Rheoencephalography: A non-invasive method for neuromonitoring.

In neurocritical care, the gold standard method is intracranial pressure (ICP) monitoring for the patient's lifesaving. Since it is an invasive method, it is desirable to use an alternative, noninvasive technique. The computerized real-time invasive cerebral blood flow (CBF) autoregulation (AR) monitoring calculates the status of CBF AR, called the pressure reactivity index (PRx). Studies documented that the electrical impedance of the head (Rheoencephalography - REG) can detect the status of CBF AR (REGx) and ICP noninvasively. We aimed to test REG to reflect ICP and CBF AR. For nineteen healthy subjects we recorded bipolar bifrontal and bitemporal REG derivations and arm bioimpedance pulses with a 200 Hz sampling rate. The challenges were a 30-second breath-holding and head-down-tilt (HDT - Trendelenburg) position. Data were stored and processed offline. REG pulse wave morphology and REGx were calculated. The most relevant finding was the significant morphological change of the REG pulse waveform (2nd peak increase) during the HDT position. Breath-holding caused REG amplitude increase, but it was not significant. REGx in male and female group averages have similar trends during HDT by indicating the active status of CBF AR. The morphological change of REG pulse wave during HDT position was identical to ICP waveform change during increased ICP, reflecting decreased intracranial compliance. A correlation study between ICP and REG was initiated in neurocritical care patients. The noninvasive REG monitoring would also be useful in space research as well as in military medicine during the transport of wounded service members as well as for fighter pilots to indicate the loss of CBF and consciousness.

Effects of 30 days bed rest and exercise countermeasures on PBMC bioenergetics.

AIM: Altered mitochondrial function across various tissues is a key determinant of spaceflight-induced physical deconditioning. In comparison to tissue biopsies, blood cell bioenergetics holds promise as a systemic and more readily accessible biomarker, which was evaluated during head-down tilt bed rest (HDTBR), an established ground-based analog for spaceflight-induced physiological changes in humans. More specifically, this study explored the effects of HDTBR and an exercise countermeasure on mitochondrial respiration in peripheral blood mononuclear cells (PBMCs). METHODS: We subjected 24 healthy participants to a strict 30-day HDTBR protocol. The control group (n = 12) underwent HDTBR only, while the countermeasure group (n = 12) engaged in regular supine cycling exercise followed by veno-occlusive thigh cuffs post-exercise for 6 h. We assessed routine blood parameters 14 days before bed rest, the respiratory capacity of PBMCs via high-resolution respirometry, and citrate synthase activity 2 days before and at day 30 of bed rest. We confirmed PBMC composition by flow cytometry. RESULTS: The change of the PBMC maximal oxidative phosphorylation capacity (OXPHOS) amounted to an 11% increase in the countermeasure group, while it decreased by 10% in the control group (p = 0.04). The limitation of OXPHOS increased in control only while other respiratory states were not affected by either intervention. Correlation analysis revealed positive associations between white blood cells, lymphocytes, and basophils with PBMC bioenergetics in both groups. CONCLUSION: This study reveals that a regular exercise countermeasure has a positive impact on PBMC mitochondrial function, confirming the potential application of blood cell bioenergetics for human spaceflight.

Mesial temporal atrophy in preoperative MRI rather than steep Trendelenburg position is associated with postoperative delirium in patients undergoing a major urologic surgery.

PURPOSE: To investigate whether steep Trendelenburg in a major urologic surgery is associated with postoperative delirium, and to examine other potential clinical and radiologic factors predictive of postoperative delirium. METHODS: 182 patients who received a major urologic surgery and underwent a 3.0-T brain MRI scan within 1 year prior to the date of surgery were retrospectively enrolled. Preoperative brain MRIs were used to analyze features related to small vessel disease burden and mesial temporal atrophy. Presence of a significant mesial temporal atrophy was defined as Scheltens' scale ≥ 2. Patients' clinico-demographic data and MRI features were used to identify significant predictors of postoperative delirium using the logistic regression analysis. Independent predictors found significant in the univariate analysis were further evaluated in the multivariate analysis. RESULTS: Incidence of postoperative delirium was 6.0%. Patients with postoperative delirium had lower body mass index (21.3 vs. 25.0 kg/m2, P = 0.003), prolonged duration of anesthesia (362.7 vs. 224.7 min, P < 0.001) and surgery (302.2 vs. 174.5 min, P < 0.001), and had more significant mesial temporal atrophy (64% vs. 30%, P = 0.046). In the univariate analysis, female sex, type of surgery (radical prostatectomy over cystectomy), prolonged duration of anesthesia (≥ 6 h), and presence of a significant mesial temporal atrophy were significant predictors (all P-values < 0.050), but only the presence of significant mesial temporal atrophy was significant in the multivariate analysis [odds ratio (OR), 3.69; 95% CI 0.99-13.75; P = 0.046]. CONCLUSION: Steep Trendelenburg was not associated with postoperative delirium. Significant mesial temporal atrophy (Scheltens' scale ≥ 2) in preoperative brain MRI was predictive of postoperative delirium. TRIAL REGISTRATION: Not applicable.

Head-down tilt reduces the heart rate in postural tachycardia syndrome in acute setting: a pilot study.

BACKGROUND: Reduced preload and thoracic blood volume accompany postural tachycardia syndrome (POTS). Head-down tilt (HDT) increases both preload and intrathoracic blood volume. The objective of this study was to assess the safety and efficacy of HDT in POTS in acute settings. METHODS: This retrospective study evaluated POTS patients. Analyzed data included heart rate, blood pressure, cerebral blood flow velocity (CBFv) in the middle cerebral artery, and capnography. The baseline supine hemodynamic data were compared with the data obtained at the second minute of the -10° HDT. A linear mixed-effects model was used to assess the effect of HDT on hemodynamic variables. RESULTS: The HDT was explored in seven POTS patients and an additional seven POTS patients without HDT served as controls. In the HDT arm, four POTS patients had overlapping diagnoses of myalgic encephalopathy/chronic fatigue syndrome (ME/CFS) and one patient had comorbidity of post-acute sequelae of SARS-CoV-2 infection (PASC). HDT lowered heart rate by 10% and increased end-tidal CO2 by 8%. There was no change in other cardiovascular variables. CONCLUSIONS: In the acute setting, HDT is safe. HDT reduces the heart rate presumably by modulating baroreflex by enhancing preload and stroke volume, which in turn increases thoracic blood volume with a net effect of parasympathetic cardiovagal activation and/or sympathetic withdrawal. This pilot study provides a foundation to proceed with longitudinal studies exploring the long-term effect of repetitive HDT in conditions associated with preload failure such as POTS, ME/CSF, and PASC.

Effects of head-down tilt on optic nerve sheath diameter in healthy subjects.

PURPOSE: Intracranial pressure increases in head-down tilt (HDT) body posture. This study evaluated the effect of HDT on the optic nerve sheath diameter (ONSD) in normal subjects. METHODS: Twenty six healthy adults (age 28 [4.7] years) participated in seated and 6° HDT visits. For each visit, subjects presented at 11:00 h for baseline seated scans and then maintained a seated or 6° HDT posture from 12:00 to 15:00 h. Three horizontal axial and three vertical axial scans were obtained at 11:00, 12:00 and 15:00 h with a 10 MHz ultrasonography probe on the same eye, randomly chosen per subject. At each time point, horizontal and vertical ONSD (mm) were quantified by averaging three measures taken 3 mm behind the globe. RESULTS: In the seated visit, ONSDs were similar across time (p > 0.05), with an overall mean (standard deviation) of 4.71 (0.48) horizontally and 5.08 (0.44) vertically. ONSD was larger vertically than horizontally at each time point (p < 0.001). In the HDT visit, ONSD was significantly enlarged from baseline at 12:00 and 15:00 h (p < 0.001 horizontal and p < 0.05 vertical). Mean (standard error) horizontal ONSD change from baseline was 0.37 (0.07) HDT versus 0.10 (0.05) seated at 12:00 h (p = 0.002) and 0.41 (0.09) HDT versus 0.12 (0.06) seated at 15:00 h (p = 0.002); mean vertical ONSD change was 0.14 (0.07) HDT versus -0.07 (0.04) seated at 12:00 h (p = 0.02) and 0.19 (0.06) HDT versus -0.03 (0.04) seated at 15:00 h (p = 0.01). ONSD change in HDT was similar between 12:00 and 15:00 h (p ≥ 0.30). Changes at 12:00 h correlated with those at 15:00 h for horizontal (r = 0.78, p < 0.001) and vertical ONSD (r = 0.73, p < 0.001). CONCLUSION: The ONSD increased when body posture transitioned from seated to HDT position without any further change at the end of the 3 h in HDT.

Effects of short-term -30° HDT on contrast sensitivity.

Potential cognitive and physiological alterations due to space environments have been investigated in long-term space flight and various microgravity-like conditions, for example, head-down tilt (HDT), confinement, isolation, and immobilization. However, little is known about the influence of simulated microgravity environments on visual function. Contrast sensitivity (CS), which indicates how much contrast a person requires to see a target, is a fundamental feature of human vision. Here, we investigated how the CS changed by 1-h -30° HDT and determined the corresponding mechanisms with a perceptual template model. A quick contrast sensitivity function procedure was used to assess the CS at ten spatial frequencies and three external noise levels. We found that (1) relative to the + 30° head-up tilt (HUT) position, 1-h -30° HDT significantly deteriorated the CS at intermediate frequencies when external noise was present; (2) CS loss was not detected in zero- or high-noise conditions; (3) HDT-induced CS loss was characterized by impaired perceptual template; and (4) self-reported questionnaires indicated that subjects felt less pleasure and more excitement, less comfort and more fatigued by screen light, less comfort in the area around the eye, and serious symptoms such as piercing pain, blur acid, strain, eye burning, and dizziness after HDT. These findings improve our understanding of the negative effects of simulated microgravity on visual function and elucidate the potential risks of astronauts during space flight.

Head Down Tilt 15° in Acute Ischemic Stroke with Poor Collaterals: A Randomized Preclinical Trial.

Cerebral collaterals are recruited after arterial occlusion with a protective effect on tissue outcome in acute ischemic stroke. Head down tilt 15° (HDT15) is a simple, low cost and accessible procedure that could be applied as an emergency treatment, before recanalization therapies, with the aim to increase cerebral collateral flow. Spontaneously hypertensive rats have been shown to display anatomical differences in morphology and function of cerebral collaterals, compared to other rat strains, resulting in an overall poor collateral circulation. We investigate the efficacy and safety of HDT15 in spontaneously hypertensive (SHR) rats, which were considered as an animal stroke model with poor collaterals. Cerebral ischemia was induced by 90 minute endovascular occlusion of the middle cerebral artery (MCA). SHR rats were randomized to HDT15 or flat position (n = 19). HDT15 was applied 30 minutes after occlusion and lasted 60 minutes, until reperfusion. HDT15 application increased cerebral perfusion (+16.6% versus +6.1%; p = 0.0040) and resulted in a small reduction of infarct size (83.6 versus 107.1 mm3; - 21.89%; p = 0.0272), but it was not associated with early neurological improvement, compared to flat position. Our study suggests that the response to HDT15 during MCA occlusion is dependent on baseline collaterals. Nonetheless, HDT15 promoted a mild improvement of cerebral hemodynamics even in subjects with poor collaterals, without safety concerns.

Postural influence on intracranial fluid dynamics: an overview.

This review focuses on the effects of different body positions on intracranial fluid dynamics, including cerebral arterial and venous flow, cerebrospinal fluid (CSF) hydrodynamics, and intracranial pressure (ICP). It also discusses research methods used to quantify these effects. Specifically, the implications of three types of body positions (orthostatic, supine, and antiorthostatic) on cerebral blood flow, venous outflow, and CSF circulation are explored, with a particular emphasis on cerebrovascular autoregulation during microgravity and head-down tilt (HDT), as well as posture-dependent changes in cerebral venous and CSF flow, ICP, and intracranial compliance (ICC). The review aims to provide a comprehensive analysis of intracranial fluid dynamics during different body positions, with the potential to enhance our understanding of intracranial and craniospinal physiology.

Segmental Tissue Resistance of Healthy Young Adults during Four Hours of 6-Degree Head-Down-Tilt Positioning.

(1) Background: One effect of microgravity on the human body is fluid redistribution due to the removal of the hydrostatic gravitational gradient. These fluid shifts are expected to be the source of severe medical risks and it is critical to advance methods to monitor them in real-time. One technique to monitor fluid shifts captures the electrical impedance of segmental tissues, but limited research is available to evaluate if fluid shifts in response to microgravity are symmetrical due to the bilateral symmetry of the body. This study aims to evaluate this fluid shift symmetry. (2) Methods: Segmental tissue resistance at 10 kHz and 100 kHz was collected at 30 min intervals from the left/right arm, leg, and trunk of 12 healthy adults over 4 h of 6° head-down-tilt body positioning. (3) Results: Statistically significant increases were observed in the segmental leg resistances, first observed at 120 min and 90 min for 10 kHz and 100 kHz measurements, respectively. Median increases were approximately 11% to 12% for the 10 kHz resistance and 9% for the 100 kHz resistance. No statistically significant changes in the segmental arm or trunk resistance. Comparing the left and right segmental leg resistance, there were no statistically significant differences in the resistance changes based on the side of the body. (4) Conclusions: The fluid shifts induced by the 6° body position resulted in similar changes in both left and right body segments (that had statistically significant changes in this work). These findings support that future wearable systems to monitor microgravity-induced fluid shifts may only require monitoring of one side of body segments (reducing the hardware needed for the system).

Lower leg blood pressure decreases while calf external pressure increases with the angulation of the Trendelenburg position in the lithotomy position with calf- and foot-supported leg holders.

Patients who underwent lower abdominopelvic surgeries in the lithotomy position (LP) and the Trendelenburg position (TP) with the leg holder are at risk of developing well leg compartment syndrome (WLCS). However, contributing factors related to the LP with TP associated with WLCS are unknown. This study aimed to investigate the associations between external pressure at the calf in the LPs at different angulations of the TP and physiological characteristics. Eighty-four university students (age, 21.7 ± 0.9; 42 men and 42 women) voluntarily participated in the study. The awake participants were placed in the LPs using the calf- and foot-supported leg holder at 0° (horizontal level), 5°, 10°, and 20° head-down tilts by moving the electric operating table. The peak contact pressure (pCP) was measured at the calf as a representative external pressure using the pressure distribution measurement system BIG-MAT®. Lower leg blood pressure significantly decreased with TP angulation, while calf pCP significantly increased with it at 0°, 5°, 10°, and 20° head-down tilts (39.4 ± 15.2, 46.5 ± 17.7, 47.2 ± 16.9, and 50.3 ± 17.6 mmHg, respectively). The calf pCP with a 10° head-down tilt was correlated positively with the calf total force (P < 0.001) and negatively with the calf contact area (P < 0.001). Blood hypoperfusion due to low lower leg blood pressure secondary to lower leg elevation and head-down tilt, and high calf external pressure due to direct external compression from the leg holder where it is loaded may contribute to WLCS.

Head down tilt 15° to preserve salvageable brain tissue in acute ischemic stroke: A pre-clinical pooled analysis, with focus on cerebral hemodynamics.

Neurological outcome after ischemic stroke depends on residual salvageable brain tissue at the time of recanalization. Head down tilt 15° (HDT15) was proven effective in reducing infarct size and improving functional outcome in rats with transient middle cerebral artery occlusion (t-MCAO) by increasing cerebral perfusion within the ischemic penumbra. In this pooled analysis, individual animal-level data from three experimental series were combined in a study population of 104 t-MCAO rats (45 in HDT15 group and 59 in flat position group). Co-primary outcomes were infarct size and functional outcome at 24 h in both groups. The secondary outcome was hemodynamic change induced by HDT15 in ischemic and non-ischemic hemispheres in a subgroup of animals. Infarct size at 24 h was smaller in HDT15 group than in flat position group (absolute mean difference 31.69 mm3 , 95% CI 9.1-54.2, Cohen's d 0.56, p = 0.006). Functional outcome at 24 h was better in HDT15 group than in flat position group (median [IQR]: 13[10-16] vs. 11), with a shift in the distribution of the neurobehavioural scores in favour of HDT15. Mean cerebral perfusion in the ischemic hemisphere was higher during HDT15 than before its application (Perfusion Unit [P.U.], mean ± SD: 52.5 ± 19.52 P.U. vs. 41.25 ± 14.54 P.U., mean of differences 13.36, 95% CI 7.5-19.18, p = 0.0002). Mean cerebral perfusion in the non-ischemic hemisphere before and during HDT15 was unchanged (P.U., mean ± SD: 94.1 ± 33.8 P.U. vs. 100.25 ± 25.34 P.U., mean of differences 3.95, 95%, CI -1.9 to 9.6, p = 0.1576). This study confirmed that HDT15 improves the outcome in t-MCAO rats by promoting cerebral perfusion in the ischemic territory, without disrupting hemodynamics in non-ischemic areas.

The effect of head-down tilt in experimental acute ischemic stroke.

BACKGROUND: Collateral therapeutics exert a promising protective effect on the outcome of acute ischemic stroke. Cerebral blood flow (CBF) may be modulated by different head positioning. The current study aimed to determine the effect of head-down tilt (HDT) on stroke in a rodent model. METHODS: The model of middle cerebral artery occlusion and reperfusion (MCAO/R) was used in this study. Neurological deficit scoring, 2,3,5-triphenyltetrazolium chloride staining, brain water content, perivascular aquaporin protein-4 (AQP4) localization, pericyte marker platelet-derived growth factor receptor ß (PDGFRß), and CBF velocity were evaluated at 24 h after MCAO/R and HDT treatment. RESULTS: In the rat model of MCAO/R, brain infarct volume and neurological deficit score were significantly alleviated in the -30° and -60° groups compared to those in the lying flat (0°) group. Compared with the 0° group, an increase in CBF velocity was detected in the -30° group through two-photon microscopy imaging at 24 h after MCAO/R. Compared with the SHAM group, a decrease in PDGFRß was observed in both the MCAO/R and HDT treatment (-30°) groups. The integrated optical density of PDGFRß was found to be higher in the HDT treatment (-30°) group than in the MCAO/R group. An impairment in perivascular AQP4 polarity and an increase in brain water content were observed after MCAO/R, which were not exacerbated by HDT treatment (-30°). CONCLUSIONS: Our findings suggest that HDT treatment at certain degrees may exert a neuroprotective effect after MCAO/R through improving CBF velocity and the protection of pericytes.

Relationships between common carotid artery blood flow and anesthesia, pneumoperitoneum, and head-down tilt position: a linear mixed-effect analysis.

This study investigated the effects of pneumoperitoneum and the head-down tilt position on common carotid artery (CCA) blood flow in surgical patients. METHODS: This prospective observational study included 20 patients who underwent gynecological surgery. CCA blood flow was measured using Doppler ultrasound at four-time points: awake in the supine position [T1], 3 min after anesthesia induction in the supine position [T2], 3 min after pneumoperitoneum in the supine position [T3], and 3 min after pneumoperitoneum in the head-down tilt position [T4]. Hemodynamic and respiratory parameters were also recorded at each time point. Linear mixed-effect analyses were performed to compare CCA blood flow across the time points and assess its relationship with hemodynamic parameters. RESULTS: Compared with T1, CCA blood flow decreased significantly at T2 (345.4 [288.0-392.9] vs. 293.1 [253.0-342.6], P = 0.048). CCA blood flow were also significantly lower at T3 and T4 compared with T1 (345.4 [288.0-392.9] vs. 283.6 [258.8-307.6] and 287.1 [242.1-321.4], P = 0.005 and 0.016, respectively). CCA blood flow at T3 and T4 did not significantly differ from that at T2. Changes in CCA blood flow were significantly associated with changes in cardiac index and stroke volume index (P = 0.011 and 0.024, respectively). CONCLUSION: CCA blood flow was significantly decreased by anesthesia induction. Inducing pneumoperitoneum, with or without the head-down tilt position, did not further decrease CCA blood flow if the cardiac index remained unchanged. The cardiac index and stroke volume index were significantly associated with CCA blood flow. CLINICAL TRIAL REGISTRATION: Clinicaltrials.gov (NCT04233177, January 18, 2020).

Trendelenburg position for internal jugular vein catheterization: A systematic review and meta-analysis.

To determine the effect of Trendelenburg position on the diameter or cross-section area of the internal jugular vein (IJV) a systematic review and metanalysis was performed. Studies that evaluated the cross-sectional area (CSA) and anteroposterior (AP) diameter of the right internal jugular vein (RIJV) with ultrasonography in supine and any degree of head-down tilt (Trendelenburg position) were analyzed. A total of 22 articles (613 study subjects) were included. A >5° Trendelenburg position statistically increases RIJV CSA and AP diameter. Further inclination from 10° does not statistically benefit IJV size. This position should be recommended for CVC placement, when patient conditions allow it, and US-guided cannulation is not available.

Are head-down tilt bedrest studies capturing the true nature of spaceflight-induced cognitive changes? A review.

Although a number of studies have examined cognitive functions in space, the reasons behind the observed changes described by space research and anecdotal reports have not yet been elucidated. A potential source of cognitive changes is the cephalad fluid shift in the body caused by the lack of hydrostatic pressure under microgravity. These alterations can be modeled under terrestrial conditions using ground-based studies, such as head-down tilt bedrest (HDBR). In this review, we compare the results of the space and HDBR cognitive research. Results for baseline and in-flight/in-HDBR comparisons, and for baseline and post-flight/post-HDBR comparisons are detailed regarding sensorimotor skills, time estimation, attention, psychomotor speed, memory, executive functions, reasoning, mathematical processing, and cognitive processing of emotional stimuli. Beyond behavioral performance, results regarding brain electrical activity during simulated and real microgravity environments are also discussed. Finally, we highlight the research gaps and suggest future directions.

MRI-based quantification of posterior ocular globe flattening during 60 days of strict 6° head-down tilt bed rest with and without daily centrifugation.

Spaceflight associated neuro-ocular syndrome (SANS) is associated with acquired optic disc edema, hyperopia, and posterior globe flattening in some astronauts during long-duration spaceflight possibly due to the headward fluid redistribution in microgravity. The goal of this study was to assess whether strict head-down tilt (HDT) bed rest as a spaceflight analog would produce globe flattening and whether centrifugation could prevent these changes. Twenty-four healthy subjects separated into three groups underwent 60 days of strict 6° HDT bed rest: one control group with no countermeasure (n = 8) and two countermeasure groups exposed to 30 min daily of short-arm centrifugation as a means of artificial gravity (AG), either intermittent (iAG, n = 8) or continuous (cAG, n = 8). Magnetic resonance images (MRI) were collected at baseline, HDT-day 14, HDT-day 52, and 3 days after bed rest. An automated method was applied to quantify posterior globe volume displacement compared with baseline scans. On average, subjects showed an increasing degree of globe volume displacement with bed rest duration (means ± SE: 1.41 ± 1.01 mm3 on HDT14 and 4.04 ± 1.19 mm3 on HDT52) that persisted post-bed rest (5.51 ± 1.26 mm3). Application of 30 min daily AG did not have a significant impact on globe volume displacement (P = 0.42 for cAG and P = 0.93 for iAG compared with control). These results indicate that strict 6° HDT bed rest produced displacement of the posterior globe with a trend of increasing displacement with longer duration that was not prevented by daily 30 min exposure to AG.NEW & NOTEWORTHY Head-down tilt (HDT) bed rest is commonly used as a spaceflight analog for investigating spaceflight associated neuro-ocular syndrome (SANS). Posterior ocular globe flattening has been identified in astronauts with SANS but until now has not been investigated during HDT bed rest. In this study, posterior ocular globe volume displacement was quantified before, during, and after HDT bed rest and countermeasures were tested for their potential to reduce the degree of globe flattening.

Effect of head-down tilt on clinical outcome and cerebral perfusion in ischemic stroke patients: A case series.

Background: The effect of head position on stroke is not clear. The current study aimed to observe the effect of head-down tilt on acute ischemic stroke (AIS) patients with large vessel occlusion. Methods: We observed the influence of head-down tilt position on clinical outcomes, myocardial enzymogram and N-terminal pro b-type Natriuretic Peptide in 4 AIS patients who suffered early neurological deterioration (END). Cerebral perfusion imaging was performed in 3 patients using arterial spin labeling. Results: In series of AIS patients with END, head down tilt (-20°) prevented further neurological deterioration and improved clinical outcomes. An increase in cerebral blood flow was observed by arterial spin labeling after head down tilt treatment. No obvious adverse events occurred. Conclusion: The case series suggest that head-down tilt may improve clinical outcome in AIS patients through increasing the cerebral perfusion with no obvious adverse events. The finding needs to be confirmed in future clinical trials.

How does head position induced intracranial pressure changes impact sympathetic activity and cerebral blood flow?

PURPOSE: Acute head-down-tilt (HDT) simulates short duration hemodynamic impact of microgravity. We sought to determine whether an increase in ICP caused by acute HDT affects sympathetic nervous system activity and cerebral blood flow velocities (CBFV) in healthy male volunteers. METHODS: HDT protocol was established as follows: basal condition immediately followed by gradual negative angles (-10°, -20° and -30°) lasting 10mn and then a return to basal condition. Velocities in the MCA (CBFV) were monitored using TCD. Sympathetic activity was assessed using MSNA. Baroreflex sensitivity (BRS) was measured using the sequence method. ICP changes were assessed using ultrasonography of the optic nerve sheath diameter (ONSD). Cerebral autoregulation (CA) was evaluated by transfer function and the autoregulatory index (Mxa). RESULTS: Twelve male volunteers (age: 35 ± 2 years) were included. Neither blood pressure nor heart rate was significantly modified during HDT. ONSD increased significantly at each step of HDT and remained elevated during Recovery. MSNA burst incidence increased at -30°. A positive correlation between variations in ONSD and variations in MSNA burst incidence was observed at -20°. CBFV were significantly diminished at -20° and -30. In the LF band, the transfer function coherence was reduced at -30° and the transfer function phase was increased at -30° and during Recovery. DISCUSSION: We found that an acute though modest increase in ICP induced by HDT was associated with an increase of sympathetic activity as assessed by MSNA, and with a reduction of CBFV with preserved CA.

Comparison of arterial to end-tidal carbon dioxide gradient P (a-ET)CO2 in volume versus pressure controlled ventilation in patients undergoing robotic abdominal surgery in the Trendelenburg position. A randomised controlled study.

Background and Aims: Robotic surgery is increasingly prevalent as an advancement in care. Steep head-down positions in pelvic surgery can increase the ventilation-perfusion mismatch and increase ventilatory requirements to offset carbon dioxide (CO2) increases consequent to pneumoperitoneum. The primary objective was to assess the impact of two ventilatory strategies, volume versus pressure-controlled ventilation on the arterial to end-tidal carbon dioxide gradient P (a-ET)CO2 in patients undergoing robotic surgery in the Trendelenburg position. The effects on alveolar to arterial oxygen gradient P (A-a)O2, peak airway pressure (Paw), dynamic compliance (Cdyn) and haemodynamics were also assessed. Methods: Fifty-one patients, 18-75 y, American Society of Anesthesiologists I-III undergoing robotic surgery in Trendelenburg position were randomised to volume-controlled ventilation (Group VCV) or pressure-controlled ventilation (Group PCV). The P (a-ET)CO2 was measured at baseline T0, 10 min after Trendelenburg position T1, 2 h of surgery T2, 4 h T3 and at Te, 10 min after deflation. The P (A-a) O2, Paw, Cdyn, heart rate and blood pressure were also measured at the same time. Results: The P (a-ET)CO2 at T1, T2, T3 and at Te was lower in Group PCV versus Group VCV. The Paw was lower at T1, T2, and T3 and Cdyn higher at T3 and Te in Group PCV at comparable minute ventilation. Haemodynamics and P (A-a)O2 were comparable between the groups. Conclusion: Pressure-controlled ventilation reduces P (a-ET)CO2 gradient, Paw and improves Cdyn but does not affect P (A-a) O2 or haemodynamics in comparison to volume-controlled ventilation in robotic surgeries in the Trendelenburg position.