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From the Department of Anesthesiology, Plastic Surgery Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, People's Republic of China.
Address correspondence and reprint requests to Prof. Fu S. Xue, MD, Department of Anesthesiology, Plastic Surgery Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Ba-Da-Chu Road, Shi-Jing-Shan District, Beijing 100041, People's Republic of China. Address e-mail to fruitxue{at}yahoo.com.cn.
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Abstract |
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METHODS: After induction of anesthesia, the ProSeal LMA was inserted using the introducer tool with cricoid pressure advanced as far as possible, and the cuff pressure was set at 60 cm H2O. Ventilation adequacy and anatomic position were scored using measures previously described for ProSeal LMA assessment. Airway seal pressure was recorded. Cricoid pressure was then released, the ProSeal LMA further advanced and reseated, and the assessment repeated.
RESULTS: Lung ventilation scores, anatomic position scores, and airway seal pressure were significantly better after release of cricoid pressure and reseating of the ProSeal LMA than in the first position, where the ProSeal LMA was seated with cricoid pressure (P < 0.05). Expiratory tidal volume during intermittent positive pressure ventilation was similar with and without cricoid pressure, but peak inspiratory pressure decreased from 28 cm H2O with cricoid pressure to 14 cm H2O without cricoid pressure (P < 0.05).
CONCLUSIONS: Cricoid pressure applied before insertion hampered proper placement of the ProSeal LMA. Temporary cricoid pressure release during insertion allowed the device to be advanced to the proper position. After correct placement of the ProSeal LMA, application of cricoid pressure did not change tidal volume, but produced a significant increase in peak inspiratory pressure.
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Introduction |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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The ProSeal LMA is a modified laryngeal mask device with a double-cuffed design, which improves the seal around the glottis. The ProSeal LMA incorporates a drainage tube to provide a channel for regurgitated fluid and placement of an orogastric tube (12). The ProSeal LMA achieves a more effective seal than the classic LMA, facilitating orogastric tube placement and isolating the glottis from the esophagus when correctly positioned (12–14). The mucosal pressures exerted by the ProSeal LMA are similar to those exerted by the classic LMA (15).
There are no published data to assess whether cricoid pressure reduces ease of ProSeal LMA placement and hampers ventilation through it. This prospective cross-over study was designed to assess the influence of cricoid pressure on insertion of, and ventilation through, the ProSeal LMA in anesthetized, paralyzed adult patients.
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METHODS |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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On arrival at the operating room, standard monitors (AS/3;Datex, Helsinki, Finland) were applied. In this study, a ProSeal LMA (LMA-PS, The Laryngeal Mask Company Ltd., Henley-on-Thames, UK) of suitable size for each patient was determined according to the manufacturer's instructions (16). Three investigators were needed during each observation. The first one, a trained anesthesiologist (CWL or YCX), provided the cricoid pressure. Before study, they practiced cricoid pressure by producing a force of 3 kg (30 N) on a scale before each case, and revalidated the force afterwards by reproducing the same force with an independent observer recording the reading on the scale. The second anesthesiologist (FSX) ventilated the patient's lungs with a facemask, inserted the ProSeal LMA, and assessed lung ventilation and LMA position. The third anesthesiologist recorded the observed variables.
After routine administration of oxygen, anesthesia was induced with fentanyl 2 µg/kg and propofol 2 mg/kg injected IV. Neuromuscular block was produced with vecuronium 0.1 mg/kg administered IV. The patient's head was placed in the sniffing position with a firm pillow (6 cm in height) under the occiput. A 10 cm-diameter gauze roll was placed under the patient's neck for support. Cricoid pressure was applied with one hand (modified bimanual cricoid pressure) before induction at a force that could be tolerated by the patient. After loss of consciousness, the force was increased to 30 N. Two minutes after vecuronium injection, the ProSeal LMA was inserted with the introducer tool according to the manufacturer's instructions (12) until obvious resistance was felt. With cricoid pressure and the introducer tool in place, the cuff was inflated with air to a pressure of 60 cm H2O using a calibrated aneroid manometer. The inflation volume was recorded. The ProSeal LMA was connected to the breathing circuit and ventilation quality was assessed as described by Keller et al. (17). A 3-point ventilation score was assigned as follows: 1 = excellent, chest expansion obvious without gas leakage, 2 = adequate, chest expansion with obvious gas leakage, and 3 = impossible, minimal chest expansion and considerable gas leakage.
After assessment of ventilation, the anatomic position of the cuff was examined visually using a fiberoptic bronchoscope according to the scoring system described by Brimacombe and Berry (18). A 4-point anatomic position score was assigned as follows: 1 = only vocal cords visible, 2 = vocal cords plus posterior epiglottis visible, 3 = vocal cords plus anterior epiglottis visible, and 4 = vocal cords invisible.
After anatomic position assessment, the airway seal pressure was determined by setting the pop-off valve to limit peak airway pressure to 40 cm H2O and allowing airway pressure to increase at a fresh gas flow of 3 L/min until no further increase in pressure was observed. When a stable airway pressure was reached, the locations of gas leak were determined as the drainage tube [bubbling of soap solution (19)], mouth (audible), or stomach (epigastric auscultation).
Afterwards, cricoid pressure was released and the ProSeal LMA was further advanced with the introducer tool until obvious resistance was felt again (the proper position). Then the introducer tool was withdrawn, the cuff was inflated to 60 cm H2O, and the final volume of air inflated was noted. Lung ventilation, anatomic position, and airway seal pressure were assessed again as described above.
After the ProSeal LMA was further advanced, if ventilation was impossible, the ProSeal LMA was removed and reinserted. A maximum of three attempts was permitted. After three failures, the ProSeal LMA was replaced by an endotracheal tube.
If no gas leaked from the drainage tube during the airway seal pressure measurement, a lubricated orogastric tube (F14) was placed through the drainage tube. The position of the orogastric tube was confirmed by epigastric auscultation with a stethoscope during the injection of 30 mL air. A failed attempt was defined as failure to advance the orogastric tube. Three attempts at orogastric tube placement were allowed. The orogastric tube was removed immediately after insertion.
After the above evaluation was completed, intermittent positive pressure ventilation (IPPV) was performed with a tidal volume of 10 mL/kg, a ventilatory frequency of 12 breaths/min, an inspiratory time-respiratory cycle time ratio of 0.33, and a fresh gas flow of 2.5 L/min. Expiratory tidal volumes and peak inspiratory pressures were measured over five consecutive breaths during IPPV with or without cricoid pressure, respectively. The mean expiratory tidal volume and peak inspiratory pressure under each circumstance were then calculated.
At the end of the operation, the ProSeal LMA was removed when protective reflexes recovered and adequate spontaneous breathing was reestablished. Complications relating to insertion, maintenance, and removal of the ProSeal LMA such as laryngospasm, respiratory tract obstruction, breath holding, coughing, aspiration, gastric inflation, nausea, vomiting, and trauma to the lips, tongue, or teeth, and blood staining the device were recorded.
The data obtained were analyzed with SPSS statistical software (Version 11.5, SPSS Inc., Chicago, IL). Parametric data were analyzed with paired Student's t-test. Nonparametric data were analyzed using a 
2 test. Previous studies showed that the success rate of excellent ventilation via the ProSeal LMA without cricoid pressure was 81%–100% (9). We considered that a clinically important difference in the success rate would be 25%. At least 50 patients would be required to detect this difference with a power of 0.8–0.9. Unless otherwise stated, data are expressed as mean ± sd or number of patients. P < 0.05 was considered significant.
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RESULTS |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Releasing cricoid pressure and advancing the ProSeal LMA significantly improved the ventilation and anatomic position scores (P < 0.05, Table 1). The inflation volume required to obtain a cuff pressure of 60 cm H2O was significantly lower when the ProSeal LMA was inserted with cricoid pressure than the volume required after cricoid pressure was released and the ProSeal LMA advanced and reseated (20 ± 5 vs 25 ± 5 mL, P < 0.05). When the ProSeal LMA was inserted with cricoid pressure, airway seal pressure obtained with a cuff pressure of 60 cm H2O was 21 ± 7 cm H2O. After the ProSeal LMA was advanced to the proper position following release of cricoid pressure, the airway seal pressure increased to 27 ± 7 cm H2O (P < 0.05).
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When airway seal pressure was measured under two circumstances, there was no gas leakage from the drainage tube or gastric insufflation in any patient. After further advancement of the ProSeal LMA to the proper position, orogastric tube placement via the drainage tube was successful on the first attempt in all patients. After the ProSeal LMA was reseated following release of cricoid pressure, the tidal volume with cricoid pressure was not different from that without cricoid pressure (501 ± 83 vs 498 ± 81 mL, P > 0.05), but the peak inspiratory pressure decreased from 28 ± 5 cm H2O with cricoid pressure to 14 ± 2 cm H2O without cricoid pressure (P < 0.05).
After device removal in the operating room, blood staining on the ProSeal LMA was found in 13 patients (26%). There were no other complications relating to insertion, maintenance, and removal of the device in any patient.
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DISCUSSION |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Our results showed that after the ProSeal LMA was inserted with cricoid pressure, only 28% of patients had excellent or adequate ventilation. The reason for impaired ventilation in the remaining patients was excessive leakage and/or airway obstruction. Fiberoptic visualization showed that the ProSeal LMA was not positioned deeply enough, so that the seal around the larynx was not complete. In contrast, after further advancement of the ProSeal LMA following release of cricoid pressure, ventilation was possible and orogastric tube placement via the drainage tube was successful in all patients. Additionally, the glottis was also seen through the fiberoptic bronchoscope in most patients. These results suggested that cricoid pressure applied before insertion impeded the ProSeal LMA advanced to the proper position. This is in agreement with the results of previous studies on the classic LMA (7–10,20,21).
Positioning the ProSeal LMA correctly in the hypopharynx is the key to preventing regurgitation and aspiration of gastric contents (12–14). A cadaveric study showed that with the drainage tube of the ProSeal LMA unclamped, aspiration occurred only when the cuff was fully deflated, suggesting that cuff inflation was necessary to isolate the glottis from the esophagus and to correctly align the drainage tube with the esophagus (23). In the present study, the results from airway seal pressure measurement, scores of the cuff anatomic positions, and orogastric tube placement indicated that the air inflated into the cuff under cricoid pressure did not impede further advancement of the ProSeal LMA to the proper position with the introducer tool after release of cricoid pressure in most patients.
Previous works confirmed that cricoid pressure harmed ventilation through the classic LMA when applied before insertion (7) and significantly decreased tidal volume during pressure-controlled ventilation when applied after the classic LMA was inserted to the proper position (11). The decrease in the tidal volume during IPPV with the classic LMA was attributed to gas leak because of the increase in the airway resistance caused by cricoid pressure (7). However, our results showed that, when applied after further advancement of the ProSeal LMA to the proper position, cricoid pressure caused a significant increase in peak inspiratory pressure, but no significant change in tidal volume. The lack of difference in tidal volume with and without cricoid pressure may account for the higher airway seal pressure obtained with the ProSeal LMA. The airway seal pressure obtained with the classic LMA has been reported to be 6 to 12 cm H2O lower than that with the ProSeal LMA (12–14,24–27).
Although use of the ProSeal LMA is generally contraindicated in patients at risk of aspiration (16), when confronted with the "cannot intubation cannot ventilate" scenario ProSeal LMA insertion with transient release of cricoid pressure may save the patient's life (3–5). Therefore, we consider that, for these patients, the ProSeal LMA may be inserted and inflated under cricoid pressure, then further advanced to the proper position with the introducer tool after temporary release of cricoid pressure. Cadaver and clinical studies have confirmed that when the drainage tube was correctly positioned, the ProSeal LMA should be able to provide protection from fluid regurgitation (23,28), which suggests that cricoid pressure reapplication after the ProSeal LMA insertion might be unnecessary. In addition, orogastric tube insertion via the drainage tube may also prevent gastric insufflation that may follow difficult ventilation by conventional facemask.
In our study, the incidence of blood staining on the ProSeal LMA after removal was 26%, which was higher than the results reported in previous studies (9% (25) to 18% (24)). This was possibly related to the application of the rigid introducer tool and the resistance to the ProSeal LMA advancement caused by cricoid pressure in this study.
In summary, the present study demonstrates that cricoid pressure applied before insertion hampers placement of the ProSeal LMA. Temporarily releasing cricoid pressure facilitates proper placement of the ProSeal LMA. After correct placement of the ProSeal LMA, cricoid pressure does not change tidal volume during IPPV, but produces a significant increase in peak inspiratory pressure.
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Footnotes |
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This manuscript has been presented as a Poster Discussion at the 2006 Annual Meeting of the American Society of Anesthesiologists in Chicago, IL.
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REFERENCES |
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