SUMMARY In order to investigate the possibility of regional variation of ventricular structure, 25 normal postmortem human hearts were studied by inspection of cavity shape and subepicardial fibre orientation, by dissection, and by the histology of sections in two orthogonal planes. Ventricular architecture was complex. Inlet and outlet long axes were separated by 30 degrees in the left ventricle. In the right the corresponding figure was 90 degrees. The thickest part of the left ventricular wall was at the base. At the apex there was potential endo-and epicardial continuity. Left ventricular cavity shape departed significantly from any simple geometric figure, there being, consistently, regions of both positive and negativecurvature on the diaphragmatic aspect. The presence of trabeculae caused considerable variation in wall thickness. Striking variation was found in the arrangement of subepicardial muscle fibres. Most pronounced was the contrast between the longitudinal arrangement of fibres observed on the oblique margin and the circumferential arrangement of those on the acute. On the diaphragmatic surface of the left ventricle, fibres near the crux and apex ran circumferentially while those between ran obliquely; those on the diaphragmatic surface of the right ventricle also ran circumferentially. Deeper in the myocardium the arrangement was simpler. In the mid-wall of the left ventricle fibres were circumferential, best developed towards the base and in the upper part of the septum. Near the apex of the left ventricle and in the mid-wall of the right ventricle such fibres were sparse. The subendocardial region consisted of longitudinally directed fibres forming the trabeculae and papillary muscles, while fibres deep to and between the trabculae coursed more obliquely. These findings were confirmed by histology. Models based on uniform myocardial fibre structure cannot explain wall movement in normal subjects, and are likely to have significant limitations if used to investigate left ventricular function in disease.Although there is a wealth of anatomical work devoted to the study of fibre orientation in the human heart, this topic has received little detailed attention over the past 20 years, apart from the studies of Streeter' and Torrent-Guasp.2 This is surprising since much attention has been focused on measurements of cavity volume, pressure, and their derivatives with time, with calculation of systolic and diastolic wall stress. Though such studies imply anatomical knowledge, little recourse has been made to actual morphology, but instead geometrical assumptions have been made concerning left ventricular architecture. With the development of improved methods of investigating the left ventricle, the need for precise anatomical information has become pressing. In this paper therefore a de-

The heart in higher vertebrates develops from a simple tube into a complex organ with four chambers specialized for efficient pumping at pressure. During this period, there is a concomitant change in the level of myocardial organization. One important event is the emergence of trabeculations in the luminal layers of the ventricles, a feature which enables the myocardium to increase its mass in the absence of any discrete coronary circulation. In subsequent development, this trabecular layer becomes solidified in its deeper part, thus increasing the compact component of the ventricular myocardium. The remaining layer adjacent to the ventricular lumen retains its trabeculations, with patterns which are both ventricle- and species-specific. During ontogenesis, the compact layer is initially only a few cells thick, but gradually develops a multilayered spiral architecture. A similar process can be charted in the atrial myocardium, where the luminal trabeculations become the pectinate muscles. Their extent then provides the best guide for distinguishing intrinsically the morphologically right from the left atrium. We review the variations of these processes during the development of the human heart and hearts from commonly used laboratory species (chick, mouse, and rat). Comparison with hearts from lower vertebrates is also provided. Despite some variations, such as the final pattern of papillary or pectinate muscles, the hearts observe the same biomechanical rules, and thus share many common points. The functional importance of myocardial organization is demonstrated by lethality of mouse mutants with perturbed myocardial architecture. We conclude that experimental studies uncovering the rules of myocardial assembly are relevant for the full understanding of development of the human heart.

Background-Radiofrequency ablation of tissues in pulmonary veins can eliminate paroxysmal atrial fibrillation. Objective-To explore the characteristics of normal pulmonary veins so as to provide more information relevant to radiofrequency ablation. Methods-20 structurally normal heart specimens were examined grossly. Histological sections were made from 65 pulmonary veins. Results-The longest myocardial sleeves were found in the superior veins. The sleeves were thickest at the venoatrial junction in the left superior pulmonary veins. For the superior veins, the sleeves were thickest along the inferior walls and thinnest superiorly. The sleeves were composed mainly of circularly or spirally oriented bundles of myocytes with additional bundles that were longitudinally or obliquely oriented, sometimes forming mesh-like arrangements. Fibrotic changes estimated at between 5% and 70% across three transverse sections were seen in 17 veins that were from individuals aged 30 to 72 years. Conclusions-The myocardial architecture in normal pulmonary veins is highly variable. The complex arrangement, stretch, and increase in fibrosis may produce greater non-uniform anisotropic properties. (Heart 2001;86:265-270) Keywords: arrhythmias; catheter ablation; fibrillation; cardiac veins Studies from various groups of investigators have suggested that certain forms of atrial fibrillation are related to the existence of an ectopic discharging focus which is frequently located within the pulmonary veins.1-4 Radiofrequency catheter ablation carried out in the pulmonary veins can eliminate paroxysmal atrial fibrillation in many cases. Stenosis of the vein is a recognised complication following catheter ablation.5 Recurrence of the arrhythmia is also a common problem.4 Both drawbacks of current techniques of catheter ablation in these patients may be avoidable if there is better understanding of the architecture of the pulmonary veins in the human heart.In this study, we explored the walls of the pulmonary veins from the venoatrial junction to the hilum in normal specimens. We then reconstructed our findings so as to provide a three dimensional impression of the architecture of the cardiac muscle, which reinforces to a varying extent the outer layer of the pulmonary veins at their junction with the left atrium. To standardise the orientation of the left and right pulmonary veins, and to emphasise the potential significance of the diVerences in the anatomical arrangements, we viewed the orifices of the veins as they would be seen in a simulated left anterior oblique projection, and used the clock face to describe the sectors of the walls. MethodsWe harvested 65 veins from 20 structurally normal heart specimens that were collected in

A large experience with fetal congenital heart disease allows the spectrum of disease to be described with accuracy and compared with that in infancy. Knowledge of the natural history of heart malformations when they present in the fetus allows accurate counseling to be offered to the parents. If the trend in parental decisions found in this series continues, a smaller number of infants and children with complex cardiac lesions will present in postnatal life.