Plus-Hex CLINICAL
Isotonic solutions are usually used in practice for maintenance. The volume to administer is calculated by using the maintenance calculation:
Bodyweight( kg) × 50 = maintenance volume( ml) per 24 hours
Box 1 shows a worked example for a 12 kg cocker spaniel that requires maintenance fluids. The calculated rate( 25 ml / hour) can then be input into an infusion pump to administer the fluid at the required rate.
Box 1. Example fluid calculation for maintenance fluids in a dog weighing 12 kg.
12 kg × 50 ml / kg = 600 ml in a 24-hour period
The hourly rate is then calculated by dividing the total maintenance volume by 24:
600 ml ÷ 24 hours = 25 ml / hour
In some cases, the VS may instruct an RVN or SVN to place a patient on a fluid rate between two and five times the maintenance rate. This is achieved by calculating the patient ' s maintenance rate and then multiplying this by the rate requested by the VS:
Maintenance requirement × 2, 3, 4 or 5 = volume per 24 hours
Box 2 shows a worked example for a 12 kg cocker spaniel requiring a rate of three times maintenance.
VS to identify the patient ' s percentage dehydration in order to calculate the fluid deficit, as follows:
Weight( kg) × dehydration(%) × 10 = fluid deficit( ml)
Box 3 shows a worked example for a 12 kg cocker spaniel that has presented with 6 % dehydration.
Box 3. Example fluid calculation incorporating a fluid deficit.
12 kg × 6 % × 10 = 720 ml fluid deficit
Add the fluid deficit to the maintenance rate, then divide the total by the number of hours over which the infusion will be given:
Maintenance rate: 12 kg × 50 ml / kg = 600 ml 600 ml maintenance + 720 ml fluid deficit = 1,320 ml
The total volume is divided by 24 to give the hourly rate to be delivered over 24 hours:
1,320 ml ÷ 24 hours = 55 ml / hour
Dehydration levels can be estimated using the patient ' s clinical signs( Table 2). In some instances, the calculated fluid deficit may fall below two or three times the maintenance requirement, for example. In these situations, the fluid rate should be based on two to three times maintenance, rather than the deficit alone, with the higher rate selected where clinically appropriate.
Table 2. Estimated dehydration levels based on clinical signs [ 1 ].
Box 2. Example fluid calculations for 3 × maintenance rate in a dog weighing 12 kg.
12 kg × 50 ml / kg = 600 ml maintenance fluids 600 ml × 3 = 1800 ml in a 24-hour period
The hourly rate is then calculated by dividing the total maintenance volume by 24:
1800 ml ÷ 24 hours = 75 ml / hour
Level Clinical signs < 5 % • No obvious clinical signs
• Increased concentration of urine 5 – 6 % • Slightly delayed skin tent
• Tacky mucous membranes
6 – 8 % • Delayed skin tent
• Dry mucous membranes
• Eyes slightly sunken
Calculating fluid deficits
It is important to include a fluid deficit calculation for patients that are dehydrated. If a patient is placed on a fluid rate calculated without the inclusion of a fluid deficit, their hydration status can take longer to normalise. Veterinary nurses( VNs) should work with the
10 – 12 % • Dry mucous membranes
• Sunken eyes
• Skin tent remains in place
• Protrusion of the third eyelid
Volume 41( 1) • February 2026
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