Shock

Chapter 156 Shock



Eric R. Schertel




Key Point


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:


• An abnormality or inadequacy of the vehicle of oxygen transport (blood)

• An abnormality of the transport system (cardiovascular system)

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:


• Blood pressure and blood flow distribution

• Blood volume distribution

• Cardiac function


Table 156-1 CLASSIFICATION AND COMMON ETIOLOGIES OF SHOCK



Shock Involving Normal Circulatory Control Mechanisms



Hypovolemia

Hypoxemia
Anemia

Hypoxia


Shock Involving Abnormal Circulatory Control Mechanisms



Blood flow maldistribution
Sepsis

Trauma—surgical and accidental

Blood volume maldistribution
Endotoxemia

Anesthesia

Neurogenic shock

Anaphylactic shock

Gastric dilatation-volvulus

Impaired cardiac function
Systolic functional failure
Cardiomyopathy

Valvular heart disease

Myocardial ischemia

Myocardial contusion

Arrhythmias

Diastolic functional failure


Pericardial disease—tamponade

Myocardial disease—decreased diastolic compliance

Shock may also develop from the presence of one or more of these conditions.



• Sepsis is an example of a condition in which shock is caused by a loss of control of blood flow distribution.

• Endotoxemia and gastric dilatation-volvulus also cause shock by interfering with the distribution of blood volume, but via different mechanisms.

• Heart failure creates shock due to inadequate cardiac function (see Table 156-1).


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.



• Causes: Decreased cerebral blood flow and oxygen delivery, circulating toxins, or 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.



• Causes: Low cardiac output or low peripheral vascular resistance.


Mucous Membrane Color


The color may be pale, gray (muddy), cyanotic, brick-red, or normal.



• Causes: Pale mucous membranes reflect hypovolemia or anemia. Gray or cyanotic mucous membranes generally indicate severe cardiovascular compromise or arterial hypoxemia. Brick-red mucous membranes are common in septic shock. Mucous membrane color may be normal.


Body/Extremity Temperature


Hypothermia (<99°F) and cold extremities are common. Patients that are septic may be hyperthermic (>103°F) with warm extremities.



• Causes: Decreased cardiac output, oxygen delivery, and ambient temperature. Cold extremities result from severe vasoconstriction. Hyperthermia results from the increased metabolic rate that commonly accompanies sepsis. Warm extremities reflect peripheral vasodilatation.


Capillary Refill Time


Capillary refill time typically is prolonged (>2 seconds), but may be normal.



• Causes: Prolonged capillary refill time reflects hypovolemia and poor peripheral blood flow.


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.



• Causes: Hypotension, hypovolemia, pain, stress, and fever may cause tachycardia. Irregular rapid heart rhythms result from ventricular tachycardia and atrial tachyarrhythmias, including atrial fibrillation (see Chapter 145).


Respiratory Rate


Increased respiratory rate (tachypnea) is common, but may be due to excitement or fever.



• Causes: Hypoxemia, metabolic acidosis, pain, fever, and excitement.


Urine Output


Urine output is decreased (normal range is 1 to 2 ml/kg/hr).



• Causes: Urine formation virtually ceases when mean arterial pressure is <60 mm Hg. Blood pressure may be normal in mild to moderate hypovolemia.

Related posts:

  1. Disorders of Muscle and Neuromuscular Junction
  2. Diseases of the Adrenal Gland
  3. Respiratory Infections
  4. Principles of Oncology

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Shock

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