Bladder Contraction EMG
Kristin La Fortune and Elese Hanson
September 29, 2003
Advisor: John Webster, Department of Biomedical Engineering
Client: Wade Bushman, M.D., Ph. D., Division of Urology, UW Medical School
(http://www.drrajmd.com/bladder)
Abstract:
While much is known about the electrical signal generated by both cardiac and skeletal muscle, very little is known about the signal generated by smooth muscle, specifically the smooth muscle of the bladder. Our client, Dr. Wade Bushman, desires a non-invasive method of detecting the contraction of the bladder, which is primarily controlled by the detrusor muscle. Thus, the device would measure the electrical signal generated by this muscle. Proposed is a method of using electromyography to measure the electrical signal. By using electrodes placed in formation about the pubic bone, we hope to design a “bladder EMG” which can be used by our client.
Problem Statement:
Our client desires an EMG that can be used to measure the electrical signal generated by the bladder. This problem has two parts: the EMG circuit design, which must be altered in order to amplify the correct signal within the desired frequency range, and the electrode design, which must be able to detect the electrical signal of the bladder. The signal should be displayed on a screen that can be used in a clinical setting.
Description of product function:
The device should be capable of obtaining and recording high-quality signal of the human bladder. It must be easy to use for the physician under all clinical circumstances that may arise. Additionally, the device must be comfortable and safe to use on a patient.
Client’s statement of product requirements:
The client wants an EMG capable of recording electrical signals of a human bladder during contraction of the detrusor muscle. The ECG that is currently used employs three external electrodes placed on the right and left arms and left leg to obtain a signal from the heart. This design is noninvasive and safe to use on a patient. Ideally, we would like our device to be designed with similar capabilities. The client would use our bladder EMG in order to learn more about electrical signal propagation through the bladder and the function of the detrusor muscle. The cost of our bladder EMG should be less than $200.
Background Information:
The urinary bladder is a balloon-like chamber located in the lower abdomen behind the pubic bone. In the male, the anal canal is right behind the bladder and the prostate is located right under the bladder, around the urethra (Figure 1). In the female the uterus and vagina lie in between the bladder and the anal canal (Figure 2).
Figures 1 & 2: Median sagittal section of male and female pelvis
(http://www.drrajmd.com/bladder)
The bladder is comprised of four layers of muscle and connective tissue, specifically the serosa, the detrusor muscle, a submucosal coat of connective tissue, and the mucosa. The openings of the dexter and sinister ureters form a triangle located near the bladder outlet that composes the beginning of the urethra (Figure 3). When the bladder fills, the ureters are compressed which results in a one-way flow, prohibiting reflux of urine from the bladder to the kidneys.
When the bladder is full, urine exists the body through a tube called the urethra. The internal and external urethral sphincter muscles surround the base of the detrusor to help control the outflow of urine. Pelvic floor muscles under the bladder also help keep the urethra closed. When the detrusor is relaxed, the internal urethral sphincter is closed. When the detrusor muscle contracts, the bladder changes shape and tends to pull open the internal urethral sphincter.
Figure 3: Labeled regions of the urinary bladder
(http://www231.pair.com/grpulse/bt/anurur.html)
The autonomic nervous system regulates detrusor muscle contraction via sympathetic and parasympathetic neurons. The state of contraction depends on the balance between the two influences. Urination is mediated by inhibition of sympathetic nerves followed by activation of the parasympathetic reflex pathway. When the bladder fills with urine, the pressure increases and triggers stretch receptors (located inside the bladder wall) to stimulate parasympathetic neurons that cause the detrusor muscle to contract. At the same time, the stretch receptors inhibit sympathetic neurons that cause the internal urethral sphincter to relax.
At the molecular level, the neurotransmitter acetylcholine in the parasympathetic nervous system causes the detrusor to contract whereas norepinephrine from the sympathetic nervous system facilitates urine storage by signaling the detrusor to relax and the urethra to contract. Both neurotransmitters affect cytoplasmic calcium concentrations, which control smooth muscle contraction through G-protein coupled receptors.
Literature Searches:
As mentioned above, detrusor muscle contraction occurs via cytoplasmic calcium concentrations. In smooth muscle, T-tubules are absent and the sarcoplasmic reticulum is poorly developed and therefore leads to slow contractile motion. Generally, smooth muscle takes thirty times longer to contract and relax as skeletal muscle in addition to using much less energy. Neuromuscular junctions are not present in smooth muscle either, so neurotransmitters acetylcholine and norepinephrine are released in close proximity to the muscle and diffuse to the cells. Neuronal input modifies rather than initiates muscle activity (unlike skeletal muscle). Local tissue factors, hormones and mechanical stretch initiate action potentials in the bladder.
According to physiologist Dr. Oretel, using electromyography to measure smooth muscle potential in the bladder would be extremely difficult due to the different mechanisms by which smooth muscles are activated. Most often, the results from electromyographic recordings on the bladder in situ are dismissed as mechanical artifact generated at the tissue/electrode interface as the muscle contracts.
The article, “A non-invasive method for bladder electromyography in humans,” discusses a possible way to measure the electrical signal of bladder contraction using six Ag-AgCl electrodes in front of the pubic bone on the lower abdomen. The researchers show that there is around a .5 mV change once voiding is initiated. This change however could be a result of skin motion artifact, which causes voltage changes of up to 5 mV across the 30 mV skin potential. One way to test whether that is the cause is to abrade the skin and reduce the skin potential.
Design Alternatives:
Since the purpose of our project is to design an EMG that can be used on the bladder, our design is twofold. First, we must design the EMG circuit. Second, we must decide the ideal location for electrodes and the type of electrodes to be used in order to attain a quality signal.
Circuit Design:
Our circuit design would be a basic EMG circuit modified for the correct frequency and bandwidth associated with the bladder. Shown in Figure 4 is a schematic of the EMG circuit. Electrodes detect the signal, which is then amplified, filtered, and converted. The signal is then recorded and processed via a computer software program.
Figure 4: Schematic of EMG circuit
(http://www.math.princeton. edu/~simas/ecg.html)
Shown in Figure 5 is a standard ECG circuit. The ECG circuit amplifies the electrical activity in cardiac tissue. Because the smooth muscle of the bladder differs from cardiac muscle, the resistors and circuit elements would need to be to detect an entirely different signal.
Figure 5: Standard ECG circuit
Electrode Design Options:
Surface Electrodes
Our first design option details surface electrodes placed in an array along the pubic bone. This design is entirely noninvasive, using Silver/Silver chloride electrodes placed on the skin of the pelvic region. Additionally, these electrodes are disposable and therefore would not need to be sterilized between uses. However, the electrical signal from the bladder would have to be detected through bone and surrounding muscle. This may prove extremely difficult due to motion artifacts and signals created by the surrounding skeletal muscle.
Figure 6 and 7: Array of Commercial Ag-AgCl Surface Electrodes (http://www.mds.qmw.ac.uk/biomed/kb/grossanatomy/basic _anat/pelvic_soft.htm and http://www.oxford-instruments.com/MDCPDP346.htm)
Catheter Electrodes
Our second design option entails inserting a catheter electrode into the urethra or vagina. Placement into the urethra allows for direct contact with the detrusor muscle. Consequently, surrounding muscle would not be as big of a problem in terms of interference. If the catheter is placed in the vagina, it is almost directly on top of the bladder and therefore, would also provide a high quality signal. Figure 8 shows a side view of the electrode placement.
Figure 8: Catheter electrode placed in the urethra
(http://www.medtronic.com/ neuro/mfd/consumables/acc_cat_2k1_trans.pdf)
With the catheter electrode, placement is not as specific as with a surface electrode. In order for the electrode to maintain stable contact during voiding, some sort of suction would be needed to hold the electrode in place. With both placement options, noise due to motion artifact caused by the bladder would be a larger problem. While the signal obtained via catheter electrodes may be of better quality than with surface electrodes, this design is not entirely in line with our client’s specifications. His intention is to have an entirely external device, thus catheter electrodes are not ideal.
Decision Matrix:
To aid in choosing a preferred design, a decision matrix was employed. Each design alternative was evaluated on how well it met the design requirement, with +2 meaning it met the specifications, -2 meaning it failed to meet the requirement, and 0 mean it is unclear or was not particularly good or bad.
Signal Quality / Ease of use / Cost / Feasibility / Invasiveness / TotalSurface Electrodes / 0 / +2 / +2 / +2 / +2 / +8
Catheter Electrodes: Inserted via the Vagina / +1 / -1 / 0 / +2 / -1 / +1
Catheter Electrodes: Inserted via the Urethra / +2 / -2 / 0 / +2 / -2 / 0
Proposed Solution:
The client selected the proposed solution for this project. It consists of replicating the system designed by the authors of “A non-invasive method for bladder electromyography in humans”. We will perform similar trials to test if the voltage change is actually a result of bladder contraction or if it is skin motion artifact. If the system does measure voltage change from the bladder, our client would like us to modify the system so that he can use it in clinical applications.
Potential Problems:
Although the proposed solution has already been designed, it will definitely be difficult to replicate the entire system. The materials and methods section in the article describe a lot of precautionary procedures used to eliminate unwanted signal contributions picked up by the electrode cables. We will also need to obtain the proper electronic equipment, which may be expensive if it is not available.
Another potential problem may arise when trying to obtain a protocol to test the device, especially when it involves humans. Reproducing the trials will require a lot of time and it might be necessary to contact the authors for their recommendations. The article stated, “Extensive control experiments have to be designed and conducted to further validate the physiological advantages of this non-invasive method for recording bladder EMG above the methods described in literature.”
Future Work:
Our future goals include designing a system similar to the device in the article entitled “A non-invasive method for bladder electromyography in humans”. Then we need to test it to see if the voltage change is a cause of skin motion artifact. We should also start researching whether we need a protocol or if that is the client’s responsibility.
References:
Ballaro A, Mundy AR, Fry CH, and Craggs MD. Bladder electrical activity: the elusive electromyogram. BJU International, 2003. 92: 78-84.
“Catheters and Transducers.” Medtronic.
http://www.medtronic.com/neuro/mfd/consumables/acc_cat_2k1_trans.pdf September 25, 2003.
Kinder MV, van Waalwijk ESC, Gommer ED, and Janknegt RA. A non-invasive method for bladder electromyography in humans. Archives of Physiology and Biochemistry, 1998. 106: 2-11.
“Pelvic Soft Tissue Structures.” Barts and the London, Queen Mary’s school of Dentistry and Medicine.
http://www.mds.qmw.ac.uk/biomed/kb/grossanatomy/basic_anat/pelvic_soft.htm September 25, 2003.
Kinder MV, van Waalwijk ESC, Gommer ED, and Janknegt RA. A non-invasive method for bladder electromyography in humans. Archives of Physiology and Biochemistry, 1998. 106: 2-11.
“TECA NCS Disposable Surface Electrodes.” Oxford Instruments. http://www.oxford-instruments.com/MDCPDP346.htm September 25, 2003.
http://www.math.princeton.edu/~simas/ecg.html September 20, 2003.
Problem Design Specification Version 1.0
Title: Bladder Contraction Detector
Group Members: Elese Hanson, Kristin la Fortune
Date: September 25, 2003
Problem Statement:
Our client desires an EMG that can be used to measure the electrical signal generated by the bladder. This problem has two parts: the EMG circuit design, which must be altered in order to amplify the correct signal within the desired frequency range, and the electrode design, which must be able to detect the electrical signal of the bladder.
Client Requirements:
Dr. Bushman desires a non-invasive method of detecting the contraction of the bladder. He would like the equivalent of a bladder EKG that could detect, by transcutaneous electrode recording, the electrical activity associated with involuntary contraction.
Design Requirements:
Performance Requirements:
The device will be used 2-3 times daily. It should be able to detect the electrical signal produced by the bladder during contraction.
Safety:
Safety considerations associated with a bladder EMG are the same as those associated with an EKG. The FDA guidelines should be followed.
Accuracy and Reliability:
Our client desires the signal obtained to be both measurable and reproducible.
Operating Environment:
The device should be able to withstand the physical environment of the human body and temperatures between 37° C and room temperature.
Weight:
The device should be maneuverable. A good estimate for maximum weight is 5 lbs.
Materials:
While the circuit component of the device will be reused, the electrodes which contact the patient must be clean. Their materials need to be able to withstand sterilization or be disposable.
Life in service/shelf life:
The circuit should be able to be used repeatedly and last up to 20 years. The electrodes will be able to be used at most 2-3 times before being disposed.
Ergonomics:
The device must be durable enough to be used on a daily basis. The circuit components must be securely placed and of high quality.
Size:
Size depends on whether the final device is chosen to be internal or external. The circuit bread board should not be larger than 5” x 5”.
Appearance:
The circuit should be professionally constructed so as to be of highest quality in appearance. The connection from the circuit to the electrodes needs to be secure and not bulky.
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