Fluid Balance

Almost every patient admitted to hospital receives IV fluids at some point in their journey. However, the body manages this, without the need for careful medical assessment and adjustment, as fluid balance is one of its core functions. Despite this, there are many situations where we need careful and controlled management. These include:

What is fluid balance?

Fluid balance is the measuring of a patient’s fluid input and output and calculating the net difference or balance. Depending on your patient’s fluid status, you will have set a 24-hour goal for their fluid balance – positive, negative, neutral.

The main reasons to record fluid balance is to prevent or correct dehydration and fluid overload.

In ITU, patients will have their input and output carefully monitored due to one-to-one nursing. Fluid balance targets can be prescribed e.g. “Mr Jones should be +500ml over the next 24 hours”. Thus, careful manipulation of the rate of IV fluids and/or furosemide can allow this to be closely achieved. Even in ITU however, it is very difficult to measure this accurately and fluid balance charts are prone to error.

The important thing to recognise is whilst we don’t have great tools to accurately assess a patient’s fluid balance, a comprehensive history and examination can give us some idea. Observing for changes in these parameters can give us an idea where the patient is heading and then we can make adjustments accordingly. Therefore monitoring a response to interventions that affect fluid balance is vital.

Fluid balance assessment

On history
  • Oral intake of food and fluids. Are they thirsty? Don’t forget about soups & hot drinks particularly if a patient is on a fluid restriction
  • Insensible losses e.g. fever, sweats, burns, trauma, tachypnoea
  • Output including vomiting/diarrhoea/stoma output, bleeding & for urine the amount and colour. It is often more helpful to ask if they’ve noticed a change in colour or amount
  • Features of dehydration (syncope or pre-syncope) or overload (shortness of breath, leg swelling, paroxysmal nocturnal dyspnoea or orthopnoea)
  • Dry weight (sometimes known to dialysis or heart failure patients)
On examination
Peripheral temperature– Impaired perfusion will lead to cool peripheries e.g. hypovolaemic shock or cardiogenic shock
– Warm peripheries can be found in a hypervolaemic patient or in vasodilatory shock e.g. sepsis
Capillary refill time– Ensure you apply at least 5 seconds of pressure
– If > 2 seconds, check if they are centrally deplete with a sternal capillary refill time
Skin turgor– Generally less helpful in the elderly
– More helpful in younger patients in whom you expect quite elastic skin
Peripheral & Central pulse– Usually very helpful
– A normal HR is reassuring that a patient isn’t significantly deplete (in an otherwise well patient)
– Beware of b-blockers that’ll blunt the compensatory tachycardia
– A weak volume pulse occurs in the later stages of shock & may suggest an emergency
Jugular venous pressure– Assessment of overload, approximates the CVP or right atrial pressure
– Difficult to assess (even for registrars). Looking for the JVP regularly can boost confidence
Mucous membranes– Usually unhelpful unless the patient is obviously dry
– They may appear wet if the patient has just had a few sips of water
Cardiac auscultation– Usually unhelpful
– A gallop rhythm or third heart sound can sometimes occur in overload (but usually is missed)
Lung auscultation– Very helpful to judge the presence & extent of pulmonary oedema
– Don’t forget percussion for pleural effusions
(Shifting dullness)– Consider whether there is any ascites
Peripheral oedema– Assess how far up it extends (it can be as far as abdominal wall oedema)
– Don’t forget sacral oedema. This can be extensive and is frequently missed!
– Documenting this allows each new assessor to know whether things are improving
(Passive leg raise)– Consider this if you want to assess whether a patient is fluid responsive
– You raise a patient’s leg for 5 minutes and repeat the observations
– It is, in effect, a fluid bolus of around 250-500 ml (without actually giving the fluid)
– A response means that fluid will increase the cardiac output (Starling’s law)
Review the charts
  • Observations (high respiratory rate and tachycardia. Hypotension occurs later when the body can no longer compensate). Note a normal blood pressure in a patient that is usually hypertensive may be low for them.
  • Lying and standing blood pressure can help differentiate hypovolaemia from euvolaemia
  • Review the fluid balance chart (see below)
  • Check their weight (really helpful to see whether diuresis is effective)
  • Review any confounding medications (antihypertensives or b-blockers) and whether the patient is on diuretics
  • Don’t assume an AKI means a patient is dehydrated. An AKI must be reviewed in context – it could be cardiogenic, hypovolaemic or a renal/post-renal AKI
Fluid Balance Chart

Nursing staff will normally document the patient’s fluid input and output on a fluid balance chart. The template may vary between trusts but the aim is the same:

  • Input will be all the fluid intake a patient is having during the day including orally, via nasogastric or PEG and intravenously.
  • Output will include the patient’s urine output (helpful if you are concerned about acute kidney injury), vomiting, stomas, NG tubes – any obvious loss of fluid
  • A running total for both input and output may also be documented (up to hourly) to aid quick calculation of the balance with 24-hour totals and balance recorded at the bottom of the chart
  • Urine output can be really helpful to judge your interventions over a few hours e.g. the effects of diuretics
  • Accuracy of fluid balance depends on staffing levels, expertise of the nursing team, patient’s concordance to having their input/output measured e.g. catheter vs using the toilet and whether they will stick to a fluid restriction

There are a lot of things to cover in a comprehensive fluid balance assessment, and it can be hard to remember them all. Fortunately, there is a useful memory aid that can be used to structure examination: The A-M approach

The A-M Approach for Assessing Fluid Balance

A: Ask the PatientDo they feel thirsty, dizzy or short of breath?
B: Blood PressureLying, sitting and/or standing as appropriate
C: CapillariesRefill time, and peripheral temperature
D: Dry Mucous MembranesDoes the patient’s mouth look moist or dry?
E: Elastic Skin TurgorIs the patient’s skin bouncy, or thin and friable?
F: Fluid intake and outputReview the charts, being sure to consider insensible losses
G: Glasgow Coma ScoreAssess consciousness level
H: HeartAuscultate and Palpate for character and rate
I: Interstitial FluidConsider the 3 A’s – Ankle oedema, Alveolar (pulmonary) oedema, and Ascites
J: Jugular Venous PressureA raised JVP could indicate fluid overload
K: PotassiumInspect the Blood Tests for Renal Impairment and/or Electrolyte Disturbance
L: Look at the PrescriptionHow appropriate is the volume and content of the IV Fluids prescribed?
M: MassDoes the patient need regular weight monitoring?
The A-M Fluid Balance Examination — with thanks to Dr Tim Dixon for giving permission to share his approach

Fluid Balance Quiz

Given how difficult fluid balance is as a topic, I have written some cases to support your learning. There are detailed explanations below.

Explanation for Question 1

The patient is usually hypertensive and is on a b-blocker. She has a normal blood pressure which would be unusual for her, particularly given her ACE inhibitor has been held. Her central perfusion is poor and therefore you would give her an IV fluid challenge looking for an improvement in her blood pressure and capillary refill time. Remember it is a “challenge” as you are seeing whether the patient is responsive by re-assessing them.

Explanation for Question 2

This patient remains significantly overloaded with pulmonary and peripheral oedema, but is responding well with around 0.5kg/day of weight loss. A mild increase of up to 20% in her creatine can be expected, but you should discuss these patients with your seniors to ensure an accurate assessment has been achieved. Remember that oral furosemide has around 50% or so bioavailability so by switching 80mg IV to oral you are effectively halving the dose.

Explanation for Question 3

This patient has confusing features where there are features to suggest the patient may be hypovolemic (peripherally cool and dry mucous membranes) but her capillary refill time is normal and she’s known to have heart failure which makes people worry about causing pulmonary oedema. There is no valid reason to be immediately concerned that this is SIADH but the hyponatraemia screen will help guide. She is likely too agitated to perform further assessments such as a lying and standing blood pressure or straight leg raise (to see whether increasing the venous return improves the heart rate, thereby saying they are fluid responsive). Rapidly correcting hyponatraemia with 3% saline is dangerous and should only be driven by a registrar or consultant. There are no features to suggest significant hypovolaemia so we can initiate maintenance fluids whilst awaiting the hyponatraemia screen. It wouldn’t be unreasonable to give the IV fluids over 8 hours; it isn’t an exact science but instead influenced by clinical judgment, oral intake of the patient, their frailty and their weight.

Explanation for Question 4

This patient is more complex and is included as this exact scenario occurred a few weeks ago with an FY1 escalating such a patient in whom they hadn’t unreasonably tried a 250ml IV fluid challenge with no success. In this scenario, raising the patient’s legs may be a helpful test but ultimately, they are overloaded but poorly perfusing their kidneys resulting in minimal urine output. They need further IV diuretics but will poorly tolerate them considering their systolic blood pressure, which therefore needs to be supported in HDU or ITU with vasoactive and inotropic agents. The resultant increased cardiac output and improved perfusion to the kidneys will get the fluid to the right place and increase her urine output. GTN infusion and b-blockers will worsen the clinical situation. Overall, unless there is a reversible cause, this situation has a poor prognosis.

References & Useful Resources

Written by Dr Sophie Legg (F3) & Dr Akash Doshi (ST3 in Endocrinology & Diabetes)
Contributions by Glen Davies (F1)

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