The present study revealed that thermal softening of DLTs before intubation decreased the incidence and severity of POST in patients with prior SARS-CoV-2 infection following thoracoscopic surgery. The airway resistance during insertion and advancement of DLTsalso decreased following their thermal softening, and the resulting airway damage also reduced.
The incidence of POST in patients undergoing general anesthesia with DLT is higher than that with SLT;this can reduce postoperative comfort, leading to unpleasantnessand irritationin patients after the surgery[20].SARS-CoV-2 infection causes local mucosal epithelial damage in the pharynx and trachea, resulting in nonspecific inflammation;moreover, in some patients with poor immunity, SARS-CoV-2 infectioncan persist and lead to chronic inflammation. SARS-CoV-2 mainly invades the airway and can persist in the airway lumen for a long time;this could increase airway sensitivity in patients and induce intubation-related complications.The incidence of POST in the present study was higher than that reported in previous studies;this might be because our patients had prior COVID-19 infection. In the present study, the thermal softening of the DLT prior to intubation decreased airway resistance during the subsequent insertion and advancement of the DLT, decreased airway injuries, and reduced the incidence and severity of POST in patients with prior SARS-CoV-2 infection. The tip of the DLT may be the main cause of resistance and airway injuries during its insertion. Rotation of the tube is often required during DLTinsertion. Following its immersion in saline at 50°C, the DLT becomes softer and more lubricated due to the application of heat;this seems to reduce the damage to the glottis and trachea during the intubation process, thus reducing the incidence of POST. In a previous study, the use of a silicon DLT decreased the incidence of POST compared to the use of a PVC DLT;this might be because of the soft texture of silicone[21].
Because of a larger outer diameter, harder texture, and deeper insertion into the trachea, DLT intubation may cause greater trauma to the airway;however, fewer methods are available to reduce the incidence of POST due to DLTs[4, 21, 22]. Several local anesthetics, systemic analgesics, and corticosteroids have been used to reduce the incidence and severity of POST[23]. Nonpharmacological methodssuch as 1800 rotational advancement[4], jaw thrust maneuver[19], and silicone catheterization[21] may reduce the incidence of POST after DLT intubation. Thermal softening is a simple physical technique that can be performed without any additional support orside effects of drugs.
The vocal cords are the most vulnerable area to sustain injury during tracheal intubation;this is because the glottis is the narrowest part of the upper respiratory tract, and it is closer to the DLT. In our present study, the total incidence of vocal cord injury was 55% in the control group. This incidence was higher than that reported in previous studies;this is possibly because the vocal cords may still be congested and edematous due to the prior SARS-CoV-2 infection. In the present study, the DLT was passed through the glottis and trachea after it was subjected to a heating treatment through immersion in saline at 50°C;this led to lower resistance during the advancement of the DLT and reduced the incidence of vocal cord and tracheal injuries;these findings indicated that saline immersion and heating of DLT reduced trauma to the throat. To ensure that the anesthesiologist was blinded to the treatment process, only the distal part of the DLT was immersedin saline and heated.
Interestingly, compared to previous studies, the laryngoscope used in the present study was a video laryngoscope rather than a direct laryngoscope;the former provides faster and better exposure of the glottis, with a better laryngoscope field of view and less intubation time, which can reduce the incidence of airway complications[24]. The incidence of POST in the control group was,however, higher than that reportedpreviously. Moreover, the control group showed more severe damage to the vocal cords and trachea than that observed in previous studies.A possible explanation for this finding might be the presence of inflammation and edema in the respiratory mucosa of patients with prior SARS-CoV-2 infection, which probably caused the vocal cords and trachea to become more prone to injury.
The present study has some limitations. First, all tracheal intubations were performed by two anesthesiologists with a rich experience in the intubation of adults with normal airways. Therefore, the findings of this study cannot be generalized to inexperienced physicians and/or patients with difficult airway. Second, although the anesthesiologists were blinded to the study details, they may be able to identify heat-softened DLTs during intubation because of the difference in texture. However, we do not believe that this limitation influenced our results, as we heated only the distal part of the DLT, and the incidence of vocal cord and tracheal injuries is an objective and clear assessment. Third, this study used only one type of polyvinyl chloride DLT for tracheal intubation. Different types of DLTs are prepared from different materialsand have varying hardness and tube diameters;this may affect the incidence of POST and airway injury. Further studies are required to determine the optimum approach for inserting different types of DLTs by using a video laryngoscope.