PROBLEMS OF AUTOMATIC VENTILATION IN INFANTS AND CHILDREN

Brit. J. Anaesth. (1962), 34, 514

PROBLEMS OF AUTOMATIC VENTILATION IN INFANTS AND CHILDREN BY

WILLIAM W. MUSHIN, WILLIAM W. MAPLESON AND JOHN N. LUNN*

Department of Anaesthetics, Welsh National School of Medicine, Cardiff, Wales SUMMARY

The use of automatic ventilators on apnoeic adult patients has become commonplace. However, in the case of children, manual ventilation by means of a bag is generally preferred. This attitude seems to indicate a lack of confidence in the suitability of ventilators for infants. Trial soon shows that some ventilators are quite unsuitable for infants. Others may, in certain circumstances, just be satisfactory. Only one or two are specially designed for infants. It seemed clear that a closer study of the mechanical and physical problems involved in ventilating infants was needed and this investigation was planned. It had as its objective the elucidation of the requirements which a ventilator must fulfil if it is to ventilate these small subjects in a proper manner. With this information clinical anaesthetists would be able to adapt many of the ventilators they already possess to this specialized use, while designers would have available the basis of a specification from which to develop the construction of a ventilator for young subjects. In the case of an adult a typical pattern of ventilation is a tidal volume of 500 ml, a respiratory rate of 15 per minute, and an inspiratory to expiratory time ratio (I: E ratio) of 1:2. Each respiratory cycle lasts 4 sec, therefore inspiration lasts 1.3 sec.

In order that 500 ml shall enter the lungs in 1.3 sec, average flow rate in inspiration 500 .. = —— ml/sec, = 22.5 l./min. As will be seen later, in the case of a newborn infant the tidal volume is typically 16 ml and the respiratory rate 40 per minute. If the I:E ratio is kept at 1:2, then inspiration lasts 0.5 sec. The average flow rate during the inspiratory period must be —— ml/ sec,

= 1.9 l./min. The differences between adult and infant in tidal volume, respiratory rate, and average flow rate during inspiration are striking. We have found from experience that many ventilatory problems arising during anaesthesia can be analyzed theoretically (Mapleson, 1954a, b, 1958, 1959; Waters and Mapleson, 1961). This enables a tentative solution based on certain assumptions to be advanced which can then be tested in the laboratory by using physical analogues or mechanical models of the physiological conditions. The ultimate confirmation lies in the application of the solution to patients. This approach was used in the present problem. In order to do this, assumptions regarding certain physiological standards had to be made. In developing the theory and in making the •Supported by the Endowment Fund of the United experiments, "normal" lungs were ventilated Cardiff Hospitals. 514

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Small children and neonates present special problems when they are to have automatic ventilation. These derive from their small tidal volumes, rapid respiratory rates, small inspiratory flow rates and low compliance pressures. In general, automatic ventilators designed for adults do not work satisfactorily for small subjects. Either the flow is too large, the respiratory rate too slow, the tidal volume too large or the cycling pressure too high. Theoretical concepts were tested by applying various ventilators to model lungs representative of different ages of children. Clinical tests support the theoretical and experimental results.

PROBLEMS OF AUTOMATIC VENTILATION IN INFANTS AND CHILDREN

PHYSIOLOGICAL STANDARDS OF VENTILATION IN INFANTS AND CHILDREN

adopted the figure of 5 ml/cm H , 0 as the total compliance. Hellieson et al. (1958) give a formula for calculating the airway resistance, relating it to a function of the height. For the neonate of average height 49.4 cm (Geigy tables), this gives an airway resistance of 31.2 cm H,O/(l./sec). The only values published are a mean of 29 cm H,O/ (l./sec) in quiet infants and 37 cm H,O/(l./sec) when crying infants are included (Cook et al., 1957). In the cases of the other two sizes of child, the standard values of the parameters were derived from interpolation formulae, relating them to either height or weight, of the form y = a H" or y = c Wd, where y is the respiratory rate, tidal volume, or compliance, H is the height, W is the weight; a, b, c, and d are constants determined by the requirements that the formulae must fit the neonate and adult data. Values for respiratory rate, tidal volume and compliance were obtained in this way and the values adopted were convenient ones near to those obtained by calculation (table II). Anatomical measurements of the trachea (table III) were obtained partly from the literature (Hull, 1955) and partly by interpolation from adult values. Recommendations given by Smith (1959) about diameters and lengths of endotracheal tubes suitable for the various ages and weights of the children were followed. The endotracheal connectors were chosen as typical of good clinical practice, that is, they were as large as would comfortably fit the endotracheal tube (table III).

There is general agreement about the range of normal values of ventilatory parameters in spontaneously breathing adults. Although there are comparatively few reports of these values in the case of neonates there is fair agreement amongst the authorities (table I). Apart from the extremes of neonate and adult, only a few isolated measurements have been made on subjects in the intermediate age groups. Nevertheless we felt it would be profitable to consider, even if only on the basis of interpolation, two intermediate child sizes between neonate and adult. Taking the average weight of a neonate to be 3.2 kg and that of an adult 66 kg, two geometrically equal steps of weight between these two figures are 8.9 kg and 25 kg. These weights correspond approximately to a child of just less than 1 year and to one of 8 years. On the basis of these weights models were made, to represent the compliance of the lungs and the resistance of the lower airways of three ages of children— neonate, 1 year and 8 years old. CONSTRUCTION OF MODELS Table I shows the measured values of tidal volume, respiratory rate and total ventilation Models (fig. 1) were constructed in which the for the neonate obtained from the literature. On compressibility of the air within a rigid container the basis of the weighted means, we chose as our reproduced the compliance of a patient. The standard a respiratory rate of 40 per minute and volume of the container was selected so that for each age of patient the compliance was approxia tidal volume of 16 ml. The few attempts to measure compliance in mately that of the child whose characteristics the the neonate (Cook et al., 1957) give a figure for model represented. The actual values for the lung compliance of about 5 ml/cm H2O. A compliance used in the models are given in similar figure for total compliance was obtained table IV. The trachea was represented by a short length by Richards and Bachman (1961), who attributed this lack of difference to the high compliance of of metal tubing of appropriate internal diameter the chest wall of the neonate. We have therefore into which the endotracheal tube was inserted.

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with "normal" tidal volumes at "normal" rates, that is to say, the tidal volumes and rates of quiet spontaneous respiration. Many anaesthetists and physicians choose to hyperventilate their patients: but if a ventilator can provide the very small ventilation of a quiet child then it can almost certainly hyperventilate the child to any desired extent. The respiratory parameters used here, therefore, should not necessarily be taken as recommendations for clinical application. They have served to provide a rigorous test for satisfactory performance of a ventilator.

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Neonatal Respiratory Function. Total Respiratory ventilation rate (ml/min) per min.

Number

Source

of

infants 50 18 17 35 23 9 21

Weighted mean

—

44 39 31 41 —

642 489 — 750 —

39-3

668

—

—

— — — — 4-9 — 4-9

16-2

—

—

16-7 19-8 16-8

721-4 8511

43-1

Compliance (ml/cm H2O)

— — —

130 —

37 0*

18-1

*See text. TABLE II

Physiological standards regarded as "normal" in this investigation. Weight (kg)

Height (cm)

Respiratory rate per min

Total ventilation (ml/min)

Neonate* 1 yearf 8 yearf Adult*

Airway Total Tidal resistance volume compliance (ml) (ml/cm H2O) (cm H,O/(l./sec))

40 3-2 49-4 640 30 90 71 0 1500 20 25 0 125 4000 16 660 171 8000 * See Table I. fValues obtained from the literature by interpolation and

16 50 200 500 calculation,

31-2 5 19-7 10 6-2 25 20 50 see text, t Accepted values

TABLE III

Sizes of trachea, tubes and connectors. Tracheal dimensions (Hull, 1955) Length (cm)

Diameter (mm)

Age

Neonate

6 7

1 year

Length (cm)

Size 00 Magill 20F

4-3

Size used

15

0 Magill Universal 1 Cardiff

18

4 Magill

10

14F

40

Connector

Endotracheal tube (Smith, 1959)

1 Magill 8 years

13*

26F

5-7

4 Magill ' By interpolation between 4 years old and adult on log/log scale. TABLE IV

Values used in models.

Model Neonate 1 year 8 year

Tidal volume (ml) 160 500 200

Total resistance (between points 4-9 Average Actual in fig. 1) at Inspiratory inspiratory Compressure average flow flow rate liance (ml in avleoli rate (cm time (sec) (l./min) /cm H8O) (cm H2O) H,O/(l./sec)) 0-5

1-92

0-66

4-5 120

10

4-3 8

20-5

3-7 6-2

10-3

28 24 9

Pressure drop across total resisActual tance at pressure typical inspi- drop at 10 l./min ratory flow rate (cm H2O) (cm H2O) 0-9 1-8

1-75

18-5 8-2 1-3

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Murphy and Thorpe (1931) Deming and Hanner (1936) Boutourline-Young and Smith (1950) Cook etal. (1955) Cook et a!. (1957) Strang (1961) Richards and Bachman (1961)

Airway resistance (cm H2O/ (l./sec))

Tidal volume (ml)

PROBLEMS OF AUTOMATIC VENTILATION IN INFANTS AND CHILDREN

Ferris, Opie and Mead (1960) and Hyatt and Wilcox (1960) showed that the upper airway is responsible for 40 per cent of the total airway resistance during mouth breathing. We followed Hyatt and Wilcox (1961) in assuming that the normal airway resistance below the endotracheal tube might reasonably be taken as 50 per cent of the total value during nasal breathing given in table II. This was reproduced by a carefully adjusted length of rubber tubing connecting the "trachea" and the compliance of the rigid container. EXPERIMENTAL ARRANGEMENT

A number of ventilators which are now available to anaesthetists was used to ventilate the models at the rates, and with the tidal volumes, indicated in table II. It was intended that the I:E ratio should be 1:2 but in those instances where a choice lay between maintaining the I:E ratio but accepting a higher ventilation, or maintaining the ventilation with an altered I:E ratio, we chose the latter. This seemed the more justifiable choice because of the lack of general agreement about the optimum I: E ratio. The two smaller sizes of endotracheal tubes

are generally not available with inflatable cuffs. In clinical practice, therefore, there is bound to be some leak round the endotracheal tube. The two possible extremes were reproduced in the models, one with the endotracheal tube inserted a few centimetres into the "trachea" and giving the maximum leakage, and the other with an airtight seal around the endotracheal tube. Flow and pressure were recorded continuously with a pneumotachograph and a pressure transducer. Pressure changes were recorded both at the endotracheal tube catheter mount and in the "alveoli". Having previously determined the compliance in each of the models, the tidal volume was regulated by observing an aneroid type manometer connected to the "alveoli". Each set of recordings was run off in a few minutes, after which the sensitivities were checked with known flows and pressures. FUNCTION AND CLASSIFICATION OF VENTILATORS

This has been considered fully in a previous publication from this department (Mapleson, 1959). It is sufficient here to consider the inspiratory phase only. In the course of this phase ventilators may act as either flow or pressure generators. Cycling at the end of the phase may be determined by either time, volume or pressure. Flow generators.

These are characterized by a flow, often constant throughout the phase, which is entirely determined by the design of the ventilator and which is unaffected by the physiological characteristics of the patient's lungs. The pressure changes induced by this flow are determined by the compliance and airway resistance of the patient. Pressure generators.

In this case it is the pressure, generally constant throughout the phase, which is entirely determined by the design of the ventilator and which is unaffected by the physiological characteristics of the patient's lungs. The flow and volume changes which result are determined by the compliance and airway resistance of the patient. At the end of the inspiratory phase both flow and pressure generators may be time, pressure or volume cycled. This implies that the phase

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

FIG. 1 The apparatus. Connection to ventilator Airway pressure tapping Connections to flow recorder Catheter mount Endotracheal tube connection Endotracheal tube Model trachea Model lower airway resistance Model compliance "Alveolar" pressure tapping Manometer

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If the criteria for the smallest subject are to be fulfilled (i.e. V T =16 ml, f=40/min, I:E 1:2), 60 inspiration must take place in sec, 40 x 3 = 0.5 sec. If the tidal volume is to enter the lungs in this time, then the inspiratory flow rate must average 16 ml/sec, 0.5 = 32 ml/sec, = 1.9 l./min. Thus a flow generator whose flow rate can be adjusted to this low level will function for the smallest model so long as its cycling mechanism can be adjusted to the appropriate value: 16 ml for volume cycling, 0.5. sec for time cycling, and 5 cm H2O* for pressure cycling. If the flow rate is much greater that 1.9 l./min then either too large a tidal volume will be delivered in the right length of time or else the desired tidal volume will be delivered in too short a time, resulting in an extreme I:E ratio. For instance, suppose the flow rate from the ventilator can be brought down to 20 l./min then a tidal volume of 16 ml will be delivered in 0.05 sec. At the end of this time the pressure in the lungs will be 3.7 cm H2O as before, but the pressure drop across the airway resistance would now be 10 cm H2O if the resistance were linear, but because of turbulence may well rise to over 40 cm H,O. It must therefore be possible to adjust the particular cycling mechanism to this high value if the ventilator is to function as

Results. In the Aintree*, a pressure-cycled flow generator, the flow rate could be adjusted to the level required, and the ventilator functioned for all model sizes. However with the Cyclator, another pressure-cycled flow generator, the flow rate is fixed at about 40 l./min by the design of the injector and the pressure of the driving gas. In the 1-year and 8-year models, the desired ventilation could be obtained, but the tidal volume was delivered in a very short time. This resulted in I:E ratios of 1:9 and 1:6 respectively. In the neonate model the maximum cycling pressure of 35 cm H2O was reached before any appreciable volume exchange occurred. The only example of a time-cycled flow generator we tested was a modified Pneumotron. Although this ventilator normally operates as a pressure generator because it has two preset safety valves which always leak, its design is fundamentally that of a flow generator and it can be made to function as such by blocking off the two valves or by replacing them with valves of better design. The former was done, and since the generated flow and the cycling time were fully adjustable the ventilator worked well on all three models. The Beaver ventilator, a volume-cycled flow generator, performed adequately for the model of the 8-year-old. In the smaller models a serious defea was found. The Beaver valve permitted total rebreathing to occur at the low volumes required. This could be eliminated by substituting a Ruben valve but a considerable volume of gas then

*This figure is the sum of 3.7 cm H.O in the alveoli due to the compliance and 1 cm H3O the pressure drop across the airway.

•Details of the construction and functioning of the various ventilators mentioned, with the exception of the Cyclator, will be found in Mushin, Rendell-Baker and Thompson (1959).

THEORY AND RESULTS

It is convenient to present the theory and the experimental results concurrently for each type of ventilator. FLOW GENERATORS

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Theory.

desired. With higher flows the cycling time will be even shorter and the cycling pressure even greater. If a time-cycled ventilator cannot be adjusted to the short time required, too large a tidal volume will be delivered. If a pressure-cycled mechanism cannot be adjusted to such a high level, the machine will cycle immediately the flow starts because the cycling mechanism will be actuated merely by the pressure difference, across the airway resistance, which appears immediately the flow begins.

ends and expiration begins either after a specified time has elapsed or when a certain pressure is reached at some point in the patient circuit, or after the passage of a given volume of gas.

PROBLEMS OF AUTOMATIC VENTILATION IN INFANTS AND CHILDREN escaped during the inspiratory phase through the expiratory port. This was wasteful, but did not prevent ventilation of the model. PRESSURE GENERATORS

because of the emptying of the bellows which contains only the tidal volume. During the rest of the inspiratory time the lungs are held inflated until the riming circuit switches the mechanism to expiration. NEGATIVE PRESSURE

The question of negative pressure in expiration has not been considered in detail but its main effect in the inspiratory phase is that it reduces the peak airway pressure. The consequences of this for any particular type of ventilator may be inferred from the foregoing results. For instance, in a pressure-cycled ventilator the cycling pressure will have to be set that much lower. SOLUTIONS OF THE PROBLEMS

From the foregoing it is evident that the main reason why many adult ventilators fail to perform satisfactorily for infants is that the flow during inspiration is too high and cannot be reduced to the low levels required. This carries with it the attendant dangers of over-inflation, or premature triggering of the cycling mechanism leading to under-inflation. The general solution of these problems lies, on the one hand, either in reducing the inspiratory flow by means of a restriction in series with a ventilator or by modifying the ventilator itself, or, on the other hand, by arranging a device in parallel with the patient which would dispose of the excess flow. Naturally, whichever method is used should interfere as little as possible with the functioning of the ventilator controls. It should also neither cause rebreathing nor interfere with any patienttriggering device which may be part of the ventilator. In the case of pressure generators either or both of these solutions can be applied. However, in the case of flow generators, by definition, the flow cannot be altered by adding external resistance; the second solution, therefore, must be adopted or else the ventilator itself must be modified. REDUCTION OF THE INFLATING FLOW

(a) A resistance in the inspiratory path. This approach was tried for the Barnet ventilator; a screw clamp was placed on the inspiratory tubing and tightened until the inspiratory flow was reduced and the required tidal volume just

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Theory. In a pressure generator which is pressure cycled the performance of the ventilator depends upon the relationship between the generated pressure and the cycling pressure. If the generated pressure is much higher than the cycling pressure the inflating flow is large and will not fall off much as inflation proceeds. The performance of such a ventilator therefore approaches that of a flow generator and the same disadvantages apply. If, on the other hand, the generated pressure is lower and only slightly in excess of the cycling pressure, the ventilator will be as suitable for small subjects as for adults, always assuming that the cycling pressure can be set low enough. In the case of volume-cycled pressure generators adequate functioning occurs so long as the cycling volume can be set to the low value required. Results. Three pressure generators were tested, all of which were time cycled, the Barnet, the Jefferson and the unmodified Pneumotron. The unmodified Pneumotron, as explained above, behaves as a time-cycled pressure generator. The timing and the generated pressure can both be varied over wide ranges and could be adjusted to suit all models. The Jefferson ventilator has a fixed inspiratory time and the rate is adjusted by changing the length of the expiratory phase. When the ventilator was set to 40 and to 30 respirations per minute for the two smaller models, it was found that the I:E ratios were 1.5:1 and 1:1 respectively. However, the generated pressure could be adjusted so that the required tidal volume was just delivered in the fixed inspiratory time. This ventilator was therefore satisfactory apart from the I:E ratio. In the Barnet the inspiratory and expiratory times can be adjusted to the required values, but the generated pressure cannot be reduced below about 40 cm H=O. Therefore the required tidal volume is delivered in a very short time (0.3 sec) in the neonate, but over-inflation does not occur

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DISPOSAL OF EXCESS FLOW

(a) Leak in circuit. Flow generators. If the flow from a flowgenerating ventilator cannot be reduced to the required level, a leak can be introduced into the system and adjusted so as to dispose of the excess flow. It can be shown that the system now behaves like a pressure generator with a low generated pressure which is equal to the pressure in the system when all the flow is passing through the leak. The flow into the patient is high at the beginning of the inspiratory phase, and tails off towards the end. Perhaps fortunately, the absence of cuffs on endotracheal tubes for infants generally leads to a variable amount of leakage around the endotracheal tube during the inspiratory phase. Such a leak operates in the direction of making an adult ventilator perform a little better than it would do otherwise. However, our measurements indicated that the effect is small and unlikely to make the difference between satisfactory and unsatisfactory ventilation. In the case of the Cyclator an adjustable leak was introduced on the patient's side of the inflating valve. This leak was adjusted for each size of model child so that ventilation was satisfactory. The adjustment, though difficult, was

quite practical and in fact the arrangement worked well for several weeks on a child with tetanus. The Blease (a P4) ventilated all the models satisfactorily. A study of its mechanisms shows that it is fundamentally a flow generator in which the flow (coming from a compressor) cannot be adjusted. However, the control labelled "length of inspiration" is in fact an adjustable leak which can be used to dispose of any excess flow. This leak happens to be in the driving (or power) circuit, and not in the patient circuit, but the effect is the same. Pressure generators. If a pressure generator will not ventilate small subjects this is because the generated pressure is too high, and its performance approaches that of a flow generator. A controlled leak would therefore be just as effective here as in a flow generator in reducing the flow rate into the patient's lungs. With both types of ventilator the use of a leak requires a higher flow of fresh gases than the minute volume to be delivered to the child. It is essential therefore that care be taken to see that the supply of fresh gases is such that the proper minute-volume ventilation of the patient is maintained and that the introduction of the leak is not followed by an undesirable diminution in the minute-volume ventilation. (b) Parallel compliance and resistance. An artificial compliance in parallel with the child's lungs can be adjusted so that the two together are equal to the compliance of an adult and will therefore accept an adult tidal volume between them. In order to ensure that the child receives its appropriate fraction of this adult tidal volume in all circumstances, it is essential that a suitable resistance be inserted in series with the artificial compliance to form a complete dummy lung. The product of compliance and resistance (i.e. the time constant) of the dummy lung should be equal to the product of the compliance and resistance of the child's lungs and airway. This will ensure that the inflation of the child's lung and of the dummy lung occurs simultaneously. In these circumstances the ventilator performs as though it were ventilating an adult, although at a much higher rate and with a much shorter

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delivered in the preset inspiratory time. The adjustment of the clamp was so critical and so difficult to get right that we came to the conclusion that this approach was not practical unless a specially designed adjustable resistor was available. The disadvantage of any period in which the pressure is held in the lungs can be avoided without the use of any external device if a more extreme I:E ratio than 1:2 is acceptable: the inspiratory time can be shortened until it is just sufficient for inflation to be completed and the expiratory time is increased to compensate. (b) Modification of the ventilator. The Cyclator was modified by varying the pressure of the driving gas between 60 lb./sq.in. and 36 lb./sq.in. The inspiratory flow rate could then be adjusted to the desired levels, but it was found in the case of the neonate that when the flow was right the cycling pressure could not be set low enough.

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inspiratory period. The cycling pressure is lower many patient-triggering devices. When the infant and any pressure-cycling mechanism must takes a small breath this is readily drawn from operate at a low enough pressure. the dummy lung without producing any pressure, A simple compact form of dummy lung which volume, or flow signal within the ventilator. we constructed for our experiments and clinical This difficulty could be overcome by inserting a cases, consisted of a 5-litre reservoir bag stuffed flow-sensitive trigger mechanism between the firmly with foam plastic chippings, with an orifice infant and the dummy lung. plate in the neck of the bag. The elasticity of the AUTOMATIC VENTILATORS SPECIALLY CONbag gave a compliance of 60 ml/cm H2O. The STRUCTED FOR INFANTS AND CHILDREN chippings held the bag slightly distended so that this compliance was operative for even the first An example is the Pneumotron which has been part of each tidal volume (Mapleson, 1954). discussed above. This ventilator worked satisThe orifice of 0.9 cm diameter gave a resistance factorily on all three models. of 4 cm H 2 O/(l./sec). The time constant of the Mention must be made of the Starling animal dummy lung was therefore 0.24 sec, while that respiration pump which was designed to ventilate of the intubated neonate model was 0.12 sec, animal lungs but has recently been used to ventithe 1-year-old 0.19 sec, and the 8-year-old 0.18 late human infants (Monro and Scurr, 1961) whose sec. The time constants of the dummy and model lungs are of the same order of size. It consists lungs were sufficiently close for practical pur- of a piston pump with mechanically operated poses. valves. It is therefore a flow generator in which The dummy lung is connected into the circuit the flow is not constant but approximately between the child and any inflating valve so that sinusoidal. The rate and tidal volume can be expiration from the child and "expiration" from adjusted as desired but the I:E ratio is fixed at the dummy lung is voided through a common 1:1.3. The ventilator worked satisfactorily on path. If the dummy lung were to be connected all three models. incorrectly on to the ventilator side of the inCONCLUSION flating valve, then, during expiration, pressure in the dummy lung may hold the inflating valve in It is clear that the proper ventilation of a small the inspiratory position thereby preventing the child does involve problems when the use of child from expiring. If the lowest cycling pressure methods and equipment normally intended for of the ventilator is too high, some resistance can ventilating adults is contemplated. Naturally the be inserted between the ventilator and the com- best solution would be the construction of bined child and dummy lung, in order to drop automatic ventilators in which the values of the the excess pressure. various parameters approximated to those of When a dummy lung was connected into the small subjects. In fact the choice of ventilators Cyclator circuit, all three child models were suitable for children is extremely small, and even ventilated satisfactorily, except that in the neonate of these, some will need modification if used for the minimum possible cycling pressure of the anaesthesia. Cyclator led to slight over-inflation. Changing The likely problem, therefore, confronting the from one Beaver valve to another, introduced clinical anaesthetist is that of adapting an adult just enough additional resistance to cure this ventilator to a child. Our studies have impressed defect. We have used a dummy lung with the us with the primary need for a clear understandCyclator in clinical practice and obtained satis- ing of the physical functioning of the particular factory ventilation. ventilator in question. Only then can the matter In principal the dummy lung should provide a be taken further. No single compensatory device general solution for all adult ventilators since in is universally applicable, though of the several effect it converts the infant into an adult, instead described here the dummy lung seems most of seeking to convert an adult ventilator into an promising. There are, however, serious practical infant ventilator. However, one limitation is that disadvantages of a bulky object near to the child's the dummy lung interferes with the operation of head and the still present need for the ventilator

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ACKNOWLEDGMENT

We record thanks to Mr. E. K. Hillard, LJ.B.S.T., Senior Technician in our Department, for technical assistance of the highest order.

Mapleson, W. W. (1958). Theoretical considerations of the effects of rebreathing in two semi-closed anaesthetic systems. Brit. mid. Bull., 14, 1, 64. (1959), in Mushin, W. W., Rendell-Baker, L., and Thompson, P. W., Automatic Ventilation of the Lungs, p. 42. Oxford: Blackwell. Monro, J. A., and Scurr, C F. (1961). The Starling pump as a ventilator for infants and children. Anaesthesia, 16, 151. Murphy, D. P., and Thorpe, E. S. (1931). Breathing measurements in normal newborn infants. /. din. Invest., 10, 545. Mushin, W. W., Rendell-Baker, L., and Thompson, P. W. (1959). Automatic Ventilation of the Lungs. Oxford: Blackwell. Richards, C. C , and Bachman, L. (1961). Lung and chest wall compliance of apneic paralyzed infants. J. din. invest., 40, 273. Smith, R. M. (1959). Anesthesia for Infants and Children, p. 136. St. Louis: C V. Mosby. Scientific Tables (1956), p. 256. Basle: Geigy. Strang, L. B. (1961). Alveolar gas and anatomical dead space measurements in normal infants. Clin. Sci., 21, 107. Waters, D. J., and Mapleson, W. W. (1961). Rebreathing during controlled respiration with various semi-closed anaesthetic systems. Brit. J. Anaesth., 33, 374.

REFERENCES

SOMMAIRE

Boutourline-Young, H. J., and Smith, C. A. (1950). Respiration of full-term and of premature infants. Amer. J. Dis. Child., »0, 753. Cook, C. D., Cherry, R. B., O'Brien, D., Karlberg, P., and Smith, C A. (1955). Studies of respiratory physiology in the newborn infant. I: Observations on normal premature and full-term infants. /. clin. Invest., 34, 975. Sutherland, J. M., Segal, S., Cherry, R. B., Mead, J., Mcllroy, M. B., and Smith, C. A. (1957). Studies of respiratory physiology in the newborn infant. Ill: Measurements of the mechanics of respiration. /. din. Invest., 36, 440. Deming, J., and Hanner, J. P. (1936). Respiration in infancy. Amer. J. Dis. Child., 51, 823. Ferris, B. G., Opie, L., and Mead, J. (1960). Partitioning of respiratory resistance in man. Fed. Proc, 19, 377. Hellieson, P. J., Cook, C. p . , Friedlander, L., and Agathon, S. (1958). Studies of respiratory physiology in children. I: Mechanics of respiration and lung volumes in 85 normal children 5 to 17 years of age. Pediatrics, 22, 80. Hull, J. E. (1955). The physiology of respiration in infants and young children. Proc. roy. Soc. Med., 48, 761. Hyatt, R. E., and Wilcox, R. E. (1960). Partitioning of pulmonary flow resistance into upper and lower airway components. Fed. Proc, 19, 237. (1961). Extrathoracic airway resistance in man. ]. appl. Physiol., 16, 326. Mapleson, W. W. (1954a). Volume pressure characteristics of the "one gallon" reservoir bag. Brit. J. Anaesth., 26, 11. (1954b). The elimination of rebreathing in various semi-closed anaesthetic systems. Brit. J, Anaesth., 26, 323.

Des enfants en bas age ainsi que les noyeaux-ne's presentent des problemes sp^ciaux lorsqu'il devient ne'cessaire de les soumettre a la "ventilation automatique". Ces problemes de'coulent du petit volume entre maximum et minimum, de la rapidity de la respiration, du taux re'duit du flux de l'inspiration et de rinsuffisance de la pression cyclique. En ge'ne'ral, les ventilateurs automatiques faits pour les adultes ne fonctionnent pas de facon satisfaisante sur des sujets menus. Ou bien Ie flux est trop grand, l'alternance respiratoire trop lente ou bien le volume entre maximum et minimum excessif ou bien la pression cyclique trop forte. Les auteurs ont e'tudie' des iddes thdoriques en appliquant divers "ventilateurs" a des modeles de poumon correspondant aux diffe'rents ages des enfants. Us obtinrent des r6sultats the'oriques et expe'rimentaux, dont la justesse fut ensuite confirmed par des tests cliniques. ZUSAMMENFASSUNG

Kleine Kinder und Neugeborene stellen spezielle Probleme dar, wenn sie kunstliche Atmung brauchen. Dabei handelt es sich urn die geringen Atemvolumina, die schnellere Atemfrequenz, kleine inspiratorische Stromungsquoten und Drucke von geringer Nachgiebigkeit. Ganz allgemein arbeiten Gerate zur kiinstlichen Atmung fur Erwachsene bei Kleinen nicht recht. Entweder ist die Durchstromung zu gross, das Ventilationsvolumen zu gross oder der alternierende Druck zu hoch. Theoretische Konzpetionen wurden durch Verwendung verschiedener Ventilatoren bei Lungenmodellen erprobt, die verschiedene Altersstufen"der Kinder darstellten. Klinische Untersuchungen bestatigten die theoretischen und experimentellen Ergebnisse.

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to function at high respiratory rates and at low cycling pressures. The insertion of a resistance or of a leak in the ventilator flow pathway may be suitable alternatives, but theory and our own experimental and clinical experience show that their adjustment is critical, and it is unlikely that simply constructed devices will be more than partially satisfactory. In any case the ultimate evidence of satisfactory ventilation must be derived from volume measurements of expiration. At the moment this cannot be done readily with the small ventilation meters on the market, like the Wright respirometer. There is clearly a need not only for specially designed ventilators but also for ventilation measuring equipment suitable for children.