In this study, CPEF was statistically significantly associated with reintubation due to decreased coughing ability in patients successfully weaned from mechanical ventilation with SBT. Moreover, a CPEF cutoff value of 60 L/min was determined for reintubation, suggesting that CPEF is useful in predicting reintubation due to decreased coughing ability. Salam et al. [26] have investigated the relationship between airway physiology and extubation success in 88 mechanically ventilated patients who successfully passed an SBT by evaluating the cough peak flow, properties of airway secretions, and responses to four commands (i.e., opening eyes, following gaze, shaking hands, and sticking out tongue). The results have shown that if the three aforementioned factors were positive, extubation would fail 100% of the time, implying a correlation between CPEF and extubation success. Studies [19] have also concluded that CPEF was an indicator of coughing ability and a factor related to reintubation, with CPEF values of 80 ± 26 L/min in the successful extubation group and 50 ± 22 L/min in the reintubation group, and a CPEF cutoff of 56 L/min for reintubation. However, both studies [19, 26] have included reintubation cases that were unrelated to decreased coughing ability, such as laryngeal edema and circulatory failure; therefore, CPEF did not directly demonstrate the association with coughing ability. In this study, reintubation cases were limited to those due to decreased coughing ability; thus, the possibility of a relationship between CPEF and reintubation due to decreased coughing ability was suggested.
Based on the results of this study, when considering extubation, it is possible that a CPEF ≤ 60 L/min reflects insufficient cough strength, which may increase the risk of reintubation due to difficulty in sputum expectoration. Therefore, care should be taken when increasing CPEF. It has been reported [27] that CPEF can be increased by step-up positioning, such as the supine position, 45° sitting, sitting at the edge of the bed, and ambulation, which are factors that can affect CPEF, in addition to posture, respiratory muscle strength, and chest expansion [20]. Therefore, advancing ambulation can be considered a care. Additionally, CPEF has been reported to be affected by pain and sedatives [28]. Thus, considering measures to address the factors causing low CPEF is necessary, such as pain control or delaying extubation time, if prolonged use of sedatives is suspected.
Similar to our study, Su et al. [16] have measured CPEF during cough reflex using an electronic flowmeter in patients admitted to ICUs who completed SBT and reported a cutoff value of 58.5 L/min as being associated with a higher risk of extubation failure. However, using a spirometer requires the addition of new equipment and training in measurement methods, which create barriers to generalization. The CPEF measurement method used in our study involved visually reading graphic waveforms from a mechanical ventilator, was noninvasive and simple, and can be easily performed by various healthcare professionals, including nurses, making it widely applicable in clinical settings.
In this study, CPEF was useful for predicting the risk of extubation failure due to decreased cough reflex and could aid in the objective assessment of cough strength in intubated patients. The numerical quantification of cough strength in patients may facilitate information sharing among healthcare professionals, including physicians and nurses, and contribute to reducing the rate of reintubation.
As a limitation of this study, the number of cases in the reintubation group was limited to only 11, which restricted the number of confounding factors that could be extracted in the multivariate analysis. In the future, adding more cases and conducting further investigations using a prospective study design to establish an evaluation index for coughing power at extubation will be necessary. Additionally, because coughing power for sputum expectoration is influenced by various factors, such as the amount and characteristics of sputum, water balance, and degree of pain, analyzing the data, including many patient factors, will be necessary.