Ada unaesth. scand. 1978, 22, 281-286

A Circle System Without Carbon Dioxide Absorption H. J . LADEGAARD-PEDERSEN Department of Anacsthesiology, Rigshospitalet, Copenhagen, Denmark

An anaesthetic circle system without a carbon dioxide absorber is described. The efficiency of the circle, i.e. the fraction of alveolar gas in' the outflow from the circle, was measured in 15 patients during halothane anaesthesia or neurolept analgesia. The fraction ranged from 0.88 to0.95 (mean 0.91), while the ratio between the alveolar ventilation and the fresh gas inflow ranged from 0.97 to 1.71. The efficiency was not correlated to this ratio. There was no need for hyperventilation if the fresh gas inflow was 10% higher than the alveolar ventilation required to maintain normal Paco,. The circle was used in 50 patients manually ventilated by nurse anaesthetists. Mean fresh gas inflow was 60 ml/kg. Mean Pacoz was 5.47 kPa (41 mmHg). In a similar group of 50 other patients, in which the standard circle used in the department was employed, the mean Pacoz was 4.80 kPa (36 mmHg). The frequency of hypercapnia was equal in the two groups, but hypocapnia was not seen when the circle without absorber was used.

Received 22 August, accepted for publication 30 September 1977

According to published descriptions of anaes- Thus the corrugated tube leading to the valve acts as a thetic circuits without carbon dioxide absorp- reservoir for fresh gas; in the expiratory phase it fills the tube backwards. During inspiration the fresh gas tion, and used with controlled ventilation, is mainly used for alveolar ventilation, while expiratory there is a need for a large fresh gas inflow or a gas is mainly used for dead space ventilation. The large respiratory minute volume, if hyper- overflow valve (Berner@) (BERNER1972) is placed capnia is to be avoided (SUWA& YAMAMURAnear the respiratory bag on the anaesthetic machine. For the investigation of this circle some measure1970, BAIN& SPOEREL 1973, SCHOLFIELD & ments using a circuit with carbon dioxide absorption WILLIAMS1974, SNOWDONet al. 1975, were needed for the calculations (see below). For this TIIOMSEN & J0RGENSEN 1976). purpose the standard circle system used in the departThis need for hyperventilation is caused ment was employed (Fig. 1 (right) ). The same double by the mixing of the fresh gas with the gas valve is fitted to the endotracheal tube, but the inflow in the circuit before it reaches the alveoli. as well as the absorber are placed on the anaesthetic machine. In the circle described in the present study such mixing is minimized. The study was Theoretical considerations designed to examine the gaseous homeostasis E&iency. The efficiency ( f ) of a circuit without carbon of the circle and the requirements of fresh dioxide absorption is, in this study, defined as the fraction of alveolar gas in the gas vented from the circle. gas inflow during controlled ventilation.

METHODS Description of the circle employed Figure 1 (left) shows the circle used on patients. A unidirectional valve (Schneider@) is attached to the endotracheal tube, and the inflow of fresh gas is placed close to the inspiratorv limb of the valve.

From this efficiency the fresh gas flow necessary to prevent hypercapnia can be calculated. If the outflow is assumed to equal the fresh gas inflow ( V I N F ) , the amount of alveolar gas vented from the circuits is fx and the alveolar concentration of carbon dioxide (FA) is determined by the following equation:

where

Vco2,

is the carbon dioxide production per

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11. J. LADEGAARD-PEDERSEN

O r in other words: How does artificial hyperventilation change the efficiency of the circuit? T o answcr this question,fwas correlated to the ratio between the alveolar ventilation and the fresh gas inflow. For the calculations, equation (2) is combined with equation (3), to give equation (5) : -V=A -

VINF

Fig. 1. Left: The circle system without carbon dioxide absorber. The inflow and outflow, as well as the direction of flow inside the circle are indicated by arrows. Right: The circle system with absorber. The absorber is indicated by a rectangle with a cross. minute. This formula has been analysed in detail by CONWAY (1976). I n a circuit with carbon dioxide absorption the alveolar concentration (FAa)is determined by:

where VAis the volume of gas entering the respiratory zone per minute, i.e. the alveolar ventilation. From equation (1) and equation (Z), it may be seen that FA equals FA, when f . VINFequals VA. Therefore the change from a circuit with absorber to a circuit without does not change the alveolar carbon dioxide concentration, if the fresh gas inflow equals VA/J Thus the efficiency of the circuit determines the necessary fresh gas flow. For the measurement o f f the above-mentioned assumption, that V I N F equals the outflow from the circuit, is used and VcOzis estimated fromequation (3) : Vcoz =

~ I N F * Fo

(3)

where Fo is the carbon dioxide concentration in the gas vented from the circuit. When equation (1) is combined with equation (3), equation (4) is obtained for the measurement off:

f =F 1 , FA

(4)

Hyperventilation and rebreathing In a circuit without carbon dioxide absorption a n artificially increased respiratory minute volume with unchanged VINF leads to rebreathing of carbon dioxide. The consequent increase in alveolar ventilation does not necessarily change the alveolar concentration of carbon dioxide. The question is rather, how rebreathing influences the efficiency of the circuit.

FO

FA,

Thus to measure the ratio between VA and VINF,a circuit without carbon dioxide absorption and a circuit with carbon dioxide absorption have to be used in the same patient, with unchanged VA and V I N F and assuming an unchanged %'coz. At the same time it is possible from equation (1) and equation (2) to calculate the inspired carbon dioxide concentration (F,) for the circuit without absorption :

Investigational design For measurement of the efficiency f (equation (4)), the ratio \fA/VINF(equation (5)) and the inspired carbon dioxide concentration (equation (6)), measurements were performed in 15 patients premedicated with diazepam and hyoscine. Body weights ranged from 45 to 93 kg. They were anaesthetized with halothane (nine patients) in N 2 0 (2 l/min) and 0 2 (2 l/min) or neurolept analgesia (six patients) with 3 1 NzO/min plus 1.5 1 0 2 / m i n . The carbon dioxide concentration was measured bya Godard@capnograph. The end-tidal concentration on the expiratory side of the Schneider valve was measured as FA. Fo was measured in a corrugated tube attached to the overflow valve. The concentration there did not show significant respiratory fluctuations. For comparison, Pace, and Paoz were measured in a Radiometer" ABL 1 automatic acid base analyser on blood samples drawn from the radial artery. Constant ventilation was secured by using a modified Bird" ventilator, which acted on the "bag-in-bottle" principle. The tidal volume was 10 ml/kg body weight at a rate of 12/min. I n each patient, Fo and FA were measured using the system without carbon dioxide absorber, and FA, using the system with absorber. Tidal volume and frequency, as well as fresh gas inflow were unchanged during the measurements of Fo, FA and FAa. The order in which the two systems were used in each patient was selected a t random. Measurements were performed after stable values of FA and FAa had been obtained by the capnograph for at least 10 min. T o compare the two circuits in clinical practice, each of the two systems was then used in 50 other patients manually ventilated by nurse anaesthetists. The body weights ranged from 30 to 114 kg (mean 67.7 kg). The operations were mainly urological or gastro-enterological surgery. The criteria for entering

CIRCLE SYSTEM WITHOUT ABSORBER

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circuit, determined as f, was fairly constant (Fig. 2), and ranged from 0.88 to 0.95 (mean 0.91). Thus about 90% of the gas vented from the circuit was alveolar gas. As a consequence of the constant efficiency, Paco2 was not correlated to VAI$'lNF (Fig. 3), while the inspired carbon dioxide concentration (calculated from equation (6)) was higher the higher ? ' ,/$,' was. It ranged Table 1 from 0.2% to 2.3%. Fresh gas flow selected for the manually ventilated The difference between the alveolar cargroups of patients. bon dioxide tension, calculated from FA and FAa,and Pacol, ranged from 0 to 1.60 kPa 0 2 N20 (0 to 12mmHg) (mean0.79kPa(5.9mmHg)). liter/min literlmin The mean Paco, was 3.87 kPa (29.0 mmHg) when the absorber was included, and 4.88 Circle system without absorber: kPa (36.6 mmHg) when it was excluded. Halothane anaesthesia 2 2 Neurolept analgesia Using the system without absorber, mean 1 2 Body weight

A circle system without carbon dioxide absorption.

Ada unaesth. scand. 1978, 22, 281-286 A Circle System Without Carbon Dioxide Absorption H. J . LADEGAARD-PEDERSEN Department of Anacsthesiology, Rigs...
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