Head-up tilt and hyperventilation produce similar changes in cerebral oxygenation and blood volume: an observational comparison study using frequency-domain near-infrared spectroscopy

Review of recent advances in frequency-domain near-infrared spectroscopy technologies [Invited]

Over the past several decades, near-infrared spectroscopy (NIRS) has become a popular research and clinical tool for non-invasively measuring the oxygenation of biological tissues, with particular emphasis on applications to the human brain. In most cases, NIRS studies are performed using continuous-wave NIRS (CW-NIRS), which can only provide information on relative changes in chromophore concentrations, such as oxygenated and deoxygenated hemoglobin, as well as estimates of tissue oxygen saturation. Another type of NIRS known as frequency-domain NIRS (FD-NIRS) has significant advantages: it can directly measure optical pathlength and thus quantify the scattering and absorption coefficients of sampled tissues and provide direct measurements of absolute chromophore concentrations. This review describes the current status of FD-NIRS technologies, their performance, their advantages, and their limitations as compared to other NIRS methods. Significant landmarks of technological progress include the development of both benchtop and portable/wearable FD-NIRS technologies, sensitive front-end photonic components, and high-frequency phase measurements. Clinical applications of FD-NIRS technologies are discussed to provide context on current applications and needed areas of improvement. The review concludes by providing a roadmap toward the next generation of fully wearable, low-cost FD-NIRS systems.

Effects of pneumoperitoneum and patient position on intracranial pressure in obese patients undergoing laparoscopic cholecystectomy

Background/Aim: Optic nerve sheath diameter (ONSD) measurement is one of the non-invasive techniques used for intracranial pressure (ICP) measurement. ICP changes have been evaluated based on ONSD measurements during many laparoscopic surgeries. However, such analyses in the obese patient populations are limited. This study aimed at investigating the effects of pneumoperitoneum and reverse Trendelenburg and head-up position on ICP based on ONSD measurements in obese patients undergoing laparoscopic cholecystectomy. Methods: This observational study included 60 female patients who were scheduled for laparoscopic cholecystectomy. Obese patients with a body mass index (BMI) of 30 and above were assigned to Group 1, while BMI < 30 patients were assigned to Group 2. The first ONSD measurement was performed just before insufflation (T1). The second measurement was taken 5 min after insufflation (T2), the third measurement 5 min after placing patients in the reverse Trendelenburg and head-up position (T3), and the last measurement 5 min after the deflation while the reverse Trendelenburg and head-up position was maintained (T4). Results: ONSD measurements at the T2 and T3 time points in Group 1 patients were higher than in Group 2 patients (P = 0.012 versus P = 0.020). Both measurement values were higher in obese patients. In Group 1 patients, T2 and T3 measurements were significantly higher than T1 and T4 measurements (T2 > T1; P < 0.001, T2 > T4; P < 0.001, T3 > T1; P < 0.001, and T3 > T4; P < 0.001). No significant difference between T2 and T3 and between T1 and T4 measurements were found. In Group 2 patients, T2 measurements were significantly higher than the T1, T3, and T4 measurements, while T3 measurements were significantly higher than T1 and T4 measurements (T2 > T1; P < 0.001, T2 > T3; P = 0.022, T2 > T4; P < 0.001, T3 > T1; P < 0.001, and T3 > T4; P = 0.048). No significant difference between T1 and T4 measurements was noted. Conclusion: Laparoscopic cholecystectomy does not cause an increase in ICP of obese patients with limited pneumoperitoneum pressure, reverse Trendelenburg and head-up position, and controlled anesthesia.

Effects on cerebral blood flow of position changes, hyperoxia, CO2 partial pressure variations and the Valsalva manoeuvre: A study in healthy volunteers

BACKGROUND

Maintaining adequate blood pressure to ensure proper cerebral blood flow (CBF) during surgery is challenging. Induced mild hypotension, sitting position or unavoidable intra-operative circumstances such as haemorrhage, added to variations in carbon dioxide and oxygen tensions, may influence perfusion. Several of these circumstances may coincide and it is unclear how these may affect CBF.

OBJECTIVE

To describe the variation in transcranial Doppler and regional cerebral oxygen saturation (rSO2), as a surrogate of CBF, after cardiac preload and gravitational positional changes.

DESIGN

Observational study.

SETTING

Operating room at Hospital Clínic de Barcelona.

VOLUNTEERS

Ten healthy volunteers, white, both sexes.

INTERVENTIONS

Measurements were performed in the supine, sitting and standing positions during hyperoxia, hypocapnia and hypercapnia protocols and after a Valsalva manoeuvre.

MAIN OUTCOME MEASURES

Cardiac index (CI), haemodynamic and respiratory variables, maximal and mean velocities (Vmax, Vmean) (transcranial Doppler) and rSO2 were acquired. Results were analysed using a generalised estimating equation technique.

RESULTS

CI increases more than 16% after a preload challenge were not accompanied by differences in rSO2 or Vmax - Vmean. With positional changes, Vmean decreased more than 7% (P = 0.042) from the supine to the seated position. Hyperoxia induced a cerebral rSO2 increase more than 6% (P = 0.0001) with decreases in Vmax, Vmean and CI values more than 3% (P = 0.001, 0.022 and 0.001) in the supine and standing position. During hypocapnia, CI rose more than 20% from supine to seated and standing (P = 0.0001) with a 4.5% decrease in cerebral rSO2 (P = 0.001) and a decrease of Vmax - Vmean more than 24% in all positions (P = 0.001). Hypercapnia increased cerebral rSO2 more than 17% (P = 0.001), Vmax - Vmean more than 30% (P = 0.001) with no changes in CI. After a Valsalva manoeuvre, rSO2 decreased more than 3% in the right hemisphere in the upright position (P = 0.001). Vmax - Vmean decreased more than 10% (P = 0.001) with no changes in CI.

CONCLUSION

CBF changes in response to cerebral vasoconstriction and vasodilatation were detected with rSO2 and transcranial Doppler in healthy volunteers during cardiac preload and in different body positions. Acute hypercapnia had a greater effect on recorded brain parameters than hypocapnia.

Enhanced recovery pathways for ambulatory surgery

PURPOSE OF REVIEW

Implementation of enhanced recovery pathways have allowed migration of complex surgical procedures from inpatient setting to the outpatient setting. These programs improve patient safety and patient-reported outcomes. The present article discusses the principles of enhanced recovery pathways in adults undergoing ambulatory surgery with an aim of improving patient safety and postoperative outcomes.

RECENT FINDINGS

Procedure and patient selection is one of the key elements that influences perioperative outcomes after ambulatory surgery. Other elements include optimization of comorbid conditions, patient and family education, minimal preoperative fasting and adequate hydration during the fasting period, use of fast-track anesthesia technique, lung-protective mechanical ventilation, maintenance of fluid balance, and multimodal pain, nausea, and vomiting prophylaxis.

SUMMARY

Implementation of enhanced recovery pathways requires a multidisciplinary approach in which the anesthesiologist should take a lead in collaborating with surgeons and perioperative nurses. Measuring compliance with enhanced recovery pathways through an audit program is essential to evaluate success and need for protocol modification. The metrics to assess the impact of enhanced recovery pathways include complication rates, patient reported outcomes, duration of postoperative stay in the surgical facility, unplanned hospital admission rate, and 7-day and 30-day readmission rates.

Reverse Trendelenburg position applied prior to pneumoperitoneum prevents excessive increase in optic nerve sheath diameter in laparoscopic cholecystectomy: randomized controlled trial

The aim of this randomized controlled trial was to determine whether applying the reverse Trendelenburg position before pneumoperitoneum has a preventive effect on increased intracranial pressure using optic nerve sheath diameter (ONSD) measurement as a noninvasive parameter. Seventy-nine patients were allocated to two groups according to whether pneumoperitoneum was applied in the supine position (group S, n = 40) or in the reverse Trendelenburg position (group RT, n = 39). The ONSD was measured at the following time points: T0: before anesthesia; T1: after endotracheal intubation; T2: after pneumoperitoneum in group S and after positioning in group RT; T3: after positioning in group S and after pneumoperitoneum in group RT; T4: 30 min after endotracheal intubation, and T5: after desufflation. The end-tidal carbon dioxide (EtCO₂), regional cerebral oxygen saturation (rSO₂), peripheral oxygen saturation (SpO₂), mean arterial pressure (MAP), heart rate (HR), peak inspiratory pressure (Ppeak), and dynamic compliance (Cdyn) were recorded. Background and perioperative characteristics were similar in both groups. In group S, the ONSD was higher at T2, T3, T4, and T5 than that in group RT (p < 0.001, p = 0.002, p = 0.001, and p = 0.012, respectively). In the same group, the number of patients with an ONSD above 5.8 mm was higher at T2, T3, and T4 (p < 0.001, p = 0.042, p = 0.036, respectively). The rSO2 and SpO2 were not different between the groups. The mean arterial pressure was lower in group RT at T2, and the HR was not different between the groups (p < 0.001). In group S, Ppeak was higher and Cdyn was lower at T2 (p < 0.001). The number of patients with nausea was higher in group S (p = 0.027). The present study demonstrates that applying the reverse Trendelenburg position before pneumoperitoneum prevented an increase in the ONSD in patients undergoing laparoscopic cholecystectomy.Trial registration The trial was registered prior to patient enrollment at https://register.clinicaltrials.gov (NCT04224532, Date of the registration: January 8, 2020).

Frequency-Domain Techniques for Cerebral and Functional Near-Infrared Spectroscopy

This article reviews the basic principles of frequency-domain near-infrared spectroscopy (FD-NIRS), which relies on intensity-modulated light sources and phase-sensitive optical detection, and its non-invasive applications to the brain. The simpler instrumentation and more straightforward data analysis of continuous-wave NIRS (CW-NIRS) accounts for the fact that almost all the current commercial instruments for cerebral NIRS have embraced the CW technique. However, FD-NIRS provides data with richer information content, which complements or exceeds the capabilities of CW-NIRS. One example is the ability of FD-NIRS to measure the absolute optical properties (absorption and reduced scattering coefficients) of tissue, and thus the absolute concentrations of oxyhemoglobin and deoxyhemoglobin in brain tissue. This article reviews the measured values of such optical properties and hemoglobin concentrations reported in the literature for animal models and for the human brain in newborns, infants, children, and adults. We also review the application of FD-NIRS to functional brain studies that focused on slower hemodynamic responses to brain activity (time scale of seconds) and faster optical signals that have been linked to neuronal activation (time scale of 100 ms). Another example of the power of FD-NIRS data is related to the different regions of sensitivity featured by intensity and phase data. We report recent developments that take advantage of this feature to maximize the sensitivity of non-invasive optical signals to brain tissue relative to more superficial extracerebral tissue (scalp, skull, etc.). We contend that this latter capability is a highly appealing quality of FD-NIRS, which complements absolute optical measurements and may result in significant advances in the field of non-invasive optical sensing of the brain.

A Case of Persistence of Normal Tissue Oxygenation Monitored by Near-Infrared Spectroscopy (NIRS) Values Despite Prolonged Perioperative Cardiac Arrest

Patient: Male, 65

Final Diagnosis: Aortic dissection

Symptoms: Hemiplegia • hypotension

Medication:—

Clinical Procedure: Emergent surgery

Specialty: Surgery

Different ventilation techniques and hemodynamic optimization to maintain regional cerebral oxygen saturation (rScO2) during laparoscopic bariatric surgery: a prospective randomized interventional study

PURPOSE

The purpose of this study was to assess the changes in regional cerebral oxygen saturation (rScO₂) in response to different ventilation strategies: inspired oxygen concentration (FiO₂), end-tidal carbon dioxide (EtCO₂), and positive end expiratory pressure (PEEP) in addition to optimizing mean arterial pressure (MAP) in obese patients subjected to laparoscopic bariatric surgery in the reverse trendelenburg position.

METHODS

50 obese patients were randomly assigned into one of two groups. Each group is 25 patients. Control patients subjected to a ventilation strategy aimed to maintain FiO₂ 0.4 and EtCO₂ 30 mmHg without PEEP. Study patients were assigned to specific protocol; T0, baseline rScO₂; T1, 5 min following induction; T2, PP/RTP (10 min after pneumoperitoneum and reverse trendelenburg position); T3, PEEP 10 cmH₂O; T4, FiO₂ 1.0; T5, EtCO₂ 40 mmHg and T6, MAP/BL; MAP back to baseline in both groups.

MAIN RESULTS

10 min after PP/RTP, there was a significant decrease in rScO₂ in both groups. At T4, with FiO₂ 1.0, there was significant improvement in rScO₂ when compared to T2. At T5, with EtCO₂ 40 mmHg, rScO₂ significantly enhanced when compared to EtCO₂ 30 mmHg. At T4 and T5, we observed highly significance difference between both groups. At the end of the procedure and when MAP increased back to baseline (T6) in both groups, rScO₂ statistically increased in both groups when compared to T2.

CONCLUSION

In obese patients, subjected to laparoscopic bariatric surgery in reverse trendelenburg position, adjustment of ventilation strategies and hemodynamic optimization succeeded to improve rScO₂.

Effects of hyperventilation on cerebral oxygen saturation estimated using near-infrared spectroscopy: A randomised comparison between propofol and sevoflurane anaesthesia

BACKGROUND

Near-infrared spectroscopy estimates cerebral regional tissue oxygen saturation (rSO2), which may decrease under hyperventilation. Propofol and sevoflurane act differently on cerebral blood vessels. Consequently, cerebral blood flow during hyperventilation with propofol and sevoflurane anaesthesia may differ.

OBJECTIVES

The first aim of this study was to compare the changes in rSO2 between propofol and sevoflurane anaesthesia during hyperventilation. The second aim was to assess changes in rSO2 with ventilation changes.

DESIGN

A randomised, open-label study.

SETTING

University of Yamanashi Hospital, Yamanashi, Japan from January 2014 to September 2014.

PARTICIPANTS

Fifty American Society of Anesthesiologists physical status 1 or 2 adult patients who were scheduled for elective abdominal surgery were assigned randomly to receive either propofol or sevoflurane anaesthesia. Exclusion criterion was a known history of cerebral disease such as cerebral infarction, cerebral haemorrhage, transient ischaemic attack and subarachnoid haemorrhage.

INTERVENTIONS

After induction of anaesthesia but before the start of surgery, rSO2, arterial carbon dioxide partial pressure (PaCO2) and arterial oxygen saturation were measured. Measurements were repeated at 5-min intervals during 15 min of hyperventilation with a PaCO2 around 30 mmHg (4 kPa), and again after ventilation was normalised.

MAIN OUTCOME MEASURES

The primary outcome was the difference of changes in rSO2 between propofol anaesthesia and sevoflurane anaesthesia during and after hyperventilation. The second outcome was change in rSO2 after the initiation of hyperventilation and after the normalisation of ventilation.

RESULTS

Changes of rSO2 during hyperventilation were -10 ± 7% (left) and -11 ± 8% (right) in the propofol group, and -10 ± 8% (left) and -9 ± 7% (right) in the sevoflurane group. After normalisation of PaCO2, rSO2 returned to baseline values. Arterial oxygen saturation remained stable throughout the measurement period. The rSO2 values were similar in the propofol and the sevoflurane groups at each time point.

CONCLUSION

The effects of hyperventilation on estimated rSO2 were similar with propofol and sevoflurane anaesthesia. Changes in rSO2 correlated well with ventilation changes.

TRIAL REGISTRATION

Japan Primary Registries Network (JPRN); UMIN-CTR ID; UMIN000010640.

Cerebral near-infrared spectroscopy (NIRS) for perioperative monitoring of brain oxygenation in children and adults

BACKGROUND

Various techniques have been employed for the early detection of perioperative cerebral ischaemia and hypoxia. Cerebral near-infrared spectroscopy (NIRS) is increasingly used in this clinical scenario to monitor brain oxygenation. However, it is unknown whether perioperative cerebral NIRS monitoring and the subsequent treatment strategies are of benefit to patients.

OBJECTIVES

To assess the effects of perioperative cerebral NIRS monitoring and corresponding treatment strategies in adults and children, compared with blinded or no cerebral oxygenation monitoring, or cerebral oxygenation monitoring based on non-NIRS technologies, on the detection of cerebral oxygen desaturation events (CDEs), neurological outcomes, non-neurological outcomes and socioeconomic impact (including cost of hospitalization and length of hospital stay).

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL 2016, Issue 12), Embase (1974 to 20 December 2016) and MEDLINE (PubMed) (1975 to 20 December 2016). We also searched the World Health Organization (WHO) International Clinical Trials Registry Platform for ongoing studies on 20 December 2016. We updated this search in November 2017, but these results have not yet been incorporated in the review. We imposed no language restriction.

SELECTION CRITERIA

We included all relevant randomized controlled trials (RCTs) dealing with the use of cerebral NIRS in the perioperative setting (during the operation and within 72 hours after the operation), including the operating room, the postanaesthesia care unit and the intensive care unit.

DATA COLLECTION AND ANALYSIS

Two authors independently selected studies, assessed risk of bias and extracted data. For binary outcomes, we calculated the risk ratio (RR) and its 95% confidence interval (CI). For continuous data, we estimated the mean difference (MD) between groups and its 95% CI. As we expected clinical and methodological heterogeneity between studies, we employed a random-effects model for analyses and we examined the data for heterogeneity (I2 statistic). We created a 'Summary of findings' table using GRADEpro.

MAIN RESULTS

We included 15 studies in the review, comprising a total of 1822 adult participants. There are 12 studies awaiting classification, and eight ongoing studies.None of the 15 included studies considered the paediatric population. Four studies were conducted in the abdominal and orthopaedic surgery setting (lumbar spine, or knee and hip replacement), one study in the carotid endarterectomy setting, and the remaining 10 studies in the aortic or cardiac surgery setting. The main sources of bias in the included studies related to potential conflict of interest from industry sponsorship, unclear blinding status or missing participant data.Two studies with 312 participants considered postoperative neurological injury, however no pooled effect estimate could be calculated due to discordant direction of effect between studies (low-quality evidence). One study (N = 126) in participants undergoing major abdominal surgery reported that 4/66 participants experienced neurological injury with blinded monitoring versus 0/56 in the active monitoring group. A second study (N = 195) in participants having coronary artery bypass surgery reported that 1/96 participants experienced neurological injury in the blinded monitoring group compared with 4/94 participants in the active monitoring group.We are uncertain whether active cerebral NIRS monitoring has an important effect on the risk of postoperative stroke because of the low number of events and wide confidence interval (RR 0.25, 95% CI 0.03 to 2.20; 2 studies, 240 participants; low-quality evidence).We are uncertain whether active cerebral NIRS monitoring has an important effect on postoperative delirium because of the wide confidence interval (RR 0.63, 95% CI 0.27 to 1.45; 1 study, 190 participants; low-quality evidence).Two studies with 126 participants showed that active cerebral NIRS monitoring may reduce the incidence of mild postoperative cognitive dysfunction (POCD) as defined by the original studies at one week after surgery (RR 0.53, 95% CI 0.30 to 0.95, I2 = 49%, low-quality evidence).Based on six studies with 962 participants, there was moderate-quality evidence that active cerebral oxygenation monitoring probably does not decrease the occurrence of POCD (decline in cognitive function) at one week after surgery (RR 0.62, 95% CI 0.37 to 1.04, I2 = 80%). The different type of monitoring equipment in one study could potentially be the cause of the heterogeneity.We are uncertain whether active cerebral NIRS monitoring has an important effect on intraoperative mortality or postoperative mortality because of the low number of events and wide confidence interval (RR 0.63, 95% CI 0.08 to 5.03, I2= 0%; 3 studies, 390 participants; low-quality evidence). There was no evidence to determine whether routine use of NIRS-based cerebral oxygenation monitoring causes adverse effects.

AUTHORS' CONCLUSIONS

The effects of perioperative active cerebral NIRS monitoring of brain oxygenation in adults for reducing the occurrence of short-term, mild POCD are uncertain due to the low quality of the evidence. There is uncertainty as to whether active cerebral NIRS monitoring has an important effect on postoperative stroke, delirium or death because of the low number of events and wide confidence intervals. The conclusions of this review may change when the eight ongoing studies are published and the 12 studies awaiting assessment are classified. More RCTs performed in the paediatric population and high-risk patients undergoing non-cardiac surgery (e.g. neurosurgery, carotid endarterectomy and other surgery) are needed.

Effects of Changes in Arterial Carbon Dioxide and Oxygen Partial Pressures on Cerebral Oximeter Performance

BACKGROUND

Cerebral oximetry (cerebral oxygen saturation; ScO2) is used to noninvasively monitor cerebral oxygenation. ScO2 readings are based on the fraction of reduced and oxidized hemoglobin as an indirect estimate of brain tissue oxygenation and assume a static ratio of arterial to venous intracranial blood. Conditions that alter cerebral blood flow, such as acute changes in PaCO2, may decrease accuracy. We assessed the performance of two commercial cerebral oximeters across a range of oxygen concentrations during normocapnia and hypocapnia.

METHODS

Casmed FORE-SIGHT Elite (CAS Medical Systems, Inc., USA) and Covidien INVOS 5100C (Covidien, USA) oximeter sensors were placed on 12 healthy volunteers. The fractional inspired oxygen tension was varied to achieve seven steady-state levels including hypoxic and hyperoxic PaO2 values. ScO2 and simultaneous arterial and jugular venous blood gas measurements were obtained with both normocapnia and hypocapnia. Oximeter bias was calculated as the difference between the ScO2 and reference saturation using manufacturer-specified weighting ratios from the arterial and venous samples.

RESULTS

FORE-SIGHT Elite bias was greater during hypocapnia as compared with normocapnia (4 ± 9% vs. 0 ± 6%; P < 0.001). The INVOS 5100C bias was also lower during normocapnia (5 ± 15% vs. 3 ± 12%; P = 0.01). Hypocapnia resulted in a significant decrease in mixed venous oxygen saturation and mixed venous oxygen tension, as well as increased oxygen extraction across fractional inspired oxygen tension levels (P < 0.0001). Bias increased significantly with increasing oxygen extraction (P < 0.0001).

CONCLUSIONS

Changes in PaCO2 affect cerebral oximeter accuracy, and increased bias occurs with hypocapnia. Decreased accuracy may represent an incorrect assumption of a static arterial-venous blood fraction. Understanding cerebral oximetry limitations is especially important in patients at risk for hypoxia-induced brain injury, where PaCO2 may be purposefully altered.

Studying cerebral hemodynamics and metabolism using simultaneous near-infrared spectroscopy and transcranial Doppler ultrasound: a hyperventilation and caffeine study

Caffeine is one of the most widely consumed psycho-stimulants in the world, yet little is known about its effects on brain oxygenation and metabolism. Using a double-blind, placebo-controlled, randomized cross-over study design, we combined transcranial Doppler ultrasound (TCD) and near-infrared spectroscopy (NIRS) to study caffeine's effect on middle cerebral artery peak blood flow velocity (Vp), brain tissue oxygenation (S_tO₂), total hemoglobin (tHb), and cerebral oxygen metabolism (CMRO₂) in five subjects. Hyperventilation-induced hypocapnia served as a control to verify the sensitivity of our measurements. During hypocapnia (∼16 mmHg below resting values), Vp decreased by 40.0 ± 2.4% (95% CI, P < 0.001), while S_tO₂ and tHb decreased by 2.9 ± 0.3% and 2.6 ± 0.4%, respectively (P = 0.003 and P = 0.002, respectively). CMRO₂, calculated using the Fick equation, was reduced by 29.3 ± 9% compared to the isocapnic-euoxia baseline (P < 0.001). In the pharmacological experiments, there was a significant decrease in Vp, S_tO₂, and tHb after ingestion of 200 mg of caffeine compared with placebo. There was no significant difference in CMRO₂ between caffeine and placebo. Both showed a CMRO₂ decline compared to baseline showing the importance of a placebo control. In conclusion, this study showed that profound hypocapnia impairs cerebral oxidative metabolism. We provide new insight into the effects of caffeine on cerebral hemodynamics. Moreover, this study showed that multimodal NIRS/TCD is an excellent tool for studying brain hemodynamic responses to pharmacological interventions and physiological challenges.

Mapping distributed brain function and networks with diffuse optical tomography

Mapping of human brain function has revolutionized systems neuroscience. However, traditional functional neuroimaging by positron emission tomography or functional magnetic resonance imaging cannot be used when applications require portability, or are contraindicated because of ionizing radiation (positron emission tomography) or implanted metal (functional magnetic resonance imaging). Optical neuroimaging offers a non-invasive alternative that is radiation free and compatible with implanted metal and electronic devices (for example, pacemakers). However, optical imaging technology has heretofore lacked the combination of spatial resolution and wide field of view sufficient to map distributed brain functions. Here, we present a high-density diffuse optical tomography imaging array that can map higher-order, distributed brain function. The system was tested by imaging four hierarchical language tasks and multiple resting-state networks including the dorsal attention and default mode networks. Finally, we imaged brain function in patients with Parkinson's disease and implanted deep brain stimulators that preclude functional magnetic resonance imaging.

Effect of ventilation on cerebral oxygenation in patients undergoing surgery in the beach chair position: a randomized controlled trial

BACKGROUND

Surgery in the beach chair position (BCP) may reduce cerebral blood flow and oxygenation, resulting in neurological injuries. The authors tested the hypothesis that a ventilation strategy designed to achieve end-tidal carbon dioxide (E'(CO₂)) values of 40-42 mm Hg would increase cerebral oxygenation (Sct(O₂)) during BCP shoulder surgery compared with a ventilation strategy designed to achieve E'(CO₂) values of 30-32 mm Hg.

METHODS

Seventy patients undergoing shoulder surgery in the BCP with general anaesthesia were enrolled in this randomized controlled trial. Mechanical ventilation was adjusted to maintain an E'(CO₂) of 30-32 mm Hg in the control group and an E'(CO₂) of 40-42 mm Hg in the study group. Cerebral oxygenation was monitored continuously in the operating theatre using near-infrared spectroscopy. Baseline haemodynamics and Sct(O₂) were obtained before induction of anaesthesia, and these values were then measured and recorded continuously from induction of anaesthesia until tracheal extubation. The number of cerebral desaturation events (CDEs) (defined as a ≥20% reduction in Sct(O₂) from baseline values) was recorded.

RESULTS

No significant differences between the groups were observed in haemodynamic variables or phenylephrine interventions during the surgical procedure. Sct(O₂) values were significantly higher in the study 40-42 group throughout the intraoperative period (P<0.01). In addition, the incidence of CDEs was lower in the study 40-42 group (8.8%) compared with the control 30-32 group (55.6%, P<0.0001).

CONCLUSIONS

Cerebral oxygenation is significantly improved during BCP surgery when ventilation is adjusted to maintain E'(CO₂) at 40-42 mm Hg compared with 30-32 mm Hg.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov NCT01546636.

The relationship between sympathetic nervous activity and cerebral hemodynamics and oxygenation: a study using skin conductance measurement and functional near-infrared spectroscopy

Simultaneous measurement of cortical and peripheral affective processing is relevant in many neuroscientific research fields. The aim was to investigate the influence of different affective task components on the coherence between cortical hemodynamic signals and peripheral autonomic skin potential signals. Seventeen healthy subjects performed four tasks, i.e. a finger-tapping task, a hyperventilation task, a working memory task and a risk-taking task. Cortical hemodynamic responses were measured using functional near-infrared spectroscopy (fNIRS). Peripheral skin conductance responses (SCRs) were assessed using electrodermal activity (EDA). Coherence between the fNIRS and the EDA time series was calculated using the S transform coherence (STC), a method that tests the temporal dynamics between two time series for consistent phase relationships and thus for a functional relationship. The following characteristics of fNIRS-EDA coherence were observed: (1) Simple motor performance was not a contributor to enhanced coherence, as revealed by the finger-tapping task. (2) Changes in respiration rate and/or heart rate acted as relevant contributors to enhanced coherence, as revealed by the hyperventilation task. (3) Working memory performance did not induce changes in coherence, (4) whereas risk-taking behavior was a significant contributor to enhanced coherence. (5) Based on all four tasks, we also observed that coherence may be subject to habituation or sensitization effects over the trial-to-trial course of a task. Increased fNIRS-EDA coherence may be an indicator of a psychophysiological link between the underlying cortical and peripheral affective systems. Our findings are relevant for several neuroscientific research areas seeking to evaluate the interplay between cortical and peripheral affective performance.

Impact of stepwise hyperventilation on cerebral tissue oxygen saturation in anesthetized patients: a mechanistic study

BACKGROUND

While the decrease in blood carbon dioxide (CO2 ) secondary to hyperventilation is generally accepted to play a major role in the decrease of cerebral tissue oxygen saturation (SctO2 ), it remains unclear if the associated systemic hemodynamic changes are also accountable.

METHODS

Twenty-six patients (American Society of Anesthesiologists I-II) undergoing nonneurosurgical procedures were anesthetized with either propofol-remifentanil (n = 13) or sevoflurane (n = 13). During a stable intraoperative period, ventilation was adjusted stepwise from hypoventilation to hyperventilation to achieve a progressive change in end-tidal CO2 (ETCO2 ) from 55 to 25 mmHg. Minute ventilation, SctO2 , ETCO2 , mean arterial pressure (MAP), and cardiac output (CO) were recorded.

RESULTS

Hyperventilation led to a SctO2 decrease from 78 ± 4% to 69 ± 5% (Δ = -9 ± 4%, P < 0.001) in the propofol-remifentanil group and from 81 ± 5% to 71 ± 7% (Δ = -10 ± 3%, P < 0.001) in the sevoflurane group. The decreases in SctO2 were not statistically different between these two groups (P = 0.5). SctO2 correlated significantly with ETCO2 in both groups (P < 0.001). SctO2 also correlated significantly with MAP (P < 0.001) and CO (P < 0.001) during propofol-remifentanil, but not sevoflurane (P = 0.4 and 0.5), anesthesia.

CONCLUSION

The main mechanism responsible for the hyperventilation-induced decrease in SctO2 is hypocapnia during both propofol-remifentanil and sevoflurane anesthesia. Hyperventilation-associated increase in MAP and decrease in CO during propofol-remifentanil, but not sevoflurane, anesthesia may also contribute to the decrease in SctO2 but to a much smaller degree.

Changes in cerebral tissue oxygen saturation during anaesthetic-induced hypotension: an interpretation based on neurovascular coupling and cerebral autoregulation

There is currently no consensus regarding how to intervene in anaesthetic-induced hypotension. Whether or not the balance between cerebral oxygen supply and demand is maintained lacks adequate elucidation. It is thus intriguing to explore how cerebral tissue oxygen saturation is affected by anaesthetic-induced hypotension. Thirty-three patients scheduled for elective non-neurosurgical procedures were included in this study. Physiological measurements were performed immediately before induction with propofol and fentanyl and after tracheal intubation. Mean (SD) Bispectral index decreased from 84.3 (9.3) to 24.4 (8.0) (p<0.001). Mean arterial pressure decreased from 84.4 (10.6) mmHg to 53.6 (11.4) mmHg (p<0.001). However, cerebral tissue oxygen saturation remained stable (67.0 (9.4) % vs 67.5 (7.8) %, p=0.6). These results imply that the fine balance between cerebral oxygen supply and demand is not disrupted by anaesthetic-induced hypotension. An interpretation based on neurovascular coupling and cerebral autoregulation is proposed.

Diffuse optical imaging using spatially and temporally modulated light

The authors describe the development of diffuse optical imaging (DOI) technologies, specifically the use of spatial and temporal modulation to control near infrared light propagation in thick tissues. We present theory and methods of DOI focusing on model-based techniques for quantitative, in vivo measurements of endogenous tissue absorption and scattering properties. We specifically emphasize the common conceptual framework of the scalar photon density wave for both temporal and spatial frequency-domain approaches. After presenting the history, theoretical foundation, and instrumentation related to these methods, we provide a brief review of clinical and preclinical applications from our research as well as our outlook on the future of DOI technology.