Endotracheal catheters have been widely used to provide continuous airway access for re-intubation following a failed extubation or for emergency oxygen insufflation and ventilation in the management of ‘can’t intubate, can’t ventilate’ patients.1–4 Oxygen and ventilation can be provided by jet ventilation through the endotracheal catheter. Such ventilation produces inflow through the lumen of the endotracheal catheter but outflow is around the endotracheal catheter, through the natural airway: this has been termed ‘coaxial ventilation’. The advantages of coaxial ventilation are a reduction in dead space5 and prevention of aspiration6,7 during ventilation. In addition, coaxial ventilation has been used during laryngomicrosurgery because, in order to provide an adequate surgical field, only a small catheter can be inserted through the glottis.8
Despite the benefits of coaxial ventilation, serious complications can arise when using jet ventilation with endotracheal catheters. Barotrauma resulting in pneumothorax has been a major concern when using jet ventilation with endotracheal catheters.2,9–13 One study found that 11% of patients suffered barotrauma from jet ventilation with endotracheal catheters.2 Other case studies have reported pneumothorax, cardiac arrest and death in association with jet ventilation via an endotracheal catheter.10–12 Because the major complications are a result of the very high driving pressure associated with jet ventilation,13 minimising the driving pressure should reduce the risk of complications, including pneumothorax.10,14 However, during jet ventilation, a reduction in driving pressure results in decreased ventilatory efficacy because the endotracheal catheters have a small inner diameter and high resistance to gas flow.
Recently, we developed an alternative method of ventilation using an endotracheal catheter modified with the addition of a functional cuff (ECFC).15 The functional cuff is only inflated during inspiration and expiratory flow is not through the lumen but rather around the endotracheal catheter, thus producing coaxial ventilation. When combined with an intensive care ventilator in pressure control mode, this novel device is capable of generating clinically relevant tidal volumes (VT) in a model lung.15
However, the ventilation efficacy and safety of this combination have not been evaluated in vivo. The aim of this study was to determine the efficacy and safety of ventilation in adult human-sized swine using the ECFC with an ICU pressure-controlled ventilator.
Materials and methods
The study was approved by the Institutional Animal Care and Use Committee (IACUC) of the Massachusetts General Hospital (Address: 149 13th Street, Charlestown, MA 02129, USA, Protocol #2010N000144/1, Date: 18 August 2010, Responsible person: Diane McCabe, IACUC Manager) and commenced on 20 March 2014. Eight 12 to 16-week-old, female Yorkshire swine, weighing 45 to 50 kg and measuring 44 to 46 inches nose to tail, were studied during general anaesthesia.
Endotracheal catheter with functional cuff
To create the ECFC, we modified Cook Airway Exchange Catheters (AEC), either 3.0 or 3.6 mm internal diameter, 14 or 19 Fr (French gauge), respectively (Cook Critical Care, Bloomington, Indiana, USA). A 5 cm long piece of thin latex tubing placed over the distal side ports of the endotracheal catheter created the balloon cuff. An internal resistor was created by placing a 1 cm long plastic tube into the distal tip of each ECFC: a piece of 14-G intravenous (i.v.) catheter for the 14-Fr AEC and a piece of 2.3 mm internal diameter (11 Fr) Cook AEC for the 19 Fr AEC.14 The lumen of the cuff thus communicates freely with the lumen of the ECF and the cuff inflates only during inspiration due to the pressure differential between the proximal ECFC and the trachea distal to the internal resistor. The intra-cuff pressure is approximately equal to the sum of the airway pressure distal to ECFC and the pressure gradient across the resistor. As the cuff is inflated automatically during inspiration, similar to the cuff of a conventional endotracheal tube, it seals the tracheal lumen. With both the 19 and 14-Fr catheters, the outer diameter of the inflated cuff was 20, 21 and 22 mm at driving pressures of 25, 50 and 70 cmH2O, respectively. These diameters were sufficient to occlude the lumen of the trachea. Expiration is passive, driven by the normal recoil pressure of the respiratory system and, as the intra-cuff pressure returns to zero at the end of inspiration, the cuff deflates, allowing expiratory flow around the ECFC: coaxial ventilation (Fig. 1).
All animals were maintained in the supine position and, preinduction, received 10 mg kg−1 of ketamine and 2 mg kg−1 of midazolam intramuscularly. Anaesthesia was then induced with an intravenous injection of propofol (2 mg kg−1) and maintained by an intermittent infusion of propofol (1 mg kg−1) during the study. Once animals were apnoeic, their tracheas were intubated with either a 7.5 or 8.0 mm internal diameter endotracheal tube (ETT). Standard monitoring was used, including electrocardiogram, transcutaneous oxyhaemoglobin saturation monitoring (SpO2) and intra-arterial blood pressure monitoring via the internal carotid artery. In addition, chest and abdominal inductance plethysmograph bands (Respitrace Calibrator; Ambulatory Monitoring, Inc., Ardsley, New York, USA) were positioned on each animal (Fig. 2). A flow/pressure sensor (NICO Cardiopulmonary Management System, Model 7300; Respironics Corp., Murrysville, Pennsylvania, USA) was placed between the proximal end of the ECFC and Y-connector of the ventilator circuit (Fig. 2). The sensor was automatically calibrated during data collection. Pressure and air flow were continuously measured by the sensor at a sampling rate of 100 Hz.
To calculate VT during ventilation through the ECFC, the inductance plethysmograph was calibrated as previously described.16 Briefly, variable sized tidal volumes were delivered during ventilation through the ETT at peak inspiratory pressures of 5, 10, 15 and 20 cmH2O. From these data, a calibration curve for each individual animal was created by plotting the tidal volumes indicated by the NICO against the amplitude change of the inductance plethysmograph. During the study, the amplitude change of the inductance plethysmograph for each individual inspiration was compared with the calibration curve for that specific animal to determine the tidal volume. If air trapping were to occur during ventilation via the ECFC, then this would be detected by a gradual increase in the baseline of the inductance plethysmograph due to the increasing lung volume.
After ventilation through the ETT, the 14-Fr ECFC was inserted through the ETT and the ETT was removed from the trachea. The proximal end of the ECFC was then connected to the ICU ventilator. The ICU ventilator (Evita 4; Dräger, Lübeck, Germany) was set to the pressure-controlled mode and three different peak inspiratory pressures (25, 50 and 70 cmH2O) were investigated. At each of these pressures, two inspiratory:expiratory (I:E) ratios (1 : 1 and 1 : 2) were studied sequentially before increasing the inspiratory pressure. Each combination of I:E ratio and inspiratory driving pressure was kept constant for 30 s. During each of these 30-s periods, data from five breaths were collected, but only the data from the last three breaths were analysed to determine VT. After completion of data collection for the 14-Fr ECFC at the three different pressures, the ETT was re-inserted over the ECFC and the ECFC was removed. Then, the 19-Fr ECFC was inserted through the ETT and the ETT was again removed. With the 19-Fr ECFC in situ, the lung was again ventilated, following the same sequence as with the 14-Fr ECFC. During the entire study, the positive end-expiratory pressure (PEEP) was 0 cmH2O, respiratory rate 10 breaths min−1 and FIO2 1.0. After completion of the second series, the ETT was re-inserted over the ECFC and the 19-Fr ECFC was removed.
Data are presented as mean ± standard deviation. An effective VT was considered as a VT greater than 150 ml. For main effects, the general linear model for univariate analysis was used to identify the significance of the I:E ratio on VT. The Friedman test, followed by Bonferroni correction for multiple comparisons, was used for overall comparisons between the various driving pressures and endotracheal catheters. Statistical analysis was done with a statistical software package (PASW Statistic 18; SPSS; Chicago, Illinois, USA). P value less than 0.05 was considered statistically significant.
VT achieved using the endotracheal catheter with functional cuffs
The mean VT generated by the ICU ventilator with both the ECFCs and the ETT is presented in Fig. 3. With the 14-Fr ECFC, the VT generated was 295 ± 92 ml (range 178 to 436 ml), Fig. 3a. With the 19-Fr ECFC, the VT generated was 419 ± 148 ml (range 236 to 648 ml), Fig. 3b. With the ETT, the VTs generated were 729 ± 107 ml with a driving pressure of 15 cmH2O and 917 ± 128 ml with a driving pressure of 20 cmH2O (Fig. 3). Considering the VT obtained through the ETT with a set peak inspiratory pressure of 15 to 20 cmH2O, the estimated peak airway pressure associated with an ECFC was within the lung protective range (Fig. 3). During the study periods, no air trapping was observed.
VT achieved with different I:E ratios
A longer inspiratory time generated a greater VT (P < 0.01) (Fig. 3).
Amplitude change during ventilation via an endotracheal catheter with a functional cuff
The baseline amplitude at the end of expiration did not increase during ventilation via an ECFC, thus air trapping did not occur (Fig. 4).
The major findings of this study are as follows:
- A conventional ITU ventilator in pressure control mode was able to generate effective coaxial ventilation using an ECFC.
- The ventilator driving pressure is much lower than the driving pressure required for jet ventilation.
- No complications were observed.
To the best of our knowledge, this is the first study to evaluate the efficacy and safety of coaxial ventilation on adult human-sized animals using a conventional ventilator rather than a jet ventilator. Our results indicate that with an ECFC, practitioners may be able to use either an ICU or operating room ventilator to achieve an adequate VT at a driving pressure much lower than that required when jet ventilation is used. The ability to use lower driving pressures may reduce the risk of barotrauma.
The ability of the ECFC to provide an adequate VT can be explained by the cuff inflating during inspiration, transforming an otherwise open airway into a closed airway. However, during expiration, the cuff deflates allowing exhalation around the endotracheal catheter, thus coaxial ventilation is established. Coaxial ventilation differs from conventional ventilation because the latter completely bypasses the natural airway and, above the cuff, bacterial growth on the surface of the endotracheal tube forms a biofilm.17 In addition, oral secretions accumulate above a conventional cuff and may be unknowingly aspirated.18 During inspiration, coaxial ventilation with an ECFC is different from that with jet ventilation. When jet ventilation is employed, there is no cuff on the endotracheal catheter and tidal volume is augmented by the Venturi effect. Because the Venturi effect creates inflow around the catheter, any material around the outside of the catheter, proximal to the distal end of the catheter, may be aspirated into the lung. With an ECFC, all inflow is through the catheter and there is no Venturi effect with additional inflow around the outside of the catheter. With both jet ventilation and the ECFC, expiratory flow is around the outside of the catheter. The ECFC design should minimise not only aspiration but should also reduce the accumulation of secretions around the catheter. Therefore, the ECFC might prevent complication associated with the silent aspiration of oral secretions. However, further study is needed to evaluate aspiration during ventilation through an ECFC.
One of the concerns regarding this new approach is inadvertent high airway pressure delivered to the lungs as a result of the driving pressures used (25 to 70 cmH2O). With the settings in this study, the largest VT was 648 ml and this VT was highly dependent on inspiratory time. An inspiratory time of 2 to 3 s generated an effective VT with estimated peak airway pressures within the lung protective range, less than 20 cmH2O. Previous studies have demonstrated that, to provide at least partial ventilatory support, conventional endotracheal catheters require a high driving pressure to maintain airway pressure: driving pressure from 7 psi (492 cmH2O) to 50 psi (3515 cmH2O) is described.13,19 In contrast, we have demonstrated that by adding a functional cuff to the endotracheal catheter, an effective VT can be achieved with much lower driving pressures, as low as 25 cmH2O. With an ordinary endotracheal catheter, the lack of a cuff allows backflow around the catheter through the open oropharynx and effective ventilation cannot be achieved until the Venturi effect is produced. The Venturi effect during jet ventilation can only be produced with high driving pressures. The addition of the cuff, with free communication of the cuff lumen to the lumen of the endotracheal catheter, and a resistor at the distal end of the endotracheal catheter allow the creation of a pressure gradient between the cuff and the trachea only during inspiration. The pressure differential between the cuff and the trachea ensures that the cuff is inflated before bulk airflow to the trachea. The resistor establishes and maintains the pressure gradient for the duration of inspiration, ensuring the intra-cuff pressure is greater than airway pressure distal to the ECFC and so the cuff remains inflated and the trachea sealed during inspiration, allowing the production of an effective inspiratory VT. At the end of inspiration, the internal cuff pressure is minimal and the elastic recoil of the cuff causes it to collapse. In addition, the elastic recoil of the lung and chest wall creates a tracheal pressure greater than the intra-cuff pressure, again causing the cuff to deflate and so expiration occurs through the natural airway around, but not through the ECFC. We did not test the application of PEEP in this study. Because no PEEP was applied, there was no inward gas flow at the end of inspiration and therefore no pressure gradient across the resistor. The outer diameter of the catheter is very small relative to diameter of the tracheal lumen; as a result, expiration is complete and auto-PEEP is zero.
Air trapping is major concern when an endotracheal catheter is used. During jet ventilation through an endotracheal catheter, several mechanisms have been proposed as the cause of air trapping and barotrauma. Jet ventilation induces dynamic hyperinflation and auto-PEEP leading to barotrauma by high driving pressures.13 This risk is increased when the endotracheal catheter is improperly placed in a smaller secondary bronchial branch, resulting in delivery of inappropriately large gas volumes into distal bronchopulmonary segments. In addition, in this latter scenario, the ratio of the external diameter of the endotracheal catheter to the cross-sectional area of the smaller bronchi is greater and thus there is an increased resistance to exhalation.10,14 Furthermore, most anaesthetised patients show partial or complete obstruction of the oropharyngeal and/or nasopharyngeal airways, leading to impairment of passive exhalation and air trapping with consequent barotrauma.13 In the current study, air trapping was not observed. One explanation is that we used much lower driving pressures than those used during jet ventilation, and we confirmed that the placement of the ECFC was appropriate. Because the outer diameter of the ECFC is much smaller than the diameter of the trachea, expiratory flow from the trachea, around the ECFC, was essentially unobstructed. It is also possible that compared with a human, the pharynx of the pig is structurally more resistant to collapse20 The ECFC may also function as a stent and prevent the upper airway from complete collapse.
When using any kind of mechanical ventilation monitoring, its effectiveness and safety is critically important. Recent advances in technology enable clinicians to monitor ventilator parameters noninvasively and accurately21 even when using an ECFC (respiratory rate, I:E ratio, tidal volume and minute ventilation). However, the ECFC used was a prototype that was not equipped with a separate channel to allow the measurement of airway pressure or end-tidal carbon dioxide (ETCO2). Despite the ability to monitor the ventilator parameters, the inability to monitor airway pressure and ETCO2 is a disadvantage of ventilation via the prototype ECFC. Further research with our next-generation prototype, with a separate channel to allow the measurement of airway pressure and ETCO2, is needed.
There are several other limitations to this study. First, the study was not conducted on humans but on human-sized apnoeic animals. However, the mechanics of the lung and size of the trachea were similar to those of an adult human. Therefore, the results obtained from this study are potentially clinically meaningful. Second, we did not evaluate the long-term efficacy and functionality of the ECFC. It is unknown whether a longer duration of ventilation will alter the findings. Mainly due to time restriction, as the animals used for this study were also being used for an additional study, we tested only five breaths at each ventilator setting, using only the last three for analysis. Because we tested a total of 12 scenarios, three driving pressures (25, 50 and 70 cmH2O), two I:E ratios (1 : 1 and 1 : 2) and two different sized ECFCs (14 and 19 Fr), we were unable to evaluate the performance of an ECFC over a longer time period. However, the primary goal of this study was to test the feasibility and efficacy of an ECFC and to provide proof of concept. We did not observe any safety issues, and clinically meaningful tidal volumes were generated as shown by our results. An outcome study with a longer duration of ventilation via the ECFC is needed. Third, we only evaluated expired tidal volume. No other parameters, such as airway pressure, inspiratory/expiratory flow, end-tidal CO2 or blood gas analysis, were evaluated. However, from our bench study with a lung model,15 we found that the airway pressure distal to the ECFC is less than 20 cmH2O even when the driving pressure was 70 cmH2O. Fourth, we did not evaluate the risk of aspiration during ventilation with the ECFC, but because there is no inflow around the cuff during inhalation or exhalation, and exhalation occurs around the cuff, secretions are more likely to be forced upwards towards the upper airway rather than pushed down towards the lung. This latter is the operational principle underlying the use of an ECFC. Theoretically, this feature may minimise aspiration, but this remains to be tested. Fifth, we did not evaluate the performance of ECFCs in states of low lung compliance. We believe that ventilation via an ECFC can generate reasonable tidal volumes, but airway pressures will reach higher levels in states of low lung compliance than with normal lung compliance. Again, this is an aspect in need of further investigation. Finally, we did not test an ECFC smaller than 14 Fr or greater than 19 Fr. It is unknown whether our observations will remain true with different ECFC sizes. In addition, we did not evaluate different sized cuffs, different sized holes in the ECFC for cuff inflation or different cuff material. Before recommending this technique be used clinically, further study is required to optimise cuff design and the appropriate selection of cuff material to maximise the efficacy of ventilation.
In conclusion, our results show that, in the short term, it is feasible, well tolerated and effective to use an ICU ventilator in pressure-control mode combined with an ECFC to ventilate a human-sized animal with clinically relevant tidal volumes via coaxial ventilation. Further investigation is required to determine whether this technique can produce well tolerated and effective ventilation for longer periods of time in human-sized animals, as well as in humans. There is also a need to assess whether this new approach to coaxial ventilation, with expiratory flow through the natural airway, has advantages over conventional ventilation.
Acknowledgements relating to this article
Assistance with this study: none.
Financial support: departmental funding.
Conflicts of interest: RMK is a consultant for Covidien and has been in receipt of a research grant from both Covidien and Venner Medical.
Presentation: preliminary data from this study were presented at the annual meeting of the American Society of Anesthesiologists, 11 to 15 October 2014, New Orleans, LA, USA.
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