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Surgical Approaches to Thoracic Disease

Raegan Wells, DVM, DACVECC
Phoenix Veterinary Referral and Emergency Center, Phoenix, AZ, USA

Thoracotomy is commonly performed in critically ill nitrogen skeleton that helps maintain normal alveolar
small animal patients with disease of the pulmonary distension. It is important to emphasize that oxygen sup-
parenchymal structures, intrathoracic cardiovascular plementation is necessary during the thoracotomy pro-
system, pleural space, thoracic wall, mediastinum, or cedure, and strongly recommended in the postoperative
esophagus. Aside from the underlying disease process, period. Nitrogen wash‐out is, however, an important
the thoracotomy procedure has the potential to induce pathophysiologic contributing factor to atelectasis in the
several negative effects that can be detrimental without thoracotomy patient. Prolonged lateral recumbency may
appropriate intensive monitoring and care. This chapter lead to pressure atelectasis, which results in significant
discusses the pathophysiologic effects of thoracic surgery, low ventilation–perfusion (V–Q) mismatching and
thoracostomy tube care, postoperative monitoring, over- intrapulmonary shunting. Hypoxemic pulmonary vaso-
all case management strategies, and potential complica- constriction is the natural defense mechanism to improve
tions incurred in postoperative thoracotomy patients. V–Q mismatching under such conditions. This empha-
sizes the importance of body positioning and frequent
rotation of recumbent patients in the immediate postop-
Pathophysiology of Thoracotomy erative period.

The most common surgical approaches to the thoracic Hypoxemia and Hypoventilation
cavity include intercostal thoracotomy, median sternot-
omy, and transdiaphragmatic incisional thoracotomy. Postthoracotomy hypoxemia can result from many eti-
Regardless of technical approach, some pathophysiologic ologies, including hypoventilation, V–Q mismatching,
consequences of entering the thoracic cavity may include and diffusion impairment. Delivery of oxygen (DO 2 ) is
atelectasis, hypoxemia, hypoventilation, hypotension, the product of arterial oxygen content (CaO 2 ) and car-
hypothermia, cardiac arrhythmias, pain, and continual diac output. In relation to hypoxemia, arterial oxygen
pneumothorax or effusion. content is dependent upon hemoglobin concentration
The four distending forces of the lungs that prevent (g/dL) and saturation as well as the partial pressure of
collapse under normal conditions are transpulmonary dissolved oxygen (PaO 2 ).
pressure (the difference between alveolar and pleural Hypoventilation is defined as a PaCO 2 greater than
pressure), tethering of the lungs to the surrounding 45 mmHg in the dog or 40 mmHg in the cat. Common
structures, pulmonary surfactant, and the nitrogen skel- causes of hypoventilation in postthoracotomy patients
eton. During thoracotomy, there is disruption of the include pain, residual anesthetics, respiratory fatigue, loss
natural subatmospheric pleural and transpulmonary of chest wall elasticity, decreased total pulmonary com-
pressure. Thoracotomy induces a positive pleural pres- pliance, increased airway resistance, and increased dead
sure which exceeds the alveolar pressure and this altera- space ventilation. There is an approximate 1:1 ratio to
tion from normal physiology rapidly leads to atelectasis. explain hypoxemia due to hypoventilation. That is, for
Nitrogen wash‐out occurs secondary to oxygen supple- every 1. mmHg elevation in PaCO 2 , there is a correspond-
mentation, which results in absorption atelectasis. This ing 1.0 mmHg decrease in PaO 2 . Calculation of the alveo-
occurs as the native nitrogen skeleton is washed out by lar to arterial oxygen gradient can be performed to rule
the greater than 21% FiO 2 , leading to a loss of the natural out pulmonary pathology as the cause for hypoxemia.

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