Cheetah Medical Technology: How Does it Work?
Cheetah’s products are based on Bioreactance®, a technology that was created by the company following years of research and development and extensive testing and validation. The basis behind this technology is the use of time delay, or phase shifts, which occur when an alternating electrical current (AC) is passed through the thorax.
The foundation of Cheetah Medical’s 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 phase shifts (or time delays) between the measured thoracic voltage and the applied AC current. 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.
WHAT IS A PHASE SHIFT?
As the AC voltage and AC current are based on the trigonometric sine function, the time delay between the two sine waves in figure 1 can also be represented in Phase (or Angle). In the figure below, the orange sine wave begins 0.25 seconds after the blue sine wave. Since the duration of a complete sine wave cycle depicted in the image is 1 second, we can say that the orange wave began a quarter of a cycle later. Given that a quarter of a cycle of a trigonometric Sine function corresponds to 90º, we can say that the orange sine wave is phase shifted from the blue sine wave by 90º.
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). In the graphic above, the X axis is converted to a time axis instead of a phase axis – a sine wave that changes in time.
BUILDING THE CHEETAH SIGNAL FROM PHASE SHIFTS
The Cheetah signal is generated by the Starling SV and the CHEETAH NICOM Monitors.
Cheetah monitors transmit the AC current to the thoracic cavity via four transmitting sensors and detects the phase shifts with an additional four receiving sensors. This signal is Phase based and is called the Cheetah signal where each point is a specific phase shift in time. Each sample on the Cheetah signal reflects the phase shift detected from the thorax at that time. The phase shift detected at any given moment is correlated with cardiac stroke volume.
FROM THE CHEETAH SIGNAL TO STROKE VOLUME
In the figure below the upper graph represents a single “beat” of the Cheetah signal. During systole, we observe a rapid buildup of the phase shifts 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 Cheetah signal is derived by time, the resulting signal represents aortic flow as presented in the lower graph which represents a single “beat” of the Cheetah Signal Derivative. Stroke volume is found by computing the area under the positive part of the Cheetah Signal Derivative, or the part of the waveform that represents systole.
COMPUTING STROKE VOLUME FROM THE CHEETAH SIGNAL
The maximum flow (dX/dtmax) is measured at the maximum point of the Cheetah signal derivative. The Ventricular Ejection Time (VET) is measured from the first zero crossing to the second zero crossing. The stroke volume is proportional to the product of dX/dtmax and VET which result in an approximation of the main positive area of the Cheetah signal derivative. Intuitively, the flow measurement derived by dX/dt is indirectly related to the strength of the hearts contractility. Greater contractility will induce higher flow and reduced contractility produces lower flow.
Once the dX/dt and VET are measured, stroke volume (SV) is obtained as follows
SV = DX/DT × VET
The Heart Rate (HR) is also detected by the same sensors that are used to detect the Cheetah signal. Stroke volume and heart rate combine by multiplication to produce Cardiac Output (CO).
CO = SV × HR
Since dX/dt is an electrical measurement that is influenced by the propagation of the signal in the thoracic cavity, the age and body surface area should be taken into account when calculating SV and CO. As such, the CO is a function of not only the HR, dX/dt, and VET, but also the patients weight, height and age.
CO=Ƒ(DX/DT,VET, HR, WEIGHT, HEIGHT, AGE)
The Cheetah Medical systems are equipped with four sensor pads. The sensors are applied around the heart on the chest or back. Cheetah sensors induces, or emits, a 75KHz AC current to the thorax via the outer sensors and detects the current and voltage via the inner sensors. Cheetah Monitors continuously measure the time delay/phase shift between the emitted current and the detected current and voltage, thus building the Cheetah signal.
Cheetah Measurement of Stroke Volume compared with Conventional Pressure Measurements
The clinical benefits of measuring flow and volume directly are presented in the table below, contrasting the Cheetah measurements with pressure measuring technology.
Cheetah measures Stroke Volume, Cardiac Output, Total Peripheral Resistance, and other hemodynamic parameters centrally, directly at the thorax. Instead of measuring pressure and converting volume (a process that requires an estimation of cardiovascular and peripheral vascular compliance), Cheetah measures flow and volume at the source – as blood leaves the left ventricle an passes through the aorta to the systemic circulatory system. Cheetah measures the “True Volume Event”.
When considering the volume management needs of the patient, the central question is to determine if fluids will improve or impede the goal of optimizing perfusion. “Will my patient respond to additional fluids by increasing cardiac output?” You ask a volume question. Cheetah provides a direct volume answer by measuring the true volume event. Stroke volume, measured continuously on a beat by beat basis.
|Dynamic volume assessments with|
Cheetah Medical Starling SV Monitor
|Conventional pressure measurements|
|Provides changes in stroke volume in response to fluid challenge1||CVP does not predict fluid responsivenes|
|Noninvasive, continuous, easy to use||Invasive|
|No central line||Invasive|
|Works on spontaneous breathing as well as mechanically ventilated patients||CVP impacted by mechanical ventilation and PEEP|
|Included as an option in NQ#0500/CM SEP-1 Severe |
Sepsis/Septic Shock Management Bundle
|Included as an option in the NQF#500/CMS SEP-1 Severe |
Sepsis/Septic Shock Management Bundle