Alveolar gas

Alveolar ventilation supplies O2 to the bloodstream while alveolar capillary perfusion provides alveolar gas with COj. Resting individuals consume approximately 250 mL 02/min and produce approximately 200 ml. COi/min because, stoichiometrically, metabolic processes require a greater supply of O, than the quantity of CO2 produced. Defining the respiratory exchange ratio, R, as... 

Alveolar gas transport Exchange of oxygen and carbon dioxide between al-... 

In order to evaluate the ventilatory status of a patient the alveolar oncygevL tension (Pj 02) must be known. It can be determined from the "alveolar gas equation" (equation 5),... 

The partial pressure of the water vapor is 47 mmHg and, as a result, the P02 is slightly decreased to 150 mmHg. The PC02 remains at 0 mmHg. By the time the air reaches the alveoli, the POz has decreased to about 100 mmHg. The P02 of the alveolar gas is determined by two processes ... 

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

Usually seen when the respiratory rate is slow. The curve starts as normal but the expiratory pause is prolonged owing to the slow rate. Fresh gas within the circuit is able to pass over the sensor causing the Pco2 to fall. During this time, the mechanical pulsations induced by the heart force small quantities of alveolar gas out of the lungs and over the sensor, causing transient spikes. Inspiration in the above example does not occur until point A. 

The above concept can be described graphically by considering the fractional concentration of an agent in the alveolar gas (Fa) as a percentage of its fractional concentration in the inhaled gas (Fi) over time. 

The alveolar gas equation is used to estimate the Pao2 of a perfect alveolus with varying fractions of inspired oxygen and it states that... 

The value for Cc o2 cannot be calculated in this way very easily as a sample is technically difficult to take without a catheter in the pulmonary vein. It is, therefore, assumed to be in equilibrium with the Pao2, which, in turn, is given by the alveolar gas equation. 

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. 

Phase 3 Plateau as alveolar gas with a steady N2 content is exhaled. Note the curve is not completely horizontal during this stage. 

During inhalation anesthesia, the partial pressure of the inhaled anesthetic in the brain equals that in the lung when steady-state conditions are achieved. Therefore, at a given level (depth) of anesthesia, measurements of the steady-state alveolar concentrations of different anesthetics provide a comparison of their relative potencies. The volatile anesthetic concentration is the percentage of the alveolar gas mixture, or partial pressure of the anesthetic as a percentage of 760 mm Hg (atmospheric pressure at sea level). The minimum alveolar anesthetic concentration (MAC ) is defined as the... 

Diffusing capacity for carbon monoxide (measurement of alveolar gas exchange)... 

Where BP is the barometric pressure measured at room temperature and PAH20 is the partial pressure of H20 in alveolar gas at body temperature (39 °C) and saturated with water. 

The alveolar surface represents a thin liquid film formed at the interface between the alveolar gas phase and a liquid hypophase covering the epithelium. This film is stabilised by the alveolar surfactant (AS), consisting mainly of phospholipids and proteins. AS plays an important role in alveolar stabilisation in the process of breathing. It is known that AS components exist as individual molecules and as various lipid and protein/lipid micellar structures present in the so-called hypophase and, according to some researchers, form a continuous lipid monolayer at the water/air interface [e.g. 1-4]. 

Venous oxygen fluctuation under various influences were measured by Yokota and Kreuzer [64,65]. These authors have used polarographic oxygen electrode to the study of alveolar oxygen tension in alveolar gas and blood of anesthetized dogs [66,67]. 

Diffusion Carbon monoxide diffusing capacity Measurement of efficiency of alveolar gas exchange decreases with thickening of alveolar blood-air barrier... 

All the static lung volumes and capaeities except FRC and RV can be measured directly through use of a simple spirometer (an apparatus traditionally consisting of a cylindrical bell immersed in water and equipped with outlets that a person can breathe into, or inhale from, to measure expiratory or inspiratory volumes). Functional residual capacity and RV are measured indirectly by using several alveolar gas dilution techniques. 

There are two techniques used to measure diffiision capacity. In one procedure, the subject takes a single vital capacity inspiration of a dilute mixture of CO and holds his breath for 10 seconds. In the second, the subject breathes a low concentration of CO (about 0.1%) for 30 seconds until a steady state has been reached. In both methods, the rate of disappearance of the CO from the alveolar gas is calculated by measuring the concentrations of CO in the inspired and expired air with an infrared analyzer. The larger the diffiising capacity (DlCO), the more CO enters the blood and the lower the amount of CO measured in the expired gas. 

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