Influence of physical exercise on baroreceptor sensitivity during pregnancy

BACKGROUND

Baroreceptor sensitivity (BRS) refers to the magnitude of change in the heart rate in response to change in blood pressure (e.g. upon standing). The impact of regular antenatal exercise on maternal BRS is unclear.

AIMS

To determine whether supervised weekly exercise influences BRS, and to determine if posture and calculation method are important in antenatal BRS measurement.

STUDY DESIGN AND SUBJECTS

Eighty-one healthy pregnant women were randomly assigned to an exercise or control group. The exercise group attended weekly classes from the 20th week of pregnancy onwards.

OUTCOME MEASURES

Cardiovascular assessments (beat-to-beat blood pressure, heart rate) were performed at 12-16, 26-28, 34-36 weeks and 12 weeks following birth. BRS was calculated using two methods ("sequence" and "beat-to-beat").

RESULTS

Fifty-one women (63%) completed the study. Mean BRS reduced progressively in all women (p < 0.025) and was lowest in those who exercised (0.046 < p < 0.002). Postnatal increases in BRS were independent of posture. Training-induced BRS (beat-to-beat) reduction occurred earlier than BRS (sequence), and only BRS (sequence) was affected by posture. Heart rate variability reduced with advancing gestation (p < 0.002) and was more pronounced in the exercise group (p < 0.029).

CONCLUSIONS

Weekly exercise exaggerated the reductions in BRS and HRV during pregnancy and is likely linked to diminished parasympathetic activity.

Influence of antenatal physical exercise on heart rate variability and QT variability

OBJECTIVE

We sought to characterise the influence of an antenatal exercise programme on ECG-derived cardiac variables.

METHODS

Fifity-one healthy pregnant women were recruited and randomly assigned (2 × 2×2 design) to an exercise group or a control group. Exercising groups attended weekly classes from the 20th week of pregnancy onwards. Cardiovascular assessments (heart rate variabiliy (HRV), QT, and the QT variability index (QTVI)) were performed at 12-16, 26-28, 34-36 weeks and 12 weeks following birth, during supine rest and exercise conditions.

RESULTS

Advancing gestation was associated with an increased maternal heart rate (p = 0.001), shorter QT interval (p = 0.003), diminished HRV (p = 0.002) and increased QTVI (p = 0.002). Each of these changes was reversed within 12 weeks postpartum (p < 0.004). The Exercise group displayed exaggerated changes for all variables (except QT) but only during supine rest in the third trimester (p < 0.029).

CONCLUSION

Advancing gestation is associated with a shift in HRV/QTVI towards values that have been associated with an elevated risk of arrhythmia. A 20-week exercise programme undertaken between mid and late pregnancy exaggerated these changes during rest in the third trimester of pregnancy.

Recruitment of pregnant women to an exercise-intervention study

We share here our experience of recruiting pregnant women into an exercise intervention study. Recruitment challenges were anticipated owing to the study design, which required four hospital visits for cardiovascular assessment, a long-term (nine-month) commitment, and adherence to a 20-week exercise programme. Fifty-three women were assigned to one of three groups (no-exercise, land exercise or water exercise) using a 2 × 2 × 2 flexible randomisation design. Seven hundred forty-four women were screened at an antenatal clinic, of whom 501 were eligible to participate in the study. One hundred forty-five women were subsequently approached: 46 (32%) of whom agreed to participate, 42 (29%) were interested but then declined and 57 (39%) declined outright. Our study design helped recruit pregnant women as it allowed them some choice of group membership. We also noted that the participant-researcher relationship is important in reducing attrition. Our experience provides indications of likely recruitment and attrition rates for future randomised controlled trials of this type.

Changes in heart rate variability and QT variability during the first trimester of pregnancy

The risk of new-onset arrhythmia during pregnancy is high, presumably relating to changes in both haemodynamic and cardiac autonomic function. The ability to non-invasively assess an individual's risk of developing arrhythmia during pregnancy would therefore be clinically significant. We aimed to quantify electrocardiographic temporal characteristics during the first trimester of pregnancy and to compare these with non-pregnant controls. Ninety-nine pregnant women and sixty-three non-pregnant women underwent non-invasive cardiovascular and haemodynamic assessment during a protocol consisting of various physiological states (postural manoeurvres, light exercise and metronomic breathing). Variables measured included stroke volume, cardiac output, heart rate, heart rate variability, QT and QT variability and QTVI (a measure of the variability of QT relative to that of RR). Heart rate (p < 0.0005, p < 0.0005, p < 0.0005) and cardiac output (p = 0.043, p < 0.0005, p < 0.0005) were greater in pregnant women in all physiological states (respectively for the supine position, light exercise and metronomic breathing state), whilst stroke volume was lower in pregnancy only during the supine position (p < 0.0005). QTe (Q wave onset to T wave end) and QTa (T wave apex) were significantly shortened (p < 0.05) and QTeVI and QTaVI were increased in pregnancy in all physiological states (p < 0.0005). QT variability (p < 0.002) was greater in pregnant women during the supine position, whilst heart rate variability was reduced in pregnancy in all states (p < 0.0005). Early pregnancy is associated with substantial changes in heart rate variability, reflecting a reduction in parasympathetic tone and an increase in sympathetic activity. QTVI shifted to a less favourable value, reflecting a greater than normal amount of QT variability. QTVI appears to be a useful method for quantifying changes in QT variability relative to RR (or heart rate) variability, being sensitive not only to physiological state but also to gestational age. We support the use of non-invasive markers of cardiac electrical variability to evaluate the risk of arrhythmic events in pregnancy, and we recommend the use of multiple physiological states during the assessment protocol.

Analysis of the QT interval and its variability in healthy adults during rest and exercise

The aim of the present study was to quantify the variability of electrocardiographic QT and RR intervals during rest and dynamic physical exercise, and to interpret these variabilities in terms of relative autonomic modulation of the atrial and ventricular myocardium. We also sought to characterize the relationships between QT, heart rate-corrected QT (QT(c)) and RR intervals, and to consider their associations with differential autonomic regulation. Nine males and eight females of similar age (22.8 +/- 4.7 years), mass (75.5 +/- 13.0 kg) and aerobic fitness (43.6 +/- 7.7 ml kg(-1) min(-1)) (mean +/- SD) undertook progressive bicycle exercise. A three-lead Holter ECG was recorded continuously during pre-exercise, exercise and recovery, and mean values of RR, QT, QT(c), QT variability index (QTVI) and mean-normalized QT variance (QTVN) were determined. At the onset of exercise QTVI increased rapidly compared with rest and remained significantly elevated throughout exercise and recovery. There were significant differences between QT(a)VI and QT(e)VI (QT measured from Q wave onset to T wave apex (QT(a)) and T wave end (QT(e)), respectively) throughout the experimental protocol. QTVI was significantly reduced in males compared with females prior to exercise but was similar thereafter. We suggest that physical exercise perturbs the resting QT-RR relationship owing to the onset of differential parasympathetic modulation of the atrial and ventricular myocardium. QTVI can be used to quantify the relative autonomic influence on the atrial and ventricular myocardium during rest and exercise, and might be related to HR-dependent and HR-independent influences on the QT interval.

Time domain correlation analysis of heart rate variability in preterm neonates

BACKGROUND AND AIM

A fuller understanding of the neural control mechanisms of heart rate during the early stages of human development would be of great value to obstetric and neonatal management. In this paper, we investigate the correlation between heart rate variability (HRV) and other physiological parameters such as blood pressure and respiration in preterm neonates with the aim of developing a numerical model to explain and predict heart rate variability.

STUDY DESIGN AND SUBJECTS

All the required data are readily available for premature babies who are routinely monitored while being nursed in intensive care, and we have collected large data sets for a random group of such neonates. For the quantitative analysis of the data, we have developed a time domain correlation method, which has a number of advantages over the more commonly used power spectral analysis. We have been able to study the dynamics of the different frequency components of HRV by this method.

RESULTS

Highly correlated behaviour of the different HRV components, previously observed in our work on fetal HRV, is also present in the neonate, with similar characteristic time constants. Furthermore, the correlation of high-frequency (HF) oscillations of HRV with respiration and that of low-frequency (LF) oscillations of HRV with blood pressure are demonstrated on timescales of a single oscillation. In neonates receiving artificial ventilation, the correlation between HRV and respiration depends on the type of ventilation involved and assumes opposite polarities for the two main types of equipment currently in use.

CONCLUSION

We demonstrate that it is possible to analyse HRV quantitatively by calculating the relative gains and characteristic time constants for the correlated parameters and components.

Power spectral analysis of the foetal magnetocardiogram

The results of power spectral analysis of foetal magnetocardiographic (FMCG) data, acquired using a DC-SQUID-based system, are reported. Similar analyses have been previously reported using foetal electrocardiographic data, but it is believed that our work represents one of the first attempts to analyse, in the frequency domain, the magnetic fields produced by foetal cardiac activity. Analysis of the data in this way may enable the integrity of the foetal nervous system, and thus the status of the foetus as a whole, to be monitored and evaluated during a significant part of the antenatal period. The results obtained are discussed with reference to the probable underlying physiological mechanisms. This preliminary study highlights some of the advantages of FMCG as a novel, non-invasive technique for obtaining clinically useful information. Fourier analysis of the FMCG data is likely to yield new information, not only about cardiac function but also about the foetal central nervous system.

Sequential recording of the abdominal fetal electrocardiogram and magnetocardiogram

The aim of this study was to demonstrate the acquisition of both the fetal magnetocardiogram (FMCG) and fetal electrocardiogram (FECG) from the abdomen of the same subject. This contrasts with previous reported studies which have generally recorded one or other of these signals but not the two together. Both signals were successfully recorded and averaged to produce a typical complex, thus allowing their direct comparison.

Magnetic susceptibility mapping of the human thorax using a SQUID biomagnetometer

Biomagnetism is essentially the study of the weak magnetic fields generated by biological organisms, in particular the human body. The human thorax is composed of a variety of tissues and organs of slightly different magnetic susceptibility. In an applied magnetic field (of the order of milliTeslas) these small differences in susceptibility lead to measurable field variations (of the order of nanoTeslas) at the body surface which may be of diagnostic value. Physiological processes such as cardiac activity, cardiac output, blood flow and respiratory related lung volume changes also contribute to the observed signal. In this study susceptibility 'maps' were obtained by measuring the magnetic field at several hundred points over the thorax. Results indicate that magnetic susceptibility mapping produces low-resolution images of internal body structures from which is should be possible to detect pathologies that cause alterations in tissue susceptibility.