A measurement system which is capable of detecting microfibrillation in the hypothermic, arrested heart has been developed. It is shown that this system can reliably measure microfibrillation signals as low as 10 microV in amplitude. Using this system, it has been shown that even after the heart has been rendered visibly arrested using cold potassium, it may still be producing a low level of electrical activity. It has also been shown that the presence of this microfibrillation during the period of the cold potassium administration and visible arrest, is very highly correlated with the decrease in high energy phosphates (ATP), mitochondrial injury, and a decreased cardiac function post-operatively. Using the information obtained from this first measurement system, a miniature, battery powered voltmeter has been designed and fabricated. This voltmeter has been tested using the initial measurement system as reference and produces the same information concerning the small signal electrical activity (i.e.: same waveform and RMS value). Because this voltmeter is battery powered and is, therefore, completely isolated from all electrical equipment in the operating theatre, it can be used to monitor the presence, or absence, of microfibrillation in the human heart. Of greatest importance, however, is the fact that the voltmeter can serve as a monitoring device which will indicate when the potassium should be administered in order to render the heart both mechanically and electrically arrested.