Radio Frequency Impedance Interrogator (RFII) Signal Overview
In biomedical engineering, thoracic bioimpedance is a measure of changes in the electrical conductivity of the thorax and heart. The measurement is based on pulsatile blood volume changes in the heart and aortic root.
Hemodynamic and bioimpedence states occur every beat ,or subcomponent of a beat, within the heart. Each action within the heart causes a physical and electrical change. Although the repetition of these actions can be viewed as a general average over a period of time, one must remain vigilant in the understanding that each heartbeat is a unique event, and thus contains a unique set of values and characteristics that provide unique and valuable therapeutic information.
Concurrently, external forces exert themselves onto these values and components of a heartbeat causing variations. These variations are generally very small and do not inject major deflections in the data ranges. External forces may include, but are not limited to the subject’s physical position such as standing or sitting. Breathing rate changes can provide a variance as well. Physical exertion, environment, and overall physical state also affects these parameters. Therefore when sudden changes occur, or deviations within a trend occurs an evaluation can be made as to the condition of the individual. When compared to the initial values or individual baseline, a determination may be made as to general health... Additionally, by predicting a future trend from the collected trend data a patient can be evaluated in terms of future outcome. This includes improvement, stability or decline in the health of the monitored individual. Therefore therapeutic intervention may be guided in order to prevent decompensation and improves outcomes.
Standard bioimpedance monitors interact with and measure electrical conductivity within an individual.. RFII however, investigates, and interacts with, their physical components; thoracic electrical conductivity, anatomy, and mechanical action. As RFII interacts with the mechanical aspects of the subject various factors affect the interaction and provide investigational information. Whereas breathing provides a minimal deflection within bioimpedance monitoring, it is a substantial subcomponent of RFII. As such, breathing not only can be ascertained, it can be quantified as well.
Within RFII, a single source of power is emitted towards the body which reflects off of all organs and tissues. When an RFII signal is sent towards the torso of the body, the first deflection occurs on the skin, followed by the ribcage. As these two physical components are static in size and composition the returned signal is a steady value. This is similar to the electrical influence the lungs or skin imparts onto a bioimpedance measurement.
Once the RFII signal passes the ribcage, a portion of it reflects from the lungs. As the lungs expand and contract, the composition of the organ changes. When the lungs are exhausted of air, the bulk of the material within the volume of the lungs is tissue. The tissue contains salt and water. As salt water is one of the most reflective materials to RFII, the signal increases due to this high reflectivity. As the subject begins inhaling, the air pocket within their lungs expands providing more surfaces with varying angles to deflect RFII waves in random angles. Concurrently the volume of the organ area begins to change in composition. The ratio of water begins to lower as more air is drawn in. As the air volume increases, the reflected signal becomes lower due to absorption as well as signal deflection. This provides a lowering of the signal which is captured and monitored.
The RFII which passes into and beyond the lungs comes into contact with the heart. A portion of the RFII is reflected back towards the monitoring device which provides a constant value as the composition of the muscle itself remains relatively constant. The RFII signal which enters the heart is modified by a number of factors resulting in a varied signal return which can be monitored and evaluated. When the heart goes through it’s cardio dynamic process, the shape and displacement of fluid levels changes. The shape of the heart changes as well.
All of these physiological events modify the signal which is deflected back out of the body. When the volume of fluid increases the signal which is returned increases due to composition. Blood plasma is 93% water thus providing an effective material for the reflection of the RFII signal. Not only does the volume of fluid affect the signal reflection, the shape of the liquid as it is defined by the container (the heart) provides a unique consistent pattern to the reflected RFII signal. RFII signals which enter the fluid volume have the opportunity to be reflected from the back or opposite surface on the fluidic shape. These reflections are smaller components however they affect the characteristics of the signal uniformly on a beat to beat basis due to the continuity of the fluidic shape as it appears within the heart.
Beyond the heart, the balance of the RFII signal will come into contact with bone material such as the spinal column, muscle and skin. During this process portions of the RFII signal will be reflected back to the detector in a consistent manner as the composition and shape of these materials remains static during the process. In order to properly understand and evaluate the RFII signal, it is important to recognize and apply the processes involved within the anatomy of the subject on a physiological and mechanical level. Components within the subject need to be differentiated and evaluated independently and concurrently as they interact with the RFII signal in unison. For example, although the blood within the subject is a liquid, therefore not maintaining a static shape it is imperative to view this fluid as a static solid which changes over time in both shape and size. As the RFII signal reflects from all surfaces within this shape, it is also important to keep in mind that the subject fluid or material is a 3 dimensional shape.
When analyzing the signal one needs to additionally factor in the effect of red shifting and Doppler radar. As the signal returns from various components at different rates due to time of travel Doppler effects can and do occur. Due to this informational interleaving being created implicitly, it becomes apparent that analysis and evaluation of the signal requires a multi compartmental approach. Even though the emission and detection of the source signal is a single data event creating a single feed of sampled information, many responses from multiple organs and tissues occur simultaneously within the data set.
A Description of RFII and its Implementation for the ETag
Radio Frequency Impedance Interrogation (RFII) is a new noninvasive impedance cardiography technology being developed at SMX.(1) Practical noninvasive impedance cardiography systems such as SMX’s IQ technology provide a noninvasive method for monitoring patient hemodynamics.(2-6) In the case of a trauma patient, practical noninvasive impedance cardiography method provides a medical clinician the ability to monitor, evaluate and optimize cardiac performance, pulmonary status and tissue perfusion. This vital and early information on cardiac performance can enable the clinician to have hard and fast data so the hard decisions to maximize survival for the trauma patient can best be made.(7-17)
Hemodynamic monitoring with a pulmonary artery catheter has become common in the care of the critically ill.(6,18,19) Accepted hemodynamic monitoring methods preceding acceptance of cardiac impedance measurement, include the Fick method, dye indicator dilution, and thermodilution.(6,18,19) Until recently, hemodynamic monitoring has been limited to the critical care unit, operating room and occasionally the emergency department due to the invasive nature of the pulmonary artery catheter, the expertise required for insertion and maintenance of the catheter, and the close vigilance required to prevent potential vital risks to the patient. Unlike invasive hemodynamic monitoring with a pulmonary artery catheter, the noninvasive system is not restricted to care of the critically ill. Noninvasive continuous hemodynamic monitoring methods such as the new RFII or the existing IQ technologies has utility in any clinical area, from the outpatient clinic to the critical care unit, where healthcare providers desire information regarding a patient’s hemodynamic status.(7-17) RFII technology will extend noninvasive continuous hemodynamic monitoring from the hospital emergency room to the medical first aid arena, covering virtually any emergency field situation, military battle field, natural disaster, or other emergency medical scenario.
The RFII method of Cardiac Impedance Measurement
In both the new Radio Frequency Impedance Interrogator (RFII) prototype technology and the established IQ technology, there is much in common between the actual electrical measurement methodologies. Both methods can be introduced as follows.
Impedance cardiography introduces a test sinusoidal (AC) signal of low magnitude into the thorax. This can be done at low frequencies using a conductive method such as electrodes on the skin, or at high radio frequency (RF) frequencies using an antenna on the chest, outside of the patient’s clothes. The motions of the cardiac system, the beating heart and the blood flow ejected by the heart, modulate the signal. The received modulated signal carries voltage information that is compared to the test signal by the receiver to extract a cardiac impedance waveform. The DC content of the waveform indicates the fixed or baseline electrical impedance of the thorax, ZO and the AC part indicates time varying cardiac impedance waveform Z/t. (6)
In the RFII prototype, an RF signal at a frequency of 915 MHz is used to measure cardiac impedance. The RFII system measures cardiac impedance by transmitting a test RF signal to the patient’s thorax, and measuring the RF impedance of the reflected wave. The RFII system uses a single patch antenna, measuring about 3” x 3”, with rounded corners and coated in a white plastic film. This is placed on the center of the sternum, aligned with the aorta, on the chest over the patient’s clothes, with no direct contact to skin required. Movement off the median of the sternum visibly reduces the received signal strength. The RFII system operates continuously on “full duplex”, meaning that the 915 MHz test signal is transmitted and the received RF reflected signal is processed at the same time, and both transmitted and reflected signals pass through the same antenna at the same time. The duplexer is an RF device that can separate the transmitted and received RFII signals. The patch antenna is designed to efficiently transmit a sufficient amount of signal strength of the single frequency 915 MHz test signal into the thorax, through the sternum and to the heart, aorta and other great vessels and receive a signal sufficiently robust to make a RF impedance measurement.
The reflected received signal from the body is picked up by the patch antenna. The received antenna signal has RF impedance that is relative to the transmitted RFII signal’s amplitude and phase. The receiver and processing circuitry measure the RF impedance of the thorax. The RF impedance is dependant on the overall conductivity and absorptiveness of the body’s blood and tissue, making it dependent on vital chemical conditions that can change RF conductivity and loss, such as body hydration, or deficient oxygen content. The resulting constant or DC component of the impedance is the baseline impedance, ZO. The moving parts of the heart and the blood flow also cause the RF impedance to change over time in a very low frequency pattern (waveform) determined by the cardiac cycle. This very low frequency pattern is a Doppler signal representing, Z/t. The mechanical motion of the heart and blood flow, relative to the antenna frequency, modulates the 915 MHz RFII signal with a frequency modulation (FM) content of 1.0 to 100 Hz. The measurement of the RF reflected wave is relative or compared to the transmitted RF signal, which is essentially the RF equivalent to the basic measurement method of the IQ system.