I can usually tell with a few seconds of echocardiography whether heart failure is the cause of the effusion.
However I believe that this patient needs a thoracocentesis before any diagnostics and would give the sedation even before the flash echo.
Many veterinarians are apprehensive about sedating an animal with a potential heart problem, particularly one that is dyspneic.There is wisdom in this viewpoint and, let's face it, you may not know on the spot whether dyspnea is due to cardiac disease or a primary respiratory condition.However I don't recommend it at typical sedative dosages for dogs with severe heart disease (and neither do anesthesiologists) because of potential for hypotension and, in my experience, arrhythmogenesis.As a past EKG interpreter for IDEXX/Cardio Pet, I would say one of the most common anesthetic misadventures veterinarians experience is to give an alpha agonist (xylazine or medetomadine) to any patient other than a young, healthy one.Nevertheless morphine has long been a staple therapy for people presented with congestive heart failure (although people are commonly intubated for assisted respiration as well).
I don't know if the decreased respiratory drive may actually be an advantage, decreasing the stress associated with the patient's perceived need for oxygen.
While there is always an exception to the rule, appropriate sedation is my preference for handling these patients, hands down! before radiographs, IV catheter, thoracocentesis, echocardiogram, etc.
Obviously I would prefer for the dyspneic patient to receive oxygen before, during, and after all interventions, but you've probably noted that some patients will struggle even if you are trying to blow some oxygen in their face; they may need sedation before you can even examine them.
Rule #1: Never perform diagnostics or procedures on a critically dyspneic patient unnecessarily.
You should have a clear idea in mind as to when risk exceeds benefit and be ready to abandon a procedure. A common clinical problem, for example, is dyspnea secondary to pleural effusion in a cat.
This is also my choice for pre-anesthesia in cardiac patients; more on this in another article.