Finger plethysmograph

The finger plethysmograph is a device designed to measure the saturation of arterial blood with oxygen, as well as to determine blood flow and heart rate. It is used in occupational medicine, angiology, neurology and laboratories of medical institutes. Patients are mainly manual workers and people whose bodies (especially hands) are exposed to various vibrations. There are one-finger, eight-finger and ten-finger plethysmographs. The next description deals mainly with eight-finger plethysmographs.

Principle of examination
The method is based on spectrophotometry and plethysmography. The presence of hemoglobin in the blood is an important value. Arterial blood oxygen saturation is expressed as the amount of oxyhemoglobin / sum of oxyhemoglobin and deoxyhemoglobin. The entire measurement is non-invasive and is performed using a sensor mounted on the finger (usually the index finger). The sensor has two LED photodiodes that emit light at a wavelength of 660/905 nm. The light passes through the layer of blood vessels and hits the photodetector, which is located opposite the photodiodes. The photodetector measures the intensity of light of different wavelengths and converts it into an electrical signal, which is then fed to the oximeter. Because oxyhemoglobin and deoxyhemoglobinthey have different absorbances, we can distinguish them and determine their amount. In an attempt to minimize the various undesirable effects, such as pigmentation of the skin, the effect of venous blood and other tissue components, these effects are separated from the data pulse absorption.

Wiring and calibration
The device consists of modules for measuring saturation, pulse and plethysmographic curve. These are connected to a control unit that communicates with the modules and the control program installed on the computer. Optical sensors inserted in the control unit sense plethysmographic curves. The sensors are strung on the fingers of both hands or feet (eight-finger scanning). Calibration takes place automatically each time the instrument is switched on or the measuring program is restarted. It consists of several consecutive steps - the number on the monitor and the display of the device informs about the number of already performed steps. Then the measuring circuits are brought into the correct mode and the device is ready for measurement.

Description and preparation of the device
The control unit ensures communication between the individual parts of the assembly and controls the examination process. In the front part there are 8 SUB9 connectors, to which finger measuring sensors and LCD display are connected, which during the measurement shows the serial number of the finger and the measurement phase. A two-color LED indicating the currently active sensor is also connected to each connector. When lit red, the fingers are measured at natural temperature, green light indicates measurement after application of the cooling bath. A printer can be connected to the device using the computer's USB connector or parallel port. An Internet connection is not recommended due to the risk of infecting your computer with viruses.

Description of measurement
Plethysmography can be divided according to the design into simple and stress.

Simple plethysmography
It is a one-time record of blood circulation in a given place. Its disadvantage is the variability of pulse waves, which limits its informative value and makes interpretation difficult.

Stress plethysmography
The examination takes longer, but is far more accurate than in the case of simple plethysmography. It consists of several stages:

1. Recording at room temperature - basic information about the state of blood circulation.

2. Cold provocation test - the most pronounced external negative stimulus.

3. Recording after the cold test - if the finding is now physiological, the examination ends, if not, the next steps continue.

4. Vasodilation - using pharmacological or thermal stimuli.

5. Record after vasodilation - shows the ability to respond to therapy.

By the patient
The day before the examination, the patient should discontinue any vasodilators after consultation with a physician. He should also refrain from alcohol and smoking. A richer breakfast is recommended in the morning before the examination, as the cold exercise test often causes hypoglycemia to collapse.

Indicating the part of the doctor
Before requesting a finger plethysmography, the physician should consider whether the patient is suffering from severe hypertension or ischemia, which could accelerate during the measurement. If he still decides to indicate an examination, he should instruct the patient about the need to discontinue vasodilators. He should also state these complications in the application.

By the medical examiner
Before examining, the examining physician must find out the anamnesis and measure the patient's blood pressure. Some experts believe that coronary artery spasm may occur during the cold test. Therefore, the presence of a doctor throughout the measurement is recommended.

During the cold stress test.
After the cold test, the change in skin color is evaluated in particular. The tone of color and border, localization and temporal manifestation of color changes are important. Mere redness after a cold test is a physiological reaction. Cyanosis is no longer physiological and its color, location, extent and time of onset must be monitored. If it was formed after the cold test and its color is typically blue-violet, it is probably the expected vasoconstriction. If this is the primary Raynaud's syndrome, the fingers and toes are affected symmetrically, and sometimes we also observe changes in the backs of the hands and insteps. In both primary and secondary Raynaud's syndrome, we can observe whitening to mild yellowing of the skin. In systemic connective tissue diseases, so-called gray cyanosis often manifests itself significantly. This phenomenon accompanies the vasoparalytic stage of vasoneurosis. In both cases, cyanosis manifests itself at room temperature and its color is unmistakably typical. If color changes occur in an atypical location, such as the chest, thighs, or abdomen, we should always investigate their origin.

During the plethysmographic stress test.
In the plethysmographic test, we observe two basic reactions to the provoking agent. In the first case, vasospasm occurs, which occurs in Raynaud's syndrome (primary and secondary). In the second case, the spastic component does not manifest itself at all or only minimally. This is the case, for example, with ischemic changes.

Related articles

 * Pulse oximetry
 * Hemoglobin saturation