Fluid Homeostasis - Surgery Flashcards
What is the water composition in the body?
Total water: 60% of 70kg = 42L
What fraction of body water is intracellular?
2/3 so 28L
What fraction of body water is extracellular?
1/3 so 14L
What are Starling’s Forces?
Osmotic and Hydrostatic Pressure
What is osmotic pressure?
- Pressure which needs to be applied to prevent the inflow of water across a semipermeable membrane.
This is the ability of a solute to attract water
Oncotic pressure: form of osmotic pressure exerted by proteins
What is hydrostatic pressure?
Pressure exerted by a fluid at equilibrium due to force of gravity.
How is fluid distributed in the body?
Distribution between ECF and ICF is driven by differences in osmotic pressure only.
Distributions within the ECF is determined by Starling’s forces.
- Capillary and interstitial oncotic pressures
- Capillary and interstitial hydrostatic pressure
- Filtration coefficient.
3rd space losses –> Decreased ECF
Fluid moving from plasma/blood cells (ECF) into the spaces between cells.
Bowel obstruction –> decreased fluid reabsorption –> 3rd space loss.
Sudden diuresis on day 2-3 post op = recovery of ileus
Peritonitis –> ascites –> 3rd space loss.
Fluid balance - total water input and food input?
1500ml + 1000ml
Total water output in urine and stool?
1500ml + 300ml
Total input from metabolism?
300ml
Total output from insensible losses?
1000ml
Total input
2000ml = 25-30ml/kg/d
Total output
2800ml= 40ml/kg/d
Minimum urine output?
0.5ml/kg/h = ~30ml/h
Glucose requirment
50-100g/day of glucose
K requirement + Na Requiremnt
1mmol/kg/d
= 80 mmol
Need to replace other losses too?
VOmiting and Diarrhoea NGT Drains Fever (+500ml for each degree) Tachypnoea High-output stomas.
CVP monitoring?
Indicate RV preload and depends on
- Venous return
- Cardiac output
Increased CVP
- Increased circulating volume
- Decreased CO: pump failure
Decreased CVP
- Decreased circulating volume
Normal value: -5-10cmH20
SIngle reading is not as useful as serial measurements before and after fluid challenge
- Unchanged: Hypovolaemic
- Increased that reverses after 30mins : euvolaemia
- Sustains increased >5cmH20: overload/failure
- Passive leg raising may be more useful than fluid challenge in determining response to fluid
- Sustained increased in CVP suggest heart failure.
24 hr NICE recommendation for fluid?
When prescribing for routine maintenance alone, consider using 25-30ml/kg/day sodium chloride 0.18% in 4% glucose with 27 mmol/l potassium on day 1.
What does NS Contain?
0.9% NaCl = 9g/L
154 mM NaCl.
Use for normal daily fluid requirements + replace losses.
If large volumes are used there is an increased risk of hyperchloraemic metabolic acidosis.
5% dextrose?
50g dextrose/L
Normal daily fluid requirement
Dextrose saline?
4% dextrose = 40g/L
0.18% NaCL = 31 mM NaCL
Normal daily fluid requirements
What does Hartmann’s contain?
Na: 131 mM Cl: 111mM K: 5mM Ca: 2.2 mM Lactate/HCO3: 29mM
Use of Hartmann’s
Resuscitations in trauma patients
pH: 6.5 but Hartmann’s is an alkalising solution
Lactate is not an acid itself: conjugate base.
Daily requirements?
3L dex-saline with 20mM K+ in each bag
1L NS + 2L dex with 20mM K+ in each bag.
Types of colloid?
Contains large molecular weight molecules - gelatin/dextrans. Preserved oncotic pressure therefore remains intravascular –> to increased intravascular volume.
Natural = Albumin, blood.
Synthetic = Gelofusin, voluven.
USe: fluid challenge: 250-500ml over 15-30mins.
Hypovolaemic shock.
Problems
- Anaphylaxis
- Volume overload.
How to assess fluid status?
Hx: balance chart, surgery, other loss, thirsty
Impression: drowsy, alert
INspection: drips, drains, stomas, catheters, CVP.
Examination of IV volume?
CRT
HR
BP lying and standing
JVP
Examination of Tissue perfusion
Skin turgor
Oedema: ankle, pulmonary ascitis
Mucus membranes
Examination of end-organ perfusion?
UO, Increased U+Cr
Consciousness
Lactate
Other assessments of perfusions?
PCWP: indirect measure of left atrial pressure
- pulmonary wedge pressure (measure the pulmonary artery)
- CVP
Post-op Fluids?
Problems
- Increased ADH, increased aldosterone, increased cortisol –> Na + H20 conservation.
Increased K+: tissue damage, transfusion, stress hormone.
Solutions to post-op fluid issues?
Use UO to guide fluid replacement but may need to decreased maintenance fluid to 2L first 24hr post op
Avoid K+ supplementation for first 24hr post-op.
Cardiac or renal failure problem + solution?
RAS activation –> NA and H20 retention
Therefore solution = avoid fluid with Na –> give 5% dextrose.
Bowel obstruction and fluid?
Pts have significant third space losses with loss of both water and electrolytes
Likely to need significantly more than standard daily requirements.
Regiments
- 0.9% NS with 20-40mml KCL to each bag
- Titrate rate of fluid to clinical findings on serial examination
- Serial U+E to guide electrolyte replacement.
Pancreatitis fluid management?
Inflammation –> significant fluid shift tinto the abdomen
Pts require aggressive fluid resus and maintenance
- Insert urinary catheter and consider CVP monitoring
- 0.9% NS with 20-40mmol KCL added to each bag.
- Keep UO >30ml/h
- serial U+E guide electrolyte replacement.
Ileostomy fluid management?
Normal output: 10-15ml/Kg/d = 700ml/day
High output = >1000ml/d
Ileum will adapt to limit fluid + electrolyte losses.
- Fluid - 0.9% NS + KCL
- Daily requirements + replaces losses, titrated to UO.
- Serial U+E guide electrolyte replacement
High output
- Loperamide
- Codeine
Reduced Urine Output Post-op
Causes
- Pre-renal: Hypovolaemia
- Renal: NSAIDs, Gentamicin
- Post-renal: Commonest cause - blocked catheter, acute urinary retention.
Anuria usually = blocked or malsited catheter
Oliguria usually = inadequate fluid replacement
Management of reduced urine output post-op?
Information
- Op Hx
- Obs Chart UO
- Drug chart: nephrotoxin
Examination
- examine fluid status
- Examinate for palpable bladder
- Inspect drips, drains, stomas, CVP
Action
- Flush with 50ml NS and aspirate back
- fluid challenge
Suspecte catheter problem
- Flush with 50ml NS and aspirate back
Suspect pre-renal problem
- Fluid challenge
250-500ml colloid bolus over 15-30 mins.
Look for CVP or UO response within minutes.
Clinical assessment of nutrition?
Hx
- weight loss
- Diet
Examination
- Skin fat
- Dry hair
- Pressure sore
- Cheilitis
- Weight + BMI (<20kg/m2)
Anthropometric assessment of nutrition?
Skin-fold thickness
Arm circumference
Investigations of nutrition?
Albumin
Transthyretin
Phosphate
What is enteral nutrition?
Requirements for body /kg/24hr.
20-40 kcal Carbs: 2g Fat: 3g Protein: 0.5-1 Nitrogen: 0.2-0.4
Enteral nutrition
- Delivery
PO is best . Consider semi-solid if risk of aspiration
- Fine bore NGT
- Percutaneous endoscopic gastrostomy
- Jejunostomy
- Build up feeds gradually to prevent diarrhoea.
What feeds are possible through enteral feeding?
Oral supplements
- Polymeric: E.g osmolite, jevity: Intact proteins, starches and long-chain FAs.
- Disease Specific. e.g decreased branched chains AA in hepatic encephalopathy
- Elemental
Simple AA and oligo/monosaccharides
Require minimal digestion and used if abnormal GIT: E.g in Crohns.
Indications for enteral feeding?
Catabolic: sepsis, burns, major surgery
Coma/ITU
Malnutrition
Dysphagia: Stricture, stroke.
Complications of enteral feeding?
NGT
- Nasal trauma
- Malposition or tube blockage
Feeding
- Feed intolerance –> Diarrhoea
- Electrolyte imbalance
- Aspiration
- Refeeding syndrome
What is parental nutrition?
May be total or used to supplement enteral feeding
- Combined with H20 to deliver total daily requirements.
Indications for parenteral nutrition?
- Prolonged obstruction or ileus (<7days)
- High output fistula
- Short bowel syndrome
- Severe Crohn’s
- Severe malnutrition
- Severe pancreatitis
- Unable to swallow: e.g oesophageal Ca.
Delivery of parenteral nutrition?
- Delivered centrally as high osmolality is toxic to veins
Short-term: CV cather
LOng-term: Hickman or PICC line
Sterility is essential: use line only for PN.
Monitoring of parenteral nutrition?
Standard
- Weight, fluid balance and urine glucose daily.
- Zn, Mg weekly.
Initially
- Blood glucose, FBC, U+E, PO4 3x a week.
- LFTs 3x /week
Once stable
- Blood glucose, FBC, U+E + PO4 daily
- LFTs weekly.
Content of parenteral nutrition
2000Kcal: 50% fat, 50% carbs
10-14g nitrogen
Vitamins, minerals and trace elements.
Complications of Parenteral nutrtion?
Line-related
- Pneumothorax/haemothorax
- Cardiac arrhythmia
- Line sepsis
- Central venous thrombosis –> PE or SVCO
Feed-related - Villous atrophy of GIT - Electrolyte disturbances Refeeding syndrome Hypercapnoea from excessive CO2 production - Hyperglycaemia and reactive hypoglycaemia - Line sepsis: increased risk with TPN - Vitamin and mineral deficiency
What is refeeding syndrome?
- Life-threatening metabolic complications of refeeding via any route after a prolonged period of starvation.
Pathophysiology of refeeding?
Decreased carbs - catabolic state with low insulin, fat and protein catabolism and depletion of intracellular phosphate.
- Refeeding –> increased insulin response to carbs and increased cellular phosphate uptake
- Hypophosphatemia Rhabdomyolysis Respiratory insufficiency Arrhythmias Shock Seizures
What is the chemistry of refeeding syndrome?
Decreased K, Decreased Mg, Decreased Po4
Patients at risk of refeeding syndrome?
Malignancy Anorexia nervosa Alcoholism GI surgery Starvation
Prevention of refeeding syndrome?
Identify and monitor at-risk patients
Liaise with dietician
Management of refeeding syndrome?
- Identify at-risk pts in advance and liaise with dietician
- Parenteral and oral PO4 supplementation
- Manage complications
