Hyperventilation, cerebral perfusion, and syncope

Effects of caffeine on cerebral blood flow

OBJECTIVE

The objective of the present study is to evaluate whether, after caffeine ingestion, there are variations in blood velocity of the middle cerebral arteries in clinically healthy young people as well as to evaluate whether this variation is dependent on the administered dose.

METHODS

We used transcranial Doppler ultrasonography to record blood velocities of the middle cerebral arteries in three groups of 15 clinically healthy young adults each: no caffeine, a45 mg, and 120 mg of caffeine groups. Transcranial Doppler ultrasonography provided simultaneous bilateral velocity of the middle cerebral arteries measurements while participants performed functional tests (hyperventilation and hypoventilation orders) and three cognitive activities (test 1, short-term memory; test 2, solving a vocabulary problem; and test 3, solving a math problem) each in 31-s tests with 1-min rests between them. Participants were assessed before and 30 min after caffeine ingestion.

RESULTS

There was a significant decrease in mean velocity, peak systolic velocity, end-diastolic velocity, and heart rate after high caffeine intake, except in hyperventilation, which was only observed in peak systolic velocity. With the intake of a lower dose, significant decreases were seen with hypoventilation and with test 1. In hyperventilation, there was only a significant decrease in end-diastolic velocity and heart rate; in test 2, it was found in mean velocity and peak systolic velocity; and in test 3, only in heart rate.

CONCLUSION

With this study, we conclude that caffeine influences the cardiovascular system acutely, interfering with the velocity of the middle cerebral arteries, causing its decrease. We also conclude that this acute effect causes vasodilation of the cerebral arteries, more accentuated with higher doses of caffeine.

Applications of flexible electronics related to cardiocerebral vascular system

Ensuring accessible and high-quality healthcare worldwide requires field-deployable and affordable clinical diagnostic tools with high performance. In recent years, flexible electronics with wearable and implantable capabilities have garnered significant attention from researchers, which functioned as vital clinical diagnostic-assisted tools by real-time signal transmission from interested targets in vivo. As the most crucial and complex system of human body, cardiocerebral vascular system together with heart-brain network attracts researchers inputting profuse and indefatigable efforts on proper flexible electronics design and materials selection, trying to overcome the impassable gulf between vivid organisms and rigid inorganic units. This article reviews recent breakthroughs in flexible electronics specifically applied to cardiocerebral vascular system and heart-brain network. Relevant sensor types and working principles, electronics materials selection and treatment methods are expounded. Applications of flexible electronics related to these interested organs and systems are specially highlighted. Through precedent great working studies, we conclude their merits and point out some limitations in this emerging field, thus will help to pave the way for revolutionary flexible electronics and diagnosis assisted tools development.

High-intensity exercise does not protect against orthostatic intolerance following bedrest in 55- to 65-yr-old men and women

Prolonged bedrest provokes orthostatic hypotension and intolerance of upright posture. Limited data are available on the cardiovascular responses of older adults to head-up tilt following bedrest, with no studies examining the potential benefits of exercise to mitigate intolerance in this age group. This randomized controlled trial of head-down bedrest (HDBR) in 55- to 65-yr-old men and women investigated if exercise could avert post-HDBR orthostatic intolerance. Twenty-two healthy older adults (11 female) underwent a strict 14-day HDBR and were assigned to either an exercise (EX) or control (CON) group. The exercise intervention included high-intensity, aerobic, and resistance exercises. Head-up tilt-testing to a maximum of 15 minutes was performed at baseline (Pre-Bedrest) and immediately after HDBR (R1), as well as 6 days (R6) and 4 weeks (R4wk) later. At Pre-Bedrest, three participants did not complete the full 15 minutes of tilt. At R1, 18 did not finish, with no difference in tilt end time between CON (422 ± 287 s) and EX (409 ± 346 s). No differences between CON and EX were observed at R6 or R4wk. At R1, just 1 participant self-terminated the test with symptoms, while 12 others reported symptoms only after physiological test termination criteria were reached. Finishers on R1 protected arterial pressure with higher total peripheral resistance relative to Pre-Bedrest. Cerebral blood velocity decreased linearly with reductions in arterial pressure, end-tidal CO2, and cardiac output. High-intensity interval exercise did not benefit post-HDBR orthostatic tolerance in older adults. Multiple factors were associated with the reduction in cerebral blood velocity leading to intolerance.

Cerebral Oxygenation Responses to Standing in Young Patients with Vasovagal Syncope

Vasovagal syncope (VVS) is common in young adults and is attributed to cerebral hypoperfusion. However, during active stand (AS) testing, only peripheral and not cerebral hemodynamic responses are measured. We sought to determine whether cerebral oxygenation responses to an AS test were altered in young VVS patients when compared to the young healthy controls. A sample of young healthy adults and consecutive VVS patients attending a Falls and Syncope unit was recruited. Continuous beat-to-beat blood pressure (BP), heart rate, near-infrared spectroscopy (NIRS)-derived tissue saturation index (TSI), and changes in concentration of oxygenated/deoxygenated Δ[O₂Hb]/Δ[HHb] hemoglobin were measured. BP and NIRS-derived features included nadir, peak, overshoot, trough, recovery rate, normalized recovery rate, and steady-state. Multivariate linear regression was used to adjust for confounders and BP. In total, 13 controls and 27 VVS patients were recruited. While no significant differences were observed in the TSI and Δ[O₂Hb], there was a significantly smaller Δ[HHb] peak-to-trough and faster Δ[HHb] recovery rate in VVS patients, independent of BP. A higher BP steady-state was observed in patients but did not remain significant after multiple comparison correction. Young VVS patients demonstrated a similar cerebral circulatory response with signs of altered peripheral circulation with respect to the controls, potentially due to a hyper-reactive autonomic nervous system. This study sets the grounds for future investigations to understand the role of cerebral regulation during standing in VVS.

[Evaluation of Disopyramide Efficacy for Refractory Syncope in Heart Failure with Preserved Ejection Fraction Using Holter Electrocardiography: A Case Report]

The management of syncope is clinically important for heart failure (HF) patients. We herein describe a case on the efficacy of disopyramide for refractory syncope in HF with preserved ejection fraction (HFpEF). An 82-year-old man was hospitalized for respiratory distress and lower limb edema and was subsequently diagnosed with HFpEF. The use of diuretics improved HF symptoms; however, on day 10 after hospitalization, a rapid decrease in blood pressure and transient loss of consciousness developed. After neurologic examination, he was diagnosed with pure autonomic failure. Although he was administered midodrine 8 mg/d, fludrocortisone 0.1 mg/d, and droxidopa 300 mg/d, syncope was observed once a day on average. According to the Holter electrocardiogram, the patient's heart rate and coefficient of variation of R-R intervals (CVRR) during the day were unstable. In addition, high frequency power (parasympathetic nerve activity) was significantly higher than low frequency power (both sympathetic and parasympathetic nerves activity), suggesting that the parasympathetic nerves may have been highly active while the sympathetic nerves would have been blocked. On day 29, a pharmacist proposed disopyramide 300 mg/d, which blocks parasympathetic nerves and improves neural-mediated syncope, to the attending doctor. After the initiation of disopyramide, transient loss of consciousness was not observed. Furthermore, the diurnal variation in the heart rate and CVRR completely disappeared. In conclusion, disopyramide would be effective for refractory syncope in patients with HFpEF, and the Holter electrocardiogram may be a useful tool for the assessment of drug efficacy by pharmacists.

Increased multimorbidity is associated with impaired cerebral and peripheral hemodynamic stabilization during active standing

Background

Age‐related morbidities and frailty are associated with impaired blood pressure (BP) and heart rate (HR) recovery after standing. Here we investigate how multimorbidity affects cerebral and peripheral hemodynamics during standing in a large sample of older patients.

Methods

Patients were recruited from a national Falls and Syncope Unit. They underwent an active stand test (5–10 min lying +3 min standing) with monitoring of continuous BP, HR, total peripheral resistance (TPR), stroke volume (SV), and a near‐infrared spectroscopy (NIRS) derived cerebral tissue saturation index (TSI). A multimorbidity count was derived from a 26‐item list of conditions. Features derived from the signals included: nadir, overshoot, value at 30 s, steady‐state and recovery rate. Robust linear regression was used to assess the association between multimorbidity, TSI and peripheral hemodynamics while correcting for covariates. A p‐value <0.05 was considered statistically significant.

Results

Multimorbidity was associated with poorer recovery of TSI at 30 s after standing (β: −0.15, CI:[−0.25–0.06], p = 0.009) independent of all peripheral hemodynamics. Impaired diastolic BP (DBP) recovery at 30s (β:−1.34, CI:[−2.29–0.40], p = 0.032), DBP steady‐state (β:−1.18, CI:[−2.04–0.32], p = 0.032), TPR overshoot‐to‐nadir difference (β:−0.041, CI:[−0.070–0.013], p = 0.045), and SV at 30s (β:1.30, CI:[0.45 2.15], p = 0.027) were also associated with increasing multimorbidity. After sex stratification, only females demonstrated impaired TSI with multimorbidity at overshoot (β: −0.19, CI: [−0.32 ‐0.07], p = 0.009), 30 s (β: −0.22 [−0.35–0.10], p = 0.005) and steady‐state (β: −0.20, CI:[−0.35–0.04], p = 0.023), independent of peripheral hemodynamics.

Conclusions

Transient cerebral oxygenation and peripheral hemodynamic responses are impaired with multimorbidity (frailty) in older patients, particularly in females. This study demonstrates the feasibility of using NIRS in this clinical context and may inform the development of clinical management strategies targeting both cerebral oxygenation and blood pressure impairments in patients with faints and falls.

Hyperventilation-Induced Hypocapnia in an Aviator

BACKGROUND: Physiological episodes are a top safety concern for aviators across the United States military. While many cases and a variety of causes for physiological episodes have been described, few cases, if any, have been reported of hyperventilation-induced hypocapnia and transient loss of consciousness.CASE REPORT: Here we describe a case of an aviator who experienced tingling extremities, confusion, and loss of consciousness during a flight. The aviator incorrectly believed he was experiencing hypoxia and continued to take multiple steps to troubleshoot the wrong underlying problem for his symptoms. Evaluation after landing suggested this was instead a stress-induced hyperventilation that resulted in symptomatic hypocapnia.DISCUSSION: We report this case to add to the body of literature in understanding this phenomenon as well as to provide aviators, physiologists, and flight surgeons with practical suggestions for recognizing hyperventilation-induced hypocapnia and awareness of how to remedy this situation when they recognize it.Kramer KEP, Anderson EE. Hyperventilation-induced hypocapnia in an aviator. Aerosp Med Hum Perform. 2022; 93(5):470-471.

Cardiovascular Autonomic Regulation, ETCO2 and the Heart Rate Response to the Tilt Table Test in Patients with Orthostatic Intolerance

Chronic orthostatic intolerance (COI) is defined by changes in heart rate (HR), blood pressure (BP), respiration, symptoms of cerebral hypoperfusion and sympathetic overactivation. Postural tachycardia syndrome (POTS) is the most common form of COI in young adults and is defined by an orthostatic increase in heart rate (HR) of ≥ 30 bpm in the absence of orthostatic hypotension. However, some patients referred for evaluation of COI symptoms do not meet the orthostatic HR response criterion of POTS despite debilitating symptoms. Such patients are ill defined, posing diagnostic and therapeutic challenges. This study explored the relationship among cardiovascular autonomic control, the orthostatic HR response, EtCO₂ and the severity of orthostatic symptoms and fatigue in patients referred for evaluation of COI. Patients (N = 108) performed standardized testing protocol of the Autonomic Reflex Screen and completed the Composite Autonomic Symptom Score (COMPASS-31) and the Fatigue Severity Scale (FSS). Greater severity of COI was associated with younger age, larger phase IV amplitude in the Valsalva maneuver and lower adrenal baroreflex sensitivity. Greater fatigue severity was associated with a larger reduction in ETCO₂ during 10 min of head-up tilt (HUT) and reduced low-frequency (LF) power of heart rate variability. This study suggests that hemodynamic changes associated with the baroreflex response and changes in EtCO₂ show a stronger association with the severity of orthostatic symptoms and fatigue than the overall orthostatic HR response in patients with COI.

Multisystem Involvement in Post-acute Sequelae of COVID-19 (PASC)

OBJECTIVE

To describe cerebrovascular, neuropathic and autonomic features of post-acute sequelae of COVID-19 (PASC).

METHODS

This retrospective study evaluated consecutive patients with chronic fatigue, brain fog and orthostatic intolerance consistent with PASC. Controls included postural tachycardia syndrome patients (POTS) and healthy participants. Analyzed data included surveys and autonomic (Valsalva maneuver, deep breathing, sudomotor and tilt tests), cerebrovascular (cerebral blood flow velocity (CBFv) monitoring in middle cerebral artery), respiratory (capnography monitoring) and neuropathic (skin biopsies for assessment of small fiber neuropathy) testing and inflammatory/autoimmune markers.

RESULTS

Nine PASC patients were evaluated 0.7±0.3 years after a mild COVID-19 infection, treated as home observations. Autonomic, pain, brain fog, fatigue and dyspnea surveys were abnormal in PASC and POTS (n=10), compared to controls (n=15). Tilt table test reproduced the majority of PASC symptoms. Orthostatic CBFv declined in PASC (-20.0±13.4%) and POTS (-20.3±15.1%), compared to controls (-3.0±7.5%,p=0.001) and was independent of end-tidal carbon dioxide in PASC, but caused by hyperventilation in POTS. Reduced orthostatic CBFv in PASC included both subjects without (n=6) and with (n=3) orthostatic tachycardia. Dysautonomia was frequent (100% in both PASC and POTS) but was milder in PASC (p=0.013). PASC and POTS cohorts diverged in frequency of small fiber neuropathy (89% vs. 60%) but not in inflammatory markers (67% vs. 70%). Supine and orthostatic hypocapnia was observed in PASC.

INTERPRETATION

PASC following mild COVID-19 infection is associated with multisystem involvement including: 1) cerebrovascular dysregulation with persistent cerebral arteriolar vasoconstriction; 2) small fiber neuropathy and related dysautonomia; 3) respiratory dysregulation; 4) chronic inflammation. This article is protected by copyright. All rights reserved.

Cerebral blood flow remains reduced after tilt testing in myalgic encephalomyelitis/chronic fatigue syndrome patients

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Cerebral blood flow in ME/CFS patients remains abnormal 5 min post-tilt test.

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Post cerebral blood flow abnormalities do not depend on hemodynamic results and on end-tidal carbon dioxide pressures during the tilt-test.

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Post cerebral blood flow abnormalities are most severe in more severely diseased ME/CFS patients.

Objective

Orthostatic symptoms in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) may be caused by an abnormal reduction in cerebral blood flow. An abnormal cerebral blood flow reduction was shown in previous studies, without information on the recovery pace of cerebral blood flow. This study examined the prevalence and risk factors for delayed recovery of cerebral blood flow in ME/CFS patients.

Methods

60 ME/CFS adults were studied: 30 patients had a normal heart rate and blood pressure response during the tilt test, 4 developed delayed orthostatic hypotension, and 26 developed postural orthostatic tachycardia syndrome (POTS) during the tilt. Cerebral blood flow measurements, using extracranial Doppler, were made in the supine position pre-tilt, at end-tilt, and in the supine position at 5 min post-tilt. Also, cardiac index measurements were performed, using suprasternal Doppler imaging, as well as end-tidal PCO2 measurements. The change in cerebral blood flow from supine to end-tilt was expressed as a percent reduction with mean and (SD). Disease severity was scored as mild (approximately 50% reduction in activity), moderate (mostly housebound), or severe (mostly bedbound).

Results

End-tilt cerebral blood flow reduction was −29 (6)%, improving to −16 (7)% at post-tilt. No differences in either end-tilt or post-tilt measurements were found when patients with a normal heart rate and blood pressure were compared to those with POTS, or between patients with normocapnia (end-tidal PCO2 ≥ 30 mmHg) versus hypocapnia (end-tidal PCO2 < 30 mmHg) at end-tilt. A significant difference was found in the degree of abnormal cerebral blood flow reduction in the supine post-test in mild, moderate, and severe ME/CFS: mild: cerebral blood flow: −7 (2)%, moderate: −16 (3)%, and severe :-25 (4)% (p all < 0.0001). Cardiac index declined significantly during the tilt test in all 3 severity groups, with no significant differences between the groups. In the supine post-test cardiac index returned to normal in all patients.

Conclusions

During tilt testing, extracranial Doppler measurements show that cerebral blood flow is reduced in ME/CFS patients and recovery to normal supine values is incomplete, despite cardiac index returning to pre-tilt values. The delayed recovery of cerebral blood flow was independent of the hemodynamic findings of the tilt test (normal heart rate and blood pressure response, POTS, or delayed orthostatic hypotension), or the presence/absence of hypocapnia, and was only related to clinical ME/CFS severity grading. We observed a significantly slower recovery in cerebral blood flow in the most severely ill ME/CFS patients.

Significance

The finding that orthostatic stress elicits a post-stress cerebral blood flow reduction and that disease severity greatly influences the cerebral blood flow reduction may have implications on the advice of energy management after a stressor and on the advice of lying down after a stressor in these ME/CFS patients.

Preventive measures for the critical postexercise period in sickle cell trait and disease

The immediate postexercise/physical activity period is critical for sickle cell trait (SCT) carriers and disease (SCD) patients. Exercise-related blood acidosis is known to trigger the cascade of HbS deoxygenation and polymerization, leading to red blood cell sickling and subsequent complications. Unfortunately, two facts worsen exercise-related blood acidosis during the initial postexercise period: First, blood lactate and H⁺ concentrations continue to increase for several minutes after exercise completion, exacerbating blood acidosis. Second, blood lactate concentration remains elevated and pH altered for 20-45 min during inactivity after intense exercise, keeping acid/base balance disturbed for a long period after exercise. Therefore, the risk of complications (including vasoocclusive crises and even sudden death) persists and even worsens several minutes after intense exercise completion in SCT carriers or SCD patients. Light physical activity following intense exercise (namely, active recovery) may, by accelerating lactate removal and acid/base balance restoration, reduce the risk of complications. Scientific evidence suggests that light exercise at or below the first lactate threshold is an appropriate strategy.

Pure SARS-CoV-2 related AVDS (Acute Vascular Distress Syndrome)

Background

SARS-CoV-2 virus which targets the pulmonary vasculature is supposed to induce an intrapulmonary right to left shunt with an increased pulmonary blood flow. Such vascular injury is difficult to observe because it is hidden by the concomitant lung injury. We report here what may be, to the best of our knowledge, the first case of a pure Covid-19 related Acute Vascular Distress Syndrome (AVDS).

Case presentation

A 43-year-old physician, tested positive for Covid-19, was addressed to the emergency unit for severe dyspnoea and dizziness. Explorations were non informative with only a doubt regarding a sub-segmental pulmonary embolism (no ground-glass lesions or consolidations related to Covid-19 disease). Dyspnoea persisted despite anticoagulation therapy and normal pulmonary function tests. Contrast-enhanced transthoracic echocardiography was performed which revealed a moderate late right-to-left shunt.

Conclusions

This case report highlights the crucial importance of the vascular component of the viral disease. The intrapulmonary shunt induced by Covid-19 which remains unrecognized because generally hidden by the concomitant lung injury, can persist for a long time. Contrast-enhanced transthoracic echocardiography is the most appropriate test to propose in case of persistent dyspnoea in Covid-19 patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12879-021-05805-5.

Cerebral vs. Cardiovascular Responses to Exercise in Type 2 Diabetic Patients

The human brain is constantly active and even small limitations to cerebral blood flow (CBF) may be critical for preserving oxygen and substrate supply, e.g., during exercise and hypoxia. Exhaustive exercise evokes a competition for the supply of oxygenated blood between the brain and the working muscles, and inability to increase cardiac output sufficiently during exercise may jeopardize cerebral perfusion of relevance for diabetic patients. The challenge in diabetes care is to optimize metabolic control to slow progression of vascular disease, but likely because of a limited ability to increase cardiac output, these patients perceive aerobic exercise to be more strenuous than healthy subjects and that limits the possibility to apply physical activity as a preventive lifestyle intervention. In this review, we consider the effects of functional activation by exercise on the brain and how it contributes to understanding the control of CBF with the limited exercise tolerance experienced by type 2 diabetic patients. Whether a decline in cerebral oxygenation and thereby reduced neural drive to working muscles plays a role for "central" fatigue during exhaustive exercise is addressed in relation to brain's attenuated vascular response to exercise in type 2 diabetic subjects.

Cerebral Blood Flow Is Reduced in Severe Myalgic Encephalomyelitis/Chronic Fatigue Syndrome Patients During Mild Orthostatic Stress Testing: An Exploratory Study at 20 Degrees of Head-Up Tilt Testing

Introduction: In a study of 429 adults with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), we demonstrated that 86% had symptoms of orthostatic intolerance in daily life. Using extracranial Doppler measurements of the internal carotid and vertebral arteries during a 30-min head-up tilt to 70 degrees, 90% had an abnormal reduction in cerebral blood flow (CBF). A standard head-up tilt test of this duration might not be tolerated by the most severely affected bed-ridden ME/CFS patients. This study examined whether a shorter 15-min test at a lower 20 degree tilt angle would be sufficient to provoke reductions in cerebral blood flow in severe ME/CFS patients. Methods and results: Nineteen severe ME/CFS patients with orthostatic intolerance complaints in daily life were studied: 18 females. The mean (SD) age was 35(14) years, body surface area (BSA) was 1.8(0.2) m² and BMI was 24.0(5.4) kg/m². The median disease duration was 14 (IQR 5–18) years. Heart rate increased, and stroke volume index and end-tidal CO₂ decreased significantly during the test (p ranging from <0.001 to <0.0001). The cardiac index decreased by 26(7)%: p < 0.0001. CBF decreased from 617(72) to 452(63) mL/min, a 27(5)% decline. All 19 severely affected ME/CFS patients met the criteria for an abnormal CBF reduction. Conclusions: Using a less demanding 20 degree tilt test for 15 min in severe ME/CFS patients resulted in a mean CBF decline of 27%. This is comparable to the mean 26% decline previously noted in less severely affected patients studied during a 30-min 70 degree head-up tilt. These observations have implications for the evaluation and treatment of severely affected individuals with ME/CFS.

Respiratory-related brain pulsations are increased in epilepsy-a two-centre functional MRI study

Resting-state functional MRI has shown potential for detecting changes in cerebral blood oxygen level-dependent signal in patients with epilepsy, even in the absence of epileptiform activity. Furthermore, it has been suggested that coefficient of variation mapping of fast functional MRI signal may provide a powerful tool for the identification of intrinsic brain pulsations in neurological diseases such as dementia, stroke and epilepsy. In this study, we used fast functional MRI sequence (magnetic resonance encephalography) to acquire ten whole-brain images per second. We used the functional MRI data to compare physiological brain pulsations between healthy controls (n = 102) and patients with epilepsy (n = 33) and furthermore to drug-naive seizure patients (n = 9). Analyses were performed by calculating coefficient of variation and spectral power in full band and filtered sub-bands. Brain pulsations in the respiratory-related frequency sub-band (0.11-0.51 Hz) were significantly (P < 0.05) increased in patients with epilepsy, with an increase in both signal variance and power. At the individual level, over 80% of medicated and drug-naive seizure patients exhibited areas of abnormal brain signal power that correlated well with the known clinical diagnosis, while none of the controls showed signs of abnormality with the same threshold. The differences were most apparent in the basal brain structures, respiratory centres of brain stem, midbrain and temporal lobes. Notably, full-band, very low frequency (0.01-0.1 Hz) and cardiovascular (0.8-1.76 Hz) brain pulses showed no differences between groups. This study extends and confirms our previous results of abnormal fast functional MRI signal variance in epilepsy patients. Only respiratory-related brain pulsations were clearly increased with no changes in either physiological cardiorespiratory rates or head motion between the subjects. The regional alterations in brain pulsations suggest that mechanisms driving the cerebrospinal fluid homeostasis may be altered in epilepsy. Magnetic resonance encephalography has both increased sensitivity and high specificity for detecting the increased brain pulsations, particularly in times when other tools for locating epileptogenic areas remain inconclusive.

Association between mild hypercapnia and hospital mortality in patients admitted to the intensive care unit after cardiac arrest: A retrospective study

BACKGROUND

Mild hypercapnia may increase cerebral oxygenation and attenuate cerebral injury in post-cardiac arrest patients. However, its association with hospital mortality has not been evaluated.

METHODS

We conducted a retrospective multi-center study of prospectively collected data of all cardiac arrest patients admitted to the ICU between 2014 and 2015. Different kinds of arterial carbon dioxide tension (PaCO₂), including time-weighted mean PaCO₂, mean PaCO₂, admission PaCO₂ and proportion of time spent in four PaCO₂ categories (hypocapnia, normocapnia, mild hypercapnia, and severe hypercapnia) were used to explore the association with outcomes. Restricted cubic splines models were built to evaluate the association between PaCO₂ and odds ratio for hospital mortality in overall population and subgroups of different pH levels (acidosis, normal pH and alkalosis).

RESULTS

2783 post-cardiac arrest patients in 150 ICUs were included. 933 (33.5%) were classified into the hypocapnia (PaCO₂ < 35 mmHg), 1088 (39.1%) into the normocapnia (35-45 mmHg), 472 (17%) into the mild hypercapnia (45-55 mmHg) and 390 (10.4%) into the severe hypercapnia (>55 mmHg) group. Compared with normocapnia, mild hypercapnia was not associated with higher hospital survival probability (OR 1.08 [95% CI 0.84-1.38, p = 0.558]). Time spent in the normocapnia was associated with good outcome (OR 0.98 [95% CI 0.97-0.99, p < 0.001], for every 5 percentage point increase in time), but mild hypercapnia was not (OR 1 [95% CI 0.98-1.01, p = 0.542]). Cox-proportional hazards models supported these findings. Associations between PaCO₂ and hospital mortality were not statistically significant in normal pH and alkalosis subgroups.

CONCLUSIONS

PaCO₂ has a U-shaped association with odds ratio for hospital mortality, with mild hypercapnia not having a higher hospital survival probability than normocapnia in post-cardiac arrest patients.

Cerebral blood flow is reduced in ME/CFS during head-up tilt testing even in the absence of hypotension or tachycardia: A quantitative, controlled study using Doppler echography

OBJECTIVE

The underlying hypothesis in orthostatic intolerance (OI) syndromes is that symptoms are associated with cerebral blood flow (CBF) reduction. Indirect CBF measurements (transcranial Doppler flow velocities), provide inconsistent support of this hypothesis. The aim of the study was to measure CBF during a 30 min head-up tilt test (HUT), using Doppler flow imaging of carotid and vertebral arteries, in individuals with chronic fatigue syndrome/myalgic encephalomyelitis (ME/CFS), a condition with a high prevalence of OI.

METHODS

429 ME/CFS patients were studied: 247 had a normal heart rate (HR) and blood pressure (BP) response to HUT, 62 had delayed orthostatic hypotension (dOH), and 120 had postural orthostatic tachycardia syndrome (POTS). We also studied 44 healthy controls (HC). CBF measurements were made at mid-tilt and end-tilt. Before mid-tilt, we administered a verbal questionnaire to ascertain for 15 OI symptoms.

RESULTS

End-tilt CBF reduction was 7% in HC versus 26% in the overall ME/CFS group, 24% in patients with a normal HR/BP response, 28% in those with dOH, and 29% in POTS patients (all P < .0005). Using a lower limit of normal of 2SD of CBF reduction in HC (13% reduction), 82% of patients with normal HR/BP response, 98% with dOH and 100% with POTS showed an abnormal CBF reduction. There was a linear correlation of summed OI symptoms with the degree of CBF reduction at mid-tilt (P < .0005).

CONCLUSIONS

During HUT, extracranial Doppler measurements demonstrate that CBF is reduced in ME/CFS patients with POTS, dOH, and even in those without HR/BP abnormalities.

SIGNIFICANCE

This study shows that orthostatic intolerance symptoms are related to CBF reduction, and that the majority of ME/CFS patients (90%) show an abnormal cerebral flow reduction during orthostatic stress testing. This may have implications for the diagnosis and treatment of ME/CFS patients.

Vital signs and impaired cognition in older emergency department patients: The APOP study

BACKGROUND/OBJECTIVES

Cognitive impairment is a frequent problem among older patients attending the Emergency Department (ED) and can be the result of pre-existing cognitive impairment, delirium, or neurologic disorders. Another cause can also be acute disturbance of brain perfusion and oxygenation, which may be reversed by optimal resuscitation. This study aimed to assess the relationship between vital signs, as a measure of acute hemodynamic changes, and cognitive impairment in older ED patients.

DESIGN

Prospective cohort study.

SETTING

ED's of two tertiary care and two secondary care hospitals in the Netherlands.

PARTICIPANTS

2629 patients aged 70-years and older.

MEASUREMENTS

Vital signs were measured at the moment of ED arrival as part of routine clinical care. Cognition was measured using the Six-Item Cognitive Impairment Test (6-CIT).

RESULTS

The median age of patients was 78 years (IQR 74-84). Cognitive impairment was present in 738 patients (28.1%). When comparing lowest with highest quartiles, a systolic blood pressure of <129 mmHg (OR 1.30, 95% confidence interval (95%CI) 0.98-1.73)was associated with increased risk of cognitive impairment. A higher respiratory rate (>21/min) was associated with increased risk of impaired cognition (OR 2.16, 95% CI 1.58-2.95) as well as oxygen saturation of <95% (OR 1.64, 95%CI 1.24-2.19).

CONCLUSION

Abnormal vital signs associated with decreased brain perfusion and oxygenation are also associated with cognitive impairment in older ED patients. This may partially be explained by the association between disease severity and delirium, but also by acute disturbance of brain perfusion and oxygenation. Future studies should establish whether normalization of vital signs will also acutely improve cognition.

Slow sinusoidal tilt movements demonstrate the contribution to orthostatic tolerance of cerebrospinal fluid movement to and from the spinal dural space

Standing up elicits a host of cardiovascular changes which all affect the cerebral circulation. Lowered mean arterial blood pressure (ABP) at brain level, change in the cerebral venous outflow path, lowered end-tidal PCO2 (PET CO2 ), and intracranial pressure (ICP) modify cerebral blood flow (CBF). The question we undertook to answer is whether gravity-induced blood pressure (BP) changes are compensated in CBF with the same dynamics as are spontaneous or induced ABP changes in a stable position. Twenty-two healthy subjects (18/4 m/f, 40 ± 8 years) were subjected to 30° and 70° head-up tilt (HUT) and sinusoidal tilts (SinTilt, 0°↨60° around 30° at 2.5-10 tilts/min). Additionally, at those three tilt levels, they performed paced breathing at 6-15 breaths/min to induce larger than spontaneous cardiovascular oscillations. We measured continuous finger BP and cerebral blood flow velocity (CBFv) in the middle cerebral artery by transcranial Doppler to compute transfer functions (TFs) from ABP- to CBFv oscillations. SinTilt induces the largest ABP oscillations at brain level with CBFv gains strikingly lower than for paced breathing or spontaneous variations. This would imply better autoregulation for dynamic gravitational changes. We demonstrate in a mathematical model that this difference is explained by ICP changes due to movement of cerebrospinal fluid (CSF) into and out of the spinal dural sack. Dynamic cerebrovascular autoregulation seems insensitive to how BP oscillations originate if the effect of ICP is factored in. CSF-movement in-and-out of the spinal dural space contributes importantly to orthostatic tolerance by its effect on cerebral perfusion pressure.

Dynamic cerebral autoregulation is impaired in Veterans with Gulf War Illness: A case-control study

Neurological dysfunction has been reported in Gulf War Illness (GWI), including abnormal cerebral blood flow (CBF) responses to physostigmine challenge. However, it is unclear whether the CBF response to normal physiological challenges and regulation is similarly dysfunctional. The goal of the present study was to evaluate the CBF velocity response to orthostatic stress (i.e., sit-to-stand maneuver) and increased fractional concentration of carbon dioxide. 23 cases of GWI (GWI+) and 9 controls (GWI) volunteered for this study. Primary variables of interest included an index of dynamic autoregulation and cerebrovascular reactivity. Dynamic autoregulation was significantly lower in GWI+ than GWI- both for autoregulatory index (2.99±1.5 vs 4.50±1.5, p = 0.017). In addition, we observed greater decreases in CBF velocity both at the nadir after standing (-18.5±6.0 vs -9.8±4.9%, p = 0.001) and during steady state standing (-5.7±7.1 vs -1.8±3.2%, p = 0.042). In contrast, cerebrovascular reactivity was not different between groups. In our sample of Veterans with GWI, dynamic autoregulation was impaired and consistent with greater cerebral hypoperfusion when standing. This reduced CBF may contribute to cognitive difficulties in these Veterans when upright.

Hypocapnic cerebral hypoperfusion: A biomarker of orthostatic intolerance

The objective of the study was to identify markers of hypocapnic cerebral hypoperfusion (HYCH) in patients with orthostatic intolerance (OI) without tachycardia and without orthostatic hypotension. This single center, retrospective study included OI patients referred for autonomic evaluation with the 10 min tilt test. Heart rate, end-tidal CO2 (ET-CO2), blood pressure, and cerebral blood flow velocity (CBFv) from middle cerebral artery were monitored. HYCH was defined by: (1) Symptoms of OI; (2) Orthostatic hypocapnia (low ET-CO2); (3) Abnormal decline in orthostatic CBFv due to hypocapnia; 4) Absence of tachycardia, orthostatic hypotension, or other causes of low CBFv or hypocapnia. Sixteen subjects met HYCH criteria (15/1 women/men, age 38.5±8.0 years) and were matched by age and gender to postural tachycardia patients (POTS, n = 16) and healthy controls (n = 16). During the tilt, CBFv decreased more in HYCH (-22.4±7.7%, p<0.0001) and POTS (-19.0±10.3%, p<0.0001) compared to controls (-3.0±5.0%). Orthostatic ET-CO2 was lower in HYCH (26.4±4.2 (mmHg), p<0.0001) and POTS (28.6±4.3, p<0.0001) compared to controls (36.9 ± 2.1 mmHg). Orthostatic heart rate was normal in HYCH (89.0±10.9 (BPM), p<0.08) and controls (80.8 ±11.2), but was higher in POTS (123.7±11.2, p<0.0001). Blood pressure was normal and similar in all groups. It is concluded that both HYCH and POTS patients have comparable decrease in CBFv which is due to vasoconstrictive effect of hypocapnia. Blood flow velocity monitoring can provide an objective biomarker for HYCH in OI patients without tachycardia.

A 56-Year-Old, Otherwise Healthy Woman Presenting With Light-headedness and Progressive Shortness of Breath

A 56-year-old white woman was referred to the pulmonary clinic for evaluation of unexplained shortness of breath. She enjoyed good health until 3 months prior to this visit when she reported experiencing recurrent episodes of shortness of breath and oppressive retrosternal chest discomfort with radiation to the neck. Episodes lasting 5 to 10 min often occurred at rest and were inconsistently related to physical activity. These symptoms became progressively worse and were often associated with light-headedness and presyncope. Her past medical history was uneventful apart from a prior diagnosis of breast cysts and suspected prolactinoma. Her symptoms escalated to such a level that she was forced to seek urgent medical attention at our institutional ED on two separate occasions in the preceding weeks. These visits precipitated a number of investigations and, eventually, a referral to the pulmonary clinic.

Respiratory and hemodynamic contributions to emotion-related pre-syncopal vasovagal symptoms

Vasovagal reactions are conventionally understood as resulting from systemic changes in cardiovascular activity; however, there exists a complementary perspective focused on specific changes in cerebral vasoconstriction associated with hyperventilation-induced hypocapnia. The present study investigated the role of cardiovascular and respiratory activity in self-reported pre-syncopal vasovagal reactions to a surgery video in a sample of 49 healthy women. Participants who indicated more previous real-life episodes of dizziness reported experiencing significantly more symptoms in the laboratory consistent with a vasovagal response. They also showed lower total peripheral resistance and higher pre-ejection period in general, suggesting lower sympathetic nervous system activity. Significant decreases in end-tidal carbon dioxide (P_ETCO₂) occurred during the surgery video among susceptible participants, without significant increases in respiration rate. Further, participants who experienced reductions from the neutral video in P_ETCO₂, systolic blood pressure, or both, reported vasovagal symptoms during the surgery video. The results suggest that patterns of respiration associated with decreases in P_ETCO₂ may contribute to vasovagal symptoms reported in non-clinical groups as well as those with blood-injection-injury phobia and are associated with susceptibility to dizziness.

Is Performance of a Modified Eucapnic Voluntary Hyperpnea Test in High Ventilation Athletes Reproducible?

Purpose

Exercise-induced bronchoconstriction (EIB) is common in “high ventilation” athletes, and the Eucapnic Voluntary Hyperpnea (EVH) airway provocation test is the standard EIB screen. Although the EVH test is widely used, the in-test performance in high ventilation athletes as well as the reproducibility of that performance has not been determined. Reproducibility of pre- and post-test spirometry and self-reported atopy/cough was also examined.

Methods

High ventilation athletes (competitive swimmers; n=11, 5 males) completed an atopy/cough questionnaire and EVH testing (operator controlled FiCO₂) on 2 consecutive days.

Results

Swimmers achieved 85%±9% and 87%±9% of target FEV1 volume on days 1 and 2, respectively, (P=0.45; ICC 0.57 [0.00-0.86]) resulting in a total ventilation of 687 vs 684 L [P=0.89, ICC 0.89 (0.65-0.97]) equating to 83%±8% and 84%±9% of predicted total volume (ICC 0.54 [0.00-0.85]) between days 1 and 2. FiCO₂ required to maintain eucapnic conditions was 2.5%. Pre-test FEV1 was less on day 2 (P=0.04; ICC >0.90). Day 1 to 2 post-test FEV1 was not different, and 4 swimmers were EIB positive (>10% fall in pre-post FEV1) on day 1 (3 on day 2).

Conclusions

EVH in-test performance is reproducible however required less FiCO₂ than standard protocol and the swimmers under-ventilated by 125 and 139 L/min for days 1 and 2, respectively. How this affects EIB diagnosis remains to be determined; however, our results indicate a post-test FEV1 fall of ≥20% may be recommended as the most consistent diagnostic criterion.

Cerebral microvascular blood flow and CO2 reactivity in pulmonary arterial hypertension

Hypocapnia and endothelial dysfunction might impair microvascular cerebral blood flow (CBF_micr) and cerebrovascular reactivity to CO₂ (CVR_CO2). Pulmonary arterial hypertension (PAH) is characteristically associated with chronic alveolar hyperventilation and microvascular endothelial dysfunction. We therefore determined CBF_micr (pre-frontal blood flow index (BFI) by the indocyanine green-near infrared spectroscopy methodology) during hypocapnia and hypercapnia in 25 PAH patients and 10 gender- and age-matched controls. Cerebral BFI was lower in patients than controls at similar transcutaneous PCO₂ (PtcCO₂) levels in both testing conditions. In fact, while BFI increased from hypocapnia to hypercapnia in all controls, it failed to increase in 17/25 (68%) patients. Thus, BFI increased to a lesser extent from hypo to hypercapnia ("Δ") in patients, i.e., they showed lower Δ BFI/Δ PtcCO₂ ratios than controls. In conclusion, CBF_micr and CVR_CO2 are lessened in clinically stable, mildly-impaired patients with PAH. These abnormalities might be associated with relevant clinical outcomes (hyperventilation and dyspnea, cognition, cerebrovascular disease) being potentially amenable to pharmacological treatment.

A review of the current and potential oxygen delivery systems and techniques utilized in cluster headache attacks

Background

Despite oxygen therapy being one of the foremost acute treatments for cluster headache (CH) attacks, little is known about the different techniques and systems.

Objectives

In this review we will examine the efficacy of the standard non-rebreather mask (NRM) with room temperature oxygen in relieving pain in CH, and try to compare it with the diversity of other oxygen gas conditions and interfaces like partial rebreathers, simple masks, nasal cannulas, tusk masks, demand valve oxygen, hyperbaric and cooled oxygen.

Method

We searched non-structured Pubmed, Medline, the Cochrane online database and instruction protocols from various oxygen delivery devices.

Conclusions and implications

Interfaces like demand valves and tusk masks are already proving to be superior or at least similar to the standard NRM in terms of fraction of inspired oxygen (FiO2), though the demand valve only showed better results than the NRM in a single study in only four participants. Furthermore, new research shows how lower temperatures of the gas may be an essential part of effective pain relief and hyperbaric treatments show potential in preventing night time attacks.

Disgust stimuli reduce heart rate but do not contribute to vasovagal symptoms

BACKGROUND AND OBJECTIVES

The vasovagal response demonstrates a unique form of stress response, common in medical settings yet provoked by a variety of blood-injury-injection stimuli. This study aimed to better understand the psychophysiological mechanisms of the vasovagal response..

METHODS

16 undergraduates with and 42 without a self-reported history of fainting watched five 3-5 min videos with different emotional content. One documentary clip (Neutral condition) described a campus environmental project while another (Blood/Injury) depicted portions of an open heart surgery. Three additional clips were also used, including Medical, Threat, and Contamination stimuli. Vasovagal symptoms and physiological variables were assessed during each video.

RESULTS

As predicted, while the disgust-related stimuli (Blood/Injury, Medical, Contamination) were associated with generally lower heart rate, the Blood/Injury video produced the highest symptoms and the only significant difference between previous fainters and non-fainters. The physiological measures also revealed that participants with a fainting history experienced higher stroke volume and lower systolic blood pressure throughout, as well as several main effects of video.

LIMITATIONS

An additional decrease in systolic blood pressure and respiration produced by watching the Blood/Injury video may have been sufficient to trigger symptoms in some, though results also suggest that systemic variables do not entirely explain susceptibility to symptoms. More careful evaluation of regional blood flow may be required.

CONCLUSIONS

Participants who had previously experienced strong vasovagal responses displayed what appeared to be an anticipatory response to the Blood/Injury video. Finally, disgust stimuli may reduce heart rate but do not appear to contribute to vasovagal symptoms.

Current approaches to the clinical assessment of syncope in pediatric population

INTRODUCTION

Syncope is one of the most common clinical problem in children. This disorder is characterized by transient, spontaneously self-terminating loss of consciousness with brief duration and complete recovery. This situation is usually alarming for the families of patients. The mechanism of syncope is transient global brain hypoperfusion to levels below those tolerated by cerebrovascular autoregulation. Syncope can occur with many different etiologies in the pediatric population.

CLASSIFICATION

Syncopes are divided into three major categories as neurally mediated syncope, cardiovascular-mediated syncope, and non-cardiovascular syncope.

CLINICAL FEATURES

The major challenge in the assessment of children with syncope is that most children are asymptomatic at the time of their presentation, therefore making a careful and detailed history and a comprehensive physical examination essential in all patients. A trigger stimulus is detected in some cases, and this is an important clinical clue for the diagnosis. Cardiac causes of syncope in children are rare but can be life threatening and have the highest risk of morbidity and mortality. Misdiagnosis of epilepsy is common in patients presenting with syncope; therefore, the differential diagnosis between epileptic seizures and syncope is very important. It should be remembered that the evaluation of syncope in children is costly and diagnostic workup has a limited diagnostic yield.

CONCLUSION

The aim of this article is to present different types of syncope and to provide new practical clinical approaches to the diagnosis, investigation, and management in the pediatric population.

Impact of hypocapnia and cerebral perfusion on orthostatic tolerance

We examined two novel hypotheses: (1) that orthostatic tolerance (OT) would be prolonged when hyperventilatory-induced hypocapnia (and hence cerebral hypoperfusion) was prevented; and (2) that pharmacological reductions in cerebral blood flow (CBF) at baseline would lower the 'CBF reserve', and ultimately reduce OT. In study 1 (n = 24; aged 25 ± 4 years) participants underwent progressive lower-body negative pressure (LBNP) until pre-syncope; end-tidal carbon dioxide (P ET , CO 2) was clamped at baseline levels (isocapnic trial) or uncontrolled. In study 2 (n = 10; aged 25 ± 4 years), CBF was pharmacologically reduced by administration of indomethacin (INDO; 1.2 mg kg(-1)) or unaltered (placebo) followed by LBNP to pre-syncope. Beat-by-beat measurements of middle cerebral artery blood flow velocity (MCAv; transcranial Doppler), heart rate (ECG), blood pressure (BP; Finometer) and end-tidal gases were obtained continuously. In a subset of subjects' arterial-to-jugular venous differences were obtained to examine the independent impact of hypocapnia or cerebral hypoperfusion (following INDO) on cerebral oxygen delivery and extraction. In study 1, during the isocapnic trial, P ET , CO 2 was successfully clamped at baseline levels at pre-syncope (38.3 ± 2.7 vs. 38.5 ± 2.5 mmHg respectively; P = 0.50). In the uncontrolled trial, P ET , CO 2 at pre-syncope was reduced by 10.9 ± 3.9 mmHg (P ≤ 0.001). Compared to the isocapnic trial, the decline in mean MCAv was 15 ± 4 cm s(-1) (35%; P ≤ 0.001) greater in the uncontrolled trial, yet the time to pre-syncope was comparable between trials (544 ± 130 vs. 572 ± 180 s; P = 0.30). In study 2, compared to placebo, INDO reduced resting MCAv by 19 ± 4 cm s(-1) (31%; P ≤ 0.001), but time to pre-syncope remained similar between trials (placebo: 1123 ± 138 s vs. INDO: 1175 ± 212 s; P = 0.53). The brain extracted more oxygen in face of hypocapnia (34% to 53%) or cerebral hypoperfusion (34% to 57%) to compensate for reductions in delivery. In summary, cerebral hypoperfusion either at rest or induced by hypocapnia at pre-syncope does not impact OT, probably due to a compensatory increase in oxygen extraction.

Hyperventilation with and without maintenance of isocapnia: a comparison of selected gasometric and respiratory parameters

The aims of this study were to examine selected respiratory and gasometric parameters during hyperventilation with and without isocapnia and to identify the possible mechanism by which isocapnic hyperventilation might be useful in the elimination of volatile substances, including CO. Ten healthy non-smoking volunteers were studied, and each underwent two procedures. During one session, CO2 was added to the respiratory circuit, and during the other session, only 100% O2 was used. The volunteers were coached to hyperventilate until the appearance of side effects. Isocapnic hyperventilation significantly increased alveolar minute ventilation and partial pressure of oxygen in arterialized capillary blood (paO2); to the best of our knowledge, these findings have not previously been reported. Isocapnic hyperventilation was associated with only mild side effects, such as dyspnea, increased respiratory effort and headache, in 30% of subjects. Side effects, including vertigo, paresthesias and muscle tremor, were present in 70% of the volunteers during hyperventilation with 100% O2, and these side effects forced them to limit their respiratory rates and tidal volumes. These increases in alveolar ventilation and the partial pressure of oxygen in the blood may play crucial roles in decreasing the half-time of carboxyhemoglobin, which is the primary goal of the treatment of CO poisoning.

Assessment of middle cerebral artery diameter during hypocapnia and hypercapnia in humans using ultra-high-field MRI

In the evaluation of cerebrovascular CO2 reactivity measurements, it is often assumed that the diameter of the large intracranial arteries insonated by transcranial Doppler remains unaffected by changes in arterial CO2 partial pressure. However, the strong cerebral vasodilatory capacity of CO2 challenges this assumption, suggesting that there should be some changes in diameter, even if very small. Data from previous studies on effects of CO2 on cerebral artery diameter [middle cerebral artery (MCA)] have been inconsistent. In this study, we examined 10 healthy subjects (5 women, 5 men, age 21-30 yr). High-resolution (0.2 mm in-plane) MRI scans at 7 Tesla were used for direct observation of the MCA diameter during hypocapnia, -1 kPa (-7.5 mmHg), normocapnia, 0 kPa (0 mmHg), and two levels of hypercapnia, +1 and +2 kPa (7.5 and 15 mmHg), with respect to baseline. The vessel lumen was manually delineated by two independent observers. The results showed that the MCA diameter increased by 6.8 ± 2.9% in response to 2 kPa end-tidal P(CO2) (PET(CO2)) above baseline. However, no significant changes in diameter were observed at the -1 kPa (-1.2 ± 2.4%), and +1 kPa (+1.4 ± 3.2%) levels relative to normocapnia. The nonlinear response of the MCA diameter to CO2 was fitted as a continuous calibration curve. Cerebral blood flow changes measured by transcranial Doppler could be corrected by this calibration curve using concomitant PET(CO2) measurements. In conclusion, the MCA diameter remains constant during small deviations of the PET(CO2) from normocapnia, but increases at higher PET(CO2) values.

Arterial pressure variations as parameters of brain perfusion in response to central blood volume depletion and repletion

Rationale: A critical reduction in central blood volume (CBV) is often characterized by hemodynamic instability. Restoration of a volume deficit may be established by goal-directed fluid therapy guided by respiration-related variation in systolic- and pulse pressure (SPV and PPV). Stroke volume index (SVI) serves as a surrogate end-point of a fluid challenge but tissue perfusion itself has not been addressed.

Objective: To delineate the relationship between arterial pressure variations, SVI and regional brain perfusion during CBV depletion and repletion in spontaneously breathing volunteers.

Methods: This study quantified in 14 healthy subjects (11 male) the effects of CBV depletion [by 30 and 70 degrees passive head-up tilt (HUT)] and a fluid challenge (by tilt back) on CBV (thoracic admittance), mean middle cerebral artery (MCA) blood flow velocity (V_mean), SVI, cardiac index (CI), PPV, and SPV.

Results: PPV (103 ± 89%, p < 0.05) and SPV (136 ± 117%, p < 0.05) increased with progression of central hypovolemia manifested by a reduction in thoracic admittance (11 ± 5%, p < 0.001), SVI (28 ± 6%, p < 0.001), CI (6 ± 8%, p < 0.001), and MCAV_mean (17 ± 7%, p < 0.05) but not in arterial pressure. The reduction in MCAV_mean correlated to the fall in SVI (R² = 0.52, p < 0.0001) and inversely to PPV and SPV [R² = 0.46 (p < 0.0001) and R² = 0.45 (p < 0.0001), respectively]. PPV and SPV predicted a ≥15% reduction in MCAV_mean and SVI with comparable sensitivity (67/67% vs. 63/68%, respectively) and specificity (89/94 vs. 89/94%, respectively). A rapid fluid challenge by tilt-back restored all parameters to baseline values within 1 min.

Conclusion: In spontaneously breathing subjects, a reduction in MCAV_mean was related to an increase in PPV and SPV during graded CBV depletion and repletion. Specifically, PPV and SPV predicted changes in both SVI and MCAV_mean with comparable sensitivity and specificity, however the predictive value is limited in spontaneously breathing subjects.

Influence of intranasal and carotid cooling on cerebral temperature balance and oxygenation

The present study evaluated the influence of intranasal cooling with balloon catheters, increased nasal ventilation, or percutaneous cooling of the carotid arteries on cerebral temperature balance and oxygenation in six healthy male subjects. Aortic arch and internal jugular venous blood temperatures were measured to assess the cerebral heat balance and corresponding paired blood samples were obtained to evaluate cerebral metabolism and oxygenation at rest, following 60 min of intranasal cooling, 5 min of nasal ventilation, and 15 min with carotid cooling. Intranasal cooling induced a parallel drop in jugular venous and arterial blood temperatures by 0.30 ± 0.08°C (mean ± SD), whereas nasal ventilation and carotid cooling failed to lower the jugular venous blood temperature. The magnitude of the arterio-venous temperature difference across the brain remained unchanged at −0.33 ± 0.05°C following intranasal and carotid cooling, but increased to −0.44 ± 0.11°C (P < 0.05) following nasal ventilation. Calculated cerebral capillary oxygen tension was 43 ± 3 mmHg at rest and remained unchanged during intranasal and carotid cooling, but decreased to 38 ± 2 mmHg (P < 0.05) following increased nasal ventilation. In conclusion, percutaneous cooling of the carotid arteries and intranasal cooling with balloon catheters are insufficient to influence cerebral oxygenation in normothermic subjects as the cooling rate is only 0.3°C per hour and neither intranasal nor carotid cooling is capable of inducing selective brain cooling.