Chapter 156 Shock
Shock is the clinical state resulting from an inadequate supply of oxygen to the tissues or an inability of the tissues to utilize oxygen properly.
Shock involves numerous physiologic disturbances and pathologic changes that affect multiple organ systems in different ways. Veterinarians often are alerted to the presence of shock in their patients by the physical findings of depressed mentation, pale mucous membranes, tachycardia, and weak pulse pressure. These clinical signs are the manifestations of a complex process and do not represent the full extent of the problem. The objective of this chapter is to provide a simplified approach to the diagnosis, monitoring, and treatment of shock. Information on the various manifestations, mechanisms, and temporal patterns of shock is beyond the scope of this book, but nonetheless is considered important to appropriate and successful therapy. Therapy for shock caused by acute heart failure is discussed in Chapter 147. See appropriate chapters for discussion of specific diseases that can cause shock.
ETIOLOGY AND CLASSIFICATION
Shock generally is classified by etiology because each cause of shock may produce distinct primary and secondary pathophysiologic changes and temporal patterns. Shock etiologies can be organized into those forms that result from:
Hypovolemia and hypoxemia (anemic, hypoxic) are examples of the first category (Table 156-1). Diseases that disrupt the cardiovascular system and its control mechanisms (circulatory control mechanisms) also cause shock. The three important circulatory control mechanisms regulate:
Table 156-1 CLASSIFICATION AND COMMON ETIOLOGIES OF SHOCK
Shock may also develop from the presence of one or more of these conditions.
CLINICAL SIGNS
Mental Attitude
Depressed mentation often is the most apparent physical finding in a patient with shock. This parameter is subjective and may be complicated by head injury.
Arterial Pulse Pressure
A weak pulse is common but not necessary for shock to exist. The pulse feels weak at mean arterial pressures <60 to 70 mm Hg. Below 40 mm Hg, the pulse may not be palpable.
Body/Extremity Temperature
Hypothermia (<99°F) and cold extremities are common. Patients that are septic may be hyperthermic (>103°F) with warm extremities.
Heart Rate
Heart rate is commonly elevated: >140 beats per minute (bpm) in large-breed dogs, >160 bpm in small-breed dogs, and >180 bpm in cats. Begin monitoring with electrocardiography (ECG) if an irregular rhythm is detected.
Respiratory Rate
Increased respiratory rate (tachypnea) is common, but may be due to excitement or fever.
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