Solid State Does Not Necessarily Mean High Fidelity

Introduction

The purpose of this article is to alert researchers to the importance of the performance of a sensor to the success and scientific acceptance of an entire research program.

A critical point in a rodent research program occurs when data from the beating heart are transduced into electrical signals for data processing. Any non-linearity, time delay, hysteresis, motion artifact, bend effect, temperature effect, electrical interference and/or drift with time will become part of the data. Effects of these errors must be minimized by careful design of the catheter and its sensors.

Millar Instruments has been at the forefront of miniature instrumentation development for pressure and pressure-volume studies in small animal research. Millar has helped to establish the reliability, repeatability, and scientific acceptance of a complex dynamic cardiovascular measuring system of which the Millar pressure transducer sensor is the critical component. Any compromise in quality and performance of the sensor will affect research results and should be avoided at the start of any research program.

Not all solid state pressure transducer catheters on the market meet the criteria to be classified as high fidelity transducers. When selecting a solid state pressure transducer catheter, it is important to understand the following characteristics of the sensor:

This article compares the sensor characteristics of a Millar Ultra-Miniature Mikro-Tip™ Pressure Transducer Catheter and another solid state pressure transducer catheter on the market, which will be referred to as Transducer B.

Frequency Response

A frequency response from 0 to 1 kHz is a basic requirement for the accurate measurement of pressure in the rapidly beating hearts of transgenic mice and rats. A flat frequency response curve insures that the transducer is sensitive enough to detect instantaneous changes in the pressure signal arising from physiological events within the heart. The graphs below illustrate the frequency responses of both a Millar transducer and Transducer B. The frequency response of the Millar transducer exceeds 5 kHz. The transducer marketed by a competitor shows a significant decrease in signal amplitude beyond 200 Hz.

Response to Rapid Pressure Changes

Pressure transducers also must respond instantaneously to rapid changes in pressure. A non-instantaneous response to a pressure drop causes distortion in the signal. This is particularly apparent after a ventricular contraction or relaxation in which the slope of the waveform and the values of the end diastolic and systolic pressures are misrepresented. The following graph illustrates the response of three transducers to an almost instantaneous pressure drop of 100 mmHg. The Millar transducer and the external high fidelity transducer respond instantaneously to the pressure drop, whereas Transducer B takes several seconds to stabilize to the new pressure.

Phase Delay

Ideally a pressure transducer should have zero phase delay. A phase delay is indicative of a transducer’s inability to accurately reproduce a sudden change in pressure . A phase delay is also a significant source of error in applications where it is important to identify the timing of the pressure waveform with respect to the EKG signal to correlate the electrical activity of the heart with the actual mechanical events. When compared with the Millar transducer and the external standard, Transducer B shows a significant phase delay in the measured signal of a complex waveform, as illustrated in the graph below.

In-Vivo Comparison

The recorded pressure waveforms obtained from a mouse and a rat are illustrated below. The effects of the inadequate frequency response of Transducer B are demonstrated in a physiological environment. A pressure transducer with an inferior frequency response can yield incorrect information for the comparison of physiological measurements between normal and diseased mice and rats. As shown in the graphs below, there are discrepancies among Transducer B’s values for Pmax, dP/dtmax, and dP/dtmin and the same values obtained from the Millar transducer. Additionally, Transducer B exhibits a significant phase delay.

* Transducer outputs were equalized at static pressures of 0 and 100 mmHg

* Transducer outputs were equalized at static pressures of 0 and 100 mmHg

Conclusion

Measuring left ventricular (LV) chamber mechanics in mice and rats is challenging because of the small size of these animals and because of their relatively fast heart rates. Fluid-filled catheters and external blood pressure transducer measuring systems and some commercial catheter-tip pressure transducers are inadequate for accurate reproduction of the blood pressure waveforms in these small animals. High fidelity catheter-tip transducers are essential for the direct measurement of blood pressure in mice and rats.

For the last 35 years, Millar Instruments has been a pioneer in the development of solid state pressure transducers to measure pressures directly at the source. The use of high accuracy pressure sensors allows for an accurate reproduction of blood pressure waveforms. It is important that researchers realize, however, that not all solid state pressure transducer catheters on the market meet the criteria to be classified as high fidelity transducers. As demonstrated in this article, a competitor’s miniature pressure sensing catheter has an inferior frequency response, introduces phase shift and amplitude errors, and in general distorts the waveform that it purports to measure.