Dead space ventilation

SSc-ILD is characterized by a restrictive defect and a decreased DLco-PFTs, and especially DLco levels, correlate well with the extent of disease on HRCT (45,95,96). Rarely, scleroderma of the chest wall may cause extrathoracic restriction. Cardiorespiratory exercise testing may detect occult lung involvement by showing increased functional dead space ventilation and widened an alveolar-arterial oxygen gradient (96,97). The six-minute walk test, a simple, inexpensive and reproducible test that gives a good indication of patient s exercise capacity, has yet to be validated in SSc-ILD as an outcome measure (98,99). 

Since VA = minute ventilation (VE) - dead-space ventilation (VD), this equation can be expressed as... 

TABLE 5.6 Effect of Dead Space Volume, Tidal Volume, and Breathing Frequency on Alveolar >fentllation at a Fixed Minute Ventilation (V = 58.0 Umin). Modified from Chemiack. ... 

Alveolar ventilation. Alveolar ventilation is less than the total ventilation because the last portion of each tidal volume remains in the conducting airways therefore, that air does not participate in gas exchange. As mentioned at the beginning of the chapter, the volume of the conducting airways is referred to as anatomical dead space. The calculation of alveolar ventilation includes the tidal volume adjusted for anatomical dead space and includes only air that actually reaches the respiratory zone ... 

In a lung unit with low blood flow and high ventilation (alveolar dead space), the level of carbon dioxide is decreased and the level of oxygen is increased. The reduced carbon dioxide causes bronchoconstriction and a decrease in ventilation. The excess oxygen causes vasodilation and an increase in perfusion and, once again, the V/Q ratio is brought closer to one and gas exchange is improved. 

Respiratory alkalosis associated with mechanical ventilation can often be corrected by decreasing the number of mechanical breaths per minute, using a capnograph and spirometer to adjust ventilator settings more precisely, or increasing dead space in the ventilator circuit. 

The patient takes a single vital capacity breath of 02 and exhales through a N2 analyser. Dead space gas, which is pure 02, passes the analyser first, followed by a mixture of dead space and alveolar gas. When pure alveolar gas passes the analyser, a plateau is reached. At closing capacity, small airways begin to close, leading to preferential exhalation from the larger-diameter upper airways. These airways contain more N2 as they are less well ventilated, so the initial concentration of N2 within them was not diluted with 02 during the 02 breath. 

The purpose of the Bohr equation is to give a ratio of physiological dead space volume to tidal volume. Dead space volume is normally around 30% of tidal volume and so the normal ratio is quoted as 0.3. Under abnormal conditions, the ratio will tend to increase and so make ventilation inefficient. 

Later, special facilities were constructed, and outfitted with heating, ventilation and air turnover (circulation system) facilities. These facilities had a relatively small volume to avoid any dead space, i.e., to conserve the rather costly insecticide. 

The volume of the nasopharyngeal chamber and airways is 150 mL, that is, dead-space volume. Thus, with a resting tidal volume of 600 mL, only 75% of the air reaches the pulmonary air spaces. There is evidence to suggest that sequential breathing patterns occur that result in poorly ventilated regions [54-56],... 

The single-breath nitrogen (Ng) test is a simple, sensitive test whose major usefulness is in its diagnostic value in the early stages of airway obstruction. This test is also valuable in the assessment of anatomical dead space, uneven ventilation, and abnormal intrapulmonary gas mixing (Buist, 1975). 

FQ series mine pneumatic fan is developed by China University of Mining and Technology, the working medium and the power of the motor are compressed air. Output shaft are installed at the both ends of fan, fan blade is installed in the front end and linen wind wheel driven by the fan airflow is installed in the back, and there has an incomplete circular air outlet in the linen wind wheel. Thus, blade and line wind wheel would be rotated by the fan when the fan is working, and a certain frequent and disturbed jet which can reach the range of 8 meters to 15 meters would be generated in a conical area, so it is suitable for blowing dead space of ventilation, especially for harmful gas accumulation in upper comer. 

In addition to the static lung volumes just identified, there are several time-dependent volumes associated with the respiratory act. The minute volume (MV) is the volume of air per breath (tidal volume) multiplied by the respiratory rate (R), that is, MV = (TV) R. It is obvious that the same minute volume can be produced by rapid shallow or slow deep breathing. However, the effectiveness is not the same, because not all the respiratory air participates in gas exchange, there being a dead space volume. Therefore the alveolar ventilation is the important quantity which is defined as the tidal volume (TV) minus the dead space (DS) multiphed by the respiratory rate R, that is, alveolar ventilation = (TV-DS) R. In a normal adult subject, the dead space amounts to about 150 mL, or 2 mL/kg. 

---