Page 296 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 11

Respiratory Acid-Base Disorders

Rebecca A. Johnson and Helio Autran de Morais

“Life is a struggle, not against sin, not against the Money Power, not against malicious animal magnetism,
but against hydrogen ions.”
Henry Louis Mencken (1919)

Respiratory acid-base disorders are those abnormalities in many other processes that affect breathing (i.e., cortical
acid-base equilibrium initiated by a change in arterial inputs, cardiovascular alterations, etc.). These inputs col-
carbon dioxide tension (PaCO 2 ). PaCO 2 is regulated by lectively establish the spatiotemporal efferent signals that
alveolar ventilation; a primary increase in PaCO 2 acidifies project to respiratory pump muscles such as the dia-
body fluids and initiates the acid-base disturbance called phragm and upper airway muscles involved in airway
resistance regulation. Altogether, these pathways result
respiratory acidosis, whereas a decrease in PaCO 2
alkalinizes body fluids and is known as respiratory alkalo- in muscular contraction, which subsequently drives
sis. The neural pathways controlling respiration initiate alveolar ventilation 50 (Figure 11-1).
and coordinate respiratory muscle contraction, thereby
generating adequate alveolar ventilation in the lungs. CHEMORECEPTORS AND
The primary responsibility of the mammalian lung is then CHEMOREFLEXES
to exchange gases at the alveolar blood-gas interface Chemoreceptors provide sensory inputs to the respira-
where carbon dioxide and oxygen move by diffusion tory control system concerning changes in CO 2 or O 2 .
from areas of high to low partial pressure. Diffusion of As a result, alveolar ventilation is altered via negative feed-
gases is directly proportional to the surface area of the back mechanisms to minimize variations in normal gas
interface and inversely proportional to the thickness levels. 6,19,50,55 The primary stimuli for changes in alveolar
of the membrane (Fick’s law). With a relatively large ventilation are hypoxemia (PaO 2 <60 mm Hg) and car-
surface area and a very thin (<1 micron) blood-gas bon dioxide-induced changes in intracellular and extra-
interface, the lungs are well suited for their role in gas cellular pH, although the magnitude of these changes is
exchange and thus, greatly influence respiratory acid-base dependent on the specific chemoreflex. For example,
regulation. arterial CO 2 levels are tightly regulated under normal
ventilatory control; increases in arterial CO 2 levels
CONTROL OF ALVEOLAR (PaCO 2 ) of 1 to 3 mm Hg will double alveolar ventilation.
VENTILATION In contrast, arterial O 2 pressures (PaO 2 ) can change up to
20 mm Hg with little alteration in ventilation. 50
The drive to breathe originates within respiratory centers In adult mammals, chemoreceptors are located in both
of the medulla (i.e., ventral respiratory column, VRC), the peripheral and central nervous systems. The most sig-
which consist of a network responsible for respiratory nificant oxygen-sensitive chemoreceptors are primarily
rhythm generation and respiratory pattern formation. found within the carotid body at the bifurcation of the
Although supramedullary brain structures, such as the internal and external carotid arteries. Although carotid
pons, cerebellum and forebrain, modulate breathing, body chemoreceptors sense changes in CO 2 and pH as
they are not required to breathe. Respiratory rhythm gen- well, these chemoreceptors dominate the hypoxic ventila-
eration and pattern formation are altered by homeostatic toryresponseandareresponsibleforapproximately90%of
control mechanisms that permit flexibility in the respira- the increase in minute ventilation seen with hypox-
tory control system (i.e., plasticity). For example, the emia. 14,18 Oxygen-sensitive receptors are also found in
medullary respiratory centers are modified by sensory the aortic bodies within the aortic arch. However, they
(i.e., chemoreceptors and mechanoreceptors) and modu- appear to play a minimal role during normal ventilation. 50
latory projections (i.e., serotonergic neurons), as well as In addition to the periphery, O 2 -sensitive chemoreceptors

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