Diaphragmatic atrophy has been observed as early as 18 h after initiation of mechanical ventilation . Another study showed that the pressure-generating ability of the diaphragm decreased by 30% after 5–6 days of mechanical ventilation . Therefore, extreme caution should be taken for diaphragm dysfunction when extubating long-term intubated patients . SBTs are commonly carried out to evaluate the readiness of extubation  Although the findings of numerous trials support the usefulness of SBTs for predicting extubation readiness in pediatric patients, it is nonetheless not 100% sensitive.
The Edi reflects the respiratory drive and its response to the diaphragm, suggesting its utility as an objective monitoring tool of diaphragm function . Especially, NVE, the value calculated as tidal volume divided by the Edi, has been reported as an index of ventilation efficiency . Again, even if tidal volume is normal, there is a possibility of extubation failure in a situation where the Edi is high and NVE is low. Therefore, in long-term intubated patients, evaluation of the Edi and NVE over time may be helpful in determining extubation readiness.
In our case, the patient required mechanical ventilation for 50 days and the presence of severe diaphragm dysfunction was expected. The first attempt of extubation was performed with successful SBTs and improvement in Crs. In addition to this, the maximal inspiratory negative pressure was − 77 cm H2O, which was indicative of the ability to generate sufficient pressure . Indeed, immediately after extubation, the respiratory pattern of the patient was normal with acceptable arterial blood gas analysis, suggesting that the patient could be weaned from mechanical ventilation for a short time. However, these facts did not predict the ability to endure high workloads for a longer period. In this regard, Edi monitoring to assess NVE was useful to predict the fatigability of the diaphragm in this patient. Immediately after reintubation, the Edi value was higher than 70 mcV. Although the maximal Edi value varies among individuals, the Edi value higher than 70 mcV observed after reintubation in this case was indisputably high . After several days of supported mechanical ventilation, the Edi values decreased while tidal volume remained constant (6–7 mL/kg), suggesting improvement of ventilatory efficiency. We finally decided to extubate the trachea of the patient on the day when the Edi during the SBTs became comparable to that during pressure support ventilation.
Moreover, the Edi refers to the myogenic potential of the diaphragmatic crura. Therefore, the Edi is not observed in fully mechanically ventilated patients. In situations when a patient does not have diaphragmatic dysfunction, weaning off from mechanical ventilation will increase tidal volume and the Edi in parallel. The optimal timing to extubate the trachea is when the tidal volume reaches acceptable values. One previous report showed that the increase in the Edi was well correlated with extubation readiness. In contrast, when patients have diaphragmatic dysfunction, the Edi and tidal volume increase with weaning from mechanical ventilation in the same manner. However, the observed Edi would reflect high effort to achieve tidal volume. If there is only information on tidal volume, clinicians might consider extubation when the tidal volume reaches an acceptable value, but this might result in extubation failure. In such cases, the Edi might be useful to assess diaphragmatic dysfunction since the Edi is expected to decrease when diaphragmatic function recovers.
Ultrasound sonography has been recently proposed for the evaluation of the structure and function of the diaphragm . It is now rapidly gaining popularity with advances in image resolution. Ultrasonography is non-invasive and carries an advantage over other methods such as chest radiography or fluoroscopy. However, the method is prone to intra- and interobserver differences . On the contrary, Edi and NVE values are easy to obtain with no interobserver differences. The technique also allows for real-time monitoring of breathing in clinical settings. Although each technique has its own strengths and weaknesses, the relationship between the methods should be assessed in the future study.
Several limitations should be acknowledged when interpreting this case study. First, the Edi value is affected by the catheter position and impedance of the esophagus and the thoracic cavity. Thus, normal values differ among patients . Second, as mentioned previously in the “Discussion” section, it is necessary to note that the Edi may increase and extubation may be possible and vice versa. Thus, it should be emphasized that these values should be used in conjunction with the clinical findings.
In conclusion, we have experienced a case in which the Edi and NVE were valuable for deciding the extubation readiness in a long-term mechanically ventilated patient.