The SPSS statistical software (version 17.0, SPSS Inc., Chicago, IL, USA) was used to analyze these data. The hemodynamic and respiratory parameters of patients in different time points were presented as mean ± standard deviation (normal distribution of data was checked by Kolmogorov-Smirnov test) and analyzed using one-way analysis of variance. A P value <0.05 was considered significant.
A total of 33 morbidly obese patients were assessed for eligibility. Among them, three patients with severe obstructive sleep apnea syndrome were excluded from the study and 30 patients were recruited into the study. The demographic data of patients were shown in Table 2.
The SAD was successfully inserted at the first attempt in all patients. Insertion conditions were excellent in nine patients (30%) and good in 21 patients (70%), respectively. Twenty patients (66.7%) presented an intermediate resistance to mouth opening and 18 patients (60%) presented an intermediate resistance to SAD insertion. Only one (3.3%) patient had slight swallowing and gagging during the SAD insertion. Coughing, head and body movement, and laryngospasm were not observed in any patient [Figure 2]. All patients obtained effective ventilation with spontaneous breathing after SAD insertion.
Before SAD insertion, both HR and RR were significantly increased compared with their baselines, but MAP, TV, and ETCO2 were significantly decreased compared with their baselines. As compared with the values before SAD insertion, HR and RR at 1 min after SAD insertion were significantly decreased, and ETCO2 at 1 min after SAD insertion was significantly increased [Table 3]. During sevoflurane inhalational induction, apnea occurred in five patients (16.7%), but spontaneous breathing resumed in all five patients after placement of an oropharyngeal airway. No hypertension, hypotension, bradycardia, laryngospasm, aspiration, and airway injury were noted throughout the observation. Furthermore, no complication associated with jaw thrust maneuver was found.
Intra-venous anesthesia induction with propofol without the use of muscle relaxants is commonly used for SAD insertion in adult patients, but it can result in a higher incidence of respiratory depression than sevoflurane inhalational induction.[23–25] This means that in patients with difficult airways, the use of intra-venous anesthesia induction with propofol may increase the risk of a sudden loss of airway control, making patients fall into the “cannot intubate cannot ventilate” situation. In contrast, the depth of anesthesia can be gradually increased with sevoflurane inhalational induction while maintaining spontaneous breathing. In this situation, adequacy of facemask ventilation during spontaneous breathing can be reliably assessed at various anesthetic levels. If airway obstruction occurs during anesthesia induction and cannot be relieved by routine airway maneuvers, sevoflurane is turned off and then the patient is woken up. Given that morbidly obese patients have an increased risk of difficult airways, sevoflurane inhalational induction with spontaneous breathing is selected in this study as to our routine practice.
Although SAD insertion is generally believed to require a lighter depth of anesthesia compared with tracheal intubation, the depth of anesthesia should be sufficient to inhibit adverse airway reflexes and circulatory responses by SAD insertion. Before SAD insertion, thus, the most important thing is how to determine whether the depth of anesthesia is adequate. An ideal test assessing appropriate depth of anesthesia for successful insertion of SAD should be easy to practice and no injurious to patients, with an ability to provide a reliable assessment. The jaw thrust is a common maneuver to prevent upper airway obstruction in anesthetized patients. As jaw thrust maneuver is a noxious stimulus that can cause significant motor reactions, the depth of anesthesia required for no motor reaction to jaw thrust may be sufficient for successful SAD insertion in morbidly obese patients receiving sevoflurane inhalational induction.
Although an adequate fiberoptic view of SAD positioning does not represent proper functioning of a SAD, an optimal fiberoptic view can facilitate subsequent blind or FOB-guided tracheal intubation via the SAD. The incidence of poor fiberoptic view of the SAD positioning in our study was 6.7%. This is with an agreement with the findings of some previous studies. In the morbidly obese patients, Shiraishi reported that the incidence of poor fiberoptic view of air-QTM intubating LMA (Mercury Medical, Clearwater, Florida, USA) positioning was 5%. In the obese patients, Wang et al reported that the incidence of poor fiberoptic view of BlockBusterTM SAD positioning was 10%. In the morbidly obese patients; however, Keller et al reported that the incidence of poor fiberoptic view of ProSeal LMA positioning was as high as 25%. The reasons for a higher incidence of poor fibreoptic view in Keller et al's study may be that patients are placed in the supine position, rather than the ramped position, which is often a position commonly recommended for anesthesia induction and airway management in the morbidly obese patients.
It must be pointed out that our design has several limitations. First, a main limitation is observational character of this study without control design. Thus, an issue that was not answered in this study is whether jaw thrust test is superior to other methods when assessing adequate depth of anesthesia for insertion of SAG in morbidly obese patients. Second, a size 4 BlockBusterTM SAD was used for all morbidly obese patients. Perhaps, the use of thyropalatal distance and IBW to guide the selection of optimal SAD size is more appropriate. In the morbidly obese patients; however, excess adipose tissue deposition in the pharyngeal space may lead to a decreased upper airway space. In our previous studies,[11,34] all obese patients obtained adequate ventilation via a size 4 BlockBuster SAD. Third, the force strength to perform jaw thrust may differ among patients and cannot be reliably quantified. Thus, we emphasize that repeated practices and experiences are needed before one attempts to use the jaw thrust test to assess adequate depth of anesthesia for successful insertion of SAD. Fourth, female patients account for a high proportion (22/30, 73.3%) in our study. It has been shown that morbidly obese males have more difficult intubation conditions than morbidly obese females. Thus, different results may be obtained when our plans are repeated in another study where male-female ratio is symmetrical or males are predominant. Fifth, the ages of the morbidly obese patients enrolled in our study are 18 to 37 years. As age can significantly influence the pharmacodynamics of inhalational anesthetics, our results may also not extrapolated to morbidly obese patients of other ages. Finally, the results of this study may not be applicable to morbidly obese patients receiving with spontaneous breathing loss under sevoflurane inhalational induction. In conclusion, the present study demonstrates that jaw thrust test is a reliable clinical indicator assessing adequate depth of anesthesia for successful insertion of SAD in spontaneous breathing morbidly obese patients receiving sevoflurane inhalational induction.
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