Approximately 300 patients with severe chronic kidney disease who are referred for a nuclear stress testing of the heart with regadenoson (Lexiscan®) will be recruited to participate in the study. Following regadenoson (administered as part of a stress routine test protocol) participants will receive either aminophylline (75 mg - intravenously) or a matching inactive placebo (sterile salt water) injection. Participants will be surveyed for gastrointestinal symptoms and other side effects related to regadenoson. The frequency and severity of such side effects will be compared between the two study groups (aminophylline vs. placebo).