Assessment of Regional Lung Ventilation Distribution During Supraglottic and Subglottic Jet Ventilation by Electrical Impedance Tomography (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.