Normalising renal tissue oxygen tension with higher inspired oxygen concentration may be falsely reassuring. Comment on Br J Anaesth 2020;125:192-200.

A recent study by Iguchi et al (Anaesth 2020;125(2):192-200) showed that measured renal tissue oxygen tension falls under anaesthesia and may apparently be normalised by increasing arterial oxygen tension. In reality tissue oxygen is a balance between supply, demand and diffusion and is not spatially homogeneous at capillary length scales. This apparent normalisation may be falsely reassuring as such measurements reflect a macroscopic spatial average and being disproportionately influenced by regions of peri-arterial hyperoxia with other areas of tissue remaining unaffected and potentially hypoxic. Therefore, clinicians should not caution before translating the apparently beneficial effects of supraphysiological arterial oxygen levels on measured renal tissue oxygen tension into clinical practice

Evaluation of the self-inflating bag-valve-mask and non-rebreather mask as preoxygenation devices in volunteers.

OBJECTIVE: To evaluate and compare the effectiveness and tolerability of preoxygenation with the self-inflating bag-valve-mask (BVM) and non-rebreather mask (NRM) as are used before emergency anaesthesia. DESIGN: Device performance evaluation. SETTING: Experimental study. PARTICIPANTS: 12 male and 12 female healthy volunteers (age range 24-47) with no history of clinically significant respiratory disease. INTERVENTIONS: End-expiration oxygen measurements (F(E)O(2)) after 3 min of preoxygenation with BVM (without mechanical assistance) and NRM devices. Mask pressures were measured and subjective difficulty of breathing was also assessed with a visual analogue score (VAS). PRIMARY AND SECONDARY OUTCOME MEASURES: The final F(E)O(2) achieved was 58.0% (SD 7.3%) for the NRM compared to 53.1% (SD 13.4%) for the BVM (p=0.072). Preoxygenation was associated with small increases in F(E)CO(2) that were greater for the BVM (0.50%; 95% CI 0.48 to 0.52) than the NRM (0.29%; 95% CI 0.31 to 0.28); this difference was statistically significant (p=0.028). Both devices were well tolerated on VAS assessment of difficulty of breathing although this was higher for the BVM than the NRM (median VAS 1.85/10 compared to 1.1/10; p=0.041). Inspiratory and expiratory mask pressures were higher for the BVM. CONCLUSIONS: In healthy volunteers, the NRM performs comparably to the BVM in terms of the degree of denitrogenation achieved although neither performed well. Although it was well tolerated, the BVM was subjectively more difficult to breathe through and was associated with greater mask pressures and a small increase in F(E)CO(2) consistent with hypoventilation or rebreathing. Our results suggest that preoxygenation with the NRM may be a preferable approach in spontaneously breathing patients

Can network science reveal structure in a complex healthcare system? A network analysis using data from emergency surgical services

Funder: Research Trainees Coordinating Centre; FundRef: http://dx.doi.org/10.13039/501100000659Introduction: Hospitals are complex systems and optimising their function is critical to the provision of high quality, cost effective healthcare. Metrics of performance have to date focused on the performance of individual elements rather than the whole system. Manipulation of individual elements of a complex system without an integrative understanding of its function is undesirable and may lead to counterintuitive outcomes and a holistic metric of hospital function might help design more efficient services. Objectives: We aimed to use network analysis to characterise the structure of the system of perioperative care for emergency surgical admissions in our tertiary care hospital. Design: We constructed a weighted directional network representation of the emergency surgical services using patient location data from electronic health records. Setting: A single-centre tertiary care hospital in the UK. Participants: We selected data from the retrospective electronic health record data of all unplanned admissions with a surgical intervention during their stay during a 3.5-year period, which resulted in a set of 16 500 individual admissions. Methods: We then constructed and analysed the structure of this network using established methods from network science such as degree distribution, betweenness centrality and small-world characteristics. Results: The analysis showed the service to be a complex system with scale-free, small-world network properties. We also identified such potential hubs and bottlenecks in the system. Conclusions: Our holistic, system-wide description of a hospital service may provide tools to inform service improvement initiatives and gives us insights into the architecture of a complex system of care. The implications for the structure and resilience of the service is that while being robust in general, the system may be vulnerable to outages at specific key nodes

Radiofrequency properties of two different Licox® parenchymal brain tissue oxygen probe designs.

Neuroimaging and continuous multimodality neuromonitoring are both central to the intensive care management of patients with severe traumatic brain injury. Guidelines recommend the use of multimodality monitors including brain tissue oxygen (PbtO2) sensors to assess the adequacy of cerebral oxygen delivery. Devices such as the Lico® sensor (Integra LifeSciences, Plainsboro, NJ) employ a miniature implantable electrochemical cell. Temperature compensation is essential for accurate measurements and the device is now available with an integrated temperature sensor (Model CC1P1). Both devices are rated as MR compatible to 1.5T by the manufacturer

In Silico Model of Critical Cerebral Oxygenation after Traumatic Brain Injury: Implications for Rescuing Hypoxic Tissue.

Cerebral oxygen delivery is central to the modern intensive care of patients with severe traumatic brain injury. Low brain tissue oxygen tension (PbtO2) results from microvascular collapse and diffusion limitation and is associated with adverse outcome. A number of therapies to improve oxygen delivery are known to be effective in improving PbtO2. Their relative effectiveness and microscopic regions of hypoxia, however, may exist/persist even in the presence of normal PbtO2. Unfortunately, there are no methods currently for assessing this quantitatively. We used an in silico (computational) simulation approach to understand the effect of common interventions on the microscopic distribution of brain tissue oxygen tension. We constructed a non-linear mathematical model of cerebral oxygen supply, diffusion, and consumption for a simplified geometry. Model parameters were chosen to agree with clinical parameters. We found that it was possible to create a plausible diffusion-limited scenario with a significant hypoxic fraction by increasing the mean diffusion distance. We found that increasing cerebral blood flow/blood oxygen content or suppressing the cerebral metabolic rate were most effective at improving PbtO2 and reduced the hypoxic fraction. Within the limitations of our modeling assumptions, increasing the arterial oxygen partial pressure was less effective and only improved PbtO2 by creating a region of hyperoxic tissue with no improvement in hypoxic fraction. The in silico simulations can be useful in understanding the likely physiological effect of complex treatments for which measurement techniques do not exist

Brainstem death and prolonged disorders of consciousness

Advances in resuscitation and the advent of modern intensive care techniques to support the circulation challenge the simple definition of death in terms of loss of spontaneous circulation (‘cardiac death’). Instead, death is now better regarded as an irreversible loss of the capacity for consciousness combined with irreversible loss of the capacity to breathe. Since the brainstem is required for both consciousness and spontaneous breathing, irreversible loss of brainstem function (for example after trauma, haemorrhage or hypoxia/ischaemia) defines the state of ‘brainstem death’. Clinical criteria for the diagnosis of brainstem death have been published, although practice varies around the world. Brainstem death lies at the extreme end of a spectrum of disorders of consciousness and is, by definition, permanent. A number of prolonged disorders of consciousness (PDOC) from coma (loss of wakefullness and awareness) through the vegetative state (VS; wakefulness without awareness) to the minimally conscious state (MCS; wakefullness with some awareness) are now recognised. Once potentially confounding conditions have been excluded, the secure diagnosis of VS and MCS is based on expert, multi-disciplinary observation and this must take place over an extended period before permanence can be declared with sufficient certainty. Unlike brainstem death, patients with PDOC may survive for many years without physiological support. The ca

Cerebrovascular Signal Complexity Six Hours after Intensive Care Unit Admission Correlates with Outcome after Severe Traumatic Brain Injury.

Disease states are associated with a breakdown in healthy interactions and are often characterized by reduced signal complexity. We applied approximate entropy (ApEn) analysis to investigate the correlation between the complexity of heart rate (ApEn-HR), mean arterial pressure (ApEn-MAP), intracranial pressure (ApEn-ICP), and a combined ApEn-product (product of the three individual ApEns) and outcome after traumatic brain injury. In 174 severe traumatic brain injured patients, we found significant differences across groups classified by the Glasgow Outcome Score in ApEn-HR (p = 0.007), ApEn-MAP (p = 0.02), ApEn-ICP (p = 0.01), ApEn-product (p = 0.001), and pressure reactivity index (PRx) (p = 0.004) in the first 6 h. This relationship strengthened in a 24 h and 72 h analysis (ApEn-MAP continued to correlate with death but was not correlated with favorable outcome). Outcome was dichotomized as survival versus death, and favorable versus unfavorable; the ApEn-product achieved the strongest statistical significance at 6 h (F = 11.0; p = 0.001 and F = 10.5; p = 0.001, respectively) and was a significant independent predictor of mortality and favorable outcome (p < 0.001). Patients in the lowest quartile for ApEn-product were over four times more likely to die (39.5% vs. 9.3%, p < 0.001) than those in the highest quartile. ApEn-ICP was inversely correlated with PRx (r = -0.39, p < 0.000001) indicating unique information related to impaired cerebral autoregulation. Our results demonstrate that as early as 6 h into monitoring, complexity measures from easily attainable vital signs, such as HR and MAP, in addition to ICP, can help triage those who require more intensive neurological management at an early stage.This is the author accepted manuscript. The final version is available from Mary Ann Liebert via http://dx.doi.org/10.1089/neu.2015.422

Cerebrovascular signal complexity six hours after ICU admission correlates with outcome following severe traumatic brain injury

Disease states are associated with a breakdown in healthy interactions and are often characterised by reduced signal complexity. We applied approximate entropy (ApEn) analysis to investigate the correlation between the complexity of heart rate (ApEn-HR), mean arterial pressure (ApEn-MAP), intracranial pressure (ApEn-ICP) and a combined ApEn-Product (product of the three individual ApEns) and outcome after traumatic brain injury. In 174 severe traumatic brain injured patients we found significant differences across groups classified by the Glasgow Outcome Score in ApEn-HR (p = 0.007), ApEn-MAP (p = 0.02), ApEn-ICP (p = 0.01), ApEn-Product (p = 0.001) and PRx (p = 0.004) in the first 6-hours. This relationship strengthened in a 24-hour and 72-hour analysis (ApEn-MAP continued to correlate with death but was not correlated with favourable outcome). Outcome was dichotomized as survival vs death, and favourable vs unfavourable; the ApEn-Product achieved the strongest statistical significance at 6-hours (F = 11.0; p = 0.001 and F = 10.5; p = 0.001, respectively) and was a significant independent predictor of mortality and favourable outcome (p < 0.001). Patients in the lowest quartile for ApEn-Product were over four times more likely to die (39 .5% vs 9.3%, p < 0.001) compared to those with the highest quartile. ApEn-ICP was inversely correlated with PRx (r = -0.39, p < 0.000001) indicating unique information related to impaired cerebral autoregulation. Our results demonstrate that as early as 6-hours into monitoring, complexity measures from easily attainable vital signs, such as heart rate and mean arterial pressure, in addition to intracranial pressure can help triage those who require more intensive neurological management at an early stage.This is the author accepted manuscript. The final version is available from Mary Ann Liebert via http://dx.doi.org/10.1089/neu.2015.422