Don’t Skip a Beat: Anesthesia for Cardiac Patients Made Easy
GIANLUCA BINI, DVM MRCVS DACVAA
Most general anesthetics can affect the cardiovascular system. A study from Carter et al. showed that there is minimal impact on the anesthetic risk in patients with heart disease as long as the anesthetic event occurs under optimal conditions. 1
Most commonly a murmur is found during physical examination or reported while collecting the patient’s history, ideally the murmur should be investigated and x-rays as well as an echocardiogram should be performed. When a complete workup is not possible, the physical exam findings can help narrow the differential as much as possible and, help determine whether the murmur is likely pathological or not. A good resource for a practical stepwise approach to differentiate them is found in a 2015 paper from Cote et al. 2
In cats the presence and/or severity of the murmur does not necessarily correlate with the presence and/or severity of the disease, some biomarkers like plasma NT-proBNP could be a useful additional test, as this is usually elevated in cats with severe disease. Recent studies found that heart disease was present only in 22% to 31% of cats with a murmur. The most common heart disease in cats is hypertrophic cardiomyopathy, which commonly manifest as primary concentric left ventricular hypertrophy, this reduces the filling volume available in the left ventricle. Additionally, secondary left atrial dilatation may occur as the disease progresses.
On the other hand, the most common heart disease in dogs is mitral valve insufficiency, this can be either due to dysplasia or degenerative changes. The intensity of the murmur may not correlate with the degree of disease either and, usually no clinical signs are seen until they develop heart failure. Congestive heart failure clinical signs include hacking cough, elevated respiratory rate and tiredness. Anesthesia of patients with heart failure should only be performed in a specialized setting and is beyond the scope of this presentation.
CARDIAC OUTPUT AND PERFUSION
Mean arterial pressure is a function of cardiac output and systemic vascular resistance (MAP = CO * SVR). Cardiac output is the amount of blood the heart pumps every minute and its unit of measure is liters per minute. Cardiac output is a function of heart rate and stroke volume (CO = HR * SV). Oversimplifying the stroke volume is affected by:
Preload = the amount of blood in the ventricle before ejection happens, which is related to the venous return
Afterload = the amount of pressure the heart has to pump against
Contractility = how hard the heart contracts
Because we need to maintain an adequate blood pressure to maintain a good perfusion, when we anesthetize a patient with heart disease we need to keep in mind our goals for maintaining cardiac output.
ANESTHETIC GOALS FOR DOGS AND CATS
In cats with hypertrophic cardiomyopathy the goals should be to decrease contractility and heart rate, while also increasing afterload and optimizing preload, especially if the patient has hypertrophic obstructive cardiomyopathy. 5 The latter is present in more than 50% of the cats with hypertrophic cardiomyopathy, in this case a mitral valve leaflet is occluding the aortic outflow tract during systole.
Decreasing their heart rate gives more time to the ventricle to fill appropriately and therefore help improving stroke volume. Dehydration should be corrected in these patients, without overloading them. Hearts suffering from hypertrophic cardiomyopathy have more than enough contractility, decreasing it may help reduce the systolic anterior motion of the mitral valve, especially in patients with hypertrophic obstructive cardiomyopathy, thus facilitating forward blood flow. Increasing the pressure after the heart (afterload) will keep the mitral valve leaflet away from the aortic outflow tract in patients with hypertrophic obstructive cardiomyopathy hence facilitating forward flow.
Avoiding drugs that increase heart rate (e.g. ketamine) or acepromazine which decrease afterload could be helpful in these patients, additionally using low doses of dexmedetomidine increasing afterload and causing bradycardia may be beneficial.
In dogs with mitral valve insufficiency the goals should be to increase contractility, maintain a normal heart rate, while also decreasing afterload and optimizing preload. 6
Patients with mitral valve insufficiency could be on diuretics, which could cause dehydration, this should be corrected, but care should be taken in avoiding to fluid overload the patient. Contractility in these patients is usually decreased, if hypotension occurs consider the use of a positive inotrope (e.g. dobutamine or dopamine). Decreasing the amount of pressure the heart has to pump against (afterload) will facilitate forward blood flow, this can be achieved by using acepromazine in premedication and avoiding alpha 2-agonists drugs like dexmedetomidine. Increasing their heart rate may decrease the amount of blood being regurgitated back in the left atria, on the other hand tachycardia should be avoided, as increasing the myocardial work can result in myocardial ischemia in these patients due to a mismatch between myocardial mass and its perfusion.
CURRENT MEDICATIONS – What shall I do?
ACE-inhibitors (or angiotensin receptor blockers) -> Skip the morning dose
Amlodipine -> Skip the morning dose
Pimobendan -> continue as normal
Diltiazem -> continue as normal
Atenolol-> continue as normal
Clopidogrel -> discontinue depending on the risk of bleeding (a risk/benefit assessment should be performed by the veterinarian on a case by case basis)
Other useful information regarding medications to continue/discontinue before anesthesia can be found in the 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats.
References
1. Carter, J. E., Motsinger-Reif, A. A., Krug, W. V., & Keene, B. W. (2017). The Effect of Heart Disease on Anesthetic Complications During Routine Dental Procedures in Dogs. Journal of the American Animal Hospital Association, 53(4), 206–213. https://doi.org/10.5326/JAAHA-MS-6512
2. Côté, E., Edwards, N. J., Ettinger, S. J., Fuentes, V. L., MacDonald, K. A., Scansen, B. A., Sisson, D. D., Abbott, J. A., & Working Group of the American College of Veterinary Internal Medicine Specialty of Cardiology on Incidentally Detected Heart Murmurs (2015). Management of incidentally detected heart murmurs in dogs and cats. Journal of the American Veterinary Medical Association, 246(10), 1076–1088. https://doi.org/10.2460/javma.246.10.1076
3. Drourr L. (2010). Heart disease in cat presented to a teaching hospital. ACVIM Forum Proceedings.
4. 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. https://doi.org/10.2460/javma.234.11.1398
5. Sato Boku, A., Morita, M., So, M., Tamura, T., Sano, F., Shibuya, Y., Harada, J., & Sobue, K. (2018). General Anesthetic Management of a Patient With Hypertrophic Cardiomyopathy for Oral Surgery: Did Digitalis Contribute to Bradycardia?. Anesthesia progress, 65(3), 192–196. https://doi.org/10.2344/anpr-65-03-12
6. Holmes, K., Gibbison, b., Vohra, H.A. (2017). Mitral valve and mitral valve disease, BJA Education, 17(1), 1–9. https://doi.org/10.1093/bjaed/mkw032
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