Updated: Nov 4, 2019
Meet ChillyDog, a 14-year-old male neutered Chihuahua with advanced dental disease, a chronic cough, and a 4/6 left apical systolic heart murmur. Actually, you already know ChillyDog. You see patients like him all the time!
The owner is aware of the heart murmur but is not interested in much of a workup; she does want to get that mouth cleaned up however.
Should you anesthetize ChillyDog for assessment, radiographs, and cleaning? His mouth smells horrible and must be painful but how risky is it to anesthetize him in the face of his heart disease and cough?
The risk vs benefit calculation is - as always - influenced by the monitoring and supportive care you have available.
Anesthesia is risky because it upsets homeostasis. If you can monitor for and manage any physiologic derangements, then you are on the path to safely anesthetizing patients with cardiac disease.
Once you have decided that you can provide adequate monitoring and supportive care, then you have to come up with a perianesthetic management plan.
To do so, I ask myself the following questions:
-What is the (likely) disease and anesthesia-relevant pathophysiology?
-How advanced is the disease?
-What are the hemodynamic goals for maintaining cardiac output and organ perfusion?
-What interventions may be necessary to maintain cardiac output?
What is the cardiac disease and the relevant pathophysiology?
Based on the signalment and murmur location, ChillyDog almost certainly has mitral regurgitation due to myxomatous degeneration of the valve (also called degenerative mitral disease and valvular endocardiosis). ChillyDog could actually be the poster dog for this disease!
Regurgitation through the mitral valve decreases forward stroke volume, reduces systemic blood flow, increases left atrial volume and pressures, and leads to chronic volume overload of the left heart with eccentric left ventricular (LV) hypertrophy. Eventually, the left ventricle and left atrium reach their limit of ability to enlarge and pressure increases are transmitted into pulmonary circulation, resulting in pulmonary venous hypertension, and eventually pulmonary congestion and edema. This increases lung stiffness and can interfere with gas exchange leading to tachypnea, even at rest. Alveolar edema causes respiratory distress and often coughing. However, couging in can also be due to bronchial compression from the left atrium or concurrent airway disease (e.g. bronchitis, tracheal collapse), which are common in small dogs. Interestingly, overt systolic dysfunction of the LV is only present in severe, end-stage mitral valve disease, and even then is uncommon to observe in small-breed dogs with congestive heart failure.
How advanced is the disease?
These are the ACVIM staging guidelines for mitral valve disease (MVD) in dogs. We will have to gather more information, in order to know for certain how advanced the MVD is but ChillyDog's owner is not interested in referral for an echocardiogram - even though it would be a great idea and would tell us if ChillyDog could benefit from pharmacologic treatment now (stage B2). So what can we do gather more information about how advanced the MVD is?
If there were no clinical signs that could possibly be related to heart disease, and the owner was able to ascertain to that the sleeping respiratory rate was under 30 bpm (with <25/minute the upper limit in normal dogs), then I would feel comfortable moving forward with a routine anesthetic approach.
But ChillyDog is coughing, so if possible, I would like to evaluate 2- or 3-view thoracic radiographs.
Mainly, I am interested in the size of the left atrium and the pulmonary vessels. If there is no evidence of left atrial enlargement, the cough is probably from airway disease, and it is likely that you can use a routine anesthetic approach for these individuals. If moderate to severe left atrial enlargement exists – or if there is any suspicion of pulmonary edema - perianesthetic management may be a bit more difficult and the risk:benefit calculation may change. For example, if the dog has sufficient cardiomegaly to fulfill Evaluation of Pimobendan in Dogs with Cardiomegaly (EPIC ) study criteria, it might be reasonable to first establish a dosing plan for pimobendan before scheduling an elective dental procedure.
What are the hemodynamic goals in the face of mitral valve disease (MVD)?
The major hemodynamic goals during anesthesia are to maintain or improve forward blood flow and avoid increases in regurgitant flow through the mitral valve. The factors that determine regurgitant flow are size of the mitral orifice, duration of systole, and the pressure gradient between the left ventricle and atrium.
Classically, the mantra for patients with MVD has been “faster, fuller, vasodilated”, ie:
1) Maintain a high normal heart rate - Bradycardia allows overfilling of the ventricle and increases regurgitant volume.
2) Maintain adequate intravascular volume. But – importantly - avoid fluid overload!!
3) Avoid increases in afterload due to vasoconstriction. Vasodilation improves forward blood flow in dogs with MVD. Conveniently, inhalant anesthetics are great vasodilators!
What interventions may be necessary to maintain cardiac output and tissue oxygen delivery?
Preoperative assessment of volume status and hydration is important and any volume deficits should be replaced prior to anesthesia. On the flip side, make sure to keep an eye on the total dose of high sodium intravenous fluids delivered to the patient. Generally, in the healthy patient with minimal perioperative fluid losses, 20 - 30 mL/kg total is plenty of volume. If the left atrium is enlarged (stage B2 or higher) see below for fluid considerations.
Use anticholinergics to maintain HR as necessary. I tend to try to keep the HR over 100 bpm and certainly over 80 bpm in dogs with MVD.
Avoid alpha2 agonists like dexmedetomidine due to their vasoconstrictive effects. The increase in afterload will worsen the regurgitant volume. In fact, dexmedetomidine can cause mitral regurgitation in normal dogs! To some extent this is dose-dependent and their are certainly dogs with MVD in whom you can use dexmedetomidine without causing a clinical disaster - you just need to select your patient carefully.
Another anesthetic consideration for dogs with MVD is how to manage hypotension. A balanced anesthetic drug plan that allows you to keep the inhalant delivery on the low side will help. Maintaining a high normal heart rate will help. Ensuring adequate volume status will help! But what if you’ve done all these things and the dog is still hypotensive?
In some clinical settings, discontinuing the anesthetic event may be best. Otherwise, pharmacologic management with catecholamines may make sense. Theoretically, you will want to avoid drugs that mainly activate alpha-receptors and cause vasoconstriction, such as phenylephrine. Dobutamine may be your best bet but low doses of ephedrine, dopamine, or even norepinephrine can be used to good effect.
For dogs with stage B2 or higher MVD, there are some other considerations:
-Consider temporarily discontinuing ACE-inhibitors just prior to anesthesia. No consensus exists on this one and outcomes assessment in the human literature is mixed. However, most anesthetists feel that hypotension is more frequent and of greater magnitude when ACE-inhibitors are given in the 24 hours prior to anesthesia.
Continue inotropes (pimobendan), diuretics (furosemide), PDE-3 inhibitors (sildenafil), and chronic vasoactive drugs such as amlodipine or hydralazine. If your patients need these medications to keep their cardiovascular act together normally, they definitely them perianesthetically!
-Be extremely careful with fluid status & keep the total dose delivered in mind, likely less than 10 – 15 mL/kg total volume unless there is significant blood or fluid loss. You may want to consider using low sodium/hypotonic fluids such as 0.45% NaCl that will quickly enter the intracellular space rather than overload the vessels (do not bolus hypotonic fluids). There may even be situations in which you place an IV catheter but do not deliver any IV fluids to these patients. This will have to be judgement call in the moment, depending on the patient’s pre-existing status and invasiveness of the procedure.
In general, patients with MVD tolerate anesthesia well. The vasodilation induced by general anesthesia is not a bad hemodynamic state for dogs with mitral valve disease and most will do fine during general anesthesia, with a little attention to detail!
Thank you to Dr. John Bonagura for reading this post and making sure I didnt say anything silly! https://twitter.com/vetcardiologist?lang=en