Regional blood flow during hyperoxic haemodilution

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

Hyperoxia does not affect oxygen delivery in healthy volunteers while causing a decrease in sublingual perfusion

Objective

To determine the human dose‐response relationship between a stepwise increase in arterial oxygen tension and its associated changes in DO₂ and sublingual microcirculatory perfusion.

Methods

Fifteen healthy volunteers breathed increasing oxygen fractions for 10 minutes to reach arterial oxygen tensions of baseline (breathing air), 20, 40, 60 kPa, and max kPa (breathing oxygen). Systemic hemodynamics were measured continuously by the volume‐clamp method. At the end of each period, the sublingual microcirculation was assessed by SDF.

Results

Systemic DO₂ was unchanged throughout the study (P_slope = .8). PVD decreased in a sigmoidal fashion (max −15% while breathing oxygen, SD18, P_slope = .001). CI decreased linearly (max −10%, SD10, P_slope < .001) due to a reduction in HR (max −10%, SD7, P_slope = .009). There were no changes in stroke volume or MAP. Most changes became apparent above an arterial oxygen tension of 20 kPa.

Conclusions

In healthy volunteers, supraphysiological arterial oxygen tensions have no effect on systemic DO₂. Sublingual microcirculatory PVD decreased in a dose‐dependent fashion. All hemodynamic changes appear negligible up to an arterial oxygen tension of 20 kPa.

Effects of hyperoxia on vascular tone in animal models: systematic review and meta-analysis

BACKGROUND

Arterial hyperoxia may induce vasoconstriction and reduce cardiac output, which is particularly undesirable in patients who already have compromised perfusion of vital organs. Due to the inaccessibility of vital organs in humans, vasoconstrictive effects of hyperoxia have primarily been studied in animal models. However, the results of these studies vary substantially. Here, we investigate the variation in magnitude of the hyperoxia effect among studies and explore possible sources of heterogeneity, such as vascular region and animal species.

METHOD

Pubmed and Embase were searched for eligible studies up to November 2017. In vivo and ex vivo animal studies reporting on vascular tone changes induced by local or systemic normobaric hyperoxia were included. Experiments with co-interventions (e.g. disease or endothelium removal) or studies focusing on lung, brain or fetal vasculature or the ductus arteriosus were not included. We extracted data pertaining to species, vascular region, blood vessel characteristics and method of hyperoxia induction. Overall effect sizes were estimated with a standardized mean difference (SMD) random effects model.

RESULTS

We identified a total of 60 studies, which reported data on 67 in vivo and 18 ex vivo experiments. In the in vivo studies, hyperoxia caused vasoconstriction with an SMD of - 1.42 (95% CI - 1.65 to - 1.19). Ex vivo, the overall effect size was SMD - 0.56 (95% CI - 1.09 to - 0.03). Between-study heterogeneity (I2) was high for in vivo (72%, 95% CI 62 to 85%) and ex vivo studies (86%, 95% CI 78 to 98%). In vivo, in comparison to the overall effect size, hyperoxic vasoconstriction was less pronounced in the intestines and skin (P = 0.03) but enhanced in the cremaster muscle region (P < 0.001). Increased constriction was seen in vessels 15-25 μm in diameter. Hyperoxic constriction appeared to be directly proportional to oxygen concentration. For ex vivo studies, heterogeneity could not be explained with subgroup analysis.

CONCLUSION

The effect of hyperoxia on vascular tone is substantially higher in vivo than ex vivo. The magnitude of the constriction is most pronounced in vessels ~ 15-25 μm in diameter and is proportional to the level of hyperoxia. Relatively increased constriction was seen in muscle vasculature, while reduced constriction was seen in the skin and intestines.

Hemodynamic effects of acute hyperoxia: systematic review and meta-analysis

Background

In clinical practice, oxygen is generally administered to patients with the intention of increasing oxygen delivery. Supplemental oxygen may, however, cause arterial hyperoxia, which is associated with hemodynamic alterations. We performed a systematic review and meta-analysis of the literature to determine the effect of hyperoxia on central hemodynamics and oxygen delivery in healthy volunteers and cardiovascular-compromised patients.

Methods

PubMed and EMBASE were searched up to March 2017. Studies with adult humans investigating changes in central hemodynamics or oxygen delivery induced by acute normobaric hyperoxia were included. Studies focusing on lung, retinal, or brain parameters were not included. We extracted subject and oxygen exposure characteristics, indexed and unindexed values for heart rate, stroke volume, cardiac output, mean arterial pressure (MAP), systemic vascular resistance, and oxygen delivery during normoxia and hyperoxia. For quantitative synthesis of the data, a random-effects ratio of means (RoM) model was used.

Results

We identified 33 studies with 42 datasets. Study categories included healthy volunteers (n = 22 datasets), patients with coronary artery disease (CAD; n = 6), heart failure (HF; n = 6), coronary artery bypass graft (CABG; n = 3) and sepsis (n = 5). Hyperoxia (arterial oxygen tension of 234–617 mmHg) reduced cardiac output (CO) by 10–15% in both healthy volunteers (−10.2%, 95% confidence interval (CI) −12.9% to −7.3%) and CAD (−9.6%, 95% CI −12.3% to −6.9%) or HF patients (−15.2%, 95% CI −21.7% to −8.2%). No significant changes in cardiac output were seen in CABG or septic patients (−3%). Systemic vascular resistance increased remarkably in patients with heart failure (24.6%, 95% CI 19.3% to 30.1%). In healthy volunteers, and those with CAD and CABG, the effect was smaller (11–16%) and was virtually absent in patients with sepsis (4.3%, 95% CI −3.2% to 12.3%). No notable effect on MAP was found in any group (2–3%). Oxygen delivery was not altered by hyperoxia. Considerable heterogeneity existed between study results, likely due to methodological differences.

Conclusions

Hyperoxia may considerably decrease cardiac output and increase systemic vascular resistance, but effects differ between patient categories. Heart failure patients were the most sensitive while no hemodynamic effects were seen in septic patients. There is currently no evidence supporting the notion that oxygen supplementation increases oxygen delivery.

Electronic supplementary material

The online version of this article (10.1186/s13054-018-1968-2) contains supplementary material, which is available to authorized users.

A reduced gastric corpus microvascular blood flow during Ivor-Lewis esophagectomy detected by laser speckle contrast imaging technique

BACKGROUND

Reduced microvascular blood flow is related to anastomotic insufficiency following esophagectomy, emphasizing a need for intraoperative monitoring of the microcirculation. This study evaluated if laser speckle contrast imaging (LSCI) was able to detect intraoperative changes in gastric microcirculation.

METHODS

Gastric microcirculation was assessed prior to and after reconstruction of gastric continuity in 25 consecutive patients operated for adenocarcinoma with open Ivor-Lewis esophagectomy while hemodynamic variables were recorded.

RESULTS

During upper laparotomy, microcirculation at the corpus decreased by 25% from baseline to mobilization of the stomach (p = .008) and decreased further (to a total decrease of 40%) following gastric pull to the thorax (p = .013). On the other hand, microcirculation at the antrum did not change significantly after gastric mobilization (p = .091). The decrease in corpus microcirculation took place unrelated to central cardiovascular variables.

CONCLUSION

Using LSCI technique, we identified a reduced microcirculation at the corpus area during open Ivor-Lewis esophagectomy. LSCI provides an option for real-time assessment of gastric microcirculation and could form basis for intraoperative stabilization of the microcirculation.

Determination of organ-specific anemia tolerance

OBJECTIVE

Utilization of anemia tolerance reduces the need for and risks of perioperative transfusion. Recent publications indicate that the critical limit for oxygen supply might not be the same for each organ system. Therefore, we investigated the effects of acute dilutional anemia on heart, brain, kidneys, liver, small intestine, and skeletal muscle to quantify organ-specific tolerance of different levels of acute anemic hypoxia. We hypothesized that, in some organs, tissue hypoxia occurs before the critical limits of systemic oxygen supply are reached.

DESIGN

Laboratory animal experiments.

SETTING

Animal research laboratory at university medical school.

SUBJECTS

A total of 18 domestic pigs of either sex (average weight: 19.6 kg).

INTERVENTIONS

Animals were anesthetized, ventilated, and randomized into three groups and then hemodiluted by exchange of 6% hydroxyethyl starch (130,000:0.4) for whole blood to the group-specific endpoint: Sham (no hemodilution), Hb4 (hemoglobin 4.3 g/dL), Hbcrit (2.7 g/dL). Subsequently, 10 mg/kg pimonidazole (which forms protein adducts in hypoxic cells) was injected. One hour after injection, tissue samples were collected and analyzed for pimonidazole-protein adduct quantification (dot blot) and as a surrogate for transcriptional activation during hypoxia the expression of vascular endothelial growth factor messenger RNA. Relevant hemodynamic and metabolic parameters were collected.

MEASUREMENTS AND MAIN RESULTS

Hemodynamics, metabolic parameters, or oxygen consumption did not indicate that tissue oxygenation was restricted before reaching Hbcrit. However, kidneys and skeletal muscle showed enhanced pimonidazole binding and vascular endothelial growth factor expression at Hb4. By contrast, liver oxygenation was actually improved at Hb4. Heart, brain, and liver showed no signs of tissue hypoxia at Hb4.

CONCLUSIONS

Heart, brain, kidneys, liver, small intestine, and skeletal muscle experience tissue hypoxia at different degrees of acute anemia, as assessed by the pimonidazole method and vascular endothelial growth factor expression. Further studies are needed to elucidate the mechanisms that determine organ-specific anemia tolerance.

Effect of hyperoxia and hypercapnia on tissue oxygen and perfusion response in the normal liver and kidney

OBJECTIVE

Inhalation of air with altered levels of oxygen and carbon dioxide to manipulate tissue oxygenation and perfusion has both therapeutic and diagnostic value. These physiological responses can be measured non-invasively with magnetic resonance (MR) relaxation times. However, interpreting MR measurements is not straight-forward in extra-cranial organs where gas challenge studies have only begun to emerge. Inconsistent results have been reported on MR, likely because different organs respond differently. The objective of this study was to elucidate organ-specific physiological responses to gas challenge underlying MR measurements by investigating oxygenation and perfusion changes in the normal liver and kidney cortex.

MATERIALS AND METHODS

Gas challenges (100% O(2), 10% CO(2), and carbogen [90% O(2)+10% CO(2)]) interleaved with room air was delivered to rabbits to investigate their effect on tissue oxygenation and perfusion. Real-time fiber-optic measurements of absolute oxygen and relative blood flow were made in the liver and kidney cortex.

RESULTS

Only the liver demonstrated a vasodilatory response to CO(2). Perfusion changes to other gases were minimal in both organs. Tissue oxygenation measurements showed the liver responding only when CO(2) was present and the kidney only when O(2) was present.

CONCLUSION

This study reveals distinct physiological response mechanisms to gas challenge in the liver and kidney. The detailed characterization of organ-specific responses is critical to improving our understanding and interpretation of MR measurements in various body organs, and will help broaden the application of MR for non-invasive studies of gas challenges.

Simultaneous measurement of pO2 and perfusion in the rabbit kidney in vivo

Recently, a combined probe has been developed capable of simultaneous measurement of local tissue pO2 (fluorescence oximetry) and microvascular perfusion (laser Doppler flux) within the same local region. The aim of the current study was to test the utility of these combined probes to measure pO2 and perfusion in the kidney. Studies were performed in anesthetized, artificially ventilated rabbits (n=7). Baseline measurements of renal medullary perfusion and pO2 obtained using combined probes (537 +/- 110 units & 28.7 +/- 6.l mmHg, respectively) were indistinguishable from those obtained using independent probes (435 +/- 102 units & 26.9 +/- 6.4 mmHg). Baseline measurements of renal cortical pO2 were also similar between combined (9.7 +/- 1.6 mmHg) and independent probes (9.5 +/- 2.3 mmHg). Baseline levels of cortical perfusion however, were significantly greater when measured using independent probes (1130 +/- 114 units) compared to combined probes (622 +/- 59 units; P < 0.02). Relative changes in perfusion and pO2 resulting from graded stimulation of the renal nerves were not significantly different when measured using combined probes to those obtained using independent probes. We conclude that combined probes are equally suitable to independent probes for tissue pO2 and microvascular perfusion measurement in the kidney. Our results raise some concerns regarding the accuracy of these OxyLite fluorescence probes for pO2 measurement in the kidney, particularly within the renal cortex.

[Tolerance to perioperative anemia. Mechanisms, influencing factors and limits]

The expected cost explosion in transfusion medicine increases the socio-economic significance of specific institutional transfusion programs. In this context the estimated use of the patient's physiologic tolerance represents an integral part of any blood conservation concept. The present article summarizes the mechanisms, influencing factors and limits of this natural tolerance to anemia and deduces the indication for perioperative red blood cell transfusion. The current recommendations coincide to the effect that perioperative transfusion is unnecessary up to a Hb concentration of 10 g/dl (6.21 mmol/l) even in older patients with cardiopulmonary comorbidity and is only recommended in cases of Hb <6 g/dl (<3.72 mmol/l) in otherwise healthy subjects including pregnant women and children. Critically ill patients with multiple trauma and sepsis do not seem to benefit from transfusions up to Hb concentrations >9 g/dl (>5.59 mmol/l). In cases of massive hemorrhaging and diffuse bleeding disorders the maintenance of a Hb concentration of 10 g/dl (6.21 mmol/l) seems to contribute to stabilization of coagulation.

Renal oxygen delivery: matching delivery to metabolic demand

The kidneys are second only to the heart in terms of O2 consumption; however, relative to other organs, the kidneys receive a very high blood flow and oxygen extraction in the healthy kidney is low. Despite low arterial-venous O2 extraction, the kidneys are particularly susceptible to hypoxic injury and much interest surrounds the role of renal hypoxia in the development and progression of both acute and chronic renal disease. Numerous regulatory mechanisms have been identified that act to maintain renal parenchymal oxygenation within homeostatic limits in the in vivo kidney. However, the processes by which many of these mechanisms act to modulate renal oxygenation and the factors that influence these processes remain poorly understood. A number of such mechanisms specific to the kidney are reviewed herein, including the relationship between renal blood flow and O2 consumption, pre- and post-glomerular arterial-venous O2 shunting, tubulovascular cross-talk, the differential control of regional kidney blood flow and the tubuloglomerular feedback mechanism. The roles of these mechanisms in the control of renal oxygenation, as well as how dysfunction of these mechanisms may lead to renal hypoxia, are discussed.

[Tolerance to perioperative anemia. Mechanisms, influencing factors and limits]

The expected cost explosion in transfusion medicine (increasing imbalance between donors and potential recipients, treatment of transfusion-associated complications) increases the socio-economic significance of specific institutional transfusion programs. In this context the estimated use of the patient's physiologic tolerance to anemia enables 1) the tolerance of larger blood losses (loss of "diluted blood"), 2) the onset of transfusion to the time after surgical control of bleeding to be delayed and 3) the perioperative collection of autologous red blood cells. The present review article summarizes the mechanisms, influencing factors and limits of this natural tolerance to anemia and deduces the indication for perioperative red blood cell transfusion. Under strictly controlled conditions (anesthesia, normovolemia, complete muscular relaxation, hyperoxemia, mild hypothermia) extremely low hemoglobin concentrations [Hb <3 g/dl (<1.86 mmol/l)] are tolerated without transfusion by individuals with no cardiopulmonary disease. In the clinical routine these situations are limited to borderline situations e.g. unexpected massive blood losses in Jehovah's Witnesses or unexpected shortcomings in blood supply. The current recommendations coincide to the effect that perioperative red blood cell transfusion 1) is unnecessary up to a Hb concentration of 10 g/dl (6.21 mmol/l) even in older patients with cardiopulmonary comorbidity and 2) is only recommended in cases of Hb <6 g/dl (<3.72 mmol/l) in otherwise healthy subjects including pregnant women and children. Critically ill patients with multiple trauma and sepsis do not seem to benefit from transfusions up to Hb concentrations >9 g/dl (>5.59 mmol/l). In cases of massive hemorrhaging and diffuse bleeding disorders the maintenance of a Hb concentration of 10 g/dl (6.21 mmol/l) seems to contribute to stabilization of coagulation.

RENAL PREGLOMERULAR ARTERIAL-VENOUS O2 SHUNTING IS A STRUCTURAL ANTI-OXIDANT DEFENCE MECHANISM OF THE RENAL CORTEX

1. High blood flow to the kidney facilitates a high glomerular filtration rate, but total renal O2 delivery greatly exceeds renal metabolic requirements. However, tissue Po2 in much of the renal cortex is lower than may be expected, being similar to that of other organs in which perfusion is closely matched to metabolic demand. 2. The lower than expected renal cortical Po2 is now attributed largely to diffusional shunting of as much as 50% of inflowing O2 from blood within preglomerular arterial vessels to post-glomerular venous vessels. However, the functional significance of this O2 shunting remains unclear. Indeed, this mechanism may appear maladaptive, given the kidney's susceptibility to hypoxic insults. 3. We hypothesize that renal preglomerular arterial-venous O2 shunting acts to protect the kidney from the potentially damaging consequences of tissue hyperoxia. The diffusion of O2 from arteries to veins within the kidney acts to reduce the O2 content of the blood before it is distributed to the renal microcirculation. Because high tissue Po2 may increase the production of reactive oxygen species, we suggest that renal arterial-venous O2 shunting may provide a physiological benefit to the organism by limiting O2 delivery to renal tissue, thereby reducing the risk of cellular oxidation.

Anemia and perioperative red blood cell transfusion: a matter of tolerance

In the past, anemia in the perioperative period has been treated by red blood cell (RBC) transfusions relatively uncritically. RBC transfusions were believed to increase oxygen delivery by increasing hemoglobin concentration. Arbitrary transfusion triggers such as the "10/30 rule" (i.e., RBC transfusion indicated below a hemoglobin concentration of 10 g/dL or a hematocrit of 30%) were applied. However, there is now increasing evidence that RBC transfusions are associated with adverse outcomes and should be avoided whenever possible. Restraining from RBC transfusions and maintaining normovolemia in patients suffering from surgical blood loss results in acute anemia. Therefore, knowing the compensatory mechanisms during acute anemia is crucial. This review focuses on acute anemia tolerance, its limits, and physiologic transfusion triggers in the perioperative period.