Anaesthesia

11. Intravenous Supportive Therapy

11. Intravenous Supportive Therapy The intravenous administration of drugs, electrolyte solutions, plasma expanders and blood are an integral part of anaesthetic management. The total body water amount s to about 48.5 % of the female an d u p to 56.8 % of the male body weight an d ca n b e classified a s extracellular and intracellular. The extracellular fluid volume is approximately 20% of the ideal body weight. It has two components: intravascular flui d o r plasma an d interstitia l fluid. The osmotically active protein present i n blood largel y determines th e proportion o f the intravascula r par t o f the extracellular fluid an d i s usually of the order of 1/5 , the other 4/5 being the interstitial fluid. The measuremen t o f bod y fluid s depend s o n th e dilutio n principle . A know n quantity o f a measurabl e substanc e i s injecte d an d th e ultimat e concentratio n i s determined afte r unifor m distributio n i n th e fluid compartmen t i s reached . Fo r example, the plasma volume can be determined by the intravenous injection o f a dye (such as Evans blue) or radioactive iodinate d human seru m albumin (RIHSA) . Th e whole bloo d volum e ma y b e estimate d b y addin g th e volum e o f th e re d cell s (de termined by the injection o f chromium tagged cells). The measurement o f extracellular fluid volume and estimatio n o f the quantity o f interstitial fluid (exclusiv e o f plasma ) ca n b e done wit h th e ai d o f tagge d sulphate , which is distributed in fluid outside the cells. The total body water ca n be measured wit h the aid of Deuterium, whic h is distributed bot h insid e an d outsid e th e cells . The amoun t o f intracellula r wate r ca n b e ascertained by subtracting the volume of the extracellular water from th e total bod y water. Parenteral fluid therapy aims to; (1) restor e a n adequat e circulatin g bloo d volum e (fo r th e maintenanc e o f bloo d pressure, tissue perfusion an d renal function) ; (2) restor e the extracellular fluid bac k to normal volume, composition and tonicity ; (3) provid e adequate nutrition . When a patien t i s subjecte d t o traum a (includin g surgica l operations) , certai n changes i n th e metaboli c processe s tak e place . Th e duratio n an d severit y o f th e changes are usually proportional to the injury. Taking an operation, such as gastrectomy for example, one would expect the following : (1) Fro m th e beginning of the induction o f anaesthesia ther e follows a reduction i n water secretion , lasting for 24-3 6 hours, probably du e to an excess of vasopressin secretion. Only about 600 ml of concentrated urine/24 hours is excreted (jus t sufficient fo r th e excretion o f waste products) an d exces s loads of water wil l b e Intravenous Supportive Therapy 209 retained. Excessiv e electrolyte-fre e fluid give n durin g thi s perio d o f oliguri a (which may last even longer in some cases) may lead to water intoxication . (2) Durin g a 3-5 da y period, sodium excretion is also diminished t o about 1 0 mEq/ 24 hours. Overloading with saline will cause expansion of the extracellular space, undue swellin g and disruptio n o f wound s an d pulmonar y a s well a s periphera l oedema. (3) Tissu e catabolism of about 4 days duration will occur when the patient consume s about 60 g of his own protein/24 hours. There will be a negative nitrogen balanc e of about 1 0 g, as well as the release of potassium and water (the latter about 50 0 ml/24 hours). (4) Fo r 3- 4 day s ther e i s a n increase d potassiu m los s i n th e urine . Thi s i s mor e marked on the first day, while the total loss amounts to about 10 0 mEq. Mobilization of intracellular potassium is probably implicated also...