Pulmonary respiration-also called ventilation-is composed of two components: alveolar ventilation and dead space ventilation. Dead space ventilation is the portion of the respiratory volume that is not involved in the exchange of carbon dioxide (CO2) for oxygen (O2). Dead space ventilation occurs because the volume of air that is in the upstream system of the respiratory tract (nasopharynx, trachea, and bronchi) after inspiration is directly exhaled during subsequent expiration without having come into contact with the alveoli.
What is dead space ventilation?
Pulmonary breathing-also called ventilation-is composed of two components: alveolar ventilation and dead space ventilation. The respiratory tract or respiratory system consists of the lungs or alveoli, which make up the functional part of respiration, and the upstream so-called dead space, which is composed of the nasopharynx, trachea, and bronchi. The functional importance of the dead space lies primarily in the conditioning of the inhaled air by heating – in extreme cases also by cooling – and enrichment with water vapor until saturation, as well as in the filtering out of solids (dust) and pathogenic germs. Respiration is not characterized by a continuous flow of air through the lungs, but is characterized by an alternating system of inhalation (inspiration) and exhalation (expiration). The part of the inhaled air that is in the dead space after inspiration is directly exhaled again during the subsequent expiration without having come into contact with the alveoli. Dead space ventilation also includes the part of respiration corresponding to the physiologically non-functional part of the lung, if such a part exists. This may be alveoli that have lost their ability to exchange gases due to disease or other exposure.
Function and task
Dead space ventilation – at least anatomic dead space ventilation – results automatically from the fact that the dead space is filled with the inspired air after inspiration has occurred, just as the alveoli of the lung are. The only difference is that the air volume of the dead space cannot participate in the gas exchange. This means that the inhaled and exhaled air of the dead space do not differ in oxygen and carbon dioxide partial pressure, but only in temperature and relative and absolute humidity. In healthy humans, the function of dead space ventilation is that the anatomical dead space serves the physiological conditioning of the respiratory air in terms of temperature and relative humidity, as well as the filtering out of dust grains and pathogenic germs. When the body is subject to a high load with a high oxygen demand and maximum ventilation sets in, dead space ventilation somewhat impairs the maximum performance of the lungs, because after expiration has taken place, the air still in the dead space, which already has a lower oxygen content compared to the ambient air with a higher carbon dioxide content at the same time, must be inhaled again for anatomical reasons. In the resting phase, without physical and mental stress, the dead space ventilation makes up a considerable part of the total ventilation. The volume of air inhaled with one breath (breath volume) during the resting phase is about 0.5 liters, of which about 0.15 liters is accounted for by the anatomical dead space, corresponding to 30%. The relative contribution of dead space ventilation to total ventilation decreases sharply at high performance demands because the respiratory volume can increase to a much higher value when the expiratory and inspiratory reserve volume is used, while the absolute dead space volume remains almost constant for anatomical reasons. The absolute size of functional dead space, including physiological dead space, can be calculated using the Bohr formula. According to this formula, the absolute size of the dead space volume, including the physiologic dead space, is a function of the respiratory volume and the carbon dioxide partial pressures of the inhaled and exhaled air.
Diseases and medical conditions
Dead space ventilation must always be considered in conjunction with alveolar ventilation, as the two are purely physically related.The alveoli cannot be filled with air without the air first passing through the anatomical dead space, being tempered, brought to 100% relative humidity, and filtering out solid components such as dust particles and possible germs. Unless the air is inhaled through a tube or tracheotomy (coniotomy). However, determination of functional dead space volume may provide evidence of the presence of alveolar dead space if the functional dead space is distinct from the anatomic dead space. In people who have a healthy respiratory system, alveolar dead space is not present, so anatomic dead space is nearly identical to functional dead space. If the functional dead space is found to significantly exceed the anatomic dead space in terms of volume, this is an indication of the presence of alveolar dead space. This is equivalent to a diagnosis that parts of the alveolar system are dysfunctional. The presence of pulmonary emphysema or pulmonary fibrosis is then suspected, which is caused by chronic inflammatory processes of the interstitial tissue in the area of the alveoli, leading to irreversible destruction of the alveolar membranes. Pulmonary fibrosis is triggered either by pathogenic germs or by noxious agents such as inorganic or organic dusts (e.g. also sprays), as an undesirable side effect of certain medications or by left heart failure and many more. Restrictive or obstructive ventilation disorders that interfere with breathing result from either lung disease, respiratory center dysfunction, respiratory muscle disease or injury, or anatomic dead space problems. Because alveolar ventilation cannot be performed independently of dead space ventilation, ventilatory disturbances always affect overall ventilation, regardless of the cause.