Reproducibility of orthostatic changes in cerebral oxygenation in healthy subjects aged 70 years or older

Cerebral Blood Deoxygenation by a Postural Change Detected by Near-Infrared Spectroscopy Has a Close Association with Cerebral Infarction

BACKGROUND

The recent introduction of near-infrared spectroscopy has enabled the monitoring of cerebral blood flow in real-time. Previous studies have shown that blood flow velocity is a predictor of cardiovascular disease. We hypothesized that cerebral oxygenation with a change in posture is a predictor for cerebral infarction. We designed a cross-sectional study to investigate the relationship between postural-related changes in cerebral oxygenation and a history of chronic cerebral infarction.

METHODS

A total of 100 consecutive participants were enrolled in this study. We evaluated changes in cerebral oxygenation with a change in posture from the supine to the upright position in the bilateral forehead. The association between a decline in cerebral oxygenation and chronic cerebral infarction was analyzed with multiple logistic regression adjusted for covariates.

RESULTS

Cerebral blood oxygenation increased in 52 participants and decreased in 48 participants with a postural change. The prevalence of decreased cerebral oxygenation was 76.3% in participants with chronic cerebral infarction. Multiple logistic regression analysis showed that a decline in cerebral oxygenation upon a postural change was strongly associated with chronic cerebral infarction (adjusted odds ratio: 3.42, p = 0.025).

CONCLUSIONS

Cerebral blood oxygenation upon a postural change could be a useful predictor for cerebral infarction.

Older Adults' Drop in Cerebral Oxygenation on Standing Correlates With Postural Instability and May Improve With Sitting Prior to Standing

BACKGROUND

Impaired blood pressure (BP) recovery with orthostatic hypotension on standing occurs in 20% of older adults. Low BP is associated with low cerebral blood flow but mechanistic links to postural instability and falls are not established. We investigated whether posture-related reductions in cerebral tissue oxygenation (tSO2) in older adults impaired stability upon standing, if a brief sit before standing improved tSO2 and stability, and if Low-tSO2 predicted future falls.

METHOD

Seventy-seven older adults (87 ± 7 years) completed (i) supine-stand, (ii) supine-sit-stand, and (iii) sit-stand transitions with continuous measurements of tSO2 (near-infrared spectroscopy). Total path length (TPL) of the center of pressure sway quantified stability. K-cluster analysis grouped participants into High-tSO2 (n = 62) and Low-tSO2 (n = 15). Fall history was followed up for 6 months.

RESULTS

Change in tSO2 during supine-stand was associated with increased TPL (R = -.356, p = .001). When separated into groups and across all transitions, the Low-tSO2 group had significantly lower tSO2 (all p < .01) and poorer postural stability (p < .04) through 3 minutes of standing compared to the High-tSO2 group. There were no effects of transition type on tSO2 or TPL for the High-tSO2 group, but a 10-second sitting pause improved tSO2 and enhanced postural stability in the Low-tSO2 group (all p < .05). During 6-month follow-up, the Low-tSO2 group had a trend (p < .1) for increased fall risk.

CONCLUSIONS

This is the first study to show an association between posture-related cerebral hypoperfusion and quantitatively assessed instability. Importantly, we found differences among older adults suggesting those with lower tSO2 and greater instability might be at increased risk of a future fall.

Vascular and haemodynamic issues of brain ageing

The population is ageing worldwide, thus increasing the burden of common age-related disorders to the individual, society and economy. Cerebrovascular diseases (stroke, dementia) contribute a significant proportion of this burden and are associated with high morbidity and mortality. Thus, understanding and promoting healthy vascular brain ageing are becoming an increasing priority for healthcare systems. In this review, we consider the effects of normal ageing on two major physiological processes responsible for vascular brain function: Cerebral autoregulation (CA) and neurovascular coupling (NVC). CA is the process by which the brain regulates cerebral blood flow (CBF) and protects against falls and surges in cerebral perfusion pressure, which risk hypoxic brain injury and pressure damage, respectively. In contrast, NVC is the process by which CBF is matched to cerebral metabolic activity, ensuring adequate local oxygenation and nutrient delivery for increased neuronal activity. Healthy ageing is associated with a number of key physiological adaptations in these processes to mitigate age-related functional and structural declines. Through multiple different paradigms assessing CA in healthy younger and older humans, generating conflicting findings, carbon dioxide studies in CA have provided the greatest understanding of intrinsic vascular anatomical factors that may mediate healthy ageing responses. In NVC, studies have found mixed results, with reduced, equivalent and increased activation of vascular responses to cognitive stimulation. In summary, vascular and haemodynamic changes occur in response to ageing and are important in distinguishing “normal” ageing from disease states and may help to develop effective therapeutic strategies to promote healthy brain ageing.

A Systematic Review of the Application of Functional Near-Infrared Spectroscopy to the Study of Cerebral Hemodynamics in Healthy Aging

Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) studies have shown that healthy aging is associated with functional brain deterioration that preferentially affects the prefrontal cortex. This article reviews the application of an alternative method, functional near-infrared spectroscopy (fNIRS), to the study of age-related changes in cerebral hemodynamics and factors that influence cerebral hemodynamics in the elderly population. We conducted literature searches in PudMed and PsycINFO, and selected only English original research articles that used fNIRS to study healthy individuals with a mean age of ≥ 55 years. All articles were published in peer-reviewed journals between 1977 and May 2019. We synthesized 114 fNIRS studies examining hemodynamic changes that occurred in the resting state and during the tasks of sensation and perception, motor control, semantic processing, word retrieval, attentional shifting, inhibitory control, memory, and emotion and motivation in healthy older adults. This review, which was not registered in a registry, reveals an age-related reduction in resting-state cerebral oxygenation and connectivity in the prefrontal cortex. It also shows that aging is associated with a reduction in functional hemispheric asymmetry and increased compensatory activity in the frontal lobe across multiple task domains. In addition, this article describes the beneficial effects of healthy lifestyles and the detrimental effects of cardiovascular disease risk factors on brain functioning among nondemented older adults. Limitations of this review include exclusion of gray and non-English literature and lack of meta-analysis. Altogether, the fNIRS literature provides some support for various neurocognitive aging theories derived from task-based PET and fMRI studies. Because fNIRS is relatively motion-tolerant and environmentally unconstrained, it is a promising tool for fostering the development of aging biomarkers and antiaging interventions.

Multimodal Monitoring of Cardiovascular Responses to Postural Changes

Background

In the poorly understood relationship between orthostatic hypotension and falls, next to blood pressure (BP), baroreflex sensitivity (BRS) and cerebral autoregulation (CAR) may be key measures. The posture- and movement dependency of orthostatic hypotension requires continuous and unobtrusive monitoring. This may be possible using simultaneous photoplethysmography (PPG), electrocardiography (ECG), and near-infrared spectroscopy (NIRS) signal recordings, from which pulse wave velocity (PWV; potentially useful for BP estimation), BRS and CAR can be derived. The PPG, NIRS and PWV signal correlation with BP and BRS/CAR reliability and validity need to be addressed.

Methods

In 34 healthy adults (mean age 25 years, inter quartile range 22–45; 10 female), wrist and finger PPG, ECG, bifrontal NIRS (oxygenated and deoxygenated hemoglobin) and continuous BP were recorded during sit to stand and supine to stand movements. Sixteen participants performed slow and rapid supine to stand movements; eighteen other participants performed a 1-min squat movement. Pulse wave velocity (PWV) was defined as the inverse of the ECG R-peak to PPG pulse delay; PPG, NIRS and PWV signal correlation with BP as their Pearson correlations with mean arterial pressure (MAP) within 30 s after the postural changes; BRS as inter beat interval drop divided by systolic BP (SBP) drop during the postural changes; CAR as oxygenated hemoglobin drop divided by MAP drop. BRS and CAR were separately computed using measured and estimated (linear regression) BP. BRS/CAR reliability was defined by the intra class correlation between repeats of the same postural change; validity as the Pearson correlation between BRS/CAR values based on measured and estimated BP.

Results

The highest correlation with MAP was found for finger PPG and oxygenated hemoglobin, ranging from 0.75–0.79 (sit to stand), 0.66–0.88 (supine to stand), and 0.82–0.94 (1-min squat). BRS and CAR reliability was highest during the different supine to stand movements, ranging from 0.17 – 0.49 (BRS) and 0.42-0.75 (CAR); validity was highest during rapid supine to stand movements, 0.54 and 0.79 respectively.

Conclusion

PPG-ECG-NIRS recordings showed high correlation with BP and enabled computation of reliable and valid BRS and CAR estimates, suggesting their potential for continuous unobtrusive monitoring of orthostatic hypotension key measures.

Transient Laterality of Cerebral Oxygenation Changes in Response to Head-of-Bed Manipulation in Acute Ischemic Stroke

Background: Cerebral oxygenation monitoring provides important information for optimizing individualized management in patients with acute ischemic stroke (AIS). Although changes in cerebral oxygenation are known to occur in response to head-of-bed (HOB) elevation within 72 h after onset, changes in cerebral oxygenation during stroke recovery are unclear. We compared changes in total- (tHb), oxygenated- (HbO2), and deoxygenated-hemoglobin (deoxyHb) concentrations in response to HOB manipulation between the timeframes within 72 h and 7–10 days after AIS onset. Methods: We measured forehead ΔtHb, ΔHbO2, and ΔdeoxyHb in response to HOB elevation (30°) within 72 h (first measurement) and 7–10 days (second measurement) after AIS onset using time-resolved near-infrared spectroscopy. Results: We enrolled 30 participants (mean age 72.8 ± 11.3 years; 13 women) with a first AIS. There were no significant differences in ΔtHb, ΔHbO2, or ΔdeoxyHb measurements on the infarct or contra-infarct side. At the first measurement, ΔtHb, ΔHbO2, and ΔdeoxyHb measured on the contra-infarct side did not correlate with those measured on the infarct side: ΔtHb (r = 0.114, p = 0.539); ΔHbO2 (r = 0.143, p = 0.440); ΔdeoxyHb (r = 0.227, p = 0.221). Notably, at the second measurement, correlation coefficients of ΔtHb and ΔHbO2 between the contra-infarct and infarct sides were statistically significant: ΔtHb (r = 0.491, p = 0.008); ΔHbO2 (r = 0.479, p = 0.010); ΔdeoxyHb (r = 0.358, p = 0.054). Conclusion: Although changes in cerebral oxygenation in response to HOB elevation had a laterality difference between hemispheres within 72 h of AIS onset, the difference had decreased, at least partially, 7–10 days after AIS onset.

Sensitivity and reliability of cerebral oxygenation responses to postural changes measured with near-infrared spectroscopy

Purpose

Cerebral oxygenation as measured by near-infrared spectroscopy (NIRS) might be useful to discriminate between physiological and pathological responses after standing up in individuals with orthostatic hypotension. This study addressed the physiological sensitivity of the cerebral oxygenation responses as measured by NIRS to different types and speeds of postural changes in healthy adults and assessed the reliability of these responses.

Methods

Cerebral oxygenated hemoglobin (O₂Hb), deoxygenated hemoglobin (HHb) and tissue saturation index (TSI) were measured bilaterally on the forehead of 15 healthy individuals (12 male, age range 18–27) using NIRS. Participants performed three repeats of sit to stand, and slow and rapid supine to stand movements. Responses were defined as the difference between mean, minimum and maximum O₂Hb, HHb and TSI values after standing up and baseline. Test–retest, interobserver and intersensor reliabilities were addressed using intraclass correlation coefficients (ICCs).

Results

The minimum O₂Hb response was most sensitive to postural changes and showed significant differences (− 4.09 µmol/L, p < 0.001) between standing up from sitting and supine position, but not between standing up at different speeds (− 0.31 µmol/L, p = 0.70). The minimum O₂Hb response was the most reliable parameter (ICC > 0.6).

Conclusions

In healthy individuals, NIRS-based cerebral oxygenation parameters are sensitive to postural change and discriminate between standing up from supine and sitting position with minimum O₂Hb response as the most sensitive and reliable parameter. The results underpin the potential value for future clinical use of NIRS in individuals with orthostatic hypotension.

Electronic supplementary material

The online version of this article (10.1007/s00421-019-04101-0) contains supplementary material, which is available to authorized users.

Posture-related changes in brain functional connectivity as assessed by wavelet phase coherence of NIRS signals in elderly subjects

Postural instability and falls are commonly seen because of aging and motor disabilities. This study aims to assess the posture-related changes in brain functional connectivity by wavelet phase coherence (WPCO) of oxyhemoglobin concentration change (Δ[HbO2]) signals measured through near-infrared spectroscopy (NIRS) in elderly subjects. The NIRS signals were continuously recorded from the prefrontal cortex and sensorimotor cortical areas in 39 healthy elderly subjects and 22 young healthy subjects during 20min resting and 10min standing states. Eight connection types were obtained from the recorded brain areas. The WPCO were calculated in five frequency intervals in each channel pair as follows: I, 0.6-2Hz; II, 0.145-0.6Hz; III, 0.052-0.145Hz; IV, 0.021-0.052Hz; and V, 0.0095-0.021Hz. Results show that posture change and age significantly interacts with the right prefrontal cortex (PFC) and left sensorimotor cortex (SMC) connectivity in interval V (F=5.010, p=0.028). The left and right PFC connectivity in interval I, the left and right SMC connectivity in interval IV, and the connectivity in interval V, including right PFC and right SMC connectivity, left PFC and left SMC connectivity, and right PFC and left SMC connectivity, showed a significant difference between the Group Elderly and Group Young in response to posture change (p<0.05). This study provides new insight into the mechanism of posture control, and results may be useful in assessing the risk of postural instability in aged persons.

Methodological comparison of active- and passive-driven oscillations in blood pressure; implications for the assessment of cerebral pressure-flow relationships

We examined the between-day reproducibility of active (squat-stand maneuvers)- and passive [oscillatory lower-body negative pressure (OLBNP) maneuvers]-driven oscillations in blood pressure. These relationships were examined in both younger (n = 10; 25 ± 3 yr) and older (n = 9; 66 ± 4 yr) adults. Each testing protocol incorporated rest (5 min), followed by driven maneuvers at 0.05 (5 min) and 0.10 (5 min) Hz to increase blood-pressure variability and improve assessment of the pressure-flow dynamics using linear transfer function analysis. Beat-to-beat blood pressure, middle cerebral artery velocity, and end-tidal partial pressure of CO2 were monitored. The pressure-flow relationship was quantified in the very low (0.02-0.07 Hz) and low (0.07-0.20 Hz) frequencies (LF; spontaneous data) and at 0.05 and 0.10 Hz (driven maneuvers point estimates). Although there were no between-age differences, very few spontaneous and OLBNP transfer function metrics met the criteria for acceptable reproducibility, as reflected in a between-day, within-subject coefficient of variation (CoV) of <20%. Combined CoV data consist of LF coherence (15.1 ± 12.2%), LF gain (15.1 ± 12.2%), and LF normalized gain (18.5 ± 10.9%); OLBNP data consist of 0.05 (12.1 ± 15.%) and 0.10 (4.7 ± 7.8%) Hz coherence. In contrast, the squat-stand maneuvers revealed that all metrics (coherence: 0.6 ± 0.5 and 0.3 ± 0.5%; gain: 17.4 ± 12.3 and 12.7 ± 11.0%; normalized gain: 16.7 ± 10.9 and 15.7 ± 11.0%; and phase: 11.6 ± 10.2 and 17.3 ± 10.8%) at 0.05 and 0.10 Hz, respectively, were considered biologically acceptable for reproducibility. These findings have important implications for the reliable assessment and interpretation of cerebral pressure-flow dynamics in humans.

Passive ankle movement increases cerebral blood oxygenation in the elderly: an experimental study

BACKGROUND

Ankle exercise has been proven to be an effective intervention to increase venous velocity. However, the efficacy of ankle exercise for improving cerebral circulation has not been determined. We hypothesized that ankle exercise in the supine position would be able to increase oxyhemoglobin levels measured at the forehead.

METHODS

Seventeen community-dwelling elderly women participated in this study. We recorded blood pressure, heart rate (HR), and oxyhemoglobin (OxyHb) levels from the participants in the supine position. Participants repeated ankle plantar flexion and dorsiflexion movements for 1 min. Two types of exercise were used: active movement and passive movement. We used two-way analysis of variance to assess the differences in mean arterial blood pressure (MAP), HR, and OxyHb between different exercises (active and passive) and times (before and after exercise).

RESULTS

The HR and MAP increased during active exercise but not during passive exercise. On the other hand, the levels of OxyHb measured at the forehead were elevated during both active and passive exercises. This increase lasted at least 1 min after exercise. There was no significant difference between active and passive exercise with regard to OxyHb; however, a significant difference was observed between before and after exercise (p < 0.05, η(2) G = 0.153).

CONCLUSIONS

The physiological response of OxyHb to ankle exercise was different from that of the other cardiovascular functions. Both active and passive ankle exercises were able to increase cerebral blood oxygenation, whereas the other cardiovascular functions did not respond to passive exercise.

Changes in cerebral blood oxygenation induced by active standing test in children with POTS and NMS

Orthostatic dysregulation (OD) has been classified into subtypes by heart rate and blood pressure; however, the hemodynamics of brains have not yet been revealed. Therefore, we investigated changes in cerebral blood flow and oxygenation during an active standing test to clarify the pathophysiology of two subtypes: postural tachycardia syndrome (POTS) and neurally mediated syncope (NMS). We studied 31 children (15 boys, 16 girls; mean age, 14.0 ± 1.7 years) who presented with OD at the Department of Pediatrics and Child Health, Nihon University School of Medicine between 2009 and 2011. OD was diagnosed using the Japanese clinical guidelines for juvenile orthostatic dysregulation. After a 10-min resting period in the supine position, patients were asked to quickly stand up and keep upright for 10 min. Cerebral blood flow and cerebral oxygenation were measured using transcranial Doppler sonography and near-infrared spectroscopy. POTS showed a significant decrease of oxy-Hb and resistance index (RI), suggesting transient ischemia with maintainable cerebral autoregulation. NMS showed a decrease of oxy-Hb and an increase of RI, suggesting ischemia and impairment of autoregulation.

Assessment of dynamic cerebral autoregulation and cerebrovascular CO2 reactivity in ageing by measurements of cerebral blood flow and cortical oxygenation

With ageing, cerebral blood flow velocity (CBFV) decreases; however, to what extent dynamic cerebral autoregulation and cerebrovascular CO2 reactivity are influenced by ageing is unknown. The aim was to examine the dynamic responses of CBFV and cortical oxygenation to changes in blood pressure (BP) and arterial CO2 across different ages. Fifty-eight participants in three age groups were included, as follows: young (n = 20, 24 ± 2 years old), elderly (n = 20, 66 ± 1 years old), and older elderly (n = 18, 78 ± 3 years old). The CBFV was measured using transcranial Doppler ultrasound, simultaneously with oxyhaemoglobin (O2Hb) using near-infrared spectroscopy and beat-to-beat BP measurements using Finapres. Postural manoeuvres were performed to induce haemodynamic fluctuations. Cerebrovascular CO2 reactivity was tested with hyperventilation and CO2 inhalation. With age, CBFV decreased (young 59 ± 12 cm s(-1), elderly 48 ± 7 cm s(-1) and older elderly 42 ± 9 cm s(-1), P < 0.05) and cerebrovascular resistance increased (1.46 ± 0.58, 1.81 ± 0.36 and 1.98 ± 0.52 mmHg cm(-1) s(-1), respectively, P < 0.05). Normalized gain (autoregulatory damping) increased with age for BP-CBFV (0.88 ± 0.18, 1.31 ± 0.30 and 1.06 ± 0.34, respectively, P < 0.05) and CBFV-O2Hb (0.10 ± 0.09, 0.12 ± 0.04 and 0.17 ± 0.08, respectively, P < 0.05) during the repeated sit-stand manoeuvre at 0.05 Hz. Even though the absolute changes in CBFV and cerebrovascular resistance index during the cerebrovascular CO2 reactivity were higher in the young group, the percentage changes in CBFV, cerebrovascular resistance index and O2Hb were similar in all age groups. In conclusion, there was no decline in dynamic cerebral autoregulation and cerebrovascular CO2 reactivity with increasing age up to 86 years. Despite the decrease in cerebral blood flow velocity and increase in cerebrovascular resistance with advancing age, CBFV and cortical oxygenation were not compromised in these elderly humans during manoeuvres that mimic daily life activities.

Cognition and Hemodynamics

The relationship between cerebral hemodynamics and cognitive performance has increasingly become recognized as a major challenge in clinical practice for older adults. Both diabetes and hypertension worsen brain perfusion and are major risk factors for cerebrovascular disease, stroke and dementia. Cerebrovascular reserve has emerged as a potential biomarker for monitoring pressure-perfusion-cognition relationships. Endothelial dysfunction and inflammation, microvascular disease, and mascrovascular disease affect cerebral hemodynamics and play an important role in pathohysiology and severity of multiple medical conditions, presenting as cognitive decline in the old age. Therefore, the identification of cerebrovascular vascular reactivity as a new therapeutic target is needed for prevention of cognitive decline late in life.

Evaluating the use of a targeted multiple intervention strategy in reducing patient falls in an acute care hospital: a randomized controlled trial

AIM

This article is a report of a randomized controlled trial to examine the effectiveness of a targeted multiple intervention strategy in reducing the number of patient falls in an acute care hospital.

BACKGROUND

Prevention of patient falls remains a challenge that has eluded healthcare institutions. The effectiveness of targeted multiple fall prevention interventions in reducing the incidences of falling has not been established.

METHODS

Patients who scored 5 and above on the Hendrich II Fall Risk Model, a fall assessment tool, were recruited in 2006. Patients who were randomized to the intervention group received targeted multiple interventions. Both the research groups received the standard fall prevention interventions from the ward nurses. The rates of fall incidences for both groups were reported with 95% CI, calculated using Wilson method and compared using the Chi-square test. The relative risk was estimated and 95% CI was calculated using the methods described by Armitage and Berry. The times to first fall events were constructed using the Kaplan-Meier method. The hazard ratio was reported at 95% CI and the comparison was made using the log-rank test.

RESULTS

There were 912 and 910 participants in the control and intervention groups, respectively. The fall incidence rates were 1·5% (95% CI: 0·9-2·6) and 0·4% (95% CI: 0·2-1·1) in the control and intervention groups, respectively. The relative risk estimate of 0·29 (95% CI: 0·1-0·87) favours the intervention group.

CONCLUSION

This study showed that targeted multiple interventions were effective in reducing the incidences of falls in patients in the acute care setting.

Signal-to-noise ratio of bilateral nonimaging transcranial Doppler recordings of the middle cerebral artery is not affected by age and sex

Differences between transcranial Doppler ultrasonography (TCD) recordings of symmetrical vessels can show true physiologic differences, but can also be caused by measurement error and other sources of noise. The aim of this project was to assess the influence of noise on estimates of dynamic cerebral autoregulation (dCA), and of age, sex and breathing manoeuvres on the signal-to-noise ratio (SNR). Cerebral blood flow (CBF) was monitored in 30 young (<40 years) and 30 older volunteers (age >60 years) during baseline conditions, breath-holding and hyperventilation. Noise was defined as the difference between beat-to-beat values of the two mean CBF velocity (CBFV) signals. Magnitude squared coherence estimates of noise vs. ABP and ABP vs. CBFV were obtained and averaged. A similar approach was adopted for the CBFV step response. The effect of age and breathing manoeuvre on the SNR was assessed using a two-way analysis of variance (ANOVA), whilst the effect of sex was investigated using a Student's t test. No significant differences were observed in SNR (baseline 6.07 ± 3.07 dB and 7.33 ± 3.84 dB, breath-hold: 13.53 ± 3.93 dB and 14.64 ± 4.52 dB, and hyperventilation: 14.69 ± 4.04 dB and 14.84 ± 4.05 dB) estimates between young and old groups, respectively. The use of breathing manoeuvres significantly improved the SNR (p < 10(-4)) without a significant difference between manoeuvres. Sex does not appear to have an effect on SNR (p = 0.365). Coherence estimates were not influenced by the SNR, but significant differences were found in the amplitude of the CBFV step response.

The relationship between blood pressure and cognitive function

The relationship between blood pressure (BP) and cognitive outcomes in elderly adults has implications for global health care. Both hypertension and hypotension affect brain perfusion and worsen cognitive outcomes. The presence of hypertension and other vascular risk factors has been associated with decreased performance in executive function and attention tests. Cerebrovascular reserve has emerged as a potential biomarker for monitoring pressure-perfusion-cognition relationships. A decline in vascular reserve capacity can lead to impaired neurovascular coupling and decreased cognitive ability. Endothelial dysfunction, microvascular disease, and mascrovascular disease in midlife could also have an important role in the manifestations and severity of multiple medical conditions underlying cognitive decline late in life. However, questions remain about the role of antihypertensive therapies for long-term prevention of cognitive decline. In this Review, we address the underlying pathophysiology and the existing evidence supporting the role of vascular factors in late-life cognitive decline.

The effects of healthy aging on cerebral hemodynamic responses to posture change

Aging is associated with an increased incidence of orthostatic hypotension, impairment of the baroreceptor reflex and lower baseline cerebral blood flow. The effect of aging on cerebrovascular autoregulation, however, remains to be fully elucidated. We used a novel optical instrument to assess microvascular cerebral hemodynamics in the frontal lobe cortex of 60 healthy subjects ranging from ages 20-78. Diffuse correlation spectroscopy (DCS) and near-infrared spectroscopy (NIRS) were used to measure relative cerebral blood flow (rCBF), total hemoglobin concentration (THC), oxyhemoglobin concentration (HbO(2)) and deoxyhemoglobin concentration (Hb). Cerebral hemodynamics were monitored for 5 min at each of the following postures: head-of-bed 30 degrees , supine, standing and supine. Supine-to-standing posture change caused significant declines in rCBF, THC and HbO(2), and an increase in Hb, across the age continuum (p < 0.01). Healthy aging did not alter postural changes in frontal cortical rCBF (p = 0.23) and was associated with a smaller magnitude of decline in HbO(2) (p < 0.05) during supine-to-standing posture change. We conclude that healthy aging does not alter postural changes in frontal cortical perfusion.

Cerebral flow velocities during daily activities depend on blood pressure in patients with chronic ischemic infarctions

BACKGROUND AND PURPOSE

Target blood pressure (BP) values for optimal cerebral perfusion after an ischemic stroke are still debated. We sought to examine the relationship between BP and cerebral blood flow velocities (BFVs) during daily activities.

METHODS

We studied 43 patients with chronic large vessel ischemic infarctions in the middle cerebral artery territory (aged 64.2+/-8.94 years; at 6.1+/-4.9 years after stroke) and 67 age-matched control subjects. BFVs in middle cerebral arteries were measured during supine baseline, sitting, standing, and tilt. A regression analysis and a dynamic phase analysis were used to quantify the BP-BFV relationship.

RESULTS

The mean arterial pressure was similar between the groups (89+/-15 mm Hg). Baseline BFVs were lower by approximately 30% in the patients with stroke compared with the control subjects (P=0.0001). BFV declined further with postural changes and remained lower in the stroke group during sitting (P=0.003), standing (P=0.003), and tilt (P=0.002) as compared with the control group. Average BFVs on the stroke side were positively correlated with BP during baseline (R=0.54, P=0.0022, the slope 0.46 cm/s/mm Hg) and tilt (R=0.52, P=0.0028, the slope 0.40 cm/s/mm Hg). Regression analysis suggested that BFV may increase approximately 30% to 50% at mean BP >100 mm Hg. Orthostatic hypotension during the first minute of tilt or standing was independently associated with lower BFV on the stroke side (P=0.0008). Baseline BP-BFV phase shift derived from the phase analysis was smaller on the stroke side (P=0.0006).

CONCLUSIONS

We found that BFVs are lower in patients with stroke and daily activities such as standing could induce hypoperfusion. BFVs increase with mean arterial pressure >100 mm Hg. Dependency of BFV on arterial pressure may have implications for BP management after stroke. Further prospective investigations are needed to determine the impact of these findings on functional recovery and strategies to improve perfusion pressure during daily activities after ischemic stroke.

HomER: a review of time-series analysis methods for near-infrared spectroscopy of the brain

Near-infrared spectroscopy (NIRS) is a noninvasive neuroimaging tool for studying evoked hemodynamic changes within the brain. By this technique, changes in the optical absorption of light are recorded over time and are used to estimate the functionally evoked changes in cerebral oxyhemoglobin and deoxyhemoglobin concentrations that result from local cerebral vascular and oxygen metabolic effects during brain activity. Over the past three decades this technology has continued to grow, and today NIRS studies have found many niche applications in the fields of psychology, physiology, and cerebral pathology. The growing popularity of this technique is in part associated with a lower cost and increased portability of NIRS equipment when compared with other imaging modalities, such as functional magnetic resonance imaging and positron emission tomography. With this increasing number of applications, new techniques for the processing, analysis, and interpretation of NIRS data are continually being developed. We review some of the time-series and functional analysis techniques that are currently used in NIRS studies, we describe the practical implementation of various signal processing techniques for removing physiological, instrumental, and motion-artifact noise from optical data, and we discuss the unique aspects of NIRS analysis in comparison with other brain imaging modalities. These methods are described within the context of the MATLAB-based graphical user interface program, HomER, which we have developed and distributed to facilitate the processing of optical functional brain data.

Quantitative study on cerebral blood volume determined by a near-infrared spectroscopy during postural change in children

AIM

To investigate changes in cerebral blood volume during standing in healthy children with or without abnormal cardiovascular responses.

METHODS

We studied 53 children (age, 10-15 years). Cerebral oxygenated haemoglobin (oxy-Hb) and deoxygenated Hb (deoxy-Hb) were non-invasively and continuously measured using near-infrared spectroscopy (NIRS) (NIRO 300, Hamamatsu Photomedics, Shizuoka, Japan) during active standing. Beat-to-beat arterial pressure was monitored by Portapres.

RESULTS

Of 49 children with complete data acquisition, 33 had a normal cardiovascular response to the test (Group I) and 16 showed an abnormal response (Group II); nine with instantaneous orthostatic hypotension, three with postural tachycardia syndrome, three with neutrally mediated syncope and one with delayed orthostatic hypotension. At the onset of standing, Group II showed a significantly larger fall of oxy-Hb than Group I did (-2.9 +/- 2.8 micromol/L vs. -6.4 +/- 7.2 micromol/L, respectively, p < 0.05). During min 1 to 7 of standing, with one exception, changes in oxy-Hb were normally distributed over the level of -4 micromol/L in Group I. Group II also showed a significantly marked decrease in oxy-Hb compared to Group I. Decreases in oxy-Hb were not correlated with blood pressure changes.

CONCLUSION

This study shows that precise change in cerebral blood volume caused by orthostatic stress can be determined by NIRS in children in a quantitative manner of NIRS. Children with abnormal circulatory responses to standing showed a significant reduction of oxy-Hb compared with normal counterparts, suggesting impairment of cerebral autoregulation in these children.

The effects of age on the spontaneous low-frequency oscillations in cerebral and systemic cardiovascular dynamics

Although the effects of ageing on cardiovascular control and particularly the response to orthostatic stress have been the subject of many studies, the interaction between the cardiovascular and cerebral regulation mechanisms is still not fully understood. Wavelet cross-correlation is used here to assess the coupling and synchronization between low-frequency oscillations (LFOs) observed in cerebral hemodynamics, as measured using cerebral blood flow velocity (CBFV) and cerebral oxygenation (O2Hb), and systemic cardiovascular dynamics, as measured using heart rate (HR) and arterial blood pressure (ABP), in both old and young healthy subjects undergoing head-up tilt table testing. Statistically significant increases in correlation values are found in the interaction of cerebral and cardiovascular LFOs for young subjects (P<0.01 for HR-ABP, P<0.001 for HR-O2Hb and ABP-O2Hb), but not in old subjects under orthostatic stress. The coupling between the cerebrovascular and wider cardiovascular systems in response to orthostatic stress thus appears to be impaired with ageing.

Effects of room temperature and body position change on cerebral blood volume and center-of-foot pressure in healthy young adults

This study aimed to examine the effects of room temperature and body position changes on cerebral blood volume, blood pressure and center-of-foot pressure (COP). Cerebral oxygenation kinetics and blood pressure were measured by near infrared spectroscopy (NIRS) and volume-compensation, respectively, in 9 males and 9 females after rapid standing from sitting and supine positions in low (12 degrees C) or normal (22 degrees C) room temperatures. COP was also measured in a static standing posture for 90 s after rapid standing. The total hemoglobin (Hb) decreased just after standing. Blood pressure after standing at normal temperature tended to decrease immediately but at low temperature tended to decrease slightly and then to increase greatly. The decreasing ratio of total Hb and blood pressure upon standing from a supine position at normal room temperatures was the largest of any condition. Total Hb recovered to a fixed level approximately 25 sec after standing from a sitting position and approximately 35 sec after standing from a supine position. All COP parameters after standing tended to change markedly in the supine position compared to the sitting position, especially at normal temperatures. The COP parameters after standing in any condition were not significantly related to the decreasing ratio of total Hb but were related to the recovery time of total Hb after standing. In conclusion, decreasing ratios of total Hb and blood pressure after standing from a supine position at normal temperatures were large and may affect body sway.

Reproducibility of cerebral blood volume measurements by near infrared spectroscopy in 16 healthy elderly subjects

Near infrared spectroscopy (NIRS) is a non-invasive method to monitor cerebral haemodynamics. Used either alone or in combination with other non-invasive methods such as transcranial Doppler sonography, this technique is well suited for use in cerebrovascular research in ageing. Reproducibility of NIRS, however, has only been determined in neonates and adults. We applied controlled desaturation (the O(2)-method) to measure the cerebral blood volume (CBV) with NIRS in 16 healthy subjects aged 65 to 88. This method uses deoxygenated haemoglobin (the concentration of which is manipulated by desaturation) as an intravascular tracer for NIRS. We determined repeatability (between tests interval: 2 min), short-term reproducibility (intervals of 20 and 40 min) and long-term reproducibility (interval > 2 weeks). We found a coefficient of variation (CV) of 12.5% for repeatability and a CV of 11.7% for short-term reproducibility. The CV for long-term reproducibility was 15%. We conclude that NIRS can reproducibly measure CBV in subjects aged 65 and older, using the O(2)-method. In this group of healthy subjects, this method was well tolerated.

Error and Individual Difference in Cardiovascular Responses to Orthostatic Stress in Humans

Variations in cardiovascular responses to orthostatic stress were investigated in terms of physiological polymorphism. Variations of physiological measurements are subdivided into individual differences and measurement errors. However, individual differences are often considered to be an error in statistical analysis due to its limitations in experimental design. In order to discuss about the relative contribution of individual difference in cardiovascular responses to postural changes, percent contribution (PC) was estimated using the Taguchi method. Six healthy male adults (age range: 21-27) were subjected to orthostatic stress by inducing a postural inclination of 60 degrees head-up-tilting to the horizontal, and the responses were measured thrice in each subject on different days. The respective changes of heart rate (HR) and stroke volume (SV) in the period from the resting supine to the head-up-tilt position were significantly increased (p < 0.01) and decreased (p < 0.01) without affecting the mean blood pressure (MBP). The PC of individual difference in HR showed a significantly higher ratio of individual difference during the head-up-tilt (71.4-76.2%) compared with supine rest (0.0-50.4%). While the main variations of HR during supine rest were not the individual differences between the subjects, the day-to-day differences within the subject were significant. The PC of individual differences in MBP and SV constantly displayed a significant difference between the subjects. These results suggest that the strategy for maintaining stable cardiovascular regulation may be different even in normal subjects. In the perspective of physiological parameters, PC monitoring may serve as an empirical approach to evaluate physiological polymorphism.

Age-correlated changes in cerebral hemodynamics assessed by near-infrared spectroscopy

Cerebral hemodynamic responses due to normal aging may interfere with hormonal changes, drug therapy, diseases, life style, and other factors. Age-correlated alterations in cerebral vasculature and autoregulatory mechanisms are the subject of interest in many studies. Near-infrared spectroscopy (NIRS) is widely used for monitoring cerebral hemodynamics and oxygenation changes at the level of small vessels. We believe that the compensatory ability of cerebral arterioles under hypoxic conditions and the dilatatory ability of cerebral vessels due to vasomotion may decline with normal aging. To test this hypothesis we used frequency-domain NIRS to measure changes in cerebral tissue oxygenation and oxy- and deoxy-hemoglobin concentrations caused by hypoxia during breath holding. We also assessed cerebral vasomotion during profound relaxation. Thirty seven healthy volunteers, 12 females and 25 males, ranging from 22 to 56 years of age (mean age 35 +/- 11 years) participated in the study. We observed age-correlated changes in the cerebral hemodynamics of normal subjects: diminished cerebral hemodynamic response to hypoxia due to breath holding in middle-aged subjects (38-56 years) and reduced amplitude of cerebral hemodynamic changes due to vasomotion during rest. Snoring related changes in cerebral hemodynamics did not allow us to observe the effect of age in a group of snorers. The prolonged supine position influenced measured changes due to hypoxia. In this investigation NIRS methodology allowed detection of age-correlated changes in cerebral oxygenation and hemodynamics. Other variables, such as snoring or posture impacted the observations in our group of healthy volunteers.

Delayed cerebrovascular autoregulatory response to ergometer exercise in normotensive elderly humans

BACKGROUND

Relatively little is known about physiological cerebrovascular haemodynamics during physical stress in elderly healthy individuals. The aim of this study was to determine the effect of ergometer stress on cerebrovascular haemodynamics in elderly healthy individuals in comparison with young healthy individuals, using non-invasive methods.

METHODS

Continuous middle cerebral artery blood flow velocity (CBFV; transcranial Doppler ultrasound), beat-to-beat blood pressure, heart rate and transcutaneous pCO(2) were measured in response to 3 min ergometer exercise stress in 18 elderly healthy subjects (mean age +/- SD 66.5 +/- 5.8 years) and 18 healthy young subjects (mean age +/- SD 29.4 +/- 4.7 years). Pulsatility index (PI) was used as a parameter for cerebrovascular resistance. The subjects were in a supine position with an elevated trunk and performed exercise by pedalling on an ergometer, generating 75-100 W. Statistical analysis was carried out using MANOVA, a general linear model with repeated measures.

RESULTS

In both groups, blood pressure increased significantly (p < 0.001) with time during exercise, with no significant differences between the groups or regarding interaction (time sequence/group factor). Heart rate increased significantly with time during exercise (p < 0.001) and was significantly more prominent (p = 0.002) and prolonged (p < 0.001) in the young group. pCO(2) did not differ with time or between the groups and with regard to interaction. Mean CBFV (MFV) increased significantly during time (p < 0.001). Between the groups, there was no significant difference (p = 0.836), but with regard to interaction (time sequence/group factor), there was a significant delay in MFV increase in the group of young subjects (p = 0.002). The PI, a measure of cerebrovascular resistance, increased significantly with time without significant differences between the groups (p = 0.061), but was significantly delayed in the elderly regarding the interaction time sequence/group factor (p < 0.001).

CONCLUSION

The cerebrovascular changes during ergometer exercise may reflect the combined activation of the cerebrovascular autoregulative mechanisms (predominantly neurogenic and myogenic). In healthy normotensive elderly subjects, cerebral autoregulatory capacity is retained but delayed in response to ergometer stress compared with young healthy subjects. We speculate that these findings may contribute to a higher risk of cerebral hypoperfusion in the elderly.

Cerebral oxygenation responses to standing in elderly patients with predominantly diastolic dysfunction

Patients with left ventricular dysfunction may have different orthostatic responses of blood pressure (BP) and cerebral oxygenation than healthy elderly subjects. We investigated orthostatic changes in systemic haemodynamic variables and cerebral oxygenation in 21 elderly patients with heart failure New York Heart Association class I-III in stable condition (age 70-83 years) after withdrawal of furosemide and captopril for 2 weeks, and in 18 healthy elderly subjects (age 70-84 years). Frontal cortical concentration changes of oxyhaemoglobin ([O2Hb]) and deoxyhaemoglobin ([HHb]) were continuously measured by near-infrared spectrophotometry and BP changes by Finapres before and during 10 min of standing. Upon standing [O2Hb] reflecting blood flow, changed by -1.2 +/- 0.9 micromol L-1 (mean +/- SEM) in the patients, whereas it decreased by -4.5 +/- 0.6 micromol L-1 (P<0.01) in the healthy subjects after standing (P<0.05 between groups). [HHb] reflecting the sum of cerebral blood flow, arterial oxygen saturation and cerebral oxygen uptake, increased by 1.5 +/- 0.5 micromol L-1 (P<0.05) and 1.7 +/- 0.6 micromol L-1 (P<0.05), respectively. Compared with healthy elderly subjects, elderly patients with left ventricular dysfunction showed smaller orthostatic [O2Hb] decreases (P<0.01), in relation to higher orthostatic BP rises (P<0.05). These findings indicate that BP changes and an altered cardiovascular balance may influence orthostatic cortical haemodynamic responses in elderly subjects.

Impaired postural cerebral hemodynamics in young patients with chronic fatigue with and without orthostatic intolerance

OBJECTIVES

To measure postural changes in cerebral hemodynamics in young patients with chronic fatigue with and without orthostatic intolerance.

STUDY DESIGN

We studied 28 patients (age, 10 to 22 years) and 20 healthy control subjects (age, 6 to 27 years). Cerebral oxygenated hemoglobin (oxy-Hb) and deoxygenated Hb were noninvasively and continuously measured with near infrared spectroscopy during active standing. Beat-to-beat arterial pressure was monitored by Finapres.

RESULTS

Orthostatic intolerance determined by cardiovascular responses to standing was observed in 16 of 28 patients: instantaneous orthostatic hypotension in 8, delayed orthostatic hypotension in 2, and postural orthostatic tachycardia in 6. A rapid recovery of oxy-Hb by near infrared spectroscopy at the onset of active standing was not found in 15 of 16 patients with chronic fatigue and orthostatic intolerance and in 6 of 12 patients with chronic fatigue without orthostatic intolerance but only in 2 of 20 control subjects. Thirteen of 16 patients with orthostatic intolerance showed prolonged reduction in oxy-Hb during standing.

CONCLUSIONS

Impaired cerebral hemodynamics in patients with chronic fatigue syndrome and postural orthostatic tachycardia suggest a link between impaired cerebral oxygenation and chronic fatigue. However, this cannot explain the symptoms in patients meeting the criteria of chronic fatigue without orthostatic intolerance.