Patients admitted to the Intensive Care Unit (ICU) frequently develop peripheral and respiratory muscle weakness. This condition is associated with delayed weaning and/or extubation failure, mainly due to respiratory muscle dysfunction and/or inadequate coughing.⁽¹⁾ Although an experienced clinician can predict the likely outcome of an attempt to wean a patient from mechanical ventilation (MV), it is desirable to have predictive indices that can be easily measured and widely applied.⁽²⁾

Several tests have been devised to predict extubation success after cardiac surgery, including the Medical Research Council (MRC) scale, which has been used to demonstrate the negative impact of muscle weakness on weaning from MV.⁽¹⁾

Determining the ideal time to discontinue MV is critical, because if extubation fails, reintubation is necessary, increasing the risk of mortality.⁽³⁾ Extubation is a complex process that requires a balance between the overall respiratory load and the ability to overcome this load to wean a patient off from invasive MV.⁽⁴⁾

Extubation failure and reintubation occur in 2-25% of mechanically ventilated patients in the ICU.⁽⁵⁾ The rate may vary depending on the type of patient and weaning protocol used. Extubation failure and reintubation are associated with increased MV time, mortality, need for tracheostomy, and hospital costs.^(5,6)

Parameters such as the ability to stick out the tongue, follow specific commands, and having a gag reflex have been investigated as reliable tools for assessing the level of consciousness and ability to protect the airway.⁽⁶⁾ Accordingly, several tests have been proposed to predict extubation success after cardiac surgery. Previous studies have used the MRC scale to demonstrate the negative impact of muscle weakness on weaning from MV.⁽¹⁾ However, no study has examined whether peripheral muscle strength can be used as a predictor of weaning success in patients undergoing cardiac surgery.

To evaluate the impact of peripheral muscle strength on extubation success after cardiac surgery.

During the study period, 72 patients required MV, six of whom were excluded from the study (Figure 1). Of the remaining 66 patients, 55 (83%) were classified as successful extubation and 11 (17%) as having failed weaning. The majority of the patients were men (38 [58%]) and the overall mean age was 62 ± 5 years. The most prevalent comorbidities were systemic arterial hypertension and sedentary lifestyle, affecting 49 (74%) and 45 (68%) participants, respectively. Other relevant information is presented in table 1.

Table 2 presents a comparison of the weaning success and failure groups based on the patient characteristics. Notably, the MRC score 30 minutes before the spontaneous breathing trial (SBT) was significantly higher in the success group (mean 44 ± 4) compared to the failure group (29 ± 5; p<0.001). Similarly, the MRC score 10 minutes before the SBT was 45 ± 5 in the success group and 34 ± 4 in the failure group (p<0.001).

The study identified a cut-off value of 44 for the MRC score 30 minutes before the SBT, which yielded a sensitivity of 77%, specificity of 84%, area under the curve (AUC) of 0.864, and 95% confidence interval (95%CI): 0.69-1.00. For the MRC score 10 minutes before the SBT, a cut-off value of 49 was associated with a sensitivity of 55%, specificity of 80%, AUC of 0.845, and 95%CI= 0.77-1.00. In comparison, the rapid shallow breathing index (RSBI) measured 10 minutes before the SBT had a cut-off value of 45, with a sensitivity of 30%, specificity of 70%, AUC of 0.476, and 95%CI= 0.22-0.71 (Table 3). These data are demonstrated in figure 2.

This study aimed to evaluate the peripheral muscle strength by using a new MV weaning success index. The sensitivity and specificity of the MRC were found to be 77% and 84%, respectively, whereas the RSBI demonstrated sensitivity and specificity of 30% and 70%, respectively. These findings reveal that the MRC is more accurate than the RSBI for assessing the peripheral muscle strength 30 and 10 minutes before the SBT.

A study of neurocritical patients by Kutchak et al. reported that the RSBI was one of the most commonly used parameters in the ICU, but it was only partially capable of predicting the risk of extubation failure. Regardless of the restitution of the primary pathology that leads to intubation in neurological patients, the motor and cognitive sequelae lead to a decreased capacity to protect the airways.⁽¹¹⁾

Previous studies have proposed that muscle weakness associated with increased respiratory work can result in a rapid and shallow breathing pattern. However, Saiphoklang et al. found a correlation between the handgrip strength and RSBI in cases of pneumonia, pulmonary edema, and bronchospasm, and demonstrated the non-specificity of the RSBI in distinguishing between patients who were successfully extubated and those who failed.⁽¹²⁾

Faadaii et al. examined the usefulness of RSBI as a predictor of weaning in the ICU, and found that it had low specificity compared to variables such as concomitant diseases and length of stay in the ICU.⁽¹³⁾ In an investigation into the correlation between decreased skeletal muscle mass and extubation failure, Woo et al. inferred that the RSBI is not capable of predicting extubation success, and has sensitivity and specificity of 0.74 and 0.00, respectively.⁽¹⁴⁾

Consequently, the MRC scale is more practical and viable for predicting extubation success. However, it does not eliminate or replace other assessment methods. Lima et al. evaluated the influence of peripheral muscle strength and other indicators on tracheostomy decannulation success, and found that the MRC value was greater than or equal to 26 in the successful decannulation group, with a sensitivity of 94.4% and specificity of 50%. This study showed that peripheral muscle strength evaluated on the day of decannulation may affect the tracheostomy decannulation success rate.⁽¹⁵⁾

The MRC scale has been frequently used as a practical means of recognizing muscle weakness acquired in the ICU. In a study by Santos et al. in critically ill oncology patients, it was shown that patients whose MRC score was lower at extubation remained on MV for a longer period of time. This finding supports an association between peripheral muscle weakness and weaning failure.⁽¹⁶⁾

Regarding the functional prognosis of patients after hospitalization in the ICU, Ferreira et al. showed that severe clinical conditions at admission can contribute to a subsequent decline in the functional performance. Thus, the MRC scale may serve as an indicator for the early implementation of rehabilitation.⁽¹⁷⁾

This study has some limitations. First, other predictive indices of weaning and extubation were not evaluated in this study. Second, a pain assessment scale was not used, which may also influence weaning. Third, the limited sample size may have influenced the study results.

Additionally, it is important to acknowledge potential confounding variables, such as age, comorbidities, surgical time, and duration of MV, which were not adjusted for in the analysis and may have significantly affected both the MRC scores and extubation outcomes. Finally, the lack of multivariate analysis limits the ability to confirm the independent effects of peripheral muscle strength on weaning success.

Therefore, peripheral muscle strength may be a useful predictor of extubation success in patients undergoing cardiac surgery. However, further studies with larger sample sizes and multivariate analyses are required to confirm this association.