Respiratory Threshold: Function, Tasks, Role & Diseases

The respiratory threshold value is the maximum respiratory time volume to be achieved and is usually calculated to one minute. Normal values average 120 to 170 liters, with age-specific variations in particular. A severely decreased respiratory threshold indicates ventilatory disorders such as hypoventilation.

What is the respiratory threshold?

The respiratory limit value is the maximum respiratory time volume that can be achieved and is usually calculated to one minute. Human respiration is physiologically characterized by different volumes. These volumes describe the respiratory air in the lungs and airways. The volumes are known as respiratory gas volumes, respiratory volumes, or lung volumes. Pulmonology measures the various volumes via procedures such as spirometry. The respiratory threshold is a respiratory volume. This is the volume of breath that can be inhaled and exhaled within a given period of time. The respiratory limit is measured at maximum respiratory volume and maximum respiratory rate and is achieved by hyperventilation. Thus, the respiratory limit value corresponds to that respiratory time volume that a subject can maximally achieve by voluntary breathing. As a rule, one minute is taken as the unit of time for the respiratory time volume. Under physiological conditions, the respiratory minute volume results from the respiratory rate times the respiratory volume. Under load or under the conditions of a breathing limit test, there is a multiplication of the physiological respiratory minute volume. In athletes, a multiplication of up to 15 times is conceivable.

Function and task

The lungs are a paired organ that enables active respiration in the human organism. The site of gas exchange is the alveoli. Oxygen is drawn from the inhaled air and passes by diffusion into the bloodstream, where a large proportion binds to hemoglobin. Through the bloodstream, oxygen reaches all areas of the body. The types of tissues depend on the supply of oxygen. If little or no oxygen reaches the organs and tissues over a period of time, they die irreversibly. In the pulmonary alveoli, in addition to the uptake of oxygen, the release of carbon monoxide also occurs. When this release is impeded, symptoms of poisoning occur. The human respiratory volumes ensure that sufficient gas exchange can take place and that the organs and tissues are thus supplied with sufficient oxygen. To this end, an adult breathes an average of around 12 to 15 times a minute. With each breath, he or she takes in a respiratory volume of about 500 to 700 milliliters. This results in an average respiratory volume of about eight liters per minute. This volume corresponds to the volume at which physiological lung breathing supplies all body tissues and organs with oxygen at an ideal rate within one minute. The respiratory threshold value is again not derived from physiological breathing conditions, but corresponds to the maximum possible respiratory minute volume. For measurement, the mouthpiece of a pneumotachograph is placed in the patient’s mouth. He is then instructed to hyperventilate maximally for ten seconds. The measured value is converted to one minute. The norm for the respiratory limit is between 120 and 170 liters per minute. Age- and size-specific fluctuations may occur. If the respiratory threshold is severely decreased, there is probably a ventilatory disorder that can be further determined by tests such as spirometry, the Tiffeneau test, or bodyplethysmography.

Diseases and complaints

Ventilatory disorders worsen ventilation of the lungs and, consequently, gas exchange in the alveoli. The disorders are either obstructive or restrictive. In addition to a pathologic decrease, a ventilatory disorder may just as easily be characterized by a pathologic increase in pulmonary ventilation. However, the respiratory threshold value usually only tells us about decreased values and can thus be used as a criterion for the diagnosis of hypoventilation. In restrictive hypoventilation, the distensibility of the lungs or thorax (chest) is limited. Trauma to the thorax is also a conceivable cause. The same applies to neuromuscular diseases, adhesions or pulmonary edema. Often, restrictive hypoventilation also corresponds to pneumonia. Obstructive ventilation disorders differ from restrictive ones in their cause.In addition to increased flow resistance, there is usually increased breathing resistance in these diseases. The airways tend to collapse and patients have problems especially when exhaling. In addition to bronchial asthma, mechanical causes such as cystic fibrosis of cystic fibrosis or chronic bronchitis can cause obstructive ventilation disorder. Also conceivable is a lack of elastic fibers, which reduces respiratory effort. In hypoventilation, pulmonary gas exchange is restricted. As a consequence, hypercapnia, hypoxemia, and respiratory acidosis set in. The patient’s CO2 exhalation is less than production. For this reason, there is an elevated partial pressure of CO2 in the blood. In addition to the diseases mentioned, possible causes include paresis of the respiratory muscles, which is usually preceded by a lesion of the phrenic nerve. Damage to the respiratory center in the central nervous system can also cause hypoventilation. Sometimes, instead of damage, there is only central nervous dysregulation, for example, due to drug influence on the central nervous system. Hypoventilation also characterizes clinical pictures such as Pickwick’s syndrome. To narrow down the cause of hypoventilation and such a decreased respiratory threshold, the aforementioned additional examinations are required.