Dexmedetomidine & Cardiac Silhouette Size

Radiographic findings of cardiac silhouette can be subjectively assessed or objectively evaluated using the vertebral heart scoring method. Cardiac silhouette enlargement is often present in cats with cardiac disease, and vertebral heart scores have been shown to be higher in cats with cardiac disorders and left-sided cardiac disease as compared with normal controls. Commonly used as sedatives, α-2 agonists can cause numerous cardiovascular effects (eg, decreased cardiac output, bradycardia, peripheral vasoconstriction, transient hypertension followed by hypotension).

Administering dexmedetomidine, an α-2 agonist, for sedation for radiographs is common. This study sought to determine if dexmedetomidine (40 mcg/kg IM) administration and reversal in healthy adult cats (n = 14) are associated with a change in radiographic cardiac silhouette size, to quantify and compare changes to established reference intervals, and to assess interobserver agreement between 2 observers of different experience levels. 

The results indicated that after administration of the studied dexmedetomidine dose in healthy cats, the radiographic cardiac silhouette size can increase above reference intervals for the vertebral heart score on the right lateral view and the percentage width on the ventrodorsal and dorsoventral projections. Enlargement may persist following reversal. 

The authors concluded that dexmedetomidine administration should be a differential diagnosis for cardiomegaly in healthy adult cats sedated with dexmedetomidine at 40 mcg/kg IM for thoracic radiographs. If cardiomegaly is observed after dexmedetomidine administration, radiographs should be repeated using an alternate sedative or without sedation. 

It is common knowledge that dexmedetomidine is contraindicated in a patient with concurrent cardiac disease. Usually, concurrent cardiac disease is determined by the presence of a heart murmur, arrhythmia, or clinical signs. However, cats can sometimes have “silent” disease in which no murmur or arrhythmia can be detected; this may cause clinicians to believe it safe to administer a drug such as dexmedetomidine for sedation. The risk for cardiac enlargement from an α-2 agonist influencing a practitioner’s assessment of cardiac disease (without clinical signs) will affect only a minority of patients—yet it should not be ignored.—Heather Troyer, DVM, DABVP, CVA, CVPP