Study explores use of calcium in resuscitation fluids in horses

Questions remain around the addition of calcium to resuscitation fluids in horses, after a study involving endurance horses in Australia.

The addition of calcium to resuscitation fluids is a common practice in horses, but studies evaluating the effects of calcium supplementation are limited, Langdon Fielding and his fellow researchers noted.

In healthy horses, decreases in heart rate and changes in serum electrolyte concentrations have been reported.

The study team set out to determine whether calcium gluconate administration at a rate of 0.4 milligrams per kilogram per minute (mg/kg/min) to eliminated endurance horses with metabolic problems would affect heart rate, gastrointestinal sounds, and serum electrolyte concentrations.

Their study, reported in the Journal of Veterinary Internal Medicine, was centered on the Tevis Cup 100-mile (160km) endurance ride. Any horses eliminated from the ride for metabolic problems and requiring intravenous fluid therapy were eligible.

Sixteen horses were randomly assigned to receive either 0.4 mg/kg/min of calcium (23% calcium gluconate solution) over 1 hour diluted in 10 litres of a non-calcium-containing isotonic crystalloid, or 10 litres of a non-calcium-containing isotonic crystalloid.

Those giving the fluids did not know whether they were giving calcium or not.

Blood samples were collected and physical examinations performed before and after treatment. Heart rates were recorded every 15 minutes during fluid administration.

Results from the two groups were then compared.

Calcium was associated with lower heart rates 45 minutes after starting the infusion, the study team reported.

Gastrointestinal sounds were less likely to improve in the calcium group compared with the control group, the researchers found.

They also noted an increase in plasma phosphorus concentration in connection with calcium administration.

In conclusion, the authors said their results suggest that 0.4 mg/kg/min of calcium in the form of 23% calcium gluconate can be infused intravenously into endurance horses requiring emergency care as an addition to 10L of isotonic crystalloids.

However, higher than normal concentrations of calcium are achieved, as well as increases in plasma phosphorus concentrations, the effects of which require further study.

Heart rates would be expected to decrease by about 10 beats per minute. Improvements seen in muscle enzyme activities and negative effects on gastrointestinal sounds require further research.

"All of these findings may be dose dependent," they said.

The authors noted that high calcium levels in the blood have been associated with a slow heart rate in many case reports or smaller studies. However, there is not widespread agreement on the mechanism.

"Previous reports in horses have shown an improvement in cardiac index (despite decreases in heart rate), which could have resulted in improved perfusion with subsequent improvement in heart rate in our study.

"Although not significant, lactate concentrations improved with calcium administration which might be consistent with an improvement in perfusion."

Improvements in cardiac index and perfusion theoretically could decrease the need for additional intravenous fluids during treatment, they said.

Another possible mechanism for the development of a slow heart rate after calcium administration could be related to calcium-mediated vasoconstriction inducing hypertension and reflex bradycardia.

Future studies could evaluate changes in blood pressure associated with calcium administration to clinical horses, they said.

High levels of calcium in the blood also may cause a slow beat by its direct effects on the heart, they said.

The finding that gastrointestinal sounds improved more in the control group than in the group that received calcium was unexpected, they said.

The study team had considered that calcium supplementation would likely improve gastrointestinal function because of the potential effects of calcium on gastrointestinal motility.

Further research is needed, they said, but this finding may represent a potential reason to avoid calcium supplementation in the resuscitation fluids of horses with normal calcium levels in their blood, especially at the higher doses used in the study.

Calcium, they said, plays a role in the vasoconstriction of vessels feeding the abdominal organs, which might lead to decreased gastrointestinal sounds.

The researchers said that, in general, the horses in the study did not have low calcium levels in their blood (hypocalcemia).

"Decreased calcium concentrations have been observed in endurance horses during competition, but development of hypocalcemia is variable.

"If the study group horses had been more hypocalcemic, it may have been possible to observe more improvement in clinical and biochemical variables.

"Similarly, the horses in our study may only have had mild to moderate clinical exhaustion based on the clinical and laboratory findings. If more severely exhausted horses had been included, the effects of calcium supplementation could have been more substantial."

The study team comprised Fielding, Emma Deane, Dustin Major, Jennifer Mayer and Juliette Love, all with the Loomis Basin Equine Medical Center in Penryn, California; Michael Peralez, with Foothill Equine in Arcadia, California; and Gary Magdesian, with the Department of Medicine and Epidemiology, part of the School of Veterinary Medicine with the University of California, Davis, California.

Fielding, CL, Deane, EL, Major, DS, et al. Effects of calcium supplementation to resuscitation fluids in endurance horses: A randomized, blinded, clinical trial. J Vet Intern Med. 2023; 1- 7. doi:10.1111/jvim.16715

The study, published under a Creative Commons License, can be read here.