2 as compared to children in industrial countries. Approxi- mately 10% of these episodes will lead to hospitalization, and it is estimated that 66,000–199,000 children will die from RSV-associated LRTI; 99% of whom will be from developing countries [12]. 2. Respiratory Syncytial Virus RSV is an enveloped RNA virus whose genome encodes several structural proteins including the (G) protein, which promotes viral attachment, and the fusion (F) protein, which promotes penetration. There are two groups, A and B, of RSV strains that circulate together during yearly outbreaks [8]. 2.1. Clinical Symptoms and Diagnosis. Typically patients have rhinorrhea and a low-grade fever that can progress to cough and respiratory distress over the next few days and can impair feeding and sleeping. Infants <3 months of age can also present with apnea [13]. Patients typically will have crackles and/or wheezes on lung exam and depending on the severity of infection, tachypnea, chest wall retractions, and hypoxia. The diagnosis is typically made clinically based on the above ﬁndings, although viral culture, direct ﬂuorescent assays, and rapid antigen detection can be used. Also, the development of real-time polymerase chain reaction (RT-PCR) testing has allowed more rapid detection of respiratory pathogens in the setting of pediatric respiratory infections. 2.2. The Impact of Viral Respiratory Infections on CHD. Cabalka [14] highlighted the physiologic risk factors in children with CHD that can contribute to worse clinical out- comes from viral respiratory infections. Children who have left-to-right shunt lesions may have increased pulmonary blood ﬂow that leads to pulmonary edema and decreased functional residual capacity. These alterations in pulmonary mechanics further lead to areas of atelectasis and ventilation- perfusion mismatch, ultimately leading to hypoxia. Patients with cyanotic congenital heart disease may experience inten- siﬁcation in their baseline level of cyanosis due to bron- chiolitis-related reductions in lung volume and airway diameter superimposed on their already reduced pulmonary blood ﬂow and obligatory right-to-left shunt. Children with CHD may also have impaired ventricular function leading to higher pulmonary venous pressures, capillary leak, and pulmonary edema which further contribute to increased ventilation-perfusion mismatch. Although earlier reports indicated mortality rates of 37% for RSV-infected CHD patients and 73% in patients with pulmonary hypertension [15], more recent studies suggest that mortality rates have decreased to <2%, even in children with pulmonary hypertension [16]. Improvements in critical care medicine have led to the decrease in mortality rates but screening for RSV prior to surgical intervention also has contributed to a reduction in RSV-related pulmonary hyper- tension and therefore a reduction in postoperative morbidity [17, 18]. Risk factors for RSV hospitalization in children with heart disease include trisomy-21, cardiomyopathy, and hemodynamically signiﬁcant heart disease. Younger patients, International Journal of Pediatrics Table 1: Cardiovascular complications associated with RSV infec- tion. Sinoatrial block Tachyarrhythmias Atrioventricular block Pericarditis Myocarditis Complete heart block Reduced right ventricular function particularly those with heart failure, can be expected to have a more severe clinical course [19]. Other viruses other than RSV are responsible for bron- chiolitis. The development of molecular testing for viruses obtained from nasopharyngeal swabs or aspirates via RT- PCR has highlighted the role of dual infections in patients with viral respiratory tract infections. However, the role of dual infection correlating to increased clinical severity is unclear. Although some investigators have reported no clinical diﬀerences in patients with dual infections [20–22], others have reported more severe bronchiolitis in patients who have had multiple viruses detected [23–26]. In the stud- ies that demonstrated a more severe clinical infection with dual respiratory pathogens, two studies speciﬁcally excluded children with CHD [23, 25] while the other two studies enrolled children with CHD but did not separately report clinical outcomes in this group [24, 26]. Richard et al. [26], however, did include children with CHD as part of a cohort of children with underlying chronic illness and reported that dual infection with RSV and rhinovirus was signiﬁcantly associated with admission to the Pediatric Intensive Care Unit. There is also no evidence that the availability to di- agnose dual pathogens via RT-PCR testing leads to shorter hospital stays or reduces antibiotic use in children with acute respiratory infection [27]. There are diﬀerent risk factors between RSV and non- RSV bronchiolitis. In one study, children hospitalized with RSV bronchiolitis were younger and had less previously known risk factors for RSV such as prematurity, chronic lung disease, and congenital heart disease; this ﬁnding may reﬂect the beneﬁt of immunoprophylaxis against RSV. In fact, the adoption of anti-RSV prophylaxis may explain why non-RSV bronchiolitis may now be more prevalent in hospitalized children with CHD and other high-risk conditions than RSV bronchiolitis [17, 28]. Disease severity was greater in children with RSV bronchiolitis versus non- RSV bronchiolitis as measured by signiﬁcant diﬀerences in length of hospitalization stay, length of stay in the PICU, intubation duration, and percentage of children requiring supplemental oxygen [7]. 2.3. Cardiac Complications of RSV. Although perhaps not as well appreciated as the pulmonary manifestations of RSV infection, the virus can lead to cardiac complications in hospitalized children [29]. Table 1 lists the cardiovascular complications that have been associated with RSV infection.