Low Flow Anesthesia

Some factors affecting low flow delivery

Marja Lajunen

Article also available in PDF: 23 KB

In this tutorial, short recommendations are made as to how to adjust fresh gas flow (FGF) at induction, during maintenance, and when emergence from general anesthesia takes place. Thereafter, a couple of interesting factors which may affect low flow delivery will be explained.

How to adjust FGF at different phases of low flow anesthesia

Premedication, pre-oxygenation and induction of sleep are performed according to the usual routines. Concerning adjustment of FGF, anesthesia can be divided into high flow, low flow and recovery phases.

1. Initial high flow phase
   At the beginning of anesthesia, high fresh gas flow of about 5.0 L/min is necessary to wash out nitrogen (N2) from patient’s body tissues. High initial flow facilitates the filling of the breathing system with the desired gas composition, which in turn influences patient uptake and distribution of the anesthetic agents.
2. Low flow phase
   After the high flow phase of some 5 to 15 minutes, or when the target gas concentrations have been achieved, FGF can be reduced to desired low flow level (e.g. 1.0 L/min). The lower the FGF, the greater the difference between vaporizer setting and inspired concentration of inhalation anesthetic agent in the breathing circuit will be. Similarly, with low FGF, time to reach the desired concentration in the inspiratory gas will be prolonged. Hence, monitoring of oxygen and anesthetic agent concentrations is essential and necessary in low flow anesthesia.
3. Recovery phase
   At the end of anesthesia, high FGF (usually with 100 % oxygen) is needed to facilitate the washout of anesthetic agent from the patient, and to remove the agent from the circuit to the scavenging system.

Special factors to consider

Humidity and low flow anesthesia

Use of low fresh gas flow increases rebreathing of gases. Consequently, an increased amount of expired CO2 will pass through the CO2 absorber. This results in increased production of heat and humidity through the CO2 absorption process.

During low flow anesthesia, humidity and heat produced by the chemical absorption process will be mixed with the humid gases expired by the patient.

Nitrogen (N2) accumulation

Typically, an adult with a body weight of 70 kg has approximately 2 liters of dissolved nitrogen in the body. Nitrogen is eliminated from the tissues by expiration and expired nitrogen can affect circuit concentrations of gases.

Hence, it is important to wash out nitrogen eliminated from the body tissues into the circle, by first using high fresh gas flow before reducing the total FGF.

Pulmonary denitrogenation with high fresh gas flows takes approximately 5 to 10 minutes. Thereafter, during maintenance of minimal flow anesthesia, small amounts of nitrogen are still accumulating into the circle. Such a minor nitrogen accumulation is usually harmless, providing that hypoxia is prevented. Therefore, decreased circuit oxygen may be a sign of nitrogen accumulation. Then, a temporary increase of fresh gas flow may be considered to wash out accumulated nitrogen from the circle.

Methane accumulation

Methane is a gas that is a product of microbial fermentation of carbohydrates in the body. Methane is eliminated via the lungs. In minimal to closed circuit anesthesia, and using very small FGF, methane may accumulate in the breathing circle.

Methane may sometimes have an effect on the anesthetic agent concentration measurements. That may occur if the gas monitor has been designed to detect anesthetic agents at an infrared absorption wavelength of 3.3 µm. There, methane may have some effect on the enflurane and isoflurane values, but it has its greatest effect on the halothane measurement. Accumulation of methane in the circle can be eliminated easily by washing it out by a temporary increase of FGF. Today, modern monitoring devices identify anesthetic agents on the 9 to 12 µm wavelengths range, at the far-end of the infrared absorption spectrum. There, methane does neither absorb nor interfere with the anesthetic agent measurements.

Side-stream gas sampling

The side stream gas analyzer suctions a constant gas sample from the circle, usually at a rate of 200 ml/min. If the FGF is clearly above the minimal flow range, the gas sample can be directed straight to the scavenging system.

However, if minimal FGF is used, the rate of sample taken is proportionally high. Then, that sample should be returned back to the circle. As said before, nitrogen may be accumulated in the circle and, hence, monitoring of oxygen concentration is mandatory. Flushing the circuit by high FGF at regular intervals may be necessary, particularly if decreasing oxygen concentrations are measured.

Last updated: 1 January 2001Created
Legal notice	© GE Healthcare 2008 ISSN 1795-6269 (Web) ISSN 1795-6277 (CD)	Webmaster