Page 571 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Shock Syndromes 559

observations have documented the phenomenon of There are three main mechanisms by which decreases
maldistribution of blood flow, but such measurements in ECV can occur (Table 23-2). 61
are not necessarily representative of all areas and are not 1. Hypovolemia may lead to inadequate blood volume
quantitative measures of the extent of maldistribution and hence inadequate preload, causing a decrease in
throughout the body. Therefore maldistribution of blood cardiac output.
flow is a physiologic concept that may be relevant to all 2. Cardiogenic causes in which abnormalities in cardiac
shock states. 130 function lead to inadequate cardiac output despite a
The various classifications of shock syndromes in vet­ normal or increased blood volume.
erinary and human medicine have been created in an 3. Distributive causes in which blood volume and cardiac
attempt to simplify a complex series of physiologic events. function are normal but there are alterations in sys­
Unfortunately, laboratory research and clinical experi­ temic vascular resistance globally or locally leading
ence have not supported any one classification of the to inadequate ECV.
shock syndromes as being the easiest to understand or The cardiovascular system has several compensatory
teach. The specific details surrounding the presentation responses to defend a fall in ECV. An understanding of
of the patient with shock should rank as most important, these responses can explain the clinical presentation of cir­
and valuable time should not be wasted deciding which culatory shock and the approach to therapy. Hemorrhagic
classification scheme best describes the patient. shock, a form of hypovolemic shock has been modeled
extensively in experimental animals and provides an ideal
PATHOPHYSIOLOGY example for this discussion.

Adequate tissue perfusion is not dependent on one simple HEMORRHAGIC SHOCK
value, rather it requires the integration of the entire cardio­
vascular system and is best described by effective Baroreceptor-Mediated Responses
circulating volume. Effective circulating volume (ECV) When ongoing hemorrhage occurs in the conscious dog
is a difficult to define term that describes the “fullness” there is an initial normotensive period (moderate hemor­
of blood vessels. In essence adequate ECV requires an ade­ rhage)followed byhypotension (severehemorrhage). 53,124
quate blood volume delivered at an adequate pressure. 11 During moderate hemorrhage, the decrease in cardiac out­
Although tissues require the maintenance of ECV for put is sensed by the high pressure baroreceptors of
long-term function, the cardiovascular system considers the carotid bodies and aortic arch. The subsequent
the maintenance of normal mean arterial pressure decrease in baroreceptor afferent traffic to the vasomotor
(MAP) as its number one priority. 55 This is primarily center of the brain causes sympathetic tone to increase,
because perfusion of the heart and the brain is pressure parasympathetic tone to decrease, and vasopressin
dependent; these vital organs need a minimum MAP for release. This results in an increase in heart rate and sys­
adequate perfusion. In terms of physics, maintaining a temic vascular resistance. Sympathetic mediated vasocon­
constant pressure is far more feasible for the cardiovascular striction is more prominent in precapillary arterioles and
system than maintaining a constant flow or volume. 11 the blood vessels of the skin, skeletal muscles, and
Figure 23-1 outlines the interrelationship of the major splanchnic viscera are vasoconstricted to a greater degree
cardiovascular parameters and how they influence MAP. than the rest of the body. This is an effort to centralize

Systemic
vascular
resistance
Preload
Mean
arterial Stroke Contractility
blood volume
pressure

Cardiac
output Afterload

Heart
rate
Figure 23-1 Cardiovascular parameters controlling mean arterial blood pressure. Mean arterial blood
pressure is the product of cardiac output and systemic vascular resistance. Cardiac output is further
dependent on both heart rate and stroke volume.