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The Cat with a Murmur: Assessing Anesthesia Risk

Kitty Poppins is a 13 year old FS DSH who needs several urgent dental extractions. She’s a bit saucy but on physical exam you hear a new 4/6 left parasternal systolic murmur. Unfortunately, like everyone else right now, the local cardiologist is booked out for months, and you feel Kitty’s dental disease can’t wait.


I will cooperate with you - but only for so long....

Is it safe to anesthetize her? How can we figure out more about her cardiovascular status? And without detailed information from an echocardiogram about her cardiovascular function, how can we make this anesthetic event safe?


In some ways, this last is a bit of loaded question because anesthesia always presents risk - even to the completely healthy patient - and it is impossible to make that risk zero. However, the best way to mitigate the risks of anesthesia is to monitor and manage the patient’s physiology for them while they can’t do it on their own. Good monitoring and supportive care are the keys to successful anesthetic outcomes. To that end, we need to figure out what might be going on with Kitty and decide if your practice setting has the resources to provide the right level of monitoring and supportive care.


When I am planning an anesthetic event for a patient with cardiac disease, I think through 3 questions:


1) What is the (likely) disease process? First, we have to figure out if the murmur represents a clinically relevant change in the size or function of the heart. That is, is there disease that is advanced enough that it would either change anesthetic management or prevent me from wanting to anesthetize the patient altogether? An echocardiogram would be the clinical gold standard but there’s neither enough money nor veterinary cardiologists in the world to screen every cat with a murmur prior to general anesthesia. Most, but not all cats, with an acquired murmur with the point of maximal intensity in the left parasternal area have hypertrophic cardiomyopathy (HCM). To complicate things, some cats with murmurs may not have any heart disease at all and some cats with heart disease have no murmur (Paige et al. 2009; Payne et al. 2015)! Also, it is completely possible for you to press too hard on the chest and cause turbulent blood flow and a murmur since the chest wall of cats is so compliant (Ferasin et al. 2020; Howell et al. 2022).

2) What is the associated pathophysiology and how advanced is it?

HCM in the cat results in a thickened ventricle that contracts normally but does not relax well. This diastolic dysfunction means that pressure rises above normal on the left side of the heart at low filling volumes. The increase in pressure causes the left atrium to stretch and, eventually, results in pulmonary edema or pleural effusion. The thickened ventricular wall also allows for part of the mitral valve to be pulled into the sub-aortic region of the outflow tract, obstructing cardiac output. Left ventricular outflow tract obstruction (LVOTO) is often the cause of the murmur in cats with HCM and this is worsened by increases in contractility (such as when the cat is sympathetically stimulated by a visit to the clinic!).


If we can’t get an echocardiogram on demand and this patient needs to be anesthetized now, several pieces of information can be combined to help give you a pretty good idea of how advanced the disease might be and how risky it is to proceed: Is the patient physically fit in your estimation? Are they exercise tolerant (ok, ok I get it – we’re talking about a cat here!)? A patient who has "slowed down" might be one who actually doesn't have a lot of cardiovascular reserve. Would you rate the patient an ASA I or II based on your physical exam or do they seem less fit overall?

Then, how about some diagnostics? Radiographs can tell you *something* about the size of the heart. At least, it can rule out a huge left atrium, though there can be quite a bit of hypertrophic change in the left ventricle that cannot be detected on radiographs.

You could also consider an NT-proBNP test. A “normal” or negative result does not rule out cardiac disease, but it makes moderate to severe HCM unlikely in the cat with a murmur (sensitivity is around 85 – 89%). If the test is elevated or positive, it does not mean that the cat definitely has HCM (these tests are around 80 – 85% specific) but it may change the discussion you have with the owner about associated risks and desirability of waiting for an echocardiogram. Here is a recent commentary about the value of screening cats for HCM with a NT-proBNP test.

3) What do I need to do to maintain homeostasis for this patient?

To maintain forward blood flow and oxygen delivery in the patient with HCM, the anesthetist’s goals should include avoiding increases in contractility or heart rate, avoiding large reductions in afterload (this can worsen LVOTO by increasing the subaortic gradient), and maintaining both a normal sinus rhythm and adequate ventricular filling. In order to avoid increases in contractility and heart rate, we want to avoid stressing Kitty as much as possible (oral anxiolytics like gabapentin can be extremely helpful) and to make sure any pain is managed appropriately. We need to keep an eye on the ECG and be prepared to intervene if malignant arrhythmias occur. We need to make sure she is volume-replete (intravascular space) and hydrated (intracellular space) but we also have to be careful to avoid excessive high sodium fluid administration. Importantly, we want to use a balanced anesthetic protocol that will avoid high doses of drugs such as inhalants that can cause profound vasodilation and reductions in afterload. Finally, we need to have plan for managing blood pressure if the usual steps haven’t improved things. This may mean discontinuing the anesthetic event or using sympathomimetic amines such as dopamine or norepinephrine.


Here is Kitty Poppins’ snap proBNP test and radiographs of her chest.



VD and lateral radiographs of Kitty Poppins' thorax. That cardiac silhouette looks pretty normal to me, as do the pulmonary vessels.


With a normal snap proBNP and normal looking chest radiographs, we can feel fairly certain that moderate to severe HCM is not present in this cat and, with a little attention to detail, a well-managed general anesthetic event can be safely conducted.





The important thing at this point is to make sure you and the owner are on the same page and communicating well. My conversation would go something like this:

“Mr. Poppins, anesthesia always carries some risk, and though we have done a pretty good job of ruling out significant heart disease for Kitty at this time, we will still be working a little bit in the dark since I don’t know with absolute certainty that Kitty’s heart is normal. We are going to do everything we can to keep Kitty safe during general anesthesia but something unexpected may occur. This could even include something terrible like death, during or after the procedure. It also could happen that we have to stop the procedure and wake Kitty up. I want to make sure you understand all that and have the opportunity to ask any questions you have before we proceed. Other options may include….”.


In a future post, we’ll discuss in detail drug choices and perianesthetic management for Kitty Poppins and also for cats with documented hypertrophic cardiomyopathy.


References:

Ferasin, L., Ferasin, H., & Kilkenny, E. (2020). Heart murmurs in apparently healthy cats caused by iatrogenic dynamic right ventricular outflow tract obstruction. Journal of veterinary internal medicine, 34(3), 1102–1107. https://doi.org/10.1111/jvim.15774


Howell, K. L., Ferasin, L., Walls, A., & Smith, N. (2022). Prevalence of iatrogenic heart murmurs in a population of apparently healthy cats. The Journal of small animal practice, 63(8), 597–602. https://doi.org/10.1111/jsap.13498


Machen, M. C., Oyama, M. A., Gordon, S. G., Rush, J. E., Achen, S. E., Stepien, R. L., Fox, P. R., Saunders, A. B., Cunningham, S. M., Lee, P. M., & Kellihan, H. B. (2014). Multi-centered investigation of a point-of-care NT-proBNP ELISA assay to detect moderate to severe occult (pre-clinical) feline heart disease in cats referred for cardiac evaluation. Journal of veterinary cardiology : the official journal of the European Society of Veterinary Cardiology, 16(4), 245–255.


Paige, C. F., Abbott, J. A., Elvinger, F., & Pyle, R. L. (2009). Prevalence of cardiomyopathy in apparently healthy cats. Journal of the American Veterinary Medical Association, 234(11), 1398–1403.


Payne, J. R., Brodbelt, D. C., & Luis Fuentes, V. (2015). Cardiomyopathy prevalence in 780 apparently healthy cats in rehoming centres (the CatScan study). Journal of veterinary cardiology : the official journal of the European Society of Veterinary Cardiology, 17 Suppl 1, S244–S257.





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