Pharyngeal airway devices can exert substantial pressures against the pharyngeal mucosa. The authors assess the relation between pharyngeal mucosal perfusion and directly measured mucosal pressure (MP) in the posterior pharynx using a fiberoptic technique with a modified cuffed oropharyngeal airway (COPA). The authors also measure in vivo intracuff pressure (CP), airway sealing pressure and MP at four locations using an unmodified COPA.
Twenty adult patients, American Society of Anesthesiologists status I or II, undergoing general anesthesia were allocated randomly to receive either (1) a COPA with a millimeter microchip sensor fixed on the external cuff surface to record distal posterior pharyngeal MP or (2) a COPA with a fiberoptic scope inserted inside the cuff to record digitized images of the distal posterior pharyngeal mucosa. MP and digitized images were obtained at the same location over an in vivo CP range of 10-160 cm H2O in 10- to 20-cm H2O increments. The digitized images were scored according to blood vessel caliber and mucosal color by two investigators blinded to MP and CP. In an additional 20 matched patients, in vivo CP, airway sealing pressure, and MP was measured at four different cuff locations (corresponding to the anterior, lateral, and posterior pharynx and the distal oropharynx) with increasing cuff volume.
Blood vessel caliber and mucosal color was normal in all patients when the mean mucosal pressure was 17 cm H2O. Blood vessel caliber was first reduced when the mean mucosal pressure was 34 cm H2O. There was a progressive incremental reduction in blood vessel caliber and mucosal color when the mean mucosal pressure increased from 34 to 80 cm H2O (P < or = 0.05). Complete blood vessel collapse and mucosal paling first occurred with the mean mucosal pressure was 73 cm H2O and was present in 90% of patients when the mean mucosal pressure was 80 cm H2O. Mean MP was always higher in the posterior pharynx compared with the other locations when the cuff volume was 20 ml or greater (P < 0.001). In vivo CP is an excellent predictor of mucosal pressure. Mean (95% confidence interval [CI]) MP in the posterior pharynx was 35 (5-67) and 78 (50-109) cm H2O when the airway sealing pressure was 10 (6-16) and 17 (13-21) cm H2O respectively.
Pharyngeal mucosal perfusion is reduced progressively in the posterior pharynx when MP is increased from 34 to 80 cm H2O with the COPA. CP provides reliable information about MP and should be less than 120 cm H2O to prevent mucosal ischemia.
PHARYNGEAL airway devices can exert mucosal pressures greater than those considered safe for the tracheal mucosa. 1Recent studies have shown that mucosal pressures for the cuffed oropharyngeal airway (COPA), 2laryngeal mask airway, 2,3and intubating laryngeal mask airway 3vary from 4–53, 1–75 and 7–160 cm H2O, respectively, depending on cuff volume and anatomic location. Tracheal mucosal perfusion in humans progressively decreases when mucosal pressures exceed 30 cm H2O and ceases when pressures are 50 cm H2O, 4but there are no published data about pharyngeal mucosal perfusion. In the following study, we assess the relation between pharyngeal mucosal perfusion and directly measured mucosal pressure in the posterior pharynx using a fiberoptic technique 4with a modified COPA. We also measure in vivo intracuff pressure (CP), airway sealing pressure (ASP), and mucosal pressure (MP) at four locations with an unmodified COPA.
Methods
Twenty adult patients, American Society of Anesthesiologists status I or II, undergoing general anesthesia for minor surgery were allocated randomly (by opening a sealed envelope) to receive either (1) a COPA with a strain-gauge silicone microchip sensor fixed on the external surface of the distal posterior cuff (group 1) or (2) a COPA with a fiberoptic scope inserted into the cuff to provide a view of the mucosa approximately 1 cm in diameter from within the distal posterior cuff (group 2). Ethics committee approval and written informed consent were obtained. Patients were excluded if they were at risk of aspiration, had respiratory tract disease, or were otherwise unsuitable for the COPA.
Mucosal pressure was measured using a strain-gauge silicone microchip sensor (Codman MicroSensor; Johnson and Johnson Medical Ltd., Bracknell, UK) attached to the external surface of the COPA with a clear adhesive dressing 45 μm thick (Tegaderm; 3M, Ontario, Canada). The sensor tip had a diameter of 1.2 mm and a length of 6 mm; the sensor cable had a diameter of 0.7 mm, a length 100 cm, a functional pressure range of −50 to 250 mmHg, a temperature sensitivity of less than 0.1 mmHg/°C, a zero drift of less than 3 mmHg/24 h, a frequency response of 0–10 Hz, and was accurate to ±2%(manufacturer's specifications). Attachment of the sensor was performed manually by placing the sensor in the correct position on the distal posterior portion of the COPA cuff and then overlaying it with the adhesive dressing (fig. 1). The sensing element in the sensor tip was orientated such that its flat surface was parallel to and directed 90° away from the COPA surface. This ensured that the flat surface of the sensing element was directly facing the mucosa. The position (orientation) of the sensor was checked in vitro over the entire inflation range before and after use in each patient by visual inspection. The sensor was zeroed after attachment to the COPA. The accuracy of the measurement system was tested in vitro before and after use in each patient by submerging the cuff portion in water at 37°C to a depth of 13.6 cm (10 mmHg) and 40.8 cm (30 mmHg) and noting the pressure readings from the sensor.
Fig. 1. Mucosal pressure sensor attached to the distal posterior cuff of the cuffed oropharyngeal airway.
Fig. 1. Mucosal pressure sensor attached to the distal posterior cuff of the cuffed oropharyngeal airway.
Digital images of the mucosa were obtained using a modification of a technique used by Seegobin and van Hassalt 4to measure tracheal mucosal perfusion. The COPA cuff, which is transparent, was modified in vitro to accommodate a fiberoptic scope with an external diameter of 3.6 mm. A small hole was made in the proximal posterior cuff, the fiberoptic scope was inserted so that it was in position within the distal posterior cuff, and the hole was closed with an adhesive dressing (fig. 2). The fiberoptic scope was connected to a recording system that produced high-resolution digital images of the mucosa (including blood vessels) in contact with the distal posterior cuff. Both the sensor and the fiberoptic scope were located in identical positions on each COPA.
Fig. 2. Fiberoptic scope within the distal posterior cuff of the cuffed oropharyngeal airway.
Fig. 2. Fiberoptic scope within the distal posterior cuff of the cuffed oropharyngeal airway.
A standard anesthesia protocol was followed. Anesthesia was induced with 1 μg/kg fentanyl and 2.5 mg/kg propofol and maintained with oxygen in air (30% O2) and a propofol infusion of 6 mg · kg−1· h−1. Muscle relaxation was with 0.6 mg/kg rocuronium. The train-of-four count was maintained at one or less. A No. 11 COPA device was used for all patients. The integrity of the cuff was checked preinsertion by inflation with 20-ml air and visual inspection. The cuff was then evacuated fully, and an experienced COPA user (C.K., > 200 uses) inserted and fixed the device according to the manufacturer's instructions. 5
Mucosal pressure, or the fiberoptic view, was documented at an CP of 10 cm H2O and after each additional 10 cm H2O, up to 80 cm H2O, and then after each additional 20 cm H2O, up to 160 cm H2O. The CPs were adjusted using a Digital Cuff Pressure Monitor (Mallinckrodt Medical, Athlone, Ireland) before each measurement. Care was taken to avoid displacement of the COPA during testing. Measurements were made at zero airway pressure with the head–neck in the neutral position, with the occiput resting on a firm pillow 7 cm in height and with chin lift applied. Between measurements, the patients’ lungs were inflated manually with a lightweight circuit connected to the COPA. During measurements, the blood pressure was measured at 2-min intervals.
The digitized images were coded and scored by two observers blinded to the CP, the MP, and to each other's scores. Blood vessel caliber was scored as follows: 1: normal diameter; 2: first reduction in diameter; 3:≥ 50% reduction in diameter; 4: completely collapsed (no vessels seen). Mucosal color was scored as follows: 1: uniform pink; 2: first pale areas detected; 3:≥ 50% pale areas; 4: completely pale.
In an additional 20 paralyzed, anesthetized patients (matched with groups 1 and 2 for age, height, weight and gender), we measured in vivo CP, ASP, and MP at four cuff locations using an unmodified No. 11 COPA (group 3). Sensors were attached to (approximate corresponding mucosal areas):(1) the anterior middle part of the cuff (base of tongue);(2) the lateral cuff (lateral pharynx);(3) the proximal posterior cuff (distal oropharynx) and (4) the posterior cuff (posterior pharynx), as in group 1. The pilot balloon was attached via a three-way tap to a 10-ml syringe and a calibrated pressure transducer accurate to ±5%. The CP was reduced to −55 cm H2O in vitro . The COPA was inserted and fixed and the sensors applied and tested as described previously. In vivo CP, ASP, and MP were documented at zero volume and after each additional 10 ml, up to 60 ml.
Statistics
The distribution of data was determined using Kolmogorov-Smirnov analysis. Statistical analysis was with paired t test for demographic data. Multinominal logistic regression analysis was used for comparing MP with blood vessel caliber and mucosal color score. The relation between MP and in vivo CP was determined using Pearson product moment correlation coefficient. Interobserver reliability for blood vessel caliber and mucosal color was determined using a κ statistic. The reliability of cuff volume and in vivo CP to predict MP was determined with the intraclass correlation coefficient (ICC). Scores for statistical measurements with the κ statistic and intraclass correlation coefficient range from 0 to 1, in which the former shows no reliability and the latter shows perfect reliability. A score greater than or equal to 0.75 is considered excellent. Unless otherwise stated, data are presented as the mean (95% confidence intervals). Significance was taken as P < 0.05.
Results
There were no demographic differences among groups, and mean blood pressure was similar during measurements between the fiberoptic and MP groups (table 1). The position and orientation of the sensors were identical and the pressures were accurate before and after usage. Mucosal pressures, blood vessel caliber and mucosal color scores are presented in table 2. Interobserver reliability for blood vessel caliber and mucosal color was excellent at 0.94 and 0.9, respectively. For group 1, there was a significant correlation between in vivo CP and MP (P < 0.001). For group 2, there was a significant correlation between in vivo CP and blood vessel caliber (P < 0.001) and mucosal color (P < 0.001). Blood vessel caliber and mucosal color were normal in all patients when the mean MP was 17 cm H2O. Blood vessel caliber was first reduced when the mean MP was 34 cm H2O. There was a significant reduction in blood vessel caliber and mucosal color when the mean MP increased from 34 to 38 cm H2O (P = 0.05), from 41 to 47 cm H2O (p = 0.05), from 68 to 73 cm H2O (P ≤ 0.04), and from 73 to 80 cm H2O (P < 0.001). Complete blood vessel collapse and mucosal paling first occurred with the mean MP was 73 cm H2O and was present in 90% of patients when the mean MP was 80 cm H2O. In vivo CP, ASP, and MP at the four cuff locations are presented in table 3. Mucosal pressure was always significantly higher in the distal pharynx compared with the other locations when the cuff volume was 20 ml or more (P < 0.001). There was a significant correlation between in vivo CP and MP at all mucosal locations (table 4). In vivo CP is an excellent predictor of MP.
Table 2. Mucosal Pressure, Blood Vessel Caliber, and Mucosal Color Scores over the Intracuff Pressure Range 0–160 cmH2O
Discussion
These data show that mucosal perfusion is reduced progressively in the posterior pharynx when MP is increased from 34 to 80 cm H2O. These values are higher than those obtained for the tracheal mucosa by Seegobin and van Hassalt 4(30–50 cm H2O) using a similar technique. We measured MP directly, whereas Seegobin and van Hassalt measured in vivo CP and assumed that MPs were similar. 4Directly measured tracheal MP tends to be lower than in vivo CP. 6,7The CP at which tracheal mucosal blood flow is reduced in humans using the fiberoptic technique 4is similar to the value obtained in dogs using the radioactive microsphere technique. 8
We measured mucosal perfusion in the posterior pharynx. This location was selected because positioning the fiberoptic scope at other intracuff locations was difficult technically. It has been shown in the rabbit 9and in humans 4that tracheal mucosal blood flow is less over the tracheal rings compared with the membranous portion. 9It is possible that blood flow might be different in mucosa overlaying protruding bony or cartilaginous pharyngeal surfaces. Ideally, we would have measured MP and perfusion in each patient. However, during a pilot study, we found that the adhesive necessary to fix the pressure probe impeded the quality of the images. We also were concerned that the pressure probe or adhesive might influence mucosal perfusion.
We found that MP for the COPA is greatest in the posterior pharynx. This supports the findings of our previous study 2and suggests that if perfusion to the posterior pharynx is adequate, it probably will be adequate in other parts of the pharynx. We also found that MP is high even when the ASP is relatively low (10 cm H2O). We speculate that this accounts for the high incidence of sore throat (35%) found in a recent double-blind trial in which the mean CP was 72 cm H2O and the ASP was 16 cm H2O. 10There are no published studies assessing the relation between pharyngeal MP, duration of application, and mucosal damage. However, Nordin 11showed that tracheal mucosal damage increases with increasing MP and, to a lesser extent, duration of application, although this finding predates the use of biocompatible cuff materials.
Finally, our data show that in vivo CP with the COPA is an excellent predictor of directly measured pharyngeal MP. By contrast, in vivo CP with the tracheal tube is only a moderate predictor of tracheal MP. 7We found that mucosal perfusion is reduced in the posterior pharynx in approximately 30% of patients when the CP is 30 cm H2O and stops in approximately 40% of patients when the CP is 140 cm H2O. We recommend that CP should be monitored and maintained at 120 cm H2O or less to prevent mucosal ischemia. Ideally, CP should be less than 30 cm H2O, but the effectiveness of the seal is inadequate for most clinical purposes at this CP.
We conclude that pharyngeal mucosal perfusion is reduced progressively in the posterior pharynx when MP is increased from 34 to 80 cm H2O with the COPA. Intracuff pressure provides reliable information about MP and should be less than 120 cm H2O to prevent mucosal ischemia.
The authors thank Alison Berry and Nick Brimacombe for advice with the manuscript.
