A group of researchers at the University of Washington have given a paper which briefly outlines some of these techniques. One simple method of measuring the expansion of the heart is to tie a thin rubber tube, filled with mercury, around the heart and record the change in resistance as the tube is stretched. A balanced resistance bridge and a pen recorder are all the electronic instrumentation needed.

Sonar can be used to measure the thickness of the heart by placing small crystal transducers at opposite sides of the heart or blood vessel and exciting one with some pulsed ultrasonic energy. The travel time of sound in tissue is about 1500 meters per second thus it takes about 16 | msec. to traverse 25 mm. of tissue. A sonar or radar type of pulse generator and time-delay measuring system is required for body tissue evaluation. In addition to the heart and aorta, successful measurements of liver and spleen have also been made by this technique. The Doppler effect, using ultrasonic signals, can be employed to measure the flow of blood without cutting into the blood vessel.

A still more sophisticated system has been devised for determining the effective power of the heart itself. It uses both an ultrasonic dimensioning arrangement of the heart and a catheter carrying a thermistor inserted into the bloodstream. The latter measures the heat carried away by the bloodstream as an indication of the velocity of the blood flow. It is also possible to utilize a pressure transducer, mounted at the end of a catheter which is inserted into the heart's left ventricle, to indicate the blood pressure in the heart itself. This pressure measurement may be made at the same time that the ultrasonic dimensioning measurement is made. A simplified version of the instrumentation for this procedure is shown in Fig. 2. Outputs of the two systems are measured by a pulse timing circuit and a resistance bridge, followed by a simple analogue computer which feeds a multichannel recorder. From this doctors can read heart rate, change in diameter, pressure, and effective heart power.