J R Brimacombe1. 1. Department of Anaesthesia and Intensive Care, Cairns Base Hospital, Australia.
Abstract
STUDY OBJECTIVE: To obtain data about the safety and efficacy of the size 5 laryngeal mask airway (LMA), which is a scaled-up version of the size 4 and is generally recommended for patients over 90 kg, for positive pressure ventilation (PPV), ease of insertion, oropharyngeal and gastric insufflation pressures, fiberoptic positioning, and complication rates. DESIGN: Prospective survey. SETTING: Teaching hospital. PATIENTS: 179 patients undergoing PPV with the size 5 LMA. INTERVENTIONS: The clinical criteria for using the size 5 LMA and the PPV technique were weight above 90 kg or an inadequate seal with a size 4 LMA and surgery estimated to last longer than 45 minutes. Anesthesia was standardized and included fentanyl/propofol for induction, N2O/O2/isoflurane 0.5% to 2% for maintenance, and atracurium for muscle relaxation. Two 20-second attempts were allowed with the standard recommended technique, followed by a single attempt with the Guedel technique. The LMA cuff was then inflated and the airway pressure at which either oropharyngeal leak or gastric insufflation occurred was determined by closure of the expiratory valve and anterior neck followed by epigastric auscultation. MEASUREMENTS AND MAIN RESULTS: The age and weight range were 15 to 82 years and 46 to 153 kg, respectively. 29% of patients had a body mass index (BMI) above 30 kg/m2. On 31 occasions the size 5 was used following an inadequate seal with the size 4. The weight range of this subgroup was 46 to 87 kg. The device was placed within 20 seconds in 94% and there were no failed placements within three attempts. Gastric insufflation was detected before oropharyngeal leak in 17% and oropharyngeal leak was detected first in 73%. In 10% of patients there was no leak at an inspiratory pressure of 45 cm H2O. Mean (range) for gastric insufflation pressure was 31 (range 23-45) cm H2O. Mean (range) for oropharyngeal leak was 33 (range 8-44) cm H2O. The mean (range) airway pressure was 17 (range 13-26) at tidal volumes of 10 ml/kg. At this tidal volume, 97.2% of patients could be ventilated without gastric insufflation and 98.3% without an oropharyngeal leak. At tidal volumes of 8 ml/kg no patient had gastric insufflation and 0.7% had an oropharyngeal leak. Oropharyngeal leak pressure of less than 15 cm H2O occurred in 11 patients. There was no correlation between fiberoptic score or Mallampati score and either gastric insufflation or oropharyngeal leak. The incidence of problems was 3% and the oxygen saturation remained above 94%. There was no correlation between problems, leak pressures, and BMI. CONCLUSIONS: Positive pressure ventilation with the size 5 LMA is safe and effective with a low failure/problem rate using tidal volumes of 8 to 10 ml/kg, even in those patients who are moderately obese. The device is suitable for patients weighing under 90 kg in whom the seal with the size 4 is inadequate.
STUDY OBJECTIVE: To obtain data about the safety and efficacy of the size 5 laryngeal mask airway (LMA), which is a scaled-up version of the size 4 and is generally recommended for patients over 90 kg, for positive pressure ventilation (PPV), ease of insertion, oropharyngeal and gastric insufflation pressures, fiberoptic positioning, and complication rates. DESIGN: Prospective survey. SETTING: Teaching hospital. PATIENTS: 179 patients undergoing PPV with the size 5 LMA. INTERVENTIONS: The clinical criteria for using the size 5 LMA and the PPV technique were weight above 90 kg or an inadequate seal with a size 4 LMA and surgery estimated to last longer than 45 minutes. Anesthesia was standardized and included fentanyl/propofol for induction, N2O/O2/isoflurane 0.5% to 2% for maintenance, and atracurium for muscle relaxation. Two 20-second attempts were allowed with the standard recommended technique, followed by a single attempt with the Guedel technique. The LMA cuff was then inflated and the airway pressure at which either oropharyngeal leak or gastric insufflation occurred was determined by closure of the expiratory valve and anterior neck followed by epigastric auscultation. MEASUREMENTS AND MAIN RESULTS: The age and weight range were 15 to 82 years and 46 to 153 kg, respectively. 29% of patients had a body mass index (BMI) above 30 kg/m2. On 31 occasions the size 5 was used following an inadequate seal with the size 4. The weight range of this subgroup was 46 to 87 kg. The device was placed within 20 seconds in 94% and there were no failed placements within three attempts. Gastric insufflation was detected before oropharyngeal leak in 17% and oropharyngeal leak was detected first in 73%. In 10% of patients there was no leak at an inspiratory pressure of 45 cm H2O. Mean (range) for gastric insufflation pressure was 31 (range 23-45) cm H2O. Mean (range) for oropharyngeal leak was 33 (range 8-44) cm H2O. The mean (range) airway pressure was 17 (range 13-26) at tidal volumes of 10 ml/kg. At this tidal volume, 97.2% of patients could be ventilated without gastric insufflation and 98.3% without an oropharyngeal leak. At tidal volumes of 8 ml/kg no patient had gastric insufflation and 0.7% had an oropharyngeal leak. Oropharyngeal leak pressure of less than 15 cm H2O occurred in 11 patients. There was no correlation between fiberoptic score or Mallampati score and either gastric insufflation or oropharyngeal leak. The incidence of problems was 3% and the oxygen saturation remained above 94%. There was no correlation between problems, leak pressures, and BMI. CONCLUSIONS: Positive pressure ventilation with the size 5 LMA is safe and effective with a low failure/problem rate using tidal volumes of 8 to 10 ml/kg, even in those patients who are moderately obese. The device is suitable for patients weighing under 90 kg in whom the seal with the size 4 is inadequate.