Tailor-made Anesthesia

Low flow anesthesia: Technical requirements and other considerations for low flow anesthesia, Part 2

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: 94KB

Technical requirements

The descriptions of low flow anesthesia in Part I make it apparent that there are a number of technical requirements for the safe performance of this system. Monitors for anesthetic agent concentration and inspired oxygen concentration are necessary. Similarly, pulse oximetry provides an important monitor when the risk of delivering low oxygen volumes is present.

A circle system is needed which provides for the ability to re-circulate gas in the circuit. Since this mandates rebreathing, the use of a carbon dioxide absorber is also a necessity. Ideally, the system would be designed to be as low in volume as possible in order to minimize the time constant. In addition, a low level of leakage from the system is needed (typically less than 100 cc/min). Remember also that sidestream gas analysis typically removes about 200 cc/min from the circuit and this must be accounted for by the addition of adequate flow [1].

Some additional design considerations make low flow anesthesia easier to perform. Flow meters that are calibrated and marked for flows less than one liter are beneficial. Newer machines have electronically controlled flow meters that deliver a precise rate of flow and are calibrated for flows less than one liter. Similarly, having vaporizers that are accurate, especially at low fresh gas flows is important. Again, newer technology allows for accurate and precisely controlled vaporizers and these are appearing on newer machines. Lastly, an ascending bellows ventilator may be helpful in cases where oxygen is the only gas being used, as it will fail to fill in situations where the oxygen flow is too low.

Other considerations

A number of other issues deserve at least brief consideration in a discussion about low flow anesthesia. (I) How should one deal with nitrogen? (II) What are the issues with low flow anesthesia, sevoflurane and Compound A? (III) What about the issue of carbon monoxide production? (IV) Should humidifiers be used if low flow anesthesia is utilized?

(I) How should one deal with nitrogen?

In the average adult patient, there is approximately one and a half liters of nitrogen contained in the functional residual capacity of the lungs when breathing room air. Additionally, there is about two liters of nitrogen dissolved in body tissues. Pre-oxygenation with 100% oxygen prior to induction of anesthesia is designed to eliminate the nitrogen content in the functional residual capacity. Once the anesthetic begins, nitrogen from the body begins to move out of the patient and into the circuit. As this occurs, the concentration of nitrogen in the circuit can reach levels as high as five to ten percent, reaching their peak in approximately thirty to forty five minutes [2].

Theoretically, this additional amount of nitrogen can reduce the concentration of oxygen and result in a hypoxic mixture. In practice, the initial high flow phase that is utilized to deepen the patient to adequate anesthetic levels will also result in sufficient de-nitrogenation to avoid any problems. Again, with proper monitoring, the risk of delivering a hypoxic mixture to the patient is largely avoided and the issue of a small amount of nitrogen in the circuit becomes a minor one.

(II) What about sevoflurane and Compound A?

Sevoflurane undergoes a chemical degradation when in contact with soda lime that forms a vinyl ether known as Compound A. This substance has been shown to be nephrotoxic in rats at levels of 50-114 ppm [3]. Higher temperatures and higher concentrations of sevoflurane have been shown to increase the production of Compound A [4]. The higher temperatures that are seen with low flow anesthesia can theoretically increase the production of Compound A.

Several studies suggest that prolonged anesthesia with sevoflurane, even with flows as low as one liter per minute, result in Compound A levels that are acceptably low (less than 25 ppm) and show no obvious clinical side effects [5,6]. Flow rates of less than one liter per minute have been used with resultant Compound A levels in the range of 50-60 ppm [7]. Estimates of the level required to cause nephrotoxicity in humans range from 150-240 ppm-h (based on studies of biochemical markers of renal failure post-operatively [8,9]) to as high as 800 ppm-h (based on studies in primates [10]).

When sevoflurane was first approved for use in the United States by the Food and Drug Administration, it carried with it the warning that fresh gas flows of less than two liters per minute should be avoided due to concerns about the accumulation of Compound A and possible renal side effects. That recommendation was subsequently changed to a statement that fresh gas flow should be kept to one liter per minute or greater. There are no such restrictions on the use of sevoflurane in the European Common Market.

(III) Carbon monoxide production?

Carbon monoxide is a byproduct of a reaction between soda lime and volatile anesthetic agents [11]. However, this seems to be a significant problem only if the soda lime is desiccated, which should not occur during low flow anesthesia. Low flow anesthesia should, therefore, help to prevent the production of carbon monoxide as compared to high flow techniques.

(IV) What about the use of humidifiers?

There is some disagreement concerning the appropriateness of using heat moisture exchangers (also known as passive humidifiers) when utilizing low flow anesthesia. The main function of these devices is to maintain the heat and moisture in expiratory gas. However, in low flow anesthesia, heat and humidity are already well maintained and it is likely that the use of these devices represents an unnecessary expense.

Summary

An understanding of the principles behind the technique is important to avoid the common drawbacks that might be encountered with low flow anesthesia. Proper monitoring is an absolute must for patient safety. Newer anesthesia machines have integrated specific design elements that make the delivery of anesthesia with low fresh gas flows technically easier and more precise.

References:

1. Bengtson JP, Bengtson J, Bengtson A, Stenqvist O. Sampled gas need not be returned during low-flow anesthesia. J Clin Monit 1993; 9:330-4.
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3. Gonsowski CT, Laste MJ, Eger EI, Ferrell LD, Kerschmann RL. Toxicity of Compound A in rats. Anesthesiology 1994; 80:556.
4. Bito H, Ikeda K. Effect of total flow rate on the concentration of degradation products generated by the reaction between sevoflurane and soda-lime. Br J Anaesth 1995; 74:667.
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8. Eger EI, Gong D, Koblin DD, Bowland T, Ionescu P, Laster MJ, Weiskop RB. Dose-related biochemical markers of renal injury after sevoflurane versus desflurane anesthesia in volunteers. Anest Analg 1997; 85:1154-63.
9. Goldberg ME, Cantillo J, Gratz I, Deal E, Vekeman D, McDougall R, Afshar M, Zafeiridis A, Larihani G. Dose of Compound A, not sevoflurane, determines changes in the biochemical markers of renal injury in healthy volunteers. Anesth Analg 1999; 88:437-45.
10. Mazze RI, Friedman M, Delgado-Herrera L, Galvez ST, Mayer DB. Renal Toxicity of Compound A plus sevoflurane compared with isoflurane in non-human primates. Anesthesiology 1998; 89:A490.
11. Fang ZX, Eger EI, Laster MJ, Chortkoff BS, Kandel L, Ionescu P. Carbon monoxide production from the degradation of desflurane, enflurane, isoflurane, halothane and sevoflurane by soda lime and Baralyme. Anesth Analg 1995; 80:1187.
    

Last updated: 1 September 2003Created
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