Confounding and reverse causality have prevented us from drawing meaningful clinical interpretation even in well-powered observational studies. Confounding may be attributed to our inability to randomize the exposure variable in observational studies. Mendelian randomization (MR) is one approach to overcome confounding. It utilizes one or more genetic polymorphisms as a proxy for the exposure variable of interest. Polymorphisms are randomly distributed in a population, they are static throughout an individual's lifetime, and may thus help in inferring directionality in exposure-outcome associations. Genome-wide association studies (GWAS) or meta-analyses of GWAS are characterized by large sample sizes and the availability of many single nucleotide polymorphisms (SNPs), making GWAS-based MR an attractive approach. GWAS-based MR comes with specific challenges, including multiple causality. Despite shortcomings, it still remains one of the most powerful techniques for inferring causality.With MR still an evolving concept with complex statistical challenges, the literature is relatively scarce in terms of providing working examples incorporating real datasets. In this chapter, we provide a step-by-step guide for causal inference based on the principles of MR with a real dataset using both individual and summary data from unrelated individuals. We suggest best possible practices and give recommendations based on the current literature.

Background-Since the early 1990s, the pulmonary autograft is predominantly implanted as a freestanding root for less aortic valve regurgitation is reported. However, there is a certain risk of dilatation of the root over time potentially impairing valve function. We favor since 8 years the original subcoronary or inclusion technique to preserve the root of the patient as a restrain to dilatation. Methods and Results-Between June 1994 and May 2002 the subcoronary (nϭ228) and inclusion technique (nϭ17) were performed in 245 patients (191 male, 54 female), mean age 45.7Ϯ13.4 (15-70) years. The underlying aortic valve disease was an aortic insufficiency in nϭ83, stenosis in nϭ48, a combined aortic valve disease in nϭ111 and an acute endocarditis in nϭ19 patients. Previous aortic valve surgery was performed in nϭ23. Last follow-up investigations (within last year) including echocardiography was performed at a mean follow-up of 29.4Ϯ24.7 months (553.7 patient years). Hospital mortality was nϭ2, late mortality nϭ4 (all noncardiac). Two patients were lost to follow-up (99% complete clinical follow-up). Reoperations were necessary in nϭ7 valves (autograft: endocarditis nϭ1, malpositioning nϭ1, leaflet prolapse nϭ1; homograft: stenosis nϭ2, insufficiency nϭ2). Autograft insufficiency (AI) was AI 0 in nϭ154, AI I nϭ66, AI II nϭ8. The maximum/mean pressure gradient across the autograft was 6.6Ϯ3.4 (2.1 to 25.9)/3.6Ϯ1.8 (1.2 to 13.2) mm Hg, respectively. Homograft insufficiency was 0 in nϭ167, I in nϭ54, II in nϭ9, and III in nϭ1. Maximum and mean transhomograft pressure gradients were 11.7Ϯ6.8 (2.2 to 42.6)/6.2Ϯ3.8 (1.2 to 24.5) mm Hg. Most patients were NYHA class I (nϭ214), class II (nϭ19), class III (nϭ2). Significant aortic root dilatation was not observed. Conclusions-Aortic

IntroductionTwo-dimensional echocardiography is a useful tool in diagnosing cardiac masses. However, the three-dimensional offline reconstruction technique of transesophageal echocardiography might be superior to two-dimensional transesophageal echocardiography in providing additional information of structural details.Case presentationWe report the case of a 76-year-old Caucasian man with a permanent dual-chamber pacemaker and a worm-like right-heart thrombus in transit. Two-dimensional transthoracic echocardiography and two-dimensional transesophageal echocardiography showed that it was debatable as to whether "the worm" was originating from the leads. Offline three-dimensional transesophageal echocardiography reconstruction technique proved superior in identifying the cardiac mass as a thrombus trapped between the leads of the pacemaker. The thrombus was successfully dissolved by systemic heparin therapy.ConclusionsThe three-dimensional transesophageal echocardiography is useful and effective in patients with implanted pacemakers or defibrillators when other closely competing imaging modalities are contraindicated, such as magnetic resonance imaging. In patients with pacemakers and trapped thrombus in transit for whom surgical therapy might be a high risk, medical therapy seems to offer a safer and convincing alternative. Whether the management of right-heart thrombi has to be modified due to the presence of pacemaker leads is controversial.

BackgroundIntestinal iron is a nutritional compound, which is essential for enteric microbiota. We evaluated the hypothesis that polymorphisms, which are known modifiers of intestinal iron uptake in adults, are associated with necrotizing enterocolitis (NEC) in preterm infants.MethodsPreterm infants (birth weight below 1,500 g) were studied. Single-nucleotide polymorphisms with known effects on serum iron levels (rs1800562, rs1799945, and rs855791) were determined using PCR. The effects of polymorphisms on NEC surgery were tested by Mendelian randomization. Outcome data were compared with χ-test, Fisher's exact test, t-test, and Cochran-Armitage test for trend and multiple logistic regression analysis.ResultsComplete genotyping data were available for 11,166 infants. High serum iron levels due to rs855791 genotype were associated with a significantly reduced risk of NEC surgery (odds ratio (OR) 0.265; 95% confidence interval (CI) 0.11-0.65; adjusted P=0.011). Carriers of the rs855791 A-allele not receiving prophylactic probiotics had a higher risk of NEC surgery (OR 1.12, 95% CI 1.08-1.70, nominal P=0.002). Prophylactic treatment with probiotics was associated with a reduced risk of NEC surgery in carriers of the rs855791 A-Allele. No differences were found with regard to other short- or long-term outcome data.ConclusionPolymorphisms inducing lower intestinal iron uptake like the rs855791 A-allele might be an underestimated risk factor for NEC.