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Laryngeal Mask Airway ProSealTM Versus Laryngeal Mask Airway-ClassicTM in Pediatric Patients
Department of Anaesthesia and Intensive Care Therapy, Philipps University Marburg, Germany
Address correspondence and reprint requests to Kai Goldmann, MD, DEAA, Department of Anesthesia and Intensive Care Therapy, Philipps University Marburg, 35033 Marburg, Germany. Address e-mail to Kaigoldmann1{at}aol.com.
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Abstract |
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P-LMA and C-LMA in random order. Ease of insertion, quality of initial airway, fiberoptic position, airway leak pressure, and maximum tidal volume were determined. G-tube placement was assessed for the P-LMA. Ease of insertion was similar for both devices. The quality of the initial airway was better for the P-LMA (P = 0.01). Airway leak pressure in neutral head position (22.6 versus 18.5 mbar; P = 0.003), maximum flexion (37 versus 26.3 mbar; P < 0.001), maximum extension (15.2 versus 13 mbar; P = 0.045), and maximum tidal volume (1088 versus 949 mL; P = 0.002) were significantly better for the P-LMA. Air entry into the stomach occurred with the C-LMA but not with the P-LMA (P = 0.014). G-tube placement was possible in all patients. The reliability of g-tube placement and the significantly increased airway leak pressure found in this investigation might have important implications for use of the size P-LMA for positive pressure ventilation in children. |
Introduction |
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The first pediatric size P-LMA recently became available. The purpose of our study was to test the hypothesis that in anesthetized pediatric patients, the size 2 P-LMA forms a more effective seal, allows for larger TV than the size C-LMA, and facilitates g-tube placement.
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Methods |
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All patients were premedicated orally with 0.4–0.5 mg/kg of midazolam, and EMLATM cream (AstraZeneca, Wilmington, DE) was applied to the back of both hands 30 min before the induction of anesthesia. Standard monitoring consisted of a precordial stethoscope, an automated arterial blood pressure monitor, electrocardiography, pulse oximetry, and capnography. In all patients, general anesthesia was induced IV with 0.02 mg/kg of alfentanil followed by 3–5 mg/kg of propofol. Anesthesia maintenance was with 10–15 mg · kg–1 · h–1 of propofol. No neuromuscular blocking drugs were used. After cessation of spontaneous ventilation, the lungs of the patients were ventilated manually (ManV) via a face mask until a sufficient depth of anesthesia, indicated by a heart rate 15%–20% less than the baseline value, was reached. If required, additional boluses of 0.01–0.02 mg/ kg of alfentanil and 1–2 mg/kg of propofol were given. The first LMA was then placed, and measurements were taken. After completion of measurements with the first mask, it was removed, the second mask placed, and the same measurements recorded for the second mask. The order of mask insertion in each patient was randomized before commencement of the trial and was contained in a sealed envelope that was opened before the induction of anesthesia by a person not participating in the investigation. Both types of masks were placed using the standard technique (12). The breathing tube was connected to a circle system of an anesthesia machine (Primus, Draeger, Luebeck, Germany), and ManV commenced after the cuff had been inflated to an intracuff pressure of 60 mbar (VBM Cuff Pressure Gauge, VBM Medizintechnik, Sulz a.N., Germany). The epigastrium and larynx were auscultated before the device was taped in the manner described in the instruction manual (13). During the initial study period, the lungs of the patient were ManV; once the measurements had been completed, pressure-controlled ventilation was used to ventilate the lungs throughout the procedure, except for those patients undergoing a very short procedure, in whom reversion to spontaneous breathing was permitted.
The total doses of propofol and alfentanil given throughout the period of measurements were recorded. The number of attempts for placement of each mask was recorded. A maximum of two attempts was allowed for each device. Removal of the mask from the mouth was considered a failed attempt. Ease of placement was recorded from 0–10 on a visual analog scale with 0 indicating placement with one movement without interruption and no resistance felt and 10 indicating that placement was not possible. The quality of initial airway was assessed during ManV, with the pop-off valve set to limit peak airway pressure to 20 mbar. The initial airway was judged as follows: excellent = no audible leak, good = an audible leak with relevant loss of air but sufficient ventilation, as indicated by a Petco2 <40 mm Hg, and unacceptable = clinically relevant loss of air and insufficient ventilation, requiring replacement of the device.
Air entry into the stomach and abnormal airway sounds over the larynx were recorded. After placement of the P-LMA, a DT-test, as described in the P-LMA instruction manual (13), was conducted to confirm correct position of the mask. A small amount of lubricant jelly was used to close the proximal end of the DT. A slight up-and-down movement of the lubricant conus was recorded as a positive DT-test, indicating that the DT did not have access to the larynx. The intracuff pressure was checked before commencement of the measurements and, if required, it was adjusted to 60 mbar. Maximum TV was determined in the neutral position of the head by squeezing the completely filled anesthesia circuit bag until an audible leak was noted in the mouth of the patient. Of three attempts, the largest expired TV indicated by the anesthesia machine was recorded. Pleak was measured after the fresh gas flow was set to 3 L/min, and the expiration valve was closed (7). The airway pressure at which an audible leak in the mouth of the patient occurred was recorded as the Pleak. The expiration valve was opened if the Pleak reached 40 mbar without an audible leak. The Pleak value on the monitor of the anesthesia machine was not visible to the observer of the audible leak. The order of the head position was neutral, maximum flexion, and maximum extension. The lungs of the child were ManV between the consecutive measurements. Fiberoptic examination of the position of the LMA was performed thereafter (2.8 mm flexible endoscope; Karl Storz, Tuttlingen, Germany). The position of the LMA was graded in accordance with the fiberoptic scoring system previously described: (a) vocal cords not seen, (b) vocal cords plus anterior epiglottis seen, (c) vocal cords plus posterior epiglottis seen, and (d) only vocal cords seen (14).
When the P-LMA was used first, a 12F g-tube was then placed, and correct placement was confirmed by auscultation of the epigastrium during injection of a small amount of air. Gastric fluid was aspirated using a syringe, and the amount of fluid was recorded. The P-LMA and g-tube were removed, and the same measurements took place for the C-LMA, which was left in place for the rest of the procedure. When the P-LMA was used second, the g-tube remained in place until the end of the case. It was removed under suction before the P-LMA was removed once the child was fully awake. Both masks were inspected after removal for signs of blood. Any adverse event, apparent oropharyngeal injury, or problems with the devices were documented. During the postoperative visit, the child was asked if he or she had a sore throat.
The primary variable tested was Pleak. The mean Pleak values for various pediatric sizes C-LMA in the literature vary depending on the study population, size of LMA, P intracuff of the LMA, and fresh gas flow used during measurements (6,15,16). We chose the Pleak of 17 mbar found in the Gursoy et al. study (16) to calculate the sample size. A 20% higher Pleak for the P-LMA was considered to be clinically relevant. A sample size of 22 patients was calculated to detect a projected difference of 20% between the groups for a type I error of 0.05 and a power of 0.9. Allowing for possible dropouts because of an inability to place one of the masks within two attempts, we chose to examine 30 patients. Results were analyzed using the SPSS (SPSS Inc., Chicago, IL) computer program. Unless otherwise stated, data are expressed as mean ± sd (95% confidence intervals) or mean (range). The distribution of data was determined using Kolmogorov-Smirnov analysis. Statistical analysis was performed using paired t-test, Wilcoxon test, and Fisher’s exact test. Results were considered statistically significant for a P value <0.05.
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Results |
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All devices were placed within two attempts. First-attempt insertion success rate was similar between P-LMA and C-LMA (26 versus 29, respectively). Ease of insertion was similar for both devices, but initial quality of airway was significantly better for the P-LMA (Table 1). Air entry into the stomach was more common with the C-LMA than the P-LMA (6 versus 0; P = 0.014). The PLMA DT-test was positive in 27 patients and inconclusive in 2 patients. The DT-test indicated access of the DT to the airway in one case, which lead to repositioning of the mask with a resulting positive DT-test, indicating correct position.
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Mean maximum TV and mean Pleak of all 3 positions tested were significantly higher for the P-LMA (Table 2). Pleak was higher for the C-LMA in 1 of 3 positions tested in 3 patients. Only in 1 of those patients was Pleak higher for the C-LMA in all 3 positions tested. Maximum TV was smaller with the P-LMA than with the C-LMA in 5 patients. A smaller TV was associated with a lower Pleak in 2 patients and a higher Pleak in 3 patients.
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Vocal cord visibility was better with the P-LMA than the C-LMA (30 versus 27 patients), and there was a statistically significant difference in the fiberoptic score between the masks (P < 0.001) (Table 3). With the P-LMA, the vocal cords seemed compressed, or supraglottic soft tissue-mucous membrane bulged into the airway, and lead to some degree of airway obstruction in 5 patients. In 2 patients, this upper airway obstruction was associated with a reduction of TV, as indicated by a smaller TV than with the C-LMA. In 1 patient, this lead to a clinically relevant impairment of ventilation, as indicated by the inability to produce an end-tidal CO2 <45 mm Hg despite adjustment of the ventilator settings (P-LMA, Pleak neutral position = 24 mbar and maximum TV = 280 mL; ventilator settings, maximum respiratory rate = 18 breaths/min, peak inspiratory pressure = 24 mbar, positive end-expiratory pressure = 3 mbar, and I:E ratio = 1:1; C-LMA, Pleak neutral position = 17 mbar and maximum TV = 480 mL; body weight of the patient = 24 kg). With the C-LMA, compression of the vocal cords, or supraglottic soft tissue-mucous membrane, was not found in any case.
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A g-tube was placed without any difficulties in 30 patients at the first attempt. Gastric fluid was aspirated in 17 patients, with the volume ranging from 2–50 mL. One patient regurgitated 10 mL of bile-stained fluid that drained out of the DT immediately after placement of the P-LMA. The correct position of the DT, indicated by a positive DT-test, and the sufficient seal of the P-LMA (Pleak, 31 mbar) helped to prevent pulmonary aspiration in this case. There were no adverse events other than the case of clinically relevant airway obstruction and the case of regurgitation. A trace of blood was seen on the mask after removal in 4 patients: 2 C-LMA and 2 P-LMA. One child complained about a sore throat afterward.
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Discussion |
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The first pediatric size P-LMA recently became available. Various authors have shown that the adult P-LMA forms a more effective seal than the C-LMA and facilitates g-tube placement (8,21,22). The results of this investigation indicate that this is also true for the size P-LMA. We found that the mean Pleak of the size P-LMA was significantly higher in all three positions studied and allowed a larger mean TV than the same size of C-LMA. In addition, air entry into the stomach was not noted with the P-LMA in any case but occurred in 6 cases with the C-LMA. These findings at the beginning of the general anesthetic and the ability in all cases to place a g-tube in the stomach through the DT of the P-LMA at the first attempt suggest that the P-LMA might be a more suitable device for PPV in pediatric patients because it avoids gastric insufflation and facilitates emptying of the stomach. The case of regurgitation and successful prevention of pulmonary aspiration indicates that a correctly positioned P-LMA can protect the airway against regurgitated fluid. A more effective seal of the P-LMA, as indicated by a higher mean Pleak, might make it a more suitable supraglottic airway in patients with poor pulmonary compliance that may require higher peak airway pressures to ventilate, such as patients with cystic fibrosis or bronchopulmonary dysplasia. Therefore, the P-LMA could be a useful alternative to the ETT in such patients. Considering the preliminary nature of our study, this certainly needs to be addressed in future studies of patients with cystic fibrosis or bronchopulmonary dysplasia before any such recommendation can be made.
In adults, use of the P-LMA can be associated with some degree of upper airway obstruction (23,24). For this reason, we used maximum TV with both masks as a simple test to aid with the diagnosis of upper airway obstruction. Our results show that a higher Pleak with the size P-LMA in general (89%) allows a larger TV. However, in 3 patients (11%), a higher Pleak was associated with a reduced TV, indicating some degree of clinically relevant upper airway obstruction. In 2 patients, fiberoptic examination revealed that upper airway obstruction was caused by compression of the vocal cords, and in 1 patient, this lead to the inability to achieve normal ventilation of the patient. Because the procedure was short, we did not exchange the P-LMA for the C-LMA but adjusted the respirator setting to produce an end-tidal CO2 of 45 mm Hg. However, for a longer procedure, we would probably have exchanged the P-LMA for a different airway. Because it is important to do this before commencement of the surgical procedure, the anesthesiologist should be alerted to the potential of clinically relevant upper airway obstruction during use of the size P-LMA. Although this investigation comprises a small number of patients, an incidence of 2 of 30 patients (6.6%) suggests that the incidence of upper airway obstruction with the size P-LMA might be similar to that reported for use of the P-LMA in an adult population (24).
In contrast to the studies conducted in adult patients, we did not find that placement of the mask was more difficult, or that fiberoptic position of the airway tube in relation to the laryngeal structures was worse, for the P-LMA than the C-LMA. In fact, the incidence of an obstructed view of the larynx was less frequent with the P-LMA. Brimacombe and Keller (21) speculated that the larger cuff of the P-LMA was responsible for difficulties in placement and an increased likelihood of epiglottic down-folding. However, in contrast to the pediatric C-LMA, the design of the size P-LMA is not just a scaled-down version of the adult P-LMA. The main difference is that it does not contain a large dorsal cuff, so the device is less bulky than the adult version. This might explain why placement of the pediatric P-LMA was as easy as the C-LMA. Another explanation could be that clinicians who have been using the P-LMA in adults for some time might benefit from their experience gained in adults when using the P-LMA in children.
In children, one can often find the long epiglottis lying against the C-LMA mask aperture bars obstructing the view of the larynx to some degree. The P-LMA does not contain aperture bars. Instead, the DT, in addition to its principle function, is believed to prevent the epiglottis from entering and obstructing the mask aperture. However, with the P-LMA, the epiglottis can enter the bowl of the mask until it comes to a hold against the DT. This might explain why an unobstructed view was more common with the P-LMA. Whether this has any clinical relevance other than avoiding obstruction of the mask aperture should be addressed in future studies.
The main limitations of this preliminary study are the small number of patients studied, their lack of comorbid diseases, and the role of PPV. Therefore, we cannot draw any conclusion about the feasibility of the P-LMA for specific subgroups of patients or PPV. However, 26 patients underwent pressure-controlled ventilation and, in all except the case described in detail above, this was possible without any problems. Another limitation lies with the fact that the measurements were taken only at the beginning of anesthesia, and different results might have been observed throughout the whole anesthetic. However, Epstein et al. (15) were able to show for the C-LMA that if the LMA seal is acceptable at the start of the procedure, it will either remain unchanged or improve.
In conclusion, we were able to show that the size P-LMA offers some advantages over the same size of C-LMA in this crossover investigation. The reliability of g-tube placement and the increased Pleak might have important implications for the use of this device for PPV in pediatric patients. The case of regurgitated gastric fluid indicates that the size P-LMA has the same potential as the adult size P-LMA to protect the airway against aspiration. The unique design of the pediatric P-LMA seems to be beneficial with respect to the first-attempt placement success rate and frequent anatomically correct position. However, the user needs to be aware that the size P-LMA can result in clinically relevant upper airway obstruction and the need to replace the P-LMA with a C-LMA or an ETT.
The authors wish to thank Dr. John Henderson for critically reviewing the manuscript.
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Footnotes |
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Accepted for publication November 16, 2004.
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