PreSSUre PORT UNBONDED STRAIN GAUGES FIGURE 114.1 An unbonded strain gauge pressure transducer. have the advantage that they are only used on one patient and they do not have to be cleaned and sterilized etween patients. By using them on only one patient, the risk of transmitting blood-borne infections is eliminated In biomedical applications pressure is generally referenced to atmospheric pressure. Therefore, the pressure in the chamber of the pressure transducer must be maintained at atmospheric pressure. This is done by means of a vent in the chamber wall or a fine bore, flexible capillary tube that couples the chamber to the atmosphere This tube is usually included in the electrical cable connecting the pressure transducer to the external instru mentation such that the tube is open to the atmosphere at the cable connecter In using this sensor to measure blood pressure the dome is coupled to a flexible plastic tube, and the dome and tube are filled with a physiological saline solution. As described by Pascals Law, the pressure in the dome, and hence against the diaphragm, will be the same as that at the tip of the tube provided the tip of the tube at the same horizontal level as the dome. Thus by threading the tube into a blood vessel, an invasive procedure, the blood pressure in that vessel can be transmitted to the dome and hence the diaphragm of the pressure transducer. The pressure transducer will, therefore, sense the pressure in the vessel. This technique is known as external direct blood pressure measurement, and the flexible plastic tube that enters the blood vessel is known catheter. It is important to remember that the horizontal level of the blood pressure transducer dome must be the same as that of the tip of the catheter in the blood vessel to accurately measure the pressure in that vessel without adding an error due to the hydrostatic pressure in the catheter In addition to problems due to hydrostatic pressure differ- DIAPHRAGM ences between the chamber and the dome, catheters introduce pressure errors as a result of the dynamic properties of L (Se EAD WIRES catheter, fluid, dome, and diaphragm. These properties as well bubbles in the catheter, or obstructions due to clotted CATHETER WALL blood or other materials, introduce resonances and damping. These problems can be minimized by utilizing miniature pressure transducers fabricated using microelectronic semi- SEMICONDUCTOR STRAIN GAUGES conductor technology that are located at the tip of a catheter FIGURE 114.2 A catheter tip pressure transducer rather than at the end that is external to the body. A general are integrated into the diaphragm of the transducer such that they detect very small deflections of this arrangement for such a pressure transducer is shown in Fig. 114.2. As with the disposable sensors, strain gaug phragm. Because of the small size, small diaphragm displacement, and lack of a catheter with a fluid column, these sensors have a much broader frequency response, give a clearer signal, and do not have any hydrostatic "It must be pointed out that the use of such a sensor is not limited to blood pressure measurement. The strain gauge pressure sensor can be used to measure the pressure of any fluid to which it is appropriately coupled e 2000 by CRC Press LLC© 2000 by CRC Press LLC have the advantage that they are only used on one patient and they do not have to be cleaned and sterilized between patients. By using them on only one patient, the risk of transmitting blood-borne infections is eliminated. In biomedical applications pressure is generally referenced to atmospheric pressure. Therefore, the pressure in the chamber of the pressure transducer must be maintained at atmospheric pressure. This is done by means of a vent in the chamber wall or a fine bore, flexible capillary tube that couples the chamber to the atmosphere. This tube is usually included in the electrical cable connecting the pressure transducer to the external instru￾mentation such that the tube is open to the atmosphere at the cable connecter. In using this sensor to measure blood pressure the dome is coupled to a flexible plastic tube, and the dome and tube are filled with a physiological saline solution.1 As described by Pascal’s Law, the pressure in the dome, and hence against the diaphragm, will be the same as that at the tip of the tube provided the tip of the tube is at the same horizontal level as the dome. Thus by threading the tube into a blood vessel, an invasive procedure, the blood pressure in that vessel can be transmitted to the dome and hence the diaphragm of the pressure transducer. The pressure transducer will, therefore, sense the pressure in the vessel. This technique is known as external direct blood pressure measurement, and the flexible plastic tube that enters the blood vessel is known as a catheter. It is important to remember that the horizontal level of the blood pressure transducer dome must be the same as that of the tip of the catheter in the blood vessel to accurately measure the pressure in that vessel without adding an error due to the hydrostatic pressure in the catheter. In addition to problems due to hydrostatic pressure differ￾ences between the chamber and the dome, catheters introduce pressure errors as a result of the dynamic properties of the catheter, fluid, dome, and diaphragm. These properties as well as air bubbles in the catheter, or obstructions due to clotted blood or other materials, introduce resonances and damping. These problems can be minimized by utilizing miniature pressure transducers fabricated using microelectronic semi￾conductor technology that are located at the tip of a catheter rather than at the end that is external to the body. A general arrangement for such a pressure transducer is shown in Fig. 114.2. As with the disposable sensors, strain gauges are integrated into the diaphragm of the transducer such that they detect very small deflections of this dia￾phragm. Because of the small size, small diaphragm displacement, and lack of a catheter with a fluid column, these sensors have a much broader frequency response, give a clearer signal, and do not have any hydrostatic pressure error. 1 It must be pointed out that the use of such a sensor is not limited to blood pressure measurement. The strain gauge pressure sensor can be used to measure the pressure of any fluid to which it is appropriately coupled. FIGURE 114.1 An unbonded strain gauge pressure transducer. FIGURE 114.2 A catheter tip pressure transducer