Magnetic resonance imaging: Noninvasive assessment of cardiac structure and function

Joseph Cava, MD, PhD, pediatric cardiologist, Children's Hospital of Wisconsin; associate professor, Pediatrics (Cardiology), Medical College of Wisconsin.

Cardiac magnetic resonance imaging (MRI) has become mainstream in the evaluation of heart disease in the adult population. MRI offers a noninvasive way to obtain detailed views of cardiac structures, as well as measure myocardial and valvular function, map blood flow and determine myocardial perfusion and viability. The ability to assess several cardiac parameters with one test makes MRI ideal for use in patients with congenital heart disease. Several centers now use MRI, in addition to echocardiography, to routinely assess and care of pediatric heart patients.

The basic principle of MRI is to create energy using a magnetic field that can be translated into an image. Tissue absorbs energy when exposed to a magnetic field and a radiofrequency pulse is applied. Various types of tissue release this energy at different times, and the collection of this energy is used to create an image of the organs and tissues selected by the operator. Different types of imaging can be employed depending on the information that is desired. A technique to obtain detailed views of cardiac structure uses a scan sequence that cancels out flowing blood, giving blood a black appearance on the final image. This is referred to as dark blood imaging. The empty or black appearance provides excellent contrast to the surrounding cardiac and vascular tissue. When information about blood flow is desired, a scan sequence is used that causes blood to appear white. This technique, called bright blood imaging, shows the heart and blood vessels filled with blood. Using bright blood imaging, data can be collected from specific points throughout the cardiac cycle. These images are played in sequence to show the heart in motion as a cine.

Echocardiography has been and continues to be the main diagnostic modality used in pediatric cardiology. There are several advantages for its use. An echocardiogram is relatively easy to obtain and can be done in a short amount of time. In general, detailed information can be obtained about structure, blood flow, and if assumptions are made, cardiac function. Unfortunately in some patients, particularly older patients, poor imaging windows are present, and adequate images cannot be obtained. Imaging the heart with MRI is not dependent on imaging windows; therefore, those patients with poor echo imaging are ideal for MRI imaging. However, MRI imaging does depend on certain variables. Motion artifact is a problem when imaging the heart. This is circumvented by the use of cardiac gating where information is collected only at specific points during the cardiac cycle for a particular image. Gating will increase scan times, and generally the cardiac MRI will take longer than the typical echocardiogram. Respiratory motion also will cause image artifacts with MRI. A cooperative patient, able to hold his or her breath, is preferable. Spontaneously breathing patients can be done, but scan time increases significantly and image resolution is sacrificed. When necessary, young children or other patients who cannot cooperate can undergo general anesthesia if detailed information is needed.

An important assessment in many congenital heart lesions is right ventricular function. Because of the right ventricle's geometric shape, the assumptions made when determining left ventricular function with echocardiography cannot be translated to the right ventricle. Cardiac MRI has been determined to be an accurate modality to measure right ventricular function. Using cine MRI imaging of the ventricles, precise measurements of ventricular volumes are possible. Right ventricular ejection fractions can then be calculated. Phase contrast MRI is another useful technique allowing the quantification of blood flow in specifically defined blood vessels. This information allows the calculation of cardiac output and regurgitant fractions across cardiac valves. Many newer techniques have been developed. These include assessment of myocardial viability and perfusion, myocardial mechanics and specialized testing such as iron deposition in thalassemias.

While echocardiography continues to be invaluable in the evaluation and detection of congenital heart disease, MRI has become a useful adjunct to echocardiography and, in certain clinical situations, is superior. Detailed imaging in any plane and location with the ability to accurately assess cardiac and valvular function makes MRI a powerful tool in the evaluation of our cardiac patients.

Vapotherm

Khris E. O'Brien, RRT, clinical projects coordinator, Respiratory Care Services, Children's Hospital of Wisconsin.

Historically, oxygen delivery devices have been limited based on the oxygen requirements of the patient. If a patient had low oxygen requirements, they were placed on a nasal cannula. Flows with a simple nasal cannula are limited to 4 liters per minute (LPM) in pediatrics and 8 LPM in adults. Fractions of inspired oxygen (FiO2s) via nasal cannula range from 0.22 to 0.45. Traditional nasal cannula systems at high flows cannot be tolerated by the patient due to complications such as dry mucous membranes, nasal irritation or bleeding, thickened secretions and airway cooling. Therefore, if a patient required additional oxygen, he or she was switched to higher flow systems such as a simple oxygen mask or non-rebreather oxygen mask. The flow ranges for these systems are generally 5-15 LPM, while the FiO2s range from 0.35 to 1.0. The mask systems supply the additional oxygen requirement but at the cost of decreased communication and inability to eat or drink. Oxygen masks also are often difficult to keep securely on small children.

Children's Hospital of Wisconsin is pleased to introduce Vapotherm as a new oxygen delivery alternative that can deliver higher oxygen levels with increased comfort to the patient. Vapotherm, quite simply, is a heated and humidified nasal cannula. Since it provides near 100 percent relative humidity at body temperature, the patient can tolerate much higher flows than a traditional nasal cannula. For comparison, the unit can be used up to 40 LPM on an adult. With the infant cartridge, the unit can deliver flows up to 8 LPM. The cannula looks exactly like a regular nasal cannula, but just below the chin, it is connected to a heated water tubing that surrounds the heated and humidified gas. The water tubing cocoon keeps the humidity in suspension and decreases rainout. Because the patient can tolerate much higher flows without the complications associated with a traditional nasal cannula, we are able to deliver FiO2s very close to 100 percent. Since the generation of flow through a regular nasal cannula can result in inadvertent continuous positive airway pressure (CPAP), we believe that inadvertent CPAP may be delivered with the Vapotherm cannula as well. Currently we are unable to measure the amount of CPAP generated at various flows, and research is underway across the country to explore this more thoroughly.

Vapotherm has been used to treat:

• Hypoxemia (not responding to low flow oxygen therapy).
• Apnea of prematurity.
• Persistent respiratory insufficiency.
• Upper airway obstructions and or anomalies.
• Airway inflammation.
• Hypothermia.
• Nasal CPAP patients experiencing excessive mucus
• plugging.
• Patients who have difficulty or the inability to consistently wear nasal CPAP; intolerance of the nasal interface (masks, prongs, pillows) or excessive breakdown of nasal area due to use of nasal CPAP and or bi-level positive airway pressure (BiPAP) therapy.

It is important to recognize that Vapotherm is not meant to be a substitute for patients requiring BiPAP, as there is no inspiratory assist offered with Vapotherm. Routine examination via chest X-ray for hyperinflation is recommended and placing an oral gastric tube may benefit some patients by decreasing gastric distention.

Experience is helping us guide starting flow rates on the Vapotherm. We usually start premature and infant populations at 3-5 LPM, pediatric patients at 5-10 LPM and adolescent and adult populations at 10-20 LPM. Flows are then increased or decreased to treat symptoms. We will be monitoring the literature carefully to help us further guide the application of this device.

Children's Hospital has been using Vapotherm for approximately three months, with a steadily increasing frequency. Patients have been very comfortable on the device, and parents are happy with the increased mobility that a nasal cannula system provides. One parent said she never had seen her child breathe so comfortably. We have seen patients with variable saturation levels on a regular cannula stabilize with the higher flow Vapotherm. Because the higher flow system allows less room air entrainment, we are able to wean the FiO2 significantly. We feel Vapotherm is another tool in our arsenal and are happy to provide this service to our patients.

Descriptive and comparison study of atrioventricular septal defect with and without Down syndrome

Kathy Hanson-Morris, RN, MS, coordinator, Wisconsin Pediatric Cardiac Registry, Herma Heart Center, Children's Hospital of Wisconsin.

While congenital heart defects are the most common birth defects in Wisconsin, the etiology of these defects continues to elude us. Approximately 5 to 10 percent are associated with a chromosomal defect, and a smaller percentage can be linked to defects in single genes. Gene-environment interaction is a theory that was postulated in the 1950s and 1960s to explain the potential etiology of the majority of cases and remains the most popular theory today.

Recently, the first analysis of data provided by families for the Wisconsin Pediatric Cardiac Registry was completed. Families participating in the registry are referred from Children's Hospital of Wisconsin, Marshfield Clinic, Prevea Medical Center in Green Bay, Dean Medical Center in Madison and recently Gundersen Lutheran in LaCrosse.

Data analyzed for this study was collected from the registry questionnaire from Jan. 1, 2000, to March 15, 2004. Exposures analyzed are among the approximately 140 environmental and infectious exposures and other potential risk factors self reported by parents on the questionnaire. Univariate analysis was completed for exposures of interest and results of odds ratio and confidence interval reported. Multivariate logistic regression was completed for exposures of significance from univariate analysis. These variables were adjusted for age.

The objectives of the study were to analyze exposures to select environmental and infectious toxicants reported by parents of infants with atrioventricular septal defects (AVSD) with (N=67) and without (N=43) Down syndrome and compare those to exposures reported by parents of infants with an isolated muscular ventricular septal defect (control/comparison group) (N=322).

AVSD is a cardiac defect that occurs in 4 to 10 percent of infants born with a congenital heart disease (CHD). It is the most common form of CHD in children with Down syndrome but also occurs as a more complex defect in infants without Down syndrome, signifying morphologic heterogeneity that may represent different etiologies.

Descriptive analysis revealed a number of interesting findings. Infants with AVSD without Down syndrome had a significantly greater percentage of births in the first and fourth quarters of all years studied (32.6 percent and 34.9 percent respectively). More females (59.7 percent) than males had AVSD with Down syndrome as compared to more males with AVSD without Down syndrome (55.8 percent). Incidence of twinning was statistically significant (p.035) in the AVSD without Down syndrome group. In addition, 63.6 percent of infants with AVSD without Down syndrome were diagnosed prenatally as compared to 44.1 percent of infant with AVSD and Down syndrome. Infants with AVSD without Down syndrome also were more often born prematurely (earlier than 38 weeks) and had associated lower birth weights.

Variables of significance from univariate analysis for the case group with Down syndrome were maternal and paternal age; maternal reports of nausea and vomiting during pregnancy; use of medications to treat nausea and vomiting; and paternal exposure to lead, ionizing radiation and chemicals to kill weeds. Paternal smoking and use of art acrylics and oils were significant for the case group without Down syndrome. Only paternal age and maternal use of medications to treat nausea and vomiting in the Down syndrome group remained significant in the multivariate logistic regression analysis.

These results suggest that examining the impact of environmental factors in the development of CHD may contribute to knowledge of etiologies and in the future may lead to preventative efforts. Identifying etiologies associated with one type of CHD may lead to a better understanding of factors affecting the development of the human heart and etiologies of other types of CHD.

This first analysis of the data provided by families participating in the registry allowed us to evaluate the design of the questionnaire and the quality of the data provided by families completing the structured, self-administered questionnaire. In addition, we were able to compare our findings with those of the Baltimore-Washington Infant Study and other studies of etiologies of congenital heart defects.

The major limitation of this study was the small sample size available. The registry currently includes more than 1,700 families, but this number encompasses all CHD diagnoses. Any study of variables from the registry utilizing a specific diagnosis will be faced with similar sample size limitations at this time. Future research initiatives will further investigate the exposures of significance from this study, correlate the increase in births of infants with AVSD without Down syndrome in the first and fourth quarters with viral databases, map addresses of affected families, correlating their location with known or suspected environmental toxicants, watershed areas and other areas of interest available from public databases within the state.

Research into etiologies of CHD remains in its infancy. Few large-scale population studies have examined associations between exposures reported by parents and the development of specific defects. The registry, which ascertains new infants born with CHDs from various centers across the state, provides a unique vehicle for obtaining information on maternal and paternal exposures, and illnesses that may be risk factors for CHD.