Spontaneous breathing (SB) is a popular mode of ventilation with the classic laryngeal mask airway (LMA) in children (1–3), but it is frequently associated with inadequate gas exchange, particularly hypercapnia. Studies of children managed with a tracheal tube (4,5) have shown that pressure support ventilation (PSV) improves gas exchange and/or work of breathing (WOB) during SB. A study of children aged 2 ± 2 yr managed with the classic LMA showed that continuous positive airway pressure (CPAP) reduces WOB during SB (6). There are no publications comparing the efficacy of PSV and CPAP with any LMA device in children. The ProSeal™ LMA (PLMA) is a new LMA with a modified cuff to improve the seal and a drain tube that prevents gastric insufflation when correctly positioned (7–9), but there are no published data about its use in children. In the following randomized, crossover study, we compared the efficacy of PSV with CPAP in healthy anesthetized pediatric patients aged 1–7 yr undergoing routine surgery managed with the PLMA.
Twenty consecutive children (ASA physical status I, aged 1–7 yr) undergoing general or urologic surgery in which the PLMA was considered suitable were studied. Exclusion criteria were prematurity (<35 wk gestation), a known or predicted difficult airway, mouth opening <1 cm, a body mass index >30 kg m−2, or risk of aspiration (fasted <4 h). Ethical committee approval and parental written informed consent were obtained.
Premedication consisted of oral midazolam 0.5 mg/kg and atropine 20 μg/kg 45 min preoperatively. A standard anesthesia protocol was followed and routine monitoring applied. Anesthesia was induced with propofol 4 mg/kg. A single experienced PLMA user (CK, >1000 uses) inserted/fixed the PLMA (size 2 in all patients) according to the manufacturer’s instructions (10). Additional boluses of propofol 1 mg/kg were given as required until an adequate level of anesthesia was achieved for placement. Anesthesia maintenance consisted of sevoflurane 2% and O2 35% in air. In all children, intraoperative analgesia was performed with a caudal block using ropivacaine 0.2% 1–1.25 mL/kg. Once an effective airway was obtained, the intracuff pressure was set and held constant at 60 cm H2O using a digital manometer (Mallinckrodt Medical, Athlone, Ireland). Oropharyngeal leak pressure was determined by closing the expiratory valve of the anesthesia breathing system at a fixed gas flow of 3 L/min and noting the airway pressure (maximum allowed, 40 cm H2O) at which equilibrium was reached (11). Epigastric auscultation was performed during oropharyngeal leak pressure testing to detect any gastric insufflation (12). The lungs were manually inflated until SB resumed. Patients were then attached to a ventilator (modified Evita 4; Draeger Medizintechnik GmbH, Luebeck, Germany) and were randomly allocated into two crossover groups. In Group 1 (n = 10), patients underwent CPAP, PSV, and CPAP in sequence. In Group 2 (n = 10), patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end-expiratory pressure (PEEP) set at 3 cm H2O and pressure support set at 10 cm H2O above PEEP. CPAP was set at 3 cm H2O. Pressure support was initiated when inspiration produced a 2-cm H2O reduction in airway pressure and a flow of 2.5 L/min. The machine had no bias flow. Each ventilatory mode was maintained for 5 min.
Respiratory variables were measured and analyzed using a pulmonary monitor (CP-100; BiCore Monitoring System, Irvine, CA) attached to a variable orifice pneumotachograph (Var flex; Allied Health Products, Riverside, CA) and an esophageal balloon catheter inserted through the drain tube of the PLMA (Smart Cath; Allied Health Products) (13–15) (Fig. 1) (14,15). The esophageal balloon catheter was 3 mm in diameter, 70 cm long, and constructed from medical-grade polyurethane. The inflated balloon was 0.9 cm in diameter and 10 cm long. The frequency response was 30 Hz. The esophageal balloon catheter was connected directly to the catheter port on the BiCore monitoring system. The BiCore monitoring system automatically performs a vacuum leak test and fills the esophageal balloon with 0.8 mL of air. The pneumotachograph was connected directly to the proximal end of the airway tube measuring airway pressure and flow. The CO2 sampling port was sited above the flow transducer. The position of the esophageal balloon catheter was checked and adjusted where necessary by observation of the cardiac artifact on the esophageal waveform, as recommended by the manufacturer. The classical airway occlusion test was not used, because the patients were not yet spontaneously breathing. The machine pop-off valve was set at 40 cm H2O.
The following data were recorded every 30 s for the last 2 min of each ventilatory mode and the average reading taken: ETco2, Spo2, noninvasive mean arterial blood pressure, end-tidal sevoflurane concentration, fraction of inspired oxygen, axillary skin temperature, expired tidal volume (VTExp), respiratory rate, peak airway pressure, WOB (sum of the physiologic work, including elastic and flow resistive work of the respiratory system and airway device), pressure time product (PTP), δ esophageal pressure, inspiratory time fraction, and respiratory drive. Any ingestion or expulsion of air via the drain tube was detected by observing movement of a column of lubricant placed in the drain tube before and after esophageal balloon catheter insertion. Measurements were made before surgery.
Sample size was selected for a type I error of 0.05 and a power of 0.9 and was based on a pilot study of 6 patients with a measured difference in the VTExp of 25% between the groups. The distribution of data was determined using Kolmogorov-Smirnov analysis (16). Statistical analysis was with paired t-test (normally distributed data) and Friedman’s two-way analysis of variance (non-normally distributed data). Within-patient variability of each set was determined using one-way analysis of variance. Significance was taken as P < 0.05.
There were no demographic differences between groups. Data are presented in Table 1. All data were normally distributed. There was no significant within-patient variability of each set of the readings. In Group 1, measurements for CPAP were similar before and after PSV. In Group 2, measurements for PSV were similar before and after CPAP. In both groups, PSV had lower ETco2 (P < 0.001), slower respiratory rate (P < 0.001), lower WOB patient (P < 0.05), lower PTP (P < 0.001), lower δ esophageal pressure (P < 0.05), and higher VTExp (P < 0.001) compared with CPAP (Table 2). In both groups, PSV had similar Spo2, respiratory drive, mean arterial blood pressure, and heart rate compared with CPAP. Data for CPAP were similar between groups and for PSV were similar between groups. Gastric insufflation was not detected during oropharyngeal leak pressure testing.
We found that PSV provided better gas exchange than CPAP. This is because PSV leads to higher airway pressures resulting in a larger lung volume being available for gas exchange. The level of PSV was selected according to Tokioka et al. (4), who found that PSV of 10 cm H2O resulted in a tidal volume of approximately 9 mL/kg body weight in children breathing through a tracheal tube. There were two studies that investigated the efficacy of PSV and/or CPAP1 (17) with the classic LMA in adults and a single study that investigated the efficacy of CPAP with the classic LMA in children. Capdevila et al.,1 in a preliminary study of 36 adults, found that PSV improves gas exchange compared with SB and decreases leaks compared with positive pressure ventilation, but with similar gas exchange. In a crossover study of 40 adults, we (17) found that PSV (with PEEP set at 5 cm H2O and inspiratory pressure support set at 5 above PEEP) provides more effective ventilation while preserving leak fraction and hemodynamic homeostasis than CPAP set at 5 cm H2O. Keidan et al. (6), in a study of 8 children aged 2 ± 2 years, found that CPAP set at 5–6 cm H2O reduces the WOB with the LMA.
WOB is increased in spontaneously breathing patients undergoing general anesthesia with an artificial airway (18). We found that PSV resulted in approximately 40% reduction in WOB compared with CPAP. This is because the ventilator is providing much of the energy required to deliver each breath. The PTP is an estimate of the oxygen consumption or metabolic work of the respiratory muscles and may be used to evaluate patient effort to overcome both mechanical and isometric force of inspiration during mechanical ventilation (19,20). The decrease in PTP from CPAP to PSV reflects the reduced respiratory muscle workload and the increased ventilator workload.
We found that that the oropharyngeal leak pressure was 25 cm H2O. This is 13 cm H2O higher than with the classic LMA using the same methodology in children of similar age (21). This suggests that the size 2 PLMA forms a better seal that the classic LMA, but a comparative study between the 2 devices is required to confirm this. We detected no episodes of gastric insufflation during oropharyngeal leak pressure testing. PSV and CPAP should be safer with the PLMA than the classic LMA, because the efficacy of the seal with the hypopharynx is probably greater (22) and any esophageal leaks should be vented from the drain tube if it is correctly positioned. We detected no differences in cardiovascular effects between PSV and CPAP. This is not surprising, because intrathoracic pressure changes were only 7 cm H2O higher for PSV.
Our study has a number of limitations. First, only healthy ASA physical status I children without underlying respiratory disease or obesity were enrolled in the study and our results may not be applicable to other patient populations. Second, we did not measure the arterial partial pressure of oxygen, which is a more sensitive test of oxygenation than oxygen saturation, and it is possible that subtle differences in oxygenation went undetected. Third, the data were collected over a short period before commencement of surgery and, in principle, our results may not be applicable to prolonged SB. However, if anything, the differences between PSV and CPAP are likely to increase with duration because of greater respiratory fatigue with CPAP.
We conclude that PSV improves gas exchange and reduces WOB during PLMA anesthesia compared with CPAP in ASA physical status I children aged 1–7 years.
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