Fluid Physiology, Assessment and Management

Note that this is a relatively large post- this is because fluid physiology, physical principles and clinical relevance are all important in understanding the topic

Total Body Water

• Essentially 3 fluid compartments
  • Intracellular (~28l or 2/3rds of TBW)
    • ~2l in RBCs and remainder in other cells
    • Composed of high potassium/low sodium; maintained by Na-K-ATPase
      • NB Insulin can drive K+ into cells
  • Extracellular (around 1/3rd of TBW)
    • Interstitial (~10l)
      • Bathing cells and in extracellular spaces e.g. plaural cavity, joint space etc
      • Composed of high sodium/low potassium (i.e. identical to blood: 135-145mmol/l Na+ and 3.5-5mmol/l K+)
    • Intravascular (~3l)
      • Plasma volume
• NB TBW, in the context of physiology, accounts for around 60% of the total body weight

Daily requirements, intake and losses

Requirements

• Water
  • Can be calculated based on weight either based on hourly or daily requirements
    • Daily
      • 1st 10kg: 100ml/kg/day
      • 2nd 10kg: 50ml/kg/day
      • 20ml/kg/day thereafter
        • e.g. a 70kg individual would be 1l + 500ml + 1l = 2500ml (in reality most people are closer to 90kg and require 2900ml)
    • Hourly
      • 1st 10kg: 4ml/kg/hour
      • 2nd 10kg: 2ml/kg/hour
      • 1ml/kg/hour thereafter
        • e.g. a 70kg individual would require 40ml + 20ml + 50ml = 110ml/hour (OR 2640ml/day) (again, a 90kg individual would require 3120ml/day)
• Sodium
  • 1-2mmol/kg/day (It is often quoted that a 70kg individual requires 100mmol/day)
• Potassium
  • ~1mmol/kg/day (usually quoted as 60mmol/day)
• Magnesium
  • ~0.1mmol/kg/day

Intake and losses for water (normal individual)

Forces governing fluid movement

• Osmotic forces
  • regulates water balance between ICF and ECF
  • dependent on osmolality
    • concentration of solute per kg of solvent
    • the normal osmolality of ECF is 280-295 mOsmol/kg
      • Aside: NB (in human physiology- the terms osmolality and osmolarity are effectively interchangeable)
        • Difference is that osmolarity is a measure of the concentration of solute per litre of solution.
          • Another is how they are measured- osmolality is measured by a machine (osmometer) and osmolarity is calculated 
            • Eqtn: 2(Na+) + 2(K+) + Glucose + Urea (all in mmol/l)
        • In the human body- most of the measurable solutes are in solution so osmolarity is almost the same as osmolality
          • An increased osmolal gap (normally <10-15mOsmol/kg) can occur and is usually a result of either
            • Increased solubility e.g. alcohol excess (alcohol is a more potent solvent)
            • Increased sugars (e.g. use of mannitol/sorbitol or in diabetes when there is increased glucose levels)
            • Increased lipids/proteins e.g. in hypertriglyceridaemia or hypergammaglobulinaemia (e.g. of B-Cell lymphoma)
• Starling forces
  • Governs movement of fluid across capillaries depending on the
    • Hydrostatic pressure (push fluids out of capillaries)
    • Oncotic pressure exerted by proteins (pulls fluids into the capillaries)
    • Permeability of the endothelium between plasma and interstitial fluid

Principles of compartment shifts and fluid resuscitation

Because, aside from oral hydration, we can only administer fluids intravenously, it is important to know roughly where the fluid goes after this.  As mentioned above, this depends on the concentrations of solutes in solution.  IV fluids can therefore be classified as:

• Isotonic (identical osmolarity to the body): for example
  •  0.9% (normal) saline (NaCl)
    • Contains 154mmol/l of both Na and Cl
  • Ringer’s lactate (NaCl, Sodium lactate, KCl, CaCl2) or Hartmann’s
    • Contains 130mmol/l of Na+; 5mmol/l of K+; 2mmol/l of Ca2+; 111mmol/l of Cl- and 29mmol/l of lactate
  • 5% Dextrose (D5W)
    • Contains 50g/l of dextrose in water
  • Gelofusine (primary colloid used in practice- see below)
    • Contains 154mmol/l of Na+; 125mmol/l of Cl-; trace potassium and 40g/l of gelatin
• Hypertonic (increased osmolarity). e.g.
  • 3% or 5% saline
    • Contains 513mmol/l and 855mmol/l, respectively, of Na/Cl
  • 5% dextrose in saline or lactate solutions
• Hypotonic
  • 0.45% (half) saline
    • Contains 77mmol/l of Na and Cl

Adding different types of fluid will affect the body’s fluid compartments in different ways:

https://www.inkling.com/read/guyton-hall-textbook-of-medical-physiology-12th/chapter-25/volume-and-osmolality-of

1. When an isotonic fluid is given, there is no change in the concentration of the ECF and thus water is distributed equally between ICF and ECF
   1. Used to replace lost fluid
   2. Na and Cl remain in the extracellular space (effectively)
2. When a hypertonic solution is given, the extracellular (and thus intracellular) osmolarity increases and osmosis occurs- water moves from inside the cells into the plasma, resulting in an increased ECF relative to ICF and which is greater than the fluid added.
   1. Used in cases of severely low electrolytes (rarely), to help with metabolic acid/base imbalances and in cases of hypoglycaemia
   2. Also used in cerebral oedema (to draw water out from the tissue); rarely but occasionally used as an alternative to colloids in hypovolaemic shock
   3. Be wary in patients with heart or kidney failure (risk of overload); or in patients with potentially hypertonic extracellular fluid already, or in those with a condition causing cellular dehydration (don’t give in dehydrated patients) e.g. in DKA
3. When a hypotonic solution is given, the osmolarity decreases and water moves into the cells (to a greater extent than which it remains in the plasma), resulting in a raised ICF (relative to the increase in ECF).
   1. Can occasionally be used in cases where the ECF may be hypertonic e.g. DKA, or where patients are receiving TPN feeding
   2. Also if a patient has been over-‘diuresed’ i.e. fluid loss due to diuretics
   3. NB Rarely are hypotonic solutions used without a bolus of isotonic fluid first (because most commonly- patients are hypovolaemic rather than hypervolaemic- and they will become hypovolaemic quite quickly on these solutions).  They should also be used short-term and with caution, as they can cause cerebral and tissue oedema (which can be life-threatening).

• A note about some particular fluids
  • The dextrose in dextrose solutions is pretty quickly incorporated into cells and metabolised.  Thus, in reality, isotonic (and even mildly hypertonic) dextrose solutions can act like hypotonic solutions- and is generally safer than hypotonic solutions e.g. 0.25% saline.
  • The crystalloid vs colloid debate…
    • Crystalloids are solutions containing natural salts e.g. saline; whilst colloids contain synthetic compounds.  The latter tend to remain in the plasma for longer and were traditionally used in patients with haemorrhagic/hypovolaemic shock.  However, recent evidence has suggested no difference in the outcomes of patients when either crystalloid or colloid was used.

Assessing fluid status

Assessing fluid status can be challenging, but a systematic approach will often help.

1. Assess symptoms/signs
   1. Any sudden (within days-weeks) change in weight is likely to be caused by fluid i.e. if a patient has lost several kilos in a day then they are likely to be dehydrated (assuming they weren’t overloaded beforehand)
   2. Blood pressure is often related to fluid status (hypertension with overload and hypotension with dehydration)
      1. classically- postural hypotension is used to assess volume status (it can be more sensitive)
   3. Cardiac: Capillary refill, JVP and heart rate
      1. JVP is often first to change but may be hard to observe (useful to have a baseline examination with and without using the hepatojugular reflex)
      2. A tachycardia may be seen in hypovolaemic patients, although beware of beta blockers and calcium channel blockers
      3. Auscultate the heart for gallop rhythm or 3rd heart sounds (hypervolaemic)
   4. Check for oedema
      1. Pulmonary oedema (auscultate lung fields)
      2. Peripheral (look for pitting sacral and/or ankle oedema)
   5. Urine output is another common measurement used in fluid assessment
      1. Be careful to try and differentiate oliguria due to dehydration (requires fluids) and oliguria due to an instrinsic/post-renal acute kidney injury (which, if treated with fluids, may cause fluid overload)
         1. This can be difficult, but in general (particularly post-op and in the acute situation), low urine output is caused by hypovolaemia (until proven otherwise).  An improved response to a fluid challenge suggests hypovolaemia.
   • Fluid overload: dyspnoea (particularly at night or lying down), oedema (swollen ankles)
   • Dehydrated: dry mouth/eyes, weakness, confusion, sunken eyes; vomiting; bleeding; decreased skin turgor
   • CHECK the patient’s U&Es
     • These can be particularly useful in determining fluid status e.g.
       • A low sodium could indicate too much fluid (dilutional result) OR increased secretion of sodium with normal/low water levels e.g. SIADH
       • A high sodium could indicated dehydration e.g. from diarrhoea/vomiting OR inadequate sodium loss with normal water levels e.g. diabetes insipidus
     • Also check urine and serum osmolarity +/- haematocrit
2. Has the patient got any conditions (or a history suggestive of any conditions) that could predispose to fluid/?
   1. e.g. renal disease; heart failure; bowel obstruction;
   2. Pre- or post-operative
3. Is the patient on any medications that might affect fluid status?
   1. Diuretics; steroids; any IV treatments/fluids

Which fluids for which patient?

• 5 Rs- Rescuscitation; Routine Maintenance; Redistribution; Replacement; Reassessment

Maintenance

• NICE guidelines say that a patient who needs routine maintenance fluids (e.g. NBM patients awaiting surgery) should receive
  • 25-30mls/kg/day of water.  For the majority of patients this is 2-3 litres.
  • around 1mmol/kg/day of potassium, sodium and chloride.  In reality, a little more sodium is given (1l of normal saline), and 60mmol of potassium is given.
  • around 50-100g/day of glucose
• A ‘normal’ maintenance regimen (‘2 sweet; 1 salty)
  • 1l 0.9% saline + 20mmol KCl over 8 hours
  • 1l dextrose 5% + 20mmol KCl over 8 hours
  • 1l dextrose 5% + 20mmol KCl over 8 hours

Pre-existing fluid loss

• If there is any evidence that the patient is shocked and might need resuscitation e.g. tachycardia (>90bpm); hypotension (SBP <100mmHg); delayed cap refill >2s; tachypnoea (>20bpm) etc
  • Give a fluid bolus
    • 500mls of 0.9% saline over 15 minutes (quicker if the patient is in severe shock)
    • NB if there are signs of heart failure give 250ml
  • NB If there is any evidence of blood loss, ideally replace blood with blood products (see management of bleeding)
  • If there is no/transient/partial response, repeat the fluid challenge
    • If not already done so, contact senior help (anaesthetist/patient consultant) after the 3rd/4th bolus
• If the patient is well enough, estimate the water/electrolyte deficit and add this to maintenance regimen.

Ongoing losses

• Estimate the amount lost per day and adjust maintenance appropriately
  • e.g. remember that electrolytes are also lost via diarrhoea/vomiting (up to 140 mmol/l of Na in the former)

Special cases

• Post op
  • K+ can increase (due to cell lysis) so if K+ levels are >4.5mmol/l, omit replacement; if <4.5mmol/l, replace at a lower rate e.g. 40mmol/24hours
  • Remember to only give fluids if necessary! – If the patient is eating/drinking normally after an operation, they are usually not required.
• Liver failure
  • Be weary of using 0.9% saline as it can precipitate ascites (consider dextrose only or hypotonic solution)
• Acute Kidney Injury/Renal Failure
  • Avoid K+ replacement
• Chronic renal failure
  • Avoid excess fluids and electrolytes.  Also avoid Hartmann’s (lactate cannot be excreted)
• Alcoholism
  • GIVE pabrinex prior to dextrose (can precipitate Korsakoff syndrome)
• Brain Haemorrhage
  • Avoid dextrose
• Anorexia/Long-term TPN patient
  • Avoid lots of dextrose (risk of re-feeding)