Real-Time Monitoring of Blood Parameters in the Intensive Care Unit: State-of-the-Art and Perspectives

The number of patients in intensive care units has increased over the past years. Critically ill patients are treated with a real time support of the instruments that offer monitoring of relevant blood parameters. These parameters include blood gases, lactate, and glucose, as well as pH and temperature. Considering the COVID-19 pandemic, continuous management of dynamic deteriorating parameters in patients is more relevant than ever before. This narrative review aims to summarize the currently available literature regarding real-time monitoring of blood parameters in intensive care. Both, invasive and non-invasive methods are described in detail and discussed in terms of general advantages and disadvantages particularly in context of their use in different medical fields but especially in critical care. The objective is to explicate both, well-known and frequently used as well as relatively unknown devices. Furtehrmore, potential future direction in research and development of realtime sensor systems are discussed. Therefore, the discussion section provides a brief description of current developments in biosensing with special emphasis on their technical implementation. In connection with these developments, the authors focus on different electrochemical approaches to invasive and non-invasive measurements in vivo.

Cerebral Alterations Following Experimental Multiple Trauma and Hemorrhagic Shock

Multiple trauma (MT) associated with hemorrhagic shock (HS) might lead to cerebral hypoperfusion and brain damage. We investigated cerebral alterations using a new porcine MT/HS model without traumatic brain injury (TBI) and assessed the neuroprotective properties of mild therapeutic hypothermia. Male pigs underwent standardized MT with HS (45% or 50% loss of blood volume) and resuscitation after 90/120 min (T90/T120). In additional groups (TH90/TH120) mild hypothermia (33°C) was induced following resuscitation. Normothermic or hypothermic sham animals served as controls. Intracranial pressure, cerebral perfusion pressure (CPP), and cerebral oxygenation (PtiO2) were recorded up to 48.5 h. Serum protein S-100B and neuron-specific enolase (NSE) were measured by ELISA. Cerebral inflammation was quantified on hematoxylin and eosin -stained brain slices; Iba1, S100, and inducible nitric oxide synthase (iNOS) expression was assessed using immunohistochemistry. Directly after MT/HS, CPP and PtiO2 were significantly lower in T90/T120 groups compared with sham. After resuscitation both parameters showed a gradual recovery. Serum protein S-100B and NSE increased temporarily as a result of MT/HS in T90 and T90/T120 groups, respectively. Cerebral inflammation was found in all groups. Iba1-staining showed significant microgliosis in T90 and T120 animals. iNOS-staining indicated a M1 polarization. Mild hypothermia reduced cerebral inflammation in the TH90 group, but resulted in increased iNOS activation. In this porcine long-term model, we did not find evidence of gross cerebral damage when resuscitation was initiated within 120 min after MT/HS without TBI. However, trauma-related microglia activation and M1 microglia polarization might be a consequence of temporary hypoxia/ischemia and further research is warranted to detail underlying mechanisms. Interestingly, mild hypothermia did not exhibit neuroprotective properties when initiated in a delayed fashion.

The influence of hyperthermia on intracranial pressure, cerebral oximetry and cerebral metabolism in traumatic brain injury

BACKGROUND

Hyperthermia is a common secondary insult in traumatic brain injury (TBI). The aim was to evaluate the relationship between hyperthermia and intracranial pressure (ICP), and if intracranial compliance and cerebral blood flow (CBF) pressure autoregulation affected that relationship. The relationships between hyperthermia and cerebral oximetry (BtipO2) and cerebral metabolism were also studied.

METHODS

A computerized multimodality monitoring system was used for data collection at the neurointensive care unit. Demographic and monitoring data (temperature, ICP, blood pressure, microdialysis, BtipO2) were analyzed from 87 consecutive TBI patients. ICP amplitude was used as measure of compliance, and CBF pressure autoregulation status was calculated using collected blood pressure and ICP values. Mixed models and comparison between groups were used.

RESULTS

The influence of hyperthermia on intracranial dynamics (ICP, brain energy metabolism, and BtipO2) was small, but individual differences were seen. Linear mixed models showed that hyperthermia raises ICP slightly more when temperature increases in the groups with low compliance and impaired CBF pressure autoregulation. There was also a tendency (not statistically significant) for increased BtipO2, and for increased pyruvate and lactate, with higher temperature, while the lactate/pyruvate ratio and glucose were stable.

CONCLUSIONS

The major finding was that the effects of hyperthermia on intracranial dynamics (ICP, brain energy metabolism, and BtipO2) were not extensive in general, but there were exceptional cases. Hyperthermia treatment has many side effects, so it is desirable to identify cases in which hyperthermia is dangerous. Information from multimodality monitoring may be used to guide treatment in individual patients.

Brain tissue oxygenation and cerebral metabolic patterns in focal and diffuse traumatic brain injury

INTRODUCTION

Neurointensive care of traumatic brain injury (TBI) patients is currently based on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) targeted protocols. There are reasons to believe that knowledge of brain tissue oxygenation (BtipO2) would add information with the potential of improving patient outcome. The aim of this study was to examine BtipO2 and cerebral metabolism using the Neurovent-PTO probe and cerebral microdialysis (MD) in TBI patients.

METHODS

Twenty-three severe TBI patients with monitoring of physiological parameters, ICP, CPP, BtipO2, and MD for biomarkers of energy metabolism (glucose, lactate, and pyruvate) and cellular distress (glutamate, glycerol) were included. Patients were grouped according to injury type (focal/diffuse) and placement of the Neurovent-PTO probe and MD catheter (injured/non-injured hemisphere).

RESULTS

We observed different patterns in BtipO2 and MD biomarkers in diffuse and focal injury where placement of the probe also influenced the results (ipsilateral/contralateral). In all groups, despite fairly normal levels of ICP and CPP, increased MD levels of glutamate, glycerol, or the L/P ratio were observed at BtipO2 <5 mmHg, indicating increased vulnerability of the brain at this level.

CONCLUSION

Monitoring of BtipO2 adds important information in addition to traditional ICP and CPP surveillance. Because of the different metabolic responses to very low BtipO2 in the individual patient groups we submit that brain tissue oximetry is a complementary tool rather than an alternative to MD monitoring.

The oxygen reactivity index and its relation to sensor technology in patients with severe brain lesions

BACKGROUND

The oxygen reactivity index (ORx) has been introduced to assess the status of cerebral autoregulation after traumatic brain injury (TBI) or subarachnoid hemorrhage (SAH). Currently, there is some controversy about whether the ORx depends on the type of PbrO2-sensor technology used for its calculation. To examine if the probe technology does matter, we compared the ORx and the resulting optimal cerebral perfusion pressures (CPPopt) of simultaneously implanted Licox (CC1.SB, Integra Neuroscience, France) and Neurovent-PTO (Raumedic, Germany) probes in patients after aneurysmal SAH or severe TBI.

METHODS

Licox and Raumedic probes were implanted side by side in 11 patients after TBI or SAH. ORx and CPPopt were recorded continuously. The equivalence of both probes was examined using Bland-Altman analyses.

RESULTS

The mean difference in ORx was 0.1, with Licox producing higher values. The limits of agreement regarding ORx ranged from -0.6 to +0.7. When both probes' ORx values were compared in each patient, no specific pattern in their relationship was seen. The mean difference in CPPopt was 0 mmHg with limits of agreement between -16.5 and +16.4 mmHg.

CONCLUSIONS

Owing to the rather limited number of patients, we view the results of this study as preliminary. The main result is that Licox and Raumedic showed consistent differences in ORx and CPPopt. Therefore, ORx values of both probes cannot be interchanged and should not be viewed as equivalent. This should be taken into consideration when discussing ORx data generated by different PbrO2 probe types.

[Multimodal monitoring in neurointensive care medicine: state of the art]

The prognosis of neurointensive care patients depends largely on the occurrence of secondary ischemic/hypoxic tissue damage, which is mediated by different pathomechanisms, such as edema formation or increased intracranial pressure. Due to the cerebral damage and need for sedation as well as intubation, clinical assessment of these patients is limited. Furthermore, clinical signs of secondary damage, such as advanced herniation syndromes are often delayed and therefore mostly indicate irreversible brain damage. To adequately predict and detect secondary neuronal damage, various neuromonitoring techniques have been developed in recent years with ongoing technical refinement. These can be used for bedside and ideally continuous monitoring of various functional systems of the brain. Neuromonitoring is used to implement early therapeutic measures before irreversible brain damage has occurred, to monitor therapeutic effects, for evaluation of the prognosis and to improve the neurological outcome of patients. Different monitoring techniques are often combined in multimodal neuromonitoring. This article gives an overview of the most promising neuromonitoring techniques available.

Parenchymal brain oxygen monitoring in the neurocritical care unit

Patients admitted to the neurocritical care unit (NCCU) often have serious conditions that can be associated with high morbidity and mortality. Pharmacologic agents or neuroprotectants have disappointed in the clinical environment. Current NCCU management therefore is directed toward identification, prevention, and treatment of secondary cerebral insults that evolve over time and are known to aggravate outcome. This strategy is based on a variety of monitoring techniques including use of intraparenchymal monitors. This article reviews parenchymal brain oxygen monitors, including the available technologies, practical aspects of use, the physiologic rationale behind their use, and patient management based on brain oxygen.

Multi frequency phase fluorimetry (MFPF) for oxygen partial pressure measurement: ex vivo validation by polarographic clark-type electrode

Background

Measurement of partial pressure of oxygen (P_O2) at high temporal resolution remains a technological challenge. This study introduces a novel P_O2 sensing technology based on Multi-Frequency Phase Fluorimetry (MFPF). The aim was to validate MFPF against polarographic Clark-type electrode (CTE) P_O2 measurements.

Methodology/Principal Findings

MFPF technology was first investigated in N = 8 anaesthetised pigs at F_IO2 of 0.21, 0.4, 0.6, 0.8 and 1.0. At each F_IO2 level, blood samples were withdrawn and P_O2 was measured in vitro with MFPF using two FOXY-AL300 probes immediately followed by CTE measurement. Secondly, MFPF-P_O2 readings were compared to CTE in an artificial circulatory setup (human packed red blood cells, haematocrit of 30%). The impacts of temperature (20, 30, 40°C) and blood flow (0.8, 1.6, 2.4, 3.2, 4.0 L min⁻¹) on MFPF-P_O2 measurements were assessed. MFPF response time in the gas- and blood-phase was determined. Porcine MFPF-P_O2 ranged from 63 to 749 mmHg; the corresponding CTE samples from 43 to 712 mmHg. Linear regression: CTE = 15.59+1.18*MFPF (R² = 0.93; P<0.0001). Bland Altman analysis: mean_diff 69.2 mmHg, range_diff -50.1/215.6 mmHg, 1.96-SD limits -56.3/194.8 mmHg. In artificial circulatory setup, MFPF-P_O2 ranged from 20 to 567 mmHg and CTE samples from 11 to 575 mmHg. Linear regression: CTE = −8.73+1.05*MFPF (R² = 0.99; P<0.0001). Bland-Altman analysis: mean_diff 6.6 mmHg, range_diff -9.7/20.5 mmHg, 1.96-SD limits -12.7/25.8 mmHg. Differences between MFPF and CTE-P_O2 due to variations of temperature were less than 6 mmHg (range 0–140 mmHg) and less than 35 mmHg (range 140–750 mmHg); differences due to variations in blood flow were less than 15 mmHg (all P-values>0.05). MFPF response-time (monoexponential) was 1.48±0.26 s for the gas-phase and 1.51±0.20 s for the blood-phase.

Conclusions/Significance

MFPF-derived P_O2 readings were reproducible and showed excellent correlation and good agreement with Clark-type electrode-based P_O2 measurements. There was no relevant impact of temperature and blood flow upon MFPF-P_O2 measurements. The response time of the MFPF FOXY-AL300 probe was adequate for real-time sensing in the blood phase.

Cortical spreading depression dynamics can be studied using intrinsic optical signal imaging in gyrencephalic animal cortex

OBJECTIVE

The aim of this study was to co-record electrical changes using electrocorticography (ECoG) and blood volume changes using intrinsic optical signal (IOS) imaging during the induction, propagation, and termination of cortical spreading depolarizations (CSDs).

METHODS

Anesthetized male swine were craniotomized and monitored over 16-20 h. A ten-contact electrode strip was placed on the cortex of one hemisphere for ECoG. An optical imaging recording was implemented using a camera with an optical bandpass filter (564 nm, FWHM:15 nm) and a full spectrum light source. CSDs were induced by mechanical and KCl stimulation. Co-occurrences of ECoG baseline shifts and blood volume changes around electrodes were identified.

RESULTS

A mean of 3 CSDs per hour were induced, in a total of 4 swine during 80 h of recording. The propagation of the CSDs increased progressively over the monitoring time. IOS enabled us to clearly visualize the induction, propagation, and termination of CSDs with a spatial resolution within the sub-millimeter range. Every CSD recorded using ECoG could also be observed in IOS imaging, although some blood volume changes of CSDs were observed that terminated before reaching any of the ECoG electrodes.

CONCLUSION

IOS imaging enables an in vivo evaluation of CSD dynamics over a large surface of gyrencephalic brain.

Brain tissue oxygenation and cerebral perfusion pressure thresholds of ischemia in a standardized pig brain death model

BACKGROUND

Neurointensive care of traumatic brain injury (TBI) patients is currently based on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) targeted protocols. Monitoring brain tissue oxygenation (BtipO2) is of considerable clinical interest, but the exact threshold level of ischemia has been difficult to establish due to the complexity of the clinical situation. The objective of this study was to use the Neurovent-PTO (NV) probe, and to define critical cerebral oxygenation- and CPP threshold levels of cerebral ischemia in a standardized brain death model caused by increasing the ICP in pig. Ischemia was defined by a severe increase of cerebral microdialysis (MD) lactate/pyruvate ratio (L/P ratio > 30).

METHODS

BtipO2, L/P ratio, Glucose, Glutamate, Glycerol and CPP were recorded using NV and MD probes during gradual increase of ICP by inflation of an epidural balloon catheter with saline until brain death was achieved.

RESULTS

Baseline level of BtipO2 was 22.9 ± 6.2 mmHg, the L/P ratio 17.7 ± 6.1 and CPP 73 ± 17 mmHg. BtipO2 and CPP decreased when intracranial volume was added. The L/P ratio increased above its ischemic levels, (>30)when CPP decreased below 30 mmHg and BtipO2 to <10 mmHg.

CONCLUSIONS

A severe increase of ICP leading to CPP below 30 mmHg and BtipO2 below 10 mmHg is associated with an increase of the L/P ratio, thus seems to be critical thresholds for cerebral ischemia under these conditions.

Brain tissue oxygen monitoring and hyperoxic treatment in patients with traumatic brain injury

Cerebral ischemia is a well-recognized contributor to high morbidity and mortality after traumatic brain injury (TBI). Standard of care treatment aims to maintain a sufficient oxygen supply to the brain by avoiding increased intracranial pressure (ICP) and ensuring a sufficient cerebral perfusion pressure (CPP). Devices allowing direct assessment of brain tissue oxygenation have showed promising results in clinical studies, and their use was implemented in the Brain Trauma Foundation Guidelines for the treatment of TBI patients in 2007. Results of several studies suggest that a brain tissue oxygen-directed therapy guided by these monitors may contribute to reduced mortality and improved outcome of TBI patients. Whether increasing the oxygen supply to supraphysiological levels has beneficial or detrimental effects on TBI patients has been a matter of debate for decades. The results of trials of hyperbaric oxygenation (HBO) have failed to show a benefit, but renewed interest in normobaric hyperoxia (NBO) in the treatment of TBI patients has emerged in recent years. With the increased availability of advanced neuromonitoring devices such as brain tissue oxygen monitors, it was shown that some patients might benefit from this therapeutic approach. In this article, we review the pathophysiological rationale and technical modalities of brain tissue oxygen monitors, as well as its use in studies of brain tissue oxygen-directed therapy. Furthermore, we analyze hyperoxia as a treatment option in TBI patients, summarize the results of clinical trials, and give insights into the recent findings of hyperoxic effects on cerebral metabolism after TBI.

Comparison of a new brain tissue oxygenation probe with the established standard

INTRODUCTION

Continuous bedside brain tissue oxygenation (p(br)O(2)) monitoring using the Licox system is an established method for detecting secondary ischemia in comatose patients with acute brain injury. The purpose of the current study was to compare the newly introduced Raumedic p(br)O(2) probe with the established standard.

METHODS

Eighteen patients with acute traumatic brain injury or aneurysmal subarachnoid hemorrhage had p(br)O(2) probes of both types implanted side by side in the same vascular territory at risk of ischemia. Data were analyzed by the Bland-Altman method as well as random effect regression models to correct for multiple measurements per individual.

RESULTS

Both types of probes were able to display spontaneous fluctuations of p(br)O(2) as well as reactions to therapy. Mean measurement difference between the Licox and Raumedic probes was -2.3 mmHg, with corresponding 95% limits of agreement of -32.3 to 27.5 mmHg. Regarding an ischemia threshold of 15 mmHg, both probes were in agreement in 78% and showed disparate results in 22%.

CONCLUSIONS

Our data suggest that the p(br)O(2) measurements of the two systems cannot be interchanged. Although we were unable to determine which system delivers more valid data, we do think that more rigorous testing is necessary before implementing the new probe in clinical routine.

Influence of respiratory rate and end-expiratory pressure variation on cyclic alveolar recruitment in an experimental lung injury model

Introduction

Cyclic alveolar recruitment/derecruitment (R/D) is an important mechanism of ventilator-associated lung injury. In experimental models this process can be measured with high temporal resolution by detection of respiratory-dependent oscillations of the paO₂ (ΔpaO₂). A previous study showed that end-expiratory collapse can be prevented by an increased respiratory rate in saline-lavaged rabbits. The current study compares the effects of increased positive end-expiratory pressure (PEEP) versus an individually titrated respiratory rate (RR_ind) on intra-tidal amplitude of Δ paO₂ and on average paO₂ in saline-lavaged pigs.

Methods

Acute lung injury was induced by bronchoalveolar lavage in 16 anaesthetized pigs. R/D was induced and measured by a fast-responding intra-aortic probe measuring paO₂. Ventilatory interventions (RR_ind (n = 8) versus extrinsic PEEP (n = 8)) were applied for 30 minutes to reduce Δ paO₂. Haemodynamics, spirometry and Δ paO₂ were monitored and the Ventilation/Perfusion distributions were assessed by multiple inert gas elimination. The main endpoints average and Δ paO₂ following the interventions were analysed by Mann-Whitney-U-Test and Bonferroni's correction. The secondary parameters were tested in an explorative manner.

Results

Both interventions reduced Δ paO₂. In the RR_ind group, ΔpaO₂ was significantly smaller (P < 0.001). The average paO₂ continuously decreased following RR_ind and was significantly higher in the PEEP group (P < 0.001). A sustained difference of the ventilation/perfusion distribution and shunt fractions confirms these findings. The RR_ind application required less vasopressor administration.

Conclusions

Different recruitment kinetics were found compared to previous small animal models and these differences were primarily determined by kinetics of end-expiratory collapse. In this porcine model, respiratory rate and increased PEEP were both effective in reducing the amplitude of paO₂ oscillations. In contrast to a recent study in a small animal model, however, increased respiratory rate did not maintain end-expiratory recruitment and ultimately resulted in reduced average paO₂ and increased shunt fraction.

A novel technique for monitoring of fast variations in brain oxygen tension using an uncoated fluorescence quenching probe (Foxy AL-300)

BACKGROUND

A novel uncoated fluorescence quenching probe allows fast measurement of oxygen tension in vessels and tissue. The present study reports the first use of the technology for dual measurements of arterial (paO(2)) and brain tissue oxygen tension (ptiO(2)) during hypoxic challenge in a pig model.

METHODS

Eight pigs were anesthetized using fentanyl and propofol. Fluorescence quenching pO(2) probes (Foxy AL-300, Ocean Optics, Dunedin, FL) were placed in the ascending aorta (Foxy-paO(2)) and subcortically at 14 mm in brain tissue (Foxy-ptiO(2)). As reference, a clark-type electrode probe (Licox-ptiO(2)) was placed into brain tissue close to the Foxy probe (Licox, Integra Neurosciences, Plainsboro, NJ). Measurements were taken at baseline (FiO(2) 1.0), during episodes of apnea, and during recovery (FiO(2) 1.0).

STATISTICS

descriptive results.

RESULTS

Individual Foxy-paO(2), Foxy-ptiO(2), and Licox-ptiO(2) courses were related to episodes of apnea. The response time of the Foxy measurements was 10 Hz. Baseline values at FiO(2) 1.0 were Foxy-paO(2) 520±120 mm Hg, Foxy-ptiO(2) 62±24 mm Hg, and Licox-ptiO(2) 55±29 mm Hg; apnea values were Foxy-paO(2) 64±10 mm Hg, Foxy-ptiO(2) 37±12 mm Hg, and Licox-ptiO(2) 31±16 mm Hg; recovery values at FiO(2) 1.0 were Foxy-paO(2) 478±98 mm Hg, Foxy-ptiO(2) 78±26 mm Hg, and Licox-ptiO(2) 62±32 mm Hg.

CONCLUSIONS

The present study demonstrates the feasibility of pO(2) measurements in macrocirculation and cerebral microcirculation using a novel uncoated fluorescence quenching probe. The technology allows for real-time investigation of pO(2) changes at a temporal resolution of 0.05 to 10 Hz.

Multiparametric characterisation of the perihemorrhagic zone in a porcine model of lobar ICH

OBJECTIVES

To describe early perihemorrhagic changes after lobar intracerebral hemorrhage (ICH) using multiparametric neuromonitoring [intracranial pressure (ICP), cerebral blood flow (CBF), tissue oxygenation (PbrO2), microdialysis (MD)].

METHODS

Seven anaesthetized male swine were examined over 12 h. Four cerebral probes were inserted around the ICH (ICP, MD, CBF and PbrO2). A right frontal autologous arterial ICH (1.5 mL) was induced in all animals.

RESULTS

Initial ICH creation was hampered by using a soft 22-G cannula. A modified injection technique with a 90° bent steel cannula (20 G) allowed for an 87.5% success rate in ICH formation. After induction of ICH, ICP significantly increased from 2 mmHg to 9 mmHg. No significant PbrO2 or CBF reduction occurred during the monitoring period. Consequently, microdialysis did not indicate overall mean deterioration in the hematoma group over time. The indicator of ischemia (extracellular lactate) did not increase significantly during the monitoring period. Individual monitoring episodes demonstrated hypoxic episodes with consecutive metabolic derangement. These effects were reversible by optimizing CPP and FiO2.

CONCLUSION

We established a reproducible cortical ICH model using multiparametric neuromonitoring. Subtle changes in ICP were observed. No evidence for the existence of a perihemorrhagic ischemic area was found, hypothetically because of the small hematoma size. Individual animals underwent critical PbrO2 and CBF decreases with consecutive metabolic derangement. The effect of larger hematoma volumes should be evaluated with this setup in future studies to study volume-dependent deterioration.

Cerebral tissue oxygenation measured by two different probes: challenges and interpretation

PURPOSE

Cerebral tissue oxygenation (PbrO(2)) is most frequently monitored using a Licox CC1.SB system (LX, Integra Neuroscience, France) but recently a new probe--the Neurovent-PTO (NV)--was introduced by a different manufacturer (Raumedic, Germany). There are no prospective data on how these probes compare in clinical routine. We therefore compared both probes in comatose patients suffering from traumatic brain injury (TBI) or subarachnoid haemorrhage (SAH) during dynamic changes of inspirational oxygen fraction (FiO(2)) and mean arterial pressure (MAP).

METHODS

PbrO(2) in 11 patients was recorded continuously using an LX and NV probe placed side by side into the same cerebrovascular region. Once a steady baseline value was reached FiO(2) was increased by 20% for 10 min. Once the baseline values were re-established MAP was increased by 20 mmHg for 10 min. Evaluation was performed using a four-parameter logistic function and Bland-Altman analyses.

RESULTS

PbrO(2) values of both probes differed significantly at all times. The LX probe reacted significantly faster to changes in FiO(2) and MAP. Limits of agreement ranged between -32.1 and 20.0 mmHg. Mean LX values were 6.1 mmHg lower than NV values.

CONCLUSIONS

Since the examined patient cohort was rather small, this study's results are preliminary. However, they suggest that LX and NV probes measure different PbrO(2) values in routine monitoring as well as during phases of dynamic changes in FiO(2) and MAP. These data therefore do not support the view that both probes can be used interchangeably.