Plasma clearance and lipaemic index of lipid emulsion used for lipid emulsion treatment

Dear Editor

I read with interest the article entitled ‘Does the use of fish-oil based lipid emulsion in the clinical setting of total parenteral nutrition (TPN) and lipid rescue therapy interfere with common laboratory analytes on Roche Cobas 6000?’ recently published in the Annals of Clinical Biochemistry. ¹ Tan and Wong describe that initial administration alone (1.5 mL/kg, 20% SMOFlipid emulsion) followed by 15 mL/kg/h for 5 min, pharmacokinetically produced a lipaemic index of 660 (660 mg/dL) and 1220 (1220 mg/dL), respectively, causing interference with aspartate aminotransferase and alanine aminotransferase measurements, on account of exceeding the lipaemic threshold for these assays. ¹ However, the in vivo plasma clearance of lipid emulsion will also affect the plasma concentration of lipid emulsion. Lipid emulsion, containing triglyceride, bypasses the gastrointestinal tract and directly enters the blood stream. ² Lipoprotein lipase hydrolyses lipid emulsion triglyceride to non-essential fatty acids. ² Medium-chain triglycerides with a relatively short carbon chain are hydrolysed much more quickly by lipoprotein lipase than long-chain triglycerides. ³ Thus, the half-life (34 ± 11 min) of triglyceride in the SMOFlipid, comprising 30% soybean oil, 30% medium-chain triglyceride, 25% olive oil and 15% fish oil, is lower than that (59 ± 25 min) of triglyceride in lipid emulsion with 100% long-chain fatty acid. ⁴ Triglyceride induced by lipid emulsion treatment is hydrolysed by lipoprotein lipase, using zero- or first-order kinetics depending on the concentration, resulting in clearance and decreased plasma concentrations of triglyceride. ⁵ The lipaemic index of SMOFlipid described in this study would be expected to decrease relatively quickly after discontinuation of infusion of SMOFlipid. ¹

Lipid emulsion is used to treat systemic toxicity of local anaesthetic drugs. ⁶ The recommended dosing regimen of lipid emulsion for this indication is as follows: initial bolus administration of lipid emulsion at 1.5 mL/kg followed by intravenous infusion of lipid emulsion at 0.25 mL/kg/min with 10–12 mL/kg for 30 min as the recommended upper dose of initial dosing. ⁶ One per cent lipid emulsion produces an inotropic and scavenging effect on lipophilic-offending drugs and has been reported to be effective in treating severe cardiovascular collapse induced by a toxic dose of non-local anaesthetic drugs (for example: verapamil and amlodipine) with high lipid solubility.^6–8 Thus, the dosing regimen of lipid emulsion suggests the maintenance of 1% plasma triglyceride (1000 mg/dL, lipaemic index: 1000) for lipid emulsion treatment of toxicity induced by non-local anaesthetic drugs. ⁹ The resultant lipaemic interference may be ameliorated by the use of liquid polymer reagents, e.g. LipoClear to ‘clear’ serum/plasma of lipids and high-speed centrifugation.^1,6