Laryngeal Resistance before and after Minor Surgery: Endotracheal Tube versus Laryngeal Mask Airway™

After obtaining the approval from the Ethics Committee of Chiba University Hospital, we obtained informed consent from 14 patients (American Society of Anesthesiologist physical status I-II). All were to undergo general anesthesia for elective minor surgeries, such as partial mastectomy or minor urological or ENT procedures. None had clinical evidence of respiratory, cardiovascular, or neuromuscular disorders. Each patient was allocated randomly by means of sealed envelopes to one of two groups (the ETT or the LMA ™ group) comprised of seven subjects each and was then placed with either an ETT or an LMA.

The experimental protocol is schematically shown in figure 1. All patients were premedicated with atropine 0.5 mg and midazolam 3 or 4 mg given intramuscularly 30 min before induction of anesthesia. Anesthesia was induced with inhalation of sevoflurane by the single-breath technique. 7After administration of succinylcholine (1 mg/kg), we inserted a polyethylene catheter into the trachea to reach 5 cm below the vocal cords for the measurement of subglottic pressure (P^sg), which was followed by the placement of an LMA ™ (size 3 or 4) for respiratory measurements (LMA-RM). Depth of the catheter was determined by the reference markers on the catheter. We confirmed the appropriate positioning of the LMA-RM with a fiberoptic endoscope (FB15H or FB10H; Pentax, Tokyo, Japan). We corrected the position of the LMA-RM so that both vocal cords were visualized in the fiberoptic image. Absence of leaks was verified by application of positive pressure. Anesthesia was maintained with sevoflurane in oxygen. End-tidal sevoflurane concentration was monitored with a respiratory gas analyzer (AS/3; Datex, Helsinki, Finland) and kept constant (sevoflurane, 1.7%, 1.0 minimum alveolar anesthetic concentration). Airflow (V̇) was measured by a pneumotachograph, connected to a differential pressure transducer (TP-602T; Nihon Koden, Tokyo, Japan). End-tidal carbon dioxide tension was measured using a side-stream capnometer (CAPNOX; Colin, Aichi, Japan). In addition to measurements of airway pressure at the proximal end of the LMA-RM (P^mask) and P^sg, pressure difference between the P^maskand P^sg(PΔ) was directly measured with a pressure transducer (23NB005G; IC sensors, Silicon Valley, CA). (V̇), P^mask, P^sg, and PΔ were recorded on an eight-channel thermal recorder (WS-682G; Nihon Koden, Tokyo, Japan) and stored simultaneously in a personal computer with a sample frequency of 50 Hz by a data-logging software package (LABDAT 5.2 RHT; Infodat, Montreal, Quebec, Canada) for later analysis.

After confirming at least 5 min of steady-state spontaneous breathing as indicated by no change in end-tidal carbon dioxide tension, tidal volume, and respiratory frequency, measurements of the ventilatory variables (V̇, P^mask, P^sg, PΔ, end-tidal carbon dioxide tension) were made during spontaneous breathing while maintaining constant anesthetic level (1 minimum alveolar concentration [MAC]) (fig. 1,1). Because it took more than 15 minutes for breathing to stabilize after completion of the experimental setting, we assumed that the effect of succinylcholine administered at the time of anesthesia induction had worn off at the time of the measurements. Following the measurements, vecuronium 8–10 mg was administered intravenously and complete paralysis was confirmed by loss of response to ulnar nerve electric stimulation. Patients were mechanically ventilated by intermittent positive pressure ventilation mode with 10 breaths/min, a tidal volume of 10 ml/kg and an inspiratory:expiratory time (I:E) ratio of 1:2. Ventilatory variables were measured again under complete paralysis (fig. 1,2). Immediately thereafter, a fiberoptic endoscope connected to a camera (ETV8; Nisco, Saitama, Japan) was passed through the LMA-RM to visualize the laryngeal aperture. The glottic aperture image during apnea at the atmosphere pressure was videotaped for later analysis of glottic aperture angle (fig. 1 3). After completion of the respiratory measurements before surgery, both the LMA-RM and the polyethylene catheter were removed and then either a cuffed ETT (ID 7.0 mm for females, 8.0 mm for male) or another LMA ™ (size 3 or 4) for the surgery was inserted. During surgery, anesthesia was maintained with sevoflurane (end-tidal concentration 1–2%) and nitrous oxide (60–66%), and vecuronium was administered as necessary. In either group, we did not measure nor control intracuff pressure of each device.

At the completion of surgery, the ETT or the LMA ™ used during surgery was removed, and then both the subglottic catheter and the LMA-RM were reinserted. Again, we confirmed the appropriate LMA-RM positioning with fiberoptic endoscope. The residual effect of muscle relaxant was fully reversed with 1.0 mg of atropine and 2.0 mg of neostigmine administered intravenously. Complete reversal of neuromuscular blockade was confirmed by ulnar nerve electric stimulation. The patient was allowed to breathe spontaneously while maintaining at sevoflurane 1 MAC. After confirming a stable breathing pattern for at least 5 min, the same respiratory measurements during spontaneous breathing as in the period immediately before the surgeries were repeated (fig. 1,4). Following the measurement, muscle paralysis was achieved again by the administration of succinylcholine (1 mg/kg) and the respiratory and endoscopic measurements under paralysis were repeated (fig. 1). 5,6 

Each laryngeal image was printed on paper for the measurement of vocal cord angle, which was defined as the angle produced by lines connecting anterior and posterior commissures. Presence or absence of laryngeal swelling was also evaluated from the endoscopic laryngeal view. The extensive adhesion of membranous portion adjacent to the anterior commissure and the extended soft-tissue swelling around the vocal cord were considered significant laryngeal swelling. An investigator [SI], who was blinded to the patient group, evaluated the glottic aperture angle and laryngeal swelling.

First, we estimated the resistive pressure reduction due to LMA-RM. because it contributes to Pδ. By measuring pressure difference across the LMA-RM while blowing 100% oxygen gas at various flow rates with flow going in both directions, we found that the resistance of the LMA-RM (R^LMA) varies with size and V̇, and can be calculated by the following Rohrer's equation for each LMA-RM size: LMA-RM of size 4: R^LMA#4= 0.785 + 0.951 *V̇ (cmH²O · l⁻¹· s), LMA-RM of size 3: R^LMA#3= 0.751 + 0.871 *V̇ (cmH²O · l⁻¹· s). Translaryngeal pressure (P^larynx) was, therefore, calculated by subtracting R^LMA*V̇ from PΔ (=P^mask− P^sg) for each flow rate. Then we plotted the P^larynx–V̇ relationship. Using computer software (ANADAT 5.1 and 5.2; RHT-Infodat Inc., Montreal, Quebec, Canada), a variety of equations were tested to fit the P^larynx–V̇ curve adequately. Finally, we calculated laryngeal resistances at a constant flow rate of 0.15 l/s (R^larynx-0.15) during inspiration and expiration. Furthermore, minute volume was calculated from the flow signal.

The Mann–Whitney rank sum test was used to compare the anthropometric characteristics and operation time between the groups. The Fisher exact test was used to evaluate differences of the incidence of laryngeal swelling between the groups. The Wilcoxon signed-rank test was used to determine the differences between the variables before and after surgeries. The Spearman rank test was used to assess correlation between the variables. All values were given as median (tenth to ninetieth percentiles). Statistical differences were considered significant when P < 0.05.

Anthropometric characteristic of the patients and operation time for the ETT and LMA ™ groups are presented in table 1. There were no significant differences in these parameters between groups. In one LMA ™ group patient, measurements under mechanical ventilation were not made and only measurements under spontaneous breathing were available for further analysis.

Both minute ventilation and end-tidal carbon dioxide did not change before and after surgery in both groups. As demonstrated by the representative images of the laryngeal aperture before and after surgery (fig. 2), the incidence of laryngeal swelling after surgery was determined to be significantly higher in the ETT group (6 of 7) than in the LMA ™ group (1 of 6) (P = 0.029). Furthermore, we found significant decrease in the glottic aperture angle after surgeries only in the ETT group, in addition to significant increase in the laryngeal resistance (fig. 3). In the ETT group, both R^larynx-0.15during inspiration and expiration significantly increased and nearly doubled after surgery (fig. 4). Conversely, in the LMA group, R^larynx-0.15during inspiration did not increase after surgery whereas R^larynx-0.15during expiration significantly increased by approximately 30% after surgery (fig. 4).

Figure 5illustrates examples of P^larynx–V̇ relationships before and after surgery in patients of both groups during spontaneous breathing and during mechanical ventilation. The pressure-flow curves were curvilinear, and marked hysteresis of the curves with clockwise rotation was observed for both conditions. In ETT group patients, the P^larynx–V̇ relationships significantly changed after surgery in both conditions. In these patients the slope of both the inspiratory and expiratory portions of the curves representing resistive components of the larynx increased after surgery, whereas in the LMA ™ group patients, P^larynx–V̇ relationships after surgery were not altered in either condition.

Because we observed marked hysteresis of the P^larynx–V̇ relationships as shown in figure 5, we adopted the modified form of the Rohrer equation 8as follows:

where P is P^larnx, V̇ is gas flow, dV̇/dt is acceleration of the gas flow calculated by differentiation from gas flow, absV̇ is absolute value of airflow, and A, K¹, and K²are constants obtained by the fitting analysis. The absV̇ is necessary for identification of inspiration and expiration. Data on inspiration and expiration were analyzed together. As the nature of the equation indicates, both K¹and K²represents resistive components of the larynx. A nonlinear least-square technique was used for the curve fitting; the quality of the fitting was provided by coefficient R²(SigmaPlot version 6.0; SSPS Inc., Chicago, IL). The results of the curve fitting analyses are presented in table 2. The pressure-flow relationships were well fitted by the equation, with small deviations between the actual relationships and fitted curves during both spontaneous and mechanical breathing. The R²values were uniformly high (> 0.979), indicating that the differential equation we adopted fitted the P^larynx–V̇ data reasonably well in our experimental conditions. Constant K¹obtained by the fitting analysis did not significantly change before and after surgeries. In contrast, constant K²significantly increased after surgery in the ETT group in both conditions (table 2).

In order to assess the physiologic meaning of the K²value, we plotted all K²values obtained during spontaneous breathing against corresponding R^larynx-0.15values. As clearly shown in the figure 6A, we found direct correlation between the variables, indicating that in our study, the K²value is likely to reflect resistance of the laryngeal airway. This was also confirmed by significant correlation between K²values obtained during mechanical ventilation and glottic aperture angles obtained under complete paralysis (fig. 6-B). On the other hand, there were no significant correlation between K¹values and R^larynx-0.15(P = 0.199) or glottic aperture angle (P = 0.410).

The major findings of this study were: (1) significant narrowing of the glottic aperture in association with marked swelling of the vocal cords was observed in patients with ETT placement; (2) the glottic aperture did not narrow after surgeries in patients with LMA ™ placement; (3) directly measured laryngeal resistance significantly increased after surgery in patients with ETT placement, whereas the resistance increased only on expiration in patients with LMA ™ placement; (4) pressure-flow relationship of the larynx was well fitted by the modified Rohrer equation during both spontaneous and mechanical ventilation; (5) the constant K²obtained by the fitting analysis was significantly correlated with measured laryngeal resistance and glottic aperture angle; and (6) the K²value in both paralyzed and nonparalyzed conditions significantly increased in patients with ETT placement, whereas the increase of K²value was observed only in nonparalyzed condition in patients with LMA ™ placement.

Until now, laryngeal edema has only been diagnosed by clinical symptoms such as dyspnea or stridor, and no objective assessment of the laryngeal swelling has been described. In this study, presence or absence of laryngeal swelling was evaluated by endoscopic laryngeal view. In addition, the vocal cord angle as an index of the laryngeal aperture size independent from the distance between the tip of the endoscope and vocal cords was measured as reported by Kuna et al.  9They demonstrated a curvilinear relationship between the vocal cord angle and cross-sectional area of the glottic aperture in awake human subjects. We adopted their methodology in this study and are first to report the glottic angle in anesthetized persons. In agreement with their results, we found significant correlation between laryngeal resistance and the vocal cord angle.

The glottic aperture size is modulated by interaction between the anatomical properties of the larynx and neural regulation of the larynx. As we previously reported, we evaluated anatomical properties of the pharynx independently of neural factors 10by eliminating the neural regulation of the larynx using a muscle relaxant under general anesthesia. Accordingly, comparison of the P^larynx–V̇ relationship during spontaneous breathing and during mechanical ventilation under paralysis allows us to estimate contribution of each factor to the laryngeal resistance. In addition, curve-fitting analysis of the P^larynx–V̇ relationship with use of the modified Rohrer equation allows assessment of dynamic behavior of the larynx, and is advantageous over calculation of the laryngeal resistance at a single point of the pressure-flow relationship. We found that in our experimental settings, K²values are particularly valuable in representing the resistive component of the laryngeal airway.

Our approach may have several methodological limitations. First, our study was performed using general anesthesia, and therefore the results of this study do not necessarily represent the laryngeal behavior during wakefulness or recovery from anesthesia in the postanesthesia care unit. Sevoflurane anesthesia at 1 MAC itself may have significant influence on the neural regulations of the laryngeal muscles. Second, the use of P^larynxfor construction of the P^larynx–V̇ curve disregards the background absolute pressure within the airway. In fact, mean airway pressure is greater during mechanical ventilation than during spontaneous breathing. Accordingly, the K²values evaluated during mechanical ventilation were possibly underestimated if the laryngeal airway wall was compliant. However, this possible underestimation of the K²values during mechanical ventilation would not have resulted in the K²differences before and after surgery. Third, because we did not measure and regulate the cuff pressure of ETT and LMA ™, this might have influenced the postoperative laryngeal resistance. Fourth, the pressure catheter below the vocal cord may possibly be influenced by subglottic edema, which could not be evaluated by fiberoptic observation.

Laryngeal resistance increases because of anatomical narrowing of the laryngeal aperture (anatomical mechanisms) and/or imbalance between the abductor and adductor of the vocal cords (neural mechanisms). We found significant increase in the K²values of the paralyzed larynx only in the ETT group, which indicates that anatomical narrowing of the larynx produced by laryngeal swelling only occurred in the ETT group. Significant increase in the K²values only during spontaneous breathing in the LMA ™ group indicates that neural mechanisms play a role in the postoperative increase in the laryngeal resistance independent of the airway managements. Although our experimental design does not allow exploration of neural mechanisms, surgical stress might possibly influence the central modulation of UA muscles as we reported previously. 6Our results indicate that laryngeal resistance increases even after minor surgeries, and that the laryngeal swelling caused by placement of an ETT significantly contributes to the increment of laryngeal resistance.

Postoperative laryngeal swelling after extubation is believed to develop rarely in adult patients who had minor surgeries, based on the absence of the clinical symptoms such as dyspnea and stridor. 1In accordance with the previous studies, neither dyspnea nor stridor developed after tracheal extubation in this study, but we clearly demonstrated significant increase in laryngeal resistance due to marked laryngeal swelling in patients with ETT placement. The increased laryngeal resistance may not manifest clinical symptoms in subjects with normal preoperative laryngeal function. However, the results should be seriously taken, alerting routine airway management procedure with ETTs, particularly in patients with preoperative laryngeal narrowing and small children who may have no clinical symptom preoperatively. While placement of LMA ™ has several disadvantages such as possible occurrence of gastric insufflations of the air and pulmonary aspiration, 11–12our results suggest the superiority of the use of the LMA ™ in maintaining laryngeal airway patency.

In conclusion, postoperative laryngeal resistance increases due to laryngeal swelling in patients with ETT placements, while alteration of laryngeal neural control mechanisms may also be present.

The authors thank Yukimasa Miyazawa, M.D., and Masato Suzuki, M.D. (both Assistant Professors in the Department of Surgery, Graduate School of Medicine, Chiba University, Chiba, Japan), for their assistance in performing this study. Megumi Shimoyama, M.D. (Assistant Professor, Department of Autonomic Physiology [C3], Graduate School of Medicine, Chiba University), greatly helped to improve this manuscript.