A spirometer is a medical instrument used to measure lung capacity and detect respiratory abnormalities in people with respiratory deficiencies of pathological origin or linked to smoking.

Spirometry is one of the examinations carried out as part of a respiratory functional exploration. The parameters measured by the spirometer, also called a volumetric spiroscope, report the volume and flow rate of the exhaled air.

Composition

Commonly used spirometers are electronic models. They can take very varied forms and be more or less sophisticated, but their overall organization is based on certain constants:

• a power supply system: by battery, rechargeable battery or USB;
• a mouthpiece through which the patient breathes: made of single-use plastic or cardboard;
• accessories: nose clip, USB cable, power adapter, antibacterial filters, color touch screen, thermal paper for printing results, PC software, carrying case, etc. ;
• connectivity: Bluetooth for models connected to smartphones, USB for models connected to PCs;
• optional additional analyzes: post bronchodilator test, calculation of lung age, measurement of oxygen saturation (by an integrated saturometer), etc.

Spirometers are used to control and monitor ventilatory function for preventive or diagnostic purposes.

Two parameters are studied:

• the volume of air exhaled by the patient in simple and forced conditions;
• exhaled air flow, that is, the rate at which air is exhaled by the patient.

The measurements are then compared to those of a healthy individual of the same age, same sex, same build (same height and weight) and same ethnic origin. The tests must also have been carried out under the same experimental conditions in order to be able to be compared.

Results interpretation

The results of the spirometry are interpreted by the doctor or pulmonologist. They are given in the form of a graph, the horizontal axis of which corresponds to the volume of air (in liters), and the vertical axis to the flow rate (in liters per second). The part of the curve below the horizontal axis corresponds to inspiration and the part above to expiration.

The shape of the expiration curve allows us to know whether we are in a “normal” or pathological situation.

• Normal situation: the ascent is rapid and the descent slow, regular and straight.
• Cough: the curve has “holes” in its descending part.
• Incomplete exhalation: the curve “stalls” at the end of the descent.
• Obstructive syndrome (asthma, COPD): the rise is normal but the descent is a concave curve typical of an obstruction of the small airways indicating difficulty in exhaling.
• Restrictive syndrome: the overall appearance of the curves is unchanged but the descent is shortened because the exhaled volume is smaller, which accounts for a weak “filling” of the lungs.
• Mixed syndrome: the descent of the curve is concave and shortened.

On reading the graph, the prescribing physician can therefore establish a diagnosis and deliver treatment or adapt treatment already in progress. He may also prescribe additional examinations.

Public concerned or at risk

Spirometry is prescribed as part of the diagnosis or monitoring of respiratory pathologies, or for preventive purposes for certain audiences. They therefore concern:

• people suffering from chronic respiratory pathologies: asthma, chronic obstructive pulmonary disease (COPD), chronic cough, chronic bronchitis, allergy, etc. ;
• from the age of 40, in smokers or former smokers showing signs of shortness of breath or persistent fatty cough;
• children with signs of asthma: wheezing, frequent coughing, recurrent ENT diseases, family history of asthma;
• people whose work requires permanent and significant breathing: divers, people breathing through a compressed air cylinder, people permanently wearing a mask, etc. ;
• athletes to assess their capacity, respiratory limit, the effectiveness of training, etc.

The stages of operation

The patient is seated, relaxed, with his back straight and feet flat on the floor. His nose is blocked by a nose clip and he holds the tip of the spirometer between his lips. He takes a deep breath and exhales through the mouthpiece in the tube that is connected to the device.

Several tests are carried out according to the patient’s profile and his pathology to establish certain measures:

• the vital capacity CV: the patient inhales and exhales slowly and fully to fill and then completely empty the lungs;
• the forced vital capacity CVF: the patient inhales deeply to fill his lungs to the maximum of their capacity, then exhales violently and as quickly as possible (3 seconds in general, ideally 6 seconds so that the lungs are completely emptied);
• maximum forced ventilation VMM: the subject inhales and exhales deeply several times in a row. This last test is mainly intended for athletes.

Each measurement is carried out 3 times so that the result obtained is as reliable as possible and sufficiently reproducible to be able to be interpreted.

The results

Simple spirometry can be used to determine vital capacity, volumes of inhaled and expired air under current conditions, and residual volume.

Forced spirometry calculates forced vital capacity and expiration speed (flow).

Precautions to take

The patient must be fully aware and cooperative. For unconscious patients and young children, there are other lung tests.

The different models

Many models of spirometers are available on the market. Here are some criteria to take into account when purchasing:

• ease of use ;
• user-friendliness: results given on a smartphone, on a PC, or directly on the spirometer;
• printing of results;
• easy maintenance;
• price of tips;
• reproducibility of results;
• consideration of ethnic differences (up to 13% between different ethnic groups);
• quality control ;
• CE standard and warranty.

Incentive spirometers

The “incentive” spirometer models are intended for training and rehabilitation of the respiratory muscles. They are used in physiotherapy practice or in postoperative treatment. They are particularly useful with children who need to exercise their lungs.