Fluid and Electrolyte Prescribing Flashcards
Describe the body’s fluid compartments and composition.
- ~60% of the adult body is fluid.
- Fluid compartments are tightly controlled (volume and composition).
- Major divisions:
- Intracellular (2/3)
- Extracellular (1/3)
- Extracellular can be further divided: plasma (1/4) and interstitial (3/4) (this includes synovial, intra-ocular, CSF etc.).
- Composition is determined by movement across the plasma membrane - ions must move through channels.
Give a summary of body fluidregulation, compartments and their separating membranes.
Blood volume is a mixture of extracellular and intracellular fluid.
Describe the exchange of fluid across the capillary membrane.
- The main protein inside the vessel is albumin.
- If there is liver failure and albumin production is reduced, the patient will get oedema.
What are the extracellular and intracellular concentrations of potasium, sodium and chloride?
- U & E is essentially measuring extracellular fluid.
- Chloride mirrors sodium.
- Bicarbonate is important in acid base balance.
- Normal K+ = 3.5-5.5mM
- K+ has to be kept in normal boundaries and if it dips it is a reflection of all the intracellular potassium that cannot be measured.
Which barrier does fluid have to move through to move:
- Between plasma and interstitial fluid?
- Between extraellular fluid and intracellular fluid?
- Between plasma and interstitial fluid
- Capillary wall
- Between extraellular fluid and intracellular fluid
- Plasma membrane
Describe typical gain and loss of body fluid.
- Gains:
- Food and water intake
- Oxidation of food
- Losses:
- Urine (variable; average 1500mL)
- Faeces (variable; average 100mL)
- Sweat (variable; average 50mL)
- Insensible losses (variable; average 900mL)
- Total losses (on average) = 2550mL
What is insensible water loss?
- Transepidermal diffusion: water that passes through the skin and is lost by evaporation.
- Evaporative loss from the respiratory tract.
- If the patient is pyrexial or they have an increased respiratory rate then factor this in when prescribing fluids - they are losing more.
- Note - Insensible losses are solute free.
What are the risks associated with prescribing IV fluids?
- Risks with IV fluids:
- Peripheral vascular cathater (PVC) required.
- Easy to give too much fluid (especially in sick people).
- Errors in prescribing.
- Note: it is easy to give fluids but it is much harder to get rid of fluid, especially in patients with renal impairment.
What information should be elicited in a history when assessing a patient’s volume status?
- Limited intake?
- Abnormal losses?
- How much?
- What kind of fluid?
- Ongoing? Can I treat the cause?
- Comorbidities?
- Current illness?
- Symptomatic?
- Fluid balance charts?
What factor is not accounted for on a fluid balance chart?
Insensible losses
Describe the examination of a patient to assess fluids?
What state would these signs be in during hypovolaemia?
- Trends, context and response to fluid challenge.
- Vital signs (in hypovolaemia):
- Systolic BP (<100mmHg)
- HR (>90bpm)
- Capillary refill (>2 secs)
- RR (20 breaths /min)
- Urine output / colour (<0.5ml / kg / min)
- (Hypovolaemic patient will also have dry mucous membranes, decreased skin turgor and responsiveness to passive leg raising to 45°).
- Postural hypotension is a sensitive marker of hypovolaemia.
What are the signs associated with fluid overload?
- History of cardiac or renal problems
- Raised JVP
- Peripheral oedema
- Inspiratory crackles at lung bases
- Hypertension
Assessment of volume status is a clinical decision, but investigations can be helpful.
Which investigations would be appropriate?
- Full blood count
- Urea & electrolytes
- CXR
- Lactate
- Urine biochemistry
What do Kerley B lines on a CXR indicate?
Cardiac failure because they show interstitial pulmonary oedema.
What are the minimum electrolyte requirements?
- Sodium: 1mmol / kg / 24 hours
- Potassium: 1mmol / kg / 24 hours
- Calories: minumum (to avoid catabolism) 400kcal / 24 hours.
- Note - keep an eye on magnesium, calcium and phosphate and replace as required. If you are keeping your patient fasting for 48 hours you need to start replacing calcium, magnesium and phosphate.
What are the different IV fluids which can be prescribed (crystalloids)?
-
5% dextrose (glucose)
- Initially distributes through interstitial fluid and plasma; glucose is metabolised, so effectively adding just water.
- Further distributes into cells as well as ISF and plasma.
- Alternatively 0.18NaCl 4 % dextrose
- 0.9% NaCl (isotonic saline)
-
Plasmalyte
- Distributes through ISF and plasma; does not enter cells.
What is fluid challenge?
Under what circumstances would you consider this?
- Oliguria or hypotension and no signs of overload - consider a fluid challenge.
- Therapeutic and diagnostic.
- 500ml balanced salt solution given quickly (<15 minutes).
- Needs to be quick because fluid will not stay in the intravascular space for a long time.
- Re-assess.
- Can repeat up to 2000ml.
