Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-05-10
2002-11-05
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
Reexamination Certificate
active
06475153
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention relates generally to the field of devices used to measure blood pressure. More particularly, the invention relates to method for continuously obtaining blood pressure data, and related information such as pulse pressure, pulse rate and arterial compliance, from a patient using a non-invasive optical sensor.
B. Statement of Related Art
Non-invasive systems for continuous monitoring of blood pressure, for example during anesthesia, currently exist. Representative patents include the patents to Shinoda et al., U.S. Pat. No. 5,165,416; the patents to Erkele et al., U.S. Pat. No. 4,802,488 and 4,799,491; Jones et al., U.S. Pat. No. 5,140,990, Jackson et al., U.S. Pat. No. 5,485,848 and Pytel et al., U.S. Pat. No. 5,195,522. It is also known to use optical sensors as the means to acquire blood pressure data. See the patents to Butterfield, et al., U.S. Patent 5,908,027; 5,158,091; 5,261,412 and 5,273,046; Cerwin, U.S. Pat. No. 5,984,874 and Tenerz et al., U.S. Pat. No. 5,018,529. The above-referenced patents are incorporated by reference herein.
Prior art mechanical sensors commonly measure blood pressure by detecting transducer changes that are proportional to the detected changes in external force measured at the skin surface during pulsation. These sensors depend on mechanical parts and are therefore more subject to breakdown due to moving parts, and are larger in size thus requiring more space for fitting it on the patient skin. They are typically large in actual size. These sensors employ the use of a single sensor, or an array of sensors from which only one (the one with the highest signal strength) is selected for measurement. Such sensors only cover a small surface area on the skin and are therefore very sensitive to initial exact placement of the sensor on top of the artery. They are also sensitive to movement or minor accidental repositioning. This typically invalidates all calibrations, requiring a need for re-calibrating the system with an air cuff pressure reference. Providing a corrective feedback mechanism for compensating for minor positional changes in sensor placement is not possible due to dependency on a single-point or single-sensor measurement. Furthermore, the resolution of these sensors to blood pressure changes at low level signal strength is not sufficient to obtain accurate results. Other sensors typically require higher hold down pressure (HDP) values in order to obtain a stronger signal due to their low sensitivity. They also offer no corrective feedback mechanism for compensating for minor variations in the hold down pressure, often requiring a need for re-calibration of the sensor at the new hold down pressure value.
Portable oscillometric wrist mounted blood pressure devices also exist, such as the Omron model HEM-609, but these are not intended for continuous blood pressure monitoring. The oscillometric method requires the patient to be at a rested state, and a cuff pressure to be applied by the device that is above the systolic blood pressure of the patient (thus temporarily cutting off circulation in the artery and causing discomfort).
Spacelabs' Modular Digital Telemetry system offers an ambulatory blood pressure (ABP) option for wireless transmission of noninvasive blood pressure data to a central computer, however it is not a tonometric optical blood pressure monitor and it is transmit only.
The above-referenced '027 Butterfield et al. patent describes a device and technique for measuring tonometric blood pressure non-invasively using a one-dimensional optical sensor array. The sensor used in the '027 patent is also described in U.S. Pat. No. 5,158,091 to Butterfield et al. The array detects photo-radiation that is reflected off of a semiconductor, thermally sensitive diaphragm, with the diaphragm deflected in response to arterial pulsation. The diaphragm's thermal properties affect how its surface is deflected. Such thermal properties are associated with calibration coefficients which are used for mapping measured deflections into mmHg blood pressure values. The calibration procedure requires taking such thermal properties into consideration, including a) thermal heating of the diaphragm, b) calibration for optimum vs. non-optimum applanation state of the underlying artery, and c) deformable and a nondeformable portions of the diaphragm so that calibration coefficients can be obtained to map measured sensor output signal into blood pressure.
The present invention is believed to be a substantial improvement over the type of sensors proposed in the prior art. The sensor itself does not depend on thermal considerations. The diaphragm or reflective surface in the present sensor is responsive to any input stress on its surface. Furthermore, a priori knowledge of the exact applanation state is not needed for proper calibration.
Additionally, the sensor is calibrated against a standard conventional air cuff for measuring blood pressure. The calibration procedure automatically compensates for variability that is inherent in patient anatomy and physiological parameters such as body weight, size, skin thickness, arterial depth, arterial wall rigidity and compliance, body fat, etc.. When the sensor is calibrated against known blood pressure (such as using an air-cuff system) all such detailed variables are individually and collectively integrated and linearized in the process of calibrating the sensor. In other words our calibration process is customized to the individual patient anatomy. Accordingly, the sensor and method of the invention produces more accurate results.
The '027 patent describes a set of detectors which are arranged in a single dimensional row. Image processing techniques are not particularly applicable in the format of arrangement of the detectors. In contrast, the sensor and method of the present invention uses a two-dimensional array of photo-sensitve elements which is cabable of producing a digitized two-dimensional image of the underlying skin surface variations due to pulsation. The number and density of elements are significantly higher. Accordingly, the array produces an image that can be processed using image processing techniques, including image transformation algorithms to detect translation or rotation of the sensor. Image processing methods can also be used for filtering, calibrating, tracking, and error-correcting the output of the sensor.
The '027 patent requires a mechanical assembly to provide a means for mechanically pushing the sensor onto the surface of skin tissue, and adjusting the force used for obtaining optimal artery applanation. The present invention does not require the need for such stress-sensing mechanical assembly for proper positioning and adjustment to achieve optimum applanation of the artery. The sensor does require a measurable hold down pressure to be applied on the sensor to produce measurable results for calibration purposes. The hold down pressure can be produced by mounting the sensor to a wrist watch band for example. Furthermore, the sensor and inventive method provide for compensating for changes in the hold down pressure between initial or calibration values of hold down pressure and values of hold down pressure later on when blood pressure data is obtained.
SUMMARY OF THE INVENTION
In a first aspect, a method is provided for obtaining blood pressure data from a patient using an optical blood pressure sensor placed against a patient's body. The sensor includes a two-dimensional array of photo-sensitive elements that obtain image data of the surface of the patient's body. Specifically, the array generates images of the deflection of the patient's body due to arterial blood flow, such as by detection of photo-radiation (i.e., light) reflecting off a flexible reflective surface placed against the patient's body. The scattering patterns are recorded as two-dimensional images. The images are in turn digitized and processed in accordance with the method of the invention.
The meth
Ghaem Sanjar
Khair Mohammad
Lopez Salvador
Ng Richard
Olson William
Brinks Hofer Gilson & Lione
Mallari Patricia
Motorola Inc.
Nasser Robert L.
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