Christopher N. McLeod

Continuous in vivo blood pressure measurements using a fully implantable wireless SAW sensor

In this paper, the development of a fully implantable wireless sensor able to provide continuous real-time accurate pressure measurements is presented. Surface Acoustic Wave (SAW) technology was used to deposit resonators on crystalline quartz wafers; the wafers were then assembled to produce a pressure sensitive device. Excitation and reading via a miniature antenna attached to the pressure sensor enables continuous external interrogation. The main advantages of such a configuration are the long term stability of quartz and the low power necessary for the interrogation, which allows 24/7 interrogation by means of a hand-held, battery powered device. Such data are of vital importance to clinicians monitoring and treating the effects of hypertension and heart failure. A prototype was designed and tested using both a bio-phantom test rig and an animal model. The pressure traces for both compare very well with a commercially available catheter tip pressure transducer. The work presented in this paper is the first known wireless pressure data from the left ventricle of the heart of a living swine.

A Pseudo-Normal-Mode Helical Antenna for Use With Deeply Implanted Wireless Sensors

A pseudo-normal-mode helical antenna as part of a deeply implanted wireless sensor was designed. Justification for using this type of antenna along with simulation, in vitro and in vivo experimental results are presented in this communication. The circumference of the helical coil is 0.43λib (wavelength in-body) and its height is 0.23λib which includes substantial insulation. While losses from such a deeply implanted antenna are inevitable, the work presented here shows accurate frequency tuning can be achieved prior to implantation. The relative size, safety of use and results presented here make this pseudo-normal-mode helical antenna an excellent candidate for use with deeply implanted wireless sensors.

FPGA design and implementation for EIT data acquisition.

OXBACT-5 was designed to meet the challenges involved in working in the intensive care hospital environment focussed particularly on thoracic imaging of patients with respiratory distress and chronic heart failure (CHF). The FPGA-based wireless LAN linked multi-channel EIT data acquisition system (DAS) providing 16 programmable excitation current channels and 64 voltage measurement channels is presented. It contains function modules of a PCI bus interface, direct digital synthesizers, dual-port memory blocks, digital demodulation and all the command and control logic in the FPGA. The whole EIT data acquisition system is fully programmable and reconfigurable from the host PC. The excitation frequency, excitation patterns, the measuring sequence and the gain of each measurement channel can be set from the host PC before each measurement. The demodulation is implemented in the FPGA chip to reduce the data rate between the DAS and the host PC. In addition, measurement process management is achieved in this FPGA chip. Complemented by analogue devices such as ADCs, DACs, analogue buffers and analogue multiplexers, the new FPGA-based EIT DAS system is implemented in a very compact way for bedside use in intensive care units of hospitals. It is intended for applications such as continuous respiration monitoring with data collection at 25 frames per second. Image reconstruction times depend on the choice of 2D or 3D imaging algorithms and the available processing power.

Effect of Stent Radial Force on Stress Pattern After Deployment: A Finite Element Study

The present article presents a method for assessing the radial stiffness of nitinol stents. An idealized stent model was created, and its radial stiffness was calculated by means of finite element modeling. The calculations were validated against experimental measurements. The variation of radial stiffness with geometrical dimensions was calculated, and the effect of increasing radial stiffness on endovascular deployment was analyzed. Peak tensile and compressive stresses as well as stent penetration were calculated in the case of an idealized pulmonary artery model having realistic dimensions as well as stiffness. The results of stress calculations were compared with a second set of simulations, where an idealized behavior of the stent (uniform expansion to a theoretical contact diameter) was modeled. The results show how in reality nitinol stents behave in a non-ideal way, having a non-uniform expansion and exerting non-uniform pressure on the contact areas with the artery. Such non-ideality decreases though with the increase in radial stiffness. The radial force alone may be insufficient in describing the stent-artery interaction, and numerical modeling proves to be necessary for capturing such complexity.

Development of an apnea detector for neonates using diaphragmatic surface electromyography

Respiratory diseases are among the most important and serious conditions that can affect the newborn baby. A cessation of breathing, longer than 15 seconds, or accompanied by hypoxia or bradycardia, is called apnea of prematurity (AOP) and has been found in more than 50% of premature infants. An apnea detector used in infant monitoring has been designed and constructed and is intended to be applied in a clinical environment. Diaphragmatic surface EMG has been used as the technique for detecting apnea episodes due to a direct relation with the respiratory drive. Both obstructive and central apnea can be determined as well as heart rate. Good performance and feasibility have been shown by the prototype.

RF communication with implantable wireless device: effects of beating heart on performance of miniature antenna

The frequency response of an implantable antenna is key to the performance of a wireless implantable sensor. If the antenna detunes significantly, there are substantial power losses resulting in loss of accuracy. One reason for detuning is because of a change in the surrounding environment of an antenna. The pulsating anatomy of the human heart constitutes such a changing environment, so detuning is expected but this has not been quantified dynamically before. Four miniature implantable antennas are presented (two different geometries) along with which are placed within the heart of living swine the dynamic reflection coefficients. These antennas are designed to operate in the short range devices frequency band (863–870 MHz) and are compatible with a deeply implanted cardiovascular pressure sensor. The measurements recorded over 27 seconds capture the effects of the beating heart on the frequency tuning of the implantable antennas. When looked at in the time domain, these effects are clearly physiological and a combination of numerical study and posthumous autopsy proves this to be the case, while retrospective simulation confirms this hypothesis. The impact of pulsating anatomy on antenna design and the need for wideband implantable antennas is highlighted.

Spatial Orientation and Morphology of the Pulmonary Artery: Relevance to Optimising Design and Positioning of a Continuous Pressure Monitoring Device.

Personalised treatment of heart disease requires an understanding of the patient-specific characteristics, which can vary over time. A newly developed implantable surface acoustic wave pressure sensor, capable of continuous monitoring of the left ventricle filling pressure, is a novel device for personalised management of patients with heart disease. However, a one-size-fits-all approach to device sizing will affect its positioning within the pulmonary artery and its relationship to the interrogating device on the chest wall on a patient-specific level. In this paper, we analyse the spatial orientation and morphology of the pulmonary artery and its main branches in patients who could benefit from the device and normal controls. The results could optimise the design of the sensor, its stent, and importantly its placement, ensuring long-term monitoring in patient groups.

The expanding role of implantable devices to monitor heart failure and pulmonary hypertension

The epidemics of heart failure and, to a lesser extent, of pulmonary arterial hypertension continue unabated worldwide and are extremely costly in terms of loss of life and earnings, as well as the burden of health-care expenditure due to repeated hospitalization. The effectiveness of newly discovered therapies for the two conditions depends on their timely application. To date, symptoms have been used to guide the application and timing of therapy. Compelling evidence now exists that symptoms are preceded by several metabolic and haemodynamic changes, particularly a rise in intravascular pressures during exercise. These observations have stimulated the development of several implantable devices for the detection of impending unstable heart failure or pulmonary arterial hypertension, necessitating admission to hospital. In this Review, we summarize the rationale for monitoring patients with heart failure or pulmonary arterial hypertension, the transition from noninvasive to implantable devices and the current and anticipated clinical uses of these devices.In this Review, Yacoub and McLeod summarize the rationale for monitoring patients with heart failure or pulmonary arterial hypertension to detect haemodynamic changes that predict the deterioration from subclinical to overt disease, the transition from noninvasive to implantable devices and the current and anticipated clinical use of these devices.Key pointsLong-term frequent or continuous haemodynamic monitoring is now feasible.Measurements of pulmonary arterial pressure from a permanent implanted device have proven efficacy in reducing rehospitalization rates for patients with chronic heart failure.Hypothetically, the integration of biomarker data, clinical signs and haemodynamic data will lead to improved care of patients with chronic cardiovascular conditions.The emerging issue is the cost–benefit ratio for a variety of conditions and for different stages in progressive chronic diseases.