Drs Grace SM Lam and Arthur CW Lau; Department of Intensive Care, Pamela Youde Nethersole Eastern Hospital. Also published in the Hong Kong Thoracic Society Newsletter (in press)
Comparison of various ETT
“Ventilator-Associated Pneumonia (VAP)”, or the more recently suggested term “Ventilation-Acquired Pneumonia”, is the commonest nosocomial infection and major cause of morbidity and mortality in the intensive care unit (ICU). VAP occurs by at least four mechanisms: (1) aspiration of microbe-laden secretion of oropharyngeal or gastric origin that has accumulated in the subglottic region above the cuff of an endotracheal tube (ETT) – the predominant mechanism; (2) inhalation of contaminated air or medical aerosols; (3) direct extension from neighboring pleural or intra-abdominal infection; and (4) haematogenous spread of distant infection. This article focuses on how some novel designs of ETT may contribute to VAP prevention.

Factors related to leakage and aspiration across ETT cuff
In addition to centering the ETT in the trachea to prevent trauma by the ETT tip, ETT cuffs provide a seal that minimizes leakage during positive pressure ventilation and gross aspiration of oropharyngeal secretions. However, even at the recommended cuff pressure of 20 to 30 cm H2O, the seal is not perfect. When fully inflated, a conventional high volume low pressure (HVLP) cuff has a diameter 1.5 - 2 times larger than an average adult trachea. The excess cuff material folds on itself and forms channels through which secretion accumulated above the cuff can seep through. This occurs especially when the accumulated volume is large, when cuff pressure is inadequate, when patient’s position changes, or when there is a negative pressure below the cuff at times of intense patient inspiratory effort or tracheal suctioning. Leakage is also affected by ventilator pressure settings. For example, Pitts et al demonstrated an inverse relationship between cuff leakage and the level of positive end-expiratory pressure (PEEP) and peak inspiratory pressure (PIP). PEEP is the predominant factor and at a PEEP level of ≥ 15cm H2O, increasing PIP no longer affected fluid leakage.1 The prophylactic application of 5 to 8 cm H2O of PEEP compared to zero PEEP was found to significantly reduce the incidence of both clinically suspected and microbiologically confirmed VAP in a prospective randomized controlled trial in non-hypoxaemic patients.2

Novel ETT designs
A comparison of various commercially available ETT is shown in Table 1.
1. Cuff contour
Most conventional cuff is cylindrical. Newer designs are conical or tapered such that the chance of having at least one point along the length of the cuff that fits perfectly into the trachea with no folds or channels formed is increased.

2. Cuff material
ETT cuff is most commonly made of polyvinyl chloride (PVC). Using polyurethane (PU), the cuff can be made much thinner (10 microns in comparison to 50 to 80 microns in PVC). This ultrathin design reduces the formation of large channels such that even when excess cuff material folds, it allows sealing of the trachea at a cuff pressure of 15 cm H2O or lower.3

In an in-vitro experimental setup that compared fluid leakage across commercially available ETT cuffs, Zanella et al demonstrated that the sealing properties improved in the order of cylindrical PVC cuff, tapered PVC cuff, and PU cuff regardless of its shape.4 In a recently published retrospective study, Miller et al compared the rates of VAP in their ICUs before and after replacement of the conventional PVC ETTs with PU ETTs.5 The use of PU ETTs was associated with a 43% decrease in the rate of VAP after adjusting for non-systematic changes in infection control measures over the study period. Given the limited evidence supporting translation of experimental findings to clinical outcomes, current expert opinions classify the use of ultrathin PU and tapered ETTs as “worth considering”.6

3. Subglottic suction
ETT with subglottic secretion drainage (SSD) has an independent channel that leads to a dorsal opening above the cuff through which continuous or intermittent suction can be applied to remove accumulated subglottic secretion. A meta-analysis7 showed that SSD appeared effective in preventing early-onset ventilator-associated pneumonia among patients expected to require >72 hours of mechanical ventilation.

In actual practice, however, suction through the subglottic suction port has been reported to fail in 34 to 48% of intubated patients. Obstruction can be due to viscous secretion if suction force is inadequate, but tracheal mucosa could block the dorsal lumen if suction is excessive. Differences in the geometry of suction channels among commercially available ETTs result in different suction performances.8 Manufacturers recommend the use of either a continuous low pressure suction not exceeding 20 mmHg, or intermittent suction for 10 to 15 seconds using a pressure of 100 to 150 mmHg. If obstruction occurs, sterile water can be instilled or air bolus injected through the subglottic lumen.

4. Silver-coated endotracheal tubes
Silver-coating on an endotracheal tube has been designed to reduce VAP incidence by preventing bacterial colonization and biofilm formation. NASCENT was a large prospective, randomized, single-blind, controlled study conducted in 54 centers in North America in patients expected to require mechanical ventilation for 24 hours or longer.9 It was found that rates of microbiologically confirmed VAP (quantitative bronchoalveolar lavage fluid culture of ≥ 104 colony-forming units/ml) were 4.8% (37/766 patients) in the group receiving the silver-coated tube and 7.5% (56/743) in the group receiving the uncoated tube (P = .03). The silver-coated endotracheal tube was associated with delayed occurrence of VAP. However, duration of mechanical ventilation, ICU stay, and hospital stay were not altered, and there was even a slight trend towards increased mortality in patients randomized to the silver-coated ETT (30.9% vs 27.3%, p = 0.08).

5. Mucus Slurpers and Shavers
These devices are designed to reduce mucus buildup and biofilm formation in the lumen of the ETT. The Mucus Slurper is an ETT with suction ports arranged radially around the tip of the tube to prevent entry and buildup of tracheal secretions in the ETT lumen.10 The Mucus Shaver is an inflatable silicone-rubber “razor” that is introduced into the lumen of the ETT to allow cleaning or “shaving” of material built up within the lumen. 11 There is as yet no published clinical data in humans.

Commercially available novel ETT designs to reduce VAP include improvements in cuff material and contour, addition of subglottic suction channel and silver-coating. However, it is unlikely that any of the above can be an effective stand-alone measure without institution of other important VAP preventive measures including shortening the duration of invasive mechanical ventilation, promoting the use of non-invasive ventilation, head-of-bed elevation to a semi-recumbent position (30 to 45o), antiseptic (e.g. chlorhexidine) oral rinse, airway humidification, maintenance of adequate cuff pressure, etc.12 More adequately powered clinical and cost-effectiveness studies of these novel ETTs are needed before wider use can be definitely recommended.

1. Pitts R, Fisher D, Sulemanji D, et al. Variables affecting leakage past endotracheal tube cuffs: a bench study. Intensive Care Med 2010; 36: 2066-2073.
2. Manzano F, Fernandez-Mondejar E, Colmenero M, et al. Positive-end expiratory pressure reduces incidence of ventilator-associated pneumonia in nonhypoxemic patients. Crit Care Med 2008; 36: 2225-2231.
3. Dullenkopf A, Gerber A, Weiss M.Fluid leakage past tracheal tube cuffs: evaluation of the new Microcuff endotracheal tube. Intensive Care Med. 2003 Oct; 29(10): 1849-53. Epub 2003 Aug 16.
4. Zanella A, Scaravilli V, Isgro S, et al. Fluid leakage across tracheal tube cuff, effect of different cuff material, shape, and positive expiratory pressure: a bench-top study. Intensive Care Med 2010 Feb; 37(2): 343-7.
5. Miller MA, Arndt JL, Konkle MA, et al. A polyurethane cuffed endotracheal tube is associated with decreased rates of ventilator-associated pneumonia. J Crit Care 2010 Jul 22. [Epub ahead of print].
6. Lorente L, Blot S, Rello J. New Issues and Controversies in the Prevention of Ventilator-associated Pneumonia. Am J Respir Crit Care Med 2010; 182: 870–876.
7. Dezfulian C, Shojania K, Collard HR, et al. Subglottic secretion drainage for preventing ventilator-associated pneumonia: a meta-analysis. Am J Med 2005; 118: 11–18.
8. Mujica-Lopez KI, Pearce MA, Narron KA, et al. In vitro evaluation of endotracheal tubes with intrinsic suction. Chest 2010; 138(4): 863-869.
9. Kollef MH, Afessa B, Anzueto A, Veremakis C, Kerr KM, Margolis BD, Craven DE, Roberts PR, Arroliga AC, Hubmayr RD, Restrepo MI, Auger WR, Schinner R; NASCENT Investigation Group. Silver-coated endotracheal tubes and incidence of ventilator-associated pneumonia: the NASCENT randomized trial. JAMA. 2008 Aug 20; 300(7): 805-13.
10. Li Bassi G, Curto F, Zanella A, Stylianou M, Kolobow T. A 72-hour study to test the efficacy and safety of the "Mucus Slurper" in mechanically ventilated sheep. Crit Care Med. 2007 Mar; 35(3): 906-11.
11. Kolobow T, Berra L, Li Bassi G, Curto F. Novel system for complete removal of secretions within the endotracheal tube: the Mucus Shaver. Anesthesiology. 2005 May; 102(5): 1063-5.
12. Scientific Committee on Infection Control, and Infection Control Branch, Centre for Health Protection, Department of Health. Recommendations on Prevention of Ventilator-associated Pneumonia. http://www.chp.gov.hk/tc/guideline1/346/365.html. Accessed on 5th Feb 2011.