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Consequences of the use of significantly high levels of polyunsaturated fatty acids (PUFA) include reduced levels of phagocytosis by macrophages (in liver and lungs) and the enhancement of reactions involving immunosuppression (sepsis) (Waitzberg D.L, 2005). Taking this into consideration, the most appropriate choices for a parenteral emulsion are Lipidem (contains Ï‰-3 acids), Omegaven (contains fish oil, highly refined) and SMOFlipid (BNF 57, 2009). In this particular instance SMOFlipid would be the most appropriate choice for the patient(s) as it contains Ï‰-6 and Ï‰-3 polyunsaturated fatty acids (PUFA), ideal for delivering a balanced supply of both fatty acids (Waitzberg D.L, 2005).
When formulating such a product one must take into account a number of factors in relation to its physiochemical nature and the likely adverse effects if such factors are not appropriately addressed.
TPN admixtures are commonly compounded resulting in infusions that are significantly unstable. The mixing of the constituent's results in the physiochemical stability, compatibility of the excepients and active ingredients to be compromised, resulting in the formation of precipitate (Allwood C.M et al, 1998). Inherent physical incompatibilities of certain additives such as electrolytes, results in the chemical degradation of specific entities or ingredients, this further contributes to the formation of precipitate and crystals (Allwood C.M et al, 1998). In vivo application of such admixtures can lead to pulmonary deposition, which is fatal if not identified early (Allwood C.M et al, 1998).
High temperatures can cause the solubility of certain ions (Ca2+) to become compromised by causing calcium to dissociate, thereby resulting further in the formation of precipitate (Allwood C.M et al, 1998). A comparison on total nutrition admixtures under varied temperature conditions was carried out. The findings from this investigation suggest that the ideal temperature for the admixtures was 18oC, higher temperatures (28oC >) were found to cause coalescence (Lee M.D et al, 2003).
UV light (sunlight) causes photolysis of the product, thereby causing permanent damage (Allwood C.M et al, 1998). Amino acids are affected greatly by the exposure to ambient light and undergo a process of photoxidation (Allwood C.M et al, 1996). This causes a colour change within the admixture, the by-product of which is known to exhibit toxicity (Allwood C.M et al, 1996). PUFA's are known to peroxidise to hydroperoxides which are potentially harmful, and have known toxicity associated when given in vivo. Again sunlight has been found to increase the level of peroxidation of PUFA's (Steger, P.T.K et al 1997).
Addition of glucose can lower the pH of the admixture (Pertkiewicz M et al, 2009), the lower the pH the increased likelihood of coalescence occurring (Schroder A.M, 2008). Amino acids play a crucial role in acting as buffers, and can vary between 5.0 and 7.4 depending on the concentration of amino acids used (Schroder A.M, 2008).
Frequently the material used to house the admixture is semi permeable. However this may not be the best option as oxidative damage is the major consequence. There is an inherent risk of embolism as oxygen can lead to the formation of bubbles within the admixture (Allwood C.M et al, 1996).
Flocculation is caused by the adherence of droplets to one another forming a cream layer. Coalescence is caused by the result of larger droplets forming an oil droplet layer. Both factors affect the physical stability and the shelf life of the admixture (C. Washington, 1996).
Due to a number of excepients used in admixtures there are inherent risks when mixing. The mixing of the excepients especially in reference to electrolytes such as divalent cations (Ca2+) and monovalent cations (Na+) have a major influence on the chemical stability of the final product. The combination of the lipid (-) and the cations (+) causes a reduction in surface potential (zeta potential) of the droplets and repulsive forces present. This results in coalescence of the emulsion. Divalent cations are known to cause significant levels of flocculation (Schroder A.M, 2008), (C. Washington, 1996).
The physical stability of the admixture can be assessed in different ways. Globule size of the emulsion can be assessed in a number of ways including laser diffraction and electrical zone sensing. However, the applications of such analytical techniques have inherent poor levels of sensitivity and are rather laborious (Driscoll D.F, 1997). More sophisticated techniques have been developed to overcome these limitations for instance laser light obscuration (Driscoll D.F, 1997). Visual examination or electron microscopy can be applied to indentify the formation of precipitate, flocculation and coalescence (Floyd G.A, 1999), (Allwood C.M et al, 1998). Concentration changes within the admixture can also be observed via the use of spectrophotometery or atomic spectroscopy (Allwood C.M et al, 1998). Flocculation of the emulsion can be measured via Turbidimetry or microscopically, however these are time consuming techniques (C. Washington, 1996). Coalescence can also be measured with the use of a particle size analyser (Coulter Counter). Nevertheless, this is limited in some respects as coalescence can only be detected in reference the relative distribution of size (C. Washington, 1996).
In reference to the handling and application of the admixture for patient use one must take into account a number of additional factors. Exposure to the admixture should be 24 hours and no longer (Lee M.D et al, 2003). In respect to shelf life, at 2-8oC the admixture should kept for 24 hours and no longer (SPC, 2006). The use of "multichamber-bags" can aid in the storage issues aforementioned as the ingredients can be kept separately and be preserved for a longer period of time (Driscoll D.F, 2008). Care must also be taken to avoid vigorous shaking or vibration of the admixture as this can result in significant oxidation of the emulsion (Lee M.D et al, 2003). Also bacterial growth may be promoted if storage of the admixture is kept at room temperature (McConell E.A, (2001), . The use of containers (bags) that are multilayered or covered with aluminium foil can aid in reducing the oxidation of the admixture (Allwood C.M et al, 1998), (Allwood C.M et al, 1996).
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