Hypertonic intravenous fluid therapy administration in acutely sick cattle

The development of the rumen and its ecosystem of micro-organisms has not only enabled ruminants to survive on a diet that largely consists of cellulose, but has also provided the animals with a large water reservoir that enables wild living ruminants to go for days without drinking any water and then rehydrate rapidly without any negative effects.

Clearly, this situation would be undesirable for domesticated ruminants – most notably, the high-yielding dairy cow. However, the essential premise remains that domesticated ruminants also have a large water reservoir in their own body that can be called upon in times of need to maintain fluid homeostasis.

Fluid movement

The movement of fluids from the rumen into the body is regulated by the ruminal epithelium. The primary force determining movement of fluids is the difference in osmolarity between the blood perfusing the ruminal epithelium and the rumen fluid; typically, the two fluids are isotonic. It is this phenomenon, along with the osmotic flow of extracellular fluid into the circulating volume, that is utilised by clinicians treating cattle with intravenous hypertonic saline solutions ().

Movement of fluid between the rumen and the circulation also occurs passively after sodium ions are actively absorbed from the forestomachs. This slower, and more sustained fluid-uptake mechanism is used when ruminants are treated with oral fluids that are hypotonic to the blood. Perhaps the situation in which hypertonic intravenous fluid therapy is most commonly used is in acute cases of endotoxaemic shock. These animals typically have a hypocholoraemic, hypokalaemic metabolic alkalosis – not the metabolic acidosis that characterises severe cases of calf scour. Hypertonic intravenous fluid therapy is most appropriate when animals are severely dehydrated (greater than or equal to eight per cent); animals that are estimated to be less dehydrated than this generally benefit from enteral rehydration therapy alone ().

Enteral rehydration has been shown to be more effective, and have a lower risk of complications, in moderately dehydrated animals than intravenous therapy with either isotonic or hypertonic fluids. However, while enteral rehydration is an important adjunct therapy; the severely dehydrated animal requires intravenous fluid therapy. In adult cattle the volume of isotonic fluids required to correct dehydration is large, at up to 60 litres within 24 hours. Unfortunately, if this is to occur successfully it requires highly skilled staff and suitable facilities – both of which are often lacking on farm.

There is some debate in the literature as to the value of hypertonic saline solutions over the equivalent amount of sodium infused rapidly in isotonic saline. While it is accepted that administering fluids too rapidly can cause respiratory and cardiac embarrassment on farm, the argument as to which is most appropriate is, to some extent, still redundant as there are considerable practical and cost implications in administering 60 litres of isotonic solution – compared to two to three litres of hypertonic solution that would be appropriate for a 600kg cow.

Dextran

The hypertonic solutions used in cattle practice are generally saline based. However, solutions based on dextrans can also be used and have considerable therapeutic benefits over simple saline solutions. Most notably, the effects of hypertonic dextran solutions are far less transient.

Dextrans are large glucosebased polymers and act as volume expanders in a similar way to albumin, providing a longeracting osmotic skeleton in the blood to which fluid is drawn. By comparison, the osmotic skeleton provided by Na+ and Cl– ions is transient, lasting just as long as it takes for these permeable ions to be redistributed in the body. While the benefits of dextrans are clear, the cost prohibits their use in most cases involving adult cattle.

The case is less clear cut for youngstock that require far smaller volumes. However, in most situations, hypertonic saline should suffice. While hypertonic solutions can be purchased, they can also be home-made. Hypertonic saline been created by adding 72g of sodium chloride to one litre of water.

Water requirement

It should be emphasised that most animals will drink a large volume of water, during or after completion of treatment with hypertonic fluids. A minimum of 40 litres of water should be available for adult cows to drink after treatment is complete. If a cow fails to drink, a minimum of 20 litres of water must be pumped into the rumen – despite the slight risk of regurgitation and subsequent inhalation pneumonia. Water is more appropriate than oral rehydration solutions in this case, as the greater the osmotic difference between the rumen fluid and the blood perfusing the rumen epithelium, the faster fluid will be absorbed into the circulation. The rapid increase in cardiac preload, combined with a transient decrease in afterload, rapidly increases cardiac output – often resulting in an almost immediate visible clinical improvement.

Hypertonic solutions should not be used intravenously if the clinician is not prepared to ensure a sufficient volume of water enters the rumen after administration. To a certain extent, this begs the question: why not simply provide oral rehydration therapy alone? This is a valid point and is the reason why only severely dehydrated patients are treated in this way.

It is also worth noting that gut stasis and changes in peripheral circulation accompany severe endotoxaemia, and treatment with intravenous fluid circumvents some of the time delay that these pathological changes would place on absorption of fluid from the gastrointestinal tract alone.

While it is quick, cheap and generally effective, hypertonic intravenous therapy is not without risk and has disadvantages when compared to conventional isotonic intravenous therapy. The drawbacks are most pronounced in patients with compromised renal function, which is not surprising, given the general principle that most supportive fluid therapy aims to increase circulating volume and cardiac output to facilitate the effective functioning of the kidney to correct ionic disturbances in the blood. Thus, where possible, the use of hypertonic therapy should be avoided if renal pathology is expected – for example, in cases of acorn poisoning.

Known complications of hypertonic intravenous therapy include hypokalaemia – resulting from a sudden expansion of the extra vascular space, hypernaturaemia and hyperosmolarity. Interestingly, a transient hypokalaemia has been reported from a number of studies, although cardiac dysrhythmias have not. As already mentioned, there is a risk of fluid regurgitation if water has to be pumped into the cow, and if the large volume of water ingested is very cold, haematoglobulinuria may be seen in the calf, but not the adult.

Speed

Attention should also be paid to the speed of administration of hypertonic fluids. Typically, infusion should occur for five to 10 minutes. If administered much slower than this, the body will be able to correct the hyperosmolarity homoeostatically before additional fluid is absorbed into the circulation. If administered too fast, cardiac output can decrease, with potentially fatal consequences.

Cardiac output also decreases as the heart rate is prone to decrease if the solution is too hypertonic – so care should be taken if making the solution. Heart rate falls by initiation of the Bezold- Jarish reflex, which involves the stimulation of receptors in the right atrium, the medullary vagus centre and the vagus nerve.

When administered at the correct concentration and rate, mean arterial blood pressure typically decreases. The mechanisms by which this occurs are complicated. However, it is thought that local effects of the hyperosmotic solution on the smooth muscle cause arterial vasodilatation, and consequent reduction in cardiac afterload.

A similar pattern of vasodilatation is observed in the capillary beds, and so a decreased resistance of blood flow to the organs – notably, the heart and brain – is observed. However, response vasodilatation is transient and generally thought to last only as long as the infusion. The animal’s body rapidly corrects for the hyperosmolarity of blood by increasing circulatory volume.

Summary

Hypertonic solutions have an important role in the treatment of severely dehydrated and endotoxaemic animals. Hypertonic dextran solutions typically offer the best clinical response, but hypertonic saline solutions are far cheaper, and easier to make on farm, making them the practical treatment of choice in many cases. The administration of highly concentrated solutions is not without potentially fatal risks. Therefore, attention to detail, concentration and speed of administration are important if deleterious consequences are to be avoided.

Equally important to a successful clinical outcome is the importance of the oral ingestion of large quantities of water after the fluids have been administered. If the clinician is not prepared to do this if necessary, the therapy is contraindicated.

Irrespective of the potential pitfalls, the administration of hypertonic solutions intravenously to acutely sick cattle is a clinically and cost-effective treatment protocol with the capacity to save many lives.