Changes in tissue oxygen tension, venous saturation, and Fick-based assessments of cardiac output during hyperoxia.

BACKGROUND: Hyperoxemia (arterial oxygen tension >100 mm Hg) may occur in critically ill patients and have effects on mixed venous saturation (SvO2 ) and on Fick-based estimates of cardiac output (CO). We investigated the effect of hyperoxemia on SvO2 and on assessments of CO using the Fick equation. METHODS: Yorkshire swine (n = 14) were anesthetized, intubated, and paralyzed for instrumentation. SvO2 (co-oximetry) and tissue oxygen tension (tPO2 , implantable electrodes) in brain and myocardium were measured during systematic manipulation of arterial oxygen tension (PaO2 ) using graded hyperoxia (fraction of inspired oxygen 0.21 → 0.8). Secondarily, oxygen- and carbon dioxide-based estimates of CO (FickO2 and FickCO2 , respectively) were compared with measurements from a flow probe placed on the aortic root. RESULTS: Independent of changes in measured oxygen delivery, cerebral and myocardial tPO2 increased in proportion to PaO2 , as did SvO2 (P < 0.001 for all). Based on mixed model analysis, each 100 mm Hg increase in PaO2 resulted in a 4.8 ± 0.9% increase in SvO2 under the conditions tested. Because neither measured oxygen consumption, arterial oxyhemoglobin saturation or cardiac output varied significantly during hyperoxia, changes in SvO2 resulted in successively increasing errors in FickO2 during hyperoxia (34% during normoxia, 72% during FiO2 0.8). FickCO2 lacked the progressively worsening errors present in FickO2 , but correlated poorly with CO. CONCLUSION: SvO2 acutely changes following changes in PaO2 even absent changes in measured DO2 . This may lead to errors in FickO2 estimates of CI. Further work is necessary to understand the impact of this phenomenon in disease states.

Hemodynamic Effects of Lipid-Based Oxygen Microbubbles via Rapid Intravenous Injection in Rodents.

PURPOSE: Low oxygen levels, or hypoxemia, is a common cause of morbidity and mortality in critically ill patients. Hypoxemia is typically addressed by increasing the fraction of inspired oxygen, the use of mechanical ventilation, or more invasive measures. Recently, the injection of oxygen gas directly into the bloodstream by packaging it within lipid-based oxygen microbubbles (LOMs) has been explored. The purpose of this work is to examine the acute hemodynamic effects of intravenous injections of LOMs. METHODS: LOMs composed of 1,2-distearoyl-sn-glycero-3-phosphocoline and cholesterol were manufactured using a process of shear homogenization under an oxygen headspace. A 5 mL aliquot of either PlasmaLyte A, or low (37%) or high (55%) concentration LOMs (n = 10 per group) was injected over a 1 min period into Sprague Dawley rats instrumented for measurement of cardiac index and pulmonary (PVR) and systemic (SVR) vascular resistance during a 60 min observation period. Hemodynamics were compared between groups by linear mixed modeling. RESULTS: Approximately 1011 LOMs with mean diameter 3.77 ± 1.19 µm were injected over the 1 min period. Relative to controls, rodents treated with high concentration LOMs exhibited a higher pulmonary artery pressure (20 ± 0.4 mmHg vs 18 ± 0.4 mmHg, P < 0.001) and higher PVR (0.31 ± 0.01 vs 0.23 ± 0.01 mmHg/mL*min*kg, P < 0.001. Despite a stable cardiac index (62.2 ± 3.5 vs 62.3 ± 3.4 mL/min*kg, P < 0.001), mean arterial blood pressure decreased significantly in LOM-treated animals (46 ± 2 vs 60 ± 2 mmHg, P < 0.001) due to a decrease in SVR. Injections with aged LOM emulsions (>48 h since manufacture) resulted in a higher incidence of hemodynamic collapse during the observation period (P = 0.02). CONCLUSIONS: LOMs may be injected in quantities sufficient to deliver clinically meaningful volumes of oxygen but cause significant decrements in blood pressure and elevations in PVR.

The proteomic fingerprint in infants with single ventricle heart disease in the interstage period: evidence of chronic inflammation and widespread activation of biological networks.

Introduction: Children with single ventricle heart disease (SVHD) experience significant morbidity across systems and time, with 70% of patients experiencing acute kidney injury, 33% neurodevelopmental impairment, 14% growth failure, and 5.5% of patients suffering necrotizing enterocolitis. Proteomics is a method to identify new biomarkers and mechanisms of injury in complex physiologic states. Methods: Infants with SVHD in the interstage period were compared to similar-age healthy controls. Serum samples were collected, stored at -80°C, and run on a panel of 1,500 proteins in single batch analysis (Somalogic Inc., CO). Partial Least Squares-Discriminant Analysis (PLS-DA) was used to compare the proteomic profile of cases and controls and t-tests to detect differences in individual proteins (FDR <0.05). Protein network analysis with functional enrichment was performed in STRING and Cytoscape. Results: PLS-DA readily discriminated between SVHD cases (n = 33) and controls (n = 24) based on their proteomic pattern alone (Accuracy = 0.96, R2 = 0.97, Q2 = 0.80). 568 proteins differed between groups (FDR <0.05). We identified 25 up-regulated functional clusters and 13 down-regulated. Active biological systems fell into six key groups: angiogenesis and cell proliferation/turnover, immune system activation and inflammation, altered metabolism, neural development, gastrointestinal system, and cardiac physiology and development. Conclusions: We report a clear differentiation in the circulating proteome of patients with SVHD and healthy controls with >500 circulating proteins distinguishing the groups. These proteomic data identify widespread protein dysregulation across multiple biologic systems with promising biological plausibility as drivers of SVHD morbidity.

Responsive monitoring of mitochondrial redox states in heart muscle predicts impending cardiac arrest.

Assessing the adequacy of oxygen delivery to tissues is vital, particularly in the fields of intensive care medicine and surgery. As oxygen delivery to a cell becomes deficient, changes in mitochondrial redox state precede changes in cellular function. We describe a technique for the continuous monitoring of the mitochondrial redox state on the epicardial surface using resonance Raman spectroscopy. We quantify the reduced fraction of specific electron transport chain cytochromes, a metric we name the resonance Raman reduced mitochondrial ratio (3RMR). As oxygen deficiency worsens, heme moieties within the electron transport chain become progressively more reduced, leading to an increase in 3RMR. Myocardial 3RMR increased from baseline values of 18.1 ± 5.9 to 44.0 ± 16.9% (P = 0.0039) after inferior vena cava occlusion in rodents (n = 8). To demonstrate the diagnostic power of this measurement, 3RMR was continuously measured in rodents (n = 31) ventilated with 5 to 8% inspired oxygen for 30 min. A 3RMR value exceeding 40% at 10 min predicted subsequent cardiac arrest with 95% sensitivity and 100% specificity [area under the curve (AUC), 0.98], outperforming all current measures, including contractility (AUC, 0.51) and ejection fraction (AUC, 0.39). 3RMR correlated with indices of intracellular redox state and energy production. This technique may permit the real-time identification of critical defects in organ-specific oxygen delivery.

Optimization and characterization of stable lipid-based, oxygen-filled microbubbles by mixture design.

Tissue hypoxia is a final common pathway that leads to cellular injury and death in a number of critical illnesses. Intravenous injections of self-assembling, lipid-based oxygen microbubbles (LOMs) can be used to deliver oxygen gas, preventing organ injury and death from systemic hypoxemia. However, current formulations exhibit high polydispersity indices (which may lead to microvascular obstruction) and poor shelf-lives, limiting the translational capacity of LOMs. In this study, we report our efforts to optimize LOM formulations using a mixture response surface methodology (mRSM). We study the effect of changing excipient proportions (the independent variables) on microbubble diameter and product loss (the dependent variables). By using mRSM analysis, the experimental data were fit using a reduced Scheffé linear mixture model. We demonstrate that formulations manufactured from 1,2-distearoyl-sn-glycero-3-phosphocholine, corn syrup, and water produce micron-sized microbubbles with low polydispersity indices, and decreased product loss (relative to previously described formulations) when stored at room temperature over a 30-day period. Optimized LOMs were subsequently tested for their oxygen-releasing ability and found to have similar release kinetics as prior formulations.

Manufacture of concentrated, lipid-based oxygen microbubble emulsions by high shear homogenization and serial concentration.

Gas-filled microbubbles have been developed as ultrasound contrast and drug delivery agents. Microbubbles can be produced by processing surfactants using sonication, mechanical agitation, microfluidic devices, or homogenization. Recently, lipid-based oxygen microbubbles (LOMs) have been designed to deliver oxygen intravenously during medical emergencies, reversing life-threatening hypoxemia, and preventing subsequent organ injury, cardiac arrest, and death. We present methods for scaled-up production of highly oxygenated microbubbles using a closed-loop high-shear homogenizer. The process can produce 2 L of concentrated LOMs (90% by volume) in 90 min. Resulting bubbles have a mean diameter of ~2 µm, and a rheologic profile consistent with that of blood when diluted to 60 volume %. This technique produces LOMs in high capacity and with high oxygen purity, suggesting that this technique may be useful for translational research labs.

Bulk manufacture of concentrated oxygen gas-filled microparticles for intravenous oxygen delivery.

Self-assembling, concentrated, lipid-based oxygen microparticles (LOMs) have been developed to administer oxygen gas when injected intravenously, preventing organ injury and death from systemic hypoxemia in animal models. Distinct from blood substitutes, LOMs are a one-way oxygen carrier designed to rescue patients who experience life-threatening hypoxemia, as caused by airway obstruction or severe lung injury. Here, we describe methods to manufacture large quantities of LOMs using an in-line, recycling, high-shear homogenizer, which can create up to 4 liters of microparticle emulsion in 10 minutes, with particles containing a median diameter of 0.93 microns and 60 volume% of gas phase. Using this process, we screen 30 combinations of commonly used excipients for their ability to form stable LOMs. LOMs composed of DSPC and cholesterol in a 1:1 molar ratio are stable for a 100 day observation period, and the number of particles exceeding 10 microns in diameter does not increase over time. When mixed with blood in vitro, LOMs fully oxygenate blood within 3.95 seconds of contact, and do not cause hemolysis or complement activation. LOMs can be manufactured in bulk by high shear homogenization, and appear to have a stability and size profile which merit further testing.

Oxygen gas-filled microparticles provide intravenous oxygen delivery.

We have developed an injectable foam suspension containing self-assembling, lipid-based microparticles encapsulating a core of pure oxygen gas for intravenous injection. Prototype suspensions were manufactured to contain between 50 and 90 ml of oxygen gas per deciliter of suspension. Particle size was polydisperse, with a mean particle diameter between 2 and 4 µm. When mixed with human blood ex vivo, oxygen transfer from 70 volume % microparticles was complete within 4 s. When the microparticles were infused by intravenous injection into hypoxemic rabbits, arterial saturations increased within seconds to near-normal levels; this was followed by a decrease in oxygen tensions after stopping the infusions. The particles were also infused into rabbits undergoing 15 min of complete tracheal occlusion. Oxygen microparticles significantly decreased the degree of hypoxemia in these rabbits, and the incidence of cardiac arrest and organ injury was reduced compared to controls. The ability to administer oxygen and other gases directly to the bloodstream may represent a technique for short-term rescue of profoundly hypoxemic patients, to selectively augment oxygen delivery to at-risk organs, or for novel diagnostic techniques. Furthermore, the ability to titrate gas infusions rapidly may minimize oxygen-related toxicity.