NICOM® Technology has now been used in tens of thousands of patients worldwide. NICOM provides continuous, accurate, non-invasive hemodynamic monitoring and empowers fluid management in virtually any clinical setting. It is based on Bioreactance®, a technology that was developed by Cheetah Medical's scientists over years of research and development and followed by extensive testing and validation. The basis behind NICOM are phase shifts that occur when an alternating electrical current (AC) is passed through the thorax. NICOM should not be confused with bioimpedance, an older and less accurate technology.
The foundation of Cheetah Medical’s NICOM technology is the discovery that when an AC current is applied to the thorax, the pulsatile blood flow taking place in the large thoracic arteries causes the amplitude of the applied thoracic voltage to change. In addition, it causes a time delay or Phase Shift between the applied current and the measured voltage. Extensive research has shown that these phase shifts are tightly correlated with stroke volume. By accurately and continuously measuring phase shifts, the stroke volume is determined. NICOM is equipped with a sensitive phase detector that can detect very small phase shifts. These phase shifts are highly correlated with the aortic blood volume and are the building blocks of the NICOM signal. The NICOM signal represents blood volume, and the derivative of the NICOM signal over time provides the dNICOM signal, which represents the change of volume in time, i.e. aortic flow. The stroke volume is then measured from the dNICOM signal.
What is a Phase Shift?
The AC current and AC voltage are based on the trigonometric sine function. The sine function is first represented as a function of the phase (or Angle) in degrees. It can be seen that the sine wave reaches a quarter of a cycle when in 90 degrees, half a cycle when in 180 degrees and it completes a full cycle when in 360 degrees. When the gray sine wave is shifted by 90 degrees, it is actually shifted by a quarter of the Sine function cycle (as 90 degrees is a quarter of 360 degrees which is the full Sine function cycle).
When dealing with sine waves such as AC current and AC voltage the change is not a function of degrees but is a function of time (in this case in seconds); thus, if we switch the X axis to a time axis instead of a phase axis, we obtain in the above figure a sine wave that changes in time. Note that in the above example the sine wave reaches a quarter of a cycle after 0.25 seconds, half a cycle after 0.5 seconds, until it completes a full cycle after 1 second. If the gray sine wave will be shifted by 0.25 seconds, it will yield a time delay between the two sine waves of 0.25 seconds (as the gray sine wave begins 0.25 seconds after the turquoise sine wave). As 0.25 seconds in the above example is a quarter of the full cycle which lasts 1 second, we can say that we shifted the gray sine wave by a quarter of a cycle and because it has the behavior of a sine Function, we can also say that we phase shifted the gray sine wave by 90 degrees (a quarter of a sine Function cycle).
From Phase Shift/ Time Delay to NICOM Signal
Thoracic pulsatility which is strong enough to induce phase shifts exists mainly in the aorta which transports virtually all of the cardiac output and does so at higher pressure variations than any other vessel. As pulsatile changes in fluid volume induce changes in phase, the NICOM signal is correlated mainly with aortic blood volume. The above figure illustrates the fact that the time delay (or phase shift) between the AC current and the AC voltage are synchronized with the heart beat and also demonstrates visually how consecutive measurements of the phase shift build the NICOM signal from which the stroke volume is later derived.
From NICOM Signal to Stroke Volume
During systole we observe a rapid build-up of the phase shift (NICOM signal) until a peak is reached in the end of the systole. This reflects the increase in aortic blood volume during ventricular ejection. Beyond the peak, during diastole we see a decrease in the phase shift representing reduction in blood volume. Since flow is defined as a dynamic change in volume, when the NICOM signal is derived by time, the resulting signal (dNICOM) represents aortic flow. The stroke volume is found by computing the area under the positive part of the dNICOM waveform, or the part of the waveform that represents systole.
The NICOM system is equipped with 4 sensor pads. Each pad contains an outer, transmitting sensor and an inner one for receiving. The sensors are applied around the heart on the chest or back (there is no need for exact positioning). The NICOM monitor induces a 75KHz AC current to the thorax via the outer sensors and receives the voltage via the inner sensors. It then measures the time delay/ phase shift between the induced current and the received voltage continuously, thus building the NICOM signal.
Click here to download the NICOM Technology PDF