The term 'tracheal gas insufflation' describes a technique used in intubated patients to increase the efficiency of lung ventilation with respect to carbon dioxide removal. Fresh gas is directed into the distal portion of the trachea, through either a special catheter or a modified tracheal tube, during all or part of the ventilatory cycle . The main advantage of this is that during expiration, when insufflated gas escapes via the expiratory limb of the ventilator circuit, expired gas is washed out of the trachea and ventilator tubing. This reduces the amount of carbon dioxide delivered to the alveoli during the next inspiration. Thus, normocapnia can be maintained using a lower tidal volume and hence a lower airway pressure, a factor which has shown to be of benefit in patients with acute lung injury [2,3].
The jet of gas from the insufflating catheter may cause damage to the tracheal mucosa over time [4-6]. This study aims to examine the effect of catheter design on pressure distribution within a model trachea in order to suggest a suitable catheter design for clinical use.
A model trachea was constructed so that the pressure imparted to the inner surface by various designs of insufflation catheter could be measured. The absolute pressure imparted at each measurement point and the evenness of pressure distribution might indicate which type of catheter would cause the least trauma to the tracheal mucosa.
A plaster of Paris cast was made of the trachea and main bronchi of a female cadaver during the course of a post-mortem examination. An exact wax copy of this cast was then made and encased in a layer of plastic approximately 2 mm in thickness. Once this outer layer had hardened, the wax inside was removed with heat. The inner surface of the resultant plastic model was therefore anatomically identical to the original trachea.
The bevels were removed from 33 disposable needles of 0.5 mm internal diameter. The needles were then glued into the tracheal model at various points so that their flattened tips were flush with the inner surface of the model. The needle holes were less than 0.8 mm in diameter, so that their effect on the gas flow within the model would be negligible . The model, therefore, had 33 needles projecting from its outer surface, each with a Luer Lok® (Becton Dickinson Co, Franklin Lakes, NJ, USA) connection. Each of these was connected to a pressure measurement port via a flexible tube and these ports were numbered 1-33 (Fig. 1a,b). To avoid any pressure attenuation by the narrow internal diameter of the needles and the length of tubing required to connect the needles to the pressure measurement port, pressure measurements were made during apnoea. By eliminating pressure fluctuations associated with mechanical ventilation, the tracheal pressures were able to attain steady-state conditions, at which point there would be no pressure gradient along the needles and tubing and thus no pressure attenuation. The right and left bronchial outlets of the tracheal model were closed off using 2 L anaesthesia reservoir bags. The final arrangement of the model trachea is shown in Fig. 2.
Five different designs of gas insufflation device were tested using this model. Four were constructed from brass tubing of 2.0 mm outside diameter and 1.7 mm internal diameter in the following designs: end hole only; two side holes opposite each other; multiple side holes consisting of three pairs of holes opposed to each other and a reverse-flow catheter. To make the reverse-flow catheter, three equally spaced side holes were drilled around a length of brass tubing just proximal to the tip. The very tip of the tube was then glued centrally into the base of a small metal object shaped like a drinking glass, approximately 4 mm in diameter and 1 cm long. Insufflation gas therefore emerged from the side holes of the tube and then moved upwards through the interior of this metal 'cap' to emerge from its open, upward-facing end. These devices are shown in Fig. 3. The fifth device tested was a Boussignac tube, as its suitability for use as a tracheal gas insufflation device has already been studied in infants . The Boussignac tube used for adults (Fig. 4) is a cuffed tracheal tube with multiple small lumens within its wall opening onto the inner surface of the tube near its distal end. Two of the lumens are available for pressure measurement, although they were not used in this study. The remainder are connected to a Luer Lok® port, located near the proximal end of the tube, which is used to supply the insufflation gas.
During testing of the brass catheter designs, a 9.0 mm internal diameter tracheal tube was placed into the tracheal part of the model so that the tube tip was 1 cm from the carina. The cuff was then inflated. A modified high-frequency jet ventilation adapter (SIMS-Portex Ltd, Hythe, UK) was attached to the proximal end of the tracheal tube. The catheter to be tested was inserted through the adapter into the centre of the tracheal tube lumen until the catheter tip was level with the proximal edge of the tracheal tube bevel. When evaluating the Boussignac tube, a 9.0 mm internal diameter version was inserted into the tracheal model such that it was located in the same position that the standard tracheal tube had been previously. When this tube was used, the insufflation gas was delivered via the Luer Lok® connection located at its proximal end.
The experiments with each catheter design were carried out with constant insufflation gas flows of 5 and 10 L min−1. For the brass tubes, this was established as follows. The brass tube was assembled such that its tip lay within a tracheal tube sealed at its proximal end. The tip of the tracheal tube was placed within the inlet of a flow measurement apparatus (RT200 Calibration Analyser®; Allied Healthcare Products (Timeter), Saint Louis, MO, USA) and the proximal end of the insufflation catheter was attached to a regulated supply of oxygen, the flow of which could be adjusted with a needle valve. This valve was adjusted until the desired flow rate was attained as indicated by the flow measurement apparatus. The insufflation catheter was then assembled within the tracheal model for the experiment, with no further adjustment of the insufflation gas flow. There was a slight increase in resistance to the flow of insufflating gas when the insufflation catheter was assembled within the tracheal model, which arguably could then reduce the total insufflation gas flow. However, this increase in resistance was insignificant when compared with the very high resistance of the insufflation catheter itself, which is the main determinant of flow rate. In the case of the Boussignac tube, the tip of the tube was placed within the inlet tube of the flow measurement apparatus and the insufflation gas was delivered via the attached Luer Lok® port. The desired flow rate was then achieved in the same manner.
The two sets of intra-tracheal pressure measurements for each insufflation device are shown in Fig. 5a-e. The pressures were affected by the rate of flow of insufflating gas as well as by tube design, and ranged from −0.64 to 4.02 cmH2O. However, for a given flow rate and tube design, there was little pressure fluctuation within the trachea, with the exception of the end hole catheter when a higher flow rate was used.
The data of the end hole catheter and, to a lesser extent, those of the side hole catheter have a large variation around the mean and exhibit particularly high pressures at individual measuring points. These high-pressure 'hot spots' could be associated with mucosal damage. In this study, only a single tracheal model was used, as each model was technically difficult to construct. We accept that different absolute pressures may have been recorded if further tracheal models had been studied. However, the distribution of the high-pressure spots is unlikely to be affected by small variations in tracheal anatomy. Different absolute pressures might also have been recorded if mechanical ventilation had been applied to the model, but again, the distribution of the high-pressure spots is unlikely to have been affected. The purpose of the study was to see if catheter design affected the distribution of pressures within the trachea. Catheters producing high-pressure spots may be more likely to cause mucosal damage than those that do not, regardless of the absolute pressures measured.
At 5 L min−1, the end hole catheter produced high-pressure spots at sample points 1, 6, 17 and 27. These correspond to the right main bronchus in Fig. 1a,b. At the higher flow rate, such high-pressure spots were found at the carina and right main bronchus. The pressure increases in the right bronchus could be due either to eccentric placement of the insufflating catheter or the fact that this bronchus is aligned more vertically than the left. The areas of higher pressure recorded for the end hole catheter were of far greater magnitude than the surrounding readings, suggesting that most of the gas flow from this catheter is impinging on only one or two small areas. This is more likely to cause damage to the tracheal mucosa over a prolonged time than a more diffuse spread of gas flow.
The three other catheters and the Boussignac tube produced similarly shaped pressure profiles, differing only in sign and magnitude. The reverse-flow catheter produced subatmospheric pressures in the main bronchi and distal trachea. This could be due to air entrainment from around the carina, leading to a reduced pressure in this area. Negative pressures around the carina may carry the risk of causing atelectasis. Increasing the end-expiratory airway pressure might lessen this risk . One advantage of the reverse-flow catheter is that the pressure profile was of a lower magnitude than the two side hole catheters or the Boussignac tube. An inverted catheter can also enhance mucus clearance .
Nahum and colleagues found that with continuous tracheal gas insufflation and volume-controlled ventilation, a reverse-flow catheter produced a smaller reduction in PaCO2 than a straight catheter . However, a more recent study found that the catheter flow rate was more important than the direction of flow in determining the efficiency of tracheal gas insufflation . Imanaka and colleagues found that with pressure-controlled ventilation and expiratory phase tracheal gas insufflation, the direction of gas flow did not markedly affect CO2 elimination .
It has been suggested that reverse-flow tracheal gas insufflation would minimize the likelihood of tracheal injury . Our experimental data support this suggestion, as the pressure changes produced in the distal trachea by the reverse-flow catheter were of low magnitude and evenly distributed.
The Boussignac tube produced the most even pressure profile, although of a greater magnitude than the other catheters. It avoids the need for a separate catheter within the trachea, since the insufflating channels are incorporated into the walls of the tube. This tube has already been successfully used to provide continuous tracheal gas insufflation in infants with hyaline membrane disease .
In conclusion, we suggest that the most suitable tracheal gas insufflation devices would be either the Boussignac tube or the reverse-flow catheter in conjunction with increased end-expiratory airway pressure.
1. Burke WC, Nahum A, Ravenscraft SA, et al.
Modes of tracheal gas insufflation: Comparison of continuous and phase-specific gas injection in normal dogs. Am Rev Respir Dis
2. Hickling KG, Henderson SJ, Jackson R. Low mortality associated with low volume pressure limited ventilation with permissive hypercapnia in severe adult respiratory distress syndrome. Int Care Med
3. Hickling KG, Walsh J, Henderson S, Jackson R. Low mortality rate in adult respiratory distress syndrome using low-volume, pressure-limited ventilation with permissive hypercapnia: a prospective study. Crit Care Med
4. Kuo PH, Wu HD, Yu CJ, Yang SC, Lai YL, Yang PC. Efficacy of tracheal gas insufflation in acute respiratory distress syndrome with permissive hypercapnia. Am J Respir Crit Care Med
5. Sznajder JI, Nahum A, Crawford G, Pollak ER, Schumacker PT, Wood LDH. Alveolar pressure inhomogeneity and gas exchange during constant-flow ventilation in dogs. J Appl Physiol
6. Delgado E, Hete B, Hoffman LA, Tasota FJ, Pinsky MR. Effects of continuous, expiratory, reverse and bi-directional tracheal gas insufflation in conjunction with a flow relief valve on delivered tidal volume, total positive end-expiratory pressure, and carbon dioxide elimination: a bench study. Respir Care
7. Necati GA. Measuring equipment and transducers. In: Hucho WH, ed. Aerodynamics of Road Vehicles.
London, UK: Butterworths, 1987: 443-444.
8. Dassieu G, Brochard L, Agudze E, Patkaï J, Janaud J-C, Danan C. Continuous tracheal gas insufflation enables a volume reduction strategy in hyaline membrane disease: technical aspects and clinical results. Int Care Med
9. Trawöger R, Kolobow T, Cereda M, et al.
Clearance of mucus from endotracheal tubes during intratracheal pulmonary ventilation. Anesthesiology
10. Nahum A, Ravenscraft SA, Nakos G, Adams AB, Burke WC, Marini JJ. Effect of catheter flow direction on CO2
removal during tracheal gas insufflation in dogs. J Appl Physiol
11. Ravenscraft SA, Shapiro RS, Nahum A, et al.
Tracheal gas insufflation: Catheter effectiveness determined by expiratory flush volume. Am J Respir Crit Care Med
12. Imanaka H, Kirmse M, Mang H, Hess D, Kacmarek RM. Expiratory phase tracheal gas insufflation and pressure control in sheep with permissive hypercapnia. Am J Respir Crit Care Med
13. Kacmarek RM. Complications of tracheal gas insufflation. Respir Care