Description
- Transducers are devices that convert one type of energy to another.
- In the perioperative setting, transducers are utilized as invasive monitors that convert pressure waveforms into an electrical signal that is then displayed onto a monitor. The most commonly used monitors are intra-arterial, central venous, pulmonary artery, intracranial catheters.
- Modern transducers are small, external, disposable chips with a fluid–air interface sample frequencies >200 Hz.
- Continuous fluid column. Changes in pressure or pulsatile fluctuations distal to the catheter are transmitted to the transducer via a continuous fluid column.
- Transducer. Within the transducer is an internal diaphragm with a “strain gauge.” Pressure fluctuations from the fluid column are conveyed from the diaphragm to a silicon chip or simple wire; this movement or stretch results in voltage output by the strain gauge that is proportional to the pressure applied at the diaphragm. Consequently, pressure fluctuations result in changes in the electrical resistance of the strain gauge (V = IR; where V is voltage, I is current, R is resistance).
Wheatstone bridge component. Describes the circuitry of the transducer chip. The unknown resistance at the strain gauge (from pressure fluctuations) can be deduced using the three known resistance values of the bridge component; application of Kirchhoff's law states that the algebraic sum of all voltages currents in a loop must equal zero. The Wheatstone bridge is capable of measuring small changes in resistance , therefore, is also suitable to measure the resistance change in a strain gauge.
FIGURE 2. Wheatstone bridge component.
R1, R2, R3 voltages are known Rx can be calculated because the algebraic sum of all voltages should equal zero.
- Electronic data. Changes in resistance are subsequently converted to electronic data (e.g., systolic, diastolic, mean arterial pressures).
- Fourier analysis. Arterial, central venous, pulmonary artery pressure tracings are complex waveforms best translated using Fourier analysis. This method allows the arterial waveform to be expressed as a summation of simple sine cosine waves. The quality of the transduced waveform varies with the characteristics of the intravascular catheter, connection tubing, the actual transducer system.
- Frequencies. The frequency of the transducer must exceed the frequency of the arterial or (venous) waveform. Most commercially available transducers have frequencies >200 Hz. If the transducing system has a frequency that is too low, frequencies of the measured waveform will approach the frequency of the transducer. Consequently, the system will resonate pressure waveforms recorded on the monitor will be amplified versions of true arterial pressure.
- Damping describes a decrease in frequency through the continuous fluid column. The waveform is “smoothed” out the dicrotic notch is often flattened or absent. This results in underestimating the systolic pressure overestimating the diastolic pressure. The mean arterial pressure remains accurate. Examples of this include
- Underdamping describes excessive resonance in the system; the tracing is characterized by a high initial spike in the waveform. This results in overestimation of systolic pressure an underestimation of diastolic pressure. The mean arterial pressure remains accurate. Examples include
- Flush bags preserve the accuracy of readings by preserving the integrity of the transducer. They are usually pumped up to 300 mm of pressure control the forward flow of flush into the system, keeping it open, at a rate of 3–4 mL/hour. Otherwise, the pressurized fluid from the patient would climb back up the catheter. Heparin is sometimes added to avoid clotting; however, it appears that the pressure itself is sufficient. Additionally, heparin should not be used in patients with heparin-induced thrombocytopenia.
stiff tubing 
- The accuracy of the transducer is heavily dependent on correct calibration zeroing. Depending on the desired level of zeroing (mid-axillary line for arterial catheters), a stopcock at the level of the desired point of measurement is opened to air (atmosphere) the system zeroed.
- for procedures in the sitting or beach chair position, the arterial pressure in the brain differs from that at the level of the heart. Thus, the transducer should be zeroed at the level of cerebral circulation, most often the external auditory meatus (approximates the level of the Circle of Willis).
- The transducer must be moved in tem with patient movement. If the transducer falls below the point of zeroing, transduced pressures will be falsely elevated. Conversely, if the transducer is raised above the point of zeroing, transduced pressures will be falsely decreased.
- Raising or lowering the transducer relative to the patient will alter the reading. A 10 cm change in height will alter the arterial line pressure reading by 7.5 mm Hg. The same change would occur for CVP since in both systems, the transducer interprets a change in fluid column level as a change in pressure (1 mm Hg = 1.36 cm H2O).
Flush test. Performing a high-pressure flush observing the initial oscillations prior to return of the pressure waveform can aid with identifying damping underdamping within the system. Overdamping is indicated by a gradual pressure decay or an undershot slow return to baseline without any oscillation after a flush. In contrast, underdamping is revealed by widely spaced oscillations.
Outline