Fluids and nutrition Flashcards
What % of the body is water?
60%
Where is water stored in the body?
2/3 intracellular = 28L
1/3 extracellular = 14L: 3L plasma, 10L interstitial, 1L transcellular
Define osmotic pressure
The pressure which needs to be applied to prevent the inflow of water across a semipermeable membrane i.e. ability of a solute to attract water
Define oncotic pressure
Form of osmotic pressure exerted by proteins
Define hydrostatic pressure
Pressure exerted by a fluid at equilibrium due to the force of gravity
What are hydrostatic and osmotic pressures collectively known as (whose forces)?
Starling’s forces
What determines the distribution of fluid in the body?
- ECF/ICF balance determined by osmotic pressure
- Distribution within ECF is deterined by Starling’s forces:
- Capillary and insterstitial oncotic pressure
- Capillary and interstitial hydrostatic pressure
- Filtration coefficient (capillary permeability)
What are some examples of 3rd space losses?
- Bowel obstruction -> decreased fluid reabsorption
- Sudden diuresis on day 2-3 post op = recovery of ileus
- Peritonitis -> ascites
What is a typical minimum urine output?
0.5ml/kg/h = about 30ml/h
What is the usual sodium requirement?
1.5-2mmol/kg/day - about 100mmol/day
What is the normal potassium requirement?
1mmol/kg/day = 60mM/day
What would be a typical fluid regimen for adults with no specific comorbidity?
- 1L normal saline + 40mmol KCl over 8h
- 1L normal saline + 40mmol KCl over 8h
- 1L dextrose (5%) over 8 hours
- =3L fluid, 200mmol Na, 80mmol KCl/24h
What is the purpose of CVP monitoring?
Indicates RV preload; it depends on venous return and cardiac output such that a raised CVP indicates either increased circulating volume or decreased cardiac output (pump failure) whereas a low CVP indicates a low circulating volume.
What is a normal CVP?
~5-10cmH2O but remember a single reading isn’t as useful as serial
How do you interpret the CVP change after a fluid challenge?
- Unchanged: hypovolaemic
- Increase that reverses after 30 minutes: euvolaemic
- Sustained increase >5cmH2O: overload/failure
What might be more useful than a fluid challenge in determining the CVP response to fluids?
Passive leg raising - a sustained increase in CVP indicates heart failure
What are some examples of crystalloid fluids?
- Normal saline
- 5% dextrose
- Dextrose-saline
- Hartmann’s/Ringer’s lactate
What are the contents of normal saline and what is its pH?
- 0.9% NaCL - 9g/L or 154mM NaCl
- pH 5-6
What can you use to give normal daily fluid requirements?
Normal saline, 5% dextrose or dextrose-saline
How many g of dextrose are in a bag of 5% dextrose?
50g
What are the contents of dextrose-saline?
- 4% dextrose = 40g/L
- 0.18% NaCl = 31mM NaCl
What are the contents of Hartmann’s/Ringer’s lactate?
- Na: 131mM
- Cl: 11mM
- K: 5mM
- Ca: 2.2mM
- Lactate/HCO3: 29mM
What do you use Hartmann’s/Ringer’s lactate for?
- Resuscitation in trauma patients
- Burns: use Parklands formula - 4 x weight x % burn = mL in 1st 24 hours
What is the pH of Hartmann’s?
6.5, but it is an alkalinising solution - lactate isn’t an acid itself but a conjugate base. It’s given exogenously as sodium lactate, and lactate is metabolised in the liver producing HCO3 (Cori cycle)
What is the physiological reason to use colloids?
- Larger molecular weight molecules e.g. gelatin, dextrans
- These preserve oncotic pressure
- Therefore remain in the intravascular space, meaning they cause a preferential increase in intravascular volume
What are some examples of colloids?
- Synthetic
- Gelofusin
- Volplex
- Haemaccel
- Voluven
- Natural
- Albumin
- Blood
What do you use colloids for?
- Fluid challenge:
- 250-500ml over 15-30 mins
- Hypovolaemic shock
- Mount Vernon formula for burns:
- (weight x % burn)/2 = ml colloid per unit time
What are some of the problems with colloids?
- Anaphylaxis
- Volume overload
- Can interfere with cross matching (so take a cross match befroe you give it)
What are some of the problems with post op fluids and how can you solve them?
- Issues:
- High ADH, aldosterone and cortisol cause Na and water conservation
- Hyperkalaemia: tissue damage, transfusion, stress hormones
- Solutions
- Use urine output (aiming for >30ml/hour) to guide fluid replacement but may need to decrease maintenance fluids to 2L first 24 hours post op
- Avoid K+ supplementation for first 24 hours post op
What are the issues with using fluids in cardiac or renal failure and how do you deal with them?
- RAS activation causes sodium and water retention
- Avoid fluids with sodium - give 5% dextrose
How do you manage fluids in patients with bowel obstruction?
- Significant third space losses of fluid and electrolytes, so probably need more than usual
- Regimen: 0.9% normal saline with 20-40 mM KCl added to each bag
- Titrate fluids to clinical findings on serial examination
- Serial U+Es to guide electrolyte replacement
What are the issues with fluids in pancreatitis and how do you manage them?
- Inflammation causes a significant fluid shift into the abdomen
- Patients need aggressive fluid resus and maintenance
- Insert urinary catheter and consider CVP monitoring
- 0.9% normal saline with 20-40mM KCl added to each bag
- Keep urine output >30ml/hour
- Serial U+Es for electrolyte replacement
What is the composition of ileal fluid?
- Na: 130mM
- Cl: 110mM
- K: 10mM
- HCO3: 30mM
What counts as normal or high output for an ileostomy?
Normal: 10-15mL/kg/day = ~700ml/day
High output = >1000ml/day
How should you manage fluid status in ileostomy patients?
- The ileum will adapt to limit fluid and electrolyte losses
- Fluids:
- 0.9% normal saline and KCl
- Daily requirements and replace losses, titrated to urine output and U+Es
- If high output give loperamide and codeine
What are the causes of post op reduced urine output?
- Post renal
- Commonest cause
- Blocked/malsited catheter
- Acute urinary retention
- Pre renal: hypovolaemia
- Renal: NSAIDs, gentamicin
- Anuria usually = blocked or malsited catheter
- Oliguria usually = inadequate fluid replacement
How do you manage reduced urine output post op?
- Information: op history, obs chart (UO), drug chart (nephrotoxins)
- Examinatino: assess fluid status, examine for palpable bladder, inspect drips, drains, stomas, CVP
- Action:
- Suspect catheter problem: flush with 50ml NS and aspirate back
- Suspect pre renal problem: fluid challenge with 250-500ml colloid bolus over 15-30 mins and look for CVP or UO response within minutes
How would you assess fluid status clinically?
- History: balance chart, surgery, other losses, thirsty
- Impression: drowsy, alert
- Inspection: drips, drains, stomas, catheters, CVP
- Examination
- IV volume
- CRT, HR, BP lying and standing, JVP
- Tissue perfusion
- Skin turgor, oedema (ankle, pulmonary, ascites), mucus membranes
- End organ
- UO, urea and creatinine, consciousness, lactate
- IV volume
- Other tests
- Pulmonary capillary wedge pressure: indirect measure of left atrial pressure
- CVP
How would you assess nutritional status of a patient?
- Clinical
- History: weight loss, diet
- Examination: skin fat, dry hair, pressure sores, cheilitis, weight and BMI (<20kg/m2)
- Anthropometric: skin fold thickness, arm circumference
- Investigations: albumin, transthyretin (prealbumin), phosphate
What are the daily nutritional requirements for an adult per kg?
- 20-40 calories
- 2g carb
- 3g fat
- 0.5-1g protein
- 0.2-0.4g nitrogen
What are the options for routes of delivery for enteral nutrition?
- PO is best - consider a semi solid diet if risk of aspiration
- Fine bore NGT (9Fr)
- Percutaneous endoscopic gastrostomy
- Jejunostomy
- Build up feeds gradually to prevent diarrhoea
What different kinds of enteral feeds are there?
- Oral supplements
- Polymeric e.g. osmolite, jevity
- Intact proteins, starches and long chain FAs
- Disease specific
- E.g. low branched chain AAs in hepatic encephalopathy
- Elemental
- Simple AAs and oligo/monosaccharides
- Require minimal digestion and used if abnormal GIT e.g. in Crohn’s
What are the indications for enteral feeding?
- Catabolic: sepsis, burns, major surgery
- Coma/ITU
- Malnutrition
- Dysphagia: stroke, stricture
What are some complications of enteral feeding?
- NGT
- Nasal trauma
- Malposition or tube blockage
- Feeding
- Feed intolerance causing diarrhoea
- Electrolyte imbalance
- Aspiration
- Refeeding syndrome
What are the indications for parenteral feeding?
- Prolonged obstruction or ileus (>7d)
- High output fistula
- Short bowel syndrome
- Severe Crohn’s
- Severe malnutrition
- Severe pancreatitis
- Unable to swallow e.g. oesophageal Ca
How is parenteral nutrition given?
- Centrally, because the high osmolality is toxic to veins
- Short term: CV catheter
- Long term: Hickmann or PICC line
- Sterility is essential, so only use the line for PN
How should parenteral nutrition be monitored?
- Standard
- Weight, fluid balance, urine glucose daily
- Zinc, magnesium weekly
- Initially
- Blood glucose, FBC, U+E, LFTs, phosphate - 3 times a week
- Once stable
- Blood glucose, FBC, U+E, phosphate daily
- LFTs weekly
What are the contents of a typical parenteral feed?
- 2000 Kcal: 50% fat, 50% carb
- 10-14g nitrogen
- Vitamins, minerals, trace elements
- Combined with H2O
What are the complications of parenteral feeding?
- 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 deficiencies
What is the definition of refeeding syndrome?
Life threatening metabolic complication of refeeding via any route after a prolonged period of starvation
What is the pathophysiology of refeeding syndrome?
- Low carbs induces a catabolic state with low insulin, fat and protein catabolism and depletion of intracellular phosphate
- Refeeding causes increased insulin in response to carbs and a high cellular phosphate uptake
- This leads to hypophsophataemia and:
- Rhabdomyolysis
- Respiratory insufficiency
- Arrhythmias
- Shock
- Seizures
What chemical abnormalities are seen in refeeding syndrome?
Low potassium, low magnesium, low phosphate
Which patients are at risk of refeeding syndrome?
- Malignancy
- Anorexia nervosa
- Alcoholism
- GI surgery
- Starvation
How do you prevent and treat refeeding syndrome?
- Prevent:
- Identify and monitor at risk patients
- Liaise with dietician
- Treat
- Parenteral and oral phosphate supplementation
- Treat complications
