Heart failure and scary arrhythmias (Proceedings)

Definitions of Heart Failure

Within the body, the cardiovascular system is responsible for maintaining normal arterial blood pressure, normal blood flow to the tissues, and normal venous and capillary pressures. Heart failure results when heart disease is so severe that the cardiovascular system can no longer maintain one or more of these functions. Heart failure is defined in simple terms as the inability of the heart to pump blood at an appropriate rate to sustain the tissues of the body. This produces a clinical syndrome divided into the categories of low cardiac output (forward) heart failure, and left or right sided congestive (backward) heart failure. Commonly, we refer to these as the two types of heart failure, forward or backward. In addition, cardiogenic shock is defined as drastically reduced forward blood flow with concurrent low blood pressure which results in inadequate oxygen delivery to the tissues of the body.

Causes of Heart Failure

The most common causes of heart failure are myocardial systolic failure, valvular regurgitation, and diastolic myocardial dysfunction. Examples of these include respectively; dilated cardiomyopathy (DCM), mitral or tricuspid regurgitation, and constrictive or hypertrophic cardiomyopathy (HCM).

Signs of Heart Failure

Forward heart failure (systolic failure or low cardiac output) typically results in poor tissue perfusion. Clinical presentation may include weakness and fatigue (exercise intolerance), cold extremities, slow capillary refill time, poor mucous membrane color, and if output is severely decreased, oliguria, azotemia, and lactic acidosis. Usually systemic arterial blood pressure is maintained by means of one or more neurohumeral responses (vasoconstriction, sodium and water retention, increased heart rate, increased contractility, or possibly an increase in myocardial mass). With a slow, progressive systolic failure (DCM), the clinical signs specific to forward failure may not be obvious until generalized heart failure (left and right backward) has occurred and the patient cannot compensate any longer. Owners may actually notice signs of right sided backward heart failure (ascites as one example) first due to visual perception of abdomen size compared to noting a change in exercise tolerance.

Left or right backward heart failure is commonly referred to as congestive heart failure (CHF) due to the signs of congestion and edema it produces. Left sided CHF typically results in tachypnea, dyspnea, coughing, crackles on auscultation, and hypoxemia secondary to pulmonary edema. Right sided CHF typically presents as ascites, hepatic enlargement, pleural effusion, and sometimes jugular vein distension. Exceptions may occur in the cat where pleural effusion often develops with left sided disease.

Examples of Heart Failure

Dilated cardiomyopathy is an example of decreased systolic myocardial function resulting in a forward, and possibly concurrent backward, heart failure. The dilated ventricles of the heart cannot produce adequate cardiac output. Usually this is a chronic development, and may go unrecognized until heart failure is severe and generalized. Presenting signs are consistent with forward and backward failure (exercise intolerance, ascites, dyspnea, pale mucous membranes). Ventricular tachyarrhythmias and atrial fibrillation are common with DCM. Treatment must address both forward and backward failure and frequently includes positive inotropic drugs to increase contractility and improve cardiac output, diuretics to reduce pulmonary edema, and venodilators to reduce systemic congestion. Treatment protocols would be tailored to each patient and would become more aggressive as needed according to lack of response to therapy. Early management of the disease might include diuretics (furosemide, spironolactone, hydrochlorothiazide), an ACE (angiotensin converting enzyme) inhibitor (enalapril, benazepril), and a positive inotrope (Digoxin). Intermediate treatments may include a B-blocker (metoprolol) or Pimobendan (a calcium sensitizer and phosphodiesterase inhibitor) which both require careful monitoring. Pimobendan has both positive inotropic and vasodilator properties. During myocardial failure and congestion treatment would be ramped up and may include diuretics, mixed veno and arteriodilator (sodium nitroprusside), and positive inotropes (dobutamine, amrinone, milrinone). Antiarrythmic drugs are usually indicated. Treatment would vary dependant on diagnosis, but they are typically tachyarrhythmias of supraventricular or ventricular origin. Large breed, male, middle age dogs are predisposed (particularly Boxers and Doberman Pinschers).

Valvular regurgitation is the leading cause of heart failure in dogs with mitral valve regurgitation being more common than tricuspid. Mitral valve regurgitation results in left sided CHF; with rupture of the chordae tendinae (fibrous strands that anchor the valve leaflets to the ventricle wall) as a common occurrence when the disease is severe. Tricuspid valve regurgitation causes right sided CHF. Early treatment is directed to manage chronic symptoms; and may include diuretics to reduce intravascular volume, ACE inhibitors to discourage sodium and water retention and reduce systemic congestion, and a sodium restricted diet. During failure, treatment may include dilator therapy (nitroglycerin-venodilator, hydralazine-arteriodilator, amlodipine-calcium channel blocker with arteriodilator properties, nitroprusside-mixed dilator), and positive inotropes (digoxin, dobutamine, Pimobendan) to improve forward output. Atrial or ventricular premature contractions may occur but may not require therapy. Tachyarrhythmias should be treated. Small breed, male dogs are predisposed with regurgitation being more common with aging.

Hypertrophic cardiomyopathy is an example of diastolic myocardial dysfunction. The thickened wall and poor relaxation of the left ventricle impedes diastolic filling. Increased pressures due to impaired ventricular filling result in left atrial enlargement and mitral regurgitation. Failure can then be due to backward failure (increased atrial pressure and regurgitation) and also forward failure (inadequate stroke volume). HCM is typically seen in cats; and cats may present with a concurrent saddle thrombus (left atrial thromboembolus that breaks away and lodges in the posterior aortic trifurcation). Presenting signs of ATE (aortic thromboembolus) include tachypnea, panting, vocalization, cool rear limbs, posterior limb pain with hard gastrocnemeus muscles, cyanotic nail beds, and poor or absent femoral pulses. Treatment of HCM is aimed at immediate management of generalized heart failure if present. Aggressive use of diuretics, oxygen supplementation, and limiting stress is crucial. Thoracocentesis would be indicated if pleural effusion is present. Further treatment is geared at improving contractility and reducing heart rate and might include use of a calcium channel blocker (Diltiazem) to promote myocardial relaxation, B-Adrenergic antagonist (atenolol) to slow heart rate, ACE inhibitor (Enalapril) to reduce systemic congestion, and diuretics to reduce pulmonary edema. Treatment for ATE may include waiting for collateral circulation to form in the rear legs, and anticoagulant drugs to discourage additional clot formation.

Common Cardiac Arrhythmias

Arrhythmias can often be intimidating or scary to see. But a systematic approach to looking at each ECG can help a technician feel more confident in their interpretation of, or reaction to, an arrhythmia. Any ECG demonstrating an arrhythmia needs to be judged as either; alarming but not life threatening (does not compromise the patient's hemodynamic state) or alarming and is life threatening (compromises the hemodynamic state and/or increases the risk of sudden death). This discussion is directed at only a few common arrhythmias, and will hopefully help a technician become more aware of when to quickly alert a clinician to an existing arrhythmia.

Sinus bradycardia is characterized by a normal sinus rhythm at a rate of less than 60 beats per minute in alert dogs, and less than 150 beats per minute in cats. Sinus bradycardia rarely results in hemodynamic imbalance, but can occasionally cause syncope related to decreased cardiac output. Treatment of symptomatic dogs may include an anticholinergic drug (atropine), or surgical pacemaker implantation if unresponsive to drug therapy.

Sinus arrest may look like a simple sinus arrhythmia on an ECG tracing at first glance. It is characterized by an ECG tracing with a period of greater than two P-P intervals without depolarization of the sinus node. Collapse or syncope can occur if a pause lasts for several seconds. Sick sinus syndrome is a common cause of sinus arrest where there are diffuse conduction system disease and non-responsive nodes. Therapy may include anticholinergic drug (atropine), sympathomimetic drugs (terbutaline, albuterol) or pacemaker implantation.

Atrioventricular (AV) block is classified as incomplete (1st or 2nd degree block) or complete (3rd degree block). First or second degree block is typically self-limiting, and usually produces no hemodynamic compromise. Third degree AV block typically produces low cardiac output, often manifested by weakness or syncope, and carries a moderate to high risk of sudden death. Pacemaker implantation is definitive treatment. Oral drug therapy of anticholinergics and bronchodilators can be attempted if pacemaker implantation is not possible.

Supraventricular tachycardia can be a life threatening arrhythmia. This arrhythmia usually appears as a narrow QRS complex and the P wave may be normal in appearance or be superimposed in the QRS-T wave. Heart rates are typically very high, but rates are dependent on the size of the patient; smaller dogs having higher heart rates than larger dogs. Supraventricular rhythms may be considered tachycardia at a rate of 160 bpm in a Golden Retriever, but the same may not apply to a small Poodle. Heart rates may reach 250-300bpm during supraventricular tachycardia, and the rhythm can rapidly deteriorate into atrial fibrillation. Treatment may include vagal maneuver (applying digital pressure to both eyes or carotid sinus) and administration of beta-blocker (atenolol, propanolol, Esmolol) or calcium channel blockers (diltiazem) in emergency situations. Long term treatment may include beta-blocker drugs, calcium channel blockers, or digoxin and other antiarrhythmics (procainamide, Sotalol, amiodarone).

Atrial fibrillation is typically referred to as a rapid, irregularly irregular rhythm. The tracing will have upright QRS complexes and irregularities in the baseline. It is a commonly presenting arrhythmia in dogs with DCM, and can be life threatening depending on the overall condition of the patient. Treatment is geared toward immediate improvement of concurrent heart failure. Other therapies may include calcium channel blockers, beta-blockers, and digoxin.

Ventricular arrhythmias or ventricular tachycardia is a commonly seen arrhythmia, and may be one of the most easily recognized. A premature ventricular contraction (PVC) typically demonstrate a wide complex QRS-T with a large T wave in the opposite direction of the QRS complex and no direct relationship with a P wave. The decision to treat an arrhythmia can depend on many factors. Clinicians often monitor the patient carefully to assess the hemodynamic state. Factors influencing treatment include; heart rates in excess of 180 beats per minute, multifocal premature ventricular contractions (PVC), greater than 30% PVC's, or sustained runs of continuous ventricular tachycardia. Ventricular arrhythmias in feline patients are often associated with significant cardiac disease or life threatening underlying disease. Tachyarrhythmias impair ventricular filling and result in decreased cardiac output. Ventricular tachyarrhythmias can progress to ventricular fibrillation; a potentially fatal arrhythmia. Treatment for ventricular arrhythmias may include management of underlying disease (GDV, sepsis, trauma) and antiarrhythmic drugs (lidocaine, procainamide). Treatment of choice for ventricular fibrillation is electrical defibrillation.

References

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Wall RE, Rush JE. Cardiac emergencies. In Murtaugh, Kaplan. Veterinary emergency and critical care Medicine. St. Louis: Mosby, 1992; 214-246.

Wingfield WE. Pathophysiology of congestive heart failure. In veterinary emergency medicine secrets. Philadelphia: Hanley & Belfus, 1997; 20-26.

Keene BW. When & how to treat common cardiac arrhythmias. Proceedings IVECCS VII, 2000; 101-105.

Rush JE. Treatment of Life-Threatening Arrythmias. Proceedings IVECCS XI, 2005: pp. 147-151.