No abstract

In 48 conscious resting subjects we examined the temporal coupling of heart beat timing and the onset of inspiration (cardioventilatory coupling), and the relationship between coupling and spectral indices of autonomic function. Using the proportional Shannon entropy (SHα) of the RI‐1 interval (interval between inspiration and the preceding ECG R wave) as a measure of coupling we detected statistically significant coupling in 32 of the 48 subjects. This was confirmed by visual inspection of time series plots of RI intervals, in which coupling was evident as horizontal banding. Coupling resulted in a significant preference for whole number heart rate/respiratory frequency ratios. The strongest coupling was associated with low ventilatory frequency and high heart rate variability in the high (0.15‐0.40 Hz) and low (0.04‐0.15 Hz) frequency ranges, but was not related to blood pressure variability, or to a spectral measure of baroreflex sensitivity (α‐index, low frequency range). There was no difference in coupling strength between males and females. We have previously described cardioventilatory coupling in spontaneously breathing anaesthetised subjects. The current study extends those observations by demonstrating that the qualitative features of coupling seen during anaesthesia are also observed in the conscious state. We conclude that the role of coupling in normal physiological respiratory control needs to be more widely explored.

Background: Introducing cardiopulmonary resuscitation (CPR) training in the high-school curriculum has been widely recommended as a long-term strategy to educate the wider community. Although CPR has been included in the New Zealand school curriculum, it is listed as an optional subject only. Aim: To assess the attitude towards and knowledge of CPR in 16-17-year-old high-school students in New Zealand. Methods: Questionnaires were administered to 494 students aged 16-17 years across six high schools in Wellington, New Zealand. Both knowledge and attitude were evaluated in the questionnaire. Results: Students showed poor theoretical knowledge, with a mean (SD) score of 5.61 (2.61) out of a maximum score of 18. Although there was no difference between male and female students, those who had received previous first-aid training (70%) showed greater knowledge (6.04 (2.56)) than their untrained counterparts (4.91 (2.24); p = 0.001). Those students with a positive attitude towards CPR and first-aid training (63%) acquired a higher knowledge score (6.12 (2.4)) than those with a negative attitude (17%; 4.65 (2.5); p = 0.001). Students with negative associations were also less likely to want to learn more about CPR and first aid (11%) when compared with those with positive associations (92%), and indicated less willingness to perform CPR on a stranger (negative v positive, 47% v 70%). Conclusions: These findings suggest that although most high-school students are willing and motivated to learn CPR, a smaller percentage of students had a negative attitude towards CPR that would act as a barrier to future learning or performance of resuscitation. Introducing CPR training to high schools is still recommended; however, this study shows the need to associate this training with positive references in an attempt to assist those for whom negative attitude may present as a barrier to learning and retaining CPR knowledge.

In 20 spontaneously breathing patients undergoing minor surgical procedures under general anaesthesia, we examined the timing relationship of heart best and ventilation. Patients received propofol 2-2.5 mg kg-1 and fentanyl 1 microgram kg-1 and breathed a mixture of 1-1.5% isoflurane and 66% nitrous oxide in oxygen. We recorded ECG R wave times and the start of each inspiration. We observed that: (1) all subjects showed evidence of phase coupling in which heart beats occurred at constant phases of the ventilatory cycle; (2) there was a significant preference for whole number ratios of heart rate to ventilatory frequency; (3) phase coupling was associated frequently with quantal changes in heart rate or ventilatory frequency, or both; (4) six coupling patterns were described according to the timing relationship between the ECG R wave and start of inspiration and according to changes in the number of heart beats within each ventilatory period; (5) phase coupling was achieved primarily by transient changes in ventilatory period. Phase coupling, in concert with respiratory sinus arrhythmia, may optimize the performance of the thoracic pump, matching cardiac filling to venous return. Coupling may therefore have anaesthetic relevance in conditions of impaired cardiac performance or hypovolaemia.

Normally, at rest, the amplitude of respiratory sinus arrhythmia (RSA) appears to correlate with cardiac vagal tone. However, recent studies showed that, under stress, RSA dissociates from vagal tone, indicating that separate mechanisms might regulate phasic and tonic vagal activity. This dissociation has been linked to the hypothesis that RSA improves pulmonary gas exchange through preferential distribution of heartbeats in inspiration. We examined the effects of hypercapnia and mild hypoxemia on RSA-vagal dissociation in relation to heartbeat distribution throughout the respiratory cycle in 12 volunteers. We found that hypercapnia, but not hypoxemia, was associated with significant increases in heart rate (HR), tidal volume, and RSA amplitude. The RSA amplitude increase remained statistically significant after adjustment for respiratory rate, tidal volume, and HR. Moreover, the RSA amplitude increase was associated with a paradoxical rise in HR and decrease in low-frequency-to-high-frequency mean amplitude ratio derived from spectral analysis, which is consistent with RSA-vagal dissociation. Although hypercapnia was associated with a significant increase in the percentage of heartbeats during inspiration, this association was largely secondary to increases in the inspiratory period-to-respiratory period ratio, rather than RSA amplitude. Additional model analyses of RSA were consistent with the experimental data. Heartbeat distribution did not change during hypoxemia. These results support the concept of RSA-vagal dissociation during hypercapnia; however, the putative role of RSA in optimizing pulmonary perfusion matching requires further experimental validation.