IV Fluids

Question 1

What are two concepts that are essential to understanding IV fluids?

Answer 1

1. What’s in the fluid?
   • Composition
2. Where will the volume go?
   • Distribution
   • Which body water compartments will be altered in volume by the infusion of the fluid?

Question 2

What is meant by what is in the fluid (aka composition)?

Answer 2

• What substances introduced into patient’s body and what effects it might have
• Crystalloids vs colloids

Question 3

Crystalloids are solutions of small molecules in water. What are some commonly used examples of crystalloid solutions?

Answer 3

• Crystalloids are superior in initial fluid resusitation
• “Normal saline” = 0.9% Sodium chloride
• Dextrose = Glucose
• Hartmann’s = Ringer’s Lactate / Sodium Lactate

Question 4

What is Hartmann’s solution?

Answer 4

• AKA Ringer’s Lactate / Sodium Lactate
• Hartmann’s is a physiological (balanced) solution
• Aim of this is to more closely approximate electrolyte composition of serum
• Lactate conc is different, but it’s there to provide a source of (roughly physiological) source of bicarbonate

Question 5

How is Hartmann’s solution different to the other crystalloids?

Answer 5

• In Hartmann’s, potassium fixed at 5 mmol/L
• Whereas other solutions generally available in 3 forms:
  
  • (a) no potassium
  • (b) 20 mmol/L (0.15%)
  • (c) 40 mmol/L (0.3%)
• These concentrations substantially higher than serum levels, can be used therapeutically if you need to provide potassium for maintenance or to replete a deficit
• In contrast, the 5mmol/L in Hartmann’s is not therapeutically useful, it makes it indifferent in respect to potassium

Question 6

How are colloids different to crystalloids?

Answer 6

• Colloids are based upon a crystalloid solution
• Defined by addition of large osmotically active molecules
  
  • eg. albumin or gelatin

In order to understand why the large osmotically active molecule is added to this solution, we need to understand distribution

Question 7

What are the body water compartments?

Answer 7

• 60% of adult body weight is water
  
  • 2/3rd of 60% → intracellular (ie. 40% body weight)
  • 1/3rd of 60% → extracellular (ie. 20% body weight)
• Extracellular further divided into:
  
  • interstitial water (80%) → in between cells
  • intravascular water (20%) → within blood vessels

Question 8

What pump resides between the intracellular and extracellular compartments?

Answer 8

• Na⁺ - K⁺ ATPase pump
• Sits between intracellular and interstitial compartment
• Constrains potassium to intracellular water
• Constrains sodium to extracellular water
• Water can cross cell membrane freely → maintains osmolality either side of the membrane

If we give a sodium load, then we increase the amount of solute in the extracellular compartment, thus water would move into EC compartment.

Also, large osmotically active molecules are constrained by the vascular endothelium (eg. albumin) into the intravascular space (ECF) → this maintains the vascular volume

Question 9

What impact does 5% glucose have on the water compartments when administered?

Answer 9

• intravenous only
• basically pure water
• small amount of glucose to make it isotonic at the point of infusion → ie. to ensure cells don’t burst
• glucose quickly mixed and taken up by cells
• pure water remains → diffuses freely across vascular endothelium and also the cell membrane
• all 3 compartments increase in volume
• equal distribution across 3 compartments (2/3rds intracellular, 1/3rd remaining extracellular)
• small amount remains in intravascular compartment

Question 10

What impact does sodium chloride 0.9% have on the water compartments?

Answer 10

• sodium excluded from intracellular compartment
• both the sodium and associated water will be constrained to the extracellular compartment
• water can’t move into cells because if it did so, then it would concentrate the solutes in the extracellular water more and dilute the solutes in the intracellular water and that creates an osmolality imbalance that can’t happen
• we can exploit this property therapeutically
• being constrained to ECF → large proportion of volume infused will remain in intravascular space, useful if we need to increase circulating blood volume
• BUT we are losing 80% (remember ECF 80/20 divide) to the interstitium, but 20% remains in intravascular space → could have a useful effect
• in an ideal world would like all of the fluid to infuse and remain in the intravascular space – this is the intention of COLLOID solutions!

Question 11

How might a colloid solution (eg. albumin 4.5% 500 mL) impact on the water compartments?

Answer 11

• contain large osmotically active molecules
• because it cannot cross vascular endothelium → set up an osmotic effect → ensures water associated with it is constrained to intravascular compartment
• not a durable effect
• unfortunately, vascular endothelium (often sick pts) is not as impenetrable as we like, can be quite leaky
• inevitably, some of the colloid molecule will leak out into interstitium and the water goes with it
• effect after a few hours will be the same as if you infused a sodium crystalloid, except that you have to bear the additional disadvantages of using a colloid

Question 12

What are disadvantages of colloids?

Answer 12

• pharmacologically more active
• gelatin → hypersensitivity rxns
• starch → serious adverse effects
• albumin → expensive and relatively scarce

Theory tells us benefits unlikely to outweight risks, therefore recommendation in major guidelines (NICE) → we should rarely use colloid solutions and almost always favour sodium based crystalloids (for intravascular volume expansion)

Question 13

The two concepts discussed that are essential to understanding IV fluids link to 2 uses of IV fluids.

What are these 2 uses?

Answer 13

• Water and electrolyte provision
• Fluid resuscitation

Question 14

When deciding which fluid to give a patient, what are the 3 considerations?

Answer 14

1. Daily maintenance requirements
2. Additional ongoing losses
3. Existing deficits

Question 15

What is meant by daily maintenance requirements?

Answer 15

• Everyday we lose a bit of water + some electrolytes through normal metabolic processes
• Replaced by eating and drinking
• Pt can’t eat/drink? Can’t be replaced by GI tract? → then must replace these requirements intravenously
• Daily requirements:
  
  • Water → 25-30 mL/kg
  • Sodium → 1 mmol/kg
  • Potassium → 1 mmol/kg
  • Glucose → 50-100g

Question 16

What is meant by additional ongoing losses?

Answer 16

• Some pts have some other losses that occur over and above their daily maintenance requirement
• Can be categoried as:
  
  • Loss of elecrolyte rich fluid → GI tract (vom/diarrhoea), mucosal surface (surg drain)
    
    • ​replace w electrolyte rich solution eg. Hartmann’s, 0.9% NaCl (good for any patient)
    • if losing fluid from stomach → losing chloride → use 0.9% NaCl
    • if losing fluid from lower in GI tract (post-pyloric) → less chloride → Hartmann’s better
    • further down GIT → more potassium lost → give 0.9% NaCl to replace potassium (as can increase potassium infusions in these, but Hartmann’s is always 5 mmol/L)
  • Loss of pure water → fever, tachypnoea, diabetes insipidus
    
    • hypernatraemia can occur due to water loss
    • these losses are not large alone, but if left for days then can lead to significant deficit
    • replace with IV fluid solution with little/no electrolytes, eg. 5% glucose (almost pure water!)
    • could also use hypotonic NaCl solution (0.18%) w/ glucose, small amount of sodium to reflect some sodium loss but principal loss is water

Question 17

What is meant by existing deficits (the third consideration)?

Answer 17

• If pt has additional ongoing loss that’s not been replaced or a daily maintenance requirement not being met then pt will progressively accumulate a deficit
• in the same way that abnormal ongoing losses can be categorised as water or electrolyte-rich, can apply the same thinking with deficits:
  
  • water deficits
  • electrolyte deficits
• serum sodium concentration is not a useful marker in evaluating deficits
  
  • hypernatraemia can reflect a water deficit
  • and normal serum sodium doesn’t in anyway reliably exclude presence of sodium deficit or water deficit (eg. losing water and sodium in proportion will keep Na conc the same) - a common scenario in someone whose been having GIT losses over several days (high urea to creatinine will show this rather than sodium conc)

Question 18

When is fluid resuscitation indicated?

Answer 18

When there is evidence of reduced circulating volume and impaired tissue perfusion

• Hypotension
• Tachycardia
• Oliguria
• AKI
• reduced GCS
• inc lactate

Fluid resus aims to improve tissue perfusion by means of sequential fluid challenges

Question 19

What kind of fluid do we use for fluid resuscitation?

Answer 19

• Use a sodium-containing crystalloid solution
• Similar to plasma serum composition
• In practice:
  
  • 0.9% sodium chloride
  • Hartmann’s solution
• We use this because sodium is an extracellular cation, it’s excluded from the ICF by Na-K-ATPase pump
• Some fluid will escape intravascular fluid into the interstitial fluid → cause oedema (sometimes necessary by-product)
• Rarely a colloid or blood product
• Never glucose
• Never with ‘added’ potassium

Question 20

What volume of fluid challenge do we give and over how long?

Answer 20

• 0.9% NaCl or Hartmann’s solution
• Vol 250-500 mL (on wards: go for higher end of range)
• Child → 20 mL/kg
• Infuse rapidly (_~5 min)

Must check response to fluid challenge (hence why it’s a ‘challenge’)

Question 21

What are you assessing for response to a fluid chalelnge?

Answer 21

• Hypotension + Tachycardia → improve over minutes
• Oligura, Lactate, GCS → may take hours
• Renal markers → may take a day (+)
• Potential adverse effects:
  
  • Oedema → predictable and inevitable
  • Breathlessness → pulm oedema concerning acutely
  • Electrolyte deragement → expected but be alert eg. hyperchloraemic metabolic acidosis develops post NaCl infusion

Question 22

What do we do next following a fluid challenge?

Answer 22

• If no change → repeat
• If deterioration → stop and discuss
• If improvement → monitor and repeat if needed

In ALL cases, get senior advice!

Question 23

Prescribe an IV fluid regimen

Answer 23

• In order to meet his sodium requirements, we can give him half of NaCl 0.9% (1L) (bag 1) which would be 500 ml, including 77 Na⁺ –> meets his sodium requirements.
• Now we also need to meet his full water requirement (_~1.6L), therefore needs 1L more as we have already given him 500mL. Don’t want to add to his sodium levels, so we can use our “pure water” of Glucose 5% 1L (bag 2).
• Finally, address potassium requirement (65 mmol), can give 40 in bag (1) and 40 in bag (2) - this will total to 60 mmol as remember, we halved bag (1).

Question 24

Prescribe an IV fluid regimen

Answer 24

• First, calculate his sodium requirement (85 mmol), can be met with 500 mL of 0.9% NaCl. In addition, he’s losing 1000 mL of gastric fluid, so will increase NaCl prescription to 1.5L (500 mls from maint requriements and 1000ml from additional ongoing loss).
• Water requirement was about 3 L (2L daily and 1L loss), already met 1.5L using NaCl 0.9%. Step 2 is to top up water requirement using 5% glucose, so add extra 1.5L by using 5% glucose
• Can distribute the _~100 mmol of potassium requirement in sensible amounts across the 4 bags of fluid - 40:20:20:20, remember can’t put 40 mmol of potassium in a 500ml bag
• The rate is worked out: total fluids (3000ml) / 24hrs = 125 ml/h