Administration of 100% oxygen before induction of anesthesia prolongs the time to onset of arterial hypoxemia . This technique increases the body’s oxygen “stores”, primarily in the functional residual capacity (FRC) of the lungs.
In humans preoxygenation can be achieved with normal tidal volume breathing for 3-5 minutes or by 4-8 voluntary deep breaths. In animals only the normal breathing strategy while 100% oxygen is administered can be used.
Uses
Used prior to induction of anesthesia in animals that are at high risk for oxygen desaturation; this includes obese animals, pregnant animals, critically ill animals and when a difficult intubation is anticipated, eg brachycephalic breeds of dog or in animals that have sustained facial trauma. Any animal with respiratory compromise may have a compromised FRC and benefit from preoxygenation. This technique can also be used when intubation will not be performed immediately after induction of anesthesia, eg when a laryngeal examination is required, or bronchoscopy or bronchiolar lavage will be performed.
Advantages
Easily performed technique, can prevent severe complications during induction and intubation, requires simple equipment.
Disadvantages
Some animals may resent the placement of a mask over their face and struggle making the procedure impossible. In addition struggling will increase the oxygen demand of the animal cancelling any potential benefits. This procedure delays the start of anesthesia and the planned procedure so the pros and cons need to be considered and weighed up against each other.
Time required Preparation
2 mins.
Procedure
3-5 mins.
Decision taking Criteria for choosing test
Include all high risk animals.
Critically ill animals.
Obese animals.
Pregnant animals.
Anticipated difficult intubation.
When intubation will not be performed immediately after induction of anesthesia.
Respiratory compromise.
Whenever FRC is reduced.
Prior to turning on the inhalant agent for mask induction.
Risk assessment
All animals may benefit from preoxygenation, and difficult intubation cannot always be predicted, therefore if in doubt as to the benefits for a specific patient this technique can be considered for all patients.
Some patients may be obligate mouth breathers and in these animals the mask must be large enough so that their mouth remains open during the procedure.
Assemble the anesthetic breathing circuit and check that there is an adequate supply of oxygen. The anesthesia machine should be checked in a routine fashion every time it is used. The machine and breathing system can be flushed with oxygen prior to use to decrease the smell of residual anesthetic gases.
Select a face mask that most closely conforms to the shape and size of the patient. Some have removable rubber diaphragms.
The closer the fit, the less likely there will be leaks that will decrease the inspired oxygen concentration. However as noted above if the animal is an obligate nasal breather the mask must not be so snug that it’s mouth becomes clamped shut.
Restraint
Animals are usually premedicated prior to induction of anesthesia and most will be sedate and easy to restrain for preoxygenation. Gentle but firm restraint should be used to avoid the animal struggling. Cats can be wrapped in a blanket or placed in a “cat bag” - both these procedures usually make cats more relaxed and less restraint is needed to perform preoxygenation. This procedure is best carried out in a quiet area of the clinic so that the animal does not become startled or distracted by noise and activity.
Open the adjustable pressure limiting valve (APL).
Turn on the oxygen flow meter to prime the anesthetic circuit with 100% oxygen. Oxygen flow rates will depend on the size of the animal and the type of circuit being used.
For non-rebreathing circuits suggested oxygen flow is 200-300 ml/kg body weight which should prevent rebreathing. If the rubber diaphragm has been removed, choose the higher flow rate.
For circle systems (larger animals) flow rates are not well defined in animals. Based on studies in humans, a rate of 150 ml/kg is suggested.
Leaks around the mask will reduce the inspired oxygen concentration.
Step 2 - Restrain the patient
Gentle but firm restraint should be used to avoid animal struggling.
Step 3 - Apply eye lubricant
Apply eye lubricant to prevent corneal damage from accidental contact with the edge of the mask and drying of the cornea by the dry gas flow.
Step 4 - Place oxygen mask
Gently and slowly place the oxygen mask over the patient’s face.
Step 5 -
If the animal does not resent the rubber diaphragm around its muzzle then it can be left in place; if the rubber diaphragm is resented it can be removed . When placing the mask, make sure the edges do not impinge on the eye.
Step 6 -
Keep the animal’s head and neck extended to ensure a patent airway .
Step 7 -
Maintain the mask in place for 3-5 minutes.
Step 8 -
Induce anesthesia (with chosen intravenous agent) while maintaining the mask in place.
Step 9 -
Remove the mask when the animal is ready to intubate, or the clinician is ready to perform a laryngeal examination or bronchoscopy.
Step 10 -
If intubation is not achieved, replace the oxygen mask and trouble shoot the problem (animal not sufficiently anesthetized, wrong size endotracheal tube, incorrect size or shape of laryngoscope).
Step 11 -
After intubation continue with 100% oxygen and check the animal’s vital signs.
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Mort T C (2005) Preoxygenation in critically ill patients requiring emergency tracheal intubation.Crit Care Med33 (11):2672-2675 PubMed.
Chiron B, Laffon M, Ferrandiere M, Pittet J F, Marret H, Mercier C (2004) Standard preoxygenation technique versus two rapid techniques in pregnant patients.Int J Obstet Anesth13( 1), 11-14 PubMed.
Pandit J J, Duncan T, Robbins P A (2003) Total oxygen uptake with two maximal breathing techniques and the tidal volume breathing technique: a physiologic study of preoxygenation.Anesthesiology99 (4), 841-846 PubMed.
Baraka A S, Taha S K, Aouad M T, El-Khatib M F, Kawkabani N I (1999) Preoxygenation: comparison of maximal breathing and tidal volume breathing techniques.Anesthesiology91 (3), 612-616 PubMed.
Vetstream contributor(s)
Graham Bilbrough MA VetMB MRCVS , Anaesthesia and Critical Care, Queen's Veterinary Hospital, University of Cambridge, Madingley Road, Cambridge CB3 0ES, UK.
Sheilah Robertson BVMS PhD DipACVA DipECVA MRCVS , Department of Large Animal Clinical Sciences, Box 100136, Gainesville, FL 32610-0136, USA.
