Closed-Circuit Anesthesia

Introduction

The basic principle of closed-circuit anesthesia is maintenance of a constant anesthetic state by adding gases and vapors to the breathing circuit at the same rate at which the patient’s body removes those same substances. Often, the desired anesthetic state is first established using a high fresh gas flow (FGF) composed of gases, such as oxygen and nitrous oxide or air, and vapors (e.g., isoflurane, sevoflurane, desflurane). Once a near-steady state is attained, inspired and end-expired gas concentrations or tensions are noted, and FGF is decreased. Throughout this chapter, the words tension and partial pressure are used interchangeably. For gases, they have the same value as concentration. This is discussed in the section on Partial Pressure. The circuit and patient gas tensions are maintained constant by adding oxygen, nitrous oxide, and agent vapor to the breathing circuit. In one common approach to closed-circuit maintenance, the amount of gas and vapor administered is determined empirically by titration. Several different titration endpoints can be used. Choices include maintenance of inspired tension, expired tension, and/or estimated anesthetic depth. When titrating against a defined endpoint, drugs can be added in a measured or quantified manner, or they can be added empirically without regard to the total amount administered. In closed-circuit anesthesia techniques, carbon dioxide is removed from exhaled gas, and the remaining exhaled gases and vapors join the FGF to produce inhaled gas. One advantage of this technique is that all gases exhaled are already warmed and humidified by the patient and are therefore well suited for rebreathing. Another advantage is that oxygen consumption is monitored by titration. A final advantage is that cost and environmental pollution are reduced dramatically.

In closed-circuit and low-flow anesthesia, the inhaled gas mixture is formed from two sources, and exhaled gas constitutes most of what the patient will breathe. In addition to exhaled gas, fresh gas is added in the correct quantity and composition to achieve the inspired gas tensions desired. The same inspired and expired gas tensions are established with a closed circuit as with a semiclosed or open (nonrebreathing) circuit. In this chapter the terms open circuit and nonrebreathing circuit are used interchangeably.

Closed-Circuit Anesthesia

Closed-circuit anesthesia can be viewed in several different ways. From one perspective, it is an anesthetic technique unlike all others. The classical closed-circuit literature describes theory and practice different from other techniques. In the classic closed-circuit approach, once a stable level of anesthesia is established with high-flow oxygen, nitrous oxide, and volatile agent, FGF is reduced to the patient’s predicted oxygen consumption (243 mL/min for a 70-kg adult), predicted nitrous oxide uptake rate (approximately 100 mL/min after the first 30 minutes), and predicted inhaled agent uptake. Nitrous oxide and agent uptake rates are calculated according to a mathematical formula based on body weight.

The traditional closed-circuit anesthesia literature uses anesthetic liquid injection or infusion rather than a vaporizer. Liquid agent is administered into the breathing circuit according to a prescribed time regimen. Specifically, the drug administration rate is inversely proportional to the square root of time (t) :

This empiric relationship was first noted by Severinghaus for nitrous oxide in 1954 and was popularized by Lowe beginning in 1972. Severinghaus’s original data demonstrated that nitrous oxide uptake followed this power function relationship fairly closely in the subjects he studied. Connor and Philip revalidated this mathematical relationship numerically and analytically.

A scientific explanation exists for this curious and unexpected relationship. It has long been known that body tissues perfused with blood of a constant drug concentration or vapor tension have an uptake rate that decreases with time. Specifically, theory and research have demonstrated that uptake into each tissue takes the form of an exponential:

where t is time and τ (Greek letter tau) is the time constant, or the time required to achieve 63% of the final value in response to a step input; e is the base of natural logarithms (2.7183...); and K is a predictable constant. It can easily be derived that

and

where t1⁄2, called the half-time, equals the time required to achieve half the final value in response to a step input.

Total body uptake is the sum of the individual uptakes by each tissue. In this case, uptake is the sum of a group of exponentials of different amplitudes (K _tissue ) and time constants (τ _tissue ). The sum of these exponentials is approximately equal to the power function, K × t ^−0.5 .

It must be emphasized that the exponential relationship for a single tissue is derived theoretically and can be demonstrated empirically. The power function relationship, however, is an empiric one that approximates the multiple exponential functions that describe the uptake into diverse tissues. ^,

Partial Pressure, Tension, and Concentration

Throughout this chapter, the words tension and partial pressure are used interchangeably. This physical variable represents the effective pressure exerted by a gas, whether it is in the gas phase alone, in combination with another gas, or dissolved in blood or tissue.

Partial pressure may be expressed as a percentage of 1 atmosphere (1 atm = 760 mm Hg). Expressing partial pressure in this way serves several purposes. When partial pressure is expressed as percent of 1 atm, partial pressure and concentration have the same numeric value for gases. For example, 1 vol% isoflurane has an isoflurane tension of 1% of 1 atm, an expression often shortened to 1%. For blood and tissues, partial pressure is also expressed as a percent. By this definition, a tissue anesthetic measure of 1% does not represent 1% concentration; rather, it represents a partial pressure of 1% of 1 atm or 1% times 760 mm Hg, or 7.6 mm Hg absolute pressure.

Next, when partial pressure is expressed as a percent of one standard atmosphere (i.e., 760 mm Hg), the numbers and concepts work equally well at any atmospheric pressure. This is because the physiologic effects of inhaled anesthetics are the result of partial pressure and not concentration. ^, Thus, anesthetic tensions in the apparatus and patient are the variables that best explain kinetics in an understandable way.

When anesthetics are present in liquid or tissue, their concentrations are equal to their partial pressure × tissue/gas solubility × atmospheric pressure, according to Henry’s law. According to Dalton’s law, for multiple gases, this applies to each as if it were alone.

It is believed that the blood leaving each tissue compartment is in equilibrium with the tissues in that compartment, therefore the partial pressure in venous blood is equal to that in the tissues; as Graham’s law states: the rate of the effusion of a gas is inversely proportional to the square root of its density or molecular mass. The blood entering each compartment has the same partial pressure of all gases as the arterial blood throughout the body. In the absence of shunt in the lung, arterial partial pressure equals alveolar partial pressure. Clinicians measure end-tidal (ET) gas concentrations or partial pressures. ET partial pressure equals alveolar partial pressure when there is no alveolar dead space. After an infinite amount of time, the partial pressures of anesthetic in inspired gas, alveolar gas, arterial blood, all tissues, and all venous blood, including mixed venous blood, become equal. At this point, there is no blood uptake of anesthetic at all, so that the inspired and exhaled partial pressures are also equal. Therefore, after an infinite amount of time, the partial pressure of anesthetic throughout the system, from vaporizer to all tissues, is the same.

The above paragraph is a simplification that serves well in most situations. Exploring this further, in the presence of shunt in the lung, arterial tension is closer to mixed venous tension than it would be otherwise. When accounting for physiologic dead space, some of the gas leaving the lungs is inspired gas, rather than alveolar gas. This makes ET partial pressure closer to inspired partial pressure than it would otherwise be. These are temporary limitations. After an infinite amount of time, these effects disappear as well, and anesthetic tension is equal everywhere. The living body continues to consume oxygen and produce carbon dioxide, therefore this equalization does not occur with these two gases.

Lowe Technique

In the classic Lowe technique for closed-circuit anesthesia, a loading dose of anesthetic vapor is administered to the breathing circuit to bring circuit tension to that desired in the patient’s alveoli. This level is somewhat higher than in the inspired gas. A unit dose for maintenance is then selected according to the patient’s body weight and is proportional to body mass to the three-quarter power (kg ^3/4 ), a relationship first shown by Kleiber that has been further described by Brody. Kleiber’s law goes on to state that many parameters of metabolic and circulatory processes—oxygen consumption, carbon dioxide production, cardiac output, and daily liquid requirement—are proportional to body mass raised to the three-quarter power.

In Lowe’s classic closed-circuit anesthesia, unit doses are administered at specific times with intervals between administrations increasing with time. This results in injections being made at 0, 1, 4, 9, 16, and 25 minutes. This is referred to as the square root of time model, in that these injection times are the series of numbers whose square roots are sequential integers. This strange number manipulation may seem farfetched; however, Connor and Philip ^, demonstrated that this relationship approximated the uptake of nitrous oxide shown by Severinghaus with remarkable accuracy.

Even closed-circuit enthusiasts advise care and careful observation of the patient, as always. Lowe recommended that after 25 minutes, the square root of time method should be modified, and that the intervals between injections should not be lengthened as much (Lowe, personal communication, 1979). This is because after 25 minutes of anesthetic at constant depth, the rate of uptake becomes almost constant. By this time, fast (vessel-rich) tissues have equilibrated with arterial blood. Meanwhile, uptake into muscle and fat are diminishing more slowly than the square root of time because of their long time constants.

It must be noted that the original 1972 theory and “cookbook” for closed-circuit anesthesia was written during the era of more primitive measurement and analysis (in 1972, the digital pocket calculator had not yet replaced the analog slide rule). Anesthetic agent analysis was uncommon, and the only clinically available device was the Dräger Narkotest (Dräger Medical, Telford, PA), which consisted of a slowly responding silicone rubber band in a box connected to a mechanical indicator. The “rubber band in a box” stretched in proportion to the potency of the single or combined inhalation anesthetic tensions administered.

With each administration technique—open circuit, semiclosed circuit with high flow, semiclosed circuit with low flow, closed circuit with vaporizer, and closed circuit with liquid injection—each patient’s inspired, end-expired, and alveolar anesthetic tensions are each the same and are independent of administration technique. Thus, from the patient’s viewpoint, all inhalation anesthetic administration techniques are equivalent. The only difference is in the way the drugs are administered and the resulting waste or lack thereof.

Although the conventional closed-circuit literature used models created by fitting experimental data to particular mathematical formulations, this chapter does not rely on these. Rather it assumes that an anesthetic agent monitor is available and in use, measuring inspired and expired anesthetic concentrations. The dosage administration scheme is adjusted on the basis of a patient’s measured and needed levels of inspired and expired vapor and nitrous oxide. Delivered oxygen flow is adjusted similarly. In the absence of a multigas monitor, closed-circuit anesthesia may still be employed, but greater vigilance and understanding of anesthetic depth and uptake of anesthetic and oxygen are required.

Limitations and Solutions

Truly closed-circuit anesthesia cannot always be performed because of technical limitations of our gas delivery systems or gas monitors. In the 1980s, gas monitoring was often performed with a multiplexed mass spectrometer shared by many operating rooms (ORs), monitoring these rooms in succession. In that situation, sampled gas could not be returned to the patient from whom it was sampled. With the advent and commonality of stand-alone or integrated gas monitors with anesthesia delivery systems, there should be no such impediment.

In 1963, Eger introduced the concept of anesthesia at constant alveolar concentration and defined the term minimum alveolar concentration (MAC). As a foundation for his approach, he used the pharmacokinetic theory explained by Seymour Kety ^, in 1950. Eger showed that constant alveolar concentration or tension can be produced and maintained when inspired anesthetic concentration is adjusted properly. To compute the proper adjustment sequence, or continuum, he calculated anesthetic uptake into each organ group and adjusted inspired concentration to maintain alveolar concentration constant. Fig. 19.1 shows the inspired concentration required to maintain 0.8% alveolar halothane, equal to 1 MAC. Inspired halothane concentration is begun at 3.3% and is slowly reduced to 2% at 5 minutes, 1.5% at 20 minutes, and 1% at 3 hours.

A more realistic clinical endpoint is constant or desired anesthetic tension in the site of interest, the target organs: the brain and the spinal cord. Producing a stepped or controlled change in brain anesthetic tension from its current level to any desired level is a useful clinical objective. This is equally true for high-flow, low-flow, and closed-circuit anesthesia.

Anesthesia Assessment and Adjustment

To the clinician who has used halothane, Eger’s calculated time course of halothane administration is quite reasonable. It mimics what was administered to many patients. Of course, in today’s clinical practice, the vaporizer is not adjusted according to the clock. Rather, the patient’s depth of anesthesia is assessed by mental integration of many signs. The pupils are observed for dilation or constriction; and blood pressure, heart rate, and possibly processed electroencephalogram (EEG; e.g., bispectral index, patient state index, or entropy) and other variables are measured and evaluated. With the convenient availability of anesthetic agent monitors, most clinicians monitor expired anesthetic concentration (tension) as an important additional sign. This represents the anesthetic level in the end-expired gas as an approximation to that in the alveoli, which is in turn an approximation to that in the blood. In the best of all situations, where end-tidal equals alveolar and arterial tension, the brain delay is still 3 to 5 minutes. This means that the measured ET value predicts what the brain concentration will be in the near future. Likewise, view of the ET value a few minutes prior is a real-time indicator of the brain and spinal cord values.

To achieve the “ideal” anesthetic, vaporizer setting and FGF are adjusted to maintain constant brain anesthetic tension. This is done by allowing a reasonable amount of alveolar overpressure (partial pressure higher than that desired at a later time) during the period when anesthesia is being deepened (described later in this chapter). Finally, an estimate or measure of depth of anesthesia can be used.

High and Low Flows

Semiclosed Circuit Anesthesia

Eger’s example (see Fig. 19.1 ) demonstrates anesthesia administration with a perfect nonrebreathing circuit (open circuit). In that system, inspired concentration is perfectly controlled. In contrast, a semiclosed breathing circuit with a carbon dioxide absorber is the one most commonly used around the world. With this circuit, inspired concentration or tension is dependent on FGF and vaporizer setting, as well as dependent on exhaled ventilatory flow and agent tension.

Fig. 19.2 shows the semiclosed circuit vaporizer settings required for constant alveolar tension of 0.8% halothane at various FGFs as calculated by Eger. As FGF into the semiclosed circuit is progressively decreased from 8 L/min to 1 L/min, vaporizer setting, and hence tension delivered to the breathing circuit, must be increased. This is again because inspired gas is a mixture of fresh gas from the anesthesia machine and gas exhaled from the patient. As FGF is decreased, a relatively greater fraction of exhaled gas is rebreathed. Because exhaled gas usually has a lower anesthetic tension than inspired gas, higher vaporizer settings are required to achieve the same inspired tension.

Note that at 1 L/min of FGF, the initial halothane vaporizer setting is more than 10%. This is difficult, if not impossible, to achieve with modern anesthesia delivery systems, because the vaporizer cannot deliver that high a concentration. It must be emphasized that the high concentration of anesthetic is delivered to the breathing circuit , where it mixes with the 0.8% in the patient’s exhaled gas to produce an inspired concentration that must be the same 3.3% as was required in an open circuit. In the two cases, the time course of anesthesia induction is identical, because inspired anesthetic concentration is the same. The patient’s body is not aware of how the anesthesiologist created the gas mixture breathed.

Nonrebreathing (Open-Circuit) and Closed-Circuit Anesthesia

Fig. 19.3 shows vaporizer settings required to maintain constant alveolar tension for a nonrebreathing circuit (open circuit) and a completely closed circuit with an FGF of 0.25 L/min. With the closed circuit, the initial vaporizer setting is 40% halothane. In practice, this is impossible for several reasons. First, modern concentration-calibrated vaporizers can be set to administer no more than 5% halothane. Second, the vapor pressure of halothane at room temperature is only 0.33 atm (33%, or 243 mm Hg), thus 40% halothane cannot exist under these conditions. Nonetheless, 40% is the theoretical concentration required. This high concentration at the low FGF of 250 mL/min into a closed circuit produces the same 3.3% inspired concentration that will achieve the same 1 MAC (0.8% end-expired) anesthetic.