Page 303 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

294 ACID-BASE DISORDERS

bring arterial pH back to normal in dogs with BOX 11-3 Causes of Respiratory
longstanding (>30 days) respiratory acidosis. Renal com-
pensation in cats with chronic respiratory acidosis is not Acidosis
known. Cats do not increase renal ammoniagenesis dur-
ing experimental metabolic acidosis. 38 Cats may not be Large Airway Obstruction
able to compensate adequately in chronic respiratory Aspiration (e.g., foreign body, vomitus)
acidosis because an increase in ammoniagenesis is the Mass (e.g., neoplasia, abscess)
most adaptive factor. Tracheal collapse
Hypochloremia is a common finding in dogs with Chronic obstructive pulmonary disease
experimentally induced chronic hypercapnia. 44,62,69,79 Asthma
During recovery from chronic hypercapnia, chloride Obstructed endotracheal tube
Brachycephalic syndrome

restriction hinders the return of plasma HCO 3 concen-
Laryngeal paralysis/laryngospasm
tration to normal. Thus the kidney needs chloride to pref-
erentially resorb chloride with sodium, excrete excess Respiratory Center Depression
Drug induced (e.g., narcotics, barbiturates, inhalant
HCO 3 in the urine, and reestablish normal SID in
the plasma. anesthesia)
Neurologic disease (e.g., brainstem or high cervical cord
CAUSES OF RESPIRATORY ACIDOSIS lesion)
Respiratory acidosis and hypercapnia can occur with any Increased CO 2 Production with Impaired
disease process involving the neural control of ventilation, Alveolar Ventilation
mechanics of ventilation, or alveolar gas exchange, Cardiopulmonary arrest
resulting in hypoventilation, ventilation-perfusion Heatstroke
mismatches, or both. Acute respiratory acidosis usually Malignant hyperthermia
results from sudden and severe primary parenchymal Neuromuscular Disease
(e.g., fulminate pulmonary edema), airway, pleural, chest
Myasthenia gravis
wall, neurologic (e.g., spinal cord injury), or neuromus- Tetanus
cular (e.g., botulism) disease. 17 Chronic respiratory aci- Botulism
dosis results in sustained hypercapnia and has many Polyradiculoneuritis
causes, including alveolar hypoventilation, abnormal Tick paralysis
respiratory drive, abnormalities of the chest wall and Electrolyte abnormalities (e.g., hypokalemia)
respiratory muscles, and increased dead space. 17 In Drug induced (e.g., neuromuscular blocking agents,
patients with neuromuscular disease leading to muscular organophosphates, aminoglycosides with anesthetics)
weakness, the degree of hypercapnia appears to be out of Restrictive Extrapulmonary Disorders
proportion to the severity of muscle disease and may be Diaphragmatic hernia
underestimated without blood gas analysis. In these Pleural space disease (e.g., pneumothorax, pleural
patients, muscle weakness and elastic load are responsible effusion)
for the modulation of central respiratory output. This Chest wall trauma/flail chest
results in a rapid shallow or dyspneic breathing pattern Intrinsic Pulmonary and Small Airway
that leads to chronic CO 2 retention. 49 A more detailed Diseases
list of causes of respiratory acidosis is found in Box 11-3.
Acute respiratory distress syndrome
As determined by the alveolar gas equations (1) and Chronic obstructive pulmonary disease
(2) above, hypercapnia can result from a decrease in alve- Asthma
olar ventilation (either through a decrease in total minute Severe pulmonary edema
ventilation or increase in the dead space to tidal volume Pulmonary thromboembolism
ratio), or an increase in metabolic production of carbon Pneumonia
Pulmonary fibrosis
dioxide. In small animal clinical practice, increased CO 2
production infrequently results in hypercapnia. In normal Diffuse metastatic disease
circumstances (e.g., exercise), an increase in CO 2 produc- Smoke inhalation
tion is matched by an increase in CO 2 elimination via the Ineffective Mechanical Ventilation (e.g.,
lung. 82 However, if CO 2 production is increased with Inadequate Minute Ventilation, Improper
impaired or fixed alveolar ventilation that is unable to CO 2 Removal)
effectively remove CO 2 , acute respiratory acidosis may
develop, as is observed in a few conditions such as heat Marked Obesity (Pickwickian Syndrome)
stroke and malignant hyperthermia. 12,74
Decreased alveolar ventilation produces hypercapnia
from either a reduction in total minute ventilation (also