Shock Due to Heart Failure in Dogs

Cardiogenic Shock

Cardiac dysfunction may be caused by an enlarged or dilated heart muscle, compression of the lining of the heart, outflow obstructions, blood clot, severe heart disease, heartworm disease, or severe arrhythmias.Cardiac pump failure may also be secondary to a systemic disease which causes the myocardial layer (middle layer of the heart) to dysfunction, such as from blood poisoning. The result is low blood pressure and compromised blood flow in the tissues, with reduced tissue oxygen delivery. Cardiogenic shock results from profound impairment of cardiac function, leading to a decrease in stroke volume (the amount of blood pumped out of each ventricle during contraction) and cardiac output, congestion of the veins, and narrowing of the blood vessels.

Most causative conditions for cardiogenic shock are associated with markedly depressed left or right ventricle function, but other conditions that can cause cardiac compression and lead to inadequate filling of the ventricles may also play a role. Low cardiac outflow can lead to severely low blood pressure, resulting in diminished blood flow to tissues. Leaking from the pericardium – the sac enclosing the heart - or conditions that cause severe inflow or outflow obstruction to the ventricles are possible culprits. Further, reduced blood flow to the tissues causes organ ischemia (loss of blood to organs) and energy depletion, leading to abnormal organ function. Secondary organs that are affected include the brain, heart, lung, liver, and kidneys. As shock progresses, congestive heart failure may develop. Abnormal rises in left atrial pressure and pulmonary venous pressure may lead to fluid being trapped in the lungs. Any breed, age, or gender can be affected by this condition.

Symptoms and Types

• Pale mucous membranes (from decreased blood flow)
• Cool extremities
• Variable heart rate and respiratory rate
• Harsh lung sounds and crackles
• Cough
• Weak pulse
• Muscle weakness
• Mental dullness
• Cardiac decompensation may be associated with a history of previously compensated heart disease and cardiac drug administration
• A suspicion of previously undiagnosed cardiac disease may result from a history of coughing, exercise intolerance, weakness, or loss of consciousness

Causes

Primary Cardiac Disease

• Dilated heart muscle ‒ large-breed dogs with taurine (aminosulfonic acid) deficiency
• Severe valve insufficiency or other end-stage disease of a heart valve
• Arrhythmia disturbances
• Pericardial constriction – tightening of sac around the heart

Secondary Cardiac Dysfunction

• Sepsis (systemic infection) may result in reduced cardiac contractility
• Excess phosphorus in the blood
• Pulmonary blood clot
• Gas in the pleural (chest) cavity

Risk Factors

• Concurrent illness causing hypoxemia (subnormal oxygenation of arterial blood), acidosis (an increase in the concentration of hydrogen ions in the arterial blood above the normal level), electrolyte imbalances.

Diagnosis

Because there are so many possible causes for this condition, your veterinarian will most likely use differential diagnosis. This process is guided by deeper inspection of the apparent outward symptoms, ruling out each of the more common causes until the correct disorder is settled upon and can be treated appropriately.

Blood pressure measurement will document low blood pressure, and electrocardiography may aid in the detection of arrhythmias. Pulse oximetry, a procedure using a device which measures oxygen saturation by fluctuations of light absorption in well-vascularized (supplied with blood) tissue during systole (contraction) and diastole (dilation) may reveal low blood pressure. Blood gas analysis may uncover metabolic acidosis, a decreased pH, and bicarbonate concentration in the body fluids, caused either by an accumulation of acids, or by abnormal losses of fixed base from the body, as in diarrhea or renal disease. A chest x-ray may reveal an enlarged heart or evidence of pulmonary edema (congestive heart failure). Echocardiography may document cardiomyopathy (disease of the heart muscle), disease of a heart valve, limited heart muscle contractability, or pericardial compression.

Treatment

If the degree of cardiac dysfunction has progressed to a condition of shock, intensive hospitalized treatment becomes necessary. Drainage of the pericardium is essential for patients displaying compression of the lining of the heart, and fluid therapy will be kept to a minimum until cardiac function is improved. This may be done with the use of positive inotropes, fluid or drug agents that alter the force or energy of muscular contractions; with vasodilators, which relax smooth muscles and widen blood vessels to improve flow; or by decompression of a pericardial (heart sac) leakage, as congestive heart failure may be exacerbated.

Cardiovascular monitoring will be conducted by electrocardiogram (ECG), which measures the electrical currents of the heart muscle, and measurement of central venous pressure and blood pressure is useful for determining the effectiveness of the given treatment. Oxygen supplementation is important, as with decreased blood flow, there is a concurrent decrease in oxygen reaching the tissues. Oxygen can be administered by oxygen cage, mask, or nasal tube. In addition, your veterinarian will choose any appropriate drugs to treat your dog's specific condition.

Living and Management

After the initial treatment, your veterinarian will want revisit your dog to monitor heart rate, pulse intensity, mucous membrane color, respiratory rate, lung sounds, urine output, mentation (mental activity), and rectal temperature.