Premedication with an opioid (eg, hydromorphone 0.05-0.1 mg/kg IM or SC or 0.01-0.03 mg/kg IV) can provide mild-to-moderate sedation with minimal cardiovascular side effects. Benzodiazepines (eg, midazolam 0.1-0.2 mg/kg IV or SC or IM) also have few to no cardiovascular effects and can be useful adjuncts in older or sicker dogs. They are, in general, less effective as sole tranquilizers in young or healthy active animals, and they may cause excitement. Alpha-2 agonists are excellent sedatives but are not recommended. They increase systemic vascular resistance and decrease the heart rate, which can increase regurgitant stroke volume while diminishing forward stroke volume. Conversely, acepromazine tends to decrease afterload but can unduly decrease preload and contractility, so its use should be carefully considered.5
Depending on premedication choice and subsequent planned anesthetic medications, an anticholinergic (eg, atropine 0.02-0.03 mg/kg IM or SC, glycopyrrolate 0.01 mg/kg IM) may be added to prevent or offset bradycardia that is commonly seen with use of opioids6 especially following IV administration. It can also maintain the heart rate in a normal range; tachycardia should be avoided, as this can negatively affect ventricular filling. Treatment with an anticholinergic may be considered as an alternate strategy only in cases of observed bradycardia or bradyarrhythmias. In the authors’ observation, this strategy commonly results in an exacerbation of bradycardia and/or bradyarrhythmias with low-dose anticholinergic administration and tachycardia with high-dose anticholinergic administration.
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For a patient with mild- (stage B1) to- moderate (early-to-intermediate stage B2) MMVD, the veterinarian may choose his or her preferred IV induction agent and titrate to the desired end point. Propofol and alfaxalone are acceptable for dogs with mild disease and may be used judiciously in patients with moderate disease.7-9 Ketamine similarly has indirect sympathomimetic actions and can help maintain heart rate and cardiac output in dogs with mild-to-moderate disease.10 A dose of ketamine (5-7 mg/kg IV) with midazolam (0.2-0.3 mg/kg IV) can provide intubation conditions in premedicated dogs. Alternatively, lower doses of ketamine (2-3 mg/kg IV) may be used in combination with similar doses of propofol titrated to effect in dogs with mild-to-moderate disease.
Anesthesia induction for patients with advanced disease can be more safely accomplished using drugs with a low likelihood of or manageable cardiovascular depression, such as a combination of an opioid (eg, fentanyl 10 µg/kg IV)11,12 or etomidate (1-2 mg/kg IV)13 and a benzodiazepine (eg, midazolam 0.1-0.2 mg/kg IV). Bradycardia is likely with IV opioids, and it can be mitigated with use of anticholinergics. A small dose (0.5-1.0 mg/kg IV) of a hypnotic agent (eg, propofol) or alfaxalone may be necessary to facilitate intubation. Data on alfaxalone’s cardiovascular effects in this patient population are limited.
Patients with severe heart failure secondary to mitral insufficiency are at greater anesthetic risk; elective surgery should be postponed for these patients until the condition is controlled or stabilized with medical management. If this is not possible, careful titration of a benzodiazepine with a nonhistamine releasing µ-opioid agonist or etomidate still provides the safest option for anesthesia induction. Preplacement of heart rate, rhythm, and blood pressure monitoring equipment and availability of supportive medications are strongly recommended in these animals.
Maintenance with inhalation anesthetics is appropriate for dogs with mild-to-moderate disease. Supplemental analgesia should be provided as needed. In dogs showing clinical signs of heart failure, a balanced technique including high doses of an opioid (eg, fentanyl 20-40 µg/kg/hr) can allow for a reduction in the dose of inhalation agents and, in turn, their dose-dependent myocardial depressant effects. High-dose opioid infusions can cause respiratory depression. Alternative approaches using anesthetic-sparing infusions of ketamine (20 µg/kg/min) and lidocaine (30-50 µg/kg/min) with lower doses of nonhistamine-releasing opioid infusions (eg, hydromorphone 0.02-0.03 mg/kg/hr) may be considered if mechanical ventilation is not possible. Consultation with or referral to a board-certified anesthesiologist should be considered in dogs with advanced (stages C and D) disease.
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