Recent developments in tracheal cuffs

Resuscitation

( I 973))

2,

77-82

Recent developments in tracheal cuffs D. E. CROSS Portex Ltd., Hythe, Kent, U.K.

Summary A model trachea and isolated cadaver tracheae were used to study floppy and standard cuff tracheostomy tubes. With floppy cuffs a relatively high intracuff pressure, compared with capillary blood pressure, was needed to give a complete seal. The mobility of this cuff and its inability to centralize the tube in the trachea could be cause for concern.

The procedure of tracheostomy is well established, and dates back to at least the first century B.C. In 1540, Vesalius demonstrated the use on an animal of intermittent positive-pressure respiration through a tracheostomy by using a bellows. However, it was not until 1869 that the first tracheostomy tube possessing an inflatable cuff was introduced (Trendelenburg, I 87 I). This eventually made possible the use of intermittent positive-pressure ventilation, which gained widespread recognition after the poliomyelitis epidemics of the 1950s. It was not long before some of the later complications of tracheostomy became recognized as a result of the longer-term survival of patients with tracheostomies. A proportion of the complications could be associated directly with the presence of a cuff on the tracheostomy tube. A selfinflating cuff was described as early as 1943, in an attempt to overcome similar problems associated with endotracheal tubes (Macintosh, 1943). Although this was not adopted generally, much work has been carried out since then to design a means whereby a seal can be made between the tracheostomy tube and trachea that will cause the minimum amount of trauma to the tracheal mucosa. The requirements for such a cuff may conflict; i.e. so as to cause minimal damage to the tracheal mucosa, the pressure should ideally be zero. However, a certain pressure is required to form a seal to prevent air leaking past the cuff in one direction and also to prevent secretions passing the cuff in the other. If such a seal is to be made, then the pressure exerted on the trachea by the cuff must be at least equal to the inilation pressure provided by the ventilator. An idealized section of the tracheal mucosa through which a blood capillary passes is shown (Fig. I). The blood pressure is normally 32 mm mercury at the artierolar end and 18 mm mercury at the venular end of the capillary, equivalent to 43.5 and 24.5 cm water respectively. It will be seen that these pressures are very close to those commonly used in ventilating tracheotomized patients. Indeed, it is likely that in some patients the inflation pressure may exceed the capillary blood pressure, and so the blood flow will be arrested whilst 77

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arteridar praaaure 435cm Hz0 (32 mm Hg)

\

trachei wall

’

capillary I

vender pressure 245 cm Hz0 (18mm kg)

I

tracheal wall pmssure (30 cm H,O)

Fig. I. Diagrammatic representation of a blood capillary in the tracheal wall showing typical pressum.

these higher pressures are applied. It will be remembered that the assumption was made that the tracheal wall pressure was equal to the inflation pressure. However, if higher pressures are used within the cuff, for instance, by injecting slightly more air than is necessary, then the tracheal wall pressure will be increased and the blood flow to the tracheal mucosa will probably be obliterated. McGinnis, Shively, Patterson & Magovern (1971) have demonstrated that very small additions of air to a standard cuff can cause alarming increases in tracheal wall pressure. To overcome excessive tracheal wall pressures, Geffin & Pontoppidan (1969) suggested that a significant improvement could be brought about by pre-stretching the standard inflatable cuff as fitted to tracheostomy tubes. Since then, several authors have indicated the value of larger floppy cuffs, and there are now a variety of tracheostomy tubes commercially available that have these larger cuffs (Grillo, Cooper & Geffin, 1971; Andrews & Pearson, 1971; Cooper & Grillo, 1972). A number of theories have been advanced to account for the improved results with these larger cuffs. It has been suggested that a standard cuff distorts the trachea, whereas a low-pressure floppy cuff conforms to the D cross-section of the trachea (Geffin & Pontoppidan, rg6g ; Cooper & Grillo, 1972). It has also been stated that the danger of over-inflating a standard cuff is much greater than for a larger volume floppy cuff, as mentioned earlier. Although both these factors could contribute to the tracheal damage seen clinically, they did not seem to be a complete explanation. To investigate this, a large floppy cuff was placed in the trachea, to study these various factors in isolation. Experimental

studies

For any inflatable cuff system, the following relationship may be used as a convenient generalization : pic=&+pf(D,S)

where Pie = intracuff pressure, PtW= tracheal wall pressure and Pi (DJ) = a pressure which is a function of the cuff diameter, D, and the stiffness of the cuff material, S. In practice thii relationship is not strictly exact over the whole area of the cuff in contact with the trachea, although it does form a useful starting-point on which to base the experimental studies. If the term Pf(o,,q were zero, for instance, then the

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intracuff pressure would be equal to the tracheal wall pressure. With the floppy cuff this should be quite a close approximation, as, by definition, the cuff is not stretched and so a pressure difference does not occur across the cuff. Therefore the inflation pressure and the intracuff pressure were measured and recorded simultaneously. The apparatus used is shown in Fig. 2. A Cape ventilator was used to provide intermittent positive pressure pulses to the tracheostomy tube under test. Two IO Ibf/ins pressure transducers were used to measure the intracuff pressure, and the pressure in the airway just below the tip of the tracheostomy tube.

itiizx& _/

I

I

recorder

1

under test

transducer

J

Fig. z. Diagram of experimental apparatus used with the ‘model trachea’ or a cadaver trachea.

All the tracheostomy tubes were modified by shortening the inflation lines, to reduce the air space external to the cuff, so that the intracuff pressure could be measured more accurately. The outputs from the pressure transducers were recorded on an ultraviolet recorder. Initially a disposable 50 ml syringe barrel was used in the test section instead of a piece of cadaver trachea. This was done so that the system could be seen through the syringe. An experimental tracheostomy tube with a large floppy cuff was introduced into the syringe barrel and the pressure transducers were connected to the inflation line and also to the bottom of the syringe barrel. The tracheostomy tube was connected to the ventilator in the normal manner, and the ventilator was set on ‘cycle’. Air was slowly in reduced into the cuff through a side arm on the inflation line. The cuff was cleafly seen to inflate, but longitudinal folds were formed along the length of the cuff because the circumference of the cuff was larger than the syringe barrel. Apparently air was leaking along these folds past the cuff, even when the intracuff pressure was equal to the inflation pressure. The cuff was inflated further to prevent this. The raised intracuff pressure ‘sealed off’ the longitudinal folds. However, it was noted that the intracuff pressure had to be raised to almost double the peak inflation pressure for a perfect seal. This was repeatedly confirmed although thinner cuffs required lower intracuff pressures to form a seal. If these small leaks along the folds were acceptable, then lower pressures were required in the cuff and these were found to be of the same order as the peak ventilator pressure. Whilst these tests were being carried out, it was noticed that there was a tendency for the tracheostomy tube to move up and down within the syringe barrel. On closer

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examination it could be seen that the 4-5 mm movement was in phase with the pulses from the ventilator; it was caused by the tendency of the inflation pressure to try and expel the tracheostomy tube from the syringe barrel, rather like a piston in a cylinder, and the movement was facilitated by the increased mobility of the floppy cuff compared to a standard cuff. Perhaps of more concern, it was noticed that the low-pressure floppy cuff did not centralize the tracheostomy tube within our artificial trachea, as does the more conventional cuff. Indeed, great care had to be taken to support the tracheostomy tube so that the tip of the tube did not come into contact with the wall of the syringe barrel. Until this point a syringe barrel had been used to simulate the trachea, whereas the trachea is D shaped in cross-section and has very elastic walls. Therefore the tests were repeated with excised cadaver tracheae obtained at post mortem. The trachea was kept moist in normal saline for the short time that it took to set up the tests. The lower end of the trachea was ligated to a suitably fabricated piece of plastic tubing, to prevent leakage, and to make the necessary connections to the pressure transducer. A floppy cuff tracheostomy tube was introduced into such a section of trachea from the open end, and the tracheostomy tube was connected to the ventilator as before. Air was slowly introduced into the cuff and the intracuff and inflation pressures were recorded. As in the previous experiments, longitudinal folding took place and air was heard to escape past the cuff along these folds. Although the opaque wall of the trachea made visual observations difficult, the ends of these folds could just be seen by looking from the top. Air was slowly injected to stop leaking along the folds and again it was found that the intracuff pressure required to form a perfect seal was approximately twice the peak inflation pressure. In fact, the pressure traces obtained with the cadaver trachea were almost identical with those obtained by usin.g the syringe barrel as a tracheal model. Air was removed from the cuff until the intracuff pressure was equal to the peak inflation pressure, and a small amount of leakage along the longitudinal folds was allowed. It was noted that the posterior wall of the trachea distended as the ventilator cycled. The posterior wall was also distended where the cuff was inflated within the trachea. The lower margins of the cuff could not be discerned when the peak inflation pressure was applied, as the shape of the posterior wall was uniform all the way down, from the top edge of the cuff. When the inflation pressure was lowered to atmospheric, the posterior wall of the trachea below the cuff returned to its normal position, but the trachea adjacent to the cuff did not return at all. During all the tests with the cadaver trachea, the tracheostomy tube moved relative to the trachea, just as it had done previously in the syringe barrel. A tracheostomy tube with a standard cuff was also tested with the cadaver trachea. An intracuff pressure of 185 cm water was required to obtain a seal for an inflation pressure of 40 cm water. No difference could be seen in the external appearance of the trachea when a floppy cuff was inserted from when the standard cuff was inserted. The standard cuff tracheostomy tube was removed without deflating the cuff, by pulling the tube clear of the trachea. The intracuff pressure was again measured, and found to be 152 cm water. By subtraction, this would tend to suggest that 33 cm water pressure had been applied to the tracheal mucosa. This figure is probably a minimum as the cuff no doubt expanded slightly during its removal, thus reducing the pressure within the cuff. It is interesting to compare this figure of35 cm water with the peak inflation pressure of 40 cm water.

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Discussion The studies carried out on the excised cadaver trachea are not necessarily directly applicable to clinical practice, as the oesophagus had been removed from the posterior wall, Furthermore, muscular tone and the effects of adjacent organs were not represented. Nevertheless, the results to date suggest that the theory that the large lowpressure cuffs do not distort the trachea requires more careful consideration. If distortion were a major cause of tracheal damage, then it would be expected that a major part of the tissue damage would occur on the posterior wall. This is well known not to be the case. A number of pathological studies have shown that injury occurs first on the anterior wall on the mucosa that lies above the cartilaginous rings (Miller & Sethi, 1970; Cooper & Grillo, 1969). It is possible that the floppy cuffs can follow the surface irregularities more easily, and therefore allow more even distribution of pressure within the trachea than does a standard cuff, which tends to be more rigid. A further feature of the present design of floppy cuff is the manner in which longitudinal folds are created, which may give rise to leakage. In the living patient, the mucus normally found on the tracheal mucosa may reduce the leakage along these folds, which might minimize leakage. However, any folding that might occur could result in the cuff being inflated so that the tracheal wall pressure could exceed the capillary blood pressure. This difficulty may be further exacerbated by the danger of over-inflation that results from the lack of ‘feel’ when inflating these low-pressure cuffs (Cooper & Grillo, 1972). The potential hazard that results from the movement of the tracheostomy tube within the trachea, coupled with the lack of ability of a floppy cuff to centralize the tube, is perhaps of more serious concern. If the tip of the tracheostomy tube were to come into contact with the tracheal wall, and even a relatively small amount of movement were to occur, there is a grave danger that the tip could erode the wall and eventually give rise to either a trachea-oesophageal or a trachea-innominate artery fistula. It is intended to repeat similar observations in the clinical situation, so that the validity of the previous findings may be assessed.

References Andrews, M. J. & Pearson, F. G. (1971) Incidence and pathogen&s of tracheal injury following cuffed tube tracheostomy and assisted ventilation: Analysis of a two year prospective study. Ann. Surg. 173, 249 Cooper, J. D. & Grillo, H. C. (xg6g) Th e evaluation of tracheal injury due to ventilatory assistance through cuffed tubes. A pathological study. Ann. Surg. 169, 334 Cooper, J. D. & Grille, M. D. (1972) Analysis of problems related to cuffs on intratracheal tubes. Chest 62, 2 I S-27s Geffin, B. & Pontoppidan, M. D (1369) Reduction of tracheal damage by the prestretching of inflatable cuffs. An&h&logy 31,462 Grille, H. C., Cooper, J. D. & Geffin, B. (1971) A low pressure cuff for tracheostomy tubes to minimize tracheal injury: A comparative clinical trial. J. Thoroz. Cardiovasc. Surg. 62, 898 McGinnis, G. E., Shively, J. G., Patterson, R. L. & Magovem, M. D. (1971) An engineering analysis of intratracheal tube cuffs. Am&h. d Analg. go, 557 Ma&nosh, R. R. (1943) Self-inflating cuff for endotracheal tubes. Brit. Med. 3. (ii), 234 Miller, E. R. & Sethi, G. (1970) Tracheal stenosis following prolonged cuff intubation. Ann. Surg. 171, 283 Trendelenburg, F. (1871) Beitrage zu den Operationen an den Luftwegen. Archiv. Klin. Chir. II, I 12