Effects of air embolism size and location on porcine hepatic microcirculation in machine perfusion

Controlled Hyperthermia With High-Intensity Focused Ultrasound and Ultrasound Contrast Agent Microbubbles in Porcine Liver

OBJECTIVE

The objective of this work was to study microbubble-enhanced temperature elevation with high-intensity focused ultrasound (HIFU) at different acoustic pressures and under image guidance. The microbubbles were administered with either local or vascular injections (that mimic systemic injections) in perfused and non-perfused ex vivo porcine liver under ultrasound image guidance.

METHODS

Porcine liver was insonified for 30 s with a single-element HIFU transducer (0.9 MHz, 0.413 ms, 82% duty cycle, focal pressures of 0.6-3.5 MPa). Contrast microbubbles were injected either locally or through the vasculature. A needle thermocouple at the focus measured temperature elevation. Diagnostic ultrasound (Philips iU22, C5-1 probe) guided placement of the thermocouple and delivery of microbubbles and monitored the procedure in real time.

RESULTS

At lower acoustic pressures (0.6 and 1.2 MPa) in non-perfused liver, inertial cavitation of the injected microbubbles led to greater temperatures at the focus compared with HIFU-only treatments. At higher pressures (2.4 and 3.5 MPa) native inertial cavitation in the tissue (without injecting microbubbles) resulted in temperature elevations similar to those after injecting microbubbles. The heated area was larger when using microbubbles at all pressures. In the presence of perfusion, only local injections provided a sufficiently high concentration of microbubbles necessary for significant temperature enhancement.

CONCLUSION

Local injections of microbubbles provide a higher concentration of microbubbles in a smaller area, avoiding acoustic shadowing, and can lead to higher temperature elevation at lower pressures and increase the size of the heated area at all pressures.

Liver ex situ machine perfusion preservation: A review of the methodology and results of large animal studies and clinical trials

Ex vivo machine perfusion (MP) is a promising way to better preserve livers prior to transplantation. Currently, no methodology has a verified benefit over simple cold storage. Before becoming clinically feasible, MP requires validation in models that reliably predict human performance. Such a model has been found in porcine liver, whose physiological, anatomical, and immunological characteristics closely resemble the human liver. Since the 1930s, researchers have explored MP as preservation, but only recently have clinical trials been performed. Making this technology clinically available holds the promise of expanding the donor pool through more effective preservation of extended criteria donor (ECD) livers. MP promises to decrease delayed graft function, primary nonfunction, and biliary strictures, which are all common failure modes of transplanted ECD livers. Although hypothermic machine perfusion (HMP) has become the standard for kidney ex vivo preservation, the precise settings and clinical role for liver MP have not yet been established. In research, there are 2 schools of thought: normothermic machine perfusion, closely mimicking physiologic conditions, and HMP, to maximize preservation. Here, we review the literature for porcine ex vivo MP, with an aim to summarize perfusion settings and outcomes pertinent to the clinical establishment of MP. Liver Transplantation 23 679-695 2017 AASLD.

Machine perfusion enhances hepatocyte isolation yields from ischemic livers

BACKGROUND

High-quality human hepatocytes form the basis of drug safety and efficacy tests, cell-based therapies, and bridge-to-transplantation devices. Presently the only supply of cells derives from an inadequate pool of suboptimal disqualified donor livers. Here we evaluated whether machine perfusion could ameliorate ischemic injury that many of these livers experience prior to hepatocyte isolation.

METHODS

Non-heparinized female Lewis rat livers were exposed to an hour of warm ischemia (34°C) and then perfused for 3h. Five different perfusion conditions that utilized the cell isolation apparatus were investigated, namely: (1) modified Williams Medium E and (2) Lifor, both with active oxygenation (95%O(2)/5%CO(2)), as well as (3) Lifor passively oxygenated with ambient air (21%O(2)/0.04%CO(2)), all at ambient temperatures (20 ± 2°C). At hypothermic temperatures (5 ± 1°C) and under passive oxygenation were (4) University of Wisconsin solution (UW) and (5) Vasosol. Negative and positive control groups comprised livers that had ischemia (WI) and livers that did not (Fresh) prior to cell isolation, respectively.

RESULTS

Fresh livers yielded 32 ± 9 million cells/g liver while an hour of ischemia reduced the cell yield to 1.6 ± 0.6 million cells/g liver. Oxygenated Williams Medium E and Lifor recovered yields of 39 ± 11 and 31 ± 2.3 million cells/g liver, respectively. The passively oxygenated groups produced 15 ± 7 (Lifor), 13 ± 7 (Vasosol), and 10 ± 6 (UW)million cells/g liver. Oxygenated Williams Medium E was most effective at sustaining pH values, avoiding the accumulation of lactate, minimizing edematous weight gain and producing bile during perfusion.

CONCLUSIONS

Machine perfusion results in a dramatic increase in cell yields from livers that have had up to an hour of warm ischemia, but perfusate choice significantly impacts the extent of recovery. Oxygenated Williams Medium E at room temperature is superior to Lifor, UW and Vasosol, largely facilitated by its high oxygen content and low viscosity.

Layer-by-layer heparinization of decellularized liver matrices to reduce thrombogenicity of tissue engineered grafts

Background:

Tissue–engineered liver grafts may offer a viable alternative to orthotopic liver transplantation and help overcome the donor organ shortage. Decellularized liver matrices (DLM) have a preserved vasculature and sustain hepatocellular function in culture, but graft survival after transplantation remains limited due to thrombogenicity of the matrix.

Aim:

To evaluate the effect of heparin immobilization on DLM thrombogenicity.

Methods:

Heparin was immobilized on DLMs by means of layer-by-layer deposition. Grafts with 4 or 8 bilayers and 2 or 4 g/L of heparin were recellularized with primary rat hepatocytes and maintained in culture for 5 days. Hemocompatibility of the graft was assessed by ex vivo diluted whole-blood perfusion and heterotopic transplantation.

Results:

Heparin was deposited throughout the matrix and the heparin content in the graft was higher with increasing number of bilayers and concentration of heparin. Recellularization and in vitro albumin and urea production were unaffected by heparinization. Resistance to blood flow during ex vivo perfusion was lower with increased heparinization and, macroscopically, no clots were visible in grafts with 8 bilayers. Following transplantation, flow through the graft was limited in all groups. Histological evidence of thrombosis was lower in heparinized DLMs, but transplantation of DLM grafts was not improved.

Conclusions:

Layer-by-layer deposition of heparin on a DLM is an effective method of immobilizing heparin throughout the graft and does not impede recellularization or hepatocellular function in vitro. Thrombogenicity during ex vivo blood perfusion was reduced in heparinized grafts and optimal with 8 bilayers, but transplantation remained unsuccessful with this method.

Relevance for patients:

Tissue engineered liver grafts may offer a viable solution to dramatic shortages in donor organs