Porth's Essentials of Pathophysiology, 4e

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Heart Failure and Circulatory Shock

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of the myocardium, and increasing coronary perfu- sion. 56,58 Fluid volume must be regulated within a level that optimizes the filling pressure and stroke volume. Pulmonary edema and arrhythmias should be corrected or prevented to increase stroke volume and decrease the oxygen demands of the heart. Coronary artery perfu- sion is increased by promoting coronary artery vasodi- lation, increasing blood pressure, decreasing ventricular wall tension, and decreasing intracardiac pressures. Currently there are two therapeutic options for patients with cardiogenic shock to support the circulation— pharmacologic therapy and mechanical support. The goal of pharmacologic treatment is to increase cardiac contractility without increasing heart rate. Dopamine, dobutamine and norepinephrine are the most commonly used inotropic and vasopressor agents. 56,58,59 Dopamine is often the drug of choice because it acts both as an inotrope as well as a vasoconstrictor. Dobutamine, an inotropic agent with arterial vasodilator properties, can be used in persons with less severe hypotension. Catecholamines, such as norepinephrine, increase car- diac contractility but also result in arterial constriction and tachycardia, which worsens the imbalance between myocardial oxygen supply and demand. Overall, these drugs must be used with caution as they have been associated with dysrhythmias, myocardial injury, and increased mortality. 56,58 Mechanical support using an intra-aortic balloon pump or extracorporeal membrane oxygenation can help to increase systemic blood flow and stabilize the patient. 60 The percutaneous intra-aortic balloon pump, also referred to as counterpulsation, enhances coronary and systemic perfusion yet decreases afterload and myo- cardial oxygen demands. 56,60 The device, which pumps in synchrony with the heart, consists of a balloon that is inserted through a catheter into the descending aorta (Fig. 20-9). The balloon is timed to inflate during ventric- ular diastole and deflate just before ventricular systole. Diastolic inflation creates a pressure wave in the ascend- ing aorta that increases coronary artery blood flow and a less intense wave in the lower aorta that enhances organ perfusion. The abrupt balloon deflation at the onset of systole results in a displacement of blood vol- ume that lowers the resistance to ejection of blood from the left ventricle. Thus, the heart’s pumping efficiency is increased, myocardial oxygen supply is increased, and myocardial oxygen consumption is decreased. Obstructive Shock The term obstructive shock describes circulatory shock that results from mechanical obstruction of the flow of blood through the central circulation (great veins, heart, or lungs; see Fig. 20-7). It is also referred to as extra- cardiac shock. Obstructive shock may be caused by a number of conditions, including dissecting aortic aneu- rysm, cardiac tamponade, pneumothorax, atrial myx- oma, and evisceration of abdominal contents into the thoracic cavity because of a ruptured hemidiaphragm. 6 The most frequent cause of obstructive shock is pulmo- nary embolism.

Left subclavian artery

Left subclavian artery

Descending aorta

Renal arteries

Deflated

Inflated

FIGURE 20-9. Aortic balloon pump placed in the descending aorta. (With permission from the Division of Cardiac Surgery; Department of Surgery; Medical University Graz, Austria.)

The primary physiologic result of obstructive shock is elevated right heart pressure due to right ventricular dysfunction. Pressures are increased despite impaired venous return to the heart. Signs of right ventricular dysfunction occur, including elevation of CVP and jug- ular venous distention. Treatment modalities focus on correcting the cause of the disorder, frequently requir- ing a procedure to remove or correct the obstruction; for example, surgical interventions such as pulmonary embolectomy, pericardiocentesis (i.e., removal of fluid from the pericardial sac) for cardiac tamponade, or the insertion of a chest tube for correction of a pneumo- thorax or hemothorax. In severe or massive pulmonary embolus, fibrinolytic drugs may be used to break down the clots causing the obstruction. Once the obstruction to blood flow is removed then normal blood flow can be re-established. Distributive Shock Distributive or vasodilatory shock is characterized by loss of blood vessel tone, enlargement of the periph- eral vascular compartment, and displacement of the vascular volume away from the heart and central cir- culation. 61 In distributive shock, the capacity of the vascular compartment expands to the extent that a normal volume of blood does not fill the circulatory system (see Fig. 20-7). Therefore, this type of shock is also referred to as normo-volemic shock. There are