What is an oscillation? You can feel it!
If you palpate radial artery, you feel beats of pulse. These beats are the same oscillations as in this note about. For better understanding the principle of the oscillometric method please try the following technique of radial artery palpation. Figure 1 will help you.
This technique refers to the old classic tactile method and allows to characterize the pulse (Do you know latin terms ”pulsus tardus”, “pulsus magnus”, “pulsus parvus” etc?).
Three fingers are used:
- the proximal finger (1) controls the position of the artery;
- the middle finger (2) compresses the artery;
- and the distal finger (3) captures resulting beats.
When the maximum compression of the artery proceeds (A), distal finger-cushion doesn’t feel any oscillations. When you squeeze the artery halves (B) this finger-cushion may feel moderate oscillations. If your fingers are all without the press over the artery (C), each finger-cushion feels same oscillations.
Oscillometric method in a blood pressure measuring
You can draw a parallel between the above-mentioned palpation technique and the oscillometric method using in 24-h blood pressure monitoring system (see figure 2).
In this method oscillations are captured from a.brachialis (1). The compression of the artery is fulfilled by the cuff (2), which is inflated and deflated by the air pump and the automatic valve (3). The role of the distal finger-cushion belongs to the pneumatic system located sensor (4) which specifies the amplitude of oscillation and transmits records in the memory chip. See also "General ABPM device structure and working principle".
For easier understanding consider of the option when there is a step-by-step (A - ... B ... - C) cuff deflation. Cuff pressure in figure is 5. At each step the amplitude of oscillations is different (6). This amplitude first gradually increases from the maximum cuff pressure step to the next one and then begins to decrease so that at the last step the amplitude is minimal. A plot of the amplitude of these oscillations vs the pressure in the cuff resembles a bell shape (7). Electronic components of device analyzing this "bell" to allow him to calculate the systolic (8) and diastolic (9) blood pressure.