When a pet arrives that is obviously having labored and difficult breathing, an immediate course of treatment is to provide
supplemental oxygen at high concentrations. This is best done with a tight-fitting face mask attached to a valve system and
In human medicine this is available commercially and is called a "non-rebreather." Its use is called non-invasive ventilation
(NIV). In veterinary medicine, the valve system needed is best met in emergencies with a bag-valve assembly ("Ambu(c)bag")
attached to a cone-shaped veterinary face mask (Photo 1).
Photo 1: A bag-valve assembly attached to a cone-shaped veterinary face mask. (Photos: courtesy of Dr. Dennis (Tim) Crowe)
The Ambu(c)bag system has an exhalation valve that directs the exhaled air into the atmosphere and an inhalation valve that
allows the inhalation of oxygen from a reservoir. To the end of the exhalation port a positive end-expiratory pressure (PEEP)
valve can be attached (Photo 2). Using this added valve increases the amount of air left in the lungs at the end of exhalation,
increasing functional residual capacity.
As the top of the valve is rotated, a spring tightens, increasing pressure on a valve that controls the resistance to airflow
out of the lungs. The reservoir is a clear-plastic, flexible bag that is filled with oxygen during the patient's exhalation
and empties during bag compression, as seen in the photo of a firefighter-paramedic providing bag-valve-mask ventilations
in emergency training (Photo 3).
Photo 2: A positive end-expiratory pressure (PEEP) valve attached to the end of the exhalation port increases the amount of
air left in the lungs at the end of exhalation, increasing functional residual capacity.
The amount of oxygen in the reser-voir needed to satisfy the patient's inhalation lung volume, which is frequently increased
in the patient's stressed state, must be easily supplied in the one-quarter to two seconds inspiratory time.
Photo 3: A clear-plastic, flexible bag is filled with oxygen during a patient's exhalation and empties during bag compression,
as seen when a firefighter-paramedic provides bag-valve-mask ventilations in emergency training.
This requires a flow rate of oxygen at the patient's airway during inhalation calculated to be between 12 and 100 liters per
minute (avg. 66 L/min). This calculation is based on the tidal volume, minute volume and inhalation time of each breath.
As can be seen, it is impossible to provide this amount of flow rate of inhaled oxygen without the use of a reservoir that
can be easily emptied and which contains 100 percent oxygen. Any other systems involving a face mask, where there are no reservoir
or valves to control the direction of inhaled and exhaled air, are not recommended in emergency settings in which the work
of breathing must not be increased and high concentrations of oxygen are required.
During the inhalation phase of breathing, if there is no reservoir attached to this same type of bag-valve-mask system, the
highest percent of oxygen that can be achieved during inhalation is 40 percent to 50 percent; the reservoir oxygen concentrations
can reach 100 percent.