Ar­rhyth­mias can also progress to con­ges­tive heart fail­ure, char­ac­ter­ized by cough­ing, ac­cu­mu­la­tion of fluid in the chest and episodes of faint­ing. How­ever, Frosty had never shown any hint of any of th­ese signs.

EQUUS - - Eq Case Report -

tend to de­velop slower heart rates while at rest, but horses do it a bit dif­fer­ently. Their heart rates are nat­u­rally slow, so as they get fit­ter they ap­pear to de­velop th­ese ar­rhyth­mias as a way of reg­u­lat­ing blood flow by skip­ping beats.”

John­son says that an ex­pe­ri­enced vet­eri­nar­ian can usu­ally iden­tify phys­i­o­log­i­cal ar­rhyth­mias sim­ply by lis­ten­ing, but to be cer­tain, the test is to ex­er­cise the horse. “Ar­rhyth­mias that are purely phys­i­o­log­i­cal and con­sid­ered nor­mal re­solve them­selves when the horse’s heart rate in­creases through ex­er­cise or ex­cite­ment,” he says. “At times when it’s crit­i­cal for the heart to be pump­ing cor­rectly, it does.”

Just as the re­fer­ring vet­eri­nar­ian had done, the Mis­souri team worked Frosty hard on the longe, then lis­tened to her heart again. Her ar­rhyth­mia was still there.

Next, the team hooked up an elec­tro­car­dio­gram (EKG) to gen­er­ate a graphic print­out of the elec­tri­cal ac­tiv­ity controllin­g Frosty’s heart­beats. The read­out en­abled the team to pin­point her di­ag­no­sis to “in­ter­mit­tent pe­ri­ods of high grade 2nd de­gree block and 3rd de­gree block.” In other words, Frosty’s ven­tri­cles were con­sis­tently re­ceiv­ing de­layed im­pulses from the up­per cham­bers, and some­times those im­pulses didn’t come at all. The EKG also showed ev­i­dence of “ven­tric­u­lar es­cape,” a cop­ing mech­a­nism that causes the ven­tri­cles to gen­er­ate elec­tri­cal im­pulses on their own.

In most cases, a horse with this type of ar­rhyth­mia would be ex­tremely ex­er­cise-in­tol­er­ant. “Th­ese horses will lose steam, not main­tain power and fatigue quickly,” says John­son. Th­ese ar­rhyth­mias can also progress to con­ges­tive heart fail­ure, char­ac­ter­ized by cough­ing, ac­cu­mu­la­tion of fluid in the chest and episodes of faint­ing. How­ever, Frosty had never shown any hint of any of th­ese signs.

The team then per­formed an echocar­dio­graph, to look at the mare’s heart via ul­tra­sound, but they found no struc­tural ab­nor­mal­i­ties that would ac­count for the ar­rhyth­mia. They also drew blood to look for un­der­ly­ing dis­ease or other in­flam­ma­tory mark­ers that can in­di­cate heart dis­ease--but again, Frosty ap­peared to be com­pletely healthy.

Still, the vet­eri­nar­i­ans rec­om­mended a two-week course of the cor­ti­cos­teroid pred­nisolone plus an omega-3 fatty acid sup­ple­ment and vi­ta­min E as an­ti­in­flam­ma­to­ries. That, they said, would ad­dress any in­flam­ma­tion that might be the cause of the prob­lem. They also asked McLean to rest the mare com­pletely, with no forced ex­er­cise that could, in the­ory at least, lead to her faint­ing and in­jur­ing her­self or a rider. They also wanted to see Frosty back at the univer­sity for a recheck in four to six weeks.

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