This article is accompanied by the following Invited Commentary:
Hinkelbein J, Greif R, Diemunsch P, Kranke P. Publication and innovation in airway management: quality not quantity! Eur J Anaesthesiol 2017; 34:408–410.
Ventilator-associated pneumonia (VAP) is a type of pneumonia acquired in the ICU that develops more than 48 h after tracheal intubation. The occurrence of a single episode of VAP not only increases the duration of ventilation but also ICU and hospital stay. VAP is associated with a high attributable mortality and adds significant cost to an ICU admission. VAP will also lead to an increased use of specific antibiotics, thus enhancing the risk of developing multi-drug-resistant bacteria.1,2
A compelling argument can be made that the presence of the tracheal tube increases susceptibility to VAP because of leakage around the cuff and subsequent micro-aspiration of bacteriologically contaminated secretions alongside voids in the cuff seal, created by cuff folds that occur even in fully inflated cuffs.3 Several benchtop models demonstrated convincingly that the single factor with the greatest impact on leakage past a tracheal tube cuff was the positive end-expiratory pressure (PEEP).4–6 Any significant drop or loss of PEEP, either accidentally or because of specific ICU procedures (e.g., preparing for extubation, tracheal suctioning) may enhance the inflow of secretions.
We designed a prototype tracheal tube which prevents leakage of secretions by creating ‘pressurised sealing’ of the airways between two cuffs. We assessed the efficacy of this novel tracheal tube in a benchtop model and compared it with four tracheal tubes in current use which differ in cuff type or shape.
The prototype tracheal tube
The prototype tracheal tube was manufactured by SUMI Company, Poland and the technical details are depicted in Fig. 1. Simply, this tracheal tube is double cuffed and equipped with a supplementary port that opens into the tracheal lumen between the two connected polyvinylchloride (PVC) cuffs. Through this port, a continuous positive pressure of 5 cmH2O is delivered and maintained by an external device (Goodknight 418A, Mallinckrodt, St Louis, Missouri, United States). The continuous positive pressure between the two cuffs creates a protective ‘shield’ preventing penetration of fluid from above the proximal inflated cuff. If the cuffs are slightly deflated, the ‘pressure seal’ results in an immediate pressure-induced upward movement of fluid.
We used the in-vitro benchtop trial model described by Dave et al.7 (Fig. 2): a tracheal tube is placed in an artificial trachea and the tracheal sealing properties of the inflated tracheal tube cuff are tested under static (gravitational, unventilated) or dynamic [PEEP alone or positive pressure ventilation (PPV) with PEEP] conditions. The prototype double-cuffed tracheal tube (PVCdc) was compared with four commercial tracheal tubes equipped with different cuff types and shapes: cylindrical-shaped PVC (PVCcyl; Portex Soft-Seal, Smiths Medical, Hythe, Kent, United Kingdom); conical-shaped PVC (PVCcon; Taperguard, Covidien, Boulder, Colorado, United States); cylindrical-shaped polyurethane (PUcyl; Microcuff, Kimberley-Clark, Roswell, Georgia, United States) and conical-shaped polyurethane, (PUcon; Sealguard, Covidien, Boulder, Colorado, United States; Table 1). The choice of the comparator tracheal tubes was not influenced by any relationship or conflict of interest with the respective manufacturers. Tubes were randomly tested by the same investigator (E.S.) and positioned with the lower cuff border 2.5 cm above the lower edge of a vertically upright acrylate trachea. The artificial trachea had an internal diameter of 18 mm and a reservoir or calibrated syringe connected to its lower end to collect and quantify leakage. The cuffs were inflated to a pressure of 25 cmH2O and this pressure was kept constant by a digital cuff pressure manometer (VBM cuff controller, VBM Medizintechnik GmbH, Sulz am Neckar, Germany). Any cuff leak (i.e., >1 cmH2O pressure drop) was detected immediately by an in-built alarm system and compensated automatically. Total 3 ml of blue-dyed water was placed above the cuff. In the static model (Fig. 2a), onset of leakage trickling alongside the cuff was visually assessed and recorded in seconds from when the dye as added until leakage was seen. Leakage volume was measured every 30 s up to 60 min or until all liquid was collected. Leakage flow across the cuff was calculated by dividing the collected liquid volume by either 60 min or by the time at which the entire 3 ml had leaked. In the dynamic models (Fig. 2b), the tracheal tubes were connected to a test lung (Siemens Maquet Adult 1 Litre Ventilator Test Lung, Siemens Healthcare, Erlangen, Germany) and ventilator (Dräger Evita XL, Lübeck, Germany). Two dynamic tests were performed using either 5 cmH2O PEEP alone or PPV plus 5 cmH2O PEEP. PPV settings were a peak inspiratory pressure of 22 cmH2O, I : E ratio 1 : 2 and respiratory rate 12 ‘breaths’ min−1. Time to onset of leakage, volume and flow were assessed in the same way as in the static experiment. At the end of the 60-min dynamic tests either PEEP was zeroed (when evaluating PEEP alone) or the ventilator was disconnected (when evaluating PPV + PEEP). A further period of observation then continued for up to 30 min or until leakage past the tracheal tube cuff was observed. Six tests were conducted for each combination of tracheal tube type/test condition, using a new tracheal tube for each test. In total, 90 tests were performed with the five types of tracheal tube. The artificial trachea was rinsed with sterile 0.9% normal saline between each test. Measurements were performed at 22°C room temperature.
For statistical analysis, IBM-SPSS version 22.0 for Windows (SPSS, Chicago, Illinois, United States) was used. Pre-experimental sample size calculation was not performed because reference data on ‘measured leakage volume’ were not available. Comparisons between tracheal tubes were performed with the Kruskal–Wallis test followed by pairwise Mann–Whitney U tests. Data are presented as medians (range). Statistical significance was accepted at a P value < 0.05.
In the static model, leakage started earlier with PVC cuffs compared with polyurethane cuffs: 2 (1 to 3) vs. 60 (33 to 136) s, PVCcyl (Portex Soft-Seal) vs. PUcyl (Microcuff) P = 0.002 and 5 (3 to 37) vs. 76 (25 to 720) s, PVCcon (Taperguard) vs. PUcon (Sealguard) P = 0.009. The leakage times were also statistically significantly different between the PVC cylindrical and conical cuffs (P < 0.004) but not between the polyurethane cylindrical and conical cuffs (P < 0.09). During the 1-h study period, the PVC cuffs with conical form allowed less dye to pass than the cylindrical ones: 9.8 (6.2 to 20) vs. 1.3 (0.2 to 3.8) ml min−1, PVCcyl (Portex Soft-Seal) vs. PVCcon (Taperguard) P = 0.004). No difference in leakage flow was noted between cylindrical and conical PU cuffs: 0.03 (0.007 to 0.1) vs. 0.04 (0.003 to 0.2) ml min−1, PUcyl (Microcuff) vs. PUcon (Sealguard) P = 0.94. Regardless of the cuff form, the polyurethane material consistently protected better against leakage than PVC: P < 0.004 for the difference between PVC and polyurethane cylindrical tracheal tubes and P < 0.005 for the difference between PVC and polyurethane conical cuffs. No leakage flow [0.0 (0.0 to 0.0) ml min−1] was observed when the PVCdc tracheal tube was used (P < 0.001, PVCdc vs. all other cuffs).
In the dynamic setting, no leakage was detected for up to 60 min with any of the cuffs studied. Only after loss of PEEP or tracheal tube disconnection did dye inflow occur alongside all cuffs except for the PVCdc (P < 0.001, PVCdc vs. all other cuffs). Leakage was rapidly detected with the PVC-cuffed tubes, but occurred significantly later with polyurethane cuffs: 2 (1 to 4) vs. 55 (15 to 120) s, PVCcyl (Portex Soft-Seal) vs. PUcyl (Microcuff) and 3 (2 to 5) vs. 114 (36 to 264) s, PVCcon (Taperguard) vs. PUcon (Sealguard); both P = 0.002 but was not influenced by cuff shape: P = 0.16 and P = 0.26 for comparisons between PVC cuffs and between polyurethane cuffs, respectively.
As regards, the prevention of leakage in a static trachea model, the prototype tracheal tube outperformed all the commercially available PVC and polyurethane-cuffed tracheal tubes studied. Under dynamic conditions, although PEEP guaranteed adequate sealing in all tracheal tubes, only the prototype tube protected against leakage after release of PEEP or tracheal tube disconnection.
The two most important mechanisms implicated in the development of tracheal tube-related VAP are biofilm formation inside the lumen of the tracheal tube and micro-aspiration around the cuff.8 Interventions to prevent biofilm formation include either antiseptic coating of the tracheal tube or mechanical removal of the biofilm. Within this context, the largest study to date demonstrated a decrease in microbiologically proven VAP in patients intubated with a silver-coated tracheal tube compared with a standard tracheal tube. However, no difference was observed in relevant outcome parameters such as time on the ventilator, length of ICU stay or mortality. Also, a worrisome, and unexplained, trend toward a higher mortality in patients randomised to the silver-coated tracheal tube was noticed.9
Experimental and clinical research has focused extensively on the prevention of airway intrusion by bacteriologically contaminated secretions that accumulate above the inflated cuff of the tracheal tube. Suggested solutions to decrease or avoid leakage and subsequent aspiration include preservation of a constant pressure (20 to 30 cmH2O) inside the cuff, the use of specific cuff materials (e.g., polyurethane) or shapes (e.g., conical) and tracheal tubes equipped with a suction lumen for aspiration of subglottic secretions.10
Madjdpour et al.11 demonstrated better air-sealing properties of conical-shaped vs. cylindrical-shaped cuffs as well as superior sealing characteristics of PU compared with PVC material. With regard to preventing fluid ingress past tracheal tube cuffs, our study results in particular under static conditions, point in the same direction and may add theoretical support for the preferential use of tracheal tubes with conical-shaped polyurethane cuffs. However, the clinical advantage of such tracheal tubes is less convincing. Poelaert et al.12 reported a lower incidence of early pneumonia in after cardiac surgery patients intubated with a conical polyurethane-cuffed tube. However, diagnosis of pneumonia was based on clinical criteria and lower airway cultures were positive in only a fraction of the study population. In a small cohort of critically ill patients, Bulpa et al.13 used scintigraphy to detect micro-aspiration alongside cuffs and found that a conical polyurethane-cuffed tracheal tube did not perform better than a PVC-cuffed one. In addition, we recently demonstrated that polyurethane-cuffed tracheal tubes were prone to internal condensation causing false manometric cuff pressure readings.14 Finally, a recent study showed that polyurethane and/or conically shaped cuffs were not superior to PVC and/or cylindrical cuffs in preventing tracheal colonisation and VAP.15
Subglottic secretion drainage (SSD) is recommended as part of many VAP prevention protocols. This recommendation is based on a recent meta-analysis showing that SSD resulted in a 50% reduction in the risk of acquiring VAP. Although SSD may shorten the duration of mechanical ventilation and ICU length of stay, it does not improve survival.16 However, other VAP prevention measures were not specified in 11 of the 13 studies analysed. Also, in one17 of the four studies marked as of ‘high methodological quality’, a polyurethane cuff was used in addition to SSD, and this prevents evaluation of their respective roles as a VAP preventive measure. In addition, SSD is not always effective and is not without risk. Rello et al.18 reported SSD failure in up to one-third of patients, whereas Girou et al.19 noted laryngeal oedema necessitating re-intubation in 25% of patients who had received continuous SSD. Data from animal models show convincingly that SSD produces tracheal injury in areas immediately adjacent to the subglottic suction port, and this injury did not recover within up to 96 h following extubation.20 We,21 and others22,23 have shown in vivo that tracheal injury occurs when mucosa is aspirated into a suction port. Recently, a tracheal tube equipped with SSD, that had provided excellent results with regard to VAP prevention,24 had to be withdrawn from the market because of repeated tube kinking which resulted in inadequate ventilation causing severe, including hypoxic injury.25
In conclusion, in terms of preventing leakage, the novel PVCdc tracheal tube outperformed all the PVC and polyurethane-cuffed comparator tracheal tubes. The sealing effect persisted after acute release of PEEP or tracheal tube disconnection. Clinically, the above may translate into a 100% prevention of micro-aspiration of the bacteria-enriched secretions which accumulate above the cuff, hence, substantially reducing the risk of patients developing ventilator-associated infection. This new tracheal tube also counters the intrusion of secretions into the lower airways in any situation or procedure characterised by abrupt loss of PEEP: for example, during an extubation procedure partial deflation of the cuffs combined with a momentary increase in the pressure between the cuffs will move secretions upward toward the oropharynx where they can be easily aspirated.
Some limitations of our study must be acknowledged. First, we used a vertically mounted benchtop rigid trachea model at room temperature which does not match the clinical situation where a tracheal tube is positioned in a more compliant trachea at body temperature in a semi-recumbent position. Second, subglottic secretions and saliva have higher viscosities compared with the liquid dye used in the study, and hence their potential for leaking past the cuff is less. Third, the study results were obtained over only a 60-min observation period, whereas VAP occurs after at least 48 h of mechanical ventilation. Nevertheless, our findings remain relevant because even a small bacterial load passing the cuff will colonise the trachea and bronchial tract. Subsequent bacterial growth and increasing inflammation then may cause tracheobronchitis, an important risk factor for VAP.26 Fourth, tracheal suctioning was not performed in our model but is unlikely to influence leakage as the inter-cuff pressure is kept constant. Finally, in contrast with the comparator tracheal tubes, the long-term in-vivo effects of a double-cuffed tracheal tube have not been investigated. Whether the advantages of the prototype tracheal tube seen in our benchtop study will translate in to clinical advantages will need to be investigated by an appropriate clinical trial.
Acknowledgements relating to this article
Assistance with the study: none.
Financial support and sponsorship: the study received a research grant from the ‘Wetenschappelijk Fonds Willy Gepts’ of the University Hospital, Vrije Universiteit, Brussels.
Conflicts of interest: none.
Presentation: the study was presented in part as an oral communication at the 33rd Congress of the Scandinavian Society of Anesthesiology and Intensive Care Medicine, 10 to 12 June 2015, Reykjavik, Iceland.
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