General anesthesia can alter the cardiovascular system in negative ways: by causing myocardial depression, sinus tachycardia, systemic hypertension, and other potentially harmful physiologic changes (Pypendop 2011). In dogs, the overall risk of anesthetic and sedation-related death is 0.17% based on a survey of 98,036 dogs anesthetized at 117 general veterinary practices (Brodbelt 2008a). In cats, the overall risk of anesthetic and sedation-related death is 0.24% based on a multicenter survey of cats presented to general veterinary practices (Brodbelt 2007). As might be expected, dogs and cats have a greater anesthetic risk when they are in poor health (Brodbelt 2007). However, the specific impact of heart disease in dogs undergoing elective general anesthesia has recently been shown to be minimal when anesthesia occurs under optimal conditions (Carter 2017).

Initial Approach

The suspicion of heart disease in dogs and cats depends on clinical presentation. The most common situation is the dog with a heart murmur and no external signs of decompensation—the so-called “asymptomatic murmur” case. In such patients, a preanesthetic cardiac evaluation that includes physical examination, thoracic radiographs, and an echocardiogram (with simultaneous electrocardiographic display) is a recommended minimum database in all cases. The information gathered from such testing identifies the cause of the murmur and the expected hemodynamic impact, the severity of the underlying disease, expected possible complications (and likelihood of such complications based on the degree of the primary problem and secondary changes), and the steps that can be followed to minimize the risk of such problems. When such a database cannot be obtained due to limitations that are logistical, financial, or perceptive (the client, the attending veterinarian, or both parties do not understand the value of the information), then the attending veterinarian can use his/her physical exam findings to narrow the differential diagnosis to the greatest degree possible (Carter 2017). An approach for doing this is described in practical terms in an open-source article (Côté 2015): https://avmajournals.avma.org.

Cats

A cat with an incidentally detected heart murmur, or incidentally detected cardiomegaly, on thoracic radiographs can have heart disease that ranges in severity from trivial to very advanced. In either situation, an echocardiogram is recommended in order to identify heart disease (if any), the extent of primary lesions, and the degree of secondary changes. Thoracic radiographs, plus circulating biomarker measurement (typically NT-proBNP, possibly cardiac troponin-I), are an alternative if echocardiography is not feasible. Physical exam findings and the medical history, together with echocardiographic results, are the core sources of information for planning and performing general anesthesia in a cat with an incidentally detected abnormality and no externally apparent clinical signs.

Key Principles for General Anesthesia in Cats with Subclinical HCM

• Avoidance of tachycardia: Premedication with vagolytic drugs probably should be avoided in cats with HCM. Rather, atropine (0.02 mg/kg IV, or approximately 0.2 mL/cat if using standard 0.54 mg/mL concentration; can repeat twice more within 10 minutes if needed) or glycopyrrolate (0.005 mg/kg IV, or approximately 0.1 mL/cat if using standard 0.2 mg/mL concentration; can repeat twice more within 10 minutes if needed) can be given if the heart rate is less than approximately 130 beats/minute in a normotensive or hypotensive anesthetized cat.
• Avoidance of tachycardia: Avoid (or minimize, if no alternatives exist) high-dose (IM) ketamine, tiletamine, and other premedications or induction agents that directly trigger sinus tachycardia. Suitable options in cats with HCM include propofol (4–8 mg/kg slow IV to effect) or alfaxalone (3–7 mg/kg slow IV to effect).
• Avoidance of reflex sinus tachycardia due to systemic hypotension: Premedication selection is important.

Drugs with minimal cardiovascular effects that are used routinely for premedication in cats with HCM include butorphanol (0.1 mg/kg IV once or 0.2 mg/kg IM once) or buprenorphine 0.005–0.01 mg/kg IV once or 0.01–0.02 mg/kg IM once). Acepromazine generally is avoided, although low dosages (e.g., 0.005 mg/kg once) given IM have historically been part of many cardiologist’s armamentaria and subjectively are associated with few to no adverse effects and a smoother, longer-lasting sedation when given with one of the other drugs just named above. Dexmedetomidine (3–5 µg/kg IV or IM once) is used routinely by cardiologists in feline cardiomyopathic patients, whereas certain anesthesiologists recommend against using it in feline heart disease due to the possibility of coronary vasoconstriction. Much higher dosages are recommended by the drug’s labeled indications, but are essentially never used.

Dexmedetomidine’s alpha-2 agonist properties cause vasoconstriction, which elicits a reflex sinus bradycardia while maintaining normo- or slight hypertension. This is desirable to increase diastolic filling time and decrease the cardiac work that is otherwise associated with tachycardia.

Maintenance of general anesthesia is almost always by inhalation (isoflurane or sevoflurane), although total intravenous anesthesia may be considered together with intubation for oxygen administration.

An important consequence of diastolic dysfunction is intolerance to intravascular volume expansion. For this reason, intravenous or subcutaneous fluids can be given at reduced dosages in cats with HCM. A typical rate is 2 mL/kg/hour, with higher rates chosen under certain circumstances (e.g., absence of atrial enlargement echocardiographically, concurrent kidney disease) and lower rates chosen in other circumstances (e.g., advanced disease as evidenced by moderate or marked atrial enlargement). An isotonic, low-sodium fluid type such as 0.45% NaCl with 2.5% dextrose is preferred in euvolemic/normally hydrated cats, although the drawbacks of dextrose administration to cats and their future predisposition to diabetes mellitus must also be considered (Zini 2009).

Dogs

In degenerative/myxomatous mitral valve disease, mitral regurgitation imposes a volume overload on the left ventricle, increasing cardiac work with each heartbeat. The degree to which this occurs depends on the volume of regurgitation, which is not reliably linked to murmur in intensity; for example, 56% of DMVD dogs with moderate-intensity (grade III/VI) heart murmurs have no significant secondary changes echocardiographically (Ljungvall 2008). However, even in dogs with secondary changes (e.g., left atrial enlargement), no difference in complication rates was found in one clinical general anesthesia study comparing 100 dogs with advanced, subclinical heart disease and 100 matched control dogs with no heart disease (Carter 2017). The study was conducted under referral conditions, which implies an optimal degree of monitoring and responding to changes when they occurred.

Key Principles for General Anesthesia in Dogs with Subclinical DMVD

• Tachycardia can be more harmful in individuals with eccentric ventricular hypertrophy than in normal individuals. This is because greater myocardial mass is not matched by increased myocardial perfusion, and the result can be ischemia during periods of increased cardiac work. Furthermore, sympathetic stimulation that causes sinus tachycardia also increases ventricular systolic function, which increases the force applied to a diseased mitral valve apparatus. As a general rule, dogs with heart disease who undergo general anesthesia should have a heart rate that is never greater than 200 beats/minute and, ideally, is always less than 170 beats/minute.
• Similarly, bradycardia can lead more quickly to a decrease in cardiac output in dogs with heart disease than in healthy dogs. As a general rule, dogs with heart disease who undergo general anesthesia should have a heart rate that remains high enough for them to maintain an adequate perfusing blood pressure, which can be defined loosely as a mean arterial pressure greater than 70 mm Hg, or a systolic arterial blood pressure greater than 90 mm Hg. In the absence of blood pressure monitoring, the patient’s heart rate should generally be maintained above 80 beats/minute.
• Dogs with heart disease have an altered tolerance to intravascular volume changes. On principle, they are at greater risk of iatrogenic pulmonary edema due to intravascular fluid overload, and might also be at greater risk of the opposite (i.e., of renal ischemic injury due to vascular underloading). This narrower margin of IV fluid tolerance typically is addressed by administering maintenance-type fluids (e.g., 0.45% NaCl in 2.5% dextrose), which are isotonic but contain less sodium, rather than replacement-type fluids (e.g., LRS, 0.9% NaCl). The rate of IV fluid administration for patients with heart disease (e.g., 1–2 mL/kg/hour) usually is lower than intraoperative maintenance rates. This approach, while logical, remains to be proven by objective evidence. Adjustments in fluid administration rate can be made in response to changes during monitoring. For example, hypotension may prompt administration of a small fluid bolus. However, other means should always be attempted first. For example, decreasing inhalant anesthetic concentration is always preferred over fluid administration in a hypotensive cardiac patient.

Drug Selection

General practical applications for premedication and induction of dogs with compensated (“asymptomatic”) heart disease include the following:

• Avoidance of tachycardia: Premedication with vagolytic drugs probably should be avoided in dogs with heart disease. Rather, atropine (0.02 mg/kg IV; can repeat twice more within 10 minutes if needed) or glycopyrrolate (0.005 mg/kg IV; can repeat twice more within 10 minutes if needed) can be given as needed during anesthesia, such as if the heart rate is less than approximately 80 beats/minute in a normotensive or hypotensive anesthetized dog.
• Avoidance of tachycardia: Avoid (or minimize, if no alternatives exist) ketamine, xylazine, and other premedications or induction agents that directly trigger sinus tachycardia. Suitable premedication options include those listed in the next item below, as well as benzodiazepines (e.g., midazolam 0.2–0.3 mg/kg IM once). Suitable options for anesthetic induction in dogs with heart disease include propofol (4–8 mg/kg slow IV to effect), alfaxalone (3–7 mg/kg slow IV to effect), or etomidate (1.5–3.5 mg/kg slow IV to effect).
• Avoidance of reflex sinus tachycardia due to systemic hypotension: Premedication selection is important. Drugs with minimal cardiovascular effects that are used routinely for premedication in dogs with heart disease include butorphanol (0.1 mg/kg IV once or 0.2 mg/kg IM once) or buprenorphine 0.005–0.01 mg/kg IV once or 0.01–0.02 mg/kg IM once). Hydromorphone has important analgesic properties, but its stimulation of panting and the increased susceptibility to noise (which can coexist with, or be mistaken for dysphoria) are important drawbacks that may make it less favorable in patients undergoing elective procedures, especially if such procedures are minimally or not painful. Acepromazine generally is avoided, although low dosages (e.g., 0.005 mg/kg once) given IM have historically been part of many cardiologists’ armamentaria and subjectively are associated with few to no adverse effects and a smoother, longer-lasting sedation when given with one of the other drugs just named above. Dexmedetomidine and other alpha-agonists are inappropriate for most canine heart disorders, including DMVD and dilated cardiomyopathy, because the arterio constriction they cause can increase preload to a degree that causes congestive heart failure iatrogenically. Other forms of heart disease where alpha-agonists can be considered are disorders that involve outflow tract obstruction (e.g., subaortic stenosis) and disorders of diastolic dysfunction (e.g., cardiac tamponade).

Maintenance of general anesthesia is almost always by inhalation (isoflurane or sevoflurane), although total intravenous anesthesia may be considered together with intubation for oxygen administration.

Monitoring

Standard monitoring of dogs with any type and degree of heart disease undergoing general anesthesia should always include:

• Continuous electrocardiographic (ECG) monitoring during anesthesia, assessing for sinus rhythm that decreases below 80 beats/minute (which justifies treatment with atropine or glycopyrrolate if associated with normo- or hypotension, as described above), or that rises above 160 beats/minute (which justifies assessment for the cause of the tachycardia—typically too light an anesthetic plane, or a different tachycardia such as ventricular tachycardia warranting confirmation/immediate consultation and possibly antiarrhythmic treatment), and for pathologic arrhythmias.
• Blood pressure (BP) monitoring, either through sphygmomanometry for continuous monitoring (observing trends) or periodic Doppler assessment.
• Pulse oximetry.

References

1.  Pypendop BH. Anesthesia in the patient with cardiac disease. In: Côté E, MacDonald KA, Meurs KM, et al. Feline Cardiology. Ames, IA: Wiley; 2011:411–421.

2.  Brodbelt DC, Blissitt KJ, Hammond RA, et al. The risk of death: the confidential enquiry into perioperative small animal fatalities. Vet Anaesth Analg. 2008a;35:365–373.

3.  Carter JE, Motsinger-Reif AA, Krug WV, et al. The effect of heart disease on anesthetic complications during routine dental procedures in dogs. J Am Anim Hosp Assoc. 2017;53:206–213.

4.  Côté E, Edwards NJ, Ettinger SJ, et al. Management of incidentally detected heart murmurs in dogs and cats. J Am Vet Med Assoc. 2015;246:1076–1088.

5.  Ljungvall I, Rishniw M, Porciello F, et al. Murmur intensity in small-breed dogs with myxomatous mitral valve disease reflects disease severity. J Small Anim Pract. 2014;55:545–550.

6.  Zini E, Osto M, Franchini M, et al. Hyperglycaemia but not hyperlipidaemia causes beta cell dysfunction and beta cell loss in the domestic cat. Diabetologia. 2009;52:336–346.