Fluids management Flashcards
A 60-year-old man was admitted to the Emergency Department after a fall, and transferred to the rehabilitation ward. While in hospital he develops hospital-acquired pneumonia and has been commenced on intravenous antimicrobials in line with your local antimicrobial guideline. You are working on-call when the nurse looking after him bleeps you and asks you to prescribe some fluids as they feel he is not drinking enough.
- Obtain a history of how much he has been eating and drinking over the last few days. Corroborate this with clinical monitoring (e.g. fluid balance charts).
- Determine the urine output.
- Check vital parameters such as temperature, blood pressure (BP), pulse and respiratory rate and what the trend has been over the last few days.
- Check the latest laboratory results (e.g. full blood count, urea and electrolytes, creatinine).
- In addition, you should check to see if he has any extra sources of fluid loss such as stomas or drains, nausea or vomiting, and what the output from these has been. The underlying rationale for this is that you need to work out what extra losses this patient is having, over-and-above his maintenance requirements.
Describe the fluid compartments of the body
The two main fluid compartments are intracellular (ICF) and extracellular (ECF). The extracellular fluid compartment is further divided into interstitial and intravascular fluid.
A typical 70 kg man will have ~ 42 L of water contained in the two compartments
- Intracellular
- 65% of the TBW (28 L)
- Extracellular
- 35% of the TBW (14 L)
- Interstitial - 10.5 L (3/4 of extracellular)
- Intravascular - 3.5 L (1/4 of extracellular)
- 35% of the TBW (14 L)
Fluid shift between compartments
Starling’s hypothesis
- The fluid movement due to filtration across the wall of a capillary is dependent on the balance between the hydrostatic pressure gradient and the oncotic pressure gradient across the capillary.
- Distribution of water between intra- and extracellular compartments is determined largely by the extracellular sodium ion concentration.
- Extracellular solute concentration determines intracellular water quantity and consequently cell volume.
- This gradient is maintained by the sodium-potassium ATPase pump (Figure 2).
What are the average maintenance requirements (with no extra losses)
Daily requirements (fluids, electrolytes) per kg
- 2.0-2.5 L fluids per day
- (~1,500 ml of urine output + 500-800 ml insensible losses)
- 25-30 ml/kg fluid
- 1 mg/kg for Na, K, Cl
- 50-100 g/day glucose to limit starvation ketosis
What are the sources of fluid losses?
-
Urine (healthy person ~ 1ml/kg/h)
- Roughly 1,700 ml in 70kg person
- Reasonable UO 1.5 - 2.5 L / day
- In fluid replacement, aim for at least 0.5 ml/kg/h
-
Gastrointestinal
- Approx 100 ml/day healthy person
- Can be prominent in diarrhoea, vomiting, high stoma output, biliary drains
-
Insensible losses
- Skin, respiratory system etc
- 500 - 800 ml
- Can increase if tachypnoeic, sweating etc
- Miscellaneous
- Surgical, pleural and peritoneal drains
- Others
- Bleeding, burns
Loss of electrolytes based on the route of fluid loss
Electrolyte replacement
Monitor electrolytes and replace accordingly
Hypovolaemia vs Hypervolaemia
What do you take into account when prescribing the type and rate of fluid?
- Type of fluid loss
- Renal function
- Cardiac function
- Concomitant electrolyte abnormalities
Compare crystalloids vs colloids
Crystalloids
- Solutions of mineral salts
Colloids
- Contain larger water-insoluble molecules such as complex branched carbohydrates or gelatins.
Assessing dehydration
Before you consider prescribing fluids, review and assess the following:
- Blood pressure
- Capillary refill time
- Fluid balance charts
- Response to straight leg raise
- Skin turgor
- Weight
You should also consider iatrogenic causes of dehydration/renal failure such as diuretics. The importance of a medication review cannot be underestimated.
Explain the mechanism of Passive Leg Raise (PLR)
PLR
- Transiently increases venous return in patients who are preload responsive, as such it is a diagnostic test, not a treatment
- It is a predictor of Fluid responsiveness
- Helps identify pts who are on the ascending portion of their Starling curve, and will have an increase in CO
Isotonic vs hypertonic solutions
Isotonic
- Stay almost entirely in the extracellular compartment
- E.g. 0.9% NaCl
Hypertonic
- Hypertonic solutions increase plasma tonicity and draw fluid out of cells
- E.g. 3% NaCl, mannitol
Hypotonic
- Lower serum osmolality, not commonly used
- E.g. 0.45% NaCl
- Emerging evidence suggests that hypotonic solutions may be comparable to isotonic solutions when administered as maintenance fluids. Previously thought adverse effects such as hypokalaemia, hyponatraemia and oedema have not been borne out in studies.
- ”Even at maintenance rate, isotonic solutions caused lower urine output, characterized by decreased aldosterone concentrations indicating (unintentional) volume expansion, than hypotonic solutions and were associated with hyperchloraemia. Despite their lower sodium and potassium content, hypotonic fluids were not associated with hyponatraemia or hypokalaemia.”
When you give 1 liter (1000 ml) of sodium chloride 0.9%, do you know how it distributes in the various fluid compartments?
1000 ml of sodium chloride 0.9%
- Is isotonic with plasma.
- Contains 154 mmol/litre of sodium.
- Contains 154 mmol/litre of chloride.
- Stays in the extracellular fluid compartment (isotonic)
Extracellular fluid distribution (normal)
- 1/4 (25%) - Intravascular
- 3/4 (75%) - Interstitial
An isotonic solution will, therefore, distribute according to the extracellular fluid distribution
- 25% of 0.9% NaCl 1 L (250 mL) will therefore go to intravascular compartment, and 75% (750 mL) will go to extravascular
If a patient loses 1000 ml of blood, what is the equivalent volume replacement with sodium chloride 0.9%, required to replace the intravascular deficit?
4,000 mL
- The equivalent volume replacement with sodium chloride 0.9% will be 4 liters.
- As only 25% of the volume administered will go to the intravascular compartment. Therefore, 4 litres will be required to expand the intravascular compartment sufficiently to replace a 1000 ml blood loss
When you give 1 liter (1000 ml) of glucose 5%, do you know how it distributes in the various fluid compartments?s
1000 ml of glucose 5%:
- Contains 50 grams/litre of glucose.
- Is isosmolar with plasma.
- Glucose enters body cells and undergoes intracellular metabolism to leave water.
The fluid distributes across all body compartments according to their relative contributions to total body water:
- 2/3 goes to the intracellular fluid.
- 1/3 goes to the extracellular fluid.
Of the one third in the extracellular fluid, only approximately 80 ml of the 1000 ml administered will stay in the intravascular compartment (Figure 5).
- (Intravascular ~ 1/4 of extracellular compartment)
-
1000 mL * 1/3 (extracellular) * 1/4 (intravascular)
- 1000 * (1/12) = 83 mL
-
1000 mL * 1/3 (extracellular) * 1/4 (intravascular)
If a patient loses 1000 ml of blood, what is the equivalent volume replacement with glucose 5%?
12,000 mL
- As only 83 ml of the volume administered will stay in the intravascular compartment, 12 litres will be required to expand the intravascular compartment sufficiently to replace the fluid lost. 1000/83 = 12.0 litres.
- This explains why glucose 5% is not commonly used in resuscitation situations secondary to blood loss.
What are colloids?
What are some examples?
What are the drawbacks of using colloids?
Colloids contain large molecules that cannot readily traverse the capillary membrane owing to their size. Colloids exert an osmotic force across the capillary membrane drawing fluid in from the interstitial to the intravascular compartment. Depending upon the nature of the infused fluid, it will distribute across the various body compartments.
- These include:
- Blood
- Dextrans
- Gelatin (e.g. gelofusine)
- Human albumin solution
- Hydroxyethyl starch (HES)
Drawbacks of colloid use include:
- Higher cost.
- Small, but well-established risk of anaphylactoid reactions and anaphylaxis.
When you give 1 litre (1000 ml) of colloid, do you know how it distributes in the various fluid compartments?
1000 ml of human albumin solution
- Stays in the intravascular compartment.
- 1000 ml of colloid raises the plasma volume by 1000 ml.
Although crystalloid solutions eventually redistribute, initially when they are rapidly infused they do cause a transient expansion of the plasma volume. So in an emergency with a volume-depleted shocked patient do not wait till the ideal fluid becomes available, give whatever is available to maintain haemodynamic stability.
The Colloid vs Crystalloid debate
In summary, from a clinical perspective, current evidence suggests there appears to be no major difference between the use of colloid and crystalloid. However, from a practical perspective, crystalloids are more widely available and considerably cheaper. For this reason, crystalloids should always be used first-line for fluid resuscitation and maintenance.
Case Vignette 1A
Over the last two days, he has been spiking temperatures of up to 38°C, with an increased respiratory rate of 24 breaths per minute. The patient has had 600 ml of oral fluids per day for the last two days, and passed 1400 ml of urine per day. He has no other documented fluid losses. His usual intake of fluid is about 2 litres per day.
Think back to our patient and click here to view his latest blood results again. You have assessed his fluid intake and output over the last two days, and have noted the following:
Total fluid intake:
- Oral 600 ml per day.
- Water from metabolism 400 ml per day.
Total fluid output:
- Urine 1400 ml per day.
Insensible losses:
- Normally about 800 ml per day.
- As he is febrile and tachypnoeic it would be reasonable to double this volume.
6,000 mL
What is the fluid deficit for the patient in the previous case 1A vignette?
4,800 mL
What is a shock?
Shock can be defined as organ hypoperfusion to the extent that cellular metabolic demands are not met.
- It is a medical emergency.
- Rapid accurate diagnosis is essential.
There are different types of shock, and as management differs from one type to another it is important you determine the underlying etiology so that the appropriate management plan is instituted.
Distributive & Hypovolaemic Shock
What is the shock severity grading?
Distributive shock
- Distributive shock results in a relative hypovolaemia.
- Causes include sepsis, anaphylaxis and neurogenic shock.
Hypovolaemic shock
- Hypovolaemic shock is the most common form of shock encountered.
- Causes include haemorrhage, burns or any cause of substantial fluid loss.
Shock owing to volume loss progresses in stages and essentially form a continuum. Determining why someone is shocked will enhance definitive treatment. Shock severity can be graded 1-4, depending on the amount of fluid lost.
All attempts to maintain vital organ perfusion should be continued while investigations are being conducted.
