The invasive arterial system and Wheatstone bridge

(Images reproduced with kind permission from Edwards Lifesciences)

The cannula is connected to a transducer (a transducer converts one form of energy to another) via a column of heparinised saline at a pressure of 300 mmHg. The saline passes through a drip chamber adjusted to allow a flow of 4 ml/hour. This continuously flushes the tubing and cannula. The ideal solution for use is dextrose, since a non-electrical conducting fluid avoids current passing down the catheter into the heart.  The transducer is a strain gauge variable transducer. If a wire is stretched it becomes longer and thinner and thus its resistance increases. This is known as a strain gauge. This is connected to an amplifier and oscilloscope.

The saline column moves back and forth with the pulsation. This causes the diaphragm to move. This movement results in a change in resistance and current flow (V=IR) through the transducer. The transducer is connected to a Wheatstone bridge. The heparinised saline allows flushing of the cannula and prevents backflow.

Sir Charles Wheatstone (1802–1875), British physicist and inventor.

Null deflection of the galvanometer implies R1/R2=Rv/Ru

The Wheatstone bridge is an electrical circuit for the precise comparison of resistances. It consists of a common source of electrical current and a galvanometer that connects two parallel branches, containing four resistors, three of which are known. One parallel branch contains one known resistance and an unknown (Ru). The other parallel branch contains resistors of known resistances. In order to determine the resistance of the unknown resistor, the resistances of the other three are adjusted and balanced until the current passing through the galvanometer decreases to zero. The Wheatstone bridge is well suited also for the measurement of small changes in resistance and, therefore, is also suitable to measure the resistance change in a strain gauge. The strain gauge transforms strain applied to it into a proportional change in resistance. Thus, changes in resistance and current are measured, then electronically converted and displayed as systolic, diastolic and mean arterial pressures. Most pressure transducers contain four strain gauges, which form the four resistances in the Wheatstone bridge. The system is designed so that the resistances of two of the strain gauges at opposite sides of the bridge increase while the resistances of the other two decrease. This results in a larger potential change at the galvanometer connections. The potential is then amplified before display.