Page 422 - Clinical Small Animal Internal Medicine

390 Section 5 Critical Care Medicine

Table 39.1 Approximate flow rates of 100% oxygen via nasal drying hypothesis is one explanation proposed for exer-
VetBooks.ir Body weight must be considered and FiO 2 values above 50% may cise‐induced asthma.
cannulae required to achieve an FiO 2 between 30% and 90%.
be difficult to achieve in large dogs using this method
Prognosis
Required flow rate in L/min to
achieve an FiO 2 of…
A specific prognosis for acute respiratory failure cannot
Body weight (kg) 0.3–0.5 0.5–0.75 0.75–0.9 be provided in a meaningful way. Clearly, acute respira-
tory failure is not good news in any context. However,
0–10 0.5–1.0 1.0–3.0 3.0–5.0 some causes may be easily resolved and carry a good
10–20 1.0–3.0 3.0–5.0 >5.0 prognosis (fluid overload) whereas others are challeng-
20–40 3.0–5.0 >5.0 ND ing to address and carry a quite guarded prognosis
(ARDS). The prognosis will naturally vary with the
ND, not determined. underlying cause. Further, prognosis is impacted by
whether a specific, effective therapy is available for that
disease process or whether one must rely solely on sup-
should be determined on an individual case basis. In portive measures. The issue of prognosis is further com-
many cases, FiO 2 will be known (e.g., 21% or 100%) but plicated by the inherent difficulty in reaching a definitive
at other times it must be estimated. In humans breath- diagnosis in many cases of acute respiratory failure.
ing with nasal cannulae in place, the rule of thumb is Patients are often too unstable to undergo the diagnos-
that one can add 4% for each L/min of oxygen flow. tics required to definitively identify the underlying cause
Thus, a person breathing with a 2 L/min flow of oxygen and thus owners are often required to make difficult
would have an estimated FiO 2 of 28–29% (i.e., 21 + (2 × decisions based on incomplete information.
4)). This rule works poorly in veterinary patients as their As mentioned above, retrospective studies of long‐term
size and tidal volumes vary so much more widely than in mechanical ventilation in veterinary patients provide
humans. Estimates of FiO 2 can be made based on a con- some insights into acute respiratory failure, but represent
sideration of both flow rate and patient size (see a highly biased sample of such cases. In one such study,
Table 39.1). Prolonged (>24 h) exposure to greater than veterinary patients with pulmonary parenchymal disease
60% oxygen can result in pulmonary oxygen toxicity in (i.e., hypoxemic respiratory failure) were able to be suc-
most mammals although the toxic range varies with cessfully weaned 36% of the time, with an overall rate of
species and age. 22% of cases surviving to hospital discharge. Outcomes
However supplemental oxygen is provided, it is impor- were better for patients experiencing hypercapneic res-
tant that the gas be properly conditioned. Conditioning piratory failure, with 50% being successfully weaned and
in this sense means appropriately warmed and humidi- 39% surviving to hospital discharge. Patients with mixed
fied. The inhalation of improperly conditioned gas respiratory failure (both hypoxemic and hypercapneic)
is associated with several adverse effects in humans, had the poorest outcomes in this study. Overall, cats fared
including increased nasal airway resistance, damage to more poorly than canine patients. Whether this is due to
the nasal mucosa (both structural and functional, includ- the types of diseases that lead to the need for IPPV in cats
ing epithelial metaplasia and keratinization), increased or whether cats are at greater risk for complications such
work of breathing, difficulty in subsequent intuba- as ventilator‐induced lung injury (VILI) remains undeter-
tion, patient discomfort, and poor patient compliance. mined. Cats are a smaller species on average than dogs
Breathing cold, dry gas from compressed sources can and smaller species develop VILI more readily than do
lead to airway dessication (particularly if the nasal pas- larger species even when identical inspiratory pressures
sages are bypassed), increases in the tonicity of airway are applied. No direct comparison of VILI incidence in
lining fluid, and mast cell degranulation. This airway cats versus 3–7 kg dogs has yet been performed.

Reference

1. Fonfara S, de la Heras Alegret L, et al. Underlying hospital because of dyspnea: 229 cases (2003–2007).
diseases in dogs referred to a veterinary teaching J Am Vet Med Assoc 2011; 239(9): 1219–24.