Reservoir Bag Selection
The reservoir bag should be at least 4 to 6 times the patient's tidal volume. Tidal volume is often quoted as 10-20 mL/kg. The middle value, 15 mL/kg, is commonly used in small animals. If 15 mL/kg is multiplied by 4 and 6 then 60 and 90, respectively, become constants.
4 x 15 = 60
6 x 15 = 90
Therefore, we can simplify this calculation by just taking the body weight in kg and multiplying it by 60 (low end) or 90 (high end). If 60 is used, then round up to the nearest bag size. If 90 is used, then round down to the nearest bag size.
Common sizes of reservoir bags include 0.5, 1, 2, 3, 4 and 5 liters for small animals. They make 0.25 L reservoir bags for birds and small exotic mammals. For comparison, large animal reservoir bags are 15 L, 20 L or 30 L.
Consequences of reservoir bag being too small:
a) It will not provide an adequate inspiratory volume during spontaneous ventilation; the reservoir bag will likely completely empty whenever the patient takes a spontaneous breath.
b) When giving a manual breath, the anesthetist will be unable to achieve an adequate chest excursion.
c) If the patient is not getting an adequate tidal volume, then that means that gas exchange at the level of the alveoli is decreased. This can lead to hypercapnia.
d) There is an increased risk of lung trauma if pressure builds up in the system (e.g. closed APL valve) because there is a smaller reserve to hold the gases.
Consequences of reservoir bag being too large:
a) Increased risk of over inflating the lungs if not watching the manometer or chest excursion during controlled manual ventilation.
b) Difficult to monitor respiratory rate because the bag movement during spontaneous ventilation will be very small.
c) Added circuit volume will increase the time it takes to reach equilibrium whenever a change is made on the vaporizer dial; this delays the ability to change the inspired inhalant concentration in the patient's brain.
When in doubt, it is always better to have a reservoir bag that is a little bit larger rather than a little bit too small.
