Systemic oxygen delivery by peritoneal perfusion of oxygen microbubbles

Abstract

Severe hypoxemia refractory to pulmonary mechanical ventilation remains life-threatening in critically ill patients. Peritoneal ventilation has long been desired for extrapulmonary oxygenation owing to easy access of the peritoneal cavity for catheterization and the relative safety compared to an extracorporeal circuit. Unfortunately, prior attempts involving direct oxygen ventilation or aqueous perfusates of fluorocarbons or hemoglobin carriers have failed, leading many researchers to abandon the method. We attribute these prior failures to limited mass transfer of oxygen to the peritoneum and have designed an oxygen formulation that overcomes this limitation. Using phospholipid-coated oxygen microbubbles (OMBs), we demonstrate 100% survival for rats experiencing acute lung trauma to at least 2 h. In contrast, all untreated rats and rats treated with peritoneal oxygenated saline died within 30 min. For rats treated with OMBs, hemoglobin saturation and heart rate were at normal levels over the 2-h timeframe. Peritoneal oxygenation with OMBs was therefore shown to be safe and effective, and the method requires less equipment and technical expertise than initiating and maintaining an extracorporeal circuit. Further translation of peritoneal oxygenation with OMBs may provide therapy for acute respiratory distress syndrome arising from trauma, sepsis, pneumonia, aspiration, burns and other pulmonary diseases.

Keywords: Acute lung injury; Acute respiratory distress syndrome; Hypoxemia; Oxygen absorption and transport; Phospholipid monolayer.

Fig. 1

Lipid-coated oxygen microbubbles. (a) Oxygen gas comprises 99% of the microbubble volume. The oxygen core is stabilized by a thin (∼3 nm) phospholipid monolayer membrane, which reduces surface tension and provides in-plane rigidity. A hydrated PEG brush (∼10 nm height) grafted to the lipid surface provides steric repulsion to prevent microbubble coalescence. (b) A scaled-up OMB manufacturing process is used to generate up to 2 L of OMBs (∼70 vol%) per day. (c) Bright field microscopy of diluted OMBs showed discrete, spherical microbubbles. (d) OMBs have a milky white appearance and can be stored and transported in 500-mL serum bottles. (e) Shown are the volume and number-weighted size distributions for freshly generated OMBs. The dashed lines are the number (blue) and volume (red) mean diameters. The shaded area covers the size range for 90% of the total OMB gas volume. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Peritoneal oxygenation with OMBs. (a) The experimental setup is shown for treatment of a right pneumothorax model for acute lung injury. (b) OMBs infused into the peritoneal cavity deliver oxygen through the parietal peritoneum and visceral peritoneum to adjacent blood and tissue. (c) The total infused volume per rat was equivalent between the OMBs and saline control. Plots show the (d) pressure (mean ± SD) in the peritoneal cavity and (e) rectal temperature (mean ± SD) versus time for OMBs and saline.

Fig. 3

Peritoneal perfusion with OMBs provides life-sustaining oxygenation. (a) All 5 animals treated with OMBs survived to the 2-h endpoint and were subsequently euthanized as directed by the IACUC protocol. The median survival times for untreated animals and those treated with oxygenated saline were 15.5 (n = 6) and 18.5 min (n = 5), respectively. Thus, OMB infusion significantly increased survival time compared to saline control (P = 0.0039, unpaired two-tailed t-test using the 125 min endpoint for OMBs). (b) Animals treated with OMBs had an SaO₂ between 80% and 90%, as measured by a paw-cuff pulse oximeter. SaO₂ rapidly declined for animals treated with saline control. (c) The heart rate for animals treated with OMBs was 240–350 bpm immediately following the right pneumothorax and increased to 300–350 bpm throughout the experiment. Animals treated with saline control experienced a rapidly declining heart rate. The pulse oximeter could read up to a maximum heart rate of 350 bpm.

Fig. 4

Oxygen transport from a microbubble. (a) Oxygen gas molecules must pass through a series of three resistances to enter the bulk fluid phase of the peritoneal cavity, including diffusion from the gas to the shell (R_gas), diffusion through the lipid shell and absorption into the liquid (R_shell) and diffusion to the bulk liquid (R_bulk). The diagram shows the relative concentrations. (b) A quantitative comparison of the three mass transfer resistances versus bubble radius at 25 °C and 1 bar. R_gas and R_shell are independent of bubble size, and R_gas is approximately six orders of magnitude less than R_shell. R_bulk is computed for a purely diffusing sphere; it dominates for large bubbles but approaches R_shell for microbubbles. The shaded region represents the OMB size range comprising 90% of the total gas volume, showing that the main mass transport limitation for these microbubbles is diffusion through the lipid shell.