Page 358 - Clinical Small Animal Internal Medicine

326 Section 4 Respiratory Disease

Decreased transpulmonary pressure, pain, and recum- erated idioventricular rhythm, bundle branch blocks,
VetBooks.ir bency impede proper ventilation via atelectasis and and ventricular tachycardia.
decreased tidal volume. As previously mentioned, gravita-
tional forces may cause significant atelectasis and decreased
alveolar ventilation within the dependent lung, but perfu- Hypotension
sion is maintained (or even increased) within these pulmo- Postoperative hypotension is a common challenge for
nary segments. The overall net result in this situation is clinicians caring for patients following thoracotomy.
moderate to low V–Q mismatching and hypoxemia. Similar to the transpulmonary pressure gradient, subat-
Finally, hypoxemia following thoracotomy may also mospheric pleural pressure is also important to maintain
result from a combination of diffusion impairment and vascular distension within the thoracic cavity.
low V–Q mismatch secondary to reexpansion pulmonary During a normal breath, the elastic recoil of the lungs
edema or increased pulmonary vascular hydrostatic pres- upon inhalation allows distension of the large intrathoracic
sure. The pulmonary vasculature possesses significant vessels (caudal and cranial vena cava), promoting venous
permeability to protein compared to the extrapulmonary return and thus increasing preload. Positive intrathoracic
blood vessels. Since there is less intravascular oncotic pressure exerted during exhalation causes mild collapse of
pressure within this vascular circuit, elevated intrapul- these vessels, resulting in slight changes in pulse pressure
monary hydrostatic pressure will favor the movement of between inspiration and expiration. During mechanical
fluid into the alveolar and interstitial space relatively ventilation, the opposite effect occurs. With positive pres-
quickly. Thus, pulmonary hydrostatic pressure is the sure ventilation, the majority of venous return occurs dur-
main determinant for the development of pulmonary ing expiration, not inspiration. Anesthesia along with
interstitial fluid accumulation and subsequent edema. underlying pulmonary pathology often necessitates the use
Net filtration in the pleural cavity (pleural effusion) is of positive end‐expiratory pressure (PEEP). Baseline PEEP
dependent upon the capillary hydrostatic pressure of the is generally set at 5.0 cm H 2 O, but decreased pulmonary
visceral and parietal pleura, intrapleural hydrostatic compliance and atelectasis often require higher values of
pressure, plasma oncotic pressure, and intrapleural PEEP. Constant, low‐grade, positive intrathoracic pressure,
oncotic pressure. Unlike the low pulmonary interstitial as PEEP increases, further impedes venous return, result-
to vascular space oncotic gradient, a substantial gradient ing in significant reductions in cardiac preload. Decreased
is generated within the pleural space. The normal colloid preload leads to compromised stroke volume, cardiac out-
osmotic pressure (COP) of the pleural space is 3.2 cmH 2 O put, and subsequent reduced delivery of oxygen. Positive
(compared to 24.5–27.0 peripherally) with the average pressure ventilation also has beneficial effects as it
pleural pressure being –5 cmH 2 O. decreases afterload during inspiration, although this can be
offset by dramatic decreases in preload.

Hypothermia Pain
Hypothermia is a common occurrence during and Postoperative pain control is of critical importance fol-
immediately following thoracotomy. With either lowing median sternotomy or lateral thoracotomy. Both
median sternotomy or lateral thoracotomy, the pleural of these procedures induce pain. In addition to surgical
(visceral and parietal) body surface area is exposed to trauma, all postthoracotomy patients have an indwelling
relatively cool ambient temperatures. This results in thoracostomy tube(s) that is a frequent source of pain.
loss of body heat via evaporation and to some extent Hypoventilation and hypoxemia may be a direct result of
convection. Unimpeded hypothermia can have serious pleurodynia, incisional pain, or thoracostomy tube‐
detrimental cardiovascular, respiratory, acid–base, and induced discomfort. Some patients are unwilling or una-
coagulopathic effects. ble to maintain an adequate tidal volume due to painful
thoracic expansion. Hypoventilation secondary to pain
Arrhythmias may also contribute to significant atelectasis resulting in
hypoxemia secondary to V–Q mismatching.
Cardiac arrhythmias may be observed during and sub-
sequent to thoracotomy. Intraoperative cardiac arrhyth- Continual Effusion and Pneumothorax
mias may result from manual cardiac manipulation
and are typically ventricular in origin. Postoperative The goal of thoracic surgery is generally to palliate or
arrhythmias, however, are classically secondary to resolve the underlying disease process to improve cardi-
underlying pathophysiology including pain, hypoxemia, opulmonary, vascular, or esophageal function. Often, the
or physical irritation from thoracostomy tube(s). underlying disease process may not be immediately
Common arrhythmias include sinus tachycardia, accel- terminated post thoracotomy, such as chylothorax,