Anesthesia of the Cardiac Patient

Chapter 19 Anesthesia of the Cardiac Patient

Thomas K. Day

INTRODUCTION

Anesthesia of the patient with heart disease can be a challenge. Most veterinary patients with heart disease that are presented for sedation or anesthesia do not have clinical signs of heart failure. Anesthetic protocols that are routinely used for normal patients without heart disease can result in acute decompensation in patients with heart disease. Differences between dogs and cats in the response to anesthetic and analgesic drugs can compound the complex nature of cardiac anesthesia. In addition, most patients with heart disease may be treated with a variety of cardiac drugs that may interact with anesthetic drugs. This chapter provides a general view of anesthetic drugs that are indicated and contraindicated in dogs and cats with heart disease. Anesthetic considerations for specific cardiac diseases are also presented.

Key Point

Suggested anesthetic protocols for dogs and cats are presented based on the functional classes of heart failure.

GENERAL PRINCIPLES

• The veterinary anesthetist must understand and recognize several factors to provide safe and effective sedation or anesthesia, including the hemodynamic changes produced by heart disease, the possible interactions between cardiac and anesthetic drugs, the presence of arrhythmias, and the potential for anesthetic drugs to predispose to the production of arrhythmias.

• Knowledge of the length of sedation or anesthesia that is desired and recognition of the need for analgesia are also important.

• Virtually all anesthetic drugs directly depress cardiac function, alter vascular tone, or modify normal cardiovascular regulatory mechanisms.

• One “magic bullet” anesthetic protocol that will safely anesthetize any dog or cat with heart disease does not exist.

• Each patient and each etiology of cardiac disease should be considered on an individual basis to provide the safest sedation, analgesic or anesthetic protocol.

Key Point

The general rule of thumb is to devise an anesthetic plan that provides minimal cardiopulmonary depression and returns the patient to preanesthetic status as soon and as safely as possible.

PREANESTHETIC CONSIDERATIONS

Diagnosis of the Etiology of Heart Disease

• The decision on which sedation or anesthetic protocol to administer to a patient with heart disease should be made primarily on the specific etiology. Once the etiology of heart disease has been determined, specific recommendations can be provided on the choice of sedation or anesthesia.

Functional Classification of Heart Failure

• Three functional classifications of heart failure are based on clinical signs. The decision on whether or not to immediately sedate or anesthetize a patient with heart disease should begin with placing the patient in one of the three classifications.

• The first classification describes the asymptomatic patient that has confirmed heart disease, yet is not exhibiting clinical signs of heart failure. Patients that fulfill criteria for this classification of heart failure can be safely anesthetized without further stabilization.

• The second classification describes when mild to moderate clinical signs of heart failure are evident at rest or with mild exercise. Stabilization of clinical signs and the lack of clinical signs for several days with drug therapy are recommended prior to sedation or anesthesia. Patients with this classification of heart failure that require life-saving, emergency surgery should have cardiac drug therapy instituted immediately by parenteral administration and clinical signs controlled as much as possible prior to anesthesia. Continuous and aggressive monitoring will be required for this classification of patient during and immediately after sedation or anesthesia. Cardiac drug therapy should continue during anesthesia and surgery and in the immediate postoperative period.

• The third classification describes when advanced clinical signs of heart failure are immediately obvious. Patients severely affected can present in cardiogenic shock, and death or severe debilitation is likely without therapy.

Key Points

• Anesthesia is contraindicated in this third category of patients until clinical signs are immediately stabilized with aggressive drug therapy.

• Clients should be advised of the increased risk of death or severe debilitation during or immediately after anesthesia if patients in this third category of heart failure are anesthetized following aggressive cardiac drug therapy.

Anesthetic Risk Classification

• The anesthetic risk of a patient can be determined based on physical status.

• Five categories of physical status have been developed for veterinary patients and parallel the classification scheme adopted in human medicine by the American Society of Anesthesiologists (ASA) (Table 19-1).

• Most clinically stable cardiac patients will be ASA II or III, depending on the presence of any other underlying disorders.

Key Points

• Unstable patients with clinical signs of cardiac decompensation and heart failure that fulfill the criteria of ASA IV should not be anesthetized until the cardiac disease has been stabilized.

• There may be patients that present in the ASA V category, especially patients with a long history of heart disease that is currently refractory to all cardiac drugs. Stabilization of the signs of heart failure may not be possible, and death during anesthesia is likely.

Table 19-1 Classification of Physical Status for Anesthetized Patients

ASA Category	Description of Physical Status	Example
I	Normal, healthy	No cardiac disease, elective surgery (spay, castration)
II	Mild systemic disease	Compensated heart disease (no cardiac medications), fracture without shock
III	Severe systemic disease	Compensated heart disease (cardiac medications), anemia, fever, compensated renal disease, dehydration
IV	Severe systemic disease and a constant threat to life	Decompensated heart disease, electrolyte imbalance, uncontrolled internal hemorrhage
V	Moribund patient not expected to live with or without surgery	Decompensated heart disease refractory to cardiac drugs, terminal malignancy

Adapted by the American Society of Anesthesiologists.

Preanesthetic Diagnostic Evaluation and Laboratory Tests

• The patient with heart disease should have a complete diagnostic cardiac evaluation.

• All patients should have the following diagnostic and laboratory tests prior to sedation or anesthesia:

Physical Examination

• Particular attention should be paid to thoracic auscultation of the heart and lungs. The character of the peripheral pulse, jugular veins, mucous membranes, and capillary refill time should be noted and the peripheral pulse should be palpated simultaneously with thoracic auscultation of the heart sounds to detect pulse deficits.

Additional Tests

• Thoracic radiography and an electrocardiogram (ECG) should be performed as well. If a complete cardiac diagnostic evaluation has been performed less than 1 to 2 weeks previous to anesthesia, and the patient’s physical status has not changed, a physical examination and electrocardiogram are the only diagnostic tests that require repetition. Complete blood count and serum chemistries should be performed at the discretion of the veterinarian, with particular attention to renal values and electrolytes.

CARDIAC DRUGS AND POTENTIAL ANESTHETIC DRUG INTERACTIONS

Diuretics

• The loop diuretic, furosemide, is the most commonly used diuretic in patients with heart disease. The most common electrolyte disturbance produced by furosemide is hypokalemia. Hypokalemia can result in tachyarrhythmias or predispose to digoxin toxicity. Furosemide may promote dehydration and predispose the patient to hypotension during sedation or anesthesia.

• The potassium sparing diuretic, spironolactone, can result in hyperkalemia if used alone for extended periods of time. Hyperkalemia may result in arrhythmias, with equal likelihood of tachyarrhythmias or bradyarrhythmias.

• The thiazide diuretic, chlorothiazide, has similar side effects as the loop diuretics with chronic use. Hypokalemia and hypomagnesemia may produce or predispose to tachyarrhythmias.

Angiotensin-Converting Enzyme Inhibitors

• Enalapril and benazepril are the most commonly used angiotensin-converting enyme inhibitors (ACEIs). Each results in arterial vasodilation that can be enhanced by acepromazine, isoflurane, and sevoflurane predisposing to arterial hypotension. Arterial blood pressure should be monitored closely during sedation protocols involving acepromazine and during isoflurane and sevoflurane anesthesia.

Key Point

All patients receiving ACEIs should be monitored with direct or indirect blood pressure during inhalation anesthesia.

Digitalis Glycosides

• A common side effect of digoxin administration is ventricular arrhythmias. There is the possibility of an increase in arrhythmogenesis with concurrent use of sympathomimetics (dopamine, dobutamine, norepinephrine, epinephrine) during anesthesia. Isoflurane, sevoflurane and opioids have not been associated with increased incidence of arrhythmias secondary to digitalis.

• Hypokalemia, most commonly caused by chronic use of loop diuretics, can exacerbate digitalis toxicity. Acute onset of hypokalemia can occur during anesthesia as a result of hyperventilation (hypocarbia and concurrent respiratory alkalosis) and can exacerbate preexisting hypokalemia caused by diuretics.

Key Point

Always obtain a blood digitalis level prior to anesthesia.

Vasodilators

• Hydralazine is an arteriodilator that can cause reflex tachycardia and fluid and water retention. Tachycardia secondary to use of sympathomimetics (dopamine, dobutamine, norepinephrine, epinephrine) during anesthesia can be exacerbated with the use of hydralazine. Fluid administration must be minimized and monitored closely. Acepromazine, isoflurane and sevoflurane may exacerbate arteriodilation and predispose to arterial hypotension.

• Prazosin causes arterial and venodilation by alpha-1 adrenergic blockade. Acepromazine is contraindicated, as arteriodilation may be excessive and produce severe hypotension. Isoflurane may exacerbate arteriodilation and may predispose to arterial hypotension.

Calcium Channel Blockers

• Diltiazem is used to treat supraventricular arrhythmias and to improve diastolic function in cats with hypertrophic cardiomyopathy (HCM). Potential side effects include vasodilation, bradycardia, and decreased myocardial contractility. Concurrent use of acepromazine, isoflurane, and sevoflurane may exacerbate vasodilation and produce hypotension. Concurrent use of opioids and inhalation anesthetics (isoflurane and sevoflurane) may exacerbate bradycardia. Decreased myocardial contractility may be exacerbated by propofol.

Antiarrhythmic Agents

• The beta-adrenergic blocking agents propranolol and atenolol are commonly used to treat arrhythmias in dogs and cats and for treatment of HCM in cats. Potential side effects include bradycardia and decreased myocardial contractile function. Bradycardia may be exacerbated with use of opioids and inhalation anesthetics. Decreased myocardial contractility may cause hypotension during anesthesia, may be less responsive to sympathomimetics (dopamine and dobutamine), and may be exacerbated by propofol.

Class I Antiarrhythmic Agents

• Procainamide and tocainide can have side effects include decreased myocardial contractility and possible bradycardia. Decreased myocardial contractility may be exacerbated by propofol.

Nonsteroidal Anti-Inflammatory Drugs

• Cats with heart disease may be prescribed aspirin to potentially prevent thromboembolic disease. Aspirin impairs platelet function. Acepromazine should be avoided in cats receiving aspirin that are presented for surgery, as acepromazine also impairs platelet function. Clopidogrel (Plavix) may be used in the future and will likely have the same anesthetic concerns (impaired platelet function) as aspirin.

Combination Drug Therapy

• Most patients with heart disease are commonly administered more than one cardiac drug. The potential side effects of each drug must first be considered individually. Potential side effects of the combination of drugs should be considered next. The addition of anesthetic drugs may introduce a greater possibility of side effects. For example, a dog with compensated mitral insufficiency could have been prescribed digoxin, furosemide, and enalapril. There is a great potential for hypotension produced by excessive arteriodilation, bradycardia, or decreased myocardial contractility based on the combined side effects of each drug. Isoflurane and sevoflurane, which minimally decrease myocardial contractility and cardiac output, could produce severe hypotension in this patient secondary to peripheral vasodilation. Inhalation anesthesia should not be considered the primary anesthetic of choice in this dog.

ANESTHETIC DRUG SELECTION AND SUPPORTIVE CARE

Choice of either Sedation or General Anesthesia

• The definition of general anesthesia is the administration of injectable anesthetics, inhalation anesthetics, or a combination to produce hypnosis (sleep), analgesia, and muscle relaxation. General anesthesia is not solely produced by the administration of inhalation anesthetics. Many useful and safe general anesthetic protocols for cardiac patients are combinations of injectable anesthetic agents.

Oxygenation and Ventilatory Support

• All cardiac patients that are sedated or anesthetized should have oxygen administered in some form. Sedated patients should have oxygen delivered by mask at an insufflation rate of no less than 5 L/min. All intubated patients should be attached to an anesthetic machine to deliver 100% oxygen, regardless of whether or not an inhalation anesthetic is delivered.

• All cardiac patients anesthetized and maintained on isoflurane, sevoflurane or using injectable anesthetics should have ventilatory support provided. Isoflurane and sevoflurane are both potent respiratory depressants and can predispose the patient to hypoxemia or hypercarbia. Hypoxemia and hypercarbia can result in the production or worsening of arrhythmias. All intubated patients should also have ventilation supported. The general rule is that four to six breaths should be delivered each minute. Expired carbon dioxide can be monitored with a capnometer (see Monitoring) to ensure adequate ventilation.

Anesthetic Drugs that are Contraindicated in Patients with Cardiac Disease

• The following sedatives and anesthetic drugs are contraindicated in patients with heart disease, regardless of the etiology. The benefits of convenience, effectiveness, or ease of administration and lower cost do not justify the use of these drugs because of the profound cardiopulmonary depression, the increased possibility of arrhythmia production, or the length of recovery time.

Alpha-2 Adrenergic Drugs

• Xylazine and medetomidine are potent respiratory and cardiac depressants. Decreased heart rate is usually responsive to anticholinergics. Xylazine can also decrease heart rate by a central mechanism of decreased sympathetic outflow that will not be responsive to anticholinergics. Xylazine decreases myocardial contractility, resulting in decreased cardiac output and hypotension. Medetomidine results in intense vasoconstriction and decreased cardiac output. Decreased heart rate is usually a result of intense vasoconstriction. Administration of an anticholinergic will greatly increase cardiac work.

Barbiturates

• Pentobarbital has a duration of action of approximately 45 to 60 minutes and results in prolonged recoveries. Pentobarbital is also a potent respiratory and cardiovascular depressant at dosages used for general anesthesia.

• Amobarbital (intermediate duration) and phenobarbital (long duration of action) are barbiturates with an extremely long duration of action and have cardiopulmonary effects inappropriate for use as anesthetics or sedatives in cardiac patients.

Inhalation Anesthetics

• Halothane is not used commonly, but is still available to some extent. Halothane is the most potent negative inotrope of the inhalation anesthetics and it predisposes the myocardium to the production of arrhythmias, especially ventricular arrhythmias. Halothane should not be used in a dog or cat with cardiac disease.

• Methoxyflurane is of more historical significance than is used in practice. The effects on inotropy and the possibility for arrhythmias fall between those of isoflurane, sevoflurane (discussed later) and halothane. Methoxyflurane is more likely to produce a prolonged recovery.

• Mask induction with isoflurane or sevoflurane is not recommended in cardiac patients. Most animals become very excited during mask induction, even with adequate preanesthetic medication, which could predispose to arrhythmias and increased myocardial work secondary to the stress response. Isoflurane has a very pungent odor and may result in laryngospasm, especially in cats, though sevoflurane is less pungent. Environmental contamination with isoflurane and sevoflurane administered by mask is a very important consideration for the safety of all personnel.

ANESTHETIC DRUGS THAT SHOULD BE USED WITH CAUTION

Preanesthetic Medication

Tranquilizers

• The phenothiazine, acepromazine, is considered a major tranquilizer owing to the high reliability of producing mental calming. It is also the most commonly used tranquilizer in small animals. The primary cardiovascular effect is peripheral vasodilation, with minimal effects on contractility and respiration. Hypotension can occur, and is primarily treated with intravenous fluids and, in severe cases, peripheral vasoconstriction agents (phenylephrine, norepinephrine). The sedative and cardiovascular effects are of long duration (4 to 6 hours), though the effects are not reversible. Acepromazine can be used effectively and safely at very low dosages in otherwise healthy cardiac patients

Anticholinergics

• Atropine and glycopyrrolate are primary used to maintain heart rate during anesthesia or sedation and are generally not recommended unless used with anesthetic drugs that are likely to lower heart rate (opioids) through increased parasympathetic tone. The potential side effects include the production of tachyarrhythmias (ventricular or supraventricular). The increase in myocardial oxygen consumption produced by an increase in heart rate above normal values may predispose the patient with heart disease to focal ischemia and the possibility of arrhythmias. There is little difference between atropine and glycopyrrolate in the effectiveness of producing an increase in heart rate, though glycopyrrolate will likely have a longer duration.

Intravenous Induction Agents

Thiobarbiturates

• Thiopental can be used safely in cardiac patients, though it is not recommended in patients with pre-existing arrhythmias. Thiopental can produce transient ventricular arrhythmias by sensitizing the heart to catecholamines. Transient decrease in blood pressure as a result of decreased contractility. Can be used effectively and safely in otherwise healthy cardiac patients at very low dosages following adequate preanesthetic medication.

Propofol

• Propofol has cardiovascular effects similar to thiopental, though the likelihood of arrhythmia production is less. Apnea can be profound, and is closely related to speed of injection. Propofol is rapidly redistributed, resulting in very rapid recovery.

Dissociatives

• Ketamine and the combination of tiletamine and zolazepam are usually very safe and effective in cardiac patients. Transient increases in heart rate can predispose to arrhythmias and are not recommended in patients with pre-existing arrhythmias. Increased heart rate is less severe if administered after preanesthetic medication. Ketamine should not be used as the sole anesthetic agent in cats with HCM. Acute fulminate congestive heart failure has been reported in cats with HCM administered ketamine and combinations of ketamine/diazepam as sole anesthetic.

Inhalation Anesthetics

• Inhalation anesthetics used as the sole anesthetic agent to induce (mask induction) and maintain anesthesia must be used with extreme caution. Both isoflurane and sevoflurane are potent vasodilators that could lead to hypotension and increased cardiac work. Both isoflurane and sevoflurane are potent respiratory depressants.

USEFUL ANESTHETIC DRUGS

Preanesthetic Medication

• The benefits of preanesthetic medication in patients with heart disease include reducing preoperative anxiety and stress, providing preemptive analgesia, lowering the requirement of intravenous induction agents and inhalation anesthetics, and ensuring a smooth recovery. Preanesthetic medication can be administered intramuscularly, subcutaneously, or intravenously. In most cardiac patients, the author recommends intramuscular administration of preanesthetic medications.

Tranquilizers

Benzodiazepines

• Diazepam and midazolam are considered minor tranquilizers because when used alone, benzodiazepines do not produce profound sedation in the normally mentated patient. Benzodiazepines may produce a profound effect on patients with advanced age or disease. The most common use is in combination with an opioid (neuroleptanalgesia). Both drugs minimally depress cardiopulmonary function. Both diazepam and midazolam are effectively absorbed after intramuscular administration, though diazepam may produce more pain on injection (propylene glycol based). Clinical effects of midazolam compared with diazepam are identical in dogs and cats, though midazolam is more expensive. The benzodiazepine antagonist, flumazenil, is available, though the effects of benzodiazepines rarely require antagonism.

Opioids

• The primary use of opioids in veterinary anesthesia is to provide analgesia. Most opioids do not possess profound sedative effects when administered alone; however, when used in combination with tranquilizers for neuroleptanalgesia (see following), adequate sedation can be achieved. Opioids do not affect myocardial contractility or vascular tone, which makes them very attractive for use in patients with heart disease. All effects produced by opioids can be antagonized by administration of naloxone, though repeat administration will likely be required as naloxone has a very short duration of action.

Morphine

• Minimal sedation is produced when used alone in normal patients, though profound sedation can occur in compromised patients. Primary side effects are vomiting and bradycardia. Bradycardia is usually anticholinergic responsive. Depression of respiration is dose dependent.

Hydromorphone and Oxymorphone

• Opioid agonists have similar clinical effects and are 10 times more potent analgesia compared with morphine. Usually more effective than morphine in producing sedation when used alone, though even more effective in a neuroleptanalgesic combination. Potential for decreased heart rate (parasympathomimetic). Less respiratory depression than with morphine. Vomiting likely, though less likely compared with morphine. Can also be used as an induction agent in compromised patients.

Butorphanol

• Opioid agonist/antagonist that is usually less effective than oxymorphone or hydromorphone in producing sedation alone or in a neuroleptanalgesic combination. There is minimal cardiopulmonary depression, and it is unlikely to produce bradycardia. A “ceiling effect” occurs regarding sedation and analgesia. This means that higher doses beyond the recommended maximum dose (approximately 0.8 mg/kg) do not produce more sedation or analgesia. Vomiting is a rare side effect. Very poor analgesic for moderate to severe pain.

Buprenorphine

• Buprenorphine is a partial opioid agonist that is 20 times more potent in producing analgesia when compared with morphine. It is generally a poor sedative when used alone, though slightly more effective in a neuroleptanalgesia combination. There is minimal cardiopulmonary depression and a ceiling effect occurs similar to butorphanol. The onset of action is 20 to 30 minutes and there is a long duration of effect. Repeat injections of naloxone are required to maintain antagonism of effects, if required.

Fentanyl

• Fentanyl is an opioid agonist that is 100 times more potent in producing analgesia when compared with morphine. The onset of action is very rapid and it can be used as an intravenous induction agent in dogs. The duration of action is extremely short, making fentanyl an ideal agent for a continuous-rate infusion (CRI) to maintain general anesthesia. Bradycardia is more likely to occur and responds to anticholinergic administration (preferred) or a decrease in the rate of infusion.

Neuroleptanalgesia

• Neuroleptanalgesia is defined as the effect produced by the combination of a tranquilizer and an opioid. The neuroleptanalgesia combinations that are recommended for patients with heart disease include any combination of a benzodiazepine and an opioid. Intravenous or intramuscular administration may be used to produce an effect.

Opioid-Diazepam Combinations

• The preferred neuroleptanalgesia for patients with heart disease is the combination of an opioid with a benzodiazepine. The most reliable sedation occurs with an opioid agonist (morphine, hydromorphone/oxymorphone, fentanyl) compared to opioid agonist/antagonist (butorphanol) and partial opioid agonist (buprenorphine) combinations. Intramuscular administration produces effects within 15 minutes. Panting is a prominent feature when opioid agonists are used in dogs (not cats), and respiratory depression can be pronounced. Bradycardia is more likely with opioid agonist combinations, and is responsive to anticholinergics.

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Tags: Manual of Canine and Feline Cardiology

Aug 15, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

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