This invention relates to a method and apparatus for monitoring and diagnosing peripheral vascular blood flow in people or animals. Part of the apparatus uses a novel heat exchanger.
Peripheral vascular disease ("PVD") is defined as impaired arterial or venous blood circulation. More than one percent of the United States population has some form of PVD.
PVD may take many different forms, but it usually affects the leg, especially the femoral and popliteal (thigh and knee) regions. A major cause of venous PVD is deep vein thrombosis. Arteriosclerosis, which occurs more frequently with increased age, is usually progressive is a major cause of PVD in the arteries. Approximately forty percent of PVD cases are a result of complications from diabetes.
As PVD becomes more advanced, blood flow can be so limited that tissues will die, which leads to gangrene. As a result amputation is required above the gangreneous tissue. Cramps, charlie-horse or calf or thigh discomfort during leg movement are symptoms of milder PVD. Pain and the other symptoms increase as PVD becomes more advanced.
There are a number of methods for diagnosing PVD, but no simple screening devices for evaluating PVD accurately are now available. A number of the diagnostic techniques involve invasive diagnosis in which veins or arteries are entered by needles or catheters. In arteriography, after a contrast dye is injected into the arteries, the arteries are viewed using X-rays. Arteriography provides a detailed image of the arterial system, and it is the accepted standard for evaluation of an arterial disease before operations. Arteriography is expensive, and there is an element of risk, discomfort and exposure to radiation. Venography is similar to arteriography except the procedure is applied to veins. Both arteriography and venography are in widespread use but they cannot be used as a simple screening test for PVD.
There are also certain non-invasive tests for PVD in which the veins and arteries are not entered and are not exposed to X-ray examination. Non-invasive techniques have substantially less risks to the patient but they require skilled technicians and are usually limited to hospital settings. The non-invasive procedures now in use include segmental plethysmography, doppler ultrasound pulse analysis, electrical impedance measuring devices and a less-invasive arteriography called digital subtraction angiography. Segmental plethysmography measures blood volume in a limb or changes in blood pressure at different positions on the extremity such as at the thigh, calf, ankle and foot. A pulse volume recorder records arterial wave forms at each location. PVD in an artery alters the wave forms. As arteriosclerosis progresses, however, plethysmography becomes progressively less accurate. Therefore, an additional wave form analysis is performed on data provided by a doppler ultrasound instrument. As is well known, moving bodies shift the frequency of sound reflected from them. Doppler measuring instruments measure the difference between the frequency of transmitted sound and that of the reflected sound, and the difference is proportional to the velocity of the moving bodies. Red blood cells are such moving bodies, and their movement have been measured using doppler measurements. Plethysmography combined with doppler ultrasound measurements have provided a moderately accurate assessment of arterial PVD with less expense and lower risk than invasive techniques. Arteriography is still needed if surgery is contemplated. Plethysmography and doppler methods also measure the relative blood volume in a limb at a given time, and they are useful in evaluating deep vein thrombosis.
Electrical impedance measuring devices have also been used to diagnose PVD. These devices operate on the principle that a decrease in the electrical resistance of a limb is directly proportional to the increase in the blood volume. Because of a relatively high occurrence of false, positive results, clinical use of these devices is declining.
Although digital subtraction angiography is not invasive in that it does not require catheterization of the arteries, it does use radiation and the injection of a radiopaque tracer dye. A computer "subtracts" the X-ray picture after the dye is in the arteries from an X-ray taken before the dye is injected. Although this system is accurate, there is still a need for a simplified, non-invasive PVD diagnostic technique that involves no radiation or injection.
It is most important that the extent of PVD be diagnosed accurately and timely. In order to prevent gangrene, if PVD cannot otherwise be treated, amputation is frequently required. Obviously, the drastic step of amputation should not be undertaken unless PVD has progressed sufficiently. Therefore, false positive tests, giving incorrect evidence requiring amputation, should be avoided. The failure of a test to yield positive results when truly indicated is also a problem in that PVD can be life threatening. Once PVD is detected, preventative treatment can be timely started to avoid or delay amputation.
Once PVD has progressed beyond the stage that it can be treated to the point that amputation is required, a surgeon must decide where on the leg to amputate. If the amputation is done too low, PVD prevents sufficient blood flow to heal the amputation wound. If the amputation is too high, larger and more complicated prostheses will be necessary. This is especially true in deciding whether to amputate above or below the knee. Therefore, it is important to be able to determine the extent of PVD and its location.
With the exception of simple blood pressure testing and some doppler applications, the systems and methods discussed are performed in a hospital. This substantially increases their expense and decreases their general availability. What is desirable, therefore, is a simple, reliable and accurate test to measure vascular circulation in a physician's office (or even in a patient's home) both for initial screening and subsequent evaluations of PVD.
It is known that changes in the skin temperature of an extremity may indicate the presence or absence of blood flow. Heat transfer from the blood in the subcutaneous capillary bed to the skin is a function of the increase or decrease in blood supply from the arteries and the rate with which the blood is emptied from the capillary system by the venous system. Although factors such as muscle tone, activity, posture, venimotor tone, disease, reflexes and drugs affect cardiovascular function and blood flow, these affects can be minimized by environmental controls. Skin thermodynamics is therefore related to cardiovascular function insofar as cardiovascular function affects peripheral and capillary blood flow. Obstructions or occlusions in the peripheral vascular system will have a direct, measurable affect on heat transfer at the skin's surface.
The present invention may also have application in diagnosing all diseases that affect skin temperature and that now are diagnosed using infrared scanning.