Gent 1998
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Different types of vaporiser in circuit.

Geoffrey Nunn BA, FRCA

Leeds, England.

The use of a vaporiser in the circle became possible in the mid 1950's with the advent of halothane. The Goldman vaporiser with its very low resistance and simple design was suitable for this function but was inherently unsafe with IPPV.

The widespread availability of reliable infra-red monitors now makes the use of in-circuit vaporisation a practical possibility.

Two methods are possible, the use of a low resistance vaporiser of the Goldman type or the volumetric addition of liquid agent.

The Goldman vaporiser is no longer manufactured, though copies are readily available in other countries where they are still used extensively.

Two companies currently manufacture vaporisers based on the original Goldman design. McKesson of England produce a device which is used mainly in dentistry. Komesaroff in Australia produces a device which is closer in some respects to the original Goldman and which is sold mainly to the veterinary market.

Like the Goldman they are capable of an output that is well in excess of that required for use in a low flow system. They are thus potentially unsafe and are also awkward to use due to the small dial movement.

What would be more suitable is a vaporiser with a lower maximum output and a more progressive change in output over the range of around 0. 1 to 0.4%.

The current McKesson vaporiser has been in production for 16 years and was preceded by an earlier type which did not feature the rotating barrel which characterises the Goldman and its later derivatives. The rotation of the barrel is responsible for the high output of the Goldman due to the rotating component given to the gas flow. The original McKesson model's barrel was moved linearly by a screw mechanism which causes it to be far less efficient. Typically the maximum output is of the order of 0.8% for Isoflurane and 0.6% for Sevoflurane. Combined with the control knob which rotates almost a full turn this vaporiser gives a very easily controllable response.

Volumetric addition of volatile agent has an even longer history and has certain advantages over an in-circuit vaporiser.

The use of a syringe pump overcomes to some extent objections to the use of an uncalibrated device such as an in-circuit vaporiser.

Volumetric addition allows for a much more rapid rise in vapour concentration than otherwise be achieved with low fresh gas flow rates.

Modem pumps have serial connection ports which allows connection to a computer which take an input from the gas analyser to allow automatic control of the end-tidal concentration.

This feedback approach is perhaps the way forward for volatile anaesthesia.

PRACTICAL USE OF A VAPORISER IN-CIRCUIT

Jan Baum,

Damme, Germany

The Komesaroff In-Circuit Vaporizcr (VIC) is a very simples device based on a truck engine oil reservoir. It can be attached directly to the inspiratory connector of the breathing system. If the non-calibrated valve is opened, a certain amount of the circulating gas by-passes the glass chamber of the VIC partially filled with the fluid agent. The graduation of the valve allows for selecting small, intermediate or large amounts of the circulating gas to bypass the chamber, thus to switch from low to high amount of anaesthetic vapor to be delivered into the system.

On the other hand, if a conventional anaesthesia machme is used, equipped with a vaporiser outside the circuit (VOC) attached to the fresh gas supply system, the amount of vapour delivered into the breathing system is bound to the fresh gas vohme. At a given setting of the vaporizer's dial the amount of anaesthetic is directly proportional to the fresh gas volume. In very low fresh gas flows even the maximum output of the VOC is comparatively small. The more, as the vaporizer's output normally is limited at a maximun being 3 times the MAC of the respective agent. Thus, at a given fresh gas flow, the maximum amount of agent vapour which can be delivered into the system is the lower the higher is the anaesthetic potency of the volatile anaesthetic. Due to the limited output of a VOC, in low flow techniques an increase of the fresh gas concentration of the volatile anaesthetic only with considerable delay in time will lead to a corresponding increase of its concentration within the breathing system. In conventional anaesthesia machines the time constant is the longer the lower is the fresh gas flow.

After institutional approval the Komesaroff vaporizer was used in a few patients undergoing inhalational anaesthesia under supervision of a second anaesthetist. The following observations were made specifically characterizing the VIC:

At a definate, setting of the bypass valve the amount of vapour delivered into the breathing system is the higher the higher is the minute volume. This is due to the increase of gas volume bypassing the absorber chamber.

At a definite setting of the bypass valve and the minute volume being constant the anaesthetic's concentration within the breathing system is the higher the lower is the fresh gas flow. This is due to the decrease of the wash out effect in low flow anaesthesia.

Due to the rough graduauon of the bypass valve only in high flow anaesthesia alterations of the setting of the bypass valve lead to calculable alterations of the agent's concentration within the breathing system. In low flow anaesthesia however, the concentrations gained within the breathing system by alterations of the valve's setting are unpredictable.

Using a VIC the disadvantage of long time constants in low flow anaesthesia - at least when an increase of the anaesthetic's concentration is intended - can be overcome. The amount of anaesthetic vapour isn't bound to the fresh gas flow any more. However, as especially in low flow anaesthesia the anaesthetic concentration, gained within the breathing system is absolutely unpredictable, this device must never be used without a reliable working gas monitor. As the gas monitor in this technical setting is a part of the anaesthetic agent metering system itself, for safety reasons to prevent a failure in the single fault condition (EN 740), the use of a redundant gas monitor would have to be stipulated. Last but not least the official regulations concerning technical safety of anaesthesia machines (EN 740) would be the definate obstacle against the use of this device, as they prohibit the use of uncalibrated vaporizers in clinical anaesthesia.

Another unsolved problem, furthermore, was the water condensation within the vaporiser chamber. After using the VIC for only a comparatively short time, there was a mixture of fluid agent and water to be found in the glass bowl. It was definately impossible to deal correctly wqith this waste. Should it be emptied to waste water or poured onto a piece of cotton for vaporization? We didn't find an acceptable solution in this matter during our tests.

Concluding, the use of a VIC is an interesting alternative for overcoming the long time constant in low flow anaesthetic techniques, however, only if an increase of the anaesthetic concentration is intended. For safety reasons this device should not be used in routine, clincal practice. With commercially available modern anaesthetic machines, equipped with VOC and all the safety tools stipulated by official regulations, low flow anaesthesia can be, performed much more safely and conveniently. The use of a VIC only partially overcomes the disadvantage of long time contants in low flow anasthesia. However, considering the shortcomings and the problems arising from the use of a VIC, one must come to the conclusion that using this device - especially in low flow techniques - for ecological and for safety reasons cannot be recommended.

NEW CONCEPTS ON GAS UPTAKE IN LOW FLOW / CLOSED CIRCUITS

Chung-Yuan Lin

Chicago USA

The prevailing model of uptake of inhalalation anaesthetics fails to recognise the existence of functional residual capacity and the alveolar membrane, leading to a misunderstanding of the uptake process. Since we do not have a good index for anaesthetic depth, the concept of minimum alveolar concentration (MAC) has been blindly accepted as the standard indicator.

With proper recognition of the existence of the functional residual capacity (FRC) and the alveolar membrane, the uptake of inhalation anaesthetics can no longer be regarded as following a power function of time. (1) Uptake actually starts from zero and reaches its highest level at the end of FRC washin, and then decreases slowly with time. (2) Uptake depends on the inspired concentration. (3) With a given inspired concentration, the uptake does not change much with time. (4) Uptake should be represented by 1 - FA / FI and not by FA / FI curve itself. (5) Uptake of anaesthetics per minute can be estimated by [Fraction of uptake x Inspired concentration (%) x Alveolar ventilation (ml / min)]. (6) We are able to determine the anesthetic concentration in the mixed venous blood without blood sampling after each breath. These concepts provide a scientific background for the safe and easy practice of closed-circuit anaesthesia.

Contrary to common belief, closed-circuit anesthesia allows much safer anesthesia practice than does the semi-closed system. This is because the anesthesia circuit and FRC together form a large space of about 9,000 ml before the alveolar membrane is reached; this space serves as a buffer.

Patient self-feedback control of the anesthetic concentration will go on to a certain degree across the alveolar membrane during closed-circuit anesthesia because of the limited supply of anesthetic gases and because of the inspired-concentration-dependent uptake. Within certain limits the patient controls his or her own anesthetic requirments by adjusting the cardiac output. Therefore, closed-circuit anesthesia provides excellent hemodynamic stability with continuous elevation of the anesthetic concentration in the mixed venous blood toward the end of surgery, without compromise of hemodynamic stability.

In the clinical arena, anesthesiologists used to judge the depth of anesthesia on the basis of the effects of the anesthetic on breathing, pupilliary reaction sweating, the circulation, and muscle relaxation. However, with the introduction of neuromuscular blocade and powerful amnesic medications in the past three decades, the problem of judging the depth of anesthesia during general anesthesia became more difficult because these drugs abolished some of the responses that are most useful for assessing the depth of anesthesia. The most commonly used index of the depth of anesthesia with inhalation anesthetics, MAC, does not include the time factor, and it only refers to the anesthetic concentration in the alveoli, which does not represent the anesthetic in the brain. However, the anesthetic concentration in the mixed venous blood should correctly reflect the anesthetic concentration in the brain because 70% of the cardiac output distributed through vessel-rich organs including the brain. Therefore, the anesthetic concentration in the return blood will much better reflect the anesthetic concentration in the brain.

With the discovery of a fundamental mistake in our past understanding of the uptake process of inhalation anesthetics, we now are able to change anesthetic practice of experience to scientific anesthesia and consider strongly that general anesthesia practice with closed-circuit provides maximum safety for the patient.

Influence of Low Solubility and Kinetics on the Practical Use of Low Flow Systems

Dr M Logan

Edinburgh Scotland.

For decades the main pursuit for circle anaesthesia protagonists has been to find an inhalational agent that is rapidly controllable within the circle. This is largely dependent on rapid saturation of the patient by the agent. The corresponding low agent uptake rapidly approximates the inspired and expired concentrations causing less dilution of the fresh gas concentrations. This results in an early reduced requirement of agent delivered into the circle from the vaporizer. The advent of volatile agents with low blood/gas partition coefiicients such as Sevoflurane and Desflurane allows early progressive elevation of agent tension within the patient. The time constant for the circle is low.

Time Constant = Volume of circle / fresh gas flow - uptake

Assuming a fixed volume of the anaesthetic breathing system for any procedure, the rate of uptake is determined by the interaction between the fresh gas flow rate and the rate of uptake. A rapid reduction in the uptake elevates the FA/Fi ratio resulting in less dilution of the fresh gas volatile concentration (FD) from the recycled expiratory limb. The clinical effect of this is to permit the use of lower concentrations delivered from the vaporiser to achieve the same inspired concentration. Alternatively, an earlier reduction in the fresh gas flow to low or minimal flow rates is possible. Unfortunately, nitrogen accumulation within the circle can be significant if low flow rates below 2 l min-1 are used early without deliberate preoxygenation / denitrogenation before induction. The less blood soluble agents, Desflurane and Sevoflurane, have the capacity, for reduction of fresh gas flows to below 2 1 min-1 within three minutes of the onset of circle system anaesthesia. In this case, the restricting factor is not the need to maintain a high input of volatile agent molecules into the circle but the avoidance of nitrogen accumulation diluting the N20/02/Volatile mixture inspired. As an illustration, the Desflurane vaporizer at capacity output of 18% delivered in a fresh gas flow of 1 l min-1 (50% 02 / 50% N20) does not significantly reduce the amount of Desflurane used over a 3 1 mim-1 conventional starting flow rate for procedures up to 40 minutes duration. The Nitrous oxide concentration is diluted by the nitrogen accumulation requiring a compensatory increase in the Desflurane concentration to maintain equivalent MAC levels. Desflurane is some twenty times more expensive than Nitrous Oxide to use for one hour. Thus very early reduction of the fresh gas flow may not necessarily lead to reduced volatile agent costs.

The high FA/Fi ratio is a consequence of low agent uptake whilst maintaining agent tension within the patient. With expired volatile concentrations approaching those inspired, less dilution of the fresh gas volatile input concentration occurs which reduces the differential between the inspired concentration and the vaporizer preset value (Fi / FD Ratio). For Desflurane and to a lesser extent Sevoflurane, the inspired volatile agent concentration is very close to that inspired unless the fresh gas flow is lower than 1 l min-1. Thus the predictability of volatile agent concentrations within the circle is improved for situations where no volatile agent monitoring is available for these agents.

When agents that permit early approximation to saturation within the patient were introduced there was the theoretical possibility that when using closed circle flows the patient uptake might become less than the vaporizer output during the maintenance phase. If this were to occur, the vapour concentration within the circle could rise higher than that preset on the vaporizer with the potentially serious consequences if in-circle volatile agent monitoring were not used. Anaesthesia up to eight hours duration produces a plateau of Desfiurane uptake rate such that the patient does not achieve saturation within this period and at no time does the circle concentration exceed that preset on the vaporizer. This may be due to losses from the wound, skin or even through porous components of the breathing system. However it may also be because of continued redistribution into the slow equilibrating compartments in the body. The almost constant uptake of Desflurane after the first hour of anaesthesia makes redistribution likely as a sole cause.

Automatic feedback control in low flow anaesthesia systems

A. M. Zbinden, University Hospital, Berne

General aspects of feedback control in anaesthesia: The rational for developing feedback control systems in anaesthesia is the assumption that automatic feedback control has advantages over human control (Table 1). Modem anaesthesia offers various parameters which can be routinely measured in the OR and which fulfil the requirements for the use in automatic control (Table 2)

Table 1

Advantages of automatic feedback control over manual control in anaesthesia

1. Better control: rapidly providing adequate anaesthesia even at intense surgical

stimulation and thus decreasing the risk of awareness without overdosing anaesthetics and consecutive prolonged recovery times.

2. More rational basis for the quantitative application of anaesthetic drugs

3. Improved efficiency by overtaking repetitive tasks from the anaesthetist

4. Reduced costs by decreasing the consumption of expensive agents.

5. Ideal research tool by providing constant and replicable anaesthesia conditions

Table 2

The prerequisites for successful feedback control

1. Good input signals, which measure true endpoints, are free of artefacts and are ideally continuous

2. Good actuators which can be controlled electronically, are safe and quantitatively accurate

3. Short delay times between changes in the: controlled variable and the reaction of the measured variable

4. Good control algorithms

5. Safety systems.

Various control loops have been applied in anesthesia  (Table 3).  The requirements for successful control are:

The development of the adequate control strategy (Table 4)

The availability of good actuators which can be controlled electronically

Reliable safety systems (intelligent supervisor system, fail proof real time computer platform, redundancy of vital measurements)

Table 3 Potential control loops

Loop	Measured	Set
Ventilation controller	FeCO2 Plateau pressure	Tidal volume Respiratory frequency
Volatile anaesthetic controller	Finspired volatile anaesthetic [1]	Vaporiser setting
	Fendtidal volatile anaesthetic	Vaporiser setting or Finspired volatile anaesthetic
	Mean arterial pressure [2]	Fexpired volatile anaesthetic
O2 /N2O controller	Finspired of O2 and N2O	Fresh gas flow of O2 and N2O
System volume controller	System volume	Flow of O2, N2O
Depth of anaesthesia	EEG parameters [3, 4] Bispectral index Median frequency Spectral edge Other parameters under investigation	Finspired volatile anaesthetic intravenous anaesthetic infusion rate
Relaxation	Electromyogram Electromechanogram Accelerometry	muscle relaxant infusion rate

Table 4

Overview on various types of controllers. Various aspects to be considered for development, tuning and

application

	Model based			Experience based
	Model Predictive	Observer Based	PID		Rule based		Training based
					Fuzzy logic	Crisp logic expert systems	Neuronal networks
Development	Mathematical model needed				Rapid first experience based prototype	Expert knowledge needed	Training basis needed
Optimisation	Numerical on line with constraints	Analytical off line without constraints	Manual		Manual		Numerical
Tuning	Efficient				Tuning problems	Difficult	Training problems
Constraints	Input and output is explicitly constrained	Input and output is not explicitly constrained. Special treatment needed			Can be taken into account	Can be taken into account
PC Power	++++	++	+		++	++	+++
Tracking behavious	++	+			++
Disturbance rejection	++	+			+

1 Curatolo M, Derighetti M, Petersen-Felix S, Feigenwinter P, Fischer M, Zbinden AM: Fuzzy logic control of inspired isoflurane and oxygen concentration using minimal flow anaesthesia .Brit J Anaesth

2 Zbinden AM, Feigenwinter P, Petersen-Felix S, Hacisalihzade S: Arterial pressure control with isoflurane using fuzzy logic. Brit J Anaesth 1995; 74:66-72

3 Schwilden H Stoeckel R and Schuttler J: Closed-loop feedback control of propofol anaesthesia by quantitative EEG analysis in humans. Brit.JAnaesth. 62:290-296, 1989.

4 Schwilden H, Schuttler J, and Stoeckel H. Closed-loop feedback control of methohexital anesthesia by quantitative EEG analysis in humans. Anesthesiology. 67:341-347, 1987.

5 Olkkola KT, Schwilden H, and Apffelstaedt C. Model-based adaptive closed-loop feedback control of atracurium- induced neuromuscular blockade. Acta.Anaesth.Scand. 35:420-423, 1991.

ASSISTED SPONTANEOUS BREATHING DURING

ANAESTHESIA WITH THE LARYNGEAL MASK AIRWAY

Dr. Nigel Harper

Manchester Royal Infirmary

It has long been known that dose-dependent respiratory depression is an invariable accompaniment to general anaesthesia (Eger 1981) but it was not until capnography came into routine use that anaesthetists realised the extent of the marked hypercapnoea that is commonly associated with spontaneous ventilation. This and other factors encouraged a move towards the widespread use of endotracheal intubation and mechanical ventilation a phenomenon that was already well established in paediatric anaesthesia and in the USA.

The more recent introduction of the laryngeal mask airway has partially reversed this trend, especially in Europe, and a higher proportion of patients are breathing spontaneously throughout their surgical procedure. Despite the introduction of new opioid drugs, new inhalational agents, and TIVA into clinical practice, hypercapnoea remains an unwelcome and persistent problem. Both the hypercarbic and the hypoxic drive to respiration are impaired by inhalational anaesthetic agents (Knill 1978).

Within minutes of inducing anaesthesia in the supine position the FRC falls by approximately 20% towards the residual volume. This occurs even if the patient is mechanically ventilated and irrespective of the presence of nitrous oxide. The cause of this reduction in the resting volume of the lung is not clear but there are deleterious effects on the patency of the airways, compliance and gas exchange.

During mechanical ventilation with large tidal volumes the physiological deadspace is increased during anaesthesia, possibly as a result of over ventilation of under perfused alveoli. This phenomenon attenuates the expected fall in arterial carbon dioxide tension. Conversely, at the small tidal volumes associated with depressed spontaneous breathing, the physiological deadspace may be markedly reduced. This can be thought of as a compensatory mechanism. In addition, carbon dioxide production is reduced by approximately 15% compared with the awake individual. Without these mechanisms, patients breathing with the small tidal volumes we observe during anaesthesia would be even more hypercarbic.

All breathing circuits are associated with an increase in the work of breathing. Only the work associated with inspiration is important: expiration is largely the result of elastic recoil of the lung and chest wall and requires very little work. The phasic contraction of the abdominal muscles during anaesthesia contribute very little to expiration.

The use of pressure support ventilation to decrease the work of breathing is well established in the Intensive Care Unit where it is employed to assist the process of weaning from full mechanical ventilation. These patients are often disadvantaged by severe respiratory muscle weakness, poor pulmonary compliance and high metabolic rate. Increasing levels of pressure support progressively reduce the respiratory rate and progressively increase the tidal volume (by approximately 25%) and minute ventilation of sedated ICU patients. These changes are associated with a reduction in the work of breathing which is reduced by approximately 50% by increasing the pressure support level from zero to 12 cm H20. The reduction in pressure support is only as great as the increase in work of breathing associated with inserting an endotracheal tube (Brochard 1991). Pressure support ventilation is an important modality in the ICU and there is no doubt that the duration of the weaning process is reduced. It is clear that the benefits of pressure support in this group of patients are not solely the direct result of a decrease in the work of breathing but are the consequence of an increase in the efficiency of ventilation.

How useful is pressure support ventilation (assisted spontaneous breathing) in anaesthetised patients undergoing surgery? There is little published work to date and many of the conclusions are contradictory. All the studies considered here have used a Siemens 900 series ventilator to deliver anaesthetic gases through an endotracheal tube.

THE CONTROVERSY

In 1992 Christie and Smith working in Tampa, Florida published the effects of pressure support on various respiratory parameters during enflurane anaesthesia in 9 patients. Their primary aim was to investigate the effect of 5 cm H20 pressure support on the work of breathing. Respiratory rate, tidal volume, PaCO2, and end-tidal C02 were unchanged but the work of breathing was reduced from 532 mJ/L breathing from a circle system to 171 mJ/L during 5 cm H20 pressure support ventilation. They did not observe any irregular breathing patterns.

Three years later, Bhatt and colleagues (1995) working in Hong Kong published a study (9 patients, isoflurane anaesthesia) in which a level of pressure support of 10 cm H2O was studied in addition to 5 cm H2O. In contrast to the Christie's work, the respiratory rate was reduced and the tidal volume was increased by 5 cm H20. These effects were more marked at 10 cm H2O support. The mean PaCO2 was unaffected but in at least half of the patients their breathing became irregular at both levels of support. When pressure support was first applied in this group of patients there was a progressive fall in end-tidal CO2 and a progressive increase in the duration of each respiratory cycle. After a period of 2 - 3 minutes the expiratory phase was greater than 10 seconds; i.e. the patient was apnoeic. They concluded that pressure support ventilation produces a fall in arterial CO2 sufficient to cause apnoea in some patients.

Under experimental conditions it is difficult to induce apnoea in healthy volunteers by deliberate hyperventilation. However, when arterial CO2 is deliberately reduced in anaesthetised patients, it is possible to induce apnoea more easily. The level of arterial CO2 at which apnoea occurs, the apnoeic threshold, is approximately 0.7 - 1.2 kPa (5.3 - 9.0 mm Hg) below the normal value (Hanks et al. 1961). Bhatt and colleagues concluded that '..it would be difficuft to justify the use of pressure support ventilation during anaesthesia in a healthy population'.

In February 1996 the Florida group (Bosek et al. 1996) published a larger series (20 patients) in which they investigated the effects of a) 5 cm H2O pressure support; or b) sufficient pressure support to result in a measured tidal volume of 8 ml/kg. Anaesthesia on this occasion was with desfiurane. Their results confirmed their earlier work in which 5 cm H2O pressure support did not alter the respiratory rate, tidal volume, PaCO2, or end-tidal CO2. Approximately 17 cm H2O support was needed to produce a tidal volume of 8 ml/kg. At this high level of support the mean tidal volume was increased (from 237 to 518 ml) but a marked reduction in respiratory rate (from 26 to 11) resulted in an overall small reduction in the minute volume. The arterial CO2 was actually reduced from 6.8 to 6.0 kPa (51 to 45 mmhg). They were able to demonstrate (using the modified Bohr equation) that the reduction in PaCO2 was the result of a pressure support-mediated reduction in physiological deadspace. In common with their earlier study and in contrast to the work of Hanks, no patient experienced an apnoeic episode of greater than 10 seconds. They concluded that ' Pressure support of spontaneous breathing appears to be a reliable alternative to controlled mechanical ventilation ...' A recent study of assisted breathing in conscious volunteers demonstrated an increase in tidal volume with no change in respiratory frequency and preservation of the regular pattern of breathing although the inspiratory time was shortened (Mecklenburgh and Mapleson 1998).

WORK IN PROGRESS

It is difficult to reconcile the apparently conflicting literature on pressure support ventilation during anaesthesia. All three studies used the Siemens 900 series ventilator. Different inhalational agents were used but enflurane, which normally produces greater respiratory depression than isofiurane or desfiurane, was not associated with irregular breathing or apnoea. The Hong Kong group performed their study in the absence of surgical stimulation. The Florida group minimised the influence of surgical stimulation by performing regional blocks.

Studies currently being undertaken by the author may cast some light on the mechanisms which produce apnoea during pressure support ventilation. In a preliminary study we have demonstrated that pressure support via a laryngeal mask airway can reduce PaCO2 towards normal levels in some patients but not others. In some patients irregular breathing and apnoea can be induced before there is any reduction in end-tidal CO2. The rapidity with which this phenomenon occurs after a step-increase in pressure support suggests that either muscle-spindle or proprioceptive afferents lead to rapid inhibition of the motor neurone pool in the anterior horn of the spinal cord. It appears that it may be possible to over-ride the irregularities in the breathing pattern in some patients.

Further work in progress may be able to identify modes of assisted breathing which may be used to restore PaCO2 to normal levels during inhalational anaesthesia without suffering the penalty of an irregular breathing pattern.

References

Eger El. lsofiurane: a review. Anesthesiology 1981; 55: 559.

Knill RL, Geib AW. Ventilatory responses to hypoxia and hypercapnia during halothane sedation and anesthesia in man. Anesthesiology 1978; 49: 244.

Brochard L, Rua F, Lorino H, Lemaire F, Harf A. Inspiratory pressure support compensates for the additional work of breathing caused by the endotracheal tube. Anesthesiology 1991; 75: 739.

Christie JM, Smith RA. Pressure support ventilation decreases inspiratory work of breathing during general anaesthesia and spontaneous ventilation. Anesthesia and Analgesia 1992; 75: 167.

Bhatt SB, Chui PT, Gin T, Tam YH, Oh TE. Pressure support ventilation during isofiurane anaesthesia. Anaesthesia 1995; 50: 1026.

Hanks EC, Ngai SH, Fink RB. The respiratory threshold for carbon dioxide in anesthetized man. Anesthesiology 1961; 22: 393.

Bosek V, Roy L, Smith RA. Pressure support improves efficiency of spontaneous breathing during inhalation anesthesia. Journal of Clinical Anesthesia 1996; 8: 9.

Mecklenburgh JS, Mapleson WW. Ventilatory assistance and respiratory muscle activity. 1: Interaction in healthy volunteers. British Journal of Anaesthesia 1998; 80: 422-433.

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