CAMBRIDGE, Mass. -- Since 1970, doctors have injected patients with iced saline to measure their hearts' blood output. Periodic injections provide reliable results but do not allow doctors to constantly monitor patients.An infrared irradiating system now under development uses diode lasers to heat the blood, potentially providing medical personnel with accurate, constant heart monitoring while reducing potential side effects.Saline injection is the most common way to monitor the heart's efficiency. Technicians insert a catheter into the right atrium of the heart. The catheter allows saline to be injected directly into the heart and enables measurement of the temperature of the blood in the pulmonary artery. By combining temperature readings with pulse rate, doctors can determine how much blood the heart pumps. Injection has several drawbacks:Injecting a foreign material represents a hazard to the body.The injections cannot be made more than once an hour because of the risk of fluid overloading.Temperature readings vary according to the operator and injection technique.The starting temperature of the saline solution can change between the cooler and the heart, reducing the accuracy of the measurements.In an effort to avoid injections, researchers investigated various methods of heating the blood, including resistors, microwave, ultrasound, radio frequency and lasers. Resistors and ultrasound heated the blood through conductive methods and allowed doctors to continually monitor cardiac output. Because too much heat at one time can "cook" and destroy the cells, however, the power of the catheter's heating element is limited by the temperature at the catheter/blood interface. This can result in uneven heating and inconclusive measurements.A technique developed by Michael G. Curley at E.P. Ltd. uses a laser to evenly heat the blood in the atrium, essentially at once. When trying to determine what kind of laser to use and at what wavelength it should operate, Curley considered the diameter of the average adult atrium (3.7 cm) and the irradiance absorption levels of blood. He selected a 980 nm laser diode from SDL Inc. of San Jose, Calif., because of its efficiency and price.As part of an experimental setup, the beam was delivered via optical fiber and emitted into the blood through a Teflon-encased, titania- doped silicon diffuser tip from Rare Earth Medical Inc. of West Yarmouth, Mass. A thermistor in the probe measured the temperature fluctuations.The laser delivers much more heat to the blood without hurting the cells and provides reproducible measurements on a par with those of the saline injections.