Making Good Choices in Veterinary Anesthesia: Affairs of the Heart
By Elana Rybak, DVM, cVMA
Our patient is Henry, a nine-year-old male neutered mixed breed dog that presents for neck pain. On his exam it is noted that he has a grade 3/6 left systolic murmur. The clients want to move forward with an MRI of his cervical spine, but they decline a cardiology consult prior to anesthesia. We now have to come up with a plan to safely anesthetize this patient for imaging.
First thing’s first: a good history!
It is very helpful to ask owners if they have noticed any excessive lethargy, difficulty breathing or increased respiratory rate while at rest, exercise intolerance, collapse episodes, or coughing. These may be clues that something more concerning may be going on with the heart.
Second: thoracic radiographs
We routinely take pre-anesthetic thoracic radiographs on our older MRI patients to check for metastases and evaluate the heart and lungs to make sure there is nothing obviously concerning that may cause issues during anesthesia. In a patient with a heart murmur, thoracic radiographs are a great tool to evaluate the heart size and shape, and note if there is any evidence of congestive heart failure.
Additional diagnostics that could be run prior to anesthesia could include checking ProBNP, an EKG, and/or blood pressure.
Henry’s owners have not noted any of the aforementioned cardiac clinical signs, and on radiographs he appears to have a normal to just slightly enlarged heart. The images are otherwise within normal limits.
Henry has also had a full bloodwork panel (CBC/chemistry/electrolytes) to assess organ function, and these test results are acceptable for general anesthesia.
What drugs do we use to anesthetize Henry?
The better question here is probably ‘what drugs shouldn’t we use?’ In general, a left-sided systolic murmur in an older dog is often going to be caused by mitral valve disease (MMVD), and this condition is what we will focus on in this discussion. There are, of course, breed-specific heart diseases and other cardiac conditions that create murmurs that we should also keep in mind (for example pulmonic stenosis in French Bulldogs).
The medication we most commonly avoid in these patients is Dexmedetomidine. This drug causes a profound peripheral vasoconstriction which does two things that are problematic for MMVD patients:
It significantly increases afterload, making the heart work harder to push blood through the constricted vasculature, and
It causes a reflex bradycardia which can further decrease cardiac output and cause hypotension.
There are some exceptions where this drug may be utilized, but in general we do try to refrain from using it in these patients.
Other drugs that should be used with some caution are high dose acepromazine (which can cause deleterious peripheral vasodilation and hypotension) and high-dose ketamine (which can increase workload on the heart via sympathetic stimulation and increased myocardial oxygen demand). We also do not want to give an anticholinergic (atropine or glycopyrrolate) unless it is clinically indicated during anesthesia.
Another consideration during anesthesia is what fluid rate to use. Generally for normotensive, euhydrated MMVD grade B1-B2 canine patients, we use a fluid rate of 2.5-3ml/kg/hr during anesthesia.
So let’s put together our drug protocol:
Henry is painful so using a pure Mu agonist opioid is indicated. Hydromorphone or methadone would both be good choices. Fentanyl is short-acting and we prefer to have something longer-acting on board for painful cases since MRI does take up to an hour or so. A fentanyl CRI could be considered.
Often, an opioid is sufficient for pre-medication to allow pre-oxygenation. If Henry won’t allow us to pre-oxygenate him with just the pain medication on board, small IV doses of an induction agent (propofol or alfaxalone) can be used to get the patient quiet enough to do so.
For induction, our practice almost always co-induces with midazolam and propofol (or alfaxalone). We often do a “midazolam sandwich” where we give a small bolus of our induction agent, then give midazolam (usually 0.2mg/kg), and then additional induction agent as needed to effect. Both propofol or alfaxalone would be acceptable induction agents in this case.
For additional pain management and anesthetic effects, if needed, a ketamine CRI or small low doses (think 0.3-0.5mg/kg) of ketamine can be used to help reduce the need for increasing your gas anesthetic (as long as your patient does not have an arrhythmia).
The final key aspect:
The final key aspect of anesthetizing cardiac patients is vigilant monitoring so that issues can be caught and treated early. These patients often have very stable anesthetic procedures and while a cardiac workup is ideal, these dogs can usually safely be anesthetized with basic diagnostics, careful planning, and close monitoring.
What if sweet Henry’s fractious brother Norman came in with the same presentation?
Sedation starts at home! As long as Norman’s neurologic problem was not emergent, we would send him home with oral sedation and schedule him to come back another day. We usually prescribe trazodone and gabapentin given the night before and morning of the appointment. There are also other “chill” protocols that include melatonin and/or acepromazine (I would avoid ace in this case). We would also send oral Cerenia home to be given the night before the appointment so it is already on board before we give any additional sedation in hospital.
In hospital, IM methadone or hydromorphone (if cerenia is on board), plus IM alfaxalone can be used to sedate for catheter placement (I sometimes prefer hydromorphone if Cerenia is on board as if offers more sedation than methadone). Alfaxalone can be a bit of an issue for larger pets due to volume of administration but it can still be helpful for many patients. You could also consider adding some midazolam or low dose ketamine (as long as the patient does not have an arrhythmia) to the other drugs. (Using midazolam and an opioid alone in a fractious patient will often cause paradoxical excitement, which is the opposite of what you want!) Once the patient has IV access, additional alfaxalone can be titrated IV as needed for things like blood draw/radiographs/etc.