Page 419 - Clinical Small Animal Internal Medicine

39 Acute Respiratory Failure 387

One recent study examined the distribution of disease typically be associated with an historical finding of vomit-
VetBooks.ir type/localization in dogs presenting for respiratory dis- ing, regurgitation, or a recent anesthetic procedure.
Alternatively, respiratory failure patients may present with
tress (termed dyspnea by the authors) [1]. In this study,
upper airway disease, lower respiratory tract disease,
pleural filling disorders, and cardiac disease represented a history of generalized weakness, regurgitation or dyspha-
gia, and megaesophagus if myasthenia gravis is present
32%, 33%, 19%, and 12% of the cases, respectively. If one concurrently. Laryngeal paralysis may be identified more
were to assume that each of these was equally likely to pro- typically in patients with a history of recent voice change,
gress to respiratory failure then period prevalence rates persistent panting, and heat or exercise intolerance. Many
might be estimated. However, this is clearly not the case. other examples could be provided, but space constraints
For example, pleural filling disorders are likely to be man- preclude an exhaustive accounting of all the historical and
aged effectively via drainage methods before overt respira- clinical signs that may precede respiratory failure.
tory failure becomes persistent. Dogs in this study with What are likely to be uniformly noted among all
lower respiratory tract disease were less likely to survive to patients with acute respiratory failure are signs of res-
hospital discharge. It is likely that these patients and piratory distress that precede overt failure. Tachypnea
patients with upper airway disease are more representa- and hyperpnea are commonly, yet not uniformly noted.
tive of patients that develop acute respiratory failure. Dogs Patients with hypercapneic ventilatory failure (excluding
with upper airway disease and pleural filling disorders those with excessive respiratory workload) may present
were younger on average than dogs with cardiac disease. with hypopnea or bradypnea which are often less obvi-
This finding may impact the epidemiology of respiratory ous than other findings in this setting. A more extensive
failure if such data were to become available. accounting of the signs of respiratory distress and dysp-
While respiratory distress is widely recognized as a nea may be found in Chapter 38.
common presenting complaint in small animal practice,
the epidemiology of respiratory failure in veterinary Diagnosis
medicine will remain largely unknown until consensus
definitions and routine surveillance measures become The diagnosis of acute respiratory failure is traditionally
commonplace.
based on arterial blood gas analysis. A sustained, repeat-
able PaCO 2 of greater than 60 mmHg is often sufficient
Signalment to diagnose acute hypercapneic respiratory failure. End‐
tidal carbon dioxide monitoring may serve as a surrogate
for PaCO 2 in many patients, but may have reduced sensi-
Acute respiratory failure can result from diverse disease tivity in patients with cardiovascular compromise and
processes and thus the signalment varies widely with the reduced pulmonary blood flows. Venous blood gas
precipitating cause. Certain breeds may be associated with measurements serve as poor surrogates for PaCO 2 when
diseases that can predispose them to the development of cardiac output is diminished and should be used cau-
respiratory failure. Select examples of such diseases for tiously, if at all, to diagnose hypercapneic respiratory fail-
which breed dispositions have been reported include feline ure. Hypoxemic respiratory failure is diagnosed based on
asthma, laryngeal paralysis, brachycephalic syndrome, cer- PaO 2 levels of less than 60 mmHg in patients breathing
vical myelopathies, pulmonary fibrosis, tracheal collapse, room air at sea level. Pulse oximetry may be substituted
bronchomalacia, metastatic neoplasia, congenital dia- if arterial blood samples cannot be obtained. Saturation
phragmatic hernia, mitral valve insufficiency, dilated car- readings obtained by these means of less than 90–92%
diomyopathy, and myasthenia gravis. The signalment will are consistent with hypoxemic respiratory failure.
thus be heavily dependent on the underlying cause. As Venous samples cannot be used to define acute hypox-
mentioned above, dogs with upper airway disease and emic respiratory failure although they may be the more
pleural filling disorders were younger on average than dogs relevant sample for defining alterations in systemic oxy-
with cardiac disease presenting for respiratory distress. gen delivery. Diagnosis of respiratory failure is discussed
Such associations might also be found in respiratory failure in more detail in Chapter 38.
case series, but such has not yet been reported.
Therapy
History and Clinical Signs
Therapy will be dependent on the form of acute respir-
History and clinical signs commonly associated with acute atory failure present. Hypercapneic and hypoxemic res-
respiratory failure will again vary substantially with the piratory failure will be discussed separately below
underlying cause. For example, aspiration pneumonia will although the two forms may occur simultaneously.