Fluids and electrolytes: the basics

Transcription

1 Fluids and electrolytes: the basics This document is based on the handout from the Surgery for Finals course. The notes provided here summarise key aspects, focusing on areas that are popular in clinical examinations. They will complement more detailed descriptions and are not intended to be comprehensive. Normal replacement 1 litre dextrose + 20mmol KCl over 8 hours 1 litre dextrose + 20mmol KCl over 8 hours 1 litre saline ie + 20mmol KCl over 8 hours dextrose-dextrose-saline with 20mmol KCl per bag Body water 2/3 intracellular 1/3 extracellular Obstruction Peritonitis - first space - second space ( 1/4 plasma 3/4 interstitial ) - third space Body water Is 3/5 of body weight For a 70kg man = 42 litres Normal requirement is 3litre per day Increased with fever, diarrhoea etc Replace as dextrose, dextrose saline Too little salt in dextrose-saline Sodium mmol Na normally required This is the skeleton on which ECF hangs Excess loss with D&V or sweating Excess loss with third space loss eg obstruction, peritonitis Potassium 97% intracellular 60mmol normally required per day Replace as 20mmol/litre x 3 per 24hrs Excess loss in D&V, alkalosis (eg pyloric stenosis) Trauma and transfusion release potassium Low K associated with long PR interval, ST depression, inverted T waves Dr R Clarke 1

2 Metabolic Response To Trauma Symp stim. Low BP Low Na Juxtaglomerular apparatus Angio I AngioII Aldosterone Lung converting enzyme Na retention K loss Low BP Symp stim. Trauma ADH release High osmolality Post pituitary Water retention Trauma Hypotension Cortisol release Maintain BP etc Post operative fluid balance Ensure urine output > 30ml / hour Compare output with previous 24hours if possible is <400ml per 24 hours Pre-renal failure: kidney concentrates well and urine: plasma osmolality is >1.5 : 1 Renal: failure to concentrate urine <1.1 : 1 plus leaks sodium into urine >20mmol/l Pre-renal: assess for signs of shock: pale anxious Sinus tachycardia Hypotension Cold Klammy Slow capillary refill Look for cause of shock hypovolaemic (inadequate replacement etc) septic cardiogenic (MI, PE etc) Dr R Clarke 2

3 Check haemodynamic status Check hydration Check for signs of sepsis- wound, drains, peritonism, canulae etc Check for a bladder Check femoral pulses (in case of aneurysm or dissection) Flush catheter with 30ml saline FBC U&E ECG for signs of hyperkalaemia Urine osmolality dehyrated > 300mosm/litre established renal failure < 300mosm/litre If negative fluid balance and no sign overload, try fluid challenge 500ml N saline over 1 hour Monitor urine output CVP can help assess fluid challenge return to baseline pressure within 5 minutes of infusion of 200ml= dehydration Principles of management Stop fluid loss Replace fluids Stop nephrotoxins eg gentamicin Exclude post renal obstruction Screen for sepsis Protect kidneys: low dose dopamine ECG for potassium status Write short notes on: Causes of a low serum sodium in a peri-operative patient. Dr R Clarke 3

4 Low Na Patient dehydrated Both sodium and water lost Patient not dehydrated Fluid overload or medical problem Renal loss High urinary Na Diuretics Diabetes Acute tub necrosis Addisons Non-renal loss Low urine Na D + V Fistula Burns Small bowel obst. No oedema Fluid overload eg dextrose drip SIADH Oedematous Probably medical Nephrotic CCF CRF Liver failure Colloids Used in shock together with crystalloids Crystalloids (eg normal saline) make up the deficit of interstitial fluid Colloids have higher osmotic pressure as they are bigger molecules- stay in the intravascular compartment longer and restore the plasma volume Help improve BP, cardiac output, & prevent respiratory distress syndrome Types of colloid Human albumin / fresh frozen plasma Dextran (polysaccharide polymer) Haemacel- (gelatin based) Starches- (glycogen based) NB: Expensive; Take blood for cross match first; Allergic reactions Dr R Clarke 4

5 CVP line Sterile subclavian puncture Seldinger technique- introducer needle, guide wire and then wide bore canula Secure with sutures and occlusive dressing Connect via 3 way tap to infusion set and manometer or pressure transducer To measure the CVP Lie patient flat Zero manometer at mid-axillary line or angle of Louis Check for flashback- lower infusion bag Fill manometer using three way tap and connect manometer to patient Read CVP once level stabilised (normal range -4cm to +3cm from angle of Louis) Manometer Patient Fluids Three-way tap for CVP measurement CVP measurement Dr R Clarke 5