Dead space alveolar

Distinguish among anatomical dead space, alveolar dead space, and physiological dead space... 

Dead space. Anatomical dead space is equal to the volume of the conducting airways. This is determined by the physical characteristics of the lungs because, by definition, these airways do not contain alveoli to participate in gas exchange. Alveolar dead space is the volume of air that enters unperfused alveoli. In other words, these alveoli receive airflow but no blood flow with no blood flow to the alveoli, gas exchange cannot take place. Therefore, alveolar dead space is based on functional considerations rather than anatomical factors. Healthy lungs have little or no alveolar dead space. Various pathological conditions, such as low cardiac output, may result in alveolar dead space. The anatomical dead space combined with the alveolar dead space is referred to as physiological dead space ... 

Alveolar dead space (obstructed blood flow) V/Q>1... 

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. 

Healthy subjects have hardly any alveolar dead space. However, disease may increase the dead space in the alveoli [15]. This might be of importance when alveolar deposition is desired, for example to obtain systemic absorption. 

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 ... 

State the first principles, for example the Bohr equation considers a single tidal exhalation comprising both dead space and alveolar gas. 

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. 

Phase 2 A mixture of dead space gas and alveolar gas. The curve rises steeply to a plateau. Demonstrate a vertical line that intercepts this curve such that area A equals area B. The anatomical dead space is taken as the volume expired at this point. 

Start with the theoretical lungs shown in the figure and remember that each Vt has a component that is dead space ( Vd) and a remainder that must take part in gas exchange at the alveolus (Vt - Vd). This is the alveolar volume. 

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. 

Measurement of physiologic dead space is based on the assumption that there is almost complete equilibrium between alveolar PCO2 and pulmonary capillary blood. Therefore, the arterial pCOj represents mean alveolar PCO2 over many breaths when an arterial blood sample is drawn for analysis of PCO2. The Bohr equation for physiologic dead space is... 

Mixed-exhaled air. This technique involves the collection of the entire volume of exhaled air. It corresponds to a mixture of the alveolar air with air from the dead space. The collection apparatus may also contribute to the dead space. Total dead space should be considered and the concentration adjusted, either by subtraction, or by regression against some other technique unaffected by the dead space. Timing of the brea collection is important here since the concentration of the air in the dead space may equal that of the air in the workroom if the sample is taken during exposure, or it may equal zero if taken after the end of exposure. 

End-exhaled air. This technique excludes air from the dead space and collects only the last part of the breath, in order to estimate the concentration in die alveolar air which is in equilibrium with the arterial blood. Use of a Haldane-Priestley tube, or simultaneous monitoring of the peaking of the temperature of the breath or its COj concentration, will assure a valid alveolar air sample. ... 

Methodologieally, the differenee between these two types of samples is the control over which portion of the breath is collected. Many techniques make use of the Haldane-Prestley tube to collect the last portion of the expiration. Earlier work used the simultaneous monitoring of COj or temperature in the exhaled breath to identify the moment (CO2 or temperature reaching a maximum) when air from the anatomical dead space has been purged and alveolar air can be sampled. 

The CO diffusing capacity, DLCO, is calculated by measuring the difference in alveolar CO concentrations at the beginning and end of a period of breath holding. The test begins by having die patient exhale completely to RV and then inspiring rapidly to TLC a breath of gas with a known CO concentration. After a 10-second breath-hold, the patient exhales rapidly (Fig. 21.14). The initial portion of this exhalation is discarded, as it contains gas from die dead space, and a portion of the subsequently exhaled gas, assumed to be well-mixed alveolar gas, is analyzed for CO content The initial alveolar concentration of CO is not the inspired concentration, as the inspired gas is diluted... 

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