# A Review of Methods and Applications for a Heart Rate Variability Analysis
## 7 Applications of HRV Analysis
### ANS Activity of Patients Undergoing Weaning from Mechanical Ventilation  
The analysis of HRV can be conducted using two separate domains: time and frequency. Each domain offers distinct insights into the cardiac function, including the potential of certain HRV features in the time and frequency domains to predict the readiness of mechanically ventilated patients for extubation. The determination of extubation readiness pertains to a patient's capacity to respire autonomously without reliance on mechanical support, and it represents a pivotal judgment within the context of the intensive care setting. Various HRV properties can be derived from ECG signals obtained from individuals who have had a spontaneous breathing trial (SBT), which is a test conducted to evaluate their preparedness for extubation. The utilization of HRV properties can assist in the classification of patients into distinct groups, namely successful or failed extubation groups.  
A significant observation was made regarding a reduction in HRV features during the transition from PSV to the spontaneous breathing trial (SBT). This decrease was shown to be significant in the unsuccessful category but not in the winning category [184]. The utilization of mechanical ventilator assistance and the process of transitioning to spontaneous breathing, also known as weaning, results in notable changes in alveolar and intrathoracic pressure. These variations have a direct impact on thoracic blood volume and flow. Compensatory adjustments in the autonomic tone take place in order to maintain a sufficient delivery of oxygen to tissues. However, these autonomic reactions might lead to cardiovascular dysfunction, ultimately resulting in the failure of weaning. Approximately 66% of the patients in a study of [PERSON] et al. (2008) exhibited a typical autonomic function, with greater severity of dysfunction observed in individuals who were not able to maintain breathing on their own [185]. In their study, [PERSON] et al. (2018) explored three fundamental strategies to ensure an appropriate autonomic cardiac control reaction during the weaning phase of cardio-respiratory stress in patients. The group model that achieved effectiveness had a decrease in overall activity accompanied by a rise in sympathetic tone and a decrease in vagal tone. The latter correlated with both the respiration rate and tidal volume. Deviations from the established HRV model serve as indicators for the occurrence of weaning failure, with a precision of 92.6% for pressure support ventilation, 81.5% for spontaneous breathing trials (SBT), and 96.3% for the combination of SBT and PSV [186]. Patients who experienced failure in the process of weaning revealed a notable drop in the RR sample entropy, LF, HF, and \(\alpha\)1 exponent in comparison to participants who successfully underwent weaning. The alterations seen in the two phases exhibited contrasting trends, with the exception of the Mean Squared Error (MSE), which showed an upsurge within as well as between the groups. Hence, [PERSON] et al. (2011) proposed that the application of a non-linear analysis to cardio-respiratory dynamics has the potential to enhance the predictive value of weaning efficacy in patients undergoing surgery [187].