Direct Methods

In 1733, Stephen Hales recorded the arterial pressures in animals by cannulation and use of a blood-filled glass column.128 Current techniques for the direct and continuous measurement of arterial pressure use the electromanometer, a transducer that converts mechanical energy into an electric signal suitable for amplification, display, and recording. The artery is cannulated with a saline-filled catheter or needle that mechanically couples the circulation to the arterial manometer. Pressures are recorded using atmospheric pressure as the "zero" reference level, and intravascular pressures are further referenced to the level of the heart by addition or subtraction of a gravitation factor. The gravitation factor is expressed by the formula pgh, where p is the density of blood (in grams per milliliter), gis the acceleration due to gravity (980 cm/s), and h is the transducer height (centimeters) above or below the horizontal plane of the heart.

The strain-gauge manometer is commonly used for the precise and accurate measurement of the arterial pressure. However, error may originate in the catheter or coupling system, in which the properties of inertia, friction, and elasticity interact to produce damping of the frequency response. Systems may be overdamped or underdamped, both of which can result in signal distortion. Nevertheless, the appropriate combination of an inelastic cardiac catheter and connecting tube filled with bubble-free fluid produces "critical" damping in which the system response is constant to some desirable frequency level and adequate for the clinical recording of intravascular pressures.125

Measurement errors also occur when an end-hole catheter is positioned axial to flow in a vessel and may become especially important during high arterial flow, when kinetic energy may exceed 10 percent of the total fluid energy. Also, pressure transients due to catheter whip can falsely elevate the measured arterial pressure.125

Miniature, self-flushing strain-gauge manometers attached directly to an intravascular catheter or needle eliminate many of the problems related to transducer mounting and flushing and overdamping by connective tubing. The most satisfactory method for reducing measurement errors, however, is the use of intravascular electromanometers mounted on cardiac catheters or surgically implanted in the vascular wall.

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