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GENERAL ARTICLES

Respiratory Efficacy of Subglottic Low-Frequency, Subglottic Combined-Frequency, and Supraglottic Combined-Frequency Jet Ventilation During Microlaryngeal Surgery

Department of Anesthesiology and General Intensive Care, University of Vienna, Austria

Address correspondence and reprint requests to Andreas Bacher, MD, Department of Anesthesiology and General Intensive Care, University of Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria. Address e-mail to andreas.bacher{at}univie.ac.at

	Abstract

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We tested the respiratory efficacy of different jet ventilation techniques (subglottic low-frequency versus subglottic combined-frequency and subglottic combined-frequency versus supraglottic combined frequency) in patients undergoing microlaryngeal surgery. The PaCO₂ and the quotient of arterial oxygen tension (PaO₂) over FIO₂ were measured. After anesthetic induction (propofol, remifentanil, vecuronium), an endotracheal Mon-Jet catheter (Xomed, Jacksonville, FL) for subglottic jet ventilation and a laryngoscope for supraglottic jet ventilation (Carl Reiner G.m.b.H., Vienna, Austria) were inserted. In Group 1 (n = 18), subglottic low-frequency (15 breaths/min), combined-frequency (600 and 15 breaths/min), and low-frequency jet ventilation was subsequently performed (15 min each). In Group 2 (n = 19), the sequence was supraglottic, subglottic, and supraglottic combined-frequency jet ventilation. The driving pressures were initially adjusted to achieve normocapnia and were not changed during the entire study period. The FIO₂ was measured endotracheally. The Wilcoxon’s signed rank test was applied. In Group 1, PaCO₂ and PaO₂/FIO₂ improved significantly after switching from subglottic low-frequency to subglottic combined-frequency jet ventilation (PaCO₂, from 46.6 ± 8.3 to 42.1 ± 8.1 mm Hg; PaO₂/FIO₂, from 311 ± 144 to 361 ± 141 mm Hg; P <0.05). In Group 2, PaCO₂ increased and PaO₂/FIO₂ decreased significantly after switching from supraglottic to subglottic combined-frequency jet ventilation (PaCO₂, from 39.4 ± 7.1 to 45.9 ± 7.5 mm Hg; PaO₂/FIO₂, from 415 ± 114 to 351 ± 129 mmHg; P <0.05). We conclude that subglottic combined-frequency jet ventilation is less effective than supraglottic combined-frequency ventilation, but more effective than subglottic low-frequency jet ventilation.

Implications: The combination of high and low respiratory frequencies (600 and 15 breaths/min) improves pulmonary gas exchange during subglottic jet ventilation via an endotracheal catheter. However, subglottic combined-frequency jet ventilation is less effective than supraglottic combined-frequency jet ventilation via a jet ventilation laryngoscope.

	Introduction

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Jet ventilation is a very useful technique for microlaryngeal surgery. It provides adequate pulmonary gas exchange as well as free surgical access and overview of the operating field (1–4). Jet ventilation may be performed in many different ways, such as high-, low-, or combined-frequency ventilation, and subglottic or supraglottic ventilation, with specially designed catheters or laryngoscopes. An optimal combination of ventilatory modes and access routes to improve the efficacy and safety of jet ventilation techniques has not been determined.

In a previous study, we compared the respiratory efficacy of supraglottic combined-frequency jet ventilation via a jet ventilation laryngoscope (Carl Reiner G.m.b.H., Vienna, Austria) and subglottic monofrequent jet ventilation via an endotracheal catheter (Mon-Jet; Xomed, Jacksonville, FL) (5). We found that supraglottic combined-frequency jet ventilation provides significantly better oxygenation and lower PaCO₂ values at a given FIO₂ and driving pressure, respectively (5). The question arising from these results was whether the observed superiority of supraglottic combined-frequency jet ventilation to subglottic monofrequent jet ventilation was a result of the simultaneous administration of combined respiratory frequencies (600 and 15 breaths/min), a result of the access route, or both. We therefore tested the hypothesis that the respiratory efficacy of subglottic jet ventilation via the Mon-Jet catheter can be improved if combined respiratory frequencies are used instead of only a single frequency. The Mon-Jet catheter was originally designed for subglottic monofrequent (i.e., either high- or low-frequency) jet ventilation (6,7). Therefore, we slightly modified the Mon-Jet catheter to enable the application of subglottic combined-frequency jet ventilation and compared this new technique with the standard technique of subglottic low-frequency (15 breaths/min) jet ventilation. To determine the impact of the access route on respiratory efficacy, we further compared subglottic combined-frequency (600 and 15 breaths/min) jet ventilation with supraglottic combined-frequency (600 and 15 breaths/min) jet ventilation.

	Methods

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The jet ventilation laryngoscope serves for the administration of supraglottic jet ventilation, for providing surgical access to laryngeal structures, and for the illumination of the operating field. It is made of steel, and two nozzles for the simultaneous administration of high- and low-frequency jet ventilation are integrated in its wall. A third tube opens close to the tip into the main lumen of the laryngoscope and serves for airway pressure monitoring. A more detailed description of the technique of supraglottic jet ventilation via this specially designed laryngoscope has been published previously (5). The Mon-Jet catheter is made of polytetrafluoroethylene and is composed of two parallel tubes. The central tube is the jet ventilation nozzle. The second lumen serves as a monitoring port. At the tip of the catheter, a basket aligns the catheter with the trachea and prevents direct contact of the jet ventilation nozzle with the tracheal wall. To enable the administration of combined respiratory frequencies, we connected a three-way stopcock to the proximal end of the jet ventilation nozzle of the Mon-Jet catheter. The respirator tubes for high- and low-frequency jet ventilation were then connected to the three-way stopcock with the high-frequency tube at a 180° angle and the low-frequency tube at a 90° angle to the jet nozzle of the Mon-Jet catheter.

Before comparing different jet ventilation techniques in patients, lung simulator tests were performed. Three ventilation techniques were tested: subglottic low-frequency jet ventilation (15 breaths/min), subglottic combined-frequency jet ventilation (15 and 600 breaths/min), and supraglottic combined-frequency jet ventilation (15 and 600 breaths/min). The tests were performed at two different lung simulators’ (LS 800; Dräger, Lübeck, Germany) settings: a) compliance of 133 mL/mm Hg, resistance of 1.5 mm Hg · L^-1 · s^-1, and b) compliance of 66.5 mL/mm Hg, resistance of 6 mm Hg · L^-1 · s^-1. The diameter of the trachea on the lung simulator was 11 mm. A jet ventilation respirator for the combined or single administration of high and low respiratory frequencies was used (LJ 4000 Jet Ventilator; Acutronic, Jona-Rappersweil, Switzerland). The inspiration/expiration time ratio was set at 1, and the FIO₂ was set at 0.5. The driving pressure was set at 1500 mm Hg. Tracheal FIO₂ was monitored with a ventilation gas analyzer (Datex, Helsinki, Finland) that was connected to the monitoring port of the Mon-Jet catheter. The amount of room air entrainment can be very precisely estimated by measuring the difference between the FIO₂ of the gas that is delivered by the respirator and the FIO₂ that is finally present in the rachea (8). The percentage of the amount of room air entrainment (E) to measured total tidal volume (VT) was calculated as follows: E/VT (%) = ([respirator FIO₂ - tracheal FIO₂] x 100/tracheal FIO₂ - 0.21)/(100 + [respirator FIO₂ - tracheal FIO₂] x 100/tracheal FIO₂ - 0.21).

After approval by our ethics committee and informed consent, 37 adult patients (ASA I–III) scheduled for elective microlaryngeal surgery were prospectively studied. Oral midazolam (7.5 mg) was administered to all patients approximately 60 min before the start of anesthesia. The usual monitoring was used. The anesthetic induction was performed with a continuous infusion of propofol 10 mg · kg^–1 · h^–1 IV and remifentanil 0.8–1 µg · kg^–1 · min^–1 IV. After patients’ loss of consciousness, vecuronium 0.1 mg/kg IV was administered, and the Mon-Jet catheter was endotracheally placed. Thereafter, a radial artery cannula was inserted and invasive arterial blood pressure was recorded. For maintenance of anesthesia, the rates of the propofol and remifentanil infusions were reduced to 3–5 mg · kg^–1 · h^–1 and 0.2–0.4 µg ·kg^–1 · min^–1, respectively. Then the otolaryngologist inserted the jet ventilation laryngoscope. The same respirator as for the lung simulator tests was used (LJ 4000 Jet Ventilator). The inspiration/expiration time ratio was set at 1 and the FIO₂ was set at 0.5. In every patient, the driving pressure was initially adjusted to achieve normocarbia but was not changed throughout the entire study period. The FIO₂ was increased by 0.2 if peripheral oxygen saturation decreased to <92%.

The patients were randomly assigned to one of two groups. In Group 1 (n = 18; 10 women, 8 men), subglottic low-frequency (15 breaths/min), subglottic combined-frequency (600 and 15 breaths/min), and again subglottic low-frequency jet ventilation (15 breaths/min) were sequentially performed via the Mon-Jet catheter. In Group 2 (n = 19; 8 women, 11 men), the sequence of ventilation techniques was supraglottic combined-frequency jet ventilation (600 and 15 breaths/min) via the laryngoscope, subglottic combined-frequency jet ventilation (600 and 15 breaths/min) via the Mon-Jet catheter, and again supraglottic combined-frequency jet ventilation (600 and 15 breaths/min) via the laryngoscope. Each of the ventilatory modes was maintained for 15 min in both groups. Arterial blood samples were drawn after 15 min of every ventilatory mode to determine PaO₂, PaCO₂, and arterial pH (pH_a). To assess the CO₂-elimination capacity of each ventilation technique, the CO₂ elimination coefficient (ECCO₂) was calculated (ECCO₂ = 56.25 · 10³ · driving pressure^–1 · PaCO₂^–1; pressures in mm Hg) (9). If we assume that a normal PaCO₂ of 37.5 mm Hg can be achieved with a driving pressure of 1500 mm Hg, ECCO₂ equals 1, and it becomes larger or smaller than 1 if CO₂ elimination is better or worse than the above described standard constellation (9). Room air entrainment was determined in the same way as during the lung simulator tests.

All statistical analyses were performed with a commercially available computer program (StatView; SAS Institute, Cary, NC). The patients’ age, weight, and height were compared with a Mann-Whitney U-test. Mean arterial pressure and heart rate were recorded in 5-min intervals. Changes in these variables were examined for statistical significance with analysis of variances for repeated measurements. The values of pH_a, PaCO₂, PaO₂/measured FIO₂, ECCO₂, and room air entrainment after 15 min of each ventilation technique were compared with Wilcoxon’s signed rank test. P < 0.05 was considered statistically significant. Data were presented as mean ± SD if not otherwise specified.

	Results

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Data obtained from the lung simulator tests are reported in Table 1. In both settings of compliance and resistance, VT during subglottic jet ventilation increased approximately 10-fold by using combined respiratory frequencies instead of only a low frequency. The volume delivered by the respirator (V_R) and E increased approximately 10-fold, whereas the ratio of E/VT remained constant. In both lung simulator settings, the largest VT was obtained during supraglottic combined-frequency jet ventilation, which was mainly caused by an increased ratio of E/VT. The V_Rs were similar between subglottic and supraglottic combined-frequency jet ventilation.

In Group 1, the driving pressure was set at 1305 ± 278 mm Hg (range, 825-1800 mm Hg). The respirator FIO₂ was set at 0.55 ± 0.11 (range, 0.50–0.90), 0.55 ± 0.11 (range, 0.50–0.90), and 0.55 ± 0.11 (range, 0.50–0.90) during subglottic low-frequency jet ventilation, subglottic combined-frequency jet ventilation, and the second episode of subglottic low-frequency jet ventilation, respectively. Fifteen minutes after switching from subglottic low-frequency to subglottic combined-frequency jet ventilation, pH_a increased (from 7.34 ± 0.06 to 7.38 ± 0.07, P < 0.05), PaCO₂ decreased (from 46.6 ± 8.3 to 42.1 ± 8.1 mm Hg, P < 0.05), and PaO₂/FIO₂ increased (from 311 ± 144 to 361 ± 141 mm Hg, P < 0.05). Fifteen min after switching back to subglottic low-frequency jet ventilation, pH_a decreased (to 7.35 ± 0.09, P < 0.05), PaCO₂ increased (to 46.2 ± 11.9 mm Hg, P < 0.05), and PaO₂/FIO₂ decreased (to 315 ± 153 mm Hg, P < 0.05). The changes in pH_a, PaCO₂, and PaO₂/FIO₂ during the sequence of ventilation techniques in Group 1 are shown in Figure 1. The ECCO₂ was significantly increased during subglottic combined-frequency jet ventilation (1.12 ± 0.36) as compared to the first (1.02 ± 0.37), but not to the second (1.05 ± 0.39, P = 0.08) episode of subglottic low-frequency jet ventilation. The calculated percentage of E/VT was 10.5% ± 10.0% during subglottic low-frequency jet ventilation, 8.2% ± 10.0% during subglottic combined-frequency jet ventilation, and 10.0% ± 10.8% during the second episode of subglottic low-frequency jet ventilation (not significant).

View larger version (14K): [in this window] [in a new window]   Figure 1. Mean ± SD of the individual changes in arterial pH (pHa) (upper panel), PaCO2 (middle panel), and the ratio of PaO2 over inspiratory oxygen fraction (PaO2/FIO2, lower panel). SBGLFSBGCF: Changes observed after switching from subglottic low-frequency (15 breaths/min) jet ventilation to subglottic combined-frequency (600 and 15 breaths/min) jet ventilation. SBGCFSBGLF: Changes observed after switching from subglottic combined-frequency jet ventilation back to subglottic low-frequency jet ventilation. Each ventilation technique was maintained for 15 min. Both types of subglottic jet ventilation were performed with the Mon-Jet catheter (Xomed, Jacksonville, FL).

View larger version (15K): [in this window] [in a new window]   Figure 2. Mean ± SD of the changes in arterial pH (pHa) (upper panel), PaCO2 (middle panel), and the ratio of PaO2 over inspiratory oxygen fraction (PaO2/FIO2, lower panel). SPGCFSBGCF: Changes observed after switching from supraglottic combined-frequency (600 and 15 breaths/min) jet ventilation to subglottic combined-frequency jet ventilation. SBGCFSPGCF: Changes observed after switching from subglottic combined-frequency jet ventilation back to supraglottic combined-frequency jet ventilation. Each ventilation technique was maintained for 15 min. Supraglottic jet ventilation was performed with a specially designed jet ventilation laryngoscope (Carl Reiner G.m.b.H., Vienna, Austria), and subglottic jet ventilation was performed with the Mon-Jet catheter (Xomed, Jacksonville, FL).

	Discussion

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Efforts to increase the respiratory efficacy of jet ventilation techniques are of great clinical importance. Although it is uncomplicated to maintain an adequate pulmonary gas exchange in many patients undergoing microlaryngeal surgery with any ventilation technique, we are increasingly challenged by older patients, or those suffering from severe cardiopulmonary or pulmonary diseases, in whom a normal PaO₂ and normocapnia cannot be easily achieved. Frequently, hypoxia and hypercapnia occur during microlaryngeal surgery even though the driving pressure is set at maximum and the FIO₂ is 100%. Hunsaker (6) repeatedly observed severe hypercapnia (PaCO₂ >55 mm Hg) in 4 of 19 study patients during the evaluation of subglottic low-frequency jet ventilation via the Mon-Jet catheter, particularly if surgery lasted for more than 30 min. In another study about subglottic jet ventilation via the Mon-Jet catheter, severe hypercapnia occurred in 5 of 164 patients despite maximum increases in driving pressure, and surgery could only be completed after switching to conventional mechanical ventilation via an endotracheal tube (9). In the present study, the incidence of severe hypercapnia and an abnormally low ECCO₂ was highest during low-frequency jet ventilation; it was lower during subglottic combined-frequency jet ventilation and lowest during supraglottic combined-frequency jet ventilation.

Of prime importance are the physiologic and physical determinants of the respiratory efficacy of a certain jet ventilation technique. Respiratory efficacy is clearly influenced by multiple factors that interact in a system consisting of the lung, the anatomy, and pathologic changes of the laryngeal structures, the ventilatory mode, and the access route by which jet streams are administered. However, we may conclude that the VT is one important determinant of respiratory efficacy. On the lung simulator, VT was markedly larger with ventilation techniques that also showed a better respiratory efficacy in patients. Further, pulmonary gas exchange during jet ventilation was improved with increasing VT, particularly with regard to CO₂ removal (5,10). The fact that the VT generated by supraglottic combined-frequency jet ventilation was larger than that generated by subglottic low-frequency and subglottic combined-frequency jet ventilation may, at least in part, be attributable to a larger amount of room air entrainment. The calculated percentages of E/VT with the three jet ventilation techniques studied were similar on the lung simulator and in the patients. The lung simulator tests showed that V_R was almost identical between subglottic and supraglottic combined-frequency jet ventilation and that the differences in VT were only caused by an increased E during supraglottic combined-frequency jet ventilation. However, E/VT was similar between subglottic low-frequency and subglottic combined-frequency jet ventilation, although the VT achieved with subglottic combined-frequency jet ventilation was considerably larger than that achieved with subglottic low-frequency jet ventilation. It is therefore impossible that the increase in VT that was generated by combined respiratory frequencies was a result of an increased percentage of E/VT. On the lung simulator, the absolute values of both E and V_R differed approximately by a factor of 10 between subglottic low-frequency and subglottic combined-frequency jet ventilation. Because the percentage of E/VT was only around 18% during subglottic jet ventilation via the Mon-Jet catheter, most of the difference in VT between subglottic low-frequency and subglottic combined-frequency jet ventilation resulted from the difference in V_R.

In patients, the creation of an (intrinsic) positive end-expiratory pressure, or an increase in gas diffusion to and from the alveolar membranes might additionally have contributed to the better respiratory efficacy of combined-frequency jet ventilation (11,12). The presence of a moderate positive end-expiratory pressure leads to a shift of the lower inflection point on the pressure-volume (compliance) curve to the right, where a large increase in volume results from only small increases in pressure. Thereby, lung areas are recruited that are otherwise collapsed because of a decrease in functional residual capacity during anesthesia (13). This could lead to an increase in PaO₂/FIO₂, as well as to an improved ECCO₂ because of a decrease in mean airway pressure required to administer a given VT. Indeed, residual capacity becomes larger if compliance increases during jet ventilation via the Mon-Jet catheter (9). Perhaps a mechanical lung simulator should not be regarded as an identical model of a lung in a live subject because of major dissimilarities in the complex and inhomogeneous dynamic properties of the human lung and thorax.

Perhaps our study could be biased by changes in oxygen consumption and carbon dioxide production resulting from variations in the metabolic state and depth of anesthesia during the study period. However, the stability of heart rate and arterial blood pressure in these patients suggests that no significant metabolic changes occurred. The anesthesia regimen was standardized and surgical stress was constant because it was primarily caused by suspension laryngoscopy, which was performed during the entire study period in all patients.

We conclude that the respiratory efficacy of subglottic jet ventilation via the Mon-Jet catheter can be improved by superimposing high and low respiratory frequencies. Compared with subglottic low-frequency jet ventilation, an average decrease in PaCO₂ by 9.5% and an increase in PaO₂/FIO₂ by 18% can be expected. However, supraglottic combined-frequency jet ventilation via a jet ventilation laryngoscope is the most efficient of the jet ventilation techniques that we tested. The observed differences in respiratory efficacy may partly be caused by differences in room air entrainment and VT.

	Acknowledgments

 
Supported by the Department of Anesthesiology and General Intensive Care of the University of Vienna.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (7) Citing Articles via Google Scholar Google Scholar Articles by Bacher, A. Articles by Aloy, A. Search for Related Content PubMed PubMed Citation Articles by Bacher, A. Articles by Aloy, A.