Small doses of four drugs-acetylcholine, bradykinin, sodium nitroprusside and BQ-123-are given through the catheter. Acetylcholine slows the heart rate. Bradykinin stimulates the release of a substance that causes blood vessels to dilate and can lower blood pressure. Sodium nitroprusside causes blood vessels to dilate and is used to treat high blood pressure and heart failure. BQ-123 blocks the blood vessel-constricting activity of endothelin.