6.11. TESTING A PACEMAKER

Transcription

1 Purpose of experiment To examine a pacemaker s signals. Tasks of experiment TESTING A PACEMAKER Using an oscilloscope to measure pacemaker s parameters: voltage amplitude, pulse repetition rate, pulse duration and duration of front and back slopes, pulse energy, pacemaker s lifetime. Theoretical topics Electrical phenomena of human body. Heart electrical system. Structure of a pacemaker. Pacemakers electrodes. Electroshock. Electrical safety. Equipment and materials Oscilloscope, pacemaker, connecting wires. Theoretical part The heart is a single muscle, which produces an electrical signal when the muscle depolarizes. The sinus node on the wall of the right atrium of the heart of generates an action potential at a repetition rate depending on the work done by the body. Those potentials travel through a conduction system of the heart forcing myocardium contraction and thus pumping blood. In each case, the pulse parameters should depend on the patient's age, body temperature, hemodynamic and other factors. The measurement of this electrical signal (the electrocardiograph) is one of the oldest medical techniques. The ECG/EKG was first recorded by Einthoven in the Netherlands and Waller in England in 1895 (please, read more in the description of lab. work No 6.10.). When the heart cannot by itself generate natural electrical signals or they are not strong enough, electrostimulation is used. The main idea of electrical stimulation is that natural biological electrical pulses are replaced by external artificial electrical pulses. The idea of electrically stimulating the heart dates back to the beginning of the last century with the work of Burns and Aldini (a nephew of Galvani). But it was 1927 by the time Hyman built the first functioning external pacemaker, a small electric clockwork-driven generator. In 1948, Shockley, Bardeen, and Brattain invented the transistor and made it possible to drastically reduce the size of electric switching units, which also advanced the development of pacemaker design. The first implantable pacemaker was implanted in 1958 by a Swedish doctor named Elmquist. This device was made of 20 discrete components and weighed about 180 g. Pacemakers today weigh FBML TESTING A PACEMAKER 1

2 about 60 g and have the functionality of a small computer. Pacemaker therapy is based on the delivery of current pulses, which lead to the artificial depolarization of some cardiac cells. They go across the conduction system of the heart, as well as across the gap junctions (intercellular ionic bonding channels, which serve to transmit the excitement directly from cell to cell), triggering a complete contraction of the heart. Based on this triggering effect of artificial pulses, this is referred to as the all-or-nothing law. Nowadays, the electrocardiostimulation system consists of the following main components: of Pulse generator Batery Connecting wires Electrode Fig Electrocardiostimulation system. Heart an external programming device and the actual cardiac pacemaker (Figure ). The pacemaker consists of a battery, electronic elements (pulse generator and wires) and electrodes, and must match the parameters of the action potential produced by a real sinus node in the heart: amplitude, frequency, duration of the pulse and its slopes. The electrode is advanced almost exclusively via a venous access (e.g., the vena cava sup.) underneath the collarbone into the right half of the heart and anchored there. A permanent pacemaker is enclosed in a case and inserted into the body. Afterward, the pacemaker is connected and implanted subcutaneously in a pocket of skin. In addition, it must conform to the medical and biological requirements: all parts must be sealed, and the materials should be non-reactive for body tissues. A tightly sealed housing made of stainless steel or titanium prevents contact between the internal electrical circuit and the heart tissue. The electronics are usually designed as a hybrid, multichip module. The once-used thick film technique is being increasingly replaced by multilayer ceramics due to the increasing complexity of the circuits. Recently, brand new PCB (printed circuit board) technologies have been attracting interest, which, thanks to their flexibility, allow electric connection technology to be considerably simplified, thus improving the device quality. When the pacemaker is programmed, the system should be optimally adapted to the needs of the patient, and the lifetime of the battery should be maximized. With the long running times of today, it cannot be assumed that the conditions on the day of implantation will remain constant over the entire lifetime of the pacemaker. It is therefore necessary to be able to adapt the pacemaker to changing conditions. External devices that are usually based on a PC platform and drive manufacturer-specific shortrange telemetric equipment are used for programming. This data transmission, usually based on inductive nearfield coupling in a range below 100 khz, allows the implant to be queried/reprogrammed. To do this, the programming head is first placed on the skin above the pacemaker housing. Afterward, the current pacemaker parameters, as well as diagnostic values (markers, event histograms, ECG, etc.), are transmitted out and evaluated. Finally, the updated parameters are reloaded into the implant. The pacemaker electrode or probe establishes the connection between the pacemaker and the heart. It consists of a connector, i. e., the plug for connection to the pacemaker, the electrode conductor, and the electrode tip (Fig ). Fig Schematic structure of a pacemaker electrode. The connector configuration has been standardized according to the IS-1 standard, so that all modern electrodes can be connected FBML TESTING A PACEMAKER 2

3 to any pacemaker. One of the main technical challenges is the high bending stress of about 40 million load changes per year. For this reason, electrode conductors nowadays are made of four individual coiled wires, which provide the highest possible reliability with high flexibility. With a coiled wire, the bending radius is reduced, which reduces the alternating load stress on the feed line material (usually special stainless steel). For this reason, electrode breakage is no longer a major problem. Special demands are placed on the insulation material. It must be biocompatible and also be able to withstand the mechanical and chemical loads in the body. Right now, insulation made of silicone and polyurethane are being used. Polyurethane has a much lower friction coefficient, which is why it is especially preferred when implanting several electrodes via the same vessel. Furthermore, a distinction is made between uni- and bipolar pacemaker electrodes). Unipolar means that the electrode tip acts as the cathode and the pacemaker housing (or another counterelectrode with a large surface area) acts as the anode. Due to the considerably greater surface area of the pacemaker compared to the electrode tip, the housing is also called an indifferent pole. Bipolar probes also work with a cathodic tip. The anode, however, is also placed in the distal electrode area (about 2.5 cm away from the tip). Bipolar probes are somewhat thicker and more rigid than unipolar electrodes since they have two feed line coils. It is hardly possible to repair electrode breakage. A widespread compromise is the implantation of bipolar probes in the atrium of the heart (a smaller potential requires a higher sensitivity and the suppression of interference signals) and unipolar probes in the ventricle area. In addition, there are two types of the electrodes connections: endocardial (Figure a) and epicardial (Figure b). a b Fig Types of the electrodes connections: endocardial (a) and epicardial (b). Usually a pacemaker produces a constant sequence of short semirectangular pulses (Figure ). The electrical pulses produced by a pacemaker travel to the heart muscles through insulated wires ending in needle electrodes which touch the heart tissue. The effective amplitude of the pulses should generate an action potential in tissue touched by an electrode. The effectiveness of T that process depends strongly also on the front U a,v slope of the pulse, being higher for an extremely steep slope (slope of a short duration). In general, the duration of the slope is of the order of microseconds, while i requirements for the back slope of the pulse are very minor. As for the potential Time, t Fig Sequence of pacemaker s output pulses. FBML TESTING A PACEMAKER 3

4 (amplitude of pulse) it should be considerably bigger than the action potential of a cell (100 mv), for most pacemakers it is in the range of several volts ( V) to overcome possible losses in the electrode-tissue contact. A pacemaker stimulates at a defined rate approximately from 25 beats/min to 155 beats/min. Frequency-adaptive pacemakers allow the heart rate to be increased in stressful situations, even in patients with sinus node dysfunction. They are therefore indicated for physically fit patients who have an insufficient heart rate increase under stress (chronotropic insufficiency). The frequency adaptation is achieved via sensors, which determine the stress-related parameters and relate these to the stimulation frequency (so-called interference coupling). Nowadays, lithium-iodide batteries (opencircuit voltage about 2.8 V, capacitance about 1Ah, inner resistance a few 100 and 50 kω) are usually used as a power source. With this type of battery, the anode is made of lithium and the cathode of iodine. In addition to its extraordinarily low selfdischarge (< 1%/year), the lithium-iodide battery offers high stability of the inner resistance beyond the discharge time. Only at the end of the service life does the open-circuit voltage rapidly drop. Lithium-iodide batteries thus offer the greatest possible reliability with their long lifetime, small dimensions, and light weight. Today, they have running times ranging from 5 to 10 years with this technology. The trend toward primary cells, or rechargeable batteries, which are at the beginning of their technical development and are occasionally discussed even today, offer no major advantages. The supposed prolonged service life must be weighed against the drastic increase in aging-related failures after about 10 years, which makes an implant change recommendable after this time, anyway. Research has recently been carried out with nuclear power sources, but in this case there is a problem of introducing radioactive material into the body. The lifetime of each source depends on its capacity - accumulated electrical charge q, which is usually expressed in ampere-hours (Ah). Knowing this value, it is possible to evaluate the maximum lifetime of a pacemaker. The pulse shape should be nearly rectangular with an extremely steep front slope. The energy required to generate a single pulse Ei can calculated by: E i 2 U m, (6.11.1) R k here Um is the amplitude of the pulse measured in Volts, Rk is the resistance of the tissue, τ duration of a pulse measured according to Figure can change from 0.1 to 2.3 ms. It is possible to evaluate the life time of a pacemaker taking into account only the battery capacity: QT t, (6.11.2) U m R k here t lifetime of the pacemaker (battery), Q initial charged energy of the battery, T period of pulses. Note that 10% of the amplitude is neglected, to prevent uncertainty when fixing the start and end of the pulse slope due to the rounded start or end of the pulse slope. The repetition rate of modern pacemaker pulses is adjustable, responding to the workload of the body and reacting in the same way as a normal heart. Fig Shape of a single pulse (image on the oscilloscope screen). FBML TESTING A PACEMAKER 4 1 0,9 0,1 0 D C E

5 Methodology We can gain information about processes in the human body by measuring such dimensions as: temperature, blood flow rate, lung volume, air flow velocity in the airways, chemical composition of inhaled and exhaled gas, and blood sugar. These and other data are recorded by the equipment based on their physical properties and finally are converted to electrical signals. The on-line device for thorough examination of electrical signals is an oscilloscope. The main technical data for the oscilloscope DSO TDS210 or TDS220 used in the laboratory: deflection values of vertical (y axis) channel: 5 V/grid to 2 mv/grid, X - axis scan speed: from 5.00 s/grid to 5.00 ns/grid. Procedures 1. Plug the power cable into the mains socket and switch the oscilloscope "POWER" on. Wait for confirmation that the oscilloscope s selfmonitoring is complete. 2. Connect the pacemaker s output wires to the oscilloscope. 3. Choose the desired x and y axis values to get a stable image of about three pulses well fitted to the oscilloscope screen (fig ). 4. Copy the pulse image into your laboratory digest for further examination. 5. Make a readout of the parameters: a) voltage amplitude Um : B A k 1 k 3 Fig Pulse signal oscillogram. U m Ak 1, (6.11.3) here A is the vertical length (in grid) of an amplitude on the screen and k1 is the value of the voltage indicator. b) peacemaker pulse period T : T Bk 3, (6.11.4) here B is the horizontal distance (in grid) between two pulses on the screen and k3 is the value of the time scale indicator. 6. Calculate pulse frequency ν : 1/T. (6.11.5) 7. Repeat the measurements of each parameter several times to improve accuracy. Calculate the mean values of the parameters. 8. Select an x axis value so that only one expanded pulse is visible on the entire screen (as shown in Figure ). FBML TESTING A PACEMAKER 5

6 9. Copy the pulse image into your laboratory digest for further examination. 10. Measure C, D, E characteristics and calculate pulse duration τ : Ck 3, (6.11.6) and also leading slope τ f1 and ending slope τ f2 : f 1 Dk 3 and f 2 Ek 3. (6.11.7) 11. Calculate the energy Ei required to generate a single pulse using equation (6.11.1). The resistance Rk used in this experimental setup is 700 Ω. 12. Evaluate the expected maximal lifetime of this pacemaker using equation (6.11.2). Initial battery energy in this experimental setup is 0,5 Ah. Table 1. Um, V T, s, Hz, ms Ei, J t, years References 1. Irving P. Herman, Physics of the human body, Berlin; Heidelberg: Springer (2007). 2. Ruediger Kramme...[et al.], Springer handbook of medical technology, Berlin etc.: Springer, 1500 p., 2011, ISBN: B H Brown, R H Smallwood, D C Barber, P V Lawford and D R Hose, Medical Physics and Biomedical Engineering, New York N.Y.; London: Taylor and Francis (1999). 4. Arthur C. Guyton, John E. Hall, Textbook of medical physiology, Philadelphia Pa.: Saunders/Elsevier (2011). FBML TESTING A PACEMAKER 6