Double Lumen Endotracheal Tube for Percutaneous Tracheostomy

Discussion

This is the first in vitro study assessing the physical properties of a DLET designed for use during percutaneous tracheostomy. Our primary finding can be summarized as follows: in a model airway for percutaneous tracheostomy, the DLET with fiberoptic bronchoscope has a lower K₂ Rohrer constant during continuous flow and a lower resistance to gas flow during simulated mechanical ventilation, than other ETTs with a fiberoptic bronchoscope in place and translaryngeal airways.

Percutaneous tracheostomy as is currently performed in the ICU is not without risks.¹ Different methods, such as laryngeal mask airways or pediatric airways, have been proposed to improve airway management, ventilation, and procedural comfort during PDT, minimizing complications such as hypoventilation and endoscopic interference.⁸ However, none of these techniques are ideal.

K₂ is an indicator of the degree of patency and thus the magnitude of ETT obstruction.⁷ In our study, the DLET had a lower value for K₂ than other devices commonly used for PDT, thus resulting in adequate airway patency and minimal obstruction. This specific characteristic of the DLET may be explained by several factors: (1) the inner diameter of the DLET is larger than the ETT with fiberoptic bronchoscope, and (2) inner diameter and cross-sectional area of the DLET were larger than F₄ and F₅ with a shorter length. Furthermore, the K₂ of F₄ was higher than any other airway devices used for PDT, and F₄ could not be evaluated for flows > 60 L/min.

Pressure drop has been used to describe ETT resistance.⁹ In our study, the DLET also had a lower pressure drop than all ETTs with fiberoptic bronchoscope, thus ensuring a lower resistance to air flow. Furthermore, DLET had a pressure drop similar to a 7 mm inner diameter ETT without fiberoptic bronchoscope. Because pressure drop across a tube is a direct measurement of the change in pressure throughout the entire tube,⁹ we believe that pressure drop across the DLET was similar to a 7 mm inner diameter ETT, as opposed to a 7.5 mm inner diameter ETT, because of (1) the elliptical shape of its distal section and (2) the minimal compression exerted by the fiberoptic bronchoscope on the lower channel.

During PDT, the presence of a bronchoscope reduces the ETT diameter available for patients' ventilation. ETT diameter is a critical factor determining airway resistance.⁹ The presence of a bronchoscope in the ETT further reduces the cross-sectional area, causing a partial obstruction to inspiratory and expiratory flow.¹⁰ In our experimental setting, the cross-sectional areas of ETTs with fiberoptic bronchoscope was reduced by 82% for 7f, 71% for 7.5f, and 66% for 8f, whereas the cross-sectional area of DLET remained stable. ETT resistance increases when the ETT inner diameter is reduced.¹¹ During the performance of flexible bronchoscopy through an ETT in patients receiving mechanical ventilation, air flow resistance across the ETT dramatically increases from 2- to 63-fold.³ During PDT, whether using an ETT or laryngeal mask airway, the airway resistance increases by introducing a fiberoptic bronchoscope, resulting in a decrease in minute ventilation and hypercapnia.² According to Reilly et al,¹² the use of fiberoptic bronchoscope during PDT is the most important factor responsible for the hypercapnia developed during the tracheostomy procedure. To minimize airway obstruction, it has been recommended to perform fiberoptic bronchoscopy through an ETT of ≥ 8 mm inner diameter, or with an ETT for which inner diameter is at least 2 mm larger than the external diameter of the bronchoscope.¹³ In our model of mechanical ventilation, although the size difference between ETT inner diameter and bronchoscope external diameter was > 2 mm, the DLET had a lower inspiratory and expiratory resistance compared with ETTs with fiberoptic bronchoscope and translaryngeal airways. This is because the bronchoscopy is performed through a dedicated lumen without affecting the mechanical ventilation flow. The DLET, allowing ventilation independent from the use of fiberoptic bronchoscopy, may allow a more complete inspiration and expiration without imposing an excessive increase in airway resistance compared with the other devices commonly used for PDT.

Bronchoscope insertion through an ETT increases peak airway pressure in volume and pressure controlled ventilation, although the peak ventilator pressure does not accurately reflect the alveolar peak pressure.^13,14 In our study, ventilator P_peak was higher in VC-CMV than in PC-CMV; however, DLET had the lowest P_peak regardless of ventilation mode during fiberoptic bronchoscopy.

P_mean is used to clinically assess mean alveolar pressure and is a major determinant of oxygenation and hemodynamic compromise during mechanical ventilation.^15,16 Our data show that P_mean was higher in ETTs with bronchoscopy, and in F₄ and F₅ compared with DLET during VC-CMV and PC-CMV.

PEEPi has been reported to increase by 105% after bronchoscope insertion, although this is not detected by ventilator sensors.¹³ The obstruction offered by the bronchoscope in the airway limits the expiratory and inspiratory flow resulting in volume trapped (incomplete exhalation) within the lung and increasing PEEPi.¹⁴ In a study by Lawson et al,¹⁴ at a compliance of 50 mL/cm H₂O, the insertion of a bronchoscope elevated the PEEPi from 0 to 6 cm H₂O. In contrast, Lindholm et al¹⁷ reported that PEEPi increased dramatically when the bronchoscopy was performed through an ETT < 8 mm inner diameter. PEEPi may compromise ventilation and hemodynamics in critically ill patients, worsening gas exchange and reducing cardiac output.¹⁸ PEEPi with DLET was lower than with the other airway devices commonly used for tracheostomy in the ICU, but similar to ETTs without bronchoscopy. This finding suggests that DLET during tracheostomy for critically ill patients does not produce any increased expiratory obstruction to flow as with the other airway devices. It is possible to decrease PEEPi by a further reduction of minute ventilation, leading to hypercapnia and acute respiratory acidosis, which may be contraindicated in critically ill patients.¹² The combination of DLET and reduction in minute ventilation can further decrease PEEPi, in cases of severe airway obstruction.

DLET may offer several clinical advantages over ETTs and laryngeal mask airways with bronchoscopy, during PDT. We hypothesize that using DLET during PDT (1) may ensure airway control avoiding the risk of accidental extubation and protection of the distal airway and lung parenchyma from bleeding and aspiration owing to the presence of a distal cuff and (2) may protect the fiberoptic bronchoscope from accidental damage as a result of puncture or inappropriate dilation by the presence of the upper tube. With the DLET in place, the posterior tracheal wall is protected during tracheal puncture. During the dilation and cannulation phase of PDT, the DLET should be withdrawn and positioned just cephlad of the site of puncture if the tracheal diameter is limited. However, in many patients, initial dilation can be accomplished with the DLET in place.

However, the use of the DLET implies an exchange of a preexisting ETT. In 2 surveys, it was reported that the ETT in place was replaced by 17% and 8% of respondents before PDT.^19,20 During translaryngeal tracheostomy, patients are frequently intubated twice, first with the tracheoscope and then with a thin ventilation tube.²¹ We hypothesize that, in clinical practice, the replacement of the ETT with the DLET may be safely performed with an appropriate tube exchanger. Furthermore, the DLET should be carefully used in the presence of a difficult airway and in the presence of increased intracranial pressure.

The use of the DLET in a clinical setting of PDT remains to be studied; a clear strategic plan on when and how to use this device is required and should be evaluated further during an in vivo study.