Br.J. Anaestk. (1979), 51, 375

SMALL ENDOTRACHEAL TUBES Ventilator and intratracheal pressures during controlled ventilation O. STENQVIST, H.

SONANDER AND K. NILSSON SUMMARY

The use of small endotracheal tubes reduces the trauma of intubation. Ventilator and tracheal pressures were measured during controlled ventilation with various tube dimensions and ventilation volumes. Ventilation with large volumes using small tracheal tubes results in high ventilator pressures. However, tracheal pressures are only marginally greater than those obtained with larger tubes. Small endotracheal tubes and high ventilation volumes result in a positive tracheal pressure at the end of expiration. The measured end-expiratory pressures are within the limits which might be used therapeutically (in PEEP). The force required to reshape endotracheal tubes of various dimensions to an "anatomical" shape was related to the tube dimensions; the beneficial effects of preformed, "anatomically shaped" endotracheal tubes can be achieved by using small tubes of standard design.

Endotracheal intubation can damage the upper airway at different anatomical levels, for example, the dorsal wall of the pharynx, the upper part of the larynx, the vocal cords, the subglottic region and the trachea itself (Bergstrom, 1962; Bryce, Briant and Pearson, 1968; Harrison and Tonkin, 1968; Lindholm, 1969; Hilding, 1971). Damage to the trachea, as caused by an inflated cuff, is a recognized problem and several techniques and designs of cuff have been devised to decrease the pressure exerted on the tracheal mucosa or the time for which pressure is applied (Carroll, McGinnis and Grenvik, 1974). Damage to the pharyngeal wall, the laryngeal entrance and the vocal cords is partly an effect of pressure and partly a result of the tube movement with erosion of mucosal surfaces. Several authors have emphasized that tubes of smaller diameter cause less laryngeal damage (Bryce, Briant and Pearson, 1968; Harrison and Tonkin, 1968; Lindholm, 1969; Hilding, 1971; McGinnis et al., 1971; Goldberg and Pearson, 1972; Mathias and Wedley, 1974). Such tubes have two advantages—the intubation procedure is easier, and these tubes may require a smaller reshaping force to adapt them to the anatomical shape of the patient's airway. O. STENQVIST,* M.D.; H. SONANDER, M.D.; K. NILSSON,

M.D., PH.D. ; Department of Anaesthesiology and Intensive Care, Sahlgrens Hospital, Fack, S-413 45 Gothenburg, Sweden. * Present address: Lidkdpings Lasarett, 53100 LidkOping, Sweden. Correspondence to Dr K. Nilsson. 0007-0912/79/040375-07 $01.00

The object of the present study was to determine if tubes of narrow bore could be used during controlled ventilation without there being marked resistance to inspiration or expiration. Also, we measured the forces necessary to reshape standard endotracheal tubes of various dimensions to conform to the anatomy of the upper airway. METHODS

Ten patients with no known airway disease, undergoing minor e.n.t. operations, were studied before surgery. Informed consent was obtained from each patient. The mean age was 41 yr. Anaesthesia was induced with fentanyl 0.2 mg and thiopentone sodium 250 mg and maintained with nitrous oxide in oxygen (3 : 1). Ventilation was controlled via a non-rebreathing system and neuromuscular blockade was produced by the administration of pancuronium bromide 0.1 mg kg- 1 body weight. Ventilator and intratracheal pressures

Each patient was intubated sequentially with Shiley low-pressure cuff endotracheal tubes no. 5, 6, 7 and 8 (i.d. 5, 6, 7 and 8 mm, length 235, 278, 290 and 325 mm respectively). All tubes were used uncut and with the enclosed connector. A polyethylene catheter (i.d. 1.2 mm) was applied to the outside of the endotracheal tube with its distal end in the trachea below the endotracheal tube and was used to measure the intratracheal pressure (transducer EMT 35, amplifier EMT 31, Siemens-Elema). © Macmillan Journals Ltd 1979

BRITISH JOURNAL OF ANAESTHESIA

376 Readings from the catheter were checked against a catheter-tip manometer (Millar PC 450). There was no damping and the time delay was found to be 0.10.15 s. In the experimental set-up a decrease in pressure of, at the most, 3 cm H2O was measured from the ventilator manometer to the ventilator end of the endotracheal tube. With each tube used, each patient was ventilated with a Servo-Ventilator 900 (Siemens-Elema) set at 8, 9, 10, 11, 12 and 13 litre min"1. Other settings on the ventilator were: frequency 20 b.p.m., inspiratory time 25%, inspiratory pause 5%, square-wave inspiratory flow. The pressure and flow in the ventilator were measured continuously by the inbuilt pneumotachograph and plotted, with the intratracheal pressures, on an ink jet recorder (Mingograph 34, Siemens-Elema). The pressure transducers were calibrated against a mercury manometer, atmospheric pressure being taken as zero. Examples of the continuous curves of flow and pressure are shown in figure 1.

fl 11

FLOW EXP

n n ^ A

MVP

fl--PVP

VENTILATOR PRESSURE ~ INTRATRACHEAL PRESSURE

_

Sakalad, 1949; Tashayod, 1967; Lindholm, 1969, 1973). Experimental studies of the force required to reshape various endotracheal tubes to an adequate form for the pharyngeal and laryngeal cavity have been presented previously (Lindholm, 1969; Lindholm and Carroll, 1975; Neuman, 1975). The force required to reshape endotracheal tubes to the contour of a Portex preformed endotracheal tube no. 8 was measured. The tubes were attached to a rack at the "oral" and "tracheal" ends and a peg used to simulate the base of the tongue. A load was attached to the tubes using a 20-mm broad strap with its centre 30 mm above the upper cuff margin (fig. 2). Loading was performed by filling a plastic bag with water until the curvature of the tube conformed to the model Portex preformed no. 8, whereafter the water in the bag was measured. Five measurements were made for each tube and five tubes of each brand and dimension were assessed.

s / \ . "ETP

A2C PERFUSING

LOAD

WATER 37°C

t

3s FIG. 1. Continuous display of flow and pressures during artificial ventilation, tube no. 9, ventilation volume 9 litre min" 1 . MIP = maximal intratracheal pressure, MVP = maximal ventilator pressure, PVP = pause ventilator pressure, EETP = end-expiratory tracheal pressure.

From these recordings pressures were measured at four points: (1) maximum ventilator pressure (MVP); (2) maximum intratracheal pressure (MIP); (3) pause ventilator pressure (PVP), representing the pressure in the ventilator during the inspiratory pause; (4) end-expiratory tracheal pressure (EETP), representing the intratracheal pressure just before the next inspiration. Reshaping of endotracheal tubes

Anatomically shaped endotracheal tubes have been devised by several workers (Dwyer, Kronenberg and

50 MM

FIG. 2. Experimental design used to evaluate the force necessary to reshape standard tubes to the contour of a preformed tube (Portex). Dotted lines indicate the standard tube, continuous lines the same tube with applied force resulting in the desired shape. Black circles indicate fixed points in the set up. The load was always applied 30 mm "oral" to the upper margin of the cuff.

Portex Blue Line Murphy and Shiley low-pressure cuff Murphy no. 5, 6, 7, 8 and 9 were assessed. All measurements were made at a temperature of 36 ± 0.5 °C achieved by perfusing the tubes with water at 37±0.1°C. RESULTS

Ventilator and intratracheal pressures

As the volume of ventilation was increased there were increases in MVP, the increase being more pronounced the smaller the tube used (fig. 3; table I). For example, MVP at a volume of ventilation of 13 litre min was 24 cm H 2 O with tube no. 8 and 42 cm H2O with tube no. 6. There was a corresponding

377

SMALL ENDOTRACHEAL TUBES but small increase in MIP as a function of the decrease in the diameter of the tube. MIP at a ventilation volume of 13 litre min" 1 was 16 cm H 2 O with tube no. 8 and 18 cm H 2 O with tube no. 6. TABLE I. Pressures in the ventilator and trachea (cm H2O) during artificial ventilation as a function of endotracheal tube size (i.d. mm) and ventilation volumes (litre min'1), mean values and SD from 10 patients Ventilation volumes (litre min"1) Tube size 5

6

7

3

MVP PVP MIP EETP MVP PVP MIP EETP MVP PVP MIP EETP MVP PVP MIP EETP

8

11

10

9

35 + 5 39±4 10 + 2 11 ±2 11 ±2 13 + 2 2±1 1±1 23 + 3 26±3 10 + 2 11+2 11+2 13 + 2 0 + 0 0±0 18±3 20 + 3 10 + 2 11+2 11 ±2 12 + 2

46 + 4 52±4 13 + 2 14 + 2 14 + 2 16±2 4+2 3±1 30 + 2 35 + 3 12 + 2 14 + 2 14 + 2 15 + 2

12

13

58 + 4

61±4

16±2

17 + 2 19 + 2

18 + 2 6±2 38 + 3 15 + 2 17 + 2 2±1 2+1 1±1 22±3 25 ±4 27 + 4 12 + 2 13 + 2 14 + 2 13 + 2 14±3 15 + 3 0±0 0±0 0 + 0 0±0 1±1 14 + 2 17 + 3 19 + 3 21 ±4 23 + 4 9 + 2 10 + 2 12 + 2 13 + 2 14 + 2 11+3 12 + 2 13±3 14 + 3 15 + 3 0±0 0±0 0+0 0+0 0+0

6±2 42 + 3 16 + 3 18 + 2 3+1 30 + 5 15 + 2 16 + 3 1+1 24 + 4 15 + 3 16 + 3 0+0

There was good agreement between the pressure developed in the ventilator during the inspiratory pause (PVP) and MIP. However, measured MIP was invariably 2-3 cm H 2 O greater than PVP (fig. 3 and table I). The end-expiratory tracheal pressure (EETP) was zero cm H 2 O for all ventilation volumes with tube no. 8 and increased at volumes of ventilation greater than 11 litre min" 1 with tube no. 7 and at 9 litre min" 1 with tube no. 6. The highest EETP measured for tube no. 6 was 3 cm H 2 O at 13 litre min- 1 (fig. 3 and table I). The difference between the maximal ventilator pressure and the maximal tracheal pressure (MVP — MIP) is an estimate of the pressure decrease across the length of the tube and is presented, as a function of the dimension of the tube and the volume of ventilation, in figure 4. The pressure decrease for a ventilation volume of 13 litre min" 1 with tube no. 8 was about 8 cm H 2 O and, for tube no. 6, 24 cm H 2 O. The corresponding pressure decrease for a volume of 8 litre min" 1 was 3 cm H 2 O and 12 cm H 2 O respectively. Since the duration of inspiration was 25% of the cycle and the inspiratory flow was delivered as a square wave, the maximum flow was four times the set volume of ventilation as indicated within brackets MVP - M A X VENTILATOR PRESSURE MIP MAX INTRATRACHEAL MIK PRESSURE PVP - PAUSE VENTILATOR PRESSURE EETP- END EXPIRATORY TRACHEAL PRESSURE

,MVP

60-

50 MVP I

E 30 MVP

20

10EETP

10 TUBE 5

13

8

10 TUBE 6

13

8

10 TUBE 7

13

8

10

13

TUBE 8

VENTILATION VOLUMES ( L I T R E M I N ' 1 ) TUBE DIMENSIONS I . D . ( M M )

FIG. 3. Pressures in the ventilator and trachea during artificial ventilation as a function of endotracheal tube size and ventilation volumes, mean values from 10 patients for each tube dimension.

BRITISH JOURNAL OF ANAESTHESIA

378

IPORTEX BLUE LINE

TUBE DIMENSION

°~50

•SHILEY HIGH VOLUME

5

O

500

I 40 o. >

f 400 2

30

|

20'

I I

300 •

U_

o z

UJ

I

200 -

CO UJ

J

a:

a.

S

100 5

10

15

(20)

WO)

(60)

VENTILATION VOLUME (LITRE MIIST1)

FIG. 4. Pressure decrease over endotracheal tubes of various sizes as a function of ventilation volumes. Values within brackets represent maximal flow during inspiration.

in figure 4. These maximum flow values should be used for comparison with measurements of the decreases in pressure over endotracheal tubes using a physical model with continuous flow (Orkin, Siegel and Rovenstine, 1954; Smith, 1961; Cave and Fletcher, 1968; Nunn, 1977). Reshaping of endotracheal tubes

The force required to reshape the tube to the chosen "anatomical" design was greater for the larger tubes (fig. 5). It was found also that brands of tube studied differed in that the increase appeared to be "linear" for the Shiley tubes, whereas the increase was more "exponential" for the large Portex tubes. The force required to reshape the tubes was, for both makes of tube, more than twice as great for a no. 8 than for a no. 6 tube. DISCUSSION

Pressure measurements

Whereas large volumes of ventilation result obviously in high MVP, high intratracheal pressures were not observed (MIP, fig. 3). The values recorded in this study were in agreement with information available from a similar investigation (Johansson and Lofstrom, 1975). High MVP as a result of high resistance in the endotracheal tube would, of course, influence artificial ventilation with any pressure-

5

6

7

8

9

TUBE DIMENSION

FIG. 5. Reshaping force for endotracheal tubes from two manufacturers as a function of tube dimension. Mean values ± SD fromfivetubes of each size. (1 pond = 0.0098 N.)

cycled ventilator (pressure generator), since the ventilator will not necessarily overcome the high resistance. The performance of the flow and timecycled ventilator (flow generator) used for the present study was not influenced. The small difference between MIP and PVP, MIP being consistently 2-3 cm H2O greater than PVP, may be explained by the inertia of the combined patient-ventilator system. An inspiratory overshoot occurred after closure of the inspiratory valve on the ventilator as an effect of the inertia of the patient (thoracic wall and diaphragm with connected abdominal structures). This overshoot was followed by a small expiratory flow during the inspiratory pause as an effect of the elasticity of the lungs and thoracic walls. A rough calculation indicates that a displacement of an expiratory volume of less than 15 ml during the inspiratory pause would result in the measured MIP —PVP difference. The increase in end-expiratory tracheal pressures (EETP, fig. 3) recorded with the narrow bore tubes was reached always within 10 respiratory cycles. During the initial respiratory cycles the resistance of the small tubes resulted in incomplete expiration during the time available. After a few cycles with ventilation at successively increasing FRC the

SMALL ENDOTRACHEAL TUBES elasticity of the lungs and the thoracic wall was sufficient to allow expiration of the inflated volume and no further increase in EETP occurred. All pressure recordings were performed with standard length tubes. In the adult e.n.t. patient the no. 5 tube was about 3 cm too short whereas the no. 8 tube was about 4 cm too long for convenient handling. This influences the comparison of the pressure decrease across tubes of various dimensions (fig. 4). The calculation of the pressure decrease presupposes that MVP and MIP occur simultaneously. With a ventilation volume of 13 litre min" 1 through a no. 5 tube MIP occurs about 0.4 s after MVP. The real driving pressure (pressure decrease) over the tube can be estimated, from the original recordings, to be 5 cm H 2 O greater than indicated by MVP — MIP in such a situation. This error is compensated for partially by the pressure decrease of about 3 cm H 2 O in the ventilator tubing. The resistance of endotracheal tubes (pressure decrease/flow) increased slightly with an increasing flow rate. This can be expected in a system in which there may be a transition between laminar and turbulent flow. However, the increase in tube resistance over the flow range studied was moderate and linear, for example with tube no. 6 from 1.4 cm H2O litre"1 min at 8 litre min" 1 ventilation volume to 1.8 cm H 2 O litre" 1 min at 13 litre min- 1 ventilation volume. Plotting the measured resistance of the tube as a function of the volume of ventilation indicated that the resistance throughout the flow range studied was mainly a resistance to laminar flow. Our results did not indicate a change to turbulent flow at the flow rates examined, nor did they indicate a transformation to turbulent flow even in small tubes at flow rates representing those in normal clinical use. A "critical" diameter of tube, as suggested by Nunn (1977), for various volumes of ventilation cannot be deduced from our results. The use of small tubes involves the risk of occlusion and kinking. Our impression is that the use of endotracheal tube no. 6 during routine anaesthesia for adult patients does not result in an increased frequency of sudden or successive occlusion of the tube. Adequate humidification and a high flow rate through the narrow lumen of the tube seem to have abolished the problem. Reshaping force

In our opinion the ideal endotracheal tube must be a compromise between adequate stiffness, allowing

379 easy intubation, and a patent lumen and sufficient adaptability to the anatomy of the airway. The mechanical properties of modern materials used in endotracheal tubes are temperature-dependent. The reshaping force was 15-60% greater at a material temperature of 26 °C than at 36 °C and larger tubes were more temperature-dependent than the smaller ones. On the other hand, the reshaping force at 36 °C did not decrease after repeated "preformations" at 36 °C. The temperature dependence is an advantage as some of the stiffness necessary for the intubation is lost when the tube is in the airway. Lindholm and Carroll (1975) presented reshaping values between 30 and 1000 ponds for tubes with 8 mm i.d.; Neuman (1975) gave values between 230 and 480 ponds for tubes with 10.2-11.5 mm o.d. The same authors transformed the measured reshaping force to a pressure by estimating a certain area of contact. It could be argued that a wider endotracheal tube offers a larger contact area resulting in a smaller pressure for the same reshaping force. If, arbitrarily, 20 mm of the tube length and half of the tube circumference is supposed to be in contact with the dorsal parts of the larynx, the resulting pressure load for various tubes can be calculated from our reshaping force (fig. 5). Approximate values would be 30 mm Hg for tube no. 6, 45 mm Hg for no. 7, 65 mm Hg for no. 8 and 90 mm Hg for no. 9. CONCLUSION

Simultaneous measurements of the pressures in the trachea and the ventilator have shown high pressures at the oral end of the tubes at high volumes of ventilation when small endotracheal tubes were used. However, there were no corresponding increases of the pressure in the trachea during the inspiratory phase. Although the small endotracheal tubes resulted in a positive tracheal pressure at the end of expiration at high ventilation volumes, the measured endexpiratory pressure was, even in extreme situations, well within the limits of clinically applicable positive end-expiratory pressure. Endotracheal tubes with inner diameters of 6 or 7 mm can be used safely together with artificial ventilation by flow cycled ventilators (flow generators) during anaesthesia. Anatomically shaped endotracheal tubes have been supposed to decrease the damage to the mucosal surfaces. The same beneficial effect can be achieved by using, a small endotracheal tube, requiring a smaller reshaping force to conform to the anatomy of the airway. However, small endotracheal tubes require symmetrical high-volume, low-pressure cuffs

BRITISH JOURNAL OF ANAESTHESIA

380

and the length of the smaller tubes should be increased. The direct measurement of intratracheal pressure is valuable in all anaesthetic procedures, but especially when using small endotracheal tubes. An additional canal in the wall of the endotracheal tube, reaching the distal end, should be introduced in the manufacturing process.

Nunn, J. F. (1977). Applied Respiratory Physiology, 2nd edn, p. 109. London: Butterworths. Orkin, L. R., Siegel, M., and Rovenstine, E. A. (1954). Resistance to breathing apparatus used in anesthesia. Anesth. Analg. {Cleve.), 33, 217. Smith, W. D. A. (1961). The effects of external resistance to respiration. Br. J. Anaesth., 33, 610. Tashayod, M. (1967). A new double-curved endotracheal tube for nasal intubation. Br. J. Anaesth., 39, 823.

ACKNOWLEDGEMENTS

This work was supported by grants, hereby gratefully acknowledged, from the Faculty of Medicine, University of Gothenburg and Goteborgs L&karesallskap.

PETITS TUBES ENDOTRACHEAUX Pressions ventilateur et endotrache'ales pendant la ventilation controlee RESUME

REFERENCES

L'usage de petits tubes endotracheaux diminue le trauBergstrSm, J. (1962). Laryngologic aspects of the treatment matisme de l'intubation. La pression trach£ale et celle du of acute barbiturate poisoning. Acta Otolaryngol. Suppl., ventilateur ont ete mesurees pendant la ventilation controlee en utilisant diverses dimensions de tubes et divers 173. Bryce, D. P., Briant, T. D. R., and Pearson, F. G. (1968). volumes de ventilation. La ventilation a l'aide de grands Laryngeal and tracheal complications of intubation. Ann. volumes par l'intermediaire de petits tubes endotracheaux entraine de fortes pressions du ventilateur. Cependant, les Owl. Rhinol. Laryngol., 77, 442. Carroll, R. G., McGinnis, G. E., and Grenvik, A. (1974). pressions tracheales sont a peine plus fortes que celles Performance characteristics of tracheal cuffs. Int. obtenues avec des tubes plus larges. De petits tubes endotracheaux et de forts volumes de ventilation entrainent Anesthesiol. Clin., 12, 111. Cave, P., and Fletcher, G. (1968). Resistance of nasotracheal une pression tracheale positive a la fin de l'expiration. Les pressions de fin d'expiration mesurees sont dans les limites tubes used in infants. Anaesthesia, 29, 588. que Ton peut utiliser therapeutiquement (PEEP). La force Dwyer, C. S., Kronenberg, S., and Sakalad, M. (1949). The requise pour modifier les tubes endotracheaux des diverses endotracheal tube: a consideration of its traumatic effect dimensions et leur dormer une forme "anatomique" a et6 with a suggestion for the modification thereof. Anes- liee aux dimensions des tubes; les effets benefiques des thesiology, 10, 714. tubes endotracheaux preformes "ayant une forme anaGoldberg, M., and Pearson, F. G. (1972). Pathogenesis of tomique" peuvent etre obtenus en utilisant de petits tubes tracheal stenosis following tracheostomy with a cuffed de conception standard. tube. Thorax, 27, 678. Harrison, G. A., and Tonkin, J. P. (1968). Prolonged (therapeutic) endotracheal intubation. Br. J. Anaesth., 40, 241. KLEINE ENDOTRACHEALROHREN Hilding, A. C. (1971). Laryngotracheal damage during Ventilator und intratracheale Drucke intratracheal anesthesia. Ann. Otol. Rhinol. Laryngol., bei kontrollierter Beliiftung 80, 565. ZUSAMMENFASSUNG Johansson, H., and Lofstrom, J. B. (1975). Effects on breathing mechanics and gas exchange of different Die Verwendung kleiner endotrachealer Rohren verringert inspiratory gas flow patterns during anaesthesia. Acta das Trauma der Intubation. Ventilator und trachealer Druck wurden gemessen bei kontrollierter Beliiftung mit Anaesthesiol. Scand., 19, 8. Lindholm, C.-E. (1969). Prolonged endotracheal intubation. verschiedenen Rohrdimensionen und Belufrungsvolumen. Grossvolumige Ventilation mit kleinen Trachealrohren Acta Anaesthesiol. Scand., Suppl. 33. fuhrte zu hoheren Ventilatordrucken. Dagegen sind (1973). Experience, with a new orotracheal tube. Acta die trachealen Drucke nur geringfiigig hoher als diese Otolaryngol., 75, 389. be grosseren Rohren. Kleine Endotrachealrohren und Carroll, R. G. (1975). Evaluation of tube deformation hohe Belufrungsvolumen fiihren zu einem positiven pressure in vitro. Crit. Care Med., 3, 196. Trachealdruck am Ende der Ausatmung. Die gemessenen McGinnis, G. E., Shively, J. G., Patterson, R. L., and Endausarmungsdrucke liegen innerhalb der therapeutisch Magovern, G. L. (1971). An engineering analysis of verwendbaren Grenzen fur PEEP. Die erforderliche Kraft intratracheal tube cuffs. Anesth. Analg. {Cleve.), 50, 557. zur Umformung endotrachealer R6hren verschiedener Mathias, D. B., and Wedley, J. R. (1974). The effects of Abmessungen zu einer "anatomischen" Form stand im cuffed endotracheal tubes on the tracheal wall. Br. J. Verhaltnis zu den Rohrenabmessungen; die vorteilhaften Wirkungen von vorgeformten, "anatomisch geformten" Anaesth., 46, 849. endotrachealen Rohren kann erzielt werden, indem kleine Neuman, O. G. (1975). Schadigungsparameter an der Rohren der Standardversion verwendet werden. Ringknorpelplarte bei Langzeitintubation. Prakt. Andsth., 10, 135.

SMALL ENDOTRACHEAL TUBES PEQUENOS TUBOS ENDOTRAQUEALES Presiones del ventilador e intratraqueal durance ventilacidn controlada SUMARIO

El empleo de tubos endotraqueales pequenos reduce el traumatismo de la intubation. Se midieron las presiones del ventilador y traqueal durante la ventilacion controlada con diversas dimensiones de tubo y volumenes de ventilacion. La ventilacion con grandes volumenes utilizando tubos traqueales pequefios da por resultado elevadas presiones de ventilador. Sin embargo, las presiones traqueales son solo

381 marginalmente superiores a las que se obtienen con los tubos de mayor tamano. Los tubos endotraqueales pequeflos dan como resultado una presion traqueal positive al final de la expiration y elevados volumenes de ventilacion. Las presiones medidas al final expiratorio quedan dentro de los limites en que pueden ser empleadas terapeuticamente en PEEP. La fuerza necesaria para dar forma "anatomica" a los tubos endotraqueales de diveras dimensiones, se relaciono con la dimensi6n de los mismos; pueden obtenerse los efectos beneficos de los tubos endotraqueales preformados "anatomicamente formados", empleando tubos pequenos de diserlo corriente.