Discussion

Parenteral nutrition (PN) is used routinely to provide nutrients and energy to critically ill horses. It can be used minimally by adding glucose to maintenance fluids or as complete nutritional support by using glucose, lipids and amino acids. The main issue with providing PN to horses is cost and the need for a high level of nursing support and monitoring. Some indications for PN include horses and foals are:
- Inappetance
- Inability to eat due to trauma
- Severe diarrhoea
- Anterior enteritis
- Ileus
- Severe entercolitis
- Septic neonates
- Premature neonates
- Post operative colics
- Oesophageal or pharyngeal dysfuntion

Energy requirements:
In human and animal studies it has been shown that the resting energy expenditure requirements (REE) of a sick patient are higher than that of a healthy patient. In severe trauma, energy requirements for humans and animals can be 1.3 - 1.4 times higher, and animals with sepsis or major burns up to 1.4 - 1.7 times higher compared to in resting state (Geor 2001). There are little data available for sick horses but the REE of a healthy horse can be estimated using the Pagan and Hintz (1986) formula:
REE = 21 kcal x bwt in kg = ? + 975 kcal = daily kcal requirement.
This can then be multiplied by 1.3 to 1.7 to accommodate sickness. For example, a 500 kg critically ill horse the calculation could be:
21 x 500 = 10500 + 975 = 11475 x 1.3 = 14917 kcal/day
The REE in foals is further complicated by their growth rate. Ousey et al. (1996, 1997) found that approximately 50% of a healthy foals estimated 120 - 150 kcal/kg bwt/day is used for growth and the rest of its basal metabolic needs, thermoregulation and activity. However Jose-Cunilleras et al. (2011) found the energy expenditure of critically ill neonate foals was in fact lower than healthy foals at 49.5 plus/minus 2.1 kcal/kg bwt/day. This result was similar to previous studies carried out on fewer numbers of critically ill foals. Based on their results, Jose-Cunilleras et al. (2011) state that 50 kcal/kg bwt/day can be used as the maintenance energy requirements for sick neonate foals. Also during this study they found that the critically ill foals that survived had a similar REE as the control foals before going home. This would then suggest that the critically ill foals, REE is increasing as their health improves, but at what rate?

PN recipes:
There are many different PN recipes that can be used and, as discussed above, rates of kcal to be delivered vary, but the ones that are used regularly in my workplace are as follows:
For a foal weighing 50 kg to provide 62 kcal/kg/day at a max rate of 96 ml/h =
Glucose 50%: 10 g/kg/day = 500 g (1 l) Aminoven 25: 2 g/kg/day = 100 g (666 ml) Intralipids 10%: 1 g/kg/day = 500 g (500 ml)

For a horse weighing 500 kg to provide 19 kcal/kg/day at a rate of 335 ml/h =
Glucose 50%: 1 g/kg/day = 500 g (1 l) Aminoven 25 =
For a horse weighing 500 kg to provide 16 - 32 kcal/kg/day given at 250 - 500 ml/h = Glucose 50%: 1 g/kg/day = 500 g (1 l) Aminoven 25 =
Also, as mentioned previously, glucose can be added to the horse's maintenance fluids to provide energy.
The above PN fluids will be supported with other fluids, electrolytes and possibly food depending on the patient's condition.

What you need:
The equipment that you use to provide PN to patients can be quite costly. The list below is a list of essentials, that can be added to later;
- IV catheter
- Freka Mix bags
- Fluid pump/accurate drip counter
- Fluid giving sets (that are compatible with the fluid pump)
- 50% glucose
- Aminoven 25
- Intralipid 10%
- Flow hood/clean working area
- Glucose monitor
- 24 h nursing care/dedicated owner or vet
- Appropriate sized stable with something to hang fluids from

Monitoring:
- Physical findings especially respiratory and neurological status
- Demeanour
- Temperature
- Blood glucose every 4 - 6 h during initial increasing phase then every 12 - 24 h
- Urine (for glucose)
- Packed cell volume/total protein
- Electrolytes
- Serum triglycerides
- Arterial or venous pH
- Weight - daily if possible
- IV catheter at least twice daily
- Fluids at the correct rate
- Any other tests related to the horses specific condition.

Parenteral nutrition can be very costly, so bear this in mind when considering its use. Horses will need to be weaned on, and off, the mixture slowly, checking blood glucose and/or urine glucose regularly. If they are hyperglycaemic it may be necessary to start the patient on insulin, which again can be expensive. Insulin can be given as a controlled rate infusion (CRI) or as a twice daily bolus. In most severely ill patients CRI is recommended.
Aseptic catheter placement is essential and strict catheter care is a must afterwards. Aseptic technique must also be used when making up the mixture, and lipids should be added last to prevent the emulsion from being disrupted by extremes of pH (Bercier 2003). The mixture should be kept in the fridge when not being used, and some people suggest blacking out the bag so that light does not degrade the amino acids in the mixture, although I have never done this. Lipids will grow harmful bacteria very easily so lines should be changed regularly, being careful not to introduce air into the mixture, and other drugs should never be injected through the same line. For this reason a double lumen catheter is ideal when providing lipids in a mixture.