Tailor-made Anesthesia

Low flow anesthesia: Advantages and disadvantages, Part 1

Paul H. Ting, MD
Department of Anesthesiology
University of Virginia Medical System
Charlottesville, Virginia

Published by permission of GASNet Inc.© 2003

The article also available in PDF: 120KB

Introduction

Low flow anesthesia is not a new concept. The technique has been in use since the 1800’s. It fell out of favor when halothane was introduced in the mid-twentieth century due to the inability of newly developed vaporizers to accurately deliver volatile agent at low flows. By the time isoflurane was introduced in the 1980’s, vaporizer technology was much improved and the introduction of this new volatile agent increased interest in low flow techniques. Low flow anesthesia provides many of the same advantages that closed circuit anesthesia does (low cost, low wastage, improved heat and humidity retention), and has now become a widely accepted technique with the introduction of new anesthetic agents that are low in solubility and relatively higher in cost [1, 2].

Low flow anesthesia does require some minor changes in practice for the practitioner that has never utilized it. However, with an understanding of some basic concepts and proper use of monitoring equipment, one can achieve the benefits of the technique with little risk. Newer anesthesia delivery machines are designed to make the delivery of anesthesia with low fresh gas flows even easier and safer by incorporating better flowmeter technology, appropriate monitors and alarms, low volume circuits, etc.

Definition of low flow

It is not uncommon to see the delivery of anesthesia with fresh gas flows of two to three liters per minute in use today [3]. Although many consider this to be “low flow” delivery of anesthesia, even lower flows are required to maximize the benefits of this technique. While there is no agreed upon definition of what qualifies as low flow anesthesia, the reduction of fresh gas flow to less than one liter per minute is possible.

Whereas closed circuit anesthesia attempts to match fresh gas delivery with patient gas uptake with no excess gas leaving the circuit, low flow anesthesia maintains delivery slightly above patient requirements and allows a small amount of excess gas to leave the circuit through the scavenger system. The intent is not to eliminate the excess gas leaving the circuit, but rather to reduce the amount of that excess. As a result of this difference in management goals, low flow anesthesia is easier to achieve and administer than closed circuit anesthesia. In addition, it is possible to increase fresh gas flows at any time without abandoning the technique altogether (unlike with closed circuit anesthesia).

Benefits of low flow anesthesia

The benefits of low flow anesthesia include [1]:

• A reduction in costs associated with volatile anesthetics
• Decreased waste anesthetics released into the environment
• Better maintenance of patient temperature and humidity

A reduction in fresh gas flow used for maintenance once the patient has achieved an adequate anesthetic depth can reduce costs significantly. A lower fresh gas flow requires a higher vaporizer setting to achieve the same desired agent delivery to the patient. High fresh gas flows with the delivery of much higher amounts of volatile agent are not necessary for maintenance of anesthesia depth, and most of the delivered agent ends up escaping the circuit into the scavenger. Reduction of the amount wasted in this way is one method of reducing costs associated with the use of volatile anesthetics. For that same reason, the reduction of fresh gas flows ultimately decreases the amount of agent that ends up escaping into the environment.

While reduction of wastage may be the primary reason that low flow anesthesia has gained renewed interest, there are tangible benefits to the patient as well. Fresh gas delivered to the circuit is cold and dry, which is heated to body temperature upon entering the lung and humidified via evaporation processes. This process not only removes heat directly from the patient, but also causes the loss of moisture from lung tissue. If lower fresh gases are utilized, the loss of heat and moisture from the patient to fresh gas is minimized.

Low flow anesthesia reduces the rate at which a change in desired anesthetic concentration (what the vaporizer is set at) is reflected in actual delivered anesthetic concentration (what the patient is actually receiving). While this can be seen as a disadvantage when a fast rate of change is desired, for example on induction of and emergence from anesthesia, it can also be a safety mechanism when a vaporizer is inadvertently set too high or too low. In these cases, the slow rate of change allows ample time to detect the error before it can affect the patient as a clinically evident event.

Disadvantages of low flow anesthesia

There are a number of possible drawbacks to the use of low flow anesthesia. These include:

• Changes in agent concentration take longer with low flows
• Inspired anesthetic concentration is lower than that set at the vaporizer
• Delivery of a hypoxic mixture is possible
• High humidity and moisture levels accumulate in the circuit

While these are easily overcome by careful practice, awareness of these issues is important to the understanding of the technique.

I. Changes in volatile agent concentration take time

The equation governing the change in concentration of a given molecule with time, such as volatile agent carried in fresh gas flow, is a first order kinetic equation of the form:

Fi = F_sete^(1-t/k)

Fi = concentration inspired (in circuit)
F_set = concentration desired (set on vaporizer)
k = time constant
t = time

The time constant is calculated by dividing the volume of the circuit in liters by the amount of fresh gas flow in liters per minute. For example, take a circle system with a volume of five liters and a fresh gas flow of five liters per minute. The time constant in that case is one minute. Now take that same circle system but change the fresh gas flow to one liter per minute. The time constant is now five minutes. The volume of the circle system includes not only the tubing but also the volume in the absorber canister, etc.

In a system with first order kinetics, the time constant is the amount of time required to reach 63% of the intended concentration. Three time constants are therefore required in order to reach 95% of the intended concentration, and the final concentration approaches 100% after five time constants. Low fresh gas flow results in a situation where changes in dialed concentrations take much longer to reach the patient.

When the clinical situation requires rapid changes in anesthetic depth, it is important to realize the effect of low flow on the time constant and increase the flow rates until the desired change has taken effect. Once the patient has reached the desired anesthetic depth and maintenance of that anesthetic level is all that is required, the use of low fresh gas flows can be resumed. [4]

II. Inspired concentration is less than set at vaporizer

Anesthetic vaporizers are set by the selection of a concentration in percent. At high gas flows, the amount of anesthetic agent coming out of the vaporizer is higher, in terms of volume, than at lower gas flows. For example, a vaporizer setting of one percent with a fresh gas flow of five liters a minute will result in a delivery of fifty milliliters of anesthetic agent per minute. The same vaporizer setting with a fresh gas flow of one liter a minute will deliver only ten milliliters of anesthetic agent per minute.

When an anesthetic is in progress, there is a continuous uptake of anesthetic vapor by the blood. Lowe has derived a formula to calculate the amount of agent uptake [5]:

Van = 1.3 x MAC x Coef x Q x t^–1/2 [ml/min]

Van = agent uptake
MAC = minimum alveolar concentration
Coef = blood/gas partition coefficient
Q = cardiac output
t = time

With low flows, this constant anesthetic uptake removes anesthetic agent from the circuit at the same time that lower volumes are being provided by the vaporizers. The end result is that the vapor concentration in the circuit falls and is less than the vaporizer setting. Although most of the adjustments that are made to the vaporizer are titrated to clinical effect, it is important to have the ability to monitor actual inspired and expired agent concentrations.

III. It is possible to deliver a hypoxic mixture

Modern anesthetic machines have safety mechanisms built in to prevent delivery of a hypoxic mixture. These systems regulate the ratio of the gas mixture, not its volume. Like the volatile anesthetics, the volume of oxygen delivered when using low flows might not meet the patient’s rate of uptake. The fresh gas flow should include at least enough oxygen to meet the patient’s needs. Oxygen uptake in normothermic awake patients can be approximated with the following formula [6], which is a modification of one described by Brody [7]. Note that uptake during anesthesia is generally about 10-30% lower.

VO₂ = 10 * BW^3/4 [ml/min]

VO₂ = oxygen uptake
BW = weight in kilograms

Another factor involves the use of nitrous oxide. Oxygen uptake stays relatively constant during a given anesthetic, the exception being a noted increase at the start of surgery. Nitrous oxide uptake is high during induction but decreases rapidly with time. According to Severinghaus, this uptake can be calculated as [8]:

VN₂O = 1000 x t ^–1/2 [ml/min]

VN₂O = uptake of nitrous oxide
t = time

Thus the amount of oxygen leaving the circuit remains constant over time but the amount of nitrous oxide leaving the circuit falls quickly. Failure to adjust fresh gas flows accordingly can lead to a decrease in the oxygen concentration. If oxygen is the only gas making up fresh gas flow, inadequate delivery will be easy to recognize because the volume in the circuit will decrease and the reservoir bag or ventilator bellows will not fill. If nitrous oxide is added to the fresh gas flow, this gas will make up the volume and bag or bellows will appear to fill normally.

Again, the solution is careful monitoring. The use of inspired oxygen monitoring and pulse oximetry should be considered mandatory for every case, but their use is magnified in importance for the practice of low flow anesthesia.

IV. Increased gas humidity

Low flow anesthesia preserves humidity better than high flow anesthesia. Moisture may condense when warm gas encounters cooler parts of the circuit, and may interfere with the normal function of the anesthetic machine such as gas sampling, inspiratory and expiratory valves, etc.

Summary

Low flow anesthesia is safe and easy for routine use. The introductions of newer anesthetic agents with low solubilities and relatively higher costs have caused a resurgence of interest in the technique. Its use results in costs savings [9], and other benefits such as a reduction of environmental wastage [10] and preservation of heat and humidity of the patient [1].

References:

1. Baxter AD. Low and minimal flow anaesthesia. Can J Anaesth. 1997; 44:643-52.
2. Cotter SM, Petros AJ, Dore CJ, Barber ND, White DC. Low-flow anaesthesia: Practice, cost implications and acceptability. Anaesthesia 1991; 46:1009-1012.
3. Cravero J, Suida E, Manzi DJ, Rice LJ. Survey of low flow anesthesia in the United States. Anesthesiology 1996; 85: A995
4. Mapleson WW. The theoretical ideal fresh-gas flow sequence at the start of low-flow anesthesia. Anaesthesia 1998; 53:264-72.
5. Lowe HJ and EA, Ernst EA. The Quantitative Practice of Anesthesia. Williams and Wilkins. Baltimore (1981).
6. 6. Brody S. Bioenergetics and Growth. Reinhold. New York. 1945
7. Kleiber M. Body size and metabolic rate. Physiol Rev 1945; 27:511-39.
8. Severinghaus JW. The rate of uptake of nitrous oxide in man. J Clin Invest 1954; 33:1183-9.
9. McKenzie AJ. Reinforcing a low flow anaesthesia policy with feedback can produce sustained reduction in isoflurane consumption. Anaesth Intensive Care 1998; 26:371-6.
10. Logan M, Farmer JG. Anaesthesia and the ozone layer. Br J Anaesth 1989; 63:645-7.
    
    
    

Last updated: 1 September 2003Created
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