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How to prescribe IV fluids

How to prescribe IV fluids for medical student exams, finals and OSCEs


Choosing a fluid regimen

  • Overall fluid requirements = replacement of fluid loss + maintenance fluids

  • Prescribing fluids is not an exact science – most of the time it is the best guess for the patient’s weight/approximate quantity of lost fluid
  • However, you should think about replacement and maintenance separately because the types and volumes of the fluids required will be different for each component
  • You should be clear what type of fluid, and how much of each type, you are giving for each component of the formula and why
    •  The type and rate of replacement fluids are based on the type and quantity of fluid lost
    • The type and rate of maintenance fluids (if needed) are based on the patient’s fluid and electrolyte requirements (dependent on their weight)


Replacement of fluid loss

  • Includes two components:
    • 1. Pre-existing fluid deficit (replace STAT)
    • 2. Ongoing losses (replace future losses as they occur)
  • The type of fluid replacement depends on the type of fluid lost i.e. fluid loss should be replaced by fluids with a similar electrolyte content to the lost fluid. Types of fluid lost and what they should be replaced with include:
    • Extracellular fluid loss (e.g. D&V, NG aspirates, stomas, burns, pancreatitis)
      • Replace with a fluid similar to plasma (e.g. Hartmann’s solution, or saline if Hartmann’s solution not available)
    • Classical dehydration (e.g. pyrexia, poor intake)
      • Should be replaced by normal maintenance fluids (e.g. dextrose-saline)
    • Blood loss
      • Should be replaced with blood (packed red cells: PRBC)
      • If the patient continues to bleed, they may also need other blood products (e.g. FFP, platelets, cryoprecipitate) to actively stop the bleeding rather than simply replacing the lost red cells


Pre-existing fluid/electrolyte deficit

  • Decide on the approximate deficit and the cause by:
    • History
      • Likely quantity vomited/bled/excreted? Over how long?
      • Are they thirsty (an often missed indicator of hypovolaemia)
    • Observations
      • Shock (tachycardic +/- low BP)?
      • Drop in postural BP?
    • Examination 
    • Fluid balance chart
      • Input over last 24 hours? Urine output?
    • Urea and electrolytes
      • Raised creatinine and urea imply dehydration, especially if acute. Urea usually rises before creatinine
      • However, note these can be late signs in dehydration and conversely will be chronically high in patients with chronic renal failure (even when well-hydrated). U&E should always be taken in clinical context
  • The quantity of fluid replacement depends on your estimate of the fluid deficit. To replace deficits, especially if the patient is unwell, use fluid challenges


What is a fluid challenge?

  • Fluid resus to replace pre-existing deficits is done in stat boluses. This is called giving a fluid challenge
  • To do this, give a bolus (e.g. 500ml 0.9% saline stat) and then reassess hydration status changes
    • A suitable response is: pulse decreased; blood pressure risen or stabilised; urine output risen
    • For reliable fluid status your patient will need to be catheterised to guide further fluid resuscitation


Maintenance fluids

  • Once your patient is euvolaemic (i.e. you feel you have replaced their previous losses) consider maintenance fluids
  • Only give maintenance fluids if the patient can’t drink enough
    • Oral/NG-tube fluids are safer than IV because they are much less likely to cause salt or fluid overload, electrolyte abnormalities and do not require a cannula (infection risk)
  • Fluid type should approximately matches the normal requirements for their weight in terms of water, sodium and potassium
  • The traditional regime = “1 salty + 2 sweet”:
    • Saline 0.9% + 20mmol potassium chloride (over 8 hours)
    • Dextrose 5% + 20mmol potassium chloride (over 8 hours)
    • Dextrose 5% + 20mmol potassium chloride (over 8 hours)
  • This gives 3L H20, 154mmol Na and 60mmol K. Note that this provides about 0.5L more water and more sodium than is required for an average patient; 2.5: per day is usually adequate
  • The fluids you give should always reflect the patient’s weight
    • e.g. for a 70kg patient there are many possible maintenance regimes (to match approx: 2.5L fluid, 70-140mmol Na and 35-70mmol K)
  • Other example regimens
    • 1L dextrose-saline with 20mmol K (over 10 hours) + 1L dextrose-saline with 20mmol K (over 10 hours) + 500ml dextrose-saline (over 4 hours)
    • 500ml saline 0.9% (over 6 hours) with 20mmol K + 1L dextrose 5% with 20mmol K (over 10 hours) + 1L dextrose 5% with 20mmol K (over 8 hours)
    • 1L Hartmanns (over 10 hours) + 1L 5% dextrose (over 8 hours) with 40mmol K + 500ml 5% dextrose (over 6 hours) with 20mmol K
  • Note: if a patient is drinking some fluids, but not enough, determine their fluid intake and top it up with IV/NG fluids
  • Regardless, ensure you always check the U&Es before prescribing any fluids and adjust the regime as necessary for any electrolyte abnormalities


Special Situations

  • Post-operatively
    • Potassium is intracellular and levels can increase due to cell lysis during surgery. As such, if K+ is over 4.5mmol/L do not give any supplementation. If K+ is normal/low, give a small volume e.g. 40mmol/24h.
    • Because of the endocrinological sodium retention mechanisms triggered by surgery and sodium containing substances commonly used in theatre (colloids, Hartmann’s solution, IV antibiotics) it is prudent to avoid too much saline post-operatively, especially as too much sodium chloride can result in hyperchloraemic acidosis and increased renal load
    • Dextrose or dextrose-saline is often a better first-line choice, especially if the patient has normal sodium pre-op
    • Only give fluids if your patient actually needs them!
  • Sepsis
    • Sepsis causes intravascular depletion due to plasma loss because of leaky capillaries and vasodilation.
    • Replace fluid with crystalloid e.g. Hartmann’s, but avoid too much sodium and chloride e.g. lots of bags of saline (for reasons above)
  • Heart failure
    • Heart failure patients are prone to fluid overload and pulmonary oedema. Watch their fluid balance carefully!
    • Requirements depend on severity but usually no more than 2L/24 hours
    • If overload develops: give furosemide if necessary, fluid restrict (e.g. 1.5L per day), advise a low sodium diet and ask for daily weights
      • Weighing your patient every day is the best way of determining how much fluid they are using (1kg less = 1L lost)
      • There is minimal logic in giving furosemide along with IV fluids!
    • Note if a patient has a low SBP and a low urine output who is at risk of LVF, you must examine the patient because there are two opposite causes for this clinical picture:
      • Dehydration: they may simply be fluid deplete (treatment = fluids)
      • LVF and overload: they may be in LVF which causes a low SBP and hence a low urine output (treatment = furosemide which will improve LVF and hence paradoxically increase SBP and urine output)
  • Liver failure
    • Excess Na+ may cause ascites so ONLY use 5% dextrose (unless hyponatraemic in which case ask for specialist input)
  • Acute renal failure
    • Avoid potassium supplementation
  • Chronic renal failure
    • Avoid excess fluids, sodium and potassium (the kidneys can’t excrete these effectively)
  • History of alcohol excess / poor nutrition
    • Give Pabrinex before giving any 5% dextrose (even if hypoglycaemic) or the glucose load can precipitate Korsakoff’s syndrome
  • Brain haemorrhage
    • Avoid dextrose (can worsen oedema due to osmotic shifts)
    • Saline would be a good first choice
  • Risk of re-feeding syndrome
    • Avoid dextrose where possible because it can precipitate re-feeding syndrome


Click here for how to examine fluid status

Perfect revision for medical student finals, OSCES and PACES

Click here for notes on shock in emergency medicine