Animals under anesthesia increase the risk of developing
airway complications as anesthesia induces a number of abnormal physiologic
Most anesthetics obtund or abolish the swallowing reflex,
a protective mechanism to prevent accidental inhalation of foreign materials
that may be present in the mouth.
Maintaining a secure airway during perianesthetic period
comes as a primary requirement for safe anesthesia and is a fundamental
responsibility of the anesthesia provider.
Failure to do so can result in brain damage and death.
In some situations, maintaining patent airway can be
performed by a simple maneuver such as extending the neck of a patient.
However, placing a tracheal tube into the trachea,
without question, ensures a more secure airway.
This lecture covers some basic anatomy and physiology of
respiratory system, the necessary devices required to perform endotracheal
intubation and techniques of airway management, and basic principles of the
ventilator, and how best to apply such knowledge into clinical practice.
- ‘Airway’ is the passage way into or out of the
lungs. Proper airway ensures adequate delivery of oxygen (O2) and
elimination of carbon dioxide (CO2) to and out of the body.
- ‘Respiration’ is the total process of oxygen
supply and carbon dioxide elimination.
- ‘Ventilation’ is the movement of gases in and out
of the alveoli.
Anatomic and Physiologic considerations
Nares, nasal cavity, pharynx, larynx
Lower airwaysbronchi, bronchioles, alveoli
- Ideal tube size:
- Select the largest possible diameter without a danger to damaging
larynx or trachea
- Length should be from the tip of the nose to the point of the shoulder
- Too long tube can result in endobronchial intubation and unintentional
one lung ventilation
- Excessive tube length extending out of the oral cavity results in
increased dead space ventilation (cut to the length if necessary)
- Laryngoscopes are composed of two parts; laryngeal blade and handle.
- The laryngoscope’s light source is the main benefit of laryngoscopy, but
the blade can be used to manipulate the tongue, soft palate, and epiglottis to
view the glottis. NB. Use the laryngoscope to depress the base of the tongue,
not the epiglottis. Depressing the epiglottis directly with the laryngeal
blade increases the risk of traumatizing the epiglottis inducing airway
- Blades come in different sizes and types, size ranging from 0 (small) to 5
(large) adequate for most small animals.
- Customized blades with extra length are available for use in some species
including pigs, llamas, and big cats.
- Two most common types available in the market are the Miller (straight)
and the McIntosh (curved), and there are numerous modifications based on these
Species specific problems and
- Ruminants carry greater risk of aspiration of the gastric contents during
anesthesia: adequate cuff inflation of the endotracheal tubes is essential.
- In ruminants, particularly in cattle, under general anesthesia the
breathing is characterized with rapid and shallow in pattern, but the exact
cause of this is unclear, and this poses little clinical significance.
- Postanesthesia laryngospasm is a significant cause of postanesthetic
airway complication in the cat, usually trauma induced. Topical administration
of lidocaine may help prevent it.
- Horses are obligatory nasal breathers and common incidence of nasal
swelling and congestion following general anesthesia is a major cause of
airway obstruction. A short nasal tube is commonly placed at recovery which
serves to provide a patent airway.
Control of respiration
- Respiratory function is controlled by central respiratory centers, central
and peripheral chemoreceptors, pulmonary reflexes and non-respiratory neural
- Blood gas tensions and hydrogen ion concentration are monitored by central
and peripheral chemoreceptors that return signals to central neural controller
to provide necessary feedback adjustments in ventilation.
- Central neural control mechanism regulate the activity of the primary and
accessory respiratory muscles.
- Central chemoreceptors are located on the ventral surface of the medulla
and bathed by cerebrospinal fluid and plays a significant regulatory role in
conscious animals by affecting total minute ventilation and alveolar
- Peripheral chemoreceptors are located on the carotid and aortic bodies and
sensitized to increase respiratory drive when PaO2 falls below 60
Drug’s effect on control of ventilation
- Anesthetics alter the central and peripheral chemoreceptor response to CO2
and O2 in a dose-dependent manner.
- Opioids shift the CO2 response curve to the right,
and cause additive depression of the respiratory center. At the clinical dose
respiration complication is very rare.
PaCO2 is not significantly altered, but a fall of PaO2
as low as 30-50 mmHg in the sheep has been reported, although in
most domestic species this severe reaction is not observed.
There is an increase of lower respiratory airway resistance
- Phenothiazine and benzodiazepines often reduce respiratory rate, but do
not appreciably alter arterial CO2
- Alpha 2 adrenergic receptor agonists produces laryngeal relaxation and an
alteration of pulmonary mechanics (compliance and resistance).
Changes of ventilation-perfusion (V/Q)
relationship during anesthesia.
- PaO2 decreases more dramatically during general
anesthesia in large animals than smaller animals. The primary attributor is
believed to be increased V/Q mismatches.
- Hypoxic pulmonary vasoconstriction (a protective mechanism) re-direct
blood flow to better ventilated area in the lung. Anesthetics cause marked
reduction in this protective response, resulting in further V/Q mismatch
- Anesthesia ventilators provide controlled ventilation to patients under
- Simply, anesthesia ventilator is a reservoir bag (a bellow or concertina
bag) in a closed container (bellows housing) that can substitute for the
reservoir bag of an anesthesia breathing system.
- The anesthesia ventilator performs the same job as the anesthetist who
periodically squeezes the breathing system’s reservoir bag to ventilate the
patient, lessening laborious workload of the anesthetist and thereby
minimizing the potential for distraction.
- Classification is through power source, drive mechanism and cycling
- Power source; electrical, or compressed gas or both
- Drive mechanism; commonly compressed gas even when electrical source is
- Cycling mechanism is typically time cycled. Other mechanisms include
pressure cycled, volume cycled, but timing mechanism plays a major role in
ventilatory function, even in these two.
- For additional details about anesthesia ventilators see pages 535-556 of
Veterinary Anesthesia (Thurmon et al. 1996).
- A variety of ventilators are manufactured by different vendors, and may
come in a stand-alone unit or built into the anesthetic machine.
- The decision to choose which type will depend on what suits better for the
clinical need a practitioner has, based on the animal species and economic
- The cost for most stand-alone ventilator adequate for small animal use
should fall into the range of $ 2000 to $ 5000.
- Most new ventilators are of this type
- Safer - if an airway leak is present, bellows no longer ascends to the
top, letting you know that a leak exists
Descending Bellows - Metomatic:
- Power: on/off
- Expiratory flow rate: used to adjust character of ventilation
- Inspiratory flow rate: used to adjust character of ventilation
- Expiratory time: controls respiratory rate
- Inspiratory hold: used to maintain inspiratory pressure to reinflate
- Inspiratory pressure: used to adjust maximal inspiratory pressure
- Inspiratory trigger effort: can be adjusted to make ventilator
function in ‘assist’ mode - patient initiates breath, ventilator just
assists to increase tidal volume (rarely used)
- Tidal volume (knob on back of bellows): adjusts tidal volume
- Note drive gas is oxygen - important safety feature!
- If a leak develops in the bellows, oxygen will enter bellows & be
delivered to patient; prevents delivering a hypoxic gas mixture
- Running a ventilator off of an E cylinder will deplete the cylinder
- Attachment of ventilator to anesthetic circuit
- A ventilator is nothing more than a mechanical rebreathing bag
- Therefore, the ventilator always attaches where the rebreathing bag
attaches to a breathing circuit!
Respiratory assist devices
- Manual resuscitator (ambu-bag): inexpensive and portable.
- Demand valves: mostly used in horses.
Clinical considerations to ensure adequate
ventilation in respiratory distressed patients
- Establish and secure a good airway
- Increase inspiratory O2 fraction of delivered gas
- Control or assist ventilation using desirable devices (mechanical
- Antagonize, when available and indicated, an overdose of respiratory
depressants with reversal agents (e.g. opioid antagonist)