Introduction

Endotracheal tube (ETT) cuffs were initially made of thick (500 μm) Hevea latex rubber and required a high inflation pressure (200–400 cmH2O) to form an adequate tracheal seal [1]. The pressure transmitted to the tracheal wall was difficult to estimate from the intracuff pressure, and consequently overinflation of the cuff was common. In some studies tracheal wall pressures were as high as 200 cmH2O, greatly above the tracheal mucosal capillary pressure about 30 cmH2O [2, 3]. Widespread use of these low-volume high-pressure cuffs resulted in frequent tracheal injury [2, 3]. In the 1970s disposable ETT cuffs made of polyvinylchloride (PVC), termed high-volume low-pressure (HVLP) inflatable cuffs, were introduced to overcome this problem, and these have remained the standard until today. PVC cuffs are inelastic and 1.5–2 times larger than the internal diameter (ID) of the trachea. The HVLP cuff fills the trachea without being stretched, transmitting the intracuff pressure entirely to the tracheal wall. When inflated at 30 cmH2O, the HVLP cuff permits mechanical ventilation and preserves tracheal capillary perfusion but invariably forms multiple longitudinal folds. Bacteria colonize oropharyngeal secretions, and gastric contents can leak along the folds into the lower airways and the lungs [4–6], a major risk factor for ventilator-associated pneumonia (VAP) [7].

We have designed, manufactured, and tested a prototype ETT cuff, created by covering a standard HVLP ETT cuff with a thin, high-compliance, low-protein guayule latex rubber cuff. As with traditional HVLP cuffs this prototype requires low-inflation pressure to fill the trachea; however, the stretching of the compliant latex covering during inflation eliminates folds, assuring a perfect seal. This cuff design may therefore prove beneficial in the prevention of ischemic damage to the trachea and the leakage of colonized secretions into the lower airways.

Materials and methods

Latex prototype ETT cuff

The guayule latex cuff is cylindrical in shape (13 mm diameter) and 50–60 μm thick (Yulex, Maricopa, AZ, USA; Fig. 1). We draped the guayule latex cuff over the Mallinckrodt Hi-Lo Evac (ID 8 mm) ETT cuff (Mallinckrodt, NY, USA) and introduced 0.5 ml sterile gel (Surgilube, Altana Inc., Melville, NY, USA) between the two cuffs to facilitate a homogeneous distribution of inflation pressure and to reduce friction between the cuffs. The outer latex cuff was secured on both ends using silk ligatures.

Fig. 1
figure 1

Left The guayule latex prototype cuff inflated at 20 cmH2O of pressure in a cylindrical glass tube (ID 20mm) shows no fold and no dye leaking. Right The guayule latex prototype cuff is a standard endotracheal tube with a high-volume low-pressure cuff, draped by a very thin, highly compliant guayule latex cuff; 0.5 ml gel is introduced between the two cuffs

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Leakage test

We tested fluid leakage across the cuff using the guayule latex ETTs and four commercially available ETTs: Euromedical (Euromedical, Malaysia), Mallinckrodt Hi-Lo Evac, Microcuff (Kimberly Clark, GA, USA), and Sheridan/CF (Hudson, NC, USA). Tests were performed using vertically positioned, cylindrical glass tubes, 20 cm long, of three internal diameters (16, 20, and 22 mm), matching the broad range of adult human tracheas [6]. All ETTs (ID 8 mm) were inflated at intracuff pressures of 20, 25, 30, 40, and 50 cmH2O. A small reservoir was positioned below the model trachea to collect water leakage. Then 15 ml water was poured above the cuff and observed until all water was collected or until the 2-h test period had ended. Leakage is reported as average flow across the cuff, calculated by dividing the volume of water collected by either 120 min or the time at which all 15 ml water had leaked. Three new ETTs of each type were tested by three different investigators. Three guayule latex prototype ETTs and three microcuff ETTs were similarly tested for 24 h, using an inflation pressure of 20 cmH2O and a 20 mm diameter model trachea.

Wall pressure measurement

Three new ETT cuffs of each type were inflated inside a glass tube (ID 22 mm) at four pressures (10, 20, 30, and 40 cmH2O). The pressure exerted against the internal wall of the glass tube was measured with a manometer connected to a flat, small (4 cm long, 1 cm wide), inelastic, partially inflated PVC cuff inserted between the ETT cuff and the glass tube.

Statistical analysis

Since we performed three observations per pressure/diameter block, we obtained an F approximation to the Friedman's test based on the generalized linear model using the within block ranks to study the overall effect of the different cuffs. The four pairwise comparisons of interest were performed only if the overall F test was significant at p < 0.05. Interactions of main effects were also investigated. Bonferroni's correction was applied. The volume of fluid leakage during the 24 h test was compared by the Wilcoxon test.

Results

Assessment of folds

The guayule latex prototype cuff inflated in all model tracheas showed no folds; however, folds always occurred with the four tested HVLP cuffs.

Assessment of leakage

The average leakage was 6.6 × 10−4 ± 2.5 × 10−3 ml/min with the guayule latex prototype ETT, 7.3 × 10−2 ± 9.3 × 10−2 ml/min with the Microcuff, 5.0 ± 4.7 ml/min with the Mallinckrodt Hi-Lo EVAC, 7.2 ± 4.4 ml/min with the Euromedical, and 41 ± 69 ml/min with the Sheridan/CF. The primary analysis results showed that the overall test of equality between the five cuffs is statistically significant (p < 0.0001). In all four paired comparisons the guayule latex cuff outperformed the other ETT cuffs (p < 0.0001). In a secondary analysis, adjusted for the significant variables (intracuff diameter and pressure), the guayule latex cuff also outperformed the other four cuffs (p < 0.0001). The latter analysis suggests that the interaction of ETT brand and diameter was significant. However, stratification by diameter analyses did not change the above conclusions. Figure 2 shows average leakage flow as a function of the intracuff pressure. The latex prototype cuff performed significantly better (p < 0.0001) than all others, followed closely only by Microcuff.

Fig. 2
figure 2

Average leakage flow (ml/min) across the five endotracheal tube cuffs at cuff pressures of 20, 25, 30, 40, and 50 cmH2O. Vertical bars Standard deviations. *p < 0.05 for the prototype guayule latex cuff vs. the commercial HVLP cuffs at the same pressure

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Twenty-four hour tests

The average volume of water leaked (ml) across the guayule latex prototype cuff was 0.9 ± 5 ml and across the Microcuff 14.1 ± 2.2 ml (p = 0.03).

Tracheal wall pressure

Figure 3 shows the relationship between intracuff pressure and the pressure transmitted to the model tracheal wall for each cuff. The guayule latex cuff exerted on average a wall pressure 7.0 ± 1.9 cmH2O lower than the intracuff pressure.

Fig. 3
figure 3

Relationship between intracuff pressure and average measured transmitted pressure to the wall of the tracheal model

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Discussion

We designed a novel ETT cuff which forms no folds when inflated in a model trachea and substantially highly reduced, almost eliminated, fluid leakage at inflation pressure as low as 20 cmH2O. In comparison, all commercial HVLP cuffs showed multiple folds and did not prevent fluid leakage, even at 50 cmH2O. The Microcuff cuff, made of thin (7 μm) polyurethane film, formed only small folds and thus performed significantly better than the PVC HVLP cuffs. Dullenkopf et al. [8] previously tested the Microcuff and reported leakage lower than that in our study. This difference may be due to the larger volume of fluid that we injected above the cuffs (15 ml vs. 5 ml) resulting in higher hydrostatic pressure. We also tested the cuff in three model tracheas with different diameters vs. the single 20-mm diameter model trachea in the Dullenkopf et al. study. Young et al. [6] developed a silicone rubber ETT cuff (LoTrach, UK) that produces no folds upon inflation and thus capable of preventing fluid leakage. We did not include the LoTrach in our report since it behaves differently than standard HVLP cuffs: to seal the trachea requires intracuff pressures above 90 cmH2O, which is partially transmitted to the tracheal wall [9] and is much higher than the inflating pressure used in our study.

Our ETT cuff differs from the others in that it uses a very thin outer layer of guayule latex to completely enclose a traditional HVLP cuff. Guayule latex rubber cuff is highly compliant, tear resistant [10, 11], requires low pressure to be stretched, allowing an almost complete transmission of the intracuff pressure to the tracheal wall, and relies on the mechanical support of the internal HVLP cuff to be uniformly expanded. Therefore our cuff creates a tight seal and prevents fluid leakage even at low pressures. Although 1–8% of the population has hypersensitivity to latex proteins, the guayule latex material used in our prototype cuff is expected to be well-tolerated by latex-sensitive persons. Latex produced from the guayule shrub contains very little protein and no epitopes that cross-react with type I latex allergy [12–14]. Additionally, guayule latex also provides a strong barrier against blood-borne pathogens [15].

In conclusion, our results confirm previous reports [4–6] that traditional HVLP cuffs do not adequately seal the trachea and contribute to leakage of bacteria-colonized oropharyngeal secretions and gastric contents into the lower airways, a major pathogenic pathway of VAP [7, 16]. The development of a leak-proof ETT is therefore a major step towards the prevention of VAP. While in vitro tests never perfectly simulate physiological conditions, we believe that our results are a valid reflection of the performance of these cuffs. Long exposure to stress, biological secretions, heavy friction, and low pH may cause the thin guayule latex rubber to deteriorate and thus reduce the efficacy of this prototype ETT cuff in preventing fluid leakage. Prolonged in vivo testing is required to evaluate this issue.