Microlaryngeal surgery involves difficulty in providing adequate mechanical ventilation during laryngoscopy because the view of the operating field must not be hindered, the laryngeal structures should be freely accessible with surgical instruments, and the application of a laser should be possible without endangering patients or personnel in the operating room. Patients undergoing microlaryngeal surgery may additionally suffer from severe obstructions in the upper airways caused by laryngeal pathologies, which inhibit gas exchange and the insertion of ventilation tubes.
One of the most promising developments that may solve these problems is jet ventilation. Several different types of jet ventilation have been investigated with regard to the ventilatory mode (i.e., respiratory frequency, driving pressure, timing of ventilatory bursts, or ventilator technologies) and to the access route (e.g., laser resistant endotracheal tubes, jet ventilation catheters, or nozzles that are integrated in the wall of the laryngoscope). The search for the ideal combination of a certain ventilatory technique and access route is therefore subject to continuing research.
Hunsaker (1) and Brooker et al. (2) have developed an endotracheal catheter (Mon-Jet; Xomed, Jacksonville, FL) that allows the application of either high- or low-frequency subglottic jet ventilation. The catheter is equipped with a basket on its tip to avoid direct contact of the jet nozzle with the tracheal wall. A second lumen that opens proximally to the jet nozzle either serves for monitoring of the ETCO2, or for the measurement of airway pressure. At our department, supraglottic combined-frequency jet ventilation has successfully been used during the last 10 yr as the standard technique (3,4). High- and low-frequency jet streams are simultaneously administered via two nozzles that are integrated in the wall of a jet ventilation laryngoscope (Carl Reiner G.m.b.H., Vienna, Austria). Therefore, no additional jet ventilation catheter is necessary.
It was our aim to compare these two jet ventilation techniques in adult patients undergoing microlaryngeal surgery. The efficacy of gas exchange and limitations in monitoring were studied. Lung simulator tests were additionally performed to demonstrate the accuracy of airway pressure measurements in both techniques.
After obtaining approval by our ethics committee and informed consent, 23 adult patients (ASA physical status I–III) scheduled for elective microlaryngeal surgery were prospectively studied.
All patients were premedicated with 7.5 mg of oral midazolam approximately 60 min before the start of anesthesia. Monitoring consisted of a five-lead electrocardiogram, noninvasive blood pressure, and pulse oximetry. Anesthetic induction was performed with fentanyl 3–5 μg/kg IV, propofol 1–2 mg/kg IV and vecuronium 0.1 mg/kg IV. Thereafter, a radial artery cannula was inserted and invasive blood pressure was recorded. Anesthesia was maintained with a continuous infusion of propofol 6 mg · kg−1 · h−1 IV. Fentanyl 2 μg/kg IV and vecuronium 0.03 mg/kg IV were supplemented as required.
After anesthetic induction, endotracheal intubation was performed in all patients with the Mon-Jet catheter. Then the jet ventilation laryngoscope was inserted by the otolaryngologist. The jet ventilation laryngoscope (Figure 1A) is made of steel and is equipped with two nozzles for the simultaneous administration of high- and low-frequency jet ventilation. The cross sections of both nozzles are circular with an inner diameter of 1.5 mm. They are integrated in the wall of the laryngoscope and open into the main lumen of the laryngoscope at distances that depend on the size of the laryngoscope (we used four different sizes, depending on the patient’s size). Airway pressure was monitored at the distal end of the jet ventilation laryngoscope via a third nozzle (circular cross section, inner diameter of 1.5 mm). Detailed construction plans of the jet ventilation laryngoscope we applied may be obtained (3). The Mon-Jet catheter (Figure 1B) was 330-mm long. It is made of polytetrafluoroethylene and is composed of two parallel tubes. The central tube is the jet ventilation nozzle (inner diameter 2.7 mm, total length 285 mm) and opens 45 mm from the tip of the catheter. The second lumen (inner diameter of 1 mm) is designed for monitoring of airway pressure or ETCO2 and opens 75 mm from the tip of the catheter. At the tip of the catheter, a basket (length, 45 mm) helps align the catheter with the trachea and prevents direct contact of the jet ventilation nozzle with the tracheal wall.
A jet ventilation respirator that allows the combined administration of high- and low-frequency jet ventilation was used (LJ 4000 Jet Ventilator; Acutronic, Jona-Rappersweil, Switzerland). In all patients, the ventilator settings were: inspiration/expiration time ratio = 1, FIO2 = 0.5, low-frequency = 15 breaths/min, high-frequency = 600 breaths/min. The driving pressures were set at 750-1500 mm Hg to achieve normocarbia during combined-frequency jet ventilation and were not changed throughout the experiment. The FIO2 was increased by 0.2 if peripheral oxygen saturation decreased below 92%.
In all patients, ventilation was started and maintained for 10 min with supraglottic combined-frequency jet ventilation via the jet ventilation laryngoscope. Thereafter, ventilation was switched to monofrequent subglottic jet ventilation via the Mon-Jet catheter. The patients were randomly assigned to receive either high-frequency jet ventilation (600 breaths/min; HF group) or low-frequency jet ventilation (15 breaths/min; LF group) via the Mon-Jet catheter. This ventilation mode was maintained for 20 min. Finally, ventilation was switched back to supraglottic combined-frequency jet ventilation in all patients. Arterial blood samples were drawn every 5 min to determine PaO2, PaCO2, and arterial pH (pHa). The FIO2 was monitored via the monitoring port of the Mon-Jet catheter during subglottic monofrequent jet ventilation and during combined-frequency jet ventilation. The ETCO2 was measured during monofrequent jet ventilation via the Mon-Jet catheter. The jet ventilation laryngoscope is not equipped with an ETCO2 monitoring port, and therefore, ETCO2 cannot be measured during combined-frequency jet ventilation. The airway pressure during monofrequent jet ventilation was measured via the monitoring port of the Mon-Jet catheter using a portable manometer (Uniflow manometer, model 59-UCAL; Baxter, Irvine, CA). During combined-frequency jet ventilation the airway pressure was measured via the pressure monitoring port of the jet ventilation laryngoscope.
The lung simulator (LS 800; Dräger, Lübeck, Germany) was set at a compliance of 66 mL/mm Hg and a resistance of 6 mm Hg · L−1 · s−1. The diameter of the trachea on the lung simulator was 11 mm. The ventilator settings were the same as in the study patients. The driving pressure on the lung simulator was set at 750 mm Hg. A tidal volume of 550 mL was achieved with combined-frequency jet ventilation via the jet ventilation laryngoscope, 100 mL with low-frequency jet ventilation via the Mon-Jet catheter, and 80 mL with high-frequency jet ventilation via the Mon-Jet catheter. A silicone pressure probe (inner diameter of 2 mm) was inserted into the laryngoscope and forwarded into the artificial trachea. The airway pressures were determined at various distances from the proximal ending of the jet ventilation laryngoscope (5.5 cm: opening of the low-frequency jet nozzle, 8 cm: opening of the high-frequency jet nozzle, 15 cm: opening of the pressure monitoring port of the jet ventilation laryngoscope, 17.5 cm: distal ending of the jet ventilation laryngoscope, 20 cm: monitoring port of the Mon-Jet catheter, 23 cm: opening of the jet nozzle of the Mon-Jet catheter, 29 cm: tip of the Mon-Jet catheter, and 32 cm: tracheal bifurcation) during combined-frequency jet ventilation and monofrequent jet ventilation. These values were compared with the airway pressures that were obtained from measurements via the monitoring port of the jet ventilation laryngoscope during combined-frequency jet ventilation, and the monitoring port of the Mon-Jet catheter during monofrequent jet ventilation.
All statistical analyses were performed with a commercially available computer program (StatView; SAS Institute Inc., Cary, NC). The patients’ age, weight, and height, as well as the difference between the measured FIO2 in the trachea and the FIO2 setting on the ventilator were compared with a Mann-Whitney U-test. Respiratory variables after 10 min of initial combined-frequency jet ventilation, after 20 min of high- or low-frequency jet ventilation via the Mon-Jet catheter, and after 15 min of the second episode of combined-frequency jet ventilation were compared using a Wilcoxon’s signed rank test. P < 0.05 was considered statistically significant. Data were presented as mean ± SD, if not otherwise specified.
No significant differences in demographic data between the two study groups (HF:n = 11, 6 women, 5 men; LF:n = 12, 4 women, 8 men) were observed (age: HF: 47.4 ± 18.6 yr, LF: 50.9 ± 17.9 yr; height: HF: 168.3 ± 9.9 cm, LF: 170.4 ± 8.7 cm; weight: HF: 74.0 ± 16.1 kg, LF: 74.8 ± 16.5 kg).
A significant difference between the measured FIO2 in the trachea and the FIO2 setting on the ventilator was observed during combined-frequency jet ventilation (LF group: 0.20 ± 0.06 at 10 min, 0.21 ± 0.06 at 45 min; HF group: 0.20 ± 0.00 at 10 min, 0.20 ± 0.01 at 45 min), but not during monofrequent jet ventilation (LF group: 0.03 ± 0.06 at 30 min; HF group: 0.03 ± 0.04 at 30 min). The means and standard deviations of pHa, PaCO2, PaO2, and the PaO2/FIO2 ratio are shown in Figure 2. pHa and the PaO2/FIO2 ratio decreased significantly, whereas PaCO2 increased significantly during ventilation with the Mon-Jet catheter in both groups. All of these changes were reversible when combined-frequency jet ventilation was reestablished. Arterial oxygen tension remained unchanged during the study in both groups.
The airway pressure measured via the measuring port of the Mon-Jet catheter was 0 mm Hg in all patients of both groups during monofrequent jet ventilation. The airway pressures measured via the measuring port of the jet ventilation laryngoscope were 8.0 ± 2.3 mm Hg during the inspiratory phase of the low-frequency ventilatory cycles and 2.4 ± 1.0 mm Hg during the expiratory phase.
On the lung simulator, the manometer continuously showed 0 mm Hg if it was connected to the pressure monitoring port of the Mon-Jet catheter, regardless of high- or low-frequency jet ventilation. In contrast, the inspiratory airway pressure was 5.0 mm Hg during low-frequency jet ventilation, and 4.0 mm Hg during high-frequency jet ventilation if measured between the tip of the Mon-Jet catheter and the level of the tracheal bifurcation with the silicone pressure probe. More proximally, the airway pressures were 0 mm Hg if measured with the silicone pressure probe during both monofrequent techniques.
During combined-frequency jet ventilation on the lung simulator, the airway pressures were 15.0 mm Hg during inspiration and 8.3 mm Hg during expiration if measured between 15 cm (opening of the pressure monitoring port of the jet ventilation laryngoscope) to 32 cm (tracheal bifurcation) from the proximal ending of the jet ventilation laryngoscope. The airway pressure measured via the monitoring port of the jet ventilation laryngoscope was 15.0 mm Hg during inspiration and 8.3 mm Hg during expiration and therefore exactly reflected the airway pressure in distal parts of the trachea. Proximal to the jet nozzles of the laryngoscope, the airway pressure ranged between −1.5 mmHg and 2.3 mm Hg, indicating that, at this location, no accurate monitoring is possible.
ETCO2 tension could not be reliably measured during high-frequency jet ventilation via the Mon-Jet catheter. The capnometer showed an ETCO2 of 0 mm Hg during the entire study in all patients. During low-frequency jet ventilation via the Mon-Jet catheter, monitoring of ETCO2 was possible. The differences between PaCO2 and ETCO2 were 5.4 ± 4.6 mm Hg (range −0.8 to 14.7 mm Hg), 6.0 ± 4.4 mm Hg (range −0.1 to 14.0 mm Hg), 6.2 ± 4.8 mm Hg (range −0.4 to 15.9 mm Hg), and 7.1 ± 6.3 mm Hg (range −0.8 to 17.8 mm Hg) at 15, 20, 25, and 30 min, respectively.
The present study demonstrates that arterial oxygenation and carbon dioxide elimination during microlaryngeal surgery can be better maintained with supraglottic combined-frequency jet ventilation than with subglottic monofrequent jet ventilation. The physiologic background of these findings is not clarified by this study, but it is possible that the tidal volumes that could be administered were greater with combined-frequency jet ventilation than with monofrequent jet ventilation. This hypothesis is based on the observation that the measured FIO2 in the trachea was lower, by 0.15–0.25, than the FIO2 that was actually set on the ventilator during combined-frequency jet ventilation. During monofrequent jet ventilation, the measured FIO2 in the trachea always matched with the FIO2 setting on the ventilator. Thus, the ventilation gas delivered by the ventilator (FIO2 ≥ 0.5) must have been mixed with environmental air (FIO2 of 0.21) between the laryngoscope and the trachea only in the case of combined-frequency jet ventilation, a phenomenon that is generally known as environmental air entrainment (5,6). In our study, the calculated amount of environmental air entrainment was at least 50%–60% of the total tidal volume during combined-frequency jet ventilation (according to a difference between measured FIO2 and the FIO2) setting on the ventilator of 0.15–0.25). It has been shown that a greater amount of environmental air entrainment results in greater tidal volumes (6), a fact that could, at least in part, have contributed to the more efficient gas exchange during combined-frequency jet ventilation. Nevertheless, it has been shown that in most cases an adequate gas exchange can be maintained with (low-frequency) subglottic monofrequent jet ventilation via the Mon-Jet catheter, provided that the driving pressures are set sufficiently high (1,7).
Monitoring of ETCO2 was only possible during low-frequency jet ventilation via the Mon-Jet catheter, but during neither high-frequency jet ventilation via the Mon-Jet catheter nor combined-frequency jet ventilation via the jet ventilation laryngoscope. The difficulties in continuous ETCO2 monitoring during any form of high-frequency ventilation are well known and have been observed by previous investigators (8–10). Reliable measurements of ETCO2 with low PaCO2-ETCO2 gradients similar to those observed during conventional mechanical ventilation were only obtained if high-frequency jet ventilation was interrupted and larger tidal volumes were applied at a low-frequency (8,9,11). The low tidal volumes delivered by high-frequency jet ventilation, mixing of expiratory and inspiratory gases in the trachea, and too long response times of ETCO2 monitors have been thought to cause this problem (12,13). In our study, rather large PaCO2-ETCO2 gradients, with a wide range, were observed during low-frequency jet ventilation, too. Similar results have been obtained in other studies on the PaCO2-ETCO2 gradient during low-frequency (10–20 breaths per minute) jet ventilation techniques (1,14). Hunsaker (1) describes PaCO2-ETCO2 gradients of 5–18 mm Hg at a respiratory frequency of 10 breaths per minute in the clinical evaluation study of the catheter. One could argue that the PaCO2-ETCO2 gradient is dependent on the ventilation/perfusion ratio of the lungs and not on the ventilatory technique, but it has been demonstrated that the gradients were significantly larger during low-frequency jet ventilation than during conventional ventilation via an endotracheal tube in one group of patients (14). Therefore, it is much more likely that the large PaCO2-ETCO2 gradients that were observed during low-frequency jet ventilation in the present and in previous studies (1) are caused by the small tidal volumes. The tidal volume that was achieved on the lung simulator was only 100 mL, which is similar to panting. Thus, the anatomical dead space volume is probably a major proportion of the total tidal volume and therefore, the expired gas is a mixture of alveolar gas (with a high carbon dioxide content) and a considerable amount of gas in the dead space (with a very low carbon dioxide content).
In contrast to the Mon-Jet catheter, monitoring of airway pressure could easily be performed via the implemented pressure monitoring port of the jet ventilation laryngoscope, and the obtained values matched well with the airway pressure in the trachea. Despite the fact that the monitoring port of the Mon-Jet catheter is purported to be designed for airway pressure monitoring (1,2, user guide), airway pressure during jet ventilation via the Mon-Jet catheter is not reported in any study on this device (1,2,7,15). We were not able to record airway pressure via the monitoring port of the Mon-Jet catheter, because the manometer continuously showed 0 mm Hg during low-frequency jet ventilation, as well as during high-frequency jet ventilation in patients and on the lung simulator. The reason for the misleading airway pressure measurements via the monitoring port of the Mon-Jet catheter is most likely the fact that the monitoring port opens proximally to the jet nozzle. Because of the acceleration of tracheal gas at the tip of the jet nozzle, a pressure trough is generated as a result of the Venturi effect, instead of positive pressure (16). In the present study, negative airway pressures were recorded proximally to the jet nozzles during combined-frequency jet ventilation on the lung simulator. The finding that airway pressure cannot be reliably measured via pressure monitoring ports that open proximally to the jet nozzle has been confirmed by an experimental study in dogs (16).
Numerous case reports of barotrauma and associated pathologies that occurred during jet ventilation have been published (17–19). Especially in cases of tracheal stenoses, the risk of hyperinflation is evident if jet ventilation is performed via catheters that are forwarded into the trachea beyond the stenosis (17–19). The gas inflation into the trachea is facilitated by inserting the catheter, but the difference between inspiratory and expiratory airway resistance might thereby increase, which could result in unintended hyperinflation. Modern jet ventilators are equipped with a pressure-triggered automatic shutoff that precludes the generation of uncontrolled peak airway pressures. However, this safety feature can only function properly if the measured pressure truly reflects the airway pressure in the trachea. The present results strongly suggest that airway pressure monitoring via the monitoring port of the Mon-Jet catheter does not fulfill this criterion.
No problems arose from the application of a CO2 laser at a power of 2–10 W. The jet ventilation laryngoscope is made of steel and, therefore, is 100% laser resistant for clinical purposes. It has been shown that the Mon-Jet catheter will be perforated if it is struck by a CO2 laser at a power of 2–10 W (1). However, a self-sustaining flame will not develop, and the material will not melt and cause further mucosal damage by flowing into the tracheobronchial system, even in an environment of pure oxygen (1).
From the otolaryngologists’ standpoint, the greatest advantage of supraglottic combined-frequency jet ventilation was an unobstructed view of the larynx and free instrumental access. In contrast, the Mon-Jet catheter had to be moved to the dorsal or ventral commissura so that it was not in the surgeon’s way. The fact that slight vocal cord vibrations occurred during combined-frequency jet ventilation was generally judged as a minor weakness of this method. During subglottic high- or low-frequency jet ventilation via the Mon-Jet catheter, these vibrations were not observed. Nevertheless, the uneventful completion of surgery was possible in all cases with either method.
We conclude that supraglottic combined-frequency jet ventilation allows for a regular pulmonary gas exchange and accurate airway pressure monitoring. The view of the glottis and surgical access are not hindered by an endotracheal catheter. In all of these points, it is superior over subglottic jet ventilation via the Mon-Jet catheter. Continuous monitoring of ETCO2 is only possible during low-frequency jet ventilation via the Mon-Jet catheter. However, a great variability of the PaCO2-ETCO2 gradient has to be considered. No clinically relevant differences were observed with regard to surgical acceptance of either technique and laser safety.
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