Assessment of Regional Lung Ventilation Distribution During Supraglottic and Subglottic Jet Ventilation by EIT.

Surgical procedures of the larynx and trachea present special challenges for surgeons and anesthesiologists. To enable an unrestricted view of the operating field, ventilation of the patient is not performed over an endotracheal tube but instead over a steel laryngoscope or a thin catheter with Jet ventilation (JV). This enables precision surgical work, good visibility of the operating area and the safe use of various laser types. Over the last decades, different modalities of JV for laryngo-tracheal surgery have been developed. The former techniques of the supraglottic approach were improved with simultaneously applied low-frequency jet ventilation additionally to the high-frequency jet ventilation, called superimposed high-frequency jet ventilation (SHFJV). The SHFJV was developed in 1990 and was applied in several clinical trials [1-4] In recent years, a number of new catheters have been developed for the trans laryngeal subglottic approach of JV, like the translaryngeal Hunsaker MonJet Ventilation catheter and its successor, the LaserJet catheter. These catheters though can be used only with the HFJV. They provide less vocal cord movement and precise surgical manipulation are possible on one side, but on the other the risk of barotrauma may result in cases of proximal obstruction of the catheter.

[5, 6] Although jet ventilation is an established method to replace patients breathing during otorhinolaryngeal surgery, research is still required on efficient lung protective ventilation in order to provide adequate carbon dioxide (CO2) elimination, adequate oxygenation and in preventing ventilator induced lung injury. Jet ventilation techniques and access routes The TwinStream jet ventilator (C. Reiner Corp, Vienna, Austria) is routinely used for high-frequency jet ventilation (HFJV) or SHFJV in case of tubeless jet laryngoscopy (laryngeal micro- and laser surgery) and tracheoscopy. The device consists of two simultaneously operating ventilation units which can be adjusted separately. The driving pressure of the device is 1.5-3 bar and respiratory rates of 10-900 per min can be provided. Supraglottic jet laryngoscope In SHFJV, jet ventilation of normal frequency and high frequency is conducted simultaneously and enables ventilation at two different pressure levels through the steel jet laryngoscope. It is equipped with two jet nozzles, which are placed at the distal end of the jet laryngoscope, with the low-frequency jet stream passing the distal cannula, and the high-frequency jet stream at the proximal cannula. A third cannula at the tip of the laryngoscope allows ventilation pressure monitoring. The jet laryngoscope is introduced by the surgeon, for examining and operating structures within the laryngotracheal system. Supraglottic JV causes more laryngeal movement induced by disturbances through the high frequent inspiratory air passing the operating field compared to the subglottic jet ventilation. [9] Furthermore it leads to greater dryness of the vocal cords. During supraglottic SHFJV the escaping exhaled gases vent blood and secretion outward, reducing the risk of fluid aspiration and of displacing cells of any entity down to the tracheobronchial tree.

Advantages of SHFJV with the supraglottic inserted steel laryngoscope:

• Unlimited surgical access and vision of the operating field
• Laser resistant
• Protection of the airway through the inherent 'auto-PEEP (positive endexpiratory pressure)' effect
• Prevention of CO2 retention, lung alveolar collapse and improvement of the oxygen index Subglottic LaserJet Catheter Through the LaserJet catheter ventilation is performed only with HFJV. It is characterized by the delivery of small tidal volumes from a high pressure jet at very high frequencies (100-400) followed by passive expiration for a very short period before delivering the next jet, creating an "auto-PEEP".

The LaserJet catheter (C. Reiner Corp, Vienna, Austria) is made of polyterafluoroethylene and is non inflammable and provides a good micro anatomical overview of the operating structures [7,8,9] However, the LaserJet catheter is not laser resistant in regards of deformation damage (perforation) under direct exposure to a continuous laser beam. The use of the new 445 nm wavelength laser, the 'blue laser' eases the surgical performance by securing important laryngeal functionality even in advanced disease [3,4,10] While the exposure of the complete pathology is visualized and treated, the surrounding tissue is not dislocated or deformed.

[11] The LaserJet catheter has the advantage of minimizing movements of the larynx and vocal cords and facilitates the precise use of the blue laser without affecting healthy tissue.

Electrical impedance tomography (EIT) When evaluating different JV devices and techniques, tidal volume and minute ventilation are difficult to assess because SHFJV and HFJV are applied in an open airway system. Air entrainment occurs to a varying extent depending on the route of JV administration, anatomical factors and jet alignment with the airway. [13] A standard intraoperative monitoring does not provide accurate predictions about changes in regional lung ventilation. The use of electrical impedance tomography (EIT) (SentecTom BB2, Landquart, Switzerland) allows the investigators to obtain a visual and quantitative representation of the areas of ventilation and aeration of the lung. The fundamental principle of lung EIT relies on the application of small alternating electrical currents into the thorax and voltage measurements using electrodes on the skin surface generating cross-sectional images representing impedance change in a slice of the thorax. It is a radiation-free imaging method which has the advantage to reveal real-time information.

An array of electrodes (textile belt with 32 integrated electrocardiographic electrodes) has to be placed around the thorax to inject currents and to measure the resulting voltages on the thoracic surface. Analyzing EIT measurements during ongoing mechanical JV is a useful tool to detect imbalances in regional lung ventilation and in further consequence enables to optimize ventilator setting during surgery. The investigators will assess changes in regional lung ventilation distribution during supra- and subglottic JV compared to controlled mask ventilation. EIT, as a non-invasive method, may become a useful tool for adjustment of optimal positioning of the JV devices and for decision-making of the optimal type of the JV device (supraglottic laryngoscope versus subglottic jet catheter) by detecting atelectasis as "silent spaces" or over distension. This additional source of information could help fine tune the jet ventilator. Its intraoperative use could provide the basis for individual optimization of jet ventilator settings, especially in patients at risk for ventilation-perfusion mismatch and impaired gas exchange. Serological biomarkers of lung damage and inflammation It is still an open question whether the type of specific mechanical ventilation like HFJV and SHFJV have an influence on changes in serological biomarkers for lung damage and inflammation like Surfactant proteins (SP)-D [14-16], Krebs van den Lungen (KL)-6 [17, 18], Clara cell protein (CC16) [19, 20], Adrenomedullin (ADM) [21] or Interleukin 6 (IL-6) and 8 (IL-8). Studies revealed that in patients with normal lungs, relatively short periods of mechanical ventilation induce lung inflammation and that these changes depend on the ventilatory parameters applied. [22-25] The investigators will study the influence of JV on these serum levels in patients undergoing elective laryngotracheal surgery. Specific aims and hypotheses The objective of this study is to compare regional ventilation distribution, blood gas oxygenation, CO2 elimination and serum biomarkers with different jet ventilatory techniques and access routes.

Our primary aim is to determine whether supraglottic JV with the jet laryngoscope leads to a shift of the Center of Ventilation (COV) towards the ventral lungs compared to mask ventilation.

The secondary objective of the study is to investigate changes in ventro-dorsal ventilation distribution, measured as ventro-dorsal COV shift, under subglottic JV compared to standardized, controlled mask ventilation. Further study objectives are the comparison of ventilation distribution parameters between the two JV techniques, supra- and subglottic JV, and the analysis of oxygenation and CO2 elimination during these two ventilation modes over time. Furthermore, special serum biomarkers of pulmonary lung damage are determined after JV application. Primary hypothesis The primary hypothesis is that during the use of supraglottic jet ventilation, the COV is shifted to more ventrally located (non-dependent) lung sections compared to controlled mask ventilation.

Secondary hypothesis The secondary hypothesis is that subglottic JV ventilation leads to a ventral displacement of the COV compared to controlled mask ventilation and that this occurs to a greater extent than during supraglottic JV. Null hypothesis There is no significant displacement of COV into ventrally located lung sections under supraglottic jet ventilation compared to controlled mask ventilation.