Fluids and electrolytes

1. Fluids and electrolytes
By
Dr. RUTAYISIRE François Xavier

2. • Fluid management is a major part of junior doctor prescribing;
whether working on a surgical firm with a patient who is nil-by-mouth
or with a dehydrated patient, this is a topic that a junior doctor
utilises on a regular basis.
• Ensuring considered fluid and haemodynamic management is
central to peri-operative patient care and has been shown to have a
significant impact on post-operative morbidity and the length of
hospital stay. Hence it is essential to gain a firm understanding of the
physiology of fluid balance and the compositions of each fluid being
prescribed.

3. Why do we prescribe Fluids
• Firstly it’s important to think about why fluids should be prescribed in
the first place. The reasons for fluid prescription are:
• Resuscitation
• Maintenance
• Replacement

4. • Fluid prescription varies depending on the individual patient and it is
essential to take individual patient characteristics into account before
prescribing fluid.
After some operations, patients are deliberately run “on the dry side”,
whilst septic patients or patients in bowel obstruction will need
aggressive fluid prescribing.

5. Fluid Compartments
• Around 2/3rd of total body weight is water (‘total body water’).
Around 2/3 of this distributes in to the intracellular fluid and the
remaining 1/3 will distribute in to the extracellular fluid.
• Of that fluid in the extracellular space, around 1/5th stays in the
intravascular space and 4/5th of this is found in the interstitium, with
a small proportion in the transcellular space.
• For the general maintenance of hydration, it is necessary for fluid to
distribute into all compartments. However, if the aim is to fluid
resuscitate a patient (improving tissue perfusion by raising the
intravascular volume), it is more important these fluids stay within
the intravascular space.

7. Fluid Compartments

8. The Septic Patient
• The tight junctions between the capillary endothelial cells break
down
• Vascular permeability increases.
• As a result, increasing hydrostatic pressures and reducing oncotic
pressure lead to fluid leaving the vasculature and entering the tissue.
• It is often therefore necessary to give relatively large volumes of
intravenous fluid to maintain the intra-vascular volume, even though
the total body water may be high. Close monitoring of the fluid
balance will be required.

9. Fluid Input-Output

10. Fluid Input
• Only 3/5th of our fluid input comes through fluids via the enteric
route.
• The remainder from both food and metabolic processes.
• Hence, when a patient is nil by mouth (NBM), it is important that all
sources are replaced via the parenteral route.

11. Fluid Output
1. Urine
2. Insensible losses(Losses from non-urine sources).
• Insensible losses will rise in unwell patients, who may be febrile,
tachypnoeic, or having increased bowel output. These factors should be
taken into account when deciding how much fluid a patients needs
replacing.
• When patients start to clinically improve, their vascular permeability
returns to baseline state. They therefore often “correct themselves” and
urinate out the excess fluid that was previously required to maintain their
intravascular volume and tissue perfusion.
• In such patent, monitor the electrolytes and allow this correction to occur,
as this is normal and is to be expected (rarely will supplementary IV fluids
will be warranted in such cases).

12. Assessment of Fluid Status
In the fluid depleted patients, one should be looking for:
• Dry mucous membranes and reduced skin turgor
• Decreasing urine output (should target >0.5 ml/kg/hr)
• Orthostatic hypotension
• In worsening stages:
• Increased capillary refill time
• Tachycardia
• Low blood pressure

13. In patients who may be fluid overloaded, one should be looking for:
• Raised JVP
• Peripheral or sacral oedema
• Pulmonary oedema

14. Daily Requirements
• Patients do not just require water, they also need Na+, K+, and glucose
replacing too, particularly if they are nil by mouth.
• Current NICE guidelines suggest the following:
• Water: 25 mL/kg/day
• Na+: 1.0 mmol/kg/day
• K+: 1.0 mmol/kg/day
• Glucose: 50g/day
• Based on these required, it is necessary to consider the fluids that are
available for prescription and what exactly they contain, to be able to
prescribe appropriately

15. Intravenous Fluids
IV fluids can be broadly categorised in to two groups, crystalloids and
colloids

16. Crystalloids
• Crystalloids are more widely used
than colloids, with research
supporting the idea that neither is
superior in replenishing
intravascular volume for
resuscitation purposes (with
crystalloids also significantly
cheaper). Therefore, crystalloids
are used very commonly in the
acute setting, in theatres, and for
maintenance fluids.
Colloids
• Colloids have a high colloid osmotic
pressure and theoretically should
raise the intravascular volume
faster than their crystalloid
counterparts, yet clinical trials have
not shown any significant benefit
or effect in practice so their use in
many hospitals is decreasing

17. Crystalloids Fluid Composition

18. • Dextrose
• 5% dextrose solution is a hypotonic (and isosmotic) fluid containing only
dextrose and water.
• Dextrose, the D-isomer of glucose, is rapidly taken up into cells to be
metabolised, leaving the remaining free water component to equilibrate
across all the body compartments.
• Only 7% of the fluid therefore stays in the intra-vascular space. So, 5%
dextrose has no role in fluid resuscitation of a patient, only in fluid
maintenance regimes.
• The main advantage of dextrose is being able to maintain hydration
without administering an excess of electrolytes, and it can also be
prescribed with supplementary potassium if required.

19. • Normal Saline
• 0.9% sodium chloride solution (commonly termed “Normal Saline”)
is an isotonic solution containing Na+, Cl–, and water. It equilibrates
throughout both the intra-vascular and interstitial spaces
(approximately 25% volume within the intra-vascular space) and this
makes it useful in both resuscitation and maintenance regimes.
• Potassium can be added to the solution too, aiding in electrolyte
management.
• It should not be used as a lone fluid maintenance however, as
excessive saline replacement can result in a hyperchloraemic
acidosis.

20. • Hartmann’s Solution
• Hartmann’s solution is a balanced isotonic solution containing Na+,
Cl–, K+, HCO3
– (as lactate), Ca2+, and water. Similar to Normal Saline, it
distributes in the intra-vascular and interstitial spaces, making it
useful for both resuscitation and fluid maintenance.
• Hartmann’s solution is considered to be more “physiological” than
Normal Saline as it contains other electrolytes in concentrations
similar to plasma. It also contains lactate, which it uses to generate
alkalising HCO3
– ions.

21. Fluid Prescribing
1. Maintenance Fluids
2. Correcting a Fluid Deficit
• Where the patient is initially dehydrated, he has a deficit that need to be
corrected with fluids, in addition to those prescribed as maintenance.
• However, in practice it is relatively uncommon to find a patient that is so
profoundly dehydrated that this deficit needs to be calculated specifically.
Instead, a subjective assessment is made based on clinical parameters,
patient size, and any comorbidities.
• Any reduced urine output (<0.5ml/kg/hr) should be managed aggressively,
giving a fluid challenge and the clinical parameters, including urine output,
subsequently rechecked (also ensuring any catheter is not blocked or
patient not retaining urine)

22. 3. Replacing Ongoing Losses
• Do subjective assessment in this aspect too. Aspects to be assessed may include:
• Are there any third-space losses?
• Third-space losses refer to fluid losses into spaces that are not visible, such as the bowel
lumen (in bowel obstruction) or the retroperitoneum (as in pancreatitis).
• Is there a diuresis?
• Is the patient tachypnoeic or febrile ?
• Is the patient passing more stool than usual (or high stoma output)?
• Are they losing electrolyte-rich fluid?
• Common scenarios of electrolyte imbalances though fluid losses that may be
encountered include dehydration (high urea:creatinine ratio), vomiting (low K+,
low Cl–, and alkalosis), or diarrhoea (low K+ and acidosis)

23. Ongoing Monitoring
• When prescribing fluids, it is important to remember to regularly
assess their fluid status, what they are managing orally, and amend
their fluid prescription accordingly.
• Use your clinical assessment, nursing charts (fluid input-output charts
± daily weights) and U&Es to guide this.