The cardiorespiratory system exhibits oscillations from a range of sources. One of the most studied oscillations is heart rate variability, which is thought to be beneficial and can serve as an index of a healthy cardiovascular system. Heart rate variability is dampened in many diseases including depression, autoimmune diseases, hypertension, and heart failure. Thus, understanding the interactions that lead to heart rate variability, and its physiological role, could help with prevention, diagnosis, and treatment of cardiovascular diseases. In this review, we consider three types of cardiorespiratory interactions: respiratory sinus arrhythmia (variability in heart rate at the frequency of breathing), cardioventilatory coupling (synchronization between the heart beat and the onset of inspiration), and respiratory stroke volume synchronization (the constant phase difference between the right and the left stroke volumes over one respiratory cycle). While the exact physiological role of these oscillations continues to be debated, the redundancies in the mechanisms responsible for its generation and its strong evolutionary conservation point to the importance of cardiorespiratory interactions. The putative mechanisms driving cardiorespiratory oscillations as well as the physiological significance of these oscillations will be reviewed. We suggest that cardiorespiratory interactions have the capacity to both dampen the variability in systemic blood flow as well as improve the efficiency of work done by the heart while maintaining physiological levels of arterial CO. Given that reduction in variability is a prognostic indicator of disease, we argue that restoration of this variability via pharmaceutical or device-based approaches may be beneficial in prolonging life.

Introduction 4 Emergency nurses are at risk of compassion fatigue. Compassion fatigue caused by 5 exposure to suffering may compromise the individual's personal wellbeing and reduce work 6 efficiency. 7 Methods: 8 A quantitative cross-sectional survey with open responses was conducted using the 9 Professional Quality of Life: Compassion Satisfaction and Compassion Fatigue (ProQOL) 10 scale and open-ended questions. Responses from a convenience sample of 86 nurses from 11 two hospital emergency departments in Victoria, Australia, were analysed. 12 Results: 13 The median score for Compassion Satisfaction was 78% with all nurses reporting average to 14 high scores. Most had average levels of Compassion Fatigue: Burnout median score was 15 53% and Secondary Traumatic Stress median score 49%. No statistically significant 16 correlation was found between scales nor with influencing demographic characteristics. A 17 qualification in emergency nursing was predictive of Compassion Satisfaction. Six 18 descriptive job-associated factors contributed to nurses' stress: human resources, the 19 organisation, job-specific components, patient mix and professional and personal 20 components. 21 Conclusion/s: 22 Average to high levels of Compassion Satisfaction and low to average levels of Compassion 23 Fatigue were found in emergency nurses. Issues contributing to stress were work and role 24related. An understanding of these stressors may help nurses and nurse managers to 25 ameliorate emergency nurses' levels of stress and help limit staff burnout. 26

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Key points Respiratory sinus arrhythmia is physiological pacing of the heart that disappears in cardiovascular disease and is associated with poor cardiac prognosis. In heart failure, cardiac pacing has little, if any, variation in rate at rest. We proposed that reinstatement of respiratory sinus arrhythmia would improve cardiac function in rats with heart failure. Heart failure rats were paced daily for 2 weeks with either respiratory sinus arrhythmia or paced monotonically at a matched heart rate; cardiac function was measured using non‐invasive echocardiography. Cardiac output and stroke volume were increased in rats paced with respiratory sinus arrhythmia compared to monotonic pacing, via improvement in systolic function that persisted beyond the pacing treatment period. We propose that respiratory sinus arrhythmia pacing reverse‐remodels the heart in heart failure and is worth considering as a new form of cardiac pacemaking. Abstract Natural pacing of the heart results in heart rate variability, an indicator of good health and cardiac function. A contributor to heart rate variability is respiratory sinus arrhythmia or RSA – an intrinsic respiratory modulated pacing of heart rate. The loss of RSA is associated with poor cardiac prognosis and sudden cardiac death. We tested if reinstatement of respiratory‐modulated heart rate (RMH) would improve cardiac performance in heart failure. Heart failure was induced in Wistar rats by ligation of the left anterior descending coronary artery. Rats were unpaced, monotonically paced and RMH paced; the latter had the same average heart rate as the monotonically paced animals. Cardiac function was assessed non‐invasively using echocardiography before and after 2 weeks of daily pacing at a time when pacing was turned off. RMH increased cardiac output by 20 ± 8% compared to monotonic pacing (−3 ± 5%; P < 0.05). This improvement in cardiac output was associated with an increase in stroke volume compared to monotonic pacing (P = 0.03) and improvement in circumferential strain (P = 0.02). Improvements in ejection fraction (P = 0.08) and surrogate measures of left ventricle compliance did not reach significance. Increases in contractility (P < 0.05) and coronary blood flow (P < 0.05) were seen in vitro during variable pacing to mimic RMH. Thus, in rats with left ventricular dysfunction, chronic RMH pacing improved cardiac function through improvements in systolic function. As these improvements were made with pacing switched off, we propose the novel idea that RMH pacing causes reverse‐remodelling.