StandardsCritical care oxygen therapy
Whenever an animal is in respiratory distress due to injury or disease, hypoxia may well be present and if severe, death may occur. With the front of a cage covered (Fig 6a) to make it a relatively confined area, oxygen therapy can significantly raise oxygen levels in this critical period. Unfortunately, because the cost of oxygen from cylinders is usually prohibitive, an animal’s survival without oxygen therapy is left to chance. This financial limitation is resolved if an oxygen concentrator is used (Fig 6b). With a 5 litre per minute flow, oxygen levels can be significantly increased within the cage, the level reached depending on the size of the cage and how well it is sealed.
Fig 6a & b: sealed cage for oxygen therapy requires large volumes of oxygen. Cylinders (left photo) can be replaced with an oxygen concentrator (right photo) which can run continuously for days at minimal cost. Clinically the first 12 hours after surgery or injury is the most critical; oxygen therapy dramatically increases the chances of recovery.
From a physiological perspective, oxygen tensions do not have to be raised by much to have a significant effect. It has been shown in humans that, by raising inspired oxygen tension to 30% or more, a significant improvement in lung alveolar oxygen occurs (Fig 7). Increasing inspired oxygen tensions from 21% to 30% increases alveolar oxygen to near normal levels even in severely compromised patients. At half normal respiration alveolar oxygenation improves from severe hypoxia levels (near death) to nearly normal tensions that will fully saturate haemoglobin.
Fig 7: The effect of raising inspired oxygen from 21% (in air) to 30%. Breathing room air (21% oxygen) at normal alveolar ventilation (red vertical arrow) the inspired O2 tension is 13.3 kPa or 100 mmHg (red diagonal arrow on right) which is enough to completely saturate haemoglobin. If through injury or disease the alveolar ventilation falls to less than half normal (blue vertical arrow), inspired oxygen tension breathing air drops to a critical level (blue diagonal arrow – almost dead!). Raising the inspired oxygen tension to 30% even at these low levels of ventilation dramatically increases oxygenation to near normal levels (red arrow on left)! This may make the difference between death and recovery.
To reach this 30% level the cage for critical care oxygenation must be an appropriate size (the smaller the better), and to achieve this easily it could be made with a variable ceiling by sliding in a panel into side slots. This can be quite low as any animal in such poor condition is unlikely to be standing. The cage must be enclosed all round; at the front the transparent covering should form a reasonable seal at the bottom and at the sides, the top being left open to the room for venting. Keeping the patient dry and warm, oxygen is best introduced near the patient‘s face with a cone. With such a set-up the patient can be left with the best chances of recovery. Oxygen is not going to run out and hence its supply needs little supervision. Running oxygen overnight becomes an affordable option, with owners being grateful to the vet for doing all they can to save their pet.
If the “Free-Ox” machine is required for anaesthesia at the same time it is needed for critical care, the concentrator can be detached and left at the cage while the anaesthetic machine is used with the back-up oxygen cylinder stored on the back of the frame. Not much is used as only 500 ml/min is required for the maintenance of anaesthesia, whereas oxygen therapy would require full flow at 5 l/min.