Non-invasive methods

Figure 3.2 shows different technique to measure the BP in non-invasive way which remain the best way to measure the BP in term of safety and usability outside clinics.

Figure 3.2: Tree schema of different ways to compute the blood pressure measure-ments [23].

Inflatable cuff

The first and the most popular measuring method is based on occlusive method.

The occlusion is provided on the artery, typically the brachial artery, by an ex-ternal pressure imposed from the outside. This vessel is used because it is at the same level of the heart. As a consequence, in this way, the hydrostatic pressure component is delated, not affecting the measurement.

The device that performed the measure is the sphygmomanometer, that was per-fected for clinical environments by prof. Riva Rocci in 1896. Even today, this method is the one most used in BP measurements. It consists of an inflatable cuff connected to both the pressure measurement system and the pump with a valve that regulates the amount of air present in the cuff.

The cuff, posed on the upper harm, is inflated to reach a pressure superior of the systolic pressure and to interrupt the blood flow. Then the pressure inside the cuff is reduced by means of the valve until the flow is restored.

By analyzing the restored process, is possible to measure the SBP and DBP.

The most used sphygmomanometer is mercury based. The mercurial sphygmo-manometer consists on a cloth cuff connected through a tube to an external pump with a valve and a mercury column. Another pressure measuring system is the aneroid sphygmomanometer, in which the mercury column is substituted by a clock manometer with mobile needle and a graduate scale in mmHg. This system does not maintain the stability of the measurements over time and it needs to be cali-brated.

New kind of sphygmomanometer, called hybrid sphygmomanometer, try to repro-duce the best features of mercury ones and replace with electrics component, with a display instead a manometer of the mercury ones and it replaces [49].

Due to the time interval between measurements, this measurement method cannot measure the pressure minute by minute, for each heartbeat, therefore it is not a continuous, but an intermittent measurement method.

Figure 3.3 shows a mercury based sphygmomanometer.

Figure 3.3: Example of mercury based sphygmomanometer [51].

Auscultatory method

With a stethoscope, placed on the antecubital fossa of the elbow, the presence or absence of a beat is auscultated during air inflection and deflection of the cuff. When the pulsatile blood flow is restored reducing the air in the cuff the stethoscope makes the possibility to check the sounds generated by the combination of the turbulent blood flow and the oscillations of the arterial wall [29].

The sounds are called Korotkoff sounds, in honor of the Russian physicist who firstly study the relation between blood pressure and blood pressure cuff. He divided the detected sound in five phases [30]:

• Phase 1: the first phase corresponds to the appearance of a clear, palpable and repetitive pulse. The pressure measured is the systolic one;

• Phase 2/Phase 3: the second one have sounds becoming less intense and more durable, with the presence of slight noise until the third one, in which the beats are less clear and low;

• Phase 4: the sounds become softer and less distinct from each other;

• Phase 5: the final corresponds to the last audible sound. The pressure mea-sured in this phase is the diastolic one.

This method can suffer from human errors because of the need of a trained operator to perform the measurement.

Oscillometric method

The oscillometric method is performed to reduce the error computed by the aus-cultatory method [31]. It is a derivative measurement technique consisting in a pneumatic circuit connected to a cuff, with similar functions to the cuff of the sphygmomanometer, but with internal pressure sensors connected to electrical cir-cuit to detect the pulse pressures inside.

The measurement is performed automatically by the device positioned in corre-spondence of the writs or of the biceps. It stars with rapid inflating air until the lumen of the artery is completely compressed and the flow is interrupted. In this way the pressure wave is completely reflected and diffuses in the cuff.

The cuff is subsequently deflated in steps of 5 mmHg and the sequence considers that the pressure in the cuff decreases by one step when two pulses of equal ampli-tude are detected. The cuff is subsequently deflated in steps of 5 mmHg and the sequence considers that the pressure in the cuff decreases by one step when two pulses of equal amplitude are detected. At each step, the system acquires the pres-sure and amplitude of the pulses. Until the meapres-sure system reaches a maximum the amplitude of the beats increasing during the cuff deflates. Once the maximum is exceeded, the detection of the beats stops permanently at the moment in which

normal flow is restored. The pressure variations are produced by the passage of the pressure wave in the artery and they are recorded thanks to the counterposed action between the blood pressure and the inflated cuff pressure. These variations induce small changes in the volume in the cuff, and then in the pressure inside it.

In the condition in which there is the maximum variation in the section of the artery during the cardiac cycle, pressure pulsations of maximum amplitude are found.

When the maximum amplitudes are detected the pressure inside the cuff correspond to the mean arterial pressure (MAP). Typically, the SBP and DBP are measured by algorithm way and often the corresponds to the 45 % - 55 % of the pulse amplitude maximum for the SBP and to the 74 % - 82 % for the DBP.

Figure 3.4 represent an example of oscillometric shygmomanometer.

Figure 3.4: Oscillometric sphygmomanometer [52].