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Mechanical Ventilation
1
Kate Hopper, BVSc, DACVESS, PhD and Mathew Mellema, DVM, PhD, DACVECC 2
1 UC-Davis School of Veterinary Medicine, Davis CA, USA
2 Applaud Medical, San Francisco, CA, USA

A respirator or mechanical ventilator is a therapeutic is impractical or unavailable. Patients with severe hypox-
device that performs some or all of the work of breath- emia will frequently have significant lung disease such as
ing. Ventilators are used in clinical practice to provide pneumonia, acute respiratory distress syndrome (ARDS),
short‐ or long‐term support of respiratory function in pulmonary edema, diffuse alveolar hemorrhage, or pul-
patients with respiratory disease or failure. monary contusions. Pulmonary vascular disorders such
Two primary functions of the lung are oxygenation of as pulmonary thromboembolism (PTE) may also cause
the arterial blood and the elimination of carbon dioxide severe hypoxemia. Other causes such as anatomic right‐
from the venous blood in order to provide metabolic to‐left shunting are far less commonly encountered in
substrate, remove metabolic waste, and maintain home- veterinary practice.
ostasis. The ability of the lung to properly oxygenate the The second primary indication for mechanical ventila-
pulmonary capillary blood depends largely on several tion is persistent severe hypercapnia (hypoventilation)
key factors: that is refractory to other therapeutic measures. Severe
hypoventilation is marked by a PACO 2 >60 mmHg. The
adequate replenishment of alveolar gases with inspired air
● PACO 2 is determined primarily by alveolar minute ven-
adequate surface area for gas exchange
● tilation and carbon dioxide production (which is often
proper matching of ventilation and perfusion
● reasonably constant). Alveolar ventilation is dependent
maintenance of the structural integrity of the gas
● on the total amount of fresh gas that reaches the alveoli
exchange barrier.
over time and is equal to the product of the respiratory
In contrast, elimination of carbon dioxide is primarily rate and effective tidal volume. Effective tidal volume is
dependent on the repeated movement of fresh gas into total tidal volume minus dead space volume. Causes of
the alveoli and removal of portions of alveolar gas, a pro- severe hypoventilation are typically diseases that impair
cess known as alveolar ventilation. Respiratory failure can the ability of patients to maintain an adequate respira-
result from diverse disease processes that interfere with tory rate and/or tidal volume. Examples include intracra-
one or more of the lung’s primary functions. Patients with nial disease, cranial cervical myelopathies, peripheral
respiratory failure can generally be divided into three neuropathies, junctionopathies, and myopathies.
groups: those with hypoxemic failure, those with hyper- Severe hypoxemia or severe hypoventilation will not
capneic failure, and those with mixed respiratory failure, always require mechanical ventilation, but the criteria
which shares aspects of both of the other two types. provided above may help to identify potential candidates
for ventilator therapy. The necessity of mechanical venti-
lation should be made on a case‐by‐case basis and knowl-
Indications for Mechanical Ventilation edge of the primary disease process and related patient
data should factor heavily into the decision. Ventilator
There are three primary indications for mechanical ven- therapy is not entirely benign and a careful weighing of
tilation. The first is persistent severe hypoxemia (PaO 2 the risks and benefits should be made before the provi-
<60 mmHg) despite supplemental oxygen therapy. Severe sion of this approach.
hypoxemia may also be recognized by the presence of The third primary indication for mechanical ventila-
cyanosis or SpO 2 <90% when arterial blood gas analysis tion is persistent, excessive respiratory effort that may

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical