Michael R. Fettiplace

Rapid cardiotonic effects of lipid emulsion infusion

Objectives:Bolus infusion of lipid emulsion can reverse cardiac pharmacotoxicity caused by local anesthetics and other lipophilic drugs. The mechanisms of this effect are not completely elucidated. The authors test the hypothesis that lipid emulsion infusion exerts direct, positive inotropic effects. Design:Prospective, randomized animal study. Setting:University research laboratory. Subjects:Adult male Sprague-Dawley rats. Interventions:Rats anesthetized with isoflurane were given intravenous infusions (9 mL/kg over 1 min) of either 20% soybean oil-based emulsion or saline. Measurements and Main Results:Arterial pressure and aortic flow were measured continuously in intact animals. Lipid infusion increased aortic flow and arterial pressure faster and to a greater degree than did the same volume of saline infusion. Isolated rat hearts were studied using an isovolumetric, constant flow, nonrecirculating system. Left ventricular pressure was monitored. The infusion of lipid emulsion in the isolated heart dose-dependently increased rate pressure product, dP/dt, –dP/dt, and myocardial oxygen demand. Conclusions:Lipid emulsion exerts rapid, positive inotropic and positive lusitropic effects in both intact animal and isolated heart models. We hypothesize that this inotropy and the resulting increase in tissue blood flow contribute to the phenomenon of lipid reversal of cardiac toxicity caused by drug overdose.

Multi-modal contributions to detoxification of acute pharmacotoxicity by a triglyceride micro-emulsion.

Triglyceride micro-emulsions such as Intralipid® have been used to reverse cardiac toxicity induced by a number of drugs but reservations about their broad-spectrum applicability remain because of the poorly understood mechanism of action. Herein we report an integrated mechanism of reversal of bupivacaine toxicity that includes both transient drug scavenging and a cardiotonic effect that couple to accelerate movement of the toxin away from sites of toxicity. We thus propose a multi-modal therapeutic paradigm for colloidal bio-detoxification whereby a micro-emulsion both improves cardiac output and rapidly ferries the drug away from organs subject to toxicity. In vivo and in silico models of toxicity were combined to test the contribution of individual mechanisms and reveal the multi-modal role played by the cardiotonic and scavenging actions of the triglyceride suspension. These results suggest a method to predict which drug toxicities are most amenable to treatment and inform the design of next-generation therapeutics for drug overdose.

Resuscitation with lipid emulsion: dose-dependent recovery from cardiac pharmacotoxicity requires a cardiotonic effect.

Background:Recent publications have questioned the validity of the “lipid sink” theory of lipid resuscitation while others have identified sink-independent effects and posed alternative mechanisms such as hemodilution. To address these issues, the authors tested the dose-dependent response to intravenous lipid emulsion during reversal of bupivacaine-induced cardiovascular toxicity in vivo. Subsequently, the authors modeled the relative contribution of volume resuscitation, drug sequestration, inotropy and combined drug sequestration, and inotropy to this response with the use of an in silico model. Methods:Rats were surgically prepared to monitor cardiovascular metrics and deliver drugs. After catheterization and instrumentation, animals received a nonlethal dose of bupivacaine to produce transient cardiovascular toxicity, then were randomized to receive one of the four treatments: 30% intravenous lipid emulsion, 20% intravenous lipid emulsion, intravenous saline, or no treatment (n = 7 per condition; 28 total animals). Recovery responses were compared with the predictions of a pharmacokinetic–pharmacodynamic model parameterized using previously published laboratory data. Results:Rats treated with lipid emulsions recovered faster than did rats treated with saline or no treatment. Intravenous lipid emulsion of 30% elicited the fastest hemodynamic recovery followed in order by 20% intravenous lipid emulsion, saline, and no treatment. An increase in arterial blood pressure underlay the recovery in both lipid emulsion–treated groups. Heart rates remained depressed in all four groups throughout the observation period. Model predictions mirrored the experimental recovery, and the model that combined volume, sequestration, and inotropy predicted in vivo results most accurately. Conclusion:Intravenous lipid emulsion accelerates cardiovascular recovery from bupivacaine toxicity in a dose-dependent manner, which is driven by a cardiotonic response that complements the previously reported sequestration effect.

Past, Present, and Future of Lipid Resuscitation Therapy

Lipid resuscitation therapy was identified in 1998 as an effective treatment for local anesthetic systemic toxicity in an animal model. Since the original observation, the field has progressed tremendously with successful clinical translation and expansion of use to treatment of other types of drug overdose. Recent work has expanded our understanding of the mechanism of this novel treatment, one that includes both a dynamic scavenging component and direct cardiotonic effect. In this review, we discuss the past, present, and future of lipid resuscitation therapy with a focus on our understanding of the mechanism and directions that the field is moving, both from a clinical and basic research side.

Sensory feedback for upper limb prostheses

In this chapter, we discuss the neurophysiological basis of how to provide sensory feedback to users with an upper limb prosthesis and discuss some of the theoretical issues that need to be considered when directly stimulating neurons in the somatosensory system. We focus on technologies that are currently available and discuss approaches that are most likely to succeed in providing natural perception from the artificial hand to the user. First, we discuss the advantages and disadvantages of providing feedback by stimulating directly the remaining afferents that originally innervated the arm and hand. In particular, we pay close attention to the normal functional roles that the peripheral afferents play in perception. What are the consequences and implications of stimulating these afferents? We then discuss whether it is reasonable to stimulate neurons in the ascending pathways that carry the information from the afferents to the cortex or directly in neurons in the primary somatosensory cortex. We show that for some modalities there are advantages for stimulating in the spinal cord, while for others it is advantageous to stimulate directly in the somatosensory cortex. Finally, we discuss results from a current experiment in which we used electrical stimuli in primary somatosensory cortex to restore the percept of the intensity of a mechanical probe indented into the hand. The results suggest that the simple percept of stimulus intensity can be provided to the animal from a single finger using four electrodes. We propose that significantly more electrodes will be needed to reproduce more complex aspects of tactile perception.

Intraosseous lipid emulsion: An effective alternative to IV delivery in emergency situations

Objective:To determine whether intraosseous infusion of a lipid emulsion reverses cardiac pharmacotoxicity in anaesthetized rats. Design:Prospective, randomized animal study. Setting:Academic research laboratory. Subjects:Adult, male Sprague-Dawley rats. Interventions:We assigned 25 male Sprague-Dawley rats into four groups: intraosseous lipid emulsion, intraosseous saline, IV lipid emulsion, and sham/null. Rats were anesthetized with 1.5% isoflurane and 95% oxygen. The left internal carotid artery and both internal jugular veins were cannulated and a flow probe was placed on the right carotid artery. Subsequently, in animals assigned to the intraosseous groups, the greater trochanter of the left proximal femur was exposed and the intraosseous space was cannulated. After surgical recovery, bupivacaine (10 mg/kg) was injected IV over 20 seconds followed 10 seconds later by treatment with one of the following: intraosseous lipid-emulsion (10 mL/kg over 180 s), intraosseous saline (10 mL/kg over 180 s), IV lipid-emulsion (10 mL/kg over 90 s), or no treatment (sham/null). Measurements and Main Results:Electrocardiogram, aortic blood pressure, and carotid blood flow were recorded continuously. Rats treated with intraosseous lipid emulsion experienced a significantly faster recovery of hemodynamic variables (return of 50% flow; median [CI]: 160 s [105–263 s]) than did rats treated with saline (471 s [283–611 s]; p < 0.05) or animals with no treatment (415 s [340–539 s], p < 0.05), but at a similar rate to animals treated with IV lipid emulsion (176 s [152–217 s], p = not significant). All groups experienced persistent negative chronotropic effects. A compensatory increase in systemic arterial pressure was observed in rats treated with lipid emulsion. Conclusion:These proof-of-principle data indicate that intraosseous infusion of lipid emulsion rapidly reverses bupivacaine-induced cardiac toxicity in rats. Further studies are warranted to optimize this novel route of lipid emulsion injection in emergency situations when intravascular access is not secured.

Insulin Signaling in Bupivacaine-induced Cardiac Toxicity: Sensitization during Recovery and Potentiation by Lipid Emulsion.

Background:The impact of local anesthetics on the regulation of glucose homeostasis by protein kinase B (Akt) and 5′-adenosine monophosphate–activated protein kinase (AMPK) is unclear but important because of the implications for both local anesthetic toxicity and its reversal by IV lipid emulsion (ILE). Methods:Sprague–Dawley rats received 10 mg/kg bupivacaine over 20 s followed by nothing or 10 ml/kg ILE (or ILE without bupivacaine). At key time points, heart and kidney were excised. Glycogen content and phosphorylation levels of Akt, p70 s6 kinase, s6, insulin receptor substrate-1, glycogen synthase kinase-3&bgr;, AMPK, acetyl-CoA carboxylase, and tuberous sclerosis 2 were quantified. Three animals received Wortmannin to irreversibly inhibit phosphoinositide-3-kinase (Pi3k) signaling. Isolated heart studies were conducted with bupivacaine and LY294002—a reversible Pi3K inhibitor. Results:Bupivacaine cardiotoxicity rapidly dephosphorylated Akt at S473 to 63 ± 5% of baseline and phosphorylated AMPK to 151 ± 19%. AMPK activation inhibited targets downstream of mammalian target of rapamycin complex 1 via tuberous sclerosis 2. Feedback dephosphorylation of IRS1 to 31 ± 8% of baseline sensitized Akt signaling in hearts resulting in hyperphosphorylation of Akt at T308 and glycogen synthase kinase-3&bgr; to 390 ± 64% and 293 ± 50% of baseline, respectively. Glycogen accumulated to 142 ± 7% of baseline. Irreversible inhibition of Pi3k upstream of Akt exacerbated bupivacaine cardiotoxicity, whereas pretreating with a reversible inhibitor delayed the onset of toxicity. ILE rapidly phosphorylated Akt at S473 and T308 to 150 ± 23% and 167 ± 10% of baseline, respectively, but did not interfere with AMPK or targets of mammalian target of rapamycin complex 1. Conclusion:Glucose handling by Akt and AMPK is integral to recovery from bupivacaine cardiotoxicity and modulation of these pathways by ILE contributes to lipid resuscitation.

Blood Density Is Nearly Equal to Water Density: A Validation Study of the Gravimetric Method of Measuring Intraoperative Blood Loss.

Purpose. The gravimetric method of weighing surgical sponges is used to quantify intraoperative blood loss. The dry mass minus the wet mass of the gauze equals the volume of blood lost. This method assumes that the density of blood is equivalent to water (1 gm/mL). This studys purpose was to validate the assumption that the density of blood is equivalent to water and to correlate density with hematocrit. Methods. 50 µL of whole blood was weighed from eighteen rats. A distilled water control was weighed for each blood sample. The averages of the blood and water were compared utilizing a Students unpaired, one-tailed t-test. The masses of the blood samples and the hematocrits were compared using a linear regression. Results. The average mass of the eighteen blood samples was 0.0489 g and that of the distilled water controls was 0.0492 g. The t-test showed P = 0.2269 and R 2 = 0.03154. The hematocrit values ranged from 24% to 48%. The linear regression R 2 value was 0.1767. Conclusions. The R 2 value comparing the blood and distilled water masses suggests high correlation between the two populations. Linear regression showed the hematocrit was not proportional to the mass of the blood. The study confirmed that the measured density of blood is similar to water.

Lipid emulsion rapidly restores contractility in stunned mouse cardiomyocytes: a comparison with therapeutic hypothermia.

Objectives:Cooling following cardiac arrest can improve survival significantly. However, delays in achieving target temperature may decrease the overall benefits of cooling. Here, we test whether lipid emulsion, a clinically approved drug reported to exert cardioprotection, can rescue heart contractility in the setting of delayed cooling in stunned mouse cardiomyocytes. Design:Cell culture study. Setting:Academic research laboratory. Subjects:Cardiomyocytes isolated from 1- to 2-day-old C57BL6 mice. Interventions:Cardiomyocytes were exposed to 30 minutes of ischemia followed by 90 minutes of reperfusion and 10 minutes of isoproterenol with nine interventions: 1) no additional treatment; 2) intraischemic cooling at 32°C initiated 10 minutes prior to reperfusion; 3) delayed cooling started 20 minutes after reperfusion; 4) lipid emulsion + delayed cooling; 5) lipid emulsion (0.25%) administered at reperfusion; 6) lipid emulsion + intraischemic cooling; 7) delayed lipid emulsion; 8) lipid emulsion + delayed cooling + Akt inhibitor (API-2, 10 µM); and 9) lipid emulsion + delayed cooling + Erk inhibitor (U0126, 10 µM). Inhibitors were given to cells 1 hour prior to ischemia. Measurements and Main Results:Contractility was recorded by real-time phase-contrast imaging and analyzed with pulse image velocimetry in MATLAB (Mathworks, Natick, MA). Ischemia diminished cell contraction. The cardioprotective effect of cooling was diminished when delayed but was rescued by lipid emulsion. Further, lipid emulsion on its own improved recovery of the contractility to a greater extent as intraischemic cooling. However, cotreatment of lipid emulsion and intraischemic cooling did not further improve the recovery compared to either treatment alone. Furthermore, Akt and Erk inhibitors blocked lipid emulsion–induced protection. Conclusions:Lipid emulsion improved contractility and rescued contractility in the context of delayed cooling. This protective effect required Akt and Erk signaling. Lipid emulsion might serve as a treatment or adjunct to cooling in ameliorating myocardial ischemia/reperfusion injury.

Lipid emulsion improves survival in animal models of local anesthetic toxicity: a meta-analysis

Abstract Introduction: The Lipid Emulsion Therapy workgroup, organized by the American Academy of Clinical Toxicology, recently conducted a systematic review, which subjectively evaluated lipid emulsion as a treatment for local anesthetic toxicity. We re-extracted data and conducted a meta-analysis of survival in animal models. Methods: We extracted survival data from 26 publications and conducted a random-effect meta-analysis based on odds ratio weighted by inverse variance. We assessed the benefit of lipid emulsion as an independent variable in resuscitative models (16 studies). We measured Cochran’s Q for heterogeneity and I2 to determine variance contributed by heterogeneity. Finally, we conducted a funnel plot analysis and Egger’s test to assess for publication bias in studies. Results: Lipid emulsion reduced the odds of death in resuscitative models (OR =0.24; 95%CI: 0.1–0.56, p = .0012). Heterogeneity analysis indicated a homogenous distribution. Funnel plot analysis did not indicate publication bias in experimental models. Discussion: Meta-analysis of animal data supports the use of lipid emulsion (in combination with other resuscitative measures) for the treatment of local anesthetic toxicity, specifically from bupivacaine. Our conclusion differed from the original review. Analysis of outliers reinforced the need for good life support measures (securement of airway and chest compressions) along with prompt treatment with lipid.