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Towards Minimally-invasive or Non-invasive Approaches to Assess Tissue Oxygenation Pre- and Post-Transfusion Workshop 2018 - Executive Summary

April 23 - 24 , 2018
Neuroscience Center (NSC), National Institutes of Health, 6001 Executive Boulevard, Bethesda, MD 20892

Description

This one-and-a-half day workshop brought together experts in multiple disciplines relevant to tissue oxygenation (transfusion, critical care, cardiology, neurology, neonatal/pediatrics, bioengineering, biochemistry, imaging) to present their latest findings, examine key challenges, and develop recommendations to facilitate the development of new technologies and/or biomarker panels to assess tissue oxygenation in a minimally to non-invasive fashion pre- and post-transfusion of Red Blood Cells (RBCs). The workshop was structured into four sessions: 1) Global Perspective; 2) Organ Systems; 3) Neonatology; and 4) Emerging Technologies. The first day of the workshop provided an overview of current approaches in the clinical setting, both from a global perspective including metabolomics for studying RBCs and tissue perfusion, as well as tissue oxygenation assessments in specific adult organ systems and neonates. The second day focused on emerging technologies for minimally-invasive to non-invasive assessment of tissue oxygenation with applications for pre- and post-transfusion of RBCs. Each day concluded with an open-microphone discussion among the speakers and the workshop participants. The workshop presentations and ensuing interdisciplinary discussions highlighted the potential of technologies to combine global "omics" signatures with additional measures (e.g., thenar eminence reading or imaging methods) to predict who needs a RBC transfusion and evaluate the efficacy of RBC transfusions. The discussion highlighted the need for collaborations across the various groups represented at the meeting to leverage existing technologies and develop novel approaches for assessing RBC transfusion efficacy in clinical settings.

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Recap

Research Needs and Challenges

The workshop focused on the tissue oxygenation assessment priority area first identified in the 2015 Transfusion Symposium. The development of minimally to non-invasive procedures to measure tissue oxygenation accurately are needed 1) pre-transfusion to predict who would benefit from RBC transfusions, and (2) post-transfusion to determine if the patient actually did benefit (and to be able to test the differential effects of various RBC products with specific characteristics). The discussion built on a previous initiative to develop assessment tools to evaluate the dynamic changes in microvascular blood flow and tissue oxygenation in clinical research applications.

Speaker presentations and panel discussions highlighted a list of promising biomarkers/assessment tools for tissue oxygenation that could be applied in clinical practice to assess tissue oxygenation pre- and post-transfusion:

  • Near-infrared spectroscopy (NIRS) is being used increasingly as a non-invasive tool for continuous monitoring of tissue oxygenation and hemodynamics, particularly of the brain. It uses near infrared light (at approximately 700–900 nm) to detect the weighted average of the saturation of hemoglobin in arterial and venous blood in a small region under the sensor. This is in contrast to pulse oximetry, which solely measures arterial oxygen saturation, and SvO2, (in pulmonary artery or “mixed venous” blood), which solely measures venous oxygen saturation. High priority areas for future work include rigorously testing, in clinical studies, to evaluate if using NIRS to guide transfusion and optimize care can improve clinical outcomes.
  • Blood-oxygen-level dependent (BOLD) imaging offers the advantage of deep tissue visualization of oxygen, but carries the disadvantage of relying on measurements of hemoglobin saturation (rather than tissue oxygen content) and requires the use of an MRI instrument. BOLD imaging utilizes the paramagnetic properties of deoxyhemoglobin to characterize local tissue oxygenation non-invasively and does not require exogenous contrast agents. It can provide reproducible, non-invasive estimation of tissue oxygenation in vascular tissues such as the kidney in human subjects.
  • A large number of candidate molecules including hypoxia-inducible factor (HIF), lactate-pyruvate, NAD/NADH, mitochondrial enzymes, and local CO2 production might serve as important biomarkers of local inadequate tissue oxygenation. Although HIF measurements are not yet clinically available to guide transfusion, assessing the metabolic status of tissue with regard to adequate tissue oxygenation appears to be a very important complement to measuring the presence of oxygen.
  • Photoacoustic microscopy (PAM) can non-invasively image oxygen delivery from single flowing RBCs in vivo with millisecond-scale temporal resolution and micrometer-scale spatial resolution. Using the intrinsic optical absorption contrast between oxyhemoglobin and deoxy-hemoglobin, PAM allows label-free imaging. Multiple single-RBC functional parameters, including total hemoglobin concentration, oxygen saturation, flow speed, oxygen release rate, and metabolic rate of oxygen can be quantified simultaneously in real time. The single-RBC functional imaging capability of PAM enables numerous biomedical studies and clinical applications.
  • Electron paramagnetic resonance (EPR) oximetry can make direct and repeated measurements of tissue oxygen in both preclinical models and human subjects. This involves a one-time, minimally invasive injection of appropriate paramagnetic materials into an area of interest. Subsequently, oxygen measurements can be made non-invasively and repetitively as frequently as desired, including continuously. Using India Ink as the oxygen sensor is especially useful, because of its minimally invasive nature (a simple needle stick, leaving a spot of India Ink similar to a tattoo in the subcutaneous tissue) and because the FDA does not require special permission for using India Ink in human subjects. Therefore, this technique could monitor the effectiveness of interventions to raise tissue pO2 using repeated measures. It could be especially valuable for monitoring oxygen in individual subjects receiving specific therapies in clinically relevant settings. EPR oximetry also may be useful to determine when widely available, but less intrinsically specific techniques, such as MRI or PET, could provide the desired information regarding tissue oxygen.
  • Oxygen tension (pO2) imaging technologies can provide quantitative transcutaneous oxygenation measurements with potential applicability to sensing tissue oxygenation changes related to transfusion. A new class of ultrabright, "clickable" porphyrin molecules have been developed that can be readily functionalized for oxygen sensing applications. These are now being used in three clinical trials to measure transcutaneous pO2 in 1) breast reconstruction surgery, 2) skin photoaging, and 3) skin infection. Fully integrated oxygen measuring devices that contain the chemistry and electronics for independent, wearable transcutaneous monitoring are also in development.
  • Novel ultrasound tools, such as elastography, calcium contact, and microvascular assessment can be applied in early gestation for the noninvasive evaluation of the human placenta. This technology could be applied to assess local tissue oxygenation.
  • Three other techniques have the potential to be used in clinical settings to measure tissue pO2: phosphorescence quenching, EPR oximetry and photoacoustic imaging of hemoglobin saturation. The latter, similar to NIRS and BOLD, measures hemoglobin oxygenation; however, in contrast to NIRS and BOLD, they have much higher spatial resolution at depth.
  • Phosphorescence quenching relies on exogenous phosphorescent (optical) probes to measure non-bound dissolved oxygen in biological tissues. The method relies on the ability of freely diffusing molecular oxygen (e.g., dissolved in blood plasma or tissue interstitial fluid) to quench phosphorescence of exogenous molecular probes (i.e., dyes). The measurement relies on phosphorescence decay time, as opposed to intensity; as such, it is independent of the local probe concentration.
  • Novel biosensors composed of tissue-like hydrogel scaffolds that reside permanently under the skin and utilize existing mobile networks can provide real-time, continuous, wireless, biochemical data for remote or cell-phone based viewing. Functionalized soft, porous oxygen sensing scaffolds with luminescent chemistries have shown their functionality in vivo for over 4 years. The first tissue-integrating sensor is approved for use in the European Union for continuous monitoring of tissue oxygen, and is currently going through the FDA approval process.

Recommendations and Opportunities

The following recommendations and opportunities were noted by the speakers and participants:

  • The ideal device would offer the ability to target deep tissues of interest (e.g., heart, brain, liver, gut, kidney), would be suitable for real-time bedside assessment of oxygen reserve (i.e., stress testing), and would provide information on both the quantity of oxygen present and the metabolic status of the target tissue.
  • Light-based reporter molecules that can penetrate deeper are more optimal because measuring oxygen levels just beneath the skin is unlikely to be satisfactory for providing meaningful tissue oxygen status in vital organs, such as heart, brain, gut, and kidney. MRI-based methods overcome this limitation, but systems that require patients to be evaluated in an MRI scanner to make transfusion decisions are unlikely to be effective. Nonetheless, a device that miniaturized MRI into a bedside scanner could be adopted.
  • Transfusion efficacy in humans could be evaluated by measuring a combination of the following parameters: SpO2, pCO2, pO2, bicarbonate, non-invasive oxygen consumption (VO2) and carbon dioxide production (VCO2), and lactate production
  • Emerging technologies in imaging detection, chemical sensors, photoacoustics, and metabolomic measurements of tissue oxygenation could have direct applications in clinical practice for assessing transfusion efficacy.
  • Several technologies exist that are chemical (Phosphorescence) or physical (Magnetic, Photoacoustic, IR) and that are non-invasive or invasive. Physical methods are generally non-invasive, but are limited by the depth of signal penetration. Although the photoacoustic method is a promising emerging technology, it is currently only being used in pre-clinical animal models and further experimentation to test its applicability to humans would be useful.
  • Most of the existing technologies are oriented to determining oxygen concentration; however, other parameters, such as acidosis measurable via carbon dioxide and lactic acid, are also important. Thus, implementing non-invasive methods to measure gas exchange of oxygen and carbon dioxide, such as indirect calorimetry, could determine the efficacy of oxygen consumption and the elimination (i.e., detoxification) of carbon dioxide.
  • Most non-invasive imaging methods detect tissue oxygenation by assessing hemoglobin saturation. It would be useful to develop tools to measure the tissue pO2 directly using minimally invasive probes to provide spatial and temporal information regarding the effect of oxygen modifying agents.
  • It would be useful to develop oxygen sensing probes that are sensitive in low oxygen ranges. That is, most oxygen sensing probes have a good sensitivity over 10 mmHg, but only a few are sensitive in the 0.1-5 mmHg range; the latter is the range of tissue hypoxia where it will be important to evaluate the effect of oxygenation agents.
  • Studies assessing tissue oxygenation responses to RBC transfusion would be strengthened by measuring donor RBC characteristics, either based on variables that can be obtained from the supplier or blood bank (e.g., age, gender, storage duration, irradiation, etc.) or through biomarker assessment of the RBC unit itself (e.g., metabolomics).
  • There are limited data regarding metabolic biomarkers that could improve on currently available blood tests (e.g., lactate) that are used, along with clinical features, to identify patients who would benefit from RBC transfusion. Priority research areas could include metabolomic analysis of the hypoxic patient:
    • To identify blood-based biomarkers (either in serum/plasma or RBCs) to categorize patients better regarding those who would, or would not, benefit from transfusion
    • To determine the extent to which these biomarkers (and the underlying physiology) can be corrected by transfusion, and what the therapeutic requirements are (e.g., how early to transfuse, what the transfusion goals are, whether RBC quality affects the ability of the transfusion to correct the underlying pathophysiology)
  • There is a need to identify other markers of blood transfusion efficacy, including markers for short-term and long-term tissue well-being after transfusion.
  • Hemoglobin threshold-driven RBC transfusion practices in preterm infants currently do not incorporate measures of tissue oxygenation. Both donor RBC and recipient factors need to be considered in assessing the effects of RBC transfusion on tissue oxygenation.
  • The currently available NIRS technologies do not integrate data on systemic arterial oxygen saturation or blood flow, limiting understanding of the underlying causes of changes in tissue oxygen saturation.
  • Study designs that account for longitudinal measures across multiple RBC exposures could provide understanding of within-patient variation in transfusion responses that could be related to variations in donor characteristics (e.g., RBC storage age, donor sex, donor age) or time-varying recipient characteristics (e.g., anemia severity). Information from such studies could help guide understanding of optimal RBC product characteristics and/or identify patient factors influencing responses to RBC transfusion.
  • A combined multi-PI, investigator-initiated, research program, which crosses multiple topical areas to bring ideas together in a set of clinical studies where combined hypotheses and data could be addressed as a group, could be very useful. This would enable the inclusion of novel hypotheses and novel technologies. Focusing initially on a specific patient population would narrow the scope and allow for testing of specific ideas. Two populations were proposed as initial models: neonates and patients with sickle-cell disease. This consortium approach would enable testing multiple hypotheses by pooling scientific and technological expertise and resources, including:
    • The quality of the RBCs contained in the bag itself
    • The quality of the RBCs post-transfusion (i.e., assessment in vivo)
    • Tissue oxygenation in response to transfusion, of deep organ structures along with correlation to peripheral oxygenation
  • There are opportunities for smaller cross-disciplinary interactions. For example, combining tools to measure blood oxygen saturation with blood oxygen tension measurements in transfusion measurements. Another example is to apply tissue oxygen tension devices, either organ specific or peripheral, with studies examining transfusion outcomes (e.g., evaluated RBC storage age or incompatibilities between donor and recipient).

Publication Plans

A Workshop Report is in preparation for publication.

NHLBI Contact

Margaret Ochocinska, PhD
Translational Blood Science and Resources Branch
Division of Blood Diseases and Resources
ochocinm@mail.nih.gov

Workshop Chair and Session Chairs

  • Steven Spitalnik, MD, Workshop Chair, Columbia University
  • Allan Doctor, MD, Session Chair, Washington University
  • Naomi Luban, MD, Session Chair, Children’s National Health System

Keynote Speakers

  • Allan Doctor, MD, Washington University
  • Ravi Mangal Patel, MD, MSc, Emory University School of Medicine

Workshop Speakers

  • Alfred Abuhamad, MD, Eastern Virginia Medical School
  • Elliott Bennett-Guerrero, PhD, Stony Brook University School of Medicine
  • Wally Carlo, MD, University of Alabama at Birmingham
  • Murali Krishna Cherukuri, PhD, National Cancer Institute, NIH
  • Sunny Dzik, MD, Harvard University
  • Conor Evans, PhD, Massachusetts General Hospital
  • Erica Forzani, PhD, Arizona State University
  • Periannan Kuppusamy, PhD, Darmouth Geisel School of Medicine
  • Natacha Le Moan, PhD, Omniox
  • Lei Li, PhD (presenting on behalf of Lihong Wang, PhD), California Institute of Technology
  • Mohandas Narla, PhD, New York Blood Center
  • John Roback, PhD, Emory University School of Medicine
  • Harold Swartz, MD, PhD, MSPH, Dartmouth University
  • Stephen Textor, MD, Mayo Clinic
  • Sergei Vinogradov, PhD, University of Pennsylvania
  • Natalie Wisniewski, PhD, Profusa

Workshop Steering Committee

  • Simone Glynn, MD, MSc, MPH, NHLBI, NIH
  • Margaret Ochocinska, PhD, NHLBI, NIH
  • Catherine Levy, RN, NHLBI, NIH
  • James Berger, MS, MT (ASCP), SBB, OASH, DHHS
  • Richard Henry, ML, MPH, OASH, DHHS