Page 433 - Clinical Small Animal Internal Medicine

40 Mechanical Ventilation 401

Ventilator‐Induced Lung Injury elevated PaCO 2 and a decreased PaO 2 is a common
VetBooks.ir Ventilator‐induced lung injury (VILI) describes pulmo- early warning sign. Airway pressure would be expected
to be increasing if a pneumothorax occurs in a patient
nary damage that occurs as a result of stresses and strains
placed on lung tissues during PPV. In addition to increas- being ventilated with volume control, while tidal vol-
ume would decrease if pressure control ventilation is
ing pulmonary pathology, in human medicine VILI has employed. Auscultation may reveal decreased lung
been found to increase the incidence of multiple organ sounds, but this finding may be difficult to appreciate in
dysfunction and patient mortality. It is believed that pul- the mechanically ventilated patient. Thoracic radio-
monary inflammation resulting from VILI leads to the graphs will provide a definitive diagnosis, but are often
elaboration of inflammatory mediators, which amplify impractical. Thoracic ultrasound can also be helpful in
the systemic inflammatory response and subsequent identifying a pneumothorax if the operator is suitably
organ damage. Indeed, some have suggested that the trained and experienced.
improved outcomes observed with lung‐protective ven- Ultimately, a pneumothorax can be rapidly fatal in the
tilator strategies are not due to improvements in pulmo- ventilator patient. If there is any doubt that one is pre-
nary performance but rather to reductions in distant sent, a diagnostic thoracocentesis should be performed
organ damage. immediately. If a pneumothorax is detected, unilateral or
There are several mechanisms by which VILI can
occur. The two major proposed mechanisms are overdis- bilateral chest tubes are likely to be required.
tension of the lung by excessive tidal volumes and repeti-
tive alveolar and peripheral airway collapse. Titration of Weaning from Mechanical Ventilation
ventilator settings should always be focused on provid- Weaning (or liberation) from the ventilator is generally
ing the lowest possible level of ventilator support that an ongoing process of gradual reduction in the ventilator
still allows the targeted goals to be achieved. When the settings. Often, as the patient improves, the mode of ven-
blood gas value goals cannot be achieved despite the use tilation may be changed to one that requires the animal
of high airway pressures and PEEP, a valid alternative to perform an ever‐increasing proportion of the work of
approach is to change the blood gas goals for this par- breathing. SIMV and pressure support modes are often
ticular patient. Permissive hypercapnia or permissive employed for this purpose. If animals have been anesthe-
hypoxemia‐based strategies may be employed in these tized for prolonged periods of time, it may take some
instances as described previously.
time (hours to days) for them to recover adequate venti-
latory capacity. Some anesthetic protocols (e.g., barbitu-
Oxygen Toxicity rates) employed for IPPV may have very prolonged
withdrawal periods. It is important to consider reducing
Pulmonary oxygen toxicity is discussed elsewhere in this or changing the anesthetic drugs administered when
text and the interested reader is referred to the appropri- patients begin to improve and weaning becomes a realis-
ate sections (see also Chapter 39). tic possibility.
Prior to discontinuance of IPPV, the patient should first
obtain certain physiologic goals, including the following:
Pneumothorax
the original disease process is stable or improving
A pneumothorax can occur in the ventilator patient ● a stable and appropriate level of respiratory drive has
when alveolar or airway rupture leads to air escaping ● been demonstrated
from the respiratory tract and accumulating in the pleu- the patient no longer requires significant ventilator
ral space. Although pneumothorax is a major concern ● support to achieve adequate gas exchange
when high airway pressures are present, studies in adequate gas exchange efficiency has been restored
human patients have not been able to demonstrate a sig- ● (i.e., PaO 2 :FiO 2 ratio consistently above 200; greater
nificant association between airway pressure and the than 300 is preferred).
occurrence of pneumothorax. Rather, the incidence of
pneumothorax seems to vary with the nature of the lung Reasons to suspect that a patient is not ready to be
pathology present. Overdistension of the lung is thought weaned include cardiovascular instability, requirement
to be the major cause of pneumothorax. Thus, maintain- of high FiO 2 (greater than 60%), high peak inspired air-
ing low tidal volumes, especially in patients with signifi- way pressures (>25 cmH 2 O), and/or high PEEP levels
cant lung disease, is currently recommended. (>5 cmH 2 O).
Pneumothorax can be rapidly progressive in patients When a patient has improved sufficiently to consider
receiving IPPV and it is imperative that it be recognized removing them from the ventilator, it is advised to first
early to avoid adverse outcomes. The acute onset of an determine if they can maintain spontaneous breathing