the lower ABP, aZFP was 23 +/- 11 mm Hg and 30 +/- 13 mm Hg (mean +/- sd) with normocapnia and hypocapnia, respectively (P < 0.001) and RAP was 0.76 +/- 0.97 mm Hg x s x cm(-1) and 1.16 +/- 0.16 mm Hg x s x cm(-1) with normocapnia and hypocapnia, respectively (P < 0.001). Similar effects of hypocapnia were seen at the higher ABP. With normocapnia, isoflurane impaired cerebral autoregulation and aZFP did not change with the increase in ABP. With hypocapnia, cerebral autoregulation was not significantly impaired and increasing ABP was associated with increased aZFP (from 30 +/- 13 to 35 +/- 13 mm Hg, P < 0.01) and increased RAP (from 1.16 +/- 0.16 to 1.52 +/- 0.20 mm Hg x s x cm(-1), P < 0.001). Calculation of the relative contributions of aZFP and RAP to the cerebral hemodynamic responses indicated that changes in RAP appeared to have a greater influence than changes in aZFP. The mean difference between the two methods of determining aZFP (Fourier-regression) was 0.5 +/- 3.6 mm Hg (mean +/- 2sd).

CONCLUSIONS

During isoflurane anesthesia, two interventions that increase cerebral arteriolar tone, hypocapnia and the autoregulatory response to increasing ABP, were associated with increased RAP and increased aZFP. The effect of changes in RAP appeared to be quantitatively greater than the effects of changes in aZFP. These results imply that arteriolar tone influences cerebral blood flow by controlling both resistance and effective downstream pressure.