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 views.

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.

Non-rebreathing Systems

In non-rebreathing 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:

Minute volume (l/min) = 250 x BW^0.8

Where BW equal body weight in kilograms. Values yielded by this formula are:

Mapleson A (Magill and Lack)

Spontaneous ventilation

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:

Fresh gas flow rate (l/min) = 175 x BW^0.8

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 be increased.

Controlled ventilation

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.

Mapleson D and E (Ayre's T-piece and Bain)

Spontaneous ventilation

It should 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 patients:

Again, these values are guidelines only--if there is evidence of rebreathing, the flow rate should be increased.

Controlled ventilation

In contrast 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).

Rebreathing systems

Closed systems

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 consumption.
    Resting oxygen consumption is approximated by the formula:

Oxygen consumption (ml/min) = 10 x BW ^0.75

where BW is the body weight (kg).

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.

Semi-closed

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 valve.
   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 consumption.
   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.

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