This study provides evidence for the existence of M cells in the human heart that contribute to heterogeneity of repolarization within the ventricular wall. Our findings provide strong support for the hypothesis that M cells contribute importantly to the manifestation of the U wave on the ECG.

These findings may explain, at least in part, the reduction of ventricular repolarization dispersion and the lower incidence of torsade de pointes observed with chronic amiodarone therapy as compared with other class III agents.

Aim: To study baroreflex maturation by measuring, longitudinally, baroreflex sensitivity in preterm (gestational age 24-37 weeks) and full term infants. Methods: Baroreflex sensitivity was quantified once a week, one to seven times, by a totally non-invasive method. Results: Baroreflex sensitivity at birth was lower in the preterm infant and increased with gestational age. It also increased with postnatal age, but the values for the preterm infants at term still tended to be lower than the values for full term babies. Conclusion: Baroreflex control of heart rate is present in the premature infant, but is underdeveloped and increases with postnatal age. Ex utero maturation seems to be delayed compared with in utero maturation assessed by full term values. These results may reflect sympathovagal imbalance in preterm infants and could identify a population more vulnerable to stress.T he short term regulation of arterial blood pressure by the autonomic nervous system involves the arterial baroreflex. This reflex minimises any variation in blood pressure by responses of both heart rate and arterial vascular tone. For example, an increase in blood pressure will be buffered by a decrease in both heart rate and peripheral vascular resistance. These responses of heart rate and vascular resistance, produced by the efferent limb of the baroreflex, are mediated by the parasympathetic and sympathetic system respectively. As both systems are involved, studies on the baroreflex give information on sympathovagal balance.Our knowledge about ontogeny of the baroreflex relies almost exclusively on animal studies, performed mainly in the lamb. Shinebourne et al 1 have shown the presence of functional arterial baroreflex responses in sheep very early during fetal and postnatal life. Furthermore, these authors and others 2 have shown that the arterial baroreflex is impaired in the fetus and newborn and increases later. Others have found, however, that the sensitivity of baroreflex control of the heart rate does not change throughout gestation 3 or is higher during fetal life than postnatally. 4 Few studies in humans have investigated the development of the baroreflex, possibly because the classical methods of investigation were not applicable to newborns for ethical reasons.The classical methods of investigating baroreflex sensitivity (BRS) are based on analysis of changes in blood pressure and heart rate experimentally induced in a controlled fashion by various manoeuvres. In these methods, the change in blood pressure is produced by either pharmacological or mechanical means. The pharmacological methods consist of giving vasoactive drugs, such as phenylephrine and nitroprusside, whereas the mechanical methods consist of carotid baroreceptor deactivation or stimulation by body tilting or use of a neck chamber device. However, these methods have major limitations. Firstly, they are not very physiological and thus cannot be easily repeated many times over a short period. As a consequence, they cannot be used to provide information on ...

Aims-To determine whether it is possible to assess baroreflex sensitivity in neonates by studying only spontaneous variations in systolic blood pressure and heart rate. Methods-ECG and non-invasive blood pressure signals were continuously studied in 14 preterm neonates (term 29-32 weeks) and five term neonates (term 40-41 weeks). Non-invasive blood pressure measures were obtained using a Finapres placed around the child's wrist. Both signals (ECG and blood pressure), sampled at 400 Hz, were digitised by an A/D converter and stored in a binary mode on magnetic disk. An inhouse software QRS detection algorithm was used to define R peaks of the QRS complexes with an accuracy greater than 2 ms. Four 4 minute periods were recorded in each infant. The slope of the linear regression of RR intervals versus systolic blood pressure was calculated in each period and the mean value of the four slopes was then considered as the index of baroreflex sensitivity (in ms/mm Hg) in each neonate. Results-Spontaneous baroreflex sensitivity was lower in preterm neonates than in term neonates (mean(SD): 4.07 (2.19) ms/mm Hg vs 10.23 (2.92) ms/mm Hg). Conclusion-Baroreflex sensitivity can be assessed in term and preterm neonates by studying spontaneous variations in systolic blood pressure alone. This method could be useful for studying the ontogeny of baroreflex sensitivity and might therefore provide information about the maturation of the autonomic nervous system. (Arch Dis Child 1997;76:F108-F112) Keywords: blood pressure; heart rate; baroreflex sensitivity; autonomic nervous system.Arterial baroreceptors have an important role in beat-to-beat modulation of eVerent cardiovascular autonomic activity, acting on the vasculature and the heart. The baroreflex acts as a negative feedback control loop of arterial blood pressure and exerts a buVering influence on its spontaneous fluctuations.Baroreflex sensitivity is defined as the slope of linear regression of RR intervals on the electrocardiogram vs systolic blood pressure. The steeper the slope, the higher the baroreflex sensitivity. A steep slope of regression line is interpreted as indicating a strong vagal reflex; a flat slope indicates a weak vagal reflex and high reflex sympathetic activity.