Anesthetizing cardiac patients (Proceedings)

Veterinary technicians are the veterinary hospital's mainstay when it comes to carefully and successfully anesthetizing critical patients. A large number of elderly canine and feline patients are affected by cardiac disease, and knowledge of how to safely monitor, anesthetize, and problem solve cardiac patients makes for a less stressful anesthesia for both the patient and technician.

Preanesthetic evaluation should include a thorough physical exam and a complete history, including what medications the patient is on. If they are currently on cardiac medications such as diuretics or ACE inhibitors, a serum biochemistry profile and electrolyte panel is even more important than usual, as these drugs can cause azotemia and changes in electrolyte levels. If arrhythmias are ausculted, they should also be characterized with an ECG prior to anesthesia. Further diagnostic testing may be necessary to evaluate the cardiac disease. As a general rule, it is much more risky to anesthetize a patient in congestive heart failure than one who is stabilized. A recent set of chest radiographs will indicate whether the patient has pulmonary edema or pleural effusion. It will also help evaluate the cardiac silhouette for enlargement. An echocardiogram may also be indicated to further characterize heart disease and evaluate severity.

Ideally, the equipment available would include blood pressure monitoring, either invasive or non-invasive; ECG; capnography; and pulse oximetry. Technicians should be familiar with the equipment in order to expedite induction and set up of the monitoring equipment. With a critical patient, it is also ideal to have a technician solely dedicated to anesthesia monitoring. Anesthesia monitoring forms should be utilized to record anesthesia dosing, track trends in blood pressure, heart rate, respiratory rate, etc. The anesthetist should also remember that monitoring equipment cannot replace their own senses, and that no one piece of equipment will give all the information about their patient's status.

As a general rule, the safest anesthetic agent for any patient is the one which the anesthetist is most familiar. However, there are a few basic guidelines for cardiac patients.

First, the patient should have a peripheral catheter placed. As with any patient, this allows the technician to deliver medications quickly in case of changes in the patient's status. The anesthetist may also wish to place an arterial catheter. This allows invasive, and therefore more accurate, blood pressure monitoring, as well as access to arterial blood to evaluate changes in respiratory status.

Next, anesthetic agents should be chosen that do not drastically change blood pressure or heart rate, unless those changes would be favorable, as indicated by findings on preanesthetic evaluation. For instance, ketamine can significantly increase the patient's heart rate. In the case of patients with HCM, this would not be a good selection. However, it will also increase the heart rate in patients with bradycardia, and therefore, it is often used to anesthetize patients receiving a pacemaker. Dexmedetomidine, an alpha 2 agonist, will considerably reduce heart rate through reflex bradycardia caused by peripheral vasoconstriction and central hypertension. Because of the degree of peripheral vasoconstriction, blood pressure monitoring and pulse oximetry can be difficult to evaluate. It will also cause an increased load on the heart, which is especially dangerous to patients with advanced valve disease.

The choice of appropriate premeds can be a huge benefit to both the technician and patient, as they allow the technician to reduce the dose of induction agents that can adversely affect blood pressure. Inhalation agents can also significantly reduce blood pressure, so reducing the reliance on them can make anesthesia less eventful. For this reason, mask and box induction should be reserved only for cardiac patients that are too fractious to handle. The stress of mask induction, followed by hypotension due to inhalant anesthetics, is not a good combination for a cardiac patient.

Valuable technicians also know how to respond to changes in their patient's status appropriately. For instance, hypotension in patients without cardiac disease can be treated with a bolus of fluids or colloids. However, fluids, especially hetastarch, may not be appropriate for a patient that already has signs of fluid overload. Sympathomimetics such as dobutamine or dopamine in a CRI are more appropriate for a cardiac patient experiencing hypotension.

The knowledge of how to use anticholinergics judiciously will also help the anesthetist respond to changes in heart rate. Glycopyrrolate typically does not cause the same increase in heart rate that atropine does, and may be more appropriate for a patient experiencing bradycardia under anesthesia. The anesthetist should also be aware that when giving an anticholinergic, the bradycardia will look more interesting before it responds fully, and they should not attempt to give more while waiting for the heart rate to respond.

Certain cardiac procedures will also cause specific changes in patient status, and being able to respond appropriately to these changes is useful for an technician performing anesthesia in these cases. For instance, in any cardiac catheter procedure, ventricular arrhythmias are common, especially when the catheters are manipulated within the heart. These arrhythmias may be benign, but they can deteriorate to ventricular fibrillation very quickly. Other examples of changes associated with cardiac procedures include: bradycardia after occlusion of PDA; extreme hypotension associated with balloon dilation during valvuloplasty; and temporary asystole while placing pacemakers. The ability to observe and react quickly when necessary while not over-reacting is a hallmark of an technician experienced with cardiac procedures.