D.C. Galletly

Respiratory sinus arrhythmia in conscious humans during spontaneous respiration.

Respiratory sinus arrhythmia (RSA) is the beat-to-beat fluctuation in heart rate at the frequency of the respiratory cycle. While it is common to study RSA under conditions of controlled breathing, where respiratory frequency, and sometimes tidal volume and inspiratory:expiratory ratio are controlled, the effect of controlled breathing on RSA is not clear. While not all studies exploring the effects of controlled breathing on RSA magnitude are consistent, some of the best-designed studies addressing this question did find a significant effect. In addition to respiratory timing influencing heartbeats, there is evidence that cardiac timing also influences respiratory timing, termed cardioventilatory coupling. Thus, the timing interactions between the cardiac and respiratory systems are complex, and bi-directional. Controlled breathing eliminates one aspect of this relationship, and studies designed to understand cardiorespiratory physiology conducted under these conditions need to be interpreted with an understanding that they may not represent normal physiology.

Arnold tongues in human cardiorespiratory systems.

Arnold tongues are phase-locking regions in parameter space, originally studied in circle-map models of cardiac arrhythmias. They show where a periodic system responds by synchronizing to an external stimulus. Clinical studies of resting or anesthetized patients exhibit synchronization between heart-beats and respiration. Here we show that these results are successfully modeled by a circle-map, neatly combining the phenomena of respiratory sinus arrhythmia (RSA, where inspiration modulates heart-rate) and cardioventilatory coupling (CVC, where the heart is a pacemaker for respiration). Examination of the Arnold tongues reveals that while RSA can cause synchronization, the strongest mechanism for synchronization is CVC, so that the heart is acting as a pacemaker for respiration.

Influence of breathing frequency on the pattern of respiratory sinus arrhythmia and blood pressure: old questions revisited

Respiratory sinus arrhythmia (RSA) is classically described as a vagally mediated increase and decrease in heart rate concurrent with inspiration and expiration, respectively. However, although breathing frequency is known to alter this temporal relationship, the precise nature of this phase dependency and its relationship to blood pressure remains unclear. In 16 subjects we systematically examined the temporal relationships between respiration, RSA, and blood pressure by graphically portraying cardiac interval (R-R) and systolic blood pressure (SBP) variations as a function of the respiratory cycle (pattern analysis), during incremental stepwise paced breathing. The principal findings were 1) the time interval between R-R maximum and expiration onset remained the same ( approximately 2.5-3.0 s) irrespective of breathing frequency (P = 0.10), whereas R-R minimum progressively shifted from expiratory onset into midinspiration with slower breathing (P < 0.0001); 2) there is a clear qualitative distinction between pre- versus postinspiratory cardiac acceleration during slow (0.10 Hz) but not fast (0.20 Hz) breathing; 3) the time interval from inspiration onset to SBP minimum (P = 0.16) and from expiration onset to SBP maximum (P = 0.26) remained unchanged across breathing frequencies; 4) SBP maximum and R-R maximum maintained an unchanged temporal alignment of approximately 1.1 s irrespective of breathing frequency (P = 0.84), whereas the alignment between SBP minimum and R-R minimum was inconstant (P > 0.0001); and 5) beta(1)-adrenergic blockade did not influence the respiration-RSA relationships or distinct RSA patterns observed during slow breathing, suggesting that temporal dependencies associated with alterations in breathing frequency are unrelated to cardiac sympathetic modulation. Collectively, these results illustrate nonlinear respiration-RSA-blood pressure relationships that may yield new insights to the fundamental mechanism of RSA in humans.

Knowledge of and attitudes towards resuscitation in New Zealand high-school students

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.

Human sinus arrhythmia: inconsistencies of a teleological hypothesis

Respiratory sinus arrhythmia (RSA) may serve an inherent function in optimizing pulmonary gas exchange efficiency via clustering and scattering of heart beats during the inspiratory and expiratory phases of the respiratory cycle. This study sought to determine whether physiological levels of RSA, enhanced by slow paced breathing, caused more heart beats to cluster in inspiration. In 12 human subjects, we analyzed the histogram distribution of heart beats throughout the respiratory cycle during paced breathing at 12, 9, and 6 breaths/min (br/min). The inspiratory period-to-respiratory period ratio was fixed at approximately 0.5. RSA and its relationship with respiration was characterized in the phase domain by average cubic-spline interpolation of electrocardiographic R wave-to-R wave interval fluctuations throughout all respiratory cycles. Although 6 br/min breathing was associated with a significant increase in RSA amplitude (P < 0.01), we observed no significant increase in the proportion of heart beats in inspiration (P = 0.34). Contrary to assumptions in the literature, we observed no significant clustering of heart beats even with high levels of RSA enhanced by slow breathing. The results of this study do not support the hypothesis that RSA optimizes pulmonary gas exchange efficiency via clustering of heart beats in inspiration.

Effect of automatic external defibrillator audio prompts on cardiopulmonary resuscitation performance.

Objectives: To determine the effectiveness of the cardiopulmonary resuscitation (CPR) audio prompts in an automatic external defibrillator in 24 lay subjects, before and after CPR training. Methods: Untrained subjects were asked to perform CPR on a manikin with and without the assistance of audio prompts. All subjects were then trained in CPR, and retested them eight weeks later. Results: Untrained subjects who performed CPR first without audio prompts performed poorly, with only (mean (SD)) 24.5% (32%) of compressions at the correct site and depth, a mean compression rate of 52 (31) per minute, and with 15% (32%) of ventilatory attempts adequate. Repeat performance by this group with audio prompts resulted in significant improvements in compression rate (91(12), p = 0.0002, paired t test), and percentage of correct ventilations (47% (40%), p = 0.01 paired t test), but not in the percentage correct compressions (23% (29%)). Those who performed CPR first with audio prompts performed significantly better in compression rate (87 (19), p = 003, unpaired t test), and the percentage of correct ventilations (51 (34), p = 0.003 unpaired t test), but not in the percentage of correct compressions (18 (27)) than those without audio prompts. After training, CPR performance was significantly better than before training, but there was no difference in performance with or without audio prompts, although 73% of subjects commented that they felt more comfortable performing CPR with audio prompts. Conclusions: For untrained subjects, the quality of CPR may be improved by using this device, while for trained subjects the willingness to perform CPR may be increased.

Spectral analysis of AC and DC components of the pulse photoplethysmograph at rest and during induction of anaesthesia

We examined spectral components of beat to beat variability in AC and DC signals of the reflectance photoplethysmograph at finger and earlobe sites in 20 resting volunteers and 20 patients during propofol, alfentanil, isoflurane, nitrous oxide anaesthesia.. We observed that at rest, the majority of spectral power at both sites and in both signals was in the low ‘thermoregulatory’ frequency band (0.01–0.08 Hz). These fluctuations were greater in the finger than in the earlobe and in the AC signal compared to the DC. With anaesthesia, low as well as mid (0.08–0.15 Hz) frequency variability decreased at both sites and in both signals whereas high frequency ‘ventilatory’ power (0.15–0.45 Hz) was maintained. During anaesthesia we found no significant differences between the spectral components of the AC or DC signals or between the finger and the earlobe sites. At all frequencies, the fluctuations in the AC and DC signals were out of phase with each other.

Interactions between heart rate variability and pulmonary gas exchange efficiency in humans

The respiratory component of heart rate variability (respiratory sinus arrhythmia, RSA) has been associated with improved pulmonary gas exchange efficiency in humans via the apparent clustering and scattering of heart beats in time with the inspiratory and expiratory phases of alveolar ventilation, respectively. However, since human RSA causes only marginal redistribution of heart beats to inspiration, we tested the hypothesis that any association between RSA amplitude and pulmonary gas exchange efficiency may be indirect. In 11 patients with fixed‐rate cardiac pacemakers and 10 healthy control subjects, we recorded R–R intervals, respiratory flow, end‐tidal gas tension and the ventilatory equivalents for carbon dioxide   and oxygen   during ‘fast’ (0.25 Hz) and ‘slow’ paced breathing (0.10 Hz). Mean heart rate, mean arterial blood pressure, mean arterial pressure fluctuations, tidal volume, end‐tidal CO2,  and   were similar between pacemaker and control groups in both the fast and slow breathing conditions. Although pacemaker patients had no RSA and slow breathing was associated with a 2.5‐fold RSA amplitude increase in control subjects (39 ± 21 versus 97 ± 45 ms, P < 0.001), comparable   (main effect for breathing frequency, F(1,19) = 76.54, P < 0.001) and   reductions (main effect for breathing frequency, F(1,19) = 23.90, P < 0.001) were observed for both cohorts during slow breathing. In addition, the degree of   (r=−0.36, P= 0.32) and   reductions (r=−0.29, P= 0.43) from fast to slow breathing were not correlated to the degree of associated RSA amplitude enhancements in control subjects. These findings suggest that the association between RSA amplitude and pulmonary gas exchange efficiency during variable‐frequency paced breathing observed in prior human work is not contingent on RSA being present. Therefore, whether RSA serves an intrinsic physiological function in optimizing pulmonary gas exchange efficiency in humans requires further experimental validation.

Fractal characteristics of breath to breath timing in sleeping infants.

We examined interbreath interval (IBI) time series of 19 term infants during active and quiet sleep for fractal properties using Fano factor analysis. For each time series we calculated the fractal exponent (alpha), comparing alpha for the original time series with two forms of surrogate data, a temporally independent surrogate set and an autoregressive surrogate set. alpha values were normally distributed between 0.79 and -0.22, and did not differ with sleep state. The fractal characteristics of the original time series were not retained in the temporally independent surrogate time series indicating that the distribution of intervals alone was not fractal, but were retained using autoregressive surrogates with an order of 10, suggesting that the fractal properties of the IBI time series were related to correlations between successive breaths. These observations suggest that some of the respiratory variability that occurs during sleep in infants, which in the past has been regarded as stochastic noise, may be the product of deterministic processes.

Junior doctor skill in the art of physical examination: a retrospective study of the medical admission note over four decades

Objectives To investigate the hypothesis that junior doctors’ examination skills are deteriorating by assessing the medical admission note examination record. Design Retrospective study of the admission record. Setting Tertiary care hospital. Methods The admission records of 266 patients admitted to Wellington hospital between 1975 and 2011 were analysed, according to the total number of physical examination observations (PEOtot), examination of the relevant system pertaining to the presenting complaint (RelSystem) and the number of body systems examined (Nsystems). Subgroup analysis proceeded according to admission year, level of experience of the admitting doctor (registrar, house surgeon (HS) and trainee intern (TI)) and medical versus surgical admission notes. Further analysis investigated the trend over time in documentation with respect to cardiac murmurs, palpable liver, palpable spleen, carotid bruit, heart rate, funduscopy and apex beat location and character. Results PEOtot declined by 34% from 1975 to 2011. Surgical admission notes had 21% fewer observations than medical notes. RelSystem occurred in 94% of admissions, with no decline over time. Medical notes documented this more frequently than surgical notes (98% and 86%, respectively). There were no differences between registrars and HS, except for the 2010s subgroup (97% and 65%, respectively). Nsystems declined over the study period. Medical admission notes documented more body systems than surgical notes. There were no differences between registrars, HSs and TIs. Fewer examinations were performed for palpable liver, palpable spleen, cardiac murmur and apex beat location and character over the study period. There was no temporal change in the positive findings of these observations or heart rate rounding. Conclusions There has been a decline in the admission record at Wellington hospital between 1975 and 2011, implying a deterioration in local doctors’ physical examination skills. Measures to counter this trend are discussed.