1 Pulse Transit Time Measuring Device BME 200/300 University of Wisconsin – Madison October 25, 2006 Team: Jonathan Baran – Team Leader Karen Chen –BSAC William Stanford – BWIG Mark Yarmarkovich – Communicator Client: Christopher G. Green, M.D. Pediatric Pulmonology University of Wisconsin Hospital Advisor: Wally Block Department of Biomedical Engineering University of Wisconsin – Madison2 Abstract Sleep apnea is a disorder affecting many children, causing them to awake from sleep to unblock their airways. Pulse transit time, the amount of time it takes for the pressure wave from the heart to travel to the finger, indirectly measures the presence of the disorder and its severity. Currently, only the tests conducted in sleep labs can test for sleep apnea and record its affects on the body. The work on this project will be in improving a device created by a previous engineering group that measures pulse transit time. The ultimate goal of the project is to create a device that can be used at home by the incorporation of a data logger that can record electrical signals from ECG and plethysmograph leads, improving the signal to noise ration by including an instrumentation amplifier and by modifying existing software to better detected the peaks of the stored voltage data.3 Table of Contents Background 3 Motivation 5 Client Requirements 5 Problem Statement 6 Previous Setup Electrocardiogram Plethysmograph circuit LabView 7 10 12 New Setup ECG circuit Design Setup Software Testing 14 17 20 25 Future Work 264 Background Information Sleep Apnea Sleep apnea is a sleep disorder where the patient has a pauses breathing during sleep. Typical breathing rates occur anywhere from 10-20 breathes per minute. During sleep apnea, the tongue blocks the airway and a 10-30 second pause in breathing occurs, causing the sufferer to miss one to two breathesi. This problem could occur repeatedly during a night of sleep, which lowers the sleeping quality. Patients would experience symptoms such as snoring and restless sleeps. Moreover, they would have poor day performance, loss of concentration, anxiety and other negative health effects. Researchers have shown that people who are over the age of 40 are at higher risks of sleep apnea. However, it can affect people at any ages. There are three different forms of sleep apnea – central sleep apnea, obstructive sleep apnea and mixed sleep apnea. Central sleep apnea causes pauses in breathing by the lack of effort in breathing. This is due to the failure of neurons in sending signals to indicate inhalation. In effect, the level of oxygen delivered to tissues decreases and oxygen is not available for cellular respiration. On the other hand, obstructive sleep apnea is where the air path inside the throat is blocked by an object, such as the tongue. As the muscles relax during sleep, the tongue can block the airway (as seen in Figure 1), which causes the patient to enter a lighter sleep stage or possibly cause the patient to awaken. Most patients suffering from obstructive apnea have trouble getting into a deep sleep state. Even though the light sleep time may be numerous, it is still not as effective as deep sleep. Finally, mixed apnea is the combination of central and obstructive sleep apnea. While obstructive sleep apnea takes place during sleep, central sleep apnea is often developed. Patients experience problems breathing and constantly wake up from sleep because of long-term obstructive apneai.5 The pauses in breathing during sleep result from a decrease in the oxygen concentration of the blood. Furthermore, the level of carbon dioxide increases. The worst-case scenario is when the oxygen level becomes so low that it causes brain damage, and eventually leads to death. Figure 1: The left figure shows a normal trachea opening. The right figure shows that of a person who has multiple obstructions. Pulse Transit Time Pulse transit time (PTT) is a noninvasive method of measuring respiratory changes in children with breathing sleep disorders. PTT is the measure of the time it takes for the pulse pressure wave to go from the heart to the peripheryii. It is measured by use of both an ECG machine and a pulse oximeter. An ECG machine generates a curve based on the depolarization of the heart while the oximeter measures the pressure wave, or pulse, at the tip of the fingeriii. A value for pulse transit time is given by calculating the difference in time between the peak of the R wave from the ECG and the peak of the pressure wave from the oximeter (Figure 2).6 Normal PTT range from 250-350 millisecondsiv; a significant variation in this time can help identify sleep apnea in two ways. First, as blood pressure decreases the arterial wall stiffness decreases. As stiffness decreases, it causes the pulse to take a longer time to reach the finger, causing on increase in PTT. This increases helps to diagnose sleep apnea. Second, the increase of blood pressure as the obstruction clears increases arterial wall. The increases in stiffness increases blood pressure, causing pulse transit time to decrease. Decreases in PTT in patients with sleep apnea can range from 15 to 50 milliseconds. Any decrease over 50 milliseconds is anatomically impossible. This decrease in PTT can help diagnose the severity of the apneav. Motivation Some consequences of prolonged sleep apnea are hyperactivity, poor daytime performance, loss of concentration and other negative health effects. This sleep disorder can occur in both adults and children. To detect this disorder, patients need to participate in sleep studies at sleep centers. This is a rather expensive study to operate on a day-to-day basis, and thus its utilization is limited. Furthermore, sleep centers are not specifically for sleep apnea studies. Thus, a small, portable instrument that is able to conduct sleep studies at home would largely benefit the patients. This medical device must be able to detect PTT. Ideally, this device should Figure 2: Calculation of pulse transit time from ECG and oximeter graphs.7 be as small as a flash drive that needs a low power supply. In addition, the number of wires should be reduced to allow more room for the patient to move. This allows easy access and operation, and the patient can self-detect sleep apnea in earlier stages. Client Requirements The design must be able to be used with children. Our client works in pediatrics and
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