How complex can a fluid balance be?

In critical patient care we want to monitor the patient's fluid balance. The fluid balance is an important aspect in assessing a patient and to treat a patient. But what is a fluid balance exactly? Simply put fluid balance can be summarized by the following formula:

balance = fluid in - fluid out

When fluid balance is positive, this means the patient is accumulating fluid, when the fluid balance is negative, the patient is losing fluid. However, this simple notion is complicated by the following factors:

1. What is the time frame of the balance?
There are different possibilities:

• Balance from start of the day until current time 
• Balance from 24 hours ago until current time 
• Balance from date time of admission 
• -Balance between from date time and to date time 

So, in general a fluid balance always refers to a time frame.

2. Insensible loss
Normally, when a fluid balance is calculated this will yield a positive estimate. This is due to the fact that the human body loses fluid through transpiration and by breathing. This undetected loss is called: insensible loss.

An estimation of insensible loss is:
500 mL/m2/day

Fever increases insensible water losses by
10% per degree Celsius above 38°, or 100-150 mL/day increase per degree Celsius above 37°.

3. Fluid balance has to be related to patient size
What does it mean if the fluid balance is, say +300 mL? If this is an adult patient, this is less than the estimated 500-1000 mL insensible loss per day, so this patient loses fluid. However, in a neonate of with a body weight of 3 kg, this means that this patients accumulates 100 mL/kg/day!

So, size matters. the fluid balance has to be related to body size. Body weight is the most convenient adjustment measure.

4. What goes in and what goes out?
To further analyze a fluid balance it is important to know the amount of what goes in (intake) and what goes out (particularly urine output).

Intake can be categorized as:

• enteral
• intravenous
• dialysis

Fluids can be further categorized as:

• crystalloids
• colloid

 Likewise, what goes out can be through:

• insensible loss
• urine
• stool
• gastric
• blood loss
• dialysis

So, in the end the fluid balance is a more complex concept which can be summarized by the folowing formula:




In which there are

• i to j intake parameters, 
• and k to l output parameters
• to is the end date time of the interval and from is the start of the interval, so
• the interval = to - from, i.e. the time difference between from and to
• BSA is the body surface area
• and temp is the body temperature with a minimum of 37

This formula still simplifies the insensible loss by assuming a constant temperature during the interval.