Monitoring of cerebral oxygenation with near infrared spectroscopy and tissue oxygen partial pressure during cardiopulmonary resuscitation in pigs

Differences in Cerebral Oxygenation in Cardiogenic and Respiratory Cardiac Arrest Before, During, and After Cardiopulmonary Resuscitation

We compared the changes in cerebral oxygen saturation (ScO₂) levels during cardiac arrest (CA) events using porcine models of ventricular fibrillation CA (VF-CA) and asphyxial CA (A-CA). Twenty female pigs were randomly divided into VF-CA and A-CA groups. We initiated cardiopulmonary resuscitation (CPR) 4 min after CA and measured the cerebral tissue oxygenation index (TOI) using near-infrared spectroscopy (NIRS) before, during, and after CPR. In both groups, the TOI was the lowest at 3-4 min after pre-CPR phase initiation (VF-CA group: 3.4 min [2.8-3.9]; A-CA group: 3.2 min [2.9-4.6]; p = 0.386). The increase in TOI differed between the groups in the CPR phase (p < 0.001); it increased more rapidly in the VF-CA group (16.6 [5.5-32.6] vs. 1.1 [0.6-3.3] %/min; p < 0.001). Seven pigs surviving for 60 min after the return of spontaneous circulation in the VF-CA group recovered limb movement, whereas only one in the A-CA group (p = 0.023) achieved movement recovery. The increase in the TOI did not differ significantly between the groups in the post-CPR phase (p = 0.341). Therefore, it is better to monitor ScO₂ concomitantly with CPR initiation using NIRS to assess the responsiveness to CPR in clinical settings.

The Influence of Extracerebral Tissue on Continuous Wave Near-Infrared Spectroscopy in Adults: A Systematic Review of In Vivo Studies

Near-infrared spectroscopy (NIRS) is a non-invasive technique for measuring regional tissue haemoglobin (Hb) concentrations and oxygen saturation (rSO₂). It may be used to monitor cerebral perfusion and oxygenation in patients at risk of cerebral ischemia or hypoxia, for example, during cardiothoracic or carotid surgery. However, extracerebral tissue (mainly scalp and skull tissue) influences NIRS measurements, and the extent of this influence is not clear. Thus, before more widespread use of NIRS as an intraoperative monitoring modality is warranted, this issue needs to be better understood. We therefore conducted a systematic review of published in vivo studies of the influence of extracerebral tissue on NIRS measurements in the adult population. Studies that used reference techniques for the perfusion of the intra- and extracerebral tissues or that selectively altered the intra- or extracerebral perfusion were included. Thirty-four articles met the inclusion criteria and were of sufficient quality. In 14 articles, Hb concentrations were compared directly with measurements from reference techniques, using correlation coefficients. When the intracerebral perfusion was altered, the correlations between Hb concentrations and intracerebral reference technique measurements ranged between |r| = 0.45-0.88. When the extracerebral perfusion was altered, correlations between Hb concentrations and extracerebral reference technique measurements ranged between |r| = 0.22-0.93. In studies without selective perfusion modification, correlations of Hb with intra- and extracerebral reference technique measurements were generally lower (|r| < 0.52). Five articles studied rSO₂. There were varying correlations of rSO₂ with both intra- and extracerebral reference technique measurements (intracerebral: |r| = 0.18-0.77, extracerebral: |r| = 0.13-0.81). Regarding study quality, details on the domains, participant selection and flow and timing were often unclear. We conclude that extracerebral tissue indeed influences NIRS measurements, although the evidence (i.e., correlation) for this influence varies considerably across the assessed studies. These results are strongly affected by the study protocols and analysis techniques used. Studies employing multiple protocols and reference techniques for both intra- and extracerebral tissues are therefore needed. To quantitatively compare NIRS with intra- and extracerebral reference techniques, we recommend applying a complete regression analysis. The current uncertainty regarding the influence of extracerebral tissue remains a hurdle in the clinical implementation of NIRS for intraoperative monitoring. The protocol was pre-registered in PROSPERO (CRD42020199053).

Extracorporeal cardiopulmonary resuscitation (eCPR) and cerebral perfusion: A narrative review

Extracorporeal cardiopulmonary resuscitation (eCPR) is emerging as an effective, lifesaving resuscitation strategy for select patients with prolonged or refractory cardiac arrest. Currently, a paucity of evidence-based recommendations is available to guide clinical management of eCPR patients. Despite promising results from initial clinical trials, neurological injury remains a significant cause of morbidity and mortality. Neuropathology associated with utilization of an extracorporeal circuit may interact significantly with the consequences of a prolonged low-flow state that typically precedes eCPR. In this narrative review, we explore current gaps in knowledge about cerebral perfusion over the course of cardiac arrest and resuscitation with a focus on patients treated with eCPR. We found no studies which investigated regional cerebral blood flow or cerebral autoregulation in human cohorts specific to eCPR. Studies which assessed cerebral perfusion in clinical eCPR were small and limited to near-infrared spectroscopy. Furthermore, no studies prospectively or retrospectively evaluated the relationship between epinephrine and neurological outcomes in eCPR patients. In summary, the field currently lacks a comprehensive understanding of how regional cerebral perfusion and cerebral autoregulation are temporally modified by factors such as pre-eCPR low-flow duration, vasopressors, and circuit flow rate. Elucidating these critical relationships may inform future strategies aimed at improving neurological outcomes in patients treated with lifesaving eCPR.

The use of 100% compared to 50% oxygen during ineffective experimental cardiopulmonary resuscitation improves brain oxygenation

INTRODUCTION

Perfusion pressure and chest compression quality are generally considered key determinants of brain oxygenation during cardiopulmonary resuscitation (CPR) and the impact of oxygen administration is less clear. We compared ventilation with 100% and 50% oxygen during ineffective manual chest compressions and hypothesized that 100% oxygen would improve brain oxygenation.

METHODS

Ventricular fibrillation (VF) was induced electrically in anaesthetized pigs and left untreated for 5 minutes, followed by randomization to ineffective manual CPR with ventilation of 50% or 100% oxygen. The first defibrillation was performed 10 minutes after induction of VF, and CPR continued with mechanical chest compressions (LUCAS2™) and defibrillation every 2 minutes until 36 minutes or return of spontaneous circulation (ROSC). Brain oxygenation was measured with near-infrared spectroscopy (rSO₂) and invasive brain tissue oxygen (PbtO₂) with a probe (NEUROVENT-PTO, RAUMEDIC) inserted into frontal brain tissue. Cerebral oxygenation was compared between groups with Mann-Whitney U tests and linear mixed models.

RESULTS

Twenty-eight pigs were included in the study: 14 subjects in each group. During ineffective chest compressions relative PbtO₂ was higher in the group ventilated with 100% compared to 50% oxygen (5.2 mmHg [1.4-20.5] vs 2.2 [0.8-6.8], p = 0.001), but there was no difference in rSO₂ (22% [16-28] vs 18 [15-25], p = 0.090). The use of 50% or 100% oxygen showed no difference in relative PbtO₂ (p = 1.00) and rSO₂ (p = 0.206) during mechanical CPR.

CONCLUSIONS

The use of 100% compared to 50% oxygen during ineffective manual CPR improved brain oxygenation measured invasively in brain tissue, but there was no difference in rSO₂.

Cerebral monitoring in a pig model of cardiac arrest with 48 h of intensive care

Background

Neurological injury is the primary cause of death after out-of-hospital cardiac arrest. There is a lack of studies investigating cerebral injury beyond the immediate post-resuscitation phase in a controlled cardiac arrest experimental setting.

Methods

The aim of this study was to investigate temporal changes in measures of cerebral injury and metabolism in a cardiac arrest pig model with clinically relevant post-cardiac arrest intensive care. A cardiac arrest group (n = 11) underwent 7 min of no-flow and was compared with a sham group (n = 6). Pigs underwent intensive care with 24 h of hypothermia at 33 °C. Blood markers of cerebral injury, cerebral microdialysis, and intracranial pressure (ICP) were measured. After 48 h, pigs underwent a cerebral MRI scan. Data are presented as median [25th; 75th percentiles].

Results

Return of spontaneous circulation was achieved in 7/11 pigs. Time to ROSC was 4.4 min [4.2; 10.9]. Both NSE and NfL increased over time (p < 0.001), and were higher in the cardiac arrest group at 48 h (NSE 4.2 µg/L [2.4; 6.1] vs 0.9 [0.7; 0.9], p < 0.001; NfL 63 ng/L [35; 232] vs 29 [21; 34], p = 0.02). There was no difference in ICP at 48 h (17 mmHg [14; 24] vs 18 [13; 20], p = 0.44). The cerebral lactate/pyruvate ratio had secondary surges in 3/7 cardiac arrest pigs after successful resuscitation. Apparent diffusion coefficient was lower in the cardiac arrest group in white matter cortex (689 × 10^–6 mm²/s [524; 765] vs 800 [799; 815], p = 0.04) and hippocampus (854 [834; 910] vs 1049 [964; 1180], p = 0.03). N-Acetylaspartate was lower on MR spectroscopy in the cardiac arrest group (− 17.2 log [− 17.4; − 17.0] vs − 16.9 [− 16.9; − 16.9], p = 0.03).

Conclusions

We have developed a clinically relevant cardiac arrest pig model that displays cerebral injury as marked by NSE and NfL elevations, signs of cerebral oedema, and reduced neuron viability. Overall, the burden of elevated ICP was low in the cardiac arrest group. A subset of pigs undergoing cardiac arrest had persisting metabolic disturbances after successful resuscitation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40635-022-00475-2.

Adrenaline improves regional cerebral blood flow, cerebral oxygenation and cerebral metabolism during CPR in a porcine cardiac arrest model using low-flow extracorporeal support

BACKGROUND

The effects of adrenaline on cerebral blood vessels during cardiopulmonary resuscitation (CPR) are not well understood. We developed an extracorporeal CPR model that maintains constant low systemic blood flow while allowing adrenaline-associated effects on cerebral vasculature to be assessed at different mean arterial pressure (MAP) levels independently of the effects on systemic blood flow.

METHODS

After eight minutes of cardiac arrest, low-flow extracorporeal life support (ECLS) (30 ml/kg/min) was started in fourteen pigs. After ten minutes, continuous adrenaline administration was started to achieve MAP values of 40 (n = 7) or 60 mmHg (n = 7). Measurements included intracranial pressure (ICP), cerebral perfusion pressure (CePP), laser-Doppler-derived regional cerebral blood flow (CBF), cerebral regional oxygen saturation (rSO₂), brain tissue oxygen tension (P_btO₂) and extracellular cerebral metabolites assessed by cerebral microdialysis.

RESULTS

During ECLS without adrenaline, regional CBF increased by only 5% (25th to 75th percentile: -3 to 14; p=0.2642) and P_btO₂ by 6% (0-15; p=0.0073) despite a significant increase in MAP to 28 mmHg (25-30; p<0.0001) and CePP to 10 mmHg (8-13; p<0.0001). Accordingly, cerebral microdialysis parameters showed a profound hypoxic-ischemic pattern. Adrenaline administration significantly improved regional CBF to 29±14% (p=0.0098) and 61±25% (p<0.001) and P_btO₂ to 15±11% and 130±82% (both p<0.001) of baseline in the MAP 40 mmHg and MAP 60 mmHg groups, respectively. Importantly, MAP of 60 mmHg was associated with metabolic improvement.

CONCLUSION

This study shows that adrenaline administration during constant low systemic blood flow increases CePP, regional CBF, cerebral oxygenation and cerebral metabolism.

Relationship of common hemodynamic and respiratory target parameters with brain tissue oxygen tension in the absence of hypoxemia or hypotension after cardiac arrest: A post-hoc analysis of an experimental study using a pig model

Brain tissue oxygen tension (PbtO2)-guided care, a therapeutic strategy to treat or prevent cerebral hypoxia through modifying determinants of cerebral oxygen delivery, including arterial oxygen tension (PaO2), end-tidal carbon dioxide (ETCO2), and mean arterial pressure (MAP), has recently been introduced. Studies have reported that cerebral hypoxia occurs after cardiac arrest in the absence of hypoxemia or hypotension. To obtain preliminary information on the degree to which PbtO2 is responsive to changes in the common target variables for PbtO2-guided care in conditions without hypoxemia or hypotension, we investigated the relationships between the common target variables for PbtO2-guided care and PbtO2 using data from an experimental study in which the animals did not experience hypoxemia or hypotension after resuscitation. We retrospectively analyzed 170 sets of MAP, ETCO2, PaO2, PbtO2, and cerebral microcirculation parameters obtained during the 60-min post-resuscitation period in 10 pigs resuscitated from ventricular fibrillation cardiac arrest. PbtO2 and cerebral microcirculation parameters were measured on parietal cortices exposed through burr holes. Multiple linear mixed effect models were used to test the independent effects of each variable on PbtO2. Despite the absence of arterial hypoxemia or hypotension, seven (70%) animals experienced cerebral hypoxia (defined as PbtO2 <20 mmHg). Linear mixed effect models revealed that neither MAP nor ETCO2 were related to PbtO2. PaO2 had a significant linear relationship with PbtO2 after adjusting for significant covariates (P = 0.030), but it could explain only 17.5% of the total PbtO2 variance (semi-partial R2 = 0.175; 95% confidence interval, 0.086-0.282). In conclusion, MAP and ETCO2 were not significantly related to PbtO2 in animals without hypoxemia or hypotension during the early post-resuscitation period. PaO2 had a significant linear association with PbtO2, but its ability to explain PbtO2 variance was small.

Effects of different adrenaline doses on cerebral oxygenation and cerebral metabolism during cardiopulmonary resuscitation in pigs

BACKGROUND

The influence of adrenaline during cardiopulmonary resuscitation (CPR) on the neurological outcome of cardiac arrest survivors is unclear. As little is known about the pathophysiological effects of adrenaline on cerebral oxygen delivery and cerebral metabolism we investigated its effects on parameters of cerebral oxygenation and cerebral metabolism in a pig model of CPR.

METHODS

Fourteen pigs were anesthetized, intubated and instrumented. After 5 min of cardiac arrest CPR was started and continued for 15 min. Animals were randomized to receive bolus injections of either 15 or 30 μg/kg adrenaline every 5 min after commencement of CPR.

RESULTS

Measurements included mean arterial pressure (MAP), intracranial pressure (ICP), cerebral perfusion pressure (CPP), cerebral regional oxygen saturation (rSO₂), brain tissue oxygen tension (P_btO₂), arterial and cerebral venous blood gases and cerebral microdialysis parameters, e.g. lactate/pyruvate ratio. Adrenaline induced a significant increase in MAP and CPP in all pigs. However, increases in MAP and CPP were short-lasting and tended to decrease with repetitive bolus administration. There was no statistical difference in any parameter of cerebral oxygenation or metabolism between study groups.

CONCLUSIONS

Both adrenaline doses resulted in short-lasting CPP peaks which did not translate into improved cerebral tissue oxygen tension and metabolism. Further studies are needed to determine whether other dosing regimens targeting a sustained increase in CPP, may lead to improved brain oxygenation and metabolism, thereby improving neurological outcome of cardiac arrest patients.

Effect of Body Temperature on Cerebral Autoregulation in Acutely Comatose Neurocritically Ill Patients

OBJECTIVES

Impaired cerebral autoregulation following neurologic injury is a predictor of poor clinical outcome. We aimed to assess the relationship between body temperature and cerebral autoregulation in comatose patients.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Neurocritical care unit of the Johns Hopkins Hospital.

PATIENTS

Eighty-five acutely comatose patients (Glasgow Coma Scale score of ≤ 8) admitted between 2013 and 2017.

INTERVENTIONS

None.

MEASUREMENT AND MAIN RESULTS

Cerebral autoregulation was monitored using multimodal monitoring with near-infrared spectroscopy-derived cerebral oximetry index. Cerebral oximetry index was calculated as a Pearson correlation coefficient between low-frequency changes in regional cerebral oxygenation saturation and mean arterial pressure. Patients were initially analyzed together, then stratified by temperature pattern over the monitoring period: no change (< 1°C difference between highest and lowest temperatures; n = 11), increasing (≥ 1°C; n = 9), decreasing (≥ 1°C; n = 9), and fluctuating (≥ 1°C difference but no sustained direction of change; n = 56). Mixed random effects models with random intercept and multivariable logistic regression analysis were used to assess the association between hourly temperature and cerebral oximetry index, as well as between temperature and clinical outcomes. Cerebral oximetry index showed a positive linear relationship with temperature (β = 0.04 ± 0.10; p = 0.29). In patients where a continual increase or decrease in temperature was seen during the monitoring period, every 1°C change in temperature resulted in a cerebral oximetry index change in the same direction by 0.04 ± 0.01 (p < 0.001) and 0.02 ± 0.01 (p = 0.12), respectively, after adjusting for PaCO2, hemoglobin, mean arterial pressure, vasopressor and sedation use, and temperature probe location. There was no significant difference in mortality or poor outcome (modified Rankin Scale score of 4-6) between temperature pattern groups at discharge, 3, or 6 months.

CONCLUSIONS

In acute coma patients, increasing body temperature is associated with worsening cerebral autoregulation as measured by cerebral oximetry index. More studies are needed to clarify the impact of increasing temperature on cerebral autoregulation in patients with acute brain injury.

Cerebral Perfusion and Cerebral Autoregulation after Cardiac Arrest

Out of hospital cardiac arrest is the leading cause of death in industrialized countries. Recovery of hemodynamics does not necessarily lead to recovery of cerebral perfusion. The neurological injury induced by a circulatory arrest mainly determines the prognosis of patients after cardiac arrest and rates of survival with a favourable neurological outcome are low. This review focuses on the temporal course of cerebral perfusion and changes in cerebral autoregulation after out of hospital cardiac arrest. In the early phase after cardiac arrest, patients have a low cerebral blood flow that gradually restores towards normal values during the first 72 hours after cardiac arrest. Whether modification of the cerebral blood flow after return of spontaneous circulation impacts patient outcome remains to be determined.

Low cerebral blood flow after cardiac arrest is not associated with anaerobic cerebral metabolism

AIM OF THE STUDY

Estimation of cerebral anaerobic metabolism in survivors and non-survivors after cardiac arrest.

METHODS

We performed an observational study in twenty comatose patients after cardiac arrest and 19 healthy control subjects. We measured mean flow velocity in the middle cerebral artery (MFV_MCA) by transcranial Doppler. Arterial and jugular blood samples were used for calculation of the jugular venous-to-arterial CO₂/arterial to-jugular venous O₂ content difference ratio.

RESULTS

After cardiac arrest, MFV_MCA increased from 26.0[18.6-40.4]cm/sec on admission to 63.9[48.3-73.1]cm/sec after 72h (p<0.0001), with no significant differences between survivors and non-survivors (p=0.4853). The MFV_MCA in controls was 59.1[52.8-69.0]cm/sec. The oxygen extraction fraction (O₂EF) was 38.9[24.4-47.7]% on admission and decreased significantly to 17.3[12.1-26.2]% at 72h (p<0.0001). The decrease in O₂EF was more pronounced in non-survivors (p=0.0173). O₂EF in the control group was 35.4[32.4-38.7]%. The jugular bulb-arterial CO₂ to arterial-jugular bulb O₂ content difference ratio was >1 at all time points after cardiac arrest and did not change during admission, with no differences between survivors and non-survivors. Values in cardiac arrest patients were similar to those in normal subjects.

CONCLUSIONS

In this study, low CBF after cardiac arrest is not associated with anaerobic metabolism. Hypoperfusion appears to be the consequence of a decrease of neuronal functioning and metabolic needs. Alternatively, hypoperfusion may decrease cerebral metabolism. Subsequently, metabolism increases in survivors, consistent with resumption of neuronal activity, whereas in non-survivors lasting low metabolism reflects irreversible neuronal damage.

Arterial pressure, end-tidal carbon dioxide, and central venous oxygen saturation in reflecting compression depth

BACKGROUND

We sought to investigate the utility of arterial pressure, end-tidal carbon dioxide (ETCO2 ), and central venous oxygen saturation (SCVO2 ) to guide compression depth adjustment. Thus, in a pig model of cardiac arrest, we observed these parameters during cardiopulmonary resuscitation (CPR) with optimal and suboptimal compression depths.

METHODS

Sixteen pigs underwent three experimental sessions after induction of ventricular fibrillation. First, the animals received two 4-min CPR trials with either optimal (20% of the anteroposterior diameter) or suboptimal (70% of the optimal depth) compression depth. Second, the animals received two 5-min CPR trials with optimal compression depth, in which adrenaline (0.02 mg/kg) or saline placebo was administered. Third, the animals randomly received compression with either optimal or suboptimal depth during advanced cardiovascular life support.

RESULTS

The systolic arterial pressure reflected compression depth most accurately and immediately (area under the curve [AUC], 0.895-0.939 without adrenaline and 0.928-1.000 with adrenaline). Although the response of ETCO2 to the change in compression depth was 0.5 min slower than that of the systolic arterial pressure, the performance of ETCO2 was comparable with that of systolic arterial pressure. SCVO2 did not reflect compression depth. Adrenaline administration remarkably increased systolic arterial pressure, diastolic arterial pressure, and coronary perfusion pressure but did not affect the ETCO2 readings.

CONCLUSION

In a pig model of cardiac arrest, systolic arterial pressure reflected compression depth immediately and accurately. The performance of ETCO2 was comparable with that of systolic arterial pressure. SCVO2 did not reflect compression depth.

Detection of cerebral arterial gas embolism using regional cerebral oxygen saturation, quantitative electroencephalography, and brain oxygen tension in the swine

BACKGROUND

Cerebral air emboli occur as a complication of invasive medical procedures. The sensitivity of cerebral monitoring methods for the detection of air emboli is not known. This study investigates the utility of electroencephalography and non-invasively measured cerebral oxygen saturation in the detection of intracerebrovascular air.

NEW METHOD

In 12 pigs oxygen saturation was continuously measured using transcranial near-infrared spectroscopy and oxygen tension was continuously measured using intraparenchymal probes. Additionally, quantitative electroencephalography and microdialysis were performed. Doses of 0.2, 0.4, 0.8, and 1.6 ml of air were injected into the cerebral arterial vasculature through a catheter.

RESULTS

Oxygen saturation and electroencephalography both reacted almost instantaneously on the air emboli, but were less sensitive than the intraparenchymal oxygen tension. There was reasonable correlation (ρ ranging from 0.417 to 0.898) between oxygen saturation, oxygen tension, electroencephalography and microdialysis values.

COMPARISON WITH EXISTING METHODS

Our study is the first to demonstrate the effects of cerebral air emboli using multimodal monitoring, specifically on oxygen saturation as measured using near-infrared spectroscopy.

CONCLUSIONS

Our results show that non-invasively measured oxygen saturation and quantitative electroencephalography can detect the local effects of air emboli on cerebral oxygenation, but with reduced sensitivity as compared to intraparenchymal oxygen tension. Prospective human studies using multimodal monitoring incorporating electroencephalography and oxygen saturation should be performed.

Monitoring the quality of cardiopulmonary resuscitation

PURPOSE OF REVIEW

Numerous recent reports have described limitations in the quality of cardiopulmonary resuscitation. Thus, there has been increasing interest in the techniques available to monitor quality. This review focuses on the major publications since the review published by the International Liaison Committee on Resuscitation in 2005. Some key articles published prior to this time period have also been included.

RECENT FINDINGS

A number of devices can monitor various components of the quality of cardiopulmonary resuscitation. End-tidal CO2 measurement assists in confirming placement of endotracheal tubes, correlates with cardiac output and detects the return of spontaneous circulation. Turbine flow-meters monitor respiratory rate and tidal volume. Transthoracic impedance monitoring measures respiratory rate, and may assist in confirmation of endotracheal tube placement. A new mechanical device (CPREzy) and a new defibrillator/monitor allow estimation of depth (and rate) of compressions. Ventricular-fibrillation waveform analysis may facilitate better timing of defibrillation. Echocardiography detects conditions that may impair the quality of cardiopulmonary resuscitation.

SUMMARY

Many options are available to monitor the quality of cardiopulmonary resuscitation. Some have significant limitations, and others are only readily available in hospital. The use of the information from this more intensive monitoring promises to improve outcomes of cardiopulmonary resuscitation.

Impact of norepinephrine and fluid on cerebral oxygenation in experimental hemorrhagic shock

Few data exist regarding resuscitation of hypovolemic shock in infants, and alternative strategies such as vasopressor therapy merit further evaluation. However, the effects of norepinephrine on cerebral perfusion and oxygenation during hemorrhagic shock in the pediatric population are still unclear. Eight anesthetized piglets were subjected to hypotension by blood withdrawal of 25 mL/kg. Norepinephrine was titrated to achieve baseline mean arterial blood pressure (MAP), and cerebral oxygenation was determined by brain tissue Po2 (Ptio2) and near-infrared spectroscopy-derived tissue oxygen index (TOI). Then, norepinephrine was stopped, MAP was allowed to decrease again below 30 mm Hg, and shed blood was retransfused. During hemorrhage, TOI dropped from 69+/-3 to 59+/-3%, and Ptio2 from 29+/-6 to 13+/-1 mm Hg (mean+/-SEM; p<0.001). Following norepinephrine, cerebral perfusion pressure (CPP) could be restored immediately, whereas TOI and Ptio2 did not increase significantly. In contrast, following retransfusion, TOI and Ptio2 increased to 68+/-3% and 27+/-7 mm Hg reaching baseline values, respectively. In conclusion, while norepinephrine increased CPP immediately, cerebral oxygenation as reflected by TOI and Ptio2 could not be improved by norepinephrine, but only by retransfusion.