It is common in veterinary critical care for patients to present with multiple serum chemistry abnormalities.¹ This makes it very difficult to choose the correct fluid for the individual patient. For this reason, fluid solutions may need to be altered in order to correct specific abnormalities.¹ It is sometimes necessary to replace glucose or certain electrolytes over a period of time making use of a constant-rate infusion (CRI). The substance in question is added to resuscitation fluid at a certain concentration and infused as part of the daily fluid. In other cases, certain drugs need to be administered as a CRI. This is the case in drugs with short half-lives, such as dobutamine or epinephrine. This paper briefly deals with the mathematics behind adding substances to fluid therapy protocols. Calculations will be demonstrated during the lecture.

Glucose

When a patient is debilitated with concurrent anorexia, a dextrose supplement is required due to the increased metabolic requirements associated with fever and sepsis. Patients with end-stage liver disease and renal disease could be hypoglycaemic due to gluconeogenic disorders. The dextrose added to the fluids is not aimed at the provision of daily calories,² but as an energy source for the brain and white blood cells. If the glucose level is below 3.3 mmol/l, addition of dextrose is indicated to a 2.5% concentration. If hypoglycaemia persists, a 5% solution is indicated.¹ This is especially important in toy breeds and paediatric patients with vomiting/diarrhoea. Glucose reserves are very low in neonates and gluconeogenesis is not very effective, resulting in rapid development of hypoglycaemia if anorexic, febrile or exposed to cold. Supplementation of dextrose to 2.5% is essential to maintain euglycaemia.

Supplementation tables

Potassium

Potassium is the most important extracellular anion in the critically ill animal.²

Hypokalaemia occurs with fluid losses via the gastrointestinal tract or urinary tract, anorexia, icterus, treated diabetic ketoacidosis (DKA) and chronic renal failure (CRF).

Potassium is cardiotoxic and in the absence of laboratory results potassium must not be supplemented in cases with urinary tract obstruction, renal failure or Addison's disease.

Ringer's lactate and Plasmalyte B contain 4 mmol/l, so potassium will need to be added to these fluids to avoid dilutional hypokalaemia.² Potassium can also be added to fluids that are being administered intraperitoneally or subcutaneously at a maximum concentration of 30 mmol/l.

Potassium can be supplemented via one of 4 methods

 Per os potassium gluconate - too slow in some critically hypokalaemic patients

 Bolus - intensive monitoring is required

 Run at the maximum rate and retest

 Run levels off table at 1–2x maintenance and retest

Bolus potassium

 Calculate the potassium difference: Diff = 3 mmol/L - K⁺ patient

 Calculate blood volume: BV = body mass x 8% (cat 6%)

 Calculate plasma volume: PV = BV x 60%

 Calculate K bolus: Bolus (mmol) = diff x PV

 Give over 1 minute while monitoring ECG

The maximum rate at which potassium can be infused is 0.5 mEq/kg/h¹ and must therefore always be administered in the fluids, not by bolus injection.¹

Potassium supplementation

Phosphate

Hypophosphataemia is caused by the refeeding syndrome, insulin therapy in ketoacidotic diabetics, use of aluminium hydroxide and sucralfate and is often seen in sepsis. If hypokalaemia and Hypophosphataemia occur concurrently, potassium can be supplemented using potassium phosphate instead of KCl.¹

Treatment of hypophosphataemia

 SiP below 1 mg/dl requires IV supplementation

 0.01–0.03 mmol/kg over 6 hours IV - in saline

 Orally at 0.5–2 mmol/kg/day

 Monitor 6 hourly

Magnesium

Magnesium usually does not need to be supplemented, but in cases with refractory hypokalaemia, the accompanying hypomagnesaemia should be addressed by supplementing magnesium at 0.375 mmol/kg/day.²

If hypomagnesaemia causes arrhythmias, it can be supplemented at 0.15–0.3 mmol/kg in saline over 15 minutes.

Calcium

Normal serum total calcium levels range from 2.2–2.9 mmol/l. Ionized calcium values vary between 1–1.2 mmol/l. In most patients, calcium does not need to be supplemented.² In some cases - for example, with puerperal tetany - calcium may need supplementation.²

Calcium is supplemented as calcium gluconate at 0.5–1.5 ml/kg of a 10% solution given slowly over 10–30 minutes while monitoring for bradycardia.

Calcium can also be added to any fluid therapy as a CRI at 10 mg/kg/h. The CRI should be discontinued if bradycardia develops.

Drugs

Various drugs can be given as CRI. Examples of these drugs include medications for sedation (barbiturates), analgesia (fentanyl), pressor or inotropic effect (dopamine or dobutamine), antiarrhythmic medications (lignocaine), and specific therapies (insulin).³ CRI allows the drug to be immediately available within the circulation avoiding the delay between its administration and its effect.³ For some drugs it is important to maintain constant plasma and tissue levels in order to optimize the effect.³ This is specifically true for drugs that have very short half-lives, as is the case with fentanyl. Drugs can be given as a pure CRI using infusion syringe pumps.³ Some drugs are added to the intravenous fluids the patient is receiving throughout the day.³ In some cases where an immediate effect is required, a bolus of the drug can be given and then followed by a CRI. Examples will be used during the lecture to demonstrate the practical aspects of calculating the infusion dose of a drug. Online "drug calculators" are also available to eliminate errors in calculations.

It is always important to remember any possible drug interactions.^3,4 For this reason, certain drugs cannot be given as CRI in the same bag of fluid. Certain drugs can also not be added to certain types of fluid. Package inserts of the individual drugs usually indicate any drug interactions that might occur.

A few examples of the most commonly used drugs in CRI solutions at Onderstepoort are listed below. For a complete list of drugs, consult the Textbook of Veterinary Internal Medicine, 6th ed., S.J. Ettinger SJ, Feldman EC. 2005.⁴

References

1.  Davis H, Jensen T, Johnson A, Knowles P, Meyer R, Rucinsky R, et al. 2013 AAHA/AAFP fluid therapy guidelines for dogs and cats. J Am Anim Hosp Assoc. 2013;49:149–59.

2.  Mazzaferro E, Powell LL. Fluid therapy for the emergent small animal patient; crystalloids, colloids and albumin products. Vet Clin North Am Small Anim Pract. 2013;43:721–734.

3.  Hackett T, Hackett E. Constant rate infusions. In: Ettinger S, Feldman E, eds. Textbook of Veterinary Internal Medicine. 7th ed. St. Louis, MO: Saunders Elsevier; 2010:361–363.

4.  Dhupa N. Constant rate infusions. In: Ettinger S, Feldman E, eds. Textbook of Veterinary Internal Medicine. St. Louis, MO: Elsevier Saunders; 2005:544–550.