Cardiac output, the average rate of blood flow through the heart, is an important diagnostic parameter for patients with heart disease. Cardiac output can be estimated using a number of methods, but the standard clinical method for estimating cardiac output is bolus thermodilution. Bolus thermodilution requires a catheter to be placed in the right side of the heart so that the injection port of the catheter is located in the right atrium and the thermistor at the end of the catheter is located in the pulmonary artery. Cardiac output is estimated by injecting 5 to 10 mls of iced saline into the right heart and recording the temperature fluctuation in the pulmonary artery. The average cardiac output is inversely proportional to the area of the thermal pulse recorded in the pulmonary artery.
Bolus thermodilution has a number of drawbacks. The most significant drawback is that bolus thermodilution can only produce intermittent measurements of cardiac output. Because of the risk of fluid overloading only 3 to 6 measurements can be taken an hour. Bolus thermodilution readings are also relatively labor intensive.
Recently, there has been significant interest in continuous measurement of cardiac output. For continuous measurement of cardiac output, the blood is usually heated with a resistive heater. Because the blood is more easily damaged by heating than by cooling, the amount of power that can be safely delivered by a heating system is much smaller than the effective power delivered by a bolus injection of iced saline. Consequently, the resulting heat pulse in the pulmonary artery is much smaller and the signal to noise ratio is much worse.
The amount of power that can be delivered to the blood is limited by the peak temperature generated in the blood. We are developing a system that uses a laser diffuser to heat blood volumetrically. In contrast, a resistive heater provides only surface heating. With volumetric heating, a laser diffuser can deliver more thermal energy to the blood than a resistive heater while maintaining the same peak temperature. By delivering more thermal energy, the laser based catheter produces a heat pulse with a better signal-to-noise ratio, allowing either faster or more accurate measurement of cardiac output.