HUMIDIFICATION AND MUCUS FLOW IN THE INTUBATED TRACHEA

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1 Brit. J. Anaesth. (1973), 45, 874 HUMIDIFICATION AND MUCUS FLOW IN THE INTUBATED TRACHEA A. R. FORBES SUMMARY Observations of mucus flow in the trachea were made on greyhounds under barbiturate anaesthesia breathing air at 37 C, at various levels of relative humidity, through an endotracheal tube. The distance travelled by a marker, lycopodium powder, in the mucus was measured at 5-minute intervals through a right-angled telescope passed down the endotracheal tube. Mucus flow at an inspired relative humidity of 1% was comparable to published values. No difference between flows at inspired relative humidities of 1% and 75% was found. A significant reduction in flows at 5% and 25% relative humidity was found, followed by cessation of flow in 5 of the 7 dogs at 5% relative humidity, and in all 7 at 25%. This would suggest that gas introduced at the top of an endotracheal tube at 37 C should have a relative humidity of over 5%, and preferably 75%, to maintain tracheal mucus flow. This would correspond to 1% relative humidity at 32 C but it remains to be seen within what temperature range mucus flow is maintained. When the upper respiratory tract is bypassed by endotracheal intubation or tracheostomy, humidification of the inspired gas is necessary to prevent inspissation of secretions and damage to the tracheal mucous membrane (Burton, 1962; Sara, 1965). Although it has been demonstrated in intact animals and excised tracheas that lack of humidity causes mucus flow and ciliary beat to stop (Dalhamn, 1956; Toremalm, 1961; Asmundsson and Kilburn, 197; Kilburn, 1967), little information is available on the level of humidity necessary to maintain mucus flow. MATERIAL AND METHOD Observations of mucus flow were made on 1 greyhounds (23-35 kg). Anaesthesia was induced with intravenous thiopentone 3 mg/kg and maintained with 12-mg increments of pentobarbitone. Each dog was intubated with a straight metal endotracheal tube, 3 cm long, 1 mm wall thickness, and 11 mm internal diameter. It was placed in the left lateral position on a warming blanket, with the trachea horizontal, and an arterial catheter and rectal thermometer were inserted. The dog breathed spon- A. R. FORBES, M.B.,CH.B., F.F.A.R.C.S., Hammersmith Hospital and Royal Postgraduate Medical School, London W12 HS. Present address: Department of Anesthesia, School of Medicine, University of California, San Francisco, California 94122, U.S.A. taneously from a T-circuit humidified air at 37 C at a rate of 8-12 b.p.m. The rate of air flow across the T-piece was at least 44 l./min, measured with a dry gas meter, whereas the maximum peak inspiratory flow rate, measured widi a pneumotachograph, was 36 l./min, so that no dilution with room air occurred. To prevent atelectasis, manual hyperinflation was performed half-hourly (Mead and Collier, 1959). Measurements of arterial Po^, Pco 2, and ph were made at three points during each experiment, which lasted approximately 5 hours. Humidified air was delivered as follows (fig. 1). Compressed air was passed through a flowmeter to a hot-water bath humidifier maintained at a constant temperature by a water heater and circulator.* The water bath was specially constructed to give a high saturation by passing air through a baffle over a turbulent water surface. Heated humidified air then passed to a Y-connector where it was mixed with heated dry air to give air at a constant temperature. Humidity could be varied by altering the relative flow rates. Before each measurement of mucus flow, the temperature and humidity of the inspired air were measured at the T-piece with a wet and dry bulb psychrometer. The accuracy of the mercury thermometers, calibrated to.2 C, was verified with a British Standard thermometer. This gives an accuracy within +1 % relative humidity. The endotracheal tube was designed with a second *Churchill Laboratory Circulator.

HUMIDIFICATION AND MUCUS FLOW IN THE INTUBATED TRACHEA lumen, lateral to and parallel to the central lumen. Through this was passed a long right-angled viewing telescope of 5 mm diameter (fig. 1).

2 HUMIDIFICATION AND MUCUS FLOW IN THE INTUBATED TRACHEA lumen, lateral to and parallel to the central lumen. Through this was passed a long right-angled viewing telescope of 5 mm diameter (fig. 1). Under direct vision through the telescope, a plastic cannula of.75 mm internal diameter was passed down the central lumen of the endotracheal tube, and a light dusting of lycopodium spores insufflated by syringe on to the lateral wall of the tracheal mucosa. As the lycopodium became incorporated into the mucus, a line of demarcation could be seen at the distal edge of the area under observation, possibly caused by fresh mucus from below pushing against a momentary area of inertia. This trailing edge was aligned in 875 RESULTS Relative humidity was maintained within the following limits. 1% over 97% 75 % from 73% to 76% 5% from48 to 51% 25% from23% to 26% The means of inspired air temperatures and rectal temperatures, taken before each insertion of lycopodium, and of arterial blood-gases, taken at three points during each experiment, are shown for all 1 dogs in table I. In the 2 dogs which acted as controls fully saturated air at 37 C was breathed for 5 hours, and mucus flow was maintained without fatigue. TABLE I. Mean inspiratory and rectal temperatures, and arterial blood-gases, in a series of dogs breathing air at 37 C, and relative humidities of 25, SO, 75 and 1%. Temperature ( C) Mean SD hst Water circulato r humidifier telescope FIG. 1. Apparatus to provide a high flow of air at 37 C, and at relative humidities of 25, 5, 75 and 1% across a T-circuit. Insp. Exp Pao2 Pao2 (mmhg) (mmhg) PH A representative chart of mucus flow in the trachea of one dog, measured at 5-min intervals, is seen in figure 2. Each line represents observations of mucus flow from the time of insertion of lycopodium, to its arrival at the tip of the endotracheal tube, or until flow had ceased. Distance travelled in cm is shown against time in 5-min intervals. The first observation the centre of the field, and a mark made on the shaft of the telescope where it entered the endotracheal tube. As the lycopodium flowed axially up the trachea in the mucus stream, the trailing edge was centred in the telescopic field and a mark made on the shaft at 5-min intervals. In this way, the rate of mucus flow could be determined. Care was taken not to touch the area under observation with telescope or cannula, although it was impossible to avoid touching mucosa at a distance from the area occasionally. Repeated measurements of mucus flow were made over 5 hours in 2 dogs breathing air at 37 C, fully saturated. In 7 dogs the relative humidity of inhaled air was varied from 25% to 5% to 75%, and mucus flow charted at each relative humidity until the tip of the endotracheal tube was reached, or flow ceased. The order of change of relative humidity FIG. 2. Representative results of mucus flow in the was randomized. Between each change in relative trachea of one of a series of dogs, breathing air at 37 C, humidity, fully saturated air was reintroduced, and and at relative humidities of 25, 5, and 75%. Between change in humidity, air at 1% relative humidity maintained until measured mucus flow was restored each was reintroduced, until mucus flow returned to control to previous values. In 1 dog, mucus flow was values. Each line represents movement of lycopodium measured whilst breathing air at 75% inspired rela- marker from its application to its arrival at the tip of the endotracheal rube, or to cessation of flow. Numbers tive humidity for 4 hours. indicate the order of change of humidity.

876 BRITISH JOURNAL OF ANAESTHESIA was made at an inspired relative humidity of 1%, showing slow movement initially, accelerating up to the tip of the endotracheal tube.

3 876 BRITISH JOURNAL OF ANAESTHESIA was made at an inspired relative humidity of 1%, showing slow movement initially, accelerating up to the tip of the endotracheal tube. The second observation, again at 1% relative humidity, shows the same pattern. The third observation, at 25 % relative humidity, shows slight initial movement which abruptly ceases. Reinstitution of 1% relative humidity (observation 4) restores movement to roughly parallel earlier values. At an inspired relative humidity of 5% (observation 5), after an initial pause when a ridge of mucus containing lycopodium formed, movement rose to equal that at 1%, then stopped. Once again, reintroduction of 1% relative humidity restored movement as seen in observation 6. Finally, change to an inspired relative humidity of 75% (observation 7) is seen to give movement similar to that at 1%. When these results were converted to flow rates, flow rates were seen to vary from 2.3 to 16.6 mm/ min, over all 1 dogs breathing air at 1% relative humidity. Thus a direct comparison of flow rates in mm/min between dogs was not possible. However, expressing flows as a percentage of the mean rate at 1% relative humidity in each of the 7 dogs in which relative humidity was varied, the results were as shown in table n. When air at 37 C and 75% relative humidity was breathed, mean mucus flow was maintained at 9% of that at 1% relative humidity. At 5% relative humidity, mean flow was reduced to 2% of that at 1%. At 25% inspired relative humidity, mean flow rate was reduced to 3% of that at 1%. Both these reductions were significant, but no significant difference was seen between flows at 25% and 5% relative humidity. Furthermore, at 25% relative humidity, flow stopped in each dog within 3 minutes. At 5% relative humidity, flow stopped in 5 dogs, and continued at a reduced rate in 2. In the dog breathing 75% inspired relative humidity for 4 hours, flow continued at a rate equal to that at 1%. DISCUSSION Mucous secretions are produced by tracheobronchial goblet cells and mucous glands to protect the respiratory mucosa and remove inhaled particles. Thus, a blanket of mucus is moved continually upwards to the larynx by ciliary action (Yeager, 1971; Hilding, 1963). The rate of normal mucus flow is determined by the interaction between ciliary beat, and volume and viscosity of secretions. The mucus-secreting mechanism is the more sensitive to environmental change or mechanical trauma, responding by a change in volume or viscosity of secretions (Rylander, 1966). As shown above, the rate of mucus flow was maintained when inspired air was at a temperature of 37 C, and a relative humidity of 75% or 1%. The slowing or cessation of flow at 5% or 25% inspired relative humidity, suggests that secretions were of increased viscosity or reduced volume, accompanied perhaps by cessation of ciliary beat when flow had ceased. Mucus flow may be studied in vivo, or in vitro, when flow in excised tracheal tissue may continue for hours, if adequate humidity is maintained TABLE II. Measured mucus flow rates in the tracheas of 7 intubated anaesthetized dogs, breathing air at 37 C, at relative humidities of 25, 5, 75 and 1%. Each value at 1% represents a mean of several observations, whilst at the other inspired relative humidities, each represents one observation only. Relative humidity Dog No % % on 1% r.h Mean of flows as % of flow on 1% r.h SD ±23.4 P.2 Significance n.s. Mucus flow rate (mm/min) % on 1% r.h ±15.7 <.1 s % on 1% r.h ±9.5 <.1 s.

HUMIDIFICATION AND MUCUS FLOW IN THE INTUBATED TRACHEA 877 (Asmundsson and Kilburn, 197). In-vitro measurements were not appropriate to this study.

4 HUMIDIFICATION AND MUCUS FLOW IN THE INTUBATED TRACHEA 877 (Asmundsson and Kilburn, 197). In-vitro measurements were not appropriate to this study. Observation over an open section of trachea would have altered the conditions under study. Bronchoscopic observation was selected in preference to the use of labelled radioactive particles (Marin and Morrow, 1969), or transillumination of the exposed trachea (Rylander, 1966), so that the pattern of mucus flow could be directly observed over a length of trachea. The successful use of lycopodium as a marker has been previously reported (Carson, Goldhamer and Carpenter, 1966; Laurenzi, Yin and Guarneri, 1968). The pattern of movement of lycopodium seen was that of acceleration from a standing start, a phenomenon observed by Stewart (1948), consistent with the application of a constant ciliary beat. When relative humidity was reduced to 5% or 25%, a ridge of mucus was seen to form. This folding was noted by Hilding (1957) in viscous mucus at points of obstruction to flow. The slower pattern of acceleration seen would also suggest increased viscosity. Restoration of flow occurred within 25 minutes of reintroducing 1% relative humidity, suggesting that no damage to the secretory or ciliary mechanism occurred over the length of this study. Rate of mucus flow varies with species, body and inspired air temperature, in-vivo or in-vitro preparation, in addition to inspired relative humidity. Low inspired oxygen has been reported to reduce mucus flow (Laurenzi, Yin and Guarneri, 1968), but this was not confirmed by other workers (Marin and Morrow, 1969). Intravenous injection of pentobarbitone prior to sacrifice has been shown not to affect subsequent mucus flow rates in the excised trachea of the dog (Asmundsson and Kilburn, 197). Nor did increments of intravenous pentobarbitone alter mucus flow rates in the anaesthetized dog (Marin and Morrow, 1969). Even when conditions are standardized, variations in flow within reported series are wide (table III). In dogs, flow has been reported to vary from 5 to 19 mm/min in the excised trachea (Asmundsson and Kilburn, 197), and from 6.6 to 2.7 mm/min in dogs breathing room air at C and 8% relative humidity, through the nose (Marin and Morrow, 1969). Flows of mm/min fall within the same range. In the human trachea, under physiological conditions, inspired air temperature varies from 34 to 36 C at mid to lower level, and is 95-1% saturated (Perwitzschky, 1927; Sara, 1965). To produce comparable conditions in the dog, a temperature of 37 C at the inspiratory limb of the T-piece was selected. Increase in inspired air temperature from 5 to 35 C was found to increase mucus flow in spontaneously nose- or mouth-breathing chicks by Baetjer (1967), who attributed it to an increase in mucosal temperature. No correlation between air temperature and mucus flow rates in rats was found by Dalhamn (1956) at temperatures between 31 and 37 C or when inspired temperature was raised in 1-degree steps to 41 C, provided that rectal temperature remained constant. Increasing the rectal TABLE III. Published results of intratracheal mucus flow rates in various species, in vitro and in vivo, under various conditions of temperature and humidity. Intratracheal mucus flow rate (mm/min) Species Preparation Authors Chicks Rats Cats Various animals from hen to horse Cows In vivo spontaneous respiration In vivo trachea open breathing air saturated at 34 C In vivo nasal breathing room air at C, 8% r h In vivo air at 37 saturated per endotracheal tube Baetjer (1967) Dalhamn (1956) Barclay and Franklin (1937) Hill (1928) Hilding (1957) Asmundsson and Kilburn (197) Marin and Morrow (1969) Present series

878 BRITISH JOURNAL OF ANAESTHESIA temperature from 37 to 39 C increased flow but increasing the rectal temperature further to 42 C reduced flow.

5 878 BRITISH JOURNAL OF ANAESTHESIA temperature from 37 to 39 C increased flow but increasing the rectal temperature further to 42 C reduced flow. In resected horse trachea, flow doubled when tissue temperature was increased from 32 to 42 C, and slowed again at 43 C (Hill, 1928). In the present study, rectal and inspired temperatures were maintained constant. Lack of humidity is known to stop mucus flow and ciliary activity (Proetz, 1933; Asmundsson and Kilburn, 197; Dalhamn, 1956; Toremalm, 1961; Kilburn, 1967). On initial exposure to dry gas, mucus thickens and slows to a halt. Cilia continue to beat, but, if drying continues, will eventually stop (Dalhamn, 1956; Kilburn, 1967). Dalhamn (1956) demonstrated cessation of ciliary beat in rats' tracheas at room temperatures in 1 minutes at 3% and 5% relative humidity, with no change in beat at 6 minutes on 75% relative humidity, nor at 12 minutes at 1% relative humidity. Toremalm (1961) showed that cessation of ciliary beat produced in excised rabbit tracheal mucosa by air 5% saturated at 21 C, could be reversed by air 8% saturated at 3 C. The clinical effects of inhalation of dry gas through an endotracheal tube or tracheostomy are well known, and consist of a progression from increased mucus viscosity to inspissation, and from inflammation of the mucous membrane to areas of sloughing and squamous metaplasia (Burton, 1962; Sara, 1965). The results presented here would suggest that inspired gas at 37 C, should have a relative humidity of over 5%, and preferably 75%, in order to maintain mucus flow. In terms of absolute humidity, 75% at 37 C, corresponds to 1% at 32 C. But it remains to be seen whether mucus flow is a function of relative or absolute humidity, and within what temperature range normal function is maintained. ACKNOWLEDGEMENTS I am indebted to Professor M. K. Sykes for his advice and constructive criticism, to the secretarial and technical staff of the Department of Anaesthetics, and to the Department of Medical Illustration, of the Royal Postgraduate Medical School. REFERENCES Asmundsson, T. 3 and Kilburn, K. H. (197). Mucociliary clearance rates at various levels in dog lungs. Amer. Rev. resp. Dis., 12, 388. Baetjer, A. M. (1967). Effect of ambient temperature and vapour pressure on cilia-mucus clearance rate. J. appl. Physiol., 23, 498. Barclay, A. E., and Franklin, K. J. (1937). The rate of excretion of indian ink, injected into the lungs. J. Physiol. (Lond.), 9, 482. Burton, J. D. K. (1962). Effects of dry anaesthetic gases on the respiratory mucous membrane. Lancet, 1, 235. Carson, S., Goldhamer, R., and Carpenter, R. (1966). Mucus transport in the respiratory tract. Amer. Rev. resp. Dis., 93, 86. Dalhamn, T. (1956). Mucous flow and ciliary activity in the trachea of rats, and rats exposed to respiratory irritant gases. Acta physiol. scand., 36, Suppl Hilding, A. C. (1957). Ciliary streaming in the lower respiratory tract. Amer. J. Physiol., 191, (2), 44. (1963). Phagocytosis, mucous flow, and ciliary action. Arch, environm. Hlth., 6, 67. Hill, L. (1928). The ciliary movement of the trachea studied in vitro. Lancet, 2, 82. Kilburn, K. H. (1967). Mucociliary clearance from bullfrog lung. J. appl. Physiol, 23, 84. Laurenzi, G. A., Yin, S., and Guarneri, J. J. (1968). Adverse effect of oxygen on tracheal mucous flow. New Engl. J. Med., 279, 333. Marin, M. G., and Morrow, P. E. (1969). Effect of changing inspired oxygen and carbon dioxide levels on tracheal mucociliary transport rate. J. appl. Physiol, 27, 385. Mead, J., and Collier, C. (1959). Relation of volume history of lungs to respiratory mechanisms in anaesthetised dogs. J. appl. Physiol., 14, 669. Perwitzschky, R. (1927"). Arch. Ohr.-, Nas.-, u. Kehlk.- Heilk., 117, 1 (quoted by Dalhamn). Proetz, A. W. (1933). Studies on nasal cilia in the living mammal. Ann. Owl. (St Louis), 42, 778. Rylander, R. (1966). Current techniques to measure alterations in the ciliary activity of intact respiratory epithelium. Amer. Rev. resp. Dis., 93, 67. Sara, C. (1965). The management of patients with a tracheostomy. Med. J. Aust., 1, 99. Stewart, W. C. (1948). Weight carrying capacity and excitability of excised ciliated epithelium. Amer. J. Physiol, 152, 1. Toremalm, N. G. (1961). Air flow patterns and ciliary activity in the trachea after tracheostomy. Acta oto- Laryng. (Stockh.), S3, 442. Yeager, H. (1971). Tracheobronchial secretions. Amer. J. Med., 5, 493.

TEMPERATURE, HUMIDITY AND MUCUS FLOW IN THE INTUBATED TRACHEA

BT. J. Anaesth. (1974), 46, 29 TEMPERATURE, HUMIDITY AND MUCUS FLOW IN THE INTUBATED TRACHEA A. R. FORBES SUMMARY Mucus flow rates were measured in the intact trachea of anaesthetized dogs, using a radioactive