Support of respiratory function in critically ill patients is extremely important because inappropriate oxygenation and/or
ventilation can quickly lead to an animal's demise. Also, respiratory infections, especially nosocomial infections, can be
quite serious. Oxygen administration and mechanical ventilation are used to optimize oxygenation and ventilatory status. Humidification
of inspired gases is performed to minimize respiratory tract injury. Aerosol therapy is performed to facilitate treatment
of respiratory infections.
The primary indication for oxygen therapy is hypoxia. Hypoxia can result from (1) inadequate inspired partial pressure of
oxygen, (2) impaired pulmonary function, (3) ineffective or inefficient oxygen transport in the blood, or (4) increased tissue
oxygen consumption that is not matched by increased oxygen delivery. The need for oxygen therapy is determined by the clinical
status of the patient. Clinical signs associated with hypoxia include cyanosis (as long as there is at least 5 g/dl of reduced
hemoglobin in the blood), dyspnea, tachypnea, tachycardia, and anxiety. Oxygen is administered to patients with these clinical
signs until it is determined that supplemental oxygen is unnecessary. Arterial blood gases (PaO2, SaO2) and pulse oximetery
(SpO2) are helpful in determining the need for oxygen therapy. Although hypoxia is defined as PaO2 < 60 mmHg and SaO2 (SpO2)
< 90%, oxygen should be administered to any critical patient with these parameters below normal values [normal PaO2 = 80 to
100 mmHg; normal SaO2 (SpO2) = 97% to 100%]. In the absence of blood gases and pulse oximetry, resolution of clinical signs
during administration of oxygen would be evidence that the patient indeed needs oxygen therapy. Finally, it should be noted
that supplemental oxygen is used to optimize oxygen delivery in patients with systemic inflammatory response syndrome or shock
regardless of the clinical data.
Methods of supplemental oxygen administration include face masks (rigid or "baggie"), hoods, special cages or chambers, intranasal
catheters, intratracheal catheters, endotracheal tubes, and tracheostomy tubes. Intranasal catheters are the most practical
and efficient method for long-term oxygen administration. At relatively low flow rates (150 to 200 ml/kg/min) intranasal administration
can achieve 40% to 50% inspired oxygen concentration. The actual flow rate used is determined by the state of pulmonary function.
Monitoring during oxygen administration includes clinical signs, pulse oximetry, and arterial blood gases. An attempt is made
to deliver the lowest possible flow rate that will abolish clinical signs and return SpO2 and PaO2 into the normal ranges.
If supplemental oxygen fails to reverse hypoxia, ventilation therapy with positive end-expiratory pressure (PEEP) or continuous
positive airway pressure (CPAP) is instituted until the patient can achieve normoxia without ventilatory support. Oxygen supplementation
is discontinued when the state of hypoxia is reversed and the patient can remain normoxic on room air. Weaning from oxygen
therapy is achieved by monitoring clinical signs, SpO2, and arterial blood gases while the oxygen flow rate is decreased and
the oxygen is eventually turned off. Return of clinical signs or abnormal SpO2 (and/or PaO2) requires re-institution of the
previous level of oxygen supplementation. During oxygen therapy an effort is made to decrease the level of supplementation
such that the inspired oxygen concentration is 50% or less as soon as possible (within the first 8 to 12 hours) in order to
prevent oxygen toxicity.