Emergency Management and Critical Care

Chapter 18 Emergency Management and Critical Care



Steven G. Cole, Kenneth J. Drobatz



INTRODUCTION


In general, cardiac emergencies may be divided into three groups:



Heart Failure


• Congestive heart failure (CHF) (commonly regarded as “backward” failure)
• CHF patients generally present with respiratory signs (due to pleural effusion or pulmonary edema) or abdominal distension (due to ascites).

• Low output (commonly regarded as “forward” failure)
• Animals with low-output heart failure most commonly have signs of weakness or collapse that are typically due to dilated cardiomyopathy or pericardial effusion.

• The term myocardial failure is used to denote the presence of reduced myocardial contractility (e.g., dilated cardiomyopathy)


Key Point


It is important to remember that the ultimate cardiac emergency is cardiopulmonary arrest. The management of cardiopulmonary arrest and strategies for cardiopulmonary resuscitation in small animals are covered in a separate chapter.



Cardiac Rhythm Disturbances


• The most common cardiac arrhythmias causing emergent presentations are third-degree heart block and tachyarrhythmias. These animals generally present with low output failure and signs of weakness or collapse.


Thromboembolism


• Thromboembolic disease typically presents with acute dysfunction of the area of compromised blood supply. In cats with cardiomyopathy, this is most often the hind limbs due to an aortic saddle thrombus, although other limbs may also be affected. Pulmonary thromboembolism may be an acute cause of respiratory distress and low output heart failure in dogs with a variety of underlying diseases. Thromboembolic disease is also encountered in animals with infectious endocarditis.


ASSESSING CARDIOVASCULAR FUNCTION IN THE EMERGENCY PATIENT



Physical Examination


• Historical complaints that support a primary cardiac emergency are varied and include weakness, lethargy, collapse or syncope, as well as cough, tachypnea, or respiratory distress. Additional complaints such as anorexia, vomiting, or diarrhea are not uncommon, as primary cardiovascular problems may have wide-ranging effects on all major organ systems.

• Physical examination findings consistent with primary cardiac emergencies are variable depending upon the specific condition.
• Mucous membranes may be pale secondary to vasoconstriction or cyanotic secondary to hypoxemia. Capillary refill time is commonly prolonged due to diminished cardiac output and hypoperfusion. Decreased tissue perfusion is more commonly seen in instances of low output failure vs. congestive failure.

• The majority of cases of canine and feline heart failure are accompanied by an audible murmur or gallop. Diminished heart sounds occur in cases of pericardial or pleural effusion or severe myocardial failure.

• Bradyarrhythmias or tachyarrhythmias are common, and are often associated with irregular rhythms and diminished pulse quality or pulse deficits.

• Pulsus paradoxus, where pulse strength gets weaker on inspiration is detected in approximately 50% of cases of pericardial effusion.

• Tachypnea and respiratory distress is often present. In cases of pulmonary edema, auscultation will commonly reveal harsh lung sounds or crackles. In cases of pleural effusion, lung sounds are commonly diminished ventrally.

• Animals suffering from low output failure often have a low body temperature and depressed mentation.

• Jugular pulses or distension is commonly detected in animals with right-sided heart failure.


Diagnostic Tests


• Diagnostic tests include electrocardiography (ECG), pulse oximetry, blood pressure measurement, chest radiographs and echocardiography.

• A lead II ECG is usually sufficient for the rapid diagnosis of most cardiac rhythm disturbances.

• Pulse oximetry provides a useful estimate of hemoglobin saturation and arterial oxygen content, findings that may be further evaluated by arterial blood gas analysis.

• Noninvasive blood pressure measurement may be accomplished via Doppler or oscillometric techniques, and reflects cardiac output and vasomotor tone. It is useful in the recognition of shock states and in monitoring therapeutic interventions.

• Chest radiography is the ideal method to assess size and shape of the cardiac silhouette, pulmonary vasculature, pulmonary parenchyma and pleural space. It represents the gold standard in documenting CHF in the form of pulmonary edema and/or pleural effusion.

• Echocardiography provides information about cardiac structure and function. It is typically used to confirm a suspected diagnosis, determine severity of disease, assess myocardial contractility, and detect intracardiac or proximal pulmonary artery thrombi. Echocardiography is particularly useful in assessing patients with pericardial effusion for cardiac neoplasia.


Key Point


Animals presenting with emergency problems related to heart disease are often not stable enough for prolonged diagnostic tests. Empiric therapy for the most likely cardiac problem based on signalment, history, physical examination, and chest radiographs is often performed. In many cases, appropriate therapy can be instituted without the need for an immediate echocardiographic exam.



EMERGENCY TREATMENT OF HEART FAILURE


• Treatment of heart failure involves the following:
• Identification and remediation of underlying causes

• Elimination of aggravating conditions (i.e., cardiac depressants, hypertension, arrhythmias)

• Control of congestion

• Improvement of myocardial contractility

• Improvement of myocardial relaxation

• Reduction of cardiac work

• Reduction of pathologic remodeling and neurohormonal activation


Congestive Heart Failure


• CHF results from elevated cardiac filling pressures that cause pulmonary or systemic venous hypertension and extravasation of fluid into the interstitial space or a body cavity.

• The location of this fluid is dependent upon the failing ventricle (left-sided, right-sided, or biventricular failure) and the subsequent signs of CHF relates to the magnitude of the fluid accumulation.

• CHF is the result of many cardiac diseases including chronic valvular disease, cardiomyopathies, infectious endocarditis, myocarditis, persistent arrhythmia, or congenital cardiovascular anomalies.


Left-Sided Congestive Heart Failure


• Left-sided congestive heart failure (LCHF) results from elevated left atrial pressure. Elevated left atrial pressure may result from mitral valve insufficiency, mitral valve stenosis, or systolic or diastolic dysfunction of the left ventricle. In dogs, LCHF causes pulmonary edema, whereas in cats it may be associated with both pulmonary edema and pleural effusion.

• Clinical signs
• Clinical signs of LCHF failure result from pulmonary compromise and include tachypnea, respiratory distress, lethargy, and exercise intolerance. Dogs with pulmonary edema frequently have a cough, whereas a cough is rare in cats with heart failure.

• Physical examination findings include harsh lung sounds and crackles in patients with pulmonary edema. Cats with pleural effusion usually have dull lung sounds in the ventral lung fields, especially when compared to the degree of respiratory effort. Severely compromised patients will present in respiratory distress, and cyanosis is not uncommon. Most patients with LCHF will be tachycardic and usually have abnormalities on auscultation such as a heart murmur, gallop rhythm, or an arrhythmia.


Diagnostics


• Confirmation of LCHF is made by obtaining chest radiographs to document the presence and severity of pulmonary edema or pleural effusion.

• In dogs, pulmonary edema tends to be most evident at the perihilar region, although all lung lobes may be affected in severe cases. Pulmonary edema in cats does not follow this pattern, and the location of affected lung tissue is variable. Pleural effusion in cats may be found either with or without concurrent pulmonary edema. Other changes supportive of LCHF include cardiomegaly, evidence of left atrial enlargement, and dilated pulmonary vasculature, particularly the pulmonary veins.

• ECG analysis may show a variety of changes, including prolongation of P wave duration (P-mitrale), increased R wave amplitude or duration, a left axis shift, bundle branch block, supraventricular or ventricular premature complexes or tachycardia, or atrial fibrillation.

• Echocardiography provides definitive information regarding the size of the cardiac chambers as well as indicators of systolic and diastolic cardiac function.

• Many patients that present with LCHF have severe respiratory distress and cannot tolerate a full diagnostic workup. Because of this, therapy is often instituted prior to obtaining radiographs or an echocardiogram. In these cases, the decision to treat for CHF is based on the history, clinical signs, and physical examination findings at the time of presentation. If access to a portable ultrasound machine is available, additional information may be obtained from a brief screening examination of the thorax for the presence of pleural effusion or grossly recognizable changes in cardiac structure (e.g., dilated left atrium, myocardial chamber dimension or wall thickness) or function (e.g., markedly diminished fractional shortening).


Treatment


• Emergency treatment of LCHF involves the use of diuretics, vasodilators, and in some cases, inotropic agents. Additionally, oxygen therapy is vital in patients with compromised pulmonary function, and some patients may benefit from the judicious use of anxiolytic drugs. Cats with significant pleural effusion require therapeutic thoracocentesis.


Diuretic Therapy


• The goal of diuretic therapy in the treatment of LCHF is to reduce the circulating blood volume, thereby reducing the preload of the left ventricle and left atrial pressure. The primary diuretic used in the acute management of CHF is furosemide.

• Furosemide may be administered intravenously, although the intramuscular route may be used in patients without vascular access.

• The dose is dependent upon the severity of clinical signs and patient response. In dogs, an initial dose of 2 to 6 mg/kg IV, IM, or SC may be followed by additional doses every 1 to 2 hours until the respiratory character improves. Following improvement, additional doses of 2 mg/kg are typically given at 6 to 12 hour intervals dependent on clinical status. Cats tend to be more sensitive to furosemide therapy, and initial doses of 1 to 2 mg/kg IV, IM, or SC every 1 to 2 hours followed by 1 to 2mg/kg every 8 to 12 hours are recommended following initial clinical response.

• Side effects of furosemide include dehydration, azotemia, hypokalemia, metabolic alkalosis, and, potentially, volume depletion.


Vasodilator Therapy


• Vasodilator therapy has two purposes. First, venodilation decreases preload by providing additional vascular capacitance. Second, arterial vasodilation reduces left ventricular afterload, thereby reducing myocardial work and promoting forward flow.

• Sodium nitroprusside (dogs; 2 to 10 mcg/kg/min constant-rate infusion [CRI]) increases the concentration of nitric oxide, a potent vasodilator, and is considered to be a balanced vasodilator providing both venous and arterial dilation. Combined with aggressive diuretic therapy, the use of sodium nitroprusside is very effective in resolving severe pulmonary edema. Because of its potency and the potential for excessive vasodilation and secondary hypotension, it is recommended that blood pressure be monitored closely during infusion.
• In all cases, the initial dose should be at the low end of the range, and the dose increased based upon clinical response and blood pressure measurement. In general, mean arterial pressure should be maintained above 60 mm Hg, while systolic pressures should not be less than 90 mm Hg.

• Sodium nitroprusside is light sensitive and may induce precipitation of co-administered IV drugs. Thus, it is ideally administered through a separate, light-protected IV set and catheter.

• Long-term nitroprusside therapy is limited by the production of molecular cyanide, although toxic doses are not usually reached until 36 to 48 hours following onset of therapy.

• Nitroglycerin ointment (2%) is a commonly used venodilator in small animal emergency patients. The ointment is applied topically to a clipped area on either the pinna or inguinal region and is dosed according to body size. A image-inch strip is used in cats and small dogs (< 10 kg), a ¼-inch strip in medium dogs (10 to 25 kg), and a ½-inch strip in large dogs (> 25 kg). The ointment may be applied every 12 hours for the first 24 to 36 hours of treatment and should be handled with gloves, as it may be absorbed transdermally.
• Although the use of nitroglycerin ointment is common, it should be noted that topically applied therapy is limited in severe CHF, especially given the profound peripheral vasoconstriction that exists in these patients, and efficacy is questionable.

• Hydralazine (dogs: 0.5 to 1 mg/kg IV or PO every 12 hours) is an arterial dilator that causes a marked reduction in afterload and is useful in cases of severe mitral regurgitation. As with other arterial dilators, hydralazine may be associated with hypotension and reflex tachycardia in the face of decreased peripheral resistance.


Positive Inotropic Agents


• Positive inotropic agents are indicated in managing LCHF associated with systolic dysfunction of the left ventricle. These drugs are administered in conjunction with diuretic and vasodilator therapy. Dilated cardiomyopathy is the most common disease producing this condition; however, myocardial failure secondary to advanced chronic valvular disease, or end-stage forms of other cardiomyopathies may also result in severe systolic dysfunction.
• Dobutamine (dogs, 2 to 15 mcg/kg/min CRI; cats, 1 to 5 mcg/kg/min CRI) is a beta-adrenergic sympathomimetic. Side effects of dobutamine may include tachycardia and ventricular arrhythmias. Cats may develop gastrointestinal or neurologic signs associated with administration.

• Dopamine (dogs and cats, 2 to 8 mcg/kg/min CRI) should be used with caution, as it has alpha-adrenergic effects at higher doses and may cause deleterious vasoconstriction and tachycardia in the face of diminished myocardial function.

• Additional options for inotropic support include phosphodiesterase inhibitors such as milrinone. Milrinone (dogs, 50 mcg/kg slow IV bolus followed by 0.40 to 0.75 mcg/kg/min CRI) is a drug that has both positive inotropic and vasodilatory properties, similar to a combination of dobutamine and sodium nitroprusside. Experience with clinical use in emergency patients is limited, but milrinone may be an effective agent for the short-term management of LCHF associated with systolic dysfunction.


Oxygen Therapy


• Oxygen therapy (40% to 60% fraction of inspired oxygen) helps to maintain the arterial oxygen content in the face of pulmonary dysfunction (in the form of ventilation-perfusion mismatch) induced by pulmonary edema. Oxygen therapy may also reduce pulmonary vascular resistance by ameliorating hypoxic pulmonary vasoconstriction.
• An oxygen cage is often the most effective method to administer supplemental oxygen to patients with heart failure, although other alternatives (such as nasal, mask, flow-by, hood, or intratracheal oxygen supplementation) exist. Although an oxygen cage provides a quiet environment that can achieve high concentrations of oxygen, these concentrations decrease rapidly when the cage is opened. Thus, an oxygen cage is less effective when patients require frequent treatments or physical examination.

• Patients should receive oxygen supplementation until their respiratory rate and effort have improved and/or objective measurements of pulmonary function (i.e., pulse oximetry or arterial blood gas) have returned to normal. If possible, the fraction of inspired oxygen should be tapered over 6 to 12 hours to allow the patient to adjust to breathing room air.

• For patients with fulminant LCHF and massive pulmonary edema, standard oxygen supplementation may not be sufficient to prevent either respiratory or ventilatory failure. In these cases, only early intubation and mechanical ventilation will provide the respiratory support necessary to sustain life. Mechanical ventilation is a significant commitment for both the owner and clinician; however, in many cases, mechanical ventilation may be weaned after only a short period (1 to 2 days) following aggressive medical management of CHF.

• The use of anxiolytic agents in the treatment of CHF is common in human medicine and may also be useful in veterinary patients. Low-dose morphine (dogs, 0.1 mg/kg IV every 4 to 6 hours as needed), butorphanol (dogs and cats, 0.1 to 0.2 mg/kg IV every 4 hours), diazepam or midazolam (dogs and cats, 0.1 to 0.3 mg/kg IV every 4 hours), or low doses of acepromazine (dogs and cats, 0.005 to 0.02 mg/kg IV every 6 to 8 hours) can be administered.


Therapeutic Thoracocentesis


• Cats with significant pleural effusions associated with LCHF experience significant benefit from therapeutic thoracocentesis. Some cats will tolerate the thoracocentesis with minimal restraint, although many cats require some degree of sedation (butorphanol [0.1 to 0.2 mg/kg] in combination with diazepam or midazolam [0.1 to 0.3 mg/kg]).
• Thoracocentesis is generally performed between the seventh and ninth intercostal spaces at the level of the costochondral junction. The area is clipped and aseptically prepared prior to initiating the procedure. Thoracocentesis in cats is generally performed with a 21-gauge butterfly catheter attached to a three-way stopcock and a 10-ml or 20-ml syringe. In extremely obese cats, a 22-gauge needle and extension set is used in place of the butterfly catheter. In larger dogs, a 16- or 18-gauge catheter with a 60-ml syringe may be used.

• The needle should be introduced just cranial to a rib to avoid the intercostal vessels and advanced into the pleural space. The needle should be redirected or the procedure terminated once lung tissue is felt at the tip of the needle or fluid is no longer able to be aspirated. Both sides of the chest should be aspirated as bilateral fluid accumulation is found in the majority of cats with pleural effusion secondary to CHF. It is not uncommon to remove 200 to 300 ml of fluid from the thorax of cats with severe pleural effusion.


Right-Sided Congestive Heart Failure


• Right-sided congestive heart failure (RCHF) is much less common in patients presenting to the emergency room. An exception is pericardial effusion and cardiac tamponade (see Pericardial Effusion below). RCHF results from elevated right atrial and central venous pressures. Conditions that may result in elevated central vencus pressure (CVP) include tricuspid valve insufficiency or stenosis, pulmonic valve insufficiency or stenosis, pulmonary hypertension, and right ventricular systolic or diastolic dysfunction.

• Clinical signs of RCHF are related to the presence of pleural effusion, ascites, or peripheral edema that results from increased right atrial pressure. Patients with large volume pleural effusion may present in respiratory distress and with dull lung sounds. Patients with ascites will present with a distended abdomen and may have respiratory compromise. Additional indications of RCHF are the presence of distended jugular veins and prominent jugular pulses, as well as the presence of a heart murmur (particularly with a maximal intensity at the left heart base or the right side of the chest). Other changes such as a split S2 sound or gallop rhythm are variable dependent upon the underlying disease.

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Aug 15, 2016 | Posted by in SMALL ANIMAL | Comments Off on Emergency Management and Critical Care

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