DIFFERENTIAL AUSCULTATORY TECHNIQUE

It is a non-invasive method for accurately measuring blood pressure.

Figure 1 is a cut away view of an arm with a cuff partially occluding the brachial artery. In this method, a special sensor is mounted between the hand and the air bag. The sensor consists of a pair of pressure sensitive elements ‘A’ and ‘B’. Sensor ‘A’ is placed over the arm and the other sensor ‘B’ is placed between the first sensor ‘A’ and airbag.

Figure 2 illustrates how high frequency pulses are created each time, the intra-arterial pressure exceeds the cuff pressure. As long as the cuff pressure exceeds the pressure in the artery, the artery is held closed, and no pulse is generated. However, as soon as the intra-arterial pressure rises to a value, which momentarily exceeds the cuff pressure, the artery "snaps" open; and a pulse is created. This process is repeated until the cuff pressure drops to a value below the diastolic.

Each time the artery opens, the signal A is created. This signal is denoted by the arrows marked A in Figure 1 transmitted from the artery to both the sensor ‘A’ and the air bag in the cuff. Note that this signal consists of a slowly rising low frequency component with a high frequency pulse superimposed on it. The low frequency component is in the range of 0.5 - 5 Hz. The frequency range of high frequency pulse is about 10 - 80 Hz.

Due to the air bag characteristics, the high frequency component in the signal A is highly attenuated, leaving only the low-frequency signal as shown in Figure 3 (b). Therefore, only the low frequency signal is transmitted to the side of the sensor ‘B’ facing the air bag, as denoted by the arrows marked B in Figure 1. Since most artefact signals (unwanted signals due to motion, etc.) fall in a frequency range below 10 Hz, they are also transmitted to both sides of the sensor.

The differential sensor subtracts the side "B" signal from the side "A" signal, thereby cancelling out the pressure wave component and the motion artifact signals, and the higher frequency Korotkoff signals are isolated.

The systolic pressure is determined as the pressure at which the first opening of the artery occurs, as shown by the first pulse in Figure 3(c), because this pulse is created the first time the artery is forced open by intra-arterial pressure.

Similarly, diastolic value is determined as the pressure at which the differential signal essentially disappears; because this corresponds to the last time the artery is forced open.