Fresh Gas Flow Requirements
As in many aspects
of anesthesia, there is no universal agreement as to the fresh gas
flow requirements for the various breathing systems. Indeed,
the opinions of many of the advocates of different systems seem
to be held with an almost religious fervor. The following represents
the author's opinion of the current state of thinking, although
it is accepted that there will be those who will hold different
The fresh gas
flow rate is the total volume of gas that flows from the anesthetic
machine into the breathing system per minute. This includes oxygen,
nitrous oxide and any other gases that may be employed.
systems, gas exhaled by the patient is expelled from the system
by fresh gas. It follows that the fresh gas flow required to prevent
rebreathing depends upon the patient's minute volume of ventilation.
Values of minute volume vary considerably,
depending upon the species, body weight, experimental conditions,
whether or not the subjects were anesthetized, and, if so, what
method and plane of anesthesia was used.
Most gas flow recommendations are presented
as a simple linear function of body weight (e.g., for the Magill
circuit, a common recommendation is 70 ml/kg/min). The problem with
this approach is that minute volume of ventilation is not a linear
function of body weight, but is greater per unit body weight in
small animals than in large. The
following formula approximates to the values of minute volume observed
in varying sized animals:
volume (l/min) = 250 x BW0.8
Where BW equal
body weight in kilograms. Values yielded by this formula are:
|l / min
||ml / kg
A (Magill and Lack)
In order to
completely prevent rebreathing, the fresh gas flow rate must equal
or exceed the minute volume. However, the last gas to be washed
out of the circuit is dead-space gas, which consists of warmed
and humidified fresh gas, and no carbon dioxide. If some rebreathing
of this dead space gas is accepted, a flow approximating to around
70% of the minute volume can be used:
gas flow rate (l/min) = 175 x BW0.8
|l / min
It must be
emphasized that these values are guidelines only--if there is
evidence of rebreathing (e.g. an increase in the end tidal CO2
concentration or unexpected hyperventilation), the flow rate should
As has been
pointed out elsewhere, Mapleon A systems are unsuitable for long
term IPPV. Practical experience suggests that gas flow rates of
double the minute volume are required to prevent rebreathing.
D and E (Ayre's T-piece and Bain)
be noted that, in contrast with Mapleson A systems, alveolar gas
is the last gas to be expelled from the circuit in Mapleson D
and E systems, so it is more important to prevent rebreathing.
Since the exhaled tidal volume must be
removed from the circuit during the expiratory pause, the required
fresh gas flow rate would be expected to be somewhat greater than
the patient's minute volume.
The original analysis of the Mapleson
E system suggested that a gas flow rate of 2.5 to 3 times the
minute volume was required to prevent rebreathing of expired gas.
However, this assumed a square-wave respiratory pattern, and investigations
using a more realistic breathing pattern have suggested that 1.5
- 2 times the minute volume is acceptable in spontaneously breathing
|l / min
||1.4 - 1.8
||2.4 - 3.2
||4.1 - 5.4
||7.2 - 9.6
values are guidelines only--if there is evidence of rebreathing,
the flow rate should be increased.
with Mapleson A systems, Mapleson D and E circuits are more efficient
during controlled than spontaneous ventilation. This is because
the tidal volume must be supplied during the expiratory pause.
With the almost sinusoidal respiratory pattern of spontaneous
respiration, there is relatively little time for this volume to
be supplied, so the fresh gas flow rate must be high. The pattern
of controlled ventilation, however, is usually one of a rapid
inspiration, expiration, and a relatively prolonged expiratory
pause. This long expiratory pause gives ample time for the tidal
volume requirement to be supplied even with a fairly low fresh
gas flow rate. Consequently, during controlled ventilation, the
recommended fresh gas flow rate is similar to that of the Mapleson
A systems during spontaneous ventilation (see above).
In truly closed
systems, the patient consumes oxygen and expires carbon dioxide
which is removed from the system by absorption. The volume of oxygen
flowing into the system must, therefore, equal the patient's oxygen
Resting oxygen consumption is approximated by
consumption (ml/min) = 10 x BW 0.75
where BW is
the body weight (kg).
(ml / min)
It will be noted
that these values are very low for small animals, which precludes
the use of out-of-circuit vaporizers with truly closed systems in
small animal patients.
The use of nitrous oxide in closed systems presents
the difficulty that, after equilibration, nitrous oxide will accumulate
in the circuit and result in a hypoxic breathing mixture. If it
is desired to use nitrous oxide in a closed system, it is mandatory
to employ an inspired oxygen concentration monitor.
When using a
semi-closed system, the oxygen flow rate must exceed the patient's
oxygen consumption. Any excess is simply lost via the pressure relief
When using an out-of-circuit vaporizer, the fresh
gas flow rates employed are a compromise between achieving a reasonable
rate of change of anesthetic concentration and economy of anesthetic
Initially, it is necessary to use both a high
flow rate and high vaporizer setting to raise the concentration
of anesthetic in the circuit. For maintenance, both the vaporizer
setting and fresh gas flow rate may be reduced.
As a general rule, a flow rate of 2 to 3 liters
per minute initially, and 500 ml to 1 liter per minute during maintenance
of anesthesia, will usually prove satisfactory.