Our study found that extubation failure is common in T-MG patients (13/38,34.2%), especially suffered from PTMC (6/8,75%), which was, which was similar to the previous studies23,24,26. Transsternal thymectomy, phrenic never or diaphragm injury and postoperative pulmonary complications may increase the risk of extubation. Besides, preextubation vital signs (HR, RR, SpO2) instead of blood gases did correlate with extubation outcome. Patients who experienced failure of extubation had much longer ICU and hospital stays.
It has been proved the thymus has a key role in inducing AChR antibodies production, however the specific mechanism remains unclear. Guidelines suggest extended thymectomy is recommended for T-MG patients, which means that maximal thymic tissue and the surrounding anterior mediastinal fat tissue have to be removed, and minimize the surgical procedure like VAST are preferred. In our study, all T-MG patients received extended thymectomy. VAST was performed in 78.9%(30/38) patients, 3 of them were converted to OT due to invasion of the great vessels. OT was performed in 8 patients and was associated with much more blood loose (800 vs.100ml, p < 0.001), longer operation time (300 vs. 170min, p < 0.001) and a higher risk of extubation failure (63.6% vs. 22.2%, p = 0.039) and longer duration of ICU stay (16 vs.22 days, p = 0.013), which is similar with previous studies[3, 23, 24]. Although VATS can reduce trauma and had significantly less blood loss, less pain, faster recovery[25–27], possible perioperative complications related to myasthenia must be involved in surgical planning, especially respiratory impairment. Respiratory insufficiency due to diaphragmatic and intercostal muscle weakness is a major challenge.
It was similar in our study, 28 (73.7%) patients have locally advanced tumors (Masaoka stage II-IV) and the mean size of thymomas is considerable as 5cm (range, 3-8cm).
As a result, the incident of phrenic never or diaphragm injured is 39.5% in all patients, which can the diaphragmatic function deteriorated sharply worsen the and is associated with greater risk of extubation failure (61.5% vs. 28%, p = 0.045).
It has been recommended that special attention have to paid to respiratory function in MG patients. Previous studies[28, 29] suggested that extubation can be attempted when passed SBT and have a MIP>-20cmH2O. However, our study shows that MIP was lower in the extubation group, the difference did not reach statistical significance (38 vs. 33, p = 0.061). Similar to the previous studies, few patients may have poor FVC results because of poor cooperation and technical reasons, even after their respiratory muscles have recovered adequate strength. For MG patients, the use of accessory muscles or paradoxical movement of the abdomen may be the first sign of respiratory muscle fatigue, which will cause an increase in respiratory rate with shallower tidal volume breaths. In our study, 76.9%(10/13) of extubation failure patients were in the late stage (Osserman Classification IIb -IV) with bulbar symptoms. Even though all patients had passed SBT, the ones who have higher RR, HR, and lower SpO2 after SBT associate with have higher ratio extubation (P < 0.05). But there was no significant difference in PCO2 or P/F before weaning(P > 0.05). Nonetheless, we observed that muscle strength would not have significant clinical improvement immediately after surgery, the variability of MIP and RR over time probably reflecting the changes for MG patients.
Additionally, PTMC is frequently associated with respiratory insufficiency and can be induced by many factors including a history of crisis, infection, chronic respiratory dysfunction, even surgery[30, 31]. In the present cohort, 9 patients (21.1%) experienced PTMC and was an independent adverse prognostic factor for extubation failure (p = 0.020). Respiratory complications also can affect the prognosis of PTMC and prolonged ventilation or reintubation. In our study, 9 patients with atelectasis, 2 with pneumonia, and 5 patients suffered both, which were associated with an increased likelihood of extubation failure (p = 0.020). Overall, atelectasis was present in all extubate failure patients (13/13). This finding is concordant with Rubinstein’s multicenter study, all extubate failure patients have atelectasis (n = 7) and indicate atelectasis as a risk factor of reintubation among 26 episodes of PTMC. Thus, strategies aimed at preventing pneumonia are needed to improve PTMC therapy and early extubation. Interventions that reportedly prevent pneumonia especially during ventilation include suction, chest physiotherapy, early mobilization, and an optimal PEEP must be used to avoid atelectasis. Timely tracheostomy may facilitate suctioning, prevent pulmonary complications and make patients feel more comfortable.
NIV was used in 5 patients after extubation, none of them need reintubation. On the contract, Seneviratne  found that the use of NIV was associated with increased chances of reintubation, and thought that may because of the untimely use. Rabinstein indicated that timely NIV use after extubation can be effective in avoid reintubation in selected MC patients. Therefore, careful monitoring of the change of RR, HR, SpO2, and using NIV before any sign of respiratory fatigue appears can be effect.