Enhanced wave overtopping simulation at vertical breakwaters using machine learning algorithms.

Accurate prediction of wave overtopping at sea defences remains central to the protection of lives, livelihoods, and infrastructural assets in coastal zones. In addressing the increased risks of rising sea levels and more frequent storm surges, robust assessment and prediction methods for overtopping prediction are increasingly important. Methods for predicting overtopping have typically relied on empirical relations based on physical modelling and numerical simulation data. In recent years, with advances in computational efficiency, data-driven techniques including advanced Machine Learning (ML) methods have become more readily applicable. However, the methodological appropriateness and performance evaluation of ML techniques for predicting wave overtopping at vertical seawalls has not been extensively studied. This study examines the predictive performance of four ML techniques, namely Random Forest (RF), Gradient Boosted Decision Trees (GBDT), Support Vector Machines-Regression (SVR), and Artificial Neural Network (ANN) for overtopping discharge at vertical seawalls. The ML models are developed using data from the EurOtop (2018) database. Hyperparameter tuning is performed to curtail algorithms to the intrinsic features of the dataset. Feature Transformation and advanced Feature Selection methods are adopted to reduce data redundancy and overfitting. Comprehensive statistical analysis shows superior performance of the RF method, followed in turn by the GBDT, SVR, and ANN models, respectively. In addition to this, Decision Tree (DT) based methods such as GBDT and RF are shown to be more computationally efficient than SVR and ANN, with GBDT performing simulations more rapidly that other methods. This study shows that ML approaches can be adopted as a reliable and computationally effective method for evaluating wave overtopping at vertical seawalls across a wide range of hydrodynamic and structural conditions.

New insights in the probability distributions of wave-by-wave overtopping volumes at vertical breakwaters.

Advances in the development of prediction tools for wave overtopping allow now for overtopping volumes to be estimated with good accuracy, with the combined use of mean overtopping rates and maximum wave by wave overtopping volumes in a sequence of wave overtopping events. While previous literature has tended to focus on mean overtopping rates at coastal structures, limited studies have investigated the wave by wave overtopping volumes at coastal sea defences; in particular, a paucity of studies have focussed on the prediction of the shape parameter in the Weibull distribution (i.e., Weibull b) of overtopping volumes. This study provides new insights on the probability distribution of individual wave overtopping volumes at plain vertical seawalls by analysing the measured Weibull b values derived from a series of laboratory experiments on seawalls performed on a wide range of wave conditions and crest freeboards. The influence of wave conditions (wave steepness, significant wave height), structural parameters (crest freeboard, toe water depth), impulsiveness, probability of overtopping waves, and overtopping discharge on Weibull b parameter were examined, and then compared with the well-established empirical formulae. For the conditions covered within this study, it was found that the probability distribution of wave-by-wave overtopping volumes follow a 2-parameter Weibull distribution. No apparent differences in Weibull b values were reported with the variation of incident wave steepness and impulsiveness parameter. Results of this study revealed that Weibull b values at vertical walls, subjected to non-impulsive wave conditions, can be predicted reasonably well using relative freeboard and relative overtopping rates. A new unified formula is proposed for the estimation of Weibull b values at vertical walls under impulsive and non-impulsive wave attack.

Modelling spatial and temporal variations of annual suspended sediment yields from small agricultural catchments.

Estimates of sediment yield are important for ecological and geomorphological assessment of fluvial systems and for assessment of soil erosion within a catchment. Many regulatory frameworks, such as the Convention for the Protection of the Marine Environment of the North-East Atlantic, derived from the Oslo and Paris Commissions (OSPAR) require reporting of annual sediment fluxes. While they may be measured in large rivers, sediment flux is rarely measured in smaller rivers. Measurements of sediment transport at a national scale can be also challenging and therefore, sediment yield models are often utilised by water resource managers for the predictions of sediment yields in the ungauged catchments. Regression based models, calibrated to field measurements, can offer an advantage over complex and computational models due to their simplicity, easy access to input data and due to the additional insights into factors controlling sediment export in the study sites. While traditionally calibrated to long-term average values of sediment yields such predictions cannot represent temporal variations. This study addresses this issue in a novel way by taking account of the variation from year to year in hydrological variables in the developed models (using annual mean runoff, annual mean flow, flows exceeded in five percentage of the time (Q5) and seasonal rainfall estimated separately for each year of observations). Other parameters included in the models represent spatial differences influenced by factors such as soil properties (% poorly drained soils and % peaty soils), land-use (% pasture or % arable lands), channel slope (S1085) and drainage network properties (drainage density). Catchment descriptors together with year-specific hydrological variables can explain both spatial differences and inter-annual variability of suspended sediment yields. The methodology is demonstrated by deriving equations from Irish data-sets (compiled in this study) with the best model efficiency of 0.84 and best model fit of adjusted R2 of 0.82. Presented approach shows the potential for regression based models to model contemporary suspended sediment yields in small river systems.

Measurement differences between turbidity instruments, and their implications for suspended sediment concentration and load calculations: A sensor inter-comparison study.

The use of turbidity for indicating environmentally detrimental levels of suspended and colloidal matter in freshwater systems, and for defining acceptable water quality standards in national and European drinking water regulations, is well established. Turbidity is therefore frequently adopted as a surrogate for suspended sediment concentrations (SSC), or as a relative and objective measure of water clarity in monitoring programmes. Through systematic, controlled experimentation, we tested the response of 12 commercially available turbidity sensors, of various designs, to gauge their measurement consistency when benchmarked against pre-prepared sediment suspensions of known SSC. Results showed that despite calibration to a Formazin standard, sensor responses to identical SSC solutions (in the range of 20-1000 mg L-1) varied considerably. For a given SSC, up to five-fold differences in recorded turbidity were recorded across the tested instruments. Furthermore, inconsistent measurements were identified across instruments, regardless of whether they operated using backscatter or side-scatter optical principles. While the findings may have implications for compliance with turbidity-based water quality standards, they are less likely to be an issue when turbidity is being used as a surrogate for SSC, provided that instrument use remains constant and that instrument drift is not an issue. In this study, a field comparison of a subset of four study sensors showed that despite very different absolute turbidity readings for a given SSC, well correlated and reliable turbidity - SSC ratings were established (as evidenced by r2 coefficients from 0.92 to 0.98). This led to reasonably consistent suspended sediment load estimates of between 64.7 and 70.8 tonnes for a rainfall event analysed. This study highlights the potential for issues to arise when interpreting water turbidity datasets that are often assumed to be comparable, in that measurement inconsistency of the type reported here may remain unknown to water resource decision-makers and practitioners.

Evaluating the relationship between biotic and sediment metrics using mesocosms and field studies.

An ongoing research challenge is the detection of biological responses to elevated sediment and the identification of sediment-specific bioassessment metrics to evaluate these biological responses. Laboratory mesocosms and field observations in rivers in Ireland were used to evaluate the relationship between a range of biological and sediment metrics and to assess which biological metrics were best at discerning the effects of excess sediment on macroinvertebrates. Results from the mesocosm study indicated a marked decrease in the abundance of sensitive taxa with increasing sediment surface cover. % EPT (Ephemeroptera, Plecoptera, Trichoptera) and % E abundances exhibited the strongest negative correlation with sediment surface cover in the mesocosm study. The field study revealed that % EPT abundance was most closely correlated with % sediment surface cover, explaining 13% of the variance in the biological metric. Both studies revealed weaker relationships with a number of other taxonomy-based metrics including total taxon abundance, total taxon richness and moderate relationships with the Proportion of Sediment-sensitive Invertebrates metric (PSI). All trait-based metrics were poorly correlated with sediment surface cover in the field study. In terms of sediment metrics, % surface cover was more closely related to biological metrics than either re-suspendable sediment or turbidity. These results indicate that % sediment surface cover and % EPT abundance may be useful metrics for assessing the effect of excessive sediment on macroinvertebrates. However, EPT metrics may not be specific to sediment impact and therefore when applied to rivers with multiple pressures should be combined with observations on sediment cover.

The impact of cattle access on ecological water quality in streams: Examples from agricultural catchments within Ireland.

Unrestricted cattle access to rivers and streams represent a potentially significant localised pressure on freshwater systems. However there is no consensus in the literature on the occurrence and extent of impact and limited research has examined the effects on aquatic biota in the humid temperate environment examined in the present study. Furthermore, this is one of the first times that research consider the potential for cattle access impacts in streams of varying water quality in Northern Europe. We investigated the effects of cattle access on macroinvertebrate communities and deposited fine sediment levels, in four rivers of high/good and four rivers of moderate water quality status which drain, low gradient, calcareous grassland catchments in Ireland. We assessed the temporal variability in macroinvertebrates communities across two seasons, spring and autumn. Site specific impacts were evident which appeared to be influenced by water quality status and season. All four high/good water status rivers revealed significant downstream changes in community structure and at least two univariate metrics (total richness and EPT richness together with taxon, E and EPT abundance). Two of the four moderate water status rivers showed significant changes in community structure, abundance and richness metrics and functional feeding groups driven in the main by downstream increases in collectors/gatherers, shredders and burrowing taxa. These two moderate water status rivers had high or prolonged livestock activity. In view of these findings, the potential for some of these sites to achieve at least high/good water quality status, as set out in the EU Water Framework Directive, may be compromised. The results presented highlight the need for additional research to further define the site specific factors and livestock management practices, under different discharge conditions, that increase the risk of impact on aquatic ecology due to these cattle-river interactions.

The relationship between gestational age, systolic blood pressure and pulse pressure in children.

This cross-sectional study investigates the relationship between gestational age and systolic blood pressure and pulse pressure in childhood. Blood pressure was measured in 483 schoolchildren, free from cardiovascular disease, aged between 6 and 16 years. Pulse pressure was estimated as the difference between the 24-h mean systolic and diastolic blood pressure values. Linear regression showed an inverse relationship between gestational age and mean 24-h systolic blood pressure (adjusted regression coefficient mm Hg per week gestation -0.631, 95% confidence interval (CI) -1.21 to -0.04, P=0.036). Further, linear regression showed a significant negative association between gestational age and log-transformed pulse pressure (adjusted antilog regression coefficient mm Hg per week of gestation -1.39, 95% CI -2.96 to -0.3, P=0.013), which after gender-specific analyses was found to be restricted to the girls in the study. The results of the present study suggest that low gestational age is associated with elevated systolic blood pressure and pulse pressure in childhood, the latter particularly in girls. This observation provides some support for the developmental origins of adult disease hypothesis-that adverse events in early life may have long-term consequences for cardiovascular health. However, as gestational age itself is unlikely to be the causal event in determining blood pressure control, further investigation is required, particularly with regard to the nutritional, physiological and molecular mechanisms that explain such epidemiological observations.

The relationship between birth weight and pulse pressure in children: cross-sectional study.

This cross-sectional study investigates the relationship between birth weight and pulse pressure in childhood, after adjusting for mean blood pressure values and for potential confounding factors. Blood pressure was measured in 937 schoolchildren, free from cardiovascular disease, aged between 6 and 16 years. Pulse pressure was estimated as the difference between the 24 h mean systolic and diastolic blood pressure values. Linear regression showed a significant negative association between birth weight and log-transformed pulse pressure, which after gender-specific analyses was found to be restricted to the girls in the study (adjusted regression coefficient log mmHg per kg -0.06, 95% CI -0.09 to -0.03). A previous investigation of this cohort reported a significant negative association between birth weight and both systolic and diastolic blood pressure, again restricted to the girls in the cohort. The results of the present study provide limited support for the hypothesis that pulse pressure in childhood is determined in utero, particularly for female subjects. However, as little research has been published in this area, further investigation is required and in particular it would be important to assess whether such gender differences are apparent in other cohorts.

Patient attitudes to sternotomy and thoracotomy scars.

BACKGROUND: There are no data comparing patient attitudes to sternotomy and thoracotomy scars following surgery for congenital heart disease (CHD). METHODS: Two hundred and one patients with a scar from CHD surgery (105 sternotomy, 36 thoracotomy, and 60 both scars) had a structured interview to explore attitudes to their scar. RESULTS: Comparable proportions of each group reported that they did not like or hated their scar (23/105 [22 %] sternotomy, 9/36 [25 %] thoracotomy, 17/60 [28 %] both scars). Significantly more patients stated that they where embarrassed by and/or their choice of clothing was affected by a thoracotomy scar (20/36, 56 %) than those with a sternotomy scar (36/105, 34 %), p = 0.04. This was also seen when comparing sternotomy alone with both scars (36/105 [34 %] vs. 34/60 [57 %], p = 0.008). CONCLUSIONS: Adults who have undergone surgery for CHD are more likely to have a negative attitude to a thoracotomy than a sternotomy scar. Before a change in surgical approach is considered based on patient preferences, the acceptability and psychological impact of the different scars following surgery needs formal study.

Life insurance and mortgage application in adults with congenital heart disease.

OBJECTIVE: To compare the outcome of life insurance and mortgage applications of adults with congenital heart disease (CHD) with controls and at different severities of CHD. METHODS: Two hundred and ninety-nine adult CHD patients underwent a questionnaire-based interview by a trained nurse. They were asked to give an identical questionnaire to a friend to act as a control. One hundred and seventy-seven controls replied. CHD patients were classified into three categories based on severity. Comparisons were made between matched controls and between different severities of CHD. RESULTS: Similar proportions of the CHD group (59%) had applied for life insurance as matched controls (56%). Compared to controls, significantly more of the adults with CHD who had applied for life insurance have been refused (34 vs 4%, P < 0.0001) or asked to pay extra (37 vs 6%, P = 0.0002). Mortgage application rate was also similar in both groups with more of the CHD patients refused than matched controls (20 vs 3%, P = 0.0004). These differences in both life insurance and mortgage remain significant when the cases and controls are matched by employment status and NYHA functional class. There was no significant difference in life insurance and mortgage application outcome between the groups of mild, significant and complex CHD. CONCLUSIONS: Adults with CHD are significantly more likely to have difficulty obtaining life insurance or a mortgage than controls. Refusal rates appear to be independent of the severity of CHD. This suggests that the label of CHD may have a negative impact despite the lesion being minor and that the outcome of an individual application is difficult to predict based on the severity of the CHD. The increasing numbers of adults with CHD suggest that this problem is likely to increase and needs to be addressed as it can have a major impact on the patient's quality of life.

Relationship between birth weight and blood pressure variability in children.

This cross-sectional study investigates the relationship between birth weight, 24-h blood pressure and blood pressure variability in childhood. Blood pressure was measured in 976 schoolchildren, free from cardiovascular disease, aged between 6 and 16 years. Blood pressure variability was estimated as the standard deviation of the 24-h mean (systolic and diastolic) blood pressure values. Linear regression showed that variation in systolic or diastolic blood pressure was not significantly associated with birth weight. Similarly, no association was found between blood pressure variability and birth weight when using the birth weight groups used by a previous study. Adjusting for other covariates, including mean 24-h blood pressure, made little difference to the observed results. No interactions were observed between birth weight and either gender or age on blood pressure variability. The results of this study do not support the suggestion of a significant association between birth weight and blood pressure variation in childhood. This might suggest that blood pressure variability is influenced mainly by environmental or lifestyle factors, but as little research has been published in this area, further investigation is required and in particular it would be important to assess the use of other measures of blood pressure variation.

Prevalence of hypertension in children after early repair of coarctation of the aorta: a cohort study using casual and 24 hour blood pressure measurement.

OBJECTIVE: To study the prevalence of hypertension in a cohort of patients using the current strategy of repair in early childhood. PATIENTS: The cohort of patients with coarctation of the aorta born between 1983 and 1992. INTERVENTION: Casual (mean of three resting readings) and 24 hour blood pressure were measured in 119 children and compared with data from 1034 normal controls. The arch repair and left ventricular parameters were assessed using Doppler echocardiography. RESULTS: Median ages at first intervention and at blood pressure measurement were 0.2 years (interquartile range 0.04-2.0) and 12.0 years (9.0-14.5), respectively. Doppler velocity in the descending aorta was significantly associated with blood pressure (r = 0.28, p = 0.002 for casual systolic blood pressure (SBP); r = 0.26, p = 0.005 for mean 24 hour SBP). Patients were classified as having "no" (n = 70) or "mild" (n = 49) arch obstruction. Casual SBP was > 95th centile in 28% (34 of 119) overall and in 21% (15 of 70) of the no arch obstruction group. Mean 24 hour SBP was > 95th centile in 30% (36 of 119) overall and in 19% (13 of 70) of the no obstruction group. The sensitivity and specificity of casual SBP in detecting increased 24 hour SBP were 66% and 88%, respectively. CONCLUSIONS: This unique study of a large cohort of patients treated for coarctation in early childhood showed that a disappointingly high prevalence of hypertension is already apparent in children aged 7-16 years in the absence of significant arch obstruction, whether assessed by 24 hour or by casual blood pressure measurement.

Antibody deficiency and autoimmunity in 22q11.2 deletion syndrome.

BACKGROUND: Although severe T cell immunodeficiency in DiGeorge anomaly is rare, previous studies of humoral function in these patients have found no antibody abnormalities but have not examined the response to polysaccharide antigens. Isolated cases of autoimmunity have been reported. Several patients with 22q11.2 deletion attending our immunology clinic suffered recurrent sinopulmonary infection or autoimmune phenomena. AIMS: To investigate humoral immunodeficiency, particularly pneumococcal polysaccharide antibody deficiency, and autoimmune phenomena in a cohort of patients with 22q11.2 deletion. METHODS: A history of severe or recurrent infection and autoimmune symptoms were noted. Lymphocyte subsets, immunoglobulins, IgG subclasses, specific vaccine antibodies, and autoantibodies were measured. Subjects were vaccinated with appropriate antigens as indicated. RESULTS: Of 32 patients identified, 26 (81%) had severe or recurrent infection, of which 13 (50%) had abnormal serum immunoglobulin measurements and 11/20 >/=4 years old (55%) had an abnormal response to pneumococcal polysaccharide. Ten of 30 patients (33%) had autoimmune phenomena; six (20%) were symptomatic. CONCLUSIONS: Humoral immunodeficiency is more common than previously recognised in patients with 22q11.2 deletion. Normal T cell function and immunoglobulin levels do not exclude poor specific antibody responses. Patients should be referred for formal immunological assessment of cellular and humoral immune function.

Palliation of systemic sclerosis-associated pulmonary hypertension by atrial septostomy.

The onset of pulmonary hypertension in patients with systemic sclerosis carries a poor prognosis. Atrial septostomy has been used successfully to palliate endstage primary pulmonary hypertension but has not been attempted in other forms of pulmonary vascular disease. We report substantial clinical improvement following atrial septostomy in a patient with systemic sclerosis complicated by severe, isolated pulmonary hypertension. After the procedure, exercise capacity was improved and exertional syncope abolished. We suggest that this procedure should be considered for other patients with this diagnosis.

Survival with congenital heart disease and need for follow up in adult life.

OBJECTIVE: To predict the growth in demand for long term follow up of adults with congenital heart disease. DESIGN: Observed diagnoses of congenital heart disease in infancy and childhood were adjusted for observed infant survival, predicted further survival to age 16 years, underascertainment in older childhood, and predicted need for long term follow up. SETTING: The resident population of one health region in the UK. PATIENTS: All confirmed cardiovascular malformations diagnosed in 1985 to 1999 in children born in 1985 to 1994. RESULTS: 1942 cases of congenital heart disease were diagnosed in infancy in a population of 377 310 live births (5.2/1000). 1588 (82%) survived to 1 year and 1514 were predicted to survive to age 16. 605 further diagnoses were made in childhood-678 when adjusted for underascertainment. Thus, 2192 children were predicted to reach age 16, of whom 784 would require long term follow up in adult life. The adult population would comprise 28% complex, 54% significant, and 18% minor congenital heart disease. These figures predict the need for adult follow up of congenital heart disease of over 200 extra cases per 100 000 live births each year or over 1600 extra cases a year every year in the UK. CONCLUSIONS: The need for follow up of congenital heart disease in adult life is likely to grow linearly, with increasing complexity and increasing need for reinvestigation and reintervention with time. Appropriate provision should be made for adequate manpower, resources, and facilities for care of these patients.

Should we standardise the pre-operative management of babies with complete transposition?

BACKGROUND: Complete transposition is the most common form of neonatal cyanotic heart disease. The management of this condition has changed markedly in the last decade and there appears to be a significant variation between centres in terms of pre-operative management. OBJECTIVES/METHODS: We surveyed all paediatric cardiac surgical centres in the United Kingdom regarding pre-operative management, particularly performance and timing of balloon atrial septostomy and aortogram, imaging techniques used and discharge prior to surgery. RESULTS: There is significant variation in pre-operative management: 10 centres now perform septostomy outside the catheter lab and 11 without general anaesthesia. Eight centres use echo control only and only 3 perform routine aortograms. Three centres do not perform routine septostomy. The most common age for arterial switch was at 1-2 weeks, but some routinely performed this procedure up to 1 month of age and others aim for arterial switch before one week of age. Only 3 centres routinely discharge patients between septostomy and switch. CONCLUSIONS: Despite a trend towards echo guided septostomy and earlier arterial switch there is still considerable variation in early management of patients with transposition of the great arteries. Debate within the profession leading to a more standardised pre-operative management strategy would protect both the patient and the doctor involved in the care of children with complete transposition.

Tracking of 24-hour and casual blood pressure: a 1-year follow-up study in adolescents.

OBJECTIVE: Tracking of blood pressure begins in childhood but the relationship between casual blood pressure in childhood and adult levels is not strong enough to predict adult hypertension. The variability of blood pressure in children might suggest that 24 recordings would have less consistency than casual readings when repeated even a relatively short time later. This study compares the short-term tracking ability of casual versus 24-h blood pressure. DESIGN: An ambulatory blood pressure device was placed on 50 teenagers. Readings were taken at rest and the device was then worn for approximately 24 h, which included the schoolday. The protocol was repeated 1 year later. RESULTS: The correlation coefficient for systolic readings taken 1 year later were: 0.4 for casual, 0.6 for school, 0.6 for home, 0.5 for night-time and 0.8 for 24-h mean systolic blood pressures. When divided into upper and lower tertiles of systolic blood pressure the relationship between tertile ranking 1 year later was stronger for 24-h blood pressure than the casual readings. Casual diastolic pressure was more consistent than the 24-h mean diastolic measurement. CONCLUSIONS: In adolescents, in whom tracking of casual blood pressure has been shown to be poor, 24-h mean systolic blood pressure tracks better than any other time period and significantly better than the casual systolic readings. This study needs to be extended and the ability of 24-h blood pressure to track from childhood to adult life investigated.

Long-term follow-up of pediatric cardiac transplant recipients on a steroid-free regime: the role of endomyocardial biopsy.

BACKGROUND: Endomyocardial biopsy is used in long-term follow-up of cardiac transplant recipients to detect sub-clinical rejection. The value of this for adults and children on triple-drug immunosuppression has been disputed. We investigated its value in children maintained on a steroid-free regime. METHODS: We used a retrospective review of annual surveillance biopsy results from children younger than 13 years at the time of cardiac transplant. RESULTS: In a series of 40 children older than 10 years, we found no evidence of rejection in 88/130 (67.7%) biopsies; 41/130 (31.5%) showed grade 1A rejection, and 1/130 (0.8%) showed grade 1B rejection. No grade 2, 3, or 4 biopsies were found. Nine patients with 1A rejection had subsequent grade 0 biopsies, without any adjustment in treatment. Seven children had treatment changes and repeat biopsies because of grade 1A biopsies. CONCLUSION: Significant late rejection is rare even in children on steroid-free maintenance. It is unlikely to be detected unexpectedly, and the practice of indefinite routine biopsy in children who are well is not justified. Future use should focus on individuals at higher risk of rejection.

Sudden death in children and adolescents.

OBJECTIVE: To identify the incidence, causes, and characteristics of sudden death at age 1-20 years. DESIGN: A review of all deaths at age 1-20 years. Death certificates were obtained from the Office for National Statistics, and further information, where appropriate, from coroners, paediatricians, physicians, and pathologists. SETTING: The resident population of one English health region in 1985-1994. RESULTS: In a population of 806 500 children and adolescents aged 1-20 years there were 2523 deaths in 10 years. Medical causes accounted for 1017 deaths (40%); 1236 (49%) were unnatural, and 270 (11%) were sudden. These sudden deaths comprised 142 with a previous diagnosis, the commonest being epilepsy 49 (34%), cardiovascular disease 33 (23%), and asthma 30 (21%); 87 attributed to a cause discovered at necropsy, which was respiratory infection in 32 (37%), other infections in 17 (20%), and unsuspected cardiovascular abnormalities in 26 (30%); 41 remained unexplained. CONCLUSIONS: Half of all sudden deaths in children or adolescents were attributed to an already diagnosed condition. Abnormalities identified at necropsy accounted for one third of sudden deaths. Undiagnosed hypertrophic cardiomyopathy caused less than one death per million person years in the population aged 1-20 years. Unexplained sudden death, which may be caused by primary cardiac arrhythmia, is probably about 10 times more common.