Characterization of cerebral macro- and microvascular hemodynamics during transient hypotension

Regional cerebral pulsatile hemodynamics during isocapnic and poikilocapnic hyperthermia in young men

Hyperthermia is known to induce hypocapnia-driven reductions in cerebral blood flow; however, it is unknown if it causes changes in hemodynamic pulsatility that negatively influence cerebrovascular function. This retrospective analysis aimed to assess cerebrovascular hemodynamic pulsatile buffering (damping factor; DFi) during poikilocapnic (HT) and isocapnic (HT-C) hyperthermia. We hypothesized that HT would reduce cerebral DFi, while HT-C would attenuate the reduction in DFi by limiting increases in resistance. Ten healthy males were passively heated +2°C from normothermia (BL). Blood flow through the internal carotid artery (ICA) and vertebral artery (VA) was measured using vascular ultrasound. Blood velocity through the middle cerebral artery (MCA) and the posterior cerebral artery (PCA) was measured using transcranial ultrasound. DFi was calculated as the ratio of proximal to distal pulsatility index (PI): Anterior cerebral DFi = PIICA/PIMCA; Posterior cerebral DFi = PIVA/PIPCA. Anterior DFi decreased in both HT (1.08 ± 0.19 a.u; p = 0.007) and HT-C (1.12 ± 0.231 a.u; p = 0.021) conditions from BL values (1.27 ± 0.14 a.u). No changes were observed in posterior DFi, p = 0.116. Irrespective of PaCO2 clamping, both hyperthermic conditions reduced anterior DFi, suggesting other mechanisms are responsible for cerebrovascular hemodynamic buffering. Posterior DFi responses were not observed, suggesting preferential buffering of the hyperthermic posterior circulation (VA-PCA).

Cerebrovascular compliance during progressive hypotension in patients with autonomic failure

The compliant nature of cerebral blood vessels may represent an important mechanical protection for sustained cerebral perfusion during reductions in arterial blood pressure (ABP). However, whether the rise in cerebrovascular compliance (Ci) with falling ABP persists and exhibits a threshold effect remains unknown. Therefore, we analyzed Ci changes during graded head-up tilt (HUT) in individuals with autonomic failure (AF), a group that tolerates graded and progressive reductions in ABP. Finger ABP and middle cerebral artery blood velocity (MCAv) were recorded from five patients with AF (61 ± 22 yr) at supine rest and during graded HUT. Tilt gradients increased incrementally between 30, 45, and 60° every 5 min until ABP reached a critically low value. The total time in HUT was 11 ± 4 min. Every 5 s during supine and HUT, individual ABP and MCAv waveforms were assessed for Ci and cerebrovascular resistance (CVR) using a modified Windkessel model. Pulse pressure (PP) was calculated as systolic ABP - diastolic ABP. A threshold value for the increase in Ci was determined using breakpoint analysis of the linear relationship between changes in Ci and PP or ABP across tilt periods. Graded HUT resulted in reduced ABP, PP, CVR, and mean MCAv, and increased Ci (all P < 0.01). Ci began to increase progressively after PP fell by 22 ± 6 mmHg and ABP fell by 20 ± 11 mmHg. In conclusion, the increase in Ci during progressive hypotension exhibited a threshold effect and persisted as ABP continued to fall.NEW & NOTEWORTHY We identify a threshold effect for the increase in cerebrovascular compliance (Ci) during progressive hypotension (baseline vs. end-tilt: 86 ± 18 vs. 50 ± 8 mmHg) in individuals with autonomic failure, such that Ci began to increase progressively after pulse pressure fell by 22 ± 6 mmHg and arterial blood pressure fell by 20 ± 11 mmHg.

Lawrence K. All-optics technique for monitoring absolute cerebral blood flow: validation against magnetic resonance imaging perfusion

Significance

The ability to monitor cerebral blood flow (CBF) at the bedside is essential to managing critical-care patients with neurological emergencies. Diffuse correlation spectroscopy (DCS) is ideal because it is non-invasive, portable, and inexpensive. We investigated a near-infrared spectroscopy (NIRS) approach for converting DCS measurements into physiological units of blood flow.

Aim

Using magnetic resonance imaging perfusion as a reference, we investigated the accuracy of absolute CBF measurements from a bolus-tracking NIRS method that used transient hypoxia as a flow tracer and hypercapnia-induced increases in CBF measured by DCS.

Approach

Twelve participants (7 female, 28 ± 6 years) completed a hypercapnia protocol with simultaneous CBF recordings from DCS and arterial spin labeling (ASL). Nine participants completed the transient hypoxia protocol while instrumented with time-resolved NIRS. The estimate of baseline CBF was subsequently used to calibrate hypercapnic DCS data.

Results

Moderately strong correlations at baseline ( slope = 0.79 andR 2 = 0.59 ) and during hypercapnia ( slope = 0.90 andR 2 = 0.58 ) were found between CBF values from calibrated DCS and ASL (range 34 to 85    mL / 100    g / min ).

Conclusions

Results demonstrated the feasibility of an all-optics approach that can both quantify CBF and perform continuous perfusion monitoring.

Rapid changes in cerebrovascular compliance during vasovagal syncope

PURPOSE

The compensatory mechanisms supporting cerebral perfusion throughout head-up tilt (HUT) in patients with vasovagal syncope (VVS) remain unclear. We tested the hypothesis that increased cerebrovascular compliance (Ci) and decreased cerebrovascular resistance (CVR) support cerebral blood velocity (CBV) during pre-syncope in VVS.

METHODS

Finger arterial blood pressure (ABP) and right middle cerebral artery blood velocity (CBV) were recorded in 15 individuals diagnosed with VVS (n = 11 female, mean age: 40 ± 16 years, mean body mass index: 24.9 ± 4.0 kg/m2) at supine rest and during HUT (80 degree angle). Individual ABP and CBV waveforms during VVS were input into a modified Windkessel model to calculate Ci and ohmic CVR. Gosling's pulsatility index (Pi; pulse amplitude/mean CBV) was calculated.

RESULTS

Diastolic ABP, systolic ABP, mean ABP (72 ± 11 to 51 ± 12 mmHg), and CVR decreased progressively during presyncope (all P ≤ 0.04). As expected, systolic CBV was sustained (all P ≥ 0.29) while diastolic and mean CBV (51 ± 13 to 38 ± 13 mmHg) fell during presyncope (all P ≤ 0.04). Both Ci and Pi increased during presyncope (128 ± 97 and 60 ± 41%, respectively; all P ≤ 0.049) and were positively correlated (R2 = 0.79, P < 0.01). Increased Ci contributed to changes in mean CBV (P < 0.01) but decreased CVR did not (P = 0.28).

CONCLUSIONS

These data provide evidence that Ci increases during presyncope in patients with VVS and is likely involved in the maintenance of systolic CBV during a fall in diastolic CBV. However, this regulation is not sufficient to preserve CBV in the presence of such extreme and progressive reductions in ABP.

A Quantitative Assessment of Cerebral Hemodynamic Perturbations Associated with Long R-R Intervals in Atrial Fibrillation: A Pilot-Case-Based Experience

Background and Objectives: Atrial fibrillation (AF) results in systemic hemodynamic perturbations which impact cerebral circulation, possibly contributing to the development of dementia. However, evidence documenting effects in cerebral perfusion is scarce. The aim of this study is to provide a quantitative characterization of the magnitude and time course of the cerebral hemodynamic response to the short hypotensive events associated with long R-R intervals, as detected by near-infrared spectroscopy (NIRS). Materials and Methods: Cerebral NIRS signals and arterial blood pressure were continuously recorded along with an electrocardiogram in twelve patients with AF undergoing elective electrical cardioversion (ECV). The top 0.5-2.5% longest R-R intervals during AF were identified in each patient and used as triggers to carry out the triggered averaging of hemodynamic signals. The average curves were then characterized in terms of the latency, magnitude, and duration of the observed effects, and the possible occurrence of an overshoot was also investigated. Results: The triggered averages revealed that long R-R intervals produced a significant drop in diastolic blood pressure (-13.7 ± 6.1 mmHg) associated with an immediate drop in cerebral blood volume (THI: -0.92 ± 0.46%, lasting 1.9 ± 0.8 s), followed by a longer-lasting decrease in cerebral oxygenation (TOI: -0.79 ± 0.37%, lasting 5.2 ± 0.9 s, p < 0.01). The recovery of the TOI was generally followed by an overshoot (+1.06 ± 0.12%). These effects were progressively attenuated in response to R-R intervals of a shorter duration. Conclusions: Long R-R intervals cause a detectable and consistent cerebral hemodynamic response which concerns both cerebral blood volume and oxygenation and outlasts the duration of the systemic perturbation. These effects are compatible with the activation of dynamic autoregulatory mechanisms in response to the hypotensive stimulus.

Protease-Activated Receptors (PARs): Biology and Therapeutic Potential in Perioperative Stroke

Perioperative stroke is a devastating complication that occurs during surgery or within 30 days following the surgical procedure. Its prevalence ranges from 0.08 to 10% although it is most likely an underestimation, as sedatives and narcotics can substantially mask symptomatology and clinical presentation. Understanding the underlying pathophysiology and identifying potential therapeutic targets are of paramount importance. Protease-activated receptors (PARs), a unique family of G-protein-coupled receptors, are widely expressed throughout the human body and play essential roles in various physiological and pathological processes. This review elucidates the biology and significance of PARs, outlining their diverse functions in health and disease, and their intricate involvement in cerebrovascular (patho)physiology and neuroprotection. PARs exhibit a dual role in cerebral ischemia, which underscores their potential as therapeutic targets to mitigate the devastating effects of stroke in surgical patients.