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 on a care of the elderly firm, this is a topic that a junior doctor utilises on a very regular basis. Hence it is essential to gain a firm understanding of the physiology of fluid balance and the compositions of each fluid being prescribed.
*Please be aware that this article talks solely about adult fluids and does not cover paediatric prescribing.
Firstly it’s important to think about why fluids should be prescribed in the first place. The reasons for fluid prescription are:
The relative importance of each of these varies between patients. Perhaps the most important point to remember therefore is that correct fluid prescription varies depending on the individual patient and it is essential to take individual patient characteristics into account before prescribing fluid.
The general key considerations to remember with every patient are:
- Is the aim of the fluid for resuscitation, maintenance, or replacement?
- What is the weight and size of the patient?
- The fluid requirements of a frail 45kg 80yr female and an obese 140kg 40yr male will be significantly different
- Are there any co-morbidities present that are important to consider, such as heart failure or chronic kidney disease?
- What is their underlying reason for admission*?
- What were their most recent electrolytes?
*After some operations, patients are deliberately run “on the dry side”, whilst septic patients or patients in bowel obstruction will need aggressive fluid prescribing.
Although this knowledge does not directly aid prescribing, a knowledge of fluid compartments and distribution does help to appreciate where fluids will go once administered. Roughly 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 extracelular 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. This concept will help us understand why different fluids are available and for what purpose they might be used.
Remember that in patients who are septic, the tight junctions between the capillary endothelial cells break down and vascular permeability increases. As a result, fluid and protein leaks out of the blood vessels into the tissues. It is often therefore necessary to give quite a lot of fluid to maintain the intra-vascular volume, even though the total body water may be high.
The proportions of fluid that are gained and lost from various sources are shown in Table 1.
Note that these figures are the average for a 70kg man. The actual amount varies considerably depending on physiological status and body weight (which in adult patients can vary from around 40kg to 200kg).
Only 3/5th of our fluid input comes from drinking, with 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.
Losses from non-urine sources are termed ‘insensible losses’. Insensible losses will rise in unwell patients, who may be febrile, tachypnoeic, or having increased bowel motions or stoma output. These factors should be taken into account when deciding how much fluid a patients needs replacing.
As patients may start to improve, their vascular permeability returns to normal. They therefore often “correct themselves” and urinate out the excess fluid that was previously required to maintain their intravascular volume and tissue perfusion when they were sick in the early stages and this is a good sign.
Consequently, in patients who are improving following large IV fluid infusions, simply monitor the electrolytes and allow them to urinate the large volumes. Any patient at the start of their stay is likely to be in positive overall balance and then at the end negative balance. This is normal and is to be expected.
Assessment of Fluid Status and Patient Monitoring
It is essential to utilise various parameters to continually assess the patient’s fluid status. A doctor’s first assessment is, of course, the patient’s clinical status.
In the fluid depleted patients, one should be looking for:
- Dry mucous membranes
- Reduced skin turgor (classically assessed over the sternum)
- Urine output (should be >0.5 ml/kg/hr)
- A patient complaining of thirst
- In worsening stages:
- Increased capillary refill time
- Low blood pressure or an orthostatic drop
Patients who may be fluid overloaded, one can assess for:
- Raised JVP
- Peripheral oedema
- Pulmonary oedema
- In worsening stages:
- Tachypnoea and / or hypoxia
Ensure that the patient has a fluid input-output chart and daily weight chart commenced; you will need to ask the nurses to begin one of these (despite commonly being poorly maintained). Also ensure to monitor the patient’s urea and electrolytes (U&Es) regularly, for any evidence of dehydration, renal hypoperfusion, or electrolyte abnormalities
Patients do not just require water; they also need Na+, K+ and glucose replacing too, particularly if they are nil by mouth. You will find numerous ways of calculating the daily requirements of these 4 components and they are invariably based on the patient’s weight.
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
As we now have an approximation as to what the fluid and electrolyte requirements are on a daily basis, it is necessary to consider the fluids that are available for prescription and what exactly they contain.
IV fluids can be broadly categorised in to two groups: crystalloids and colloids. Table 2 sets out the components of each of the commonly prescribed fluids.
Crystalloids are far cheaper than colloids and research supports that idea that neither is superior in replenishing intravascular volume for resuscitation purposes. Therefore, crystalloids are used very commonly in the acute setting, in theatres, and for maintenance fluids.
Colloids have a high colloid osmotic pressure and theoretically should raise the intravascular volume faster than their crystalloid counterparts, yet randomised trials have not shown any benefit or effect in practice so their use in the UK is decreasing.
There are a number of different types of colloid, but the main 3 you will see prescribed* are gelatins (Volplex® and Gelofusine®), albumin and blood products, the former of which is most often prescribed in acute blood loss where blood products are not yet available.
*The main concern with the use of colloids is anaphylactic reactions, coagulopathy, and their cost, so they should be used with caution. They are occasionally useful in resuscitation but have no role in maintenance fluid prescription.
Let us say that our patient is a 70kg male. From the above section, we know in total, we need to prescribe fluids over 24 hours that provide 1750mL of water (70kg x 25mL/kg/day), 70mmol of Na+ (70kg x 1.0mmol/kg/day), 70mmol of K+ (70kg x 1.0mmol/kg/day), and 50g (50g/day) of glucose.
Providing the patient’s renal function is adequate and they are not fluid overloaded, it is normally okay to over-replace these volumes slightly, to permit ease of prescribing. Consequently, a typical fluid maintenance regimen is as follows:
- First bag: 500mL of 0.9% saline with 20mmol/L K+ to be run over 8 hours.
- This provides all of their Na+,~1/3rd of their K+, and a quarter of their water.
- Second bag: 1L of 5% dextrose with 20mmol/L K+ to run over 8 hours.
- This provides a further 1/3rd of their K+ and half of their water, as well as glucose.
- Third bag: 500mL of 5% dextrose with 20mmol/L K+ to run over 8 hours.
- This provides the remaining 1/3rd of their K+ and a quarter of their water, as well as glucose.
Correcting a Fluid Deficit
Where the patient is initially dehydrated, you will need to correct this deficit 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.
NICE guidance suggests that any reduced urine output (<0.5ml/kg/hr) should be managed aggressively, giving a fluid challenge and urine output subsequently rechecked, also ensuring any catheter is not blocked.
The fluid challenge should be either 250ml or 500ml Hartmann’s over 1hr, depending on the patient’s size and comorbidities. For example a 100kg 50yr male may need 500 ml to make any difference to their intravascular volume, whereas in a frail, elderly lady with ischaemic heart disease, 250ml may be more appropriate.
Replacing Ongoing Losses
Like much of fluid prescribing, there is a degree of subjective assessment in this aspect, too. With reference to Table 1, one should assess if there are excess losses in any of the 4 secretions. Ask yourself therefore:
- 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?
- Is the patient febrile and therefore sweating more than ordinary?
- Is the patient passing more stool than usual or do they have a high output stoma?
- Are they losing electrolyte rich fluid?
Common examples of ongoing losses that may be encountered include:
- Dehydration (high urea:creatinine ratio and high PCV)
- Vomiting (low K+, low Cl–, and alkalosis)
- Diarrhoea (low K+ and acidosis)
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.