P. Larsen

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.

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.

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.

Methodological considerations for the assessment of ADP induced platelet aggregation using the Multiplate® analyser.

Factors affecting the Multiplate® assays analytical precision have not been well defined. We investigated the effect of methodological factors on the measurement of ADP-induced platelet aggregation using the Multiplate® assay. ADP-induced platelet aggregation was analysed in whole blood using the Multiplate® assay. We tested the reproducibility of measurement, the effect of different anticoagulants (hirudin, citrate and heparin) and the effect of time delay (15, 30, 45, 60, 120 and 180 minutes) between sampling and analysis in patients. The use of a manual calibrated pipette with the Multiplate® analyser was also tested. The mean coefficient of variation (CV) using the manufacturers recommended methods was 10.8 ± 8.7% (n = 30). When compared to hirudin (359.5 ± 309 AU*min) the use of heparin (521.0 ± 316 AU*min, p = 0.0015) increased platelet aggregation, while the use of sodium citrate (245.0 ± 209 AU*min, p = 0.003) decreased the platelet aggregation (n = 20). The addition of CaCl2 to the citrate-anticoagulated blood resulted in platelet aggregation levels similar to hirudin. Platelet aggregation varied with time delay (n = 20). When compared to platelet aggregation at 30 minutes (391.1 ± 283 AU*min), platelet aggregation was reduced at 60 minutes (335.2 ± 251.6 AU*min, p < 0.05), 120 minutes (198.8 ± 122.9 AU*min, p < 0.001) and 180 minutes (160.7 ± 92 AU*min, p < 0.001). The use of a manual calibrated pipette did not significantly reduce the mean CV in the assay (n = 20). Methodological factors such as the anticoagulant used and the time delay should be standardised where possible to reduce variability, and allow thresholds derived from one study to be comparable across multiple studies.

Patterns of external chest compression.

We studied the performance of external chest compression by 20 fourth year medical students on 2 study days, separated by 5-7 weeks, 4-8 months after they had been initially trained in cardiopulmonary resuscitation (CPR). Our hypotheses were (1) that a given individual would perform external chest compressions in the same manner each time CPR was performed and (2) that the pattern of performance of chest compressions would be determined, in part, by the anthropometric characteristics of the rescuer. A Laerdal Skillmeter Resusci-Anne CPR manikin chest compression transducer was interfaced with an analogue-to-digital conversion board in a Macintosh LC III computer. Each subject on each study day performed four cycles of 15 compressions and two ventilations twice, once on a table (which raised the surface of the manikin sternum to 95 cm) and once on the floor. For each individual, on each attempt, we calculated the depth and rate of compressions, duty cycle, peak compression velocity, time to peak compression velocity and time to peak compression depth. In addition, we calculated the regression slope of compression depth versus compression number for each cycle of 15 chest compressions and over four cycles of compressions. Statistically significant correlations were observed between the first and second study days in each of the variables of chest compression measured, indicating that the performance of chest compressions was constant over time for a given individual. We observed that the depth of compression, duty cycle, time to peak compression, time to peak velocity and regression slope of depth of compressions versus compression number were significantly related to the height and weight of the rescuer.

A national survey of clinician's knowledge of and attitudes towards implantable cardioverter defibrillators.

AIMS This study surveyed referring clinicians to identify barriers that may contribute to New Zealands low national implantable cardioverter defibrillator (ICD) implant rate. METHODS We conducted a telephone survey of 100 cardiologists and general physicians working at 30 different New Zealand hospitals who routinely manage patients with ischaemic heart disease and heart failure. RESULTS The majority of those surveyed (76%) rated their knowledge as satisfactory or better, although only 62% reported familiarity with international guidelines for ICD therapy. When asked to identify ICD indications 80% identified symptomatic or sustained ventricular arrhythmias and 73% left ventricular dysfunction. While 82% believed that the use of ICD therapy for secondary prevention was cost effective, only 53% believed they were cost effective for primary prevention. Lack of financial resource (88%), lack of local expertise (61%), lack of New Zealand guidelines (51%), and the referral process (43%) were seen as significant barriers to ICD referral by many participants. The majority of rural clinicians (71%) identified restricted access to investigations as a barrier to implantation, significantly higher than urban clinicians (18%, P = 0.001). CONCLUSION We have identified a number of potential barriers that will need to be addressed to raise the New Zealand ICD implantation rate.

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.