Capacitor Device for Air Bubbles Monitoring (PDF Download Available)

International Journal of Electrical & Computer Sciences IJECS Vol: 9 No: 10

12

Capacitor Device for Air Bubbles Monitoring Mawahib Gafare Abdalrahman Ahmed Applied physics Electrical and Instrumentation, Faculty of Engineering & Technology, University of Gezira Wad Medani, Sudan [email protected] Abdallah Belal Adam Abstract— Emboli or Air bubble detection is vital for many medical procedures involving Extracorporeal Blood Circuits or ECBC e.g. hemodialysis, hemofiltration and cardiopulmonary bypass .This risk has increased as negative pressures are developed between the arterial fistula needle and the blood pump in the ECBC machine. This study suggests a capacitor device to detect the presence of air bubbles by measuring change in the output voltage. The capacitor has two platinum plates with area 0.75 cm2. The distance between the plates is 1 cm and Dextran70 fluid is introduced as the dielectric material between the plates. This solution replaces blood to avoid clotting whilst conducting the experiment. This device is connected with a resistor i.e. 100 kΩ to form a low pass filter circuit. The change of output voltage is measured when there is a change of dielectric material of the capacitor due to the appearance of an air bubble in the stream. The sensitivity of the capacitor device is found to be 8.28 mV/ nF at 2 MHz Keywords-air bubbles; extracorporeal blood circuits; capacitor; dextran70; dielectric material

I.

INTRODUCTION

The rapid advances of technology in the past few decades have brought new areas to medical practice. One recent development is the detection of air bubbles in many medical procedures. Using different types of detectors, clinicians and researchers have found that the phenomenon of air bubbles is widespread [1]. Air bubbles originate mainly in extracorporeal blood circuits and devices such as cardiopulmonary bypass and hemodialysis machines. Air bubbles circulate in the blood stream and lodge in the capillary bed of various organs, causing local reactions such as tissue ischemia, necroses which are sometime fatal. Air bubbles usually originate in extracorporeal tubing, infusing with the fluids into the blood stream. The bubbles may be present while priming and preparing the lines for use; formed by the blood pump [2]; or newly formed as a result of turbulent flow in the tubing and at the vascular access. Differences in temperature are another possible reason for bubble generation in lines [3].The movement of the bubble in an extracorporeal infusion set is affected by two principally opposing forces: firstly, the buoyant force of a bubble, which takes it upward in a standard drip chamber; and secondly, the driving force of the fluid flow, by which the bubble is carried into the patient’s body.

Physics Dept. Faculty of Science, University of Hail, Saudi Arabia [email protected]; [email protected] John Ojur Dennis, and Gail Sylvia Steele , Universiti Teknologi PETRONAS, Bandar Seri Iskendar, Tronoh, Perak, Malaysia

Micro bubbles as a medical hazard were first recognized in open heart surgery several decades ago with ultrasound and Doppler technology used as detection tools [4, 5, 6]. These tools uncovered signals of air emboli originating in extracorporeal lines and tubing [7]. Many studies have shown that rapid infusion of air bubbles may be fatal [8, 9, 10]. They conclude that the entry of air into the venous or arterial system is a risk in virtually all areas of clinical care. Venous emboli may lead to cardiovascular collapse or to paradoxical arterial emboli. Arterial emboli may occlude end arteries throughout the body and may cause serious diseases or death if they occlude cardiac or cerebral vessels. Irrespective of the mechanism responsible for the embolism, rapid aggressive treatment is essential to preserve life and functions. The clinical outcome of air embolism depends on the size of the bubble, location i.e. organ or tissue, general status, comorbidity of the patient, plus many known and unknown factors [11]. Air bubbles in the blood stream cause stroke, memory loss and other undesirable effects in the patient. Scientific evidence from humans is limited; nevertheless, it supports most of the laboratory findings [12] Several different physical principles have been employed in air bubble detection such as infrared light (IR) source and photocell receptor. The earliest IR based air detectors consisted of a light source triggering a photocell situated on the opposite side of the bubble trap. The photo cell did not react if blood obstructed the light path [13]. A recent study using ultrasonic detector was conducted to remove air bubbles from the venous line of the extracorporeal circuit before they reached the circulation of the patient’s body. They designed an air bubble trapper contained in a main channel connected to the tubing of the hemodialysis machine. They found that when the bubble trapper was deactivated the number of micro emboli signal was higher than when the bubble trapper was activated [14]. But the accuracy of ultrasound measurements is poor for small diameter bubbles. Based on the literature reviewed, embolism by large or microscopic bubbles remains a concern and a known risk of all extracorporeal blood circuits [15, 16, 17].

93110-7878 IJECS-IJENS @ International Journals of Engineering and Sciences IJENS

International Journal of Electrical & Computer Sciences IJECS Vol: 9 No: 10 This study utilizes a capacitor device and Dextran70 to detect air bubbles ranging in size from 820 µm to 4 mm. II.

METHODOLOGY AND MATERIALS

To fabricate the detector cell, two plates of Platinum of area 0.75 cm2 are encased within an acrylic material to form a capacitor into which the Dextran70, fluid is introduced. Platinum was used because it is highly malleable, soft, and extremely resistant to oxidation and corrosion by high temperatures or chemical elements. More importantly, it is a very good conductor of electricity [18]. The structural design of the capacitor detector cell is shown in Figure 1. The distance between the two plates is nominally fixed at 1.0 cm with copper wires attached to measure the changes in the capacitance and the output voltage. The Dextran70 solution is then introduced between the plates. The Dextran70 solution used in the tubing system is a mixture of 1L isotonic water with 9 g NaCl and 40 g/ L Dextran70, manufactured by Sigma Aldrich and 50 Ml of a 20% concentrated Albumin solution [19] .This composition selected for the fluid mixture has been shown to closely mimic blood rheology. Measurements were taken for density using, a Densito30p made by Mettler Toledo, Japan with accuracy ± 0.0005 at temperature 36.8 ºC, viscosity using viscometer No.2F145 made by Cannon Instrumentation Company, USA. The viscometer constant was 0.0979 mm2/s2. To obtain the viscosity of the solution in mPa.s the efflux time in seconds was multiplied by the viscometer constant, multiplied by the density in g/mL. The viscosity was measured at 37ºC, temperature using a thermometer with accuracy ±1ºC made in China.

13

viscosity of the solution in mPa.s the efflux time in seconds was multiplied by the viscometer constant, multiplied by the density in g/mL. The viscosity was measured at 37ºC, temperature using a thermometer with accuracy ±1ºC made in China. To conduct the experiment, the prepared Dextran70 solution is introduced between the capacitor plates through inlet and outlet valves attached at both ends of the capacitor device. This device is then connected to a resistor with value 100 kΩ to form a RC circuit, and a frequency of 2 MHz is applied as shown in Figure 2. The input voltage from the signal generator is set at 2.5 volts. The valves are first opened to allow the solution to move between the capacitor plates, and then closed to keep the liquid in place.

Figure 2. The proposed circuit "A series RC filter circuit”

The output voltage is then measured via the lead wires connected to the capacitor plates without air bubbles. An air bubble of unknown diameter is then introduced in the Dextran70 solution contained between the capacitor plates, and the output voltage measured. Air bubbles with different diameters are used in order to investigate the change in capacitance as a function of the bubble diameter using two types of needles: one glass needle with 10 µm diameters and the second made of plastic with 980 µm diameters. Air bubble diameter was measured using Absolute coolant proof caliper and a magnification lamp for accuracy. The diameter of air bubble used ranges from 820 µm to 4 mm. III. Figure 1. Schematic of the capacitive detector device

The Dextran70 solution used in the tubing system is a mixture of isotonic water with 9 g NaCl and 40 g/ L Dextran70 [19]. This composition selected for the fluid mixture has been shown to closely mimic blood rheology. Measurements were taken for density using, a Densito30p made by Mettler Toledo, Japan with accuracy ± 0.0005 at temperature 36.8 ºC, viscosity using viscometer No.2F145 made by Cannon Instrumentation Company, USA. The viscometer constant was 0.0979 mm2/s2. To obtain the

RESULTS AND DISCUSSION

For this study the presence of air bubbles was detected by measuring the change in the output voltage across the capacitor in the circuit Table 1 shows the condition of the Dextran70 during the experiment. The solution appears to display close similarities to blood i.e. human blood has an average density of 1.06 g/cm3 [20] and an average viscosity of 3.3 mPa.s [19].


International Journal of Electrical & Computer Sciences IJECS Vol: 9 No: 10

14

TABLE 1 EXPERIMENTAL DEXTRAN70 SOLUTION COMPARED TO HUMAN BLOOD Density

Viscosity

Temperature

Material

(g/cm3)

(mPa.s)

(ºC)

Dextran70

1.057

3.41

37.1

Human Blood

1.060

3.30

36-37

The Dextran70 solution without air bubbles is introduced between the plates of the capacitor and the output voltage determined. Individual air bubbles of various diameters are then injected into the Dextran70 solution between the plates of the capacitor and the output voltage measured and recorded in each instance. Table 2 shows the experimental variation in the capacitance of the capacitor when the device contained Dextran70 with or without air bubbles and output voltage for air bubbles of different diameters. It is clear that when the air bubble diameter increases the output voltage increases. And the capacitance of the device decreases by increasing the air bubbles diameter. TABLE 2 MEASURED CAPACITANCE AND VOLTAGE FOR DIFFERENT AIR BUBBLE DIAMETERS Capacitance (nF)

Air bubbles diameter

Output voltage

(mm)

(mV)

43.50

0

18.60

42.63

0.82

20.3

42.54

1.00

22.01

41.07

2.97

40.50

40.44

3.55

43.70

39.38

4.00

47.90

Figure 3. The output voltage from the device at different air bubble diameters

The same figure illustrates the output voltage from the capacitor device when an even larger bubble diameter i.e. 2.97mm is introduced between the capacitor plates. It was found to be 40.5 mV. Therefore, the change in the output voltage between this diameter and a 1mm diameter air bubble was found to be 18.49 mV From Figure 4 it was clear that the relation between the air bubble diameters and the output voltage from the device is directly proportional. The linearity error was computed to be 1.9% and the sensitivity was found to be 7.926mV/mm. Figure 5 shows the relation between the capacitance and the output voltage from the device at 2 MHz is inversely proportional. It was found that when the capacitance decreased due to the appearance of the air bubble the output voltage increased. The linearity was found to be 5.5% and the sensitivity was found to be 8.28 mV/nF.

For example, change in the capacitance when a single air bubble of 820 µm diameter is introduced is 0.87 nF = 870 pF. The difference in the capacitance between an 820 µm bubble and a 1 mm diameter bubble is0.09 nF = 90 pF. These results seem to indicate that the device is capable of achieving greater sensitivity. Figure 3 is an oscilloscope reading of the capacitance device, before and after injection of air bubbles of different diameters. The Root Mean Squared Voltage (Vrms) was measured and found to be 18.6 mV, using a digital meter for the output voltage when there were no air bubbles. The figure also indicates an increase in the output voltage following injection of an 820 µm diameter air bubble. This change was measured and found to be 1.7 mV. The figure also shows the output voltage across the capacitor device when there is an air bubble with a diameter of 1mm in the stream. The output voltage increased and was found to be 22.01 mV compared to the output voltage from the previous diameter which was 20.3 mV.

Figure 4. The output voltage from the capacitor device as a function of air bubble diameter


International Journal of Electrical & Computer Sciences IJECS Vol: 9 No: 10 [1] [2]

[3]

[4]

[5]

[6]

[7] Figure 5. The output voltage as a function of capacitance device for several air bubble diameters

[8] [9]

IV.

CONCULSION

A single air bubble was injected into the Dextran70 solution between the plates of the device and a significant change in the capacitance of the capacitor device was observed. It was found that when the air bubble diameter increased the capacitance of the capacitor device decreased whereas the output voltage increased. Therefore, the relation between the air bubble diameters and the output voltage from the device is directly proportional. The linearity error was computed to be 1.9% and the sensitivity was found to be 7.926 mV/mm. The relation between the capacitance and the output voltage due to the air bubbles appearance is inversely proportional. The sensitivity of the device at 2 MHz was found to be 8.28 mV/nF.

ACKNOWLEDGMENT The authors would like to thank all staff at Ipoh Specialist Hospital and Hospital Raja Permaisuri Bainon. Special thanks to Dr. Norain Karim the head of Pathology Department and Dr. Kalaichelvi Muthiah and Dr. Ida Marhainis Bt. Isahak. Also thanks and gratitude must be given to the members of the Electrical and Electronic Engineering Department at Universiti Teknologi PETRONAS. Finally, Thanks and gratitude must be given to the members of the Faculty of Engineering and Technology, University of Gezira.

[10] [11]

[12]

[13]

15

Barak, M., and Katz, Y. (2005). Micro bubbles pathophysiology and clinical implications. Chest, 128, 2918- 2932. Woltmann, D., Fatica, A. R., Rubin, M. J., and Weitzel, W., “Ultrasound detection of micro embolic signals in hemodialysis accesses,” American Journal of Kidney Diseases, vol. 35, pp. 526- 528, 2000. Hartmannsgruber, M. W. B., and Gravenstein, N., “Very limited air elimination capability of the level 1 fluid warmer,” Journal of Clinical Anesthesia, vol. 9, pp. 233- 235, 1997. Gallagher, E. G., Pearson, D. T., (1973). Ultrasonic identification of sources of gaseous micro emboli during open heart surgery. Thorax, 28,295- 305. Daniels, S., Davies, J. M., Paton, W. D., and Smith, E. B. (1980). The detection of gas bubbles in guinea-pigs after decompression from air saturation dives using ultrasonic imaging. Journal of Physiology, 308, 369– 383. Ringelstein, E. B., Droste, D. W., Babikian, V.L., Evans, D. H., Grosset, D.G., Kaps, M., Makus, H. S., Russell, D., and Siebler, M. (1998). Consensus on micro embolus detection by TCD. International Consensus Group on Micro embolus Detection, Stroke, 29, 725-729. Barak, M., Nakhoul, F., and Katz, Y. (2008) Pathophysiology and clinical implications of micro bubbles during hemodialysis. Seminars in Dialysis, 21, 232- 238. Muth C. M., Shank E. S. (2000). Gas embolism. New England Journal Medicine, 342:476–482. Moehring, M. A. and Klepper, J. R. (1994). Pulse Doppler ultrasound detection characterization and size estimation of emboli in flowing blood. IEEE Transaction on Biomedical Engineering, 41, 35- 44. Petts, J. S., and Presson, R. G. (1992). A review of the detection and treatment of venous air embolism. Anesthesiol Rev, 19, 13-21. Sanfeld, A., Steinchen, A., and Steinchen, A., “Does the size of small objects influence chemical reactivity in living systems,” C R Biological, vol. 326, pp. 141–147, 2003. Kapoor, T., and Gutierrez, G., “Air embolism as a cause of the systemic inflammatory response syndrome: a case report,” Critical Care, vol. 7, pp. R98–R100, 2003. Keshaviah, P. R., and Shaldon, S., Hemodialysis monitors and monitoring, in replacement of renal function by dialysis a textbook of dialysis, 3rd Ed. Kulwer Academic Publishers, 1989, pp. 276- 299.

[14] [15] Jonsson, P., Karlsson, L., Forsberg, U., Gref, M., Stegmayr C., and Stegmayr, B. (2007). Air bubbles pass the security system of the dialysis device without alarming. Journal of Artificial Organs, 31,132- 139. [16] Kurusz, M., and Butler, B. D. (2004). Bubbles and bypass: an update. Perfusion, 19, S49- S55. [17] Jones, J. T., Deal, D. D., Vernon, C. J., Blackburn, N., and Stump, A. D. (2002). How effective are cardiopulmonary bypass circuits at removing Gaseous micro emboli? Journal of the American Society of ExtraCorporeal Technology, 34: 34- 39. [18] Stegmayr, C. J., Jonsson, P., Forsberg U., and Stegmayr B. G. (2007). Development of air micro bubbles in the venous outlet line: an in vitro analysis of various air traps used for hemodialysis. Journal of Artificial Organs, 31, 483-488. [19] Cutnell D. J., and Johnson K. W. (1998). Physics (4th Edition). Wiley: New York, p. 308. [20] Lagowski J. J., Chemistry Foundations and Applications, 3rd ed. United States of America: Thompson Gale, 2004, pp. 267- 268.

REFERENCES
