Intra-arterial Pressure Measurement with Strain Gauge and Amplifier *

The purpose of this paper is briefly to describe intra-arterial pressure recording by means of a strain gauge pressure transducert and amplifiers. The use of resistance wire strain gauges for blood pressure recording with galvanometers has already been described.1"8 The use of the strain gauge with electronic amplification as a plethysmograph has also been described.2 Since alternating input current may be used to energize these transmitters, we found it possible to register blood pressure along with other variables on a multichannel recorder with conventional condenser coupled amplifiers. The blood pressure records thus obtained, as shown in Figure 1, consist of bands or ribbons formed by sine-wave oscillating current, rather than single lines. The whole envelope of the record represents the pressure (the increase and decrease of the latter indicated by the broadening and narrowing of the envelope), and the pulse pressure curve is bilaterally symmetrical. It is only when the pressure is zero, as indicated in the calibration, that the envelope constricts bilaterally to a line. The choice of frequency of input current is determined principally by the frequency responses of the registering instrument. Thus, in the Grass electroencephalographic unit we found 50 cycles per second as the most convenient frequency. In the Sanborn cardiette the frequency response of the galvanometer limits the frequency input to an optimum of 100 cycles per second. If it is desirable to use a cathode ray oscillograph, the frequency can obviously be much higher. As may be seen in B and C portions of Figure 1, the sine-wave oscillations may serve, in addition, as small-time interval markers. Calibration of the system with a mercury manometer shows a linear relationship in the range of 0-150 mm. Hg. For the registration of systolic and diastolic pressures the precaution against bubbles in the liquid system is well known. This was particularly important in our experiments in which the animal and strain gauge were subjected to partial vacuums in order to simulate high altitudes, where bubbles that may be microscopic in size expand enormously with marked reduction in atmospheric pressure. We therefore used thick walled transparent polyethylene tubing (5.5 mm. o.d., 2 mm. i.d.) instead of lead to facilitate the visual detection of bubbles in the entire system. This tubing