Vagal withdrawal and psychological distress during ventilator weaning and the related outcomes
Introduction
Mechanical ventilation (MV) is typically used to support the respiratory function of critically ill patients [1], [2], [3]. After resolving the underlying problems necessitating MV, most patients can be readily weaned from MV. Prolonged MV is associated with high morbidity and mortality, as well as poor quality of life [4], [5]. Accordingly, previous studies have explored the physiological predictors for facilitating successful MV weaning; however, a 20%–30% variance in weaning failure remains unexplained [6], [7]. The most recently published guidelines have provided evidence-based recommendations that assist clinicians in safely and effectively weaning critically ill patients from MV [8], [9], [10]. Of these, one strong recommendation is using noninvasive mechanical ventilation to prevent extubation failure for high-risk patients [9], [10]. There are also five conditional (weak) recommendations: conducting spontaneous breathing trials with inspiratory pressure augmentation, using protocols to minimize sedation, using protocolized rehabilitation directed toward early mobilization, using ventilator liberation protocols, performing a cuff leak test (CLT) for extubation criteria, and administering systemic steroids at least 4 h prior to extubation for patients who failed a CLT [8], [9], [10]. However, it is unclear whether these guidelines considered all individual contexts, especially psychological factors, which also critically affect ventilator weaning outcomes [11]. Nevertheless, clinicians have become more aware that successful weaning can be hindered not only by physiological dysfunction but also by psychological distress [4], [7].
During the MV weaning process, patients can experience the following psychological distress: fear [4], anxiety [12], and dyspnea [13]. In addition, patients on MV have reported experiencing high degrees of uncertainty, discomfort, and fatigue [12], [14], as well as feelings of being “out of control” or “tortured by helplessness” [15]. These psychological factors may exert a compromising effect on patients' physiological performance (e.g., respiratory functions) during weaning trials, contributing to weaning failure [4]. Simultaneously considering both psychological and physiological variables has a greater predictive effect regarding weaning success than does solely considering physiological variables [4], [16].
Ventilator weaning with a spontaneous breathing trial changes the intrathoracic pressure and thoracic blood volume and flow, which can lead to compensatory autonomic tone alterations that occur to maintain adequate cardiac output and oxygen delivery [17], [18]. Autonomic nervous dysfunction is a key factor that renders MV weaning difficult; although no significant changes in cardiac output were noted, one study did observe a significant increase in the catecholamine levels in response to a weaning trial [18]. Heart rate variability (HRV), measured as the variation in sequential inter-beat or R-R intervals on an electrocardiogram (ECG), is a valid noninvasive measure of autonomic nervous system (ANS) function [19]. HRV has also been assessed as a physiological variable related to weaning outcomes [20], [21]. Specifically, patients with significantly lower baseline HRVs when receiving MV and a greater reduction in HRVs during a weaning trial or weaning ventilation mode transitions have been found to be at a higher risk for MV weaning failure [18], [20].
HRV not only directly links the heart to the central ANS [19], but may also be influenced by patients' emotional responses [22], [23], [24]. Indeed, HRV has been considered an indicator derived from psychological and physiological interactions when treating medical and psychological conditions [22], [23]. For example, some patients experience psychological distress such as anxiety during the weaning process, which may lead to the downregulation of β-adrenergic receptors, presenting a blunted ability to respond to further elevation in catecholamines and contributing to the reduction in HRV [18]. A high rapid shallow breathing index (RSBI), which is measured as the ratio of respiratory frequency f to tidal volume VT (f/VT) [25], is believed to represent a respiratory center response to both respiratory loading and anxiety during weaning trials [5], [26]. According to previous studies, the decrease in the high-frequency component of HRV (HRV-HF), indicating vagal withdrawal, is associated with a higher RSBI in patients who fail to wean from MV [20], [21].
Clinical evidence has indicated that even some patients who meet the weaning criteria still fail to pass the weaning trial, which suggests that they had encountered a decline in physiological and psychological status during the weaning process. Therefore, simultaneously exploring changes in ANS function and psychological status during the MV weaning process, and testing the association of those changes with weaning outcomes, are crucial but absent research pursuit. Accordingly, this study was conducted to identify whether the changes in HRV, RSBI, and psychological distress (i.e., self-rated anxiety, fear, and dyspnea) in response to weaning trials were associated with MV weaning outcomes. Our multivariate logistic regression findings confirmed a significant association.
Section snippets
Study design and population
In this prospective study, we recruited a convenience sample of patients on MV from a 22-bed medical intensive care unit and 21-bed respiratory care center at a medical center in northern Taiwan. Patients were included if they (a) had received MV support for > 24 h; (b) were ready for T-piece weaning; (c) were hemodynamically stable (i.e., systolic blood pressure [SBP] > 90 mm Hg and requiring no or low-dose vasopressors [e.g., dopamine or dobutamine < 5 g/kg/min]); (d) did not require vasoactive
Results
Of the 67 patients in this study, 49 successfully passed the 2-h weaning trial, and none failed the T-piece trail within 30 min of its commencement. The main reason for the patients' use of MV was acute respiratory failure (35.8%). The two groups did not differ significantly in age, sex, disease severity (APACHE II scores), length of time on MV before weaning, or medication use (Table 1).
Discussion
This study revealed that higher ANS function (i.e., better reservation of ANS function before weaning and less vagal withdrawal during the weaning trial) and less psychological distress (e.g., fear) in response to the weaning trial were significantly associated with 2-h T-piece weaning success (Fig. 1B).
Consistent with previous findings that have suggested that ANS dysfunction is prevalent in critically ill patients on MV [37], [38], the time- and frequency-domains of HRV indices of the
Limitations
This study has several limitations. First, we did not consider the confounding effect of respiratory variables and blood pressure on HRV [48], although the effectiveness of using respiratory parameters to predict weaning outcomes remains unconfirmed [18]. Second, the small sample limits the generalizability of our findings. Although recruiting patients from this critically ill and vulnerable population is highly challenging, a larger sample size is necessary to validate the findings of this
Conclusions
Our results indicated that a successful T-piece weaning outcome was associated with a higher reservation of ANS function before weaning, and less psychological and physiological distress (perceived fear and vagal withdrawal, respectively) during weaning among patients on MV. Accordingly, these results provide a more accurate understanding of the dynamic changes in psychological and physiological functioning that may be present during the weaning process. Future studies with a larger sample size
Acknowledgments
This work was supported by a national research grant (99-2314-B-016-040) from the Ministry of Science and Technology in Taiwan. In addition, the authors gratefully acknowledge the help and support from the Tri-Service General Hospital in Taipei, Taiwan. Great appreciation is also extended to Dr. Amy H.T. Davis for providing scientific consultation on this manuscript.
Author contributions
Conception and design of the study: Y.J. Chen, S.L. Huang, C.C. Yang, K.L. Huang; collection, analysis, and interpretation of the data: Y.J. Chen, C.C. Yang, C.R. Li, C.Y. Lin, C.Y. Lee; and preparation of the manuscript: Y.J. Chen, S.L. Huang, C.C. Yang, K.L. Huang, C.R. Li, C.Y. Lin. All authors gave approval for publication.
Conflict of interest
The authors declare no conflicts of interest.
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