Pressure Transducer System

In clinical settings, arterial and venous blood pressures and waveforms are displayed using a pressure transducer monitoring system. A typical pressure transducer monitoring system includes: (1) an indwelling intravascular catheter; (2) pressure tubing; (3) a pressure transducer; (4) a stopcock and flush valve; (5) a high-pressure fluid bag; and (6) a graphical display monitor and microprocessor (Fig. 12A,B).

The pressure wave derived from the transducer system is a summation of sine waves at different frequencies and amplitudes. The fundamental frequency (first harmonic) is equal to the heart rate. Therefore, at a heart rate of 120 beats/min, the fundamental frequency is 2 Hz. Because the first 10 harmonics of the fundamental frequency make significant contributions to the arterial waveform (2), frequencies up to 20 Hz make major contributions to the pressure waveform. The maximum significant frequency in the arterial blood pressure signal is approx 20 Hz (3).

All materials have a natural frequency, also known as the resonant frequency. The natural frequency of the monitoring system is the frequency at which the pressure-monitoring system resonates and amplifies the actual blood pressure signal (3,4). If the natural frequency of the system is near the fundamental frequency, the blood pressure waveform will be amplified, giving an inaccurate pressure recording. The natural frequency is defined by the following equation (5):

fn = (d/8) * (3/ttLpVd) A1/2 รง = (16n/d A3) * (3LVd/:rcp) A1/2 (damping coefficient)

where fn is the natural frequency, d is the tubing diameter, n is the viscosity of fluid, L is the tubing length, p is the density of fluid, and Vd is the transducer fluid volume displacement.

Pressure Transducer Block

Pressure lubing fill&d with Flura

Fig 12. (A) and (B) Schematics of a pressure transducer monitoring system (see text for details).

Pressure lubing fill&d with Flura

Fig 12. (A) and (B) Schematics of a pressure transducer monitoring system (see text for details).

Underdamped Line Waveform

Fig. 13. Effect of damping on the arterial pressure wave. In an underdamped pressure-monitoring system, the pressure wave overestimates the systolic blood pressure and underestimates the diastolic blood pressure. In an overdamped system, the pressure wave underestimates the systolic blood pressure and overestimates the diastolic blood pressure. The mean arterial pressure remains essentially unchanged.

Fig. 13. Effect of damping on the arterial pressure wave. In an underdamped pressure-monitoring system, the pressure wave overestimates the systolic blood pressure and underestimates the diastolic blood pressure. In an overdamped system, the pressure wave underestimates the systolic blood pressure and overestimates the diastolic blood pressure. The mean arterial pressure remains essentially unchanged.

To increase accuracy of the blood pressure waveform assessment, the natural frequency needs to be increased as the amount of distortion is reduced. The optimal natural frequency should be at least 10 times the fundamental frequency, which is then greater than the 10th harmonic of the fundamental frequency (2,3). Therefore, the natural frequency should be greater than 20 Hz. In clinical settings, the input frequency is usually close to the monitoring system's natural frequency, which ranges from 10 to 20 Hz. When the input frequency is close to the natural frequency, the system amplifies the actual pressure signal. Ideally, the natural frequency should exceed the maximum significant frequency in a blood pressure signal, which is about 20 Hz (3). An amplified system typically requires damping to minimize distortion; yet, an underdamped system will result in amplification, and an overdamped system will result in reduced amplification.

The ability of the system to extinguish oscillations through viscous and frictional forces is the damping coefficient g (6). Some degree of damping may be required to prevent overam-plifications of blood pressure waveform. More damping may be required, especially in patients with higher heart rates, such as neonates. At higher heart rates, the 10th harmonic of the fundamental frequency will approach the natural frequency, and the waveform is amplified. Overamplification, or ringing, can be adjusted by increasing the damping coefficient. Specifically, in an overamplified system, a connector with an air bubble can intentionally be placed in line with the pressure transducer; the air bubble serves to damp the system to diminish ringing.

The accuracy of pressure transducers is considered optimal in the following situations: low compliance of the pressure catheter and tubing, low density of fluid in the pressure tubing, and short tubing with a minimal number of connectors. Note that a suboptimal pressure system may produce an underdamped or overdamped pressure waveform; an underdamped waveform will overestimate systolic blood pressure, and an overdamped waveform will underestimate systolic blood pressure.

Damping occurs when factors such as compliance of tubing, air bubbles, and blood clots decrease the peaks and troughs of the pressure sine waves by absorbing energy and diminish the waveform. In an underdamped system, the pressure waves can generate additive harmonics, which may also lead to an overestimated blood pressure. In an overdamped system, a pressure wave may be impeded from adequately propagating forward. Overdamping may occur because of air bubbles in the pressure lines, kinks, blood clots, low-flush bag pressures, and multiple stopcocks or injection ports. This often results in underestimation of systolic blood pressure and overestimation of diastolic blood pressure. Fortunately, the MAP in all such situations is minimally affected by dampening (Fig. 13).

Optimal pressure waveforms can be obtained when there is balance between the degree of damping and distortion from the pressure tubing system. A simple way to assess damping is to observe the results from a high-pressure fluid flush. In such a flush test, the pressure transducer system is flushed, and the resulting oscillations (ringing) are observed. In an optimally damped system, baseline results after one oscillation (Fig. 14). In an overdamped system, the baseline is reached without oscillations, and the waveform is blunted. In an underdamped system, the flush test results in multiple oscillations, before the waveform returns to baseline.

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