Anesthesia for patients with cardiac diseases (Proceedings)

Small animal patients with cardiac disease and/or congenital abnormalities can be challenging and sometimes intimidating patients. An understanding of the physiology of the particular cardiac disease and how it affects the function of the heart is paramount to choosing a safe anesthetic protocol and can help to alleviate the intimidation factor.

Because stress activates the sympathetic nervous system and increases cardiac work techniques should be employed to reduce or try to avoid stress in the cardiac compromised patient. Each patient must be individually assessed to determine if a particular procedure will create more stress than the benefit afforded.  Having a good understanding of anesthetic agents and their effects on the cardiovascular system can provide the anesthetist with the tools required to successfully anesthetize the patient with cardiac disease.

It is also important to realize and understand the effects of current cardiac medication being given and the potential for adverse reactions to those drugs when mixed with anesthetic drugs. Some cardiac drugs can potentiate the side effects of anesthetic drugs.  For instance, enalapril along with iso or sevoflurane can cause profound vasodilation and hypotension.  Beta blockers like propanolol and atenolol slow the heart rate.  Opioids on top of beta blockers may result in severe bradycardia.  Calcium channel blockers can potentiate bradycardia, hypotension and vasodilation when they interact with inhalants and opioids.

The beneficial aspects of providing pre-anesthetic tranquilizers and/or analgesics are significant. A reduction of fear and anxiety along with pre-emptive analgesia should reduce the amount of induction drug needed and also allow for a lower percentage of inhalant anesthetic during the maintenance period (lowering MAC).  Stress and anxiety during induction can cause tachycardia, a rise in blood pressure and increases in myocardial oxygen demand.  An anxious and struggling patient during induction can lead to catecholamine release and subsequent arrhythmias.  All patients should be closely monitored following pre-medication.  If possible, all cardiac patients should be pre-oxygenated prior to induction to maximize oxygen delivery and saturation during induction.

The International Small Animal Cardiac Health Council has designed guidelines for three classifications of heart disease based on clinical signs.  These classifications can be integrated into the American Society of Anesthesiologists (ASA) classifications for anesthetic patients to help assess risk status and to aid in choosing and preparing a safe anesthetic plan for the cardiac patient.

Anesthetic drugs and their effects on cardiovascular function

Alpha-2 agonists

These drugs should be avoided in most cardiac patients because they cause severe cardiovascular side effects including transient vasoconstriction, bradycardia, decreased contractility, arrhythmias and increased myocardial workload.

Opioids

These drugs are seldom contraindicated in cardiac patients and are considered a safe and effective choice. Oxymorphone, hydromorphone and fentanyl are the most popular and useful mu agonists.  They provide reliable sedation and excellent analgesia.  Butorphanol is a mu antagonist and a kappa receptor agonist.  It provides less sedation and analgesia than the mu agonists. Buprenorphine is a partial mu agonist with good analgesic properties.  It has a long onset of action, is non-titratable and is usually best reserved for post-operative use.

Thiobarbiturates

Cause potential arrhythmias, hypotension, apnea and myocardial depression.  The thiobarbiturates should be used with caution in the cardiac patient.

Benzodiazepines

These drugs have minimal cardiovascular effects and are considered a safe and sound choice in cardiac patients.

Anticholinergics

Can be used judiciously in cardiac patients to counteract bradycardia caused by opioids and in situations where it is crucial to maintain heart rate.  Beware of potential tachycardia which can cause reduced coronary perfusion, diastolic filling and increased myocardial work. Glycopyrrolate may be the preferred anticholinergic for pre-medication because it tends to cause less tachycardia.

Etomidate

Very minimal cardiovascular effects.  Heart rate, stroke volume and cardiac output remain basically the same following injection.  Etomidate is usually the induction drug of choice for patients in severe heart failure.

Phenothiazines

Acepromazine causes vasodilation which can lead to hypotension.  Its effects are non-reversible.  This drug should be used very cautiously, if at all, in most cardiac patients pre-operatively and with those patients already on vasodilators. Acepromazine may have some beneficial effects in those patients with ventricular or aortic septal defect (VSD or ASD) and mitral valve insufficiency where the vasodilating effect can be helpful in reducing afterload.  It may have advantages when used post-operatively in low doses to reduce anxiety.

Propofol

Propofol's side effects include hypotension, apnea and vasodilation.

Ketamine

Ketamine causes an increase in heart rate, blood pressure and cardiac output.  These effects cause the heart to work harder, but they can be beneficial in cardiac patients that need and can handle this cardiovascular support.  These patients include those with mitral or tricuspid valvular disease, those with ventricular defects, patients with hypovolemia, dilated cardiomyopathy (DCM) and patent ductus arterioisis (PDA).  Ketamine use should be avoided in cats with hypertrophic cardiomyopathy (HCM) or in other patients where an increase in heart rate, blood pressure and myocardial oxygen consumption can be detrimental.

Classification of Physical Status for Anesthetized Patients with Emphasis on Cardiac Disease Category Description Example I Normal, healthy No discernable disease II Mild systemic disease Compensated cardiac disease (no cardiac meds) III Severe systemic disease Compensated cardiac disease (on cardiac meds) IV Severe systemic disease Decompensated heart disease (constant threat to life) V Moribund, not expected to Terminal heart disease, unresponsive to live 24 hours cardiac meds

Inhalant anesthetics

Both isoflurane and sevoflurane are acceptable choices for maintenance of anesthesia.  Both drugs can cause potent dose dependant vasodilation and so should be used judiciously in those patients who cannot tolerate decreases in blood pressure.  These patients include cats with HCM and patients with left to right shunting.  The use of “balanced anesthesia” or the concurrent administration of opioids during the maintenance period can help to lower the inhalant concentration required and thus reduce the cardiovascular side effects.

Explanation of specific cardiovascular defects and acceptable anesthetic protocols

Dilated cardiomyopathy (DCM)  causes decreased myocardial contractility, ventricular and atrial enlargement. Goals: maintain preload, contractility, and avoid excessive increases in HR and fluid overload.  LRS at 5 ml/kg/hr.   Glycopyrrolate at 0.01 mg/kg can be used for bradycardia. An opioid (hydromorphone or oxymorphone at 0.1 mg/kg) with a benzodiazepine (diazepam or midazolam at 0.1-0.2 mg/kg) drug may be used for pre-medication ( a technique called neuroleptanalgesia).  Ketamine/midazolam (1 ml of 50/50 mix per 20 # may be used for induction).  Dobutamine can be used to improve blood pressure by improving contractility.  Include the infusion as part of the maintenance fluid plan to avoid overhydrating.  Isoflurane or sevoflurane are acceptable choices for maintenance inhalant although the use of a concurrent opioid CRI such as fentanyl will help to lower the concentrations required and therefore reduce the vasodilating effects of the inhalant.

Mitral regurgitation: Mitral valve insufficiency causes blood to flow back into the atrium with less ejected out of the ventricle.  The goals for anesthetizing the patient with mild mitral regurge include maintaining heart rate and contractility and the avoidance of arterial vasoconstriction.  These patients (who are not in heart failure) are considered low risk and may be pre-medicated with an opioid/benzodiazepine combination (neuroleptanalgesia).  They should be given an anticholinergic to prevent bradycardia.  Ketamine/diazepam (or midazolam) is an acceptable choice for induction or low doses of propofol may be used.  Blood pressure, contractility and cardiac output can be improved and maintained with dobutamine at 2-5 µg/kg/min.  Pressor agents that can potentially cause vasoconstriction and tachycardia should be avoided.  Avoid fluid overload.

Hypertrophic cardiomyopathy (HCM):  Feline HCM is the most common cardiac disease seen in cats.  It is characterized by thickened ventricles and impaired diastolic filling. The impaired diastolic filling is due to increased left ventricular stiffness which impairs its ability to fill with blood.  These cats often have myocardial ischemia and may have resultant pulmonary edema from impaired left ventricular filling.  Changes in chamber size and a dynamically narrowed left ventricular outflow tract can cause mitral regurge.  Goals for anesthetizing these cats include reducing stress and anxiety pre-operatively, and maintaining heart rate while avoiding brady and tachycardia. 

Fluid maintenance can also be tricky, but IV fluids such as LRS can be given at 5 ml/kg/hr.  Measurement of CVP can help guide fluid therapy.  Any arrhythmias or pulmonary edema should be treated and stabilized prior to anesthesia induction.  HCM cats may be pre-medicated with oxymorphone at 0.01 mg/kg along with diazepam or midazolam at 0.1-0.2 mg/kg.  For cats that are not in failure and are asymptomatic induction can be achieved with ketamine/diazepam (midazolam) or low doses of propofol.  Cats who are symptomatic or are in failure should be induced with etomidate 0.5-1.0  mg/kg +/- a benzodiazepine.  These cats could also be masked or boxed down, but this method causes extreme stress in cats which can actually lead to sudden death in some patients, or at the very least, can cause arrhythmias.

Dilated cardiomyopathy (DCM)- Since the myocardium cannot pump blood out of the left heart effectively, blood begins to “back up” within the left side of the heart and within the pulmonary veins that feed into the left heart.  This leads to an enlargement of the heart in an attempt to compensate for the ineffective pumping. As blood “backs up,” left-sided congestive heart failure (CHF) or pulmonary edema (fluid within the lungs) develops.  This is not a feature exclusive to DCM, but is a common feature of many types of left-heart disease. Goals for anesthetizing these patients include avoiding tachycardia yet maintaining heart rate, maintaining myocardial contractility and avoiding fluid over load. 

Glycopyrrolate can be used to maintain heart rate. A neuroleptanalgesic technique using oxymorphone or hydromorphone at 0.1 mg/kg along with a benzodiazepine at 0.1-0.2 mg/kg can be used for premedication. Induction of patients with early signs of DCM can be induced with ketamine/midazolam, propofol or etomidate.  Patients with advanced DCM should be induced with etomidate +/- a benzodiazepine.  The addition of an opioid CRI such as fentanyl, or intermittent small boluses of an opioid can help lower MAC and reduce the vasodilating side effects of the inhalant.  Crystalloids can be given at 5 ml/kg/hr.

Ventricular and atrial septal defects: Patients with small lesions are generally considered low risk anesthesia patients.  Patients with large left to right shunts and heart failure are considered riskier patients.  In these patients it is paramount to avoid volume overloading which can lead to congestion.  Tachycardia should also be avoided therefore ketamine, a sympathetic stimulant, should not be used.  Stress and anxiety can be detrimental in these patients. 

These patients can actually benefit from a reduction in systemic vascular resistance.  Low dose acepromazine and the inhalant anesthetics are good choices for these patients.  A neuroleptanalgesic approach to premedication with attention paid to heart rate can be used.  Etomidate is the induction agent of choice for patients in failure.  Ventilation with 100 % oxygen is recommended to help with avoidance or worsening of pulmonary vascular resistance.

Patent ductus arteriosis (PDA): Patients are usually young when diagnosed with this congenital abnormality and therefore are not yet usually in failure.  When diagnosed early, flow is usually from the systemic circulation to the pulmonary artery.  After induction systemic vascular resistance (SVR) decreases and this can cause a reversal of the shunt flow.  This can be detected with a pulse oximeter because it causes de-saturation of arterial hemoglobin.  Phenylephrine, a potent vasoconstrictor can be used to increase SVR and reestablish left to right flow.  Also, during surgery, when the patent ductus is ligated often a reflex bradycardia in response to increased blood pressure.  This is normal and usually normalizes on its own.  Anticholinergics can be used if heart rate does not rebound. 

The anesthetic protocol for surgical patients is similar to any thoracotomy protocol although etomidate may be the induction drug of choice for patients in failure. The protocol for coil procedures will be similar except the post-op analgesia plan may not need to be as profound. Remember that blood pressure in young patients is often heart rate dependant so an anticholinergic may be needed to offset bradycardia from opioids. Hypothermia should be avoided.  Good post-op analgesia through an opioid CRI, wound soaker catheter and local blocks (bupivicaine in the chest tube) is paramount in assuring a smooth recovery for these young patients.

Regardless of the procedure being performed constant and thorough anesthetic monitoring of cardiac patients is essential.  In most cases, ECG, arterial blood pressure, pulse oximetry, and temperature monitoring are necessary and helpful parameters to monitor.  Blood gases and capnography are ideal. Central venous pressure (CVP) monitoring is often the only method of evaluating tricky cardio patient fluid therapy.  Patients in congestive heart failure may be currently managed with diuretics.  The potential for hypovolemia, or conversely fluid overload, is high.  Measuring CVP is an ideal way to stay on top of intra-operative fluid therapy.  If blood pressure therapy is needed in patients where fluid overload is a concern, the CRIs (dopamine, dobutamine, phenylephrine can be prepared at double or even triple the normal strength used by your hospital to reduce the amount of fluids given.

Careful constant monitoring is essential to maintaining an adequate plane of anesthesia.  Too deep a plane of anesthesia can significantly depress the cardiovascular system which can worsen cardiac disease.  Too light a plane of anesthesia can result in tachycardia, hypertension, and can cause the release of catecholamines and potential arrhythmias. Many times the cardiac surgeon will have certain preferences as to what monitoring and supportive care they prefer.  Depending on the procedure to be performed, certain accessible veins may be “off limits” because they require use by the cardiac surgeon (as in coil or balloon procedures).  Some cardiologists may prefer that mechanical ventilation not be used. 

Intermittent positive pressure ventilation can be beneficial in maintaining normal CO2 and preventing respiratory acidosis. However excessive intrathoracic pressure can cause a reduction in venous return and cardiac output. Gentle occasional hand bagging may be preferred. The cardiologist can also be helpful in the monitoring process of interventional radiologic procedures letting the anesthetist know when to treat arrhythmias, when more or less volume is needed and when to treat hypotension. Good communication with the cardiologist/surgeon peri-operatively is key to success.

References

Day, Thomas K: Anesthesia of patients with cardiac disease: In Greene, Stephen A. Veterinary Anesthesia and Pain Management Secrets, Philadelphia, Hanley and Belfus, Inc. 2002, 27: 157-163

Caulkett, Nigel: Anesthesia and Sedation of the Cardiovascular Patient. In Abbott, Jonathan Small Animal Cardiology Secrets. Philadelphia, Hanley and Belfus, 2000, 26: 151-158.

Harvey, Ralph C., Ettinger, Stephen J. Anesthesia and analgesia of patients with specific disease: Cardiovascular disease. In Tranquilli, William J., Thurmon, John C., Grimm, Kurt A. Lumb and Jones' Veterinary Anesthesia and Analgesia. 4th ed. Ames, Blackwell Publishing, 2007, 36: 891-894.

 Johnson, Craig. Patient Monitoring, in Seymour, Chris and Gleed, Robin. BSAVA Manual of Small Animal Anesthesia and Analgesia. British Small Animal Veterinary Association. United Kingdom; 1999, 5: 52-53.