Alveolar Gases

Inspired Gases

Air at sea level has an average barometric pressure of 760 mmHg and contains approximately 21% oxygen and 0.04% carbon dioxide, with nitrogen making up most of the remainder. Thus, the partial pressures of oxygen and carbon dioxide in the air at sea level are 159 and 0.3 mmHg, respectively ( Table^^-l).

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TABLE 22-1 Partial Pressure of Gases while Breathing Room Air, mmHg

Alveolar air does not have the same concentration of gases as atmospheric air. The reasons for the difference include the following: (1) dry atmospheric air that enters the respiratory passages is humidified before it reaches the alveoli, (2) alveolar air is only partially replaced by atmospheric air with each breath, (3) oxygen is constantly being absorbed from the alveolar air, and (4) carbon dioxide is constantly diffusing from the pulmonary blood into the alveoli.

Humidification of Inspired Air

When air enters the respiratory passageways, water immediately evaporates from the surfaces of these passages and humidifies the inhaled air. At 37°C (98.6°F), water has a vapor pressure of 47 mmHg. Therefore, once the gas mixture is fully humidified, the partial pressure of the water vapor in the gas mixture is 47 mmHg (regardless of barometric pressure). This partial pressure is designated P h2o and must be subtracted from atmospheric pressure (760 mmHg) prior to calculation of the partial pressures of the dry gases in the alveolus (760 - 47 = 713). The remaining gases are predominantly nitrogen (79%, or 563 mmHg), oxygen (21%, or 149 mmHg), and CO? (0.04%, or 0.3 mmHg) (Table. . . .2.2-1). If the patient is breathing 60% oxygen (fraction of inspired oxygen [Fi o2]) = 0.6, the inspired oxygen pressure (Pio2) in the trachea or bronchi is determined as follows:

Pioj = (PB - PuiOjFioj where PB is barometric pressure (assumed to be 760 mmHg at sea level). Rate at Which Alveolar Air Is Renewed by Atmospheric Air

The functional residual capacity of the lungs, which is the amount of air remaining in the lungs at the end of normal expiration, is approximately 2500 to 3000 mL (35 to 45 mL/kg). Only 350 mL of new air is brought into the alveoli with each new V T and the same amount of old alveolar air is expired.

The slow replacement of alveolar air helps prevent sudden changes in gas concentrations in the blood. This helps to prevent excessive changes in tissue oxygenation, tissue carbon dioxide concentration, and tissue pH when ventilation is temporarily interrupted. This is also the basis of preoxygenation of patients prior to elective intubation, which could be more accurately described as denitrogenation, or replacing the alveolar nitrogen with oxygen.

Oxygen Concentration and Partial Pressure in the Alveoli

Oxygen is continually being absorbed into the blood in the alveolar capillaries, and new oxygen is continually entering the alveoli from the atmosphere. The more rapidly oxygen is absorbed, the lower its concentration in the alveoli. The more rapidly new oxygen is brought into the alveoli from the atmosphere, the higher its concentration becomes. Therefore, oxygen concentration in the alveoli is controlled by the rate of absorption of oxygen into the blood and the rate of entry of new oxygen into the lung.

Carbon Dioxide Concentration in the Alveoli

Carbon dioxide is continually formed and discharged into the alveoli and continually removed from the alveoli by ventilation. Therefore, the two factors that determine the PAco2 are (1) the rate of excretion of carbon dioxide from the blood into the alveoli and (2) the rate at which carbon dioxide is removed from the alveoli by V A.

At a normal rate of Va of 4.2 L/min, the PAco2 is usually 40 mmHg. If Va is doubled, the PAco2 is reduced to 20 mmHg. If Va is decreased to 2.1 L/min, the PAco2 rises to 80 mmHg. These estimations change with metabolic activity, nutritional state, temperature, and so on.

Alveolar Gas Equation

Inspired gas in the trachea has a partial pressure of oxygen (P o2) of about 149 mmHg and a Pco2 of about 0.3 mmHg. As the warm, water-saturated air enters the alveoli, oxygen diffuses through the alveolar capillary membranes into the plasma and carbon dioxide diffuses from the blood into the alveoli. The mixed venous blood brought to the pulmonary capillaries normally has a P o2 of about 40 mmHg and a Pco2 of 46 mmHg. On the average, for each milliliter of oxygen that leaves the alveolus, 0.8 to 1.0 mL of carbon dioxide enters it. This relationship is defined as the respiratory quotient (RQ), which can be expressed as rateofCOj produclion rate of 03 consumption

In order to determine how well the lungs are functioning at oxygenation, the difference between PA o2 and Pao2 is often estimated. To estimate PAo2 from the Pio2 and Paco2, one needs a correction factor to determine how much oxygen is consumed for each 1.0 mmHg of Paco2 resulting from carbon dioxide that enters the alveoli. Thus, for the usual circumstances, in which the RQ is 0.8, the alveolar gas equation is

PAOj = (PB - PHjoXFiOi) - (PacOiVRQ In room air (Fio2 = 0.21) at sea level with a Paco2 of 40 mmHg, the PAo2 is expected to be

PAo: = (760 - 47K0.21) - (W0.3 = !50 - 50 = 100 The normal difference between PAo2 and Pao2 is 2 to 10 mmHg. End-Tidal Gases

Expired air is a combination of dead space air and alveolar air, and its overall composition is determined by the proportion of each in expired air. Dead space air is expired first. Then progressively more alveolar air becomes mixed with the dead space air until all the dead space air has finally been washed out. At the end of normal exhalation nothing but alveolar air is expired. Therefore, to collect alveolar air for study, one simply collects end-tidal gas. Determination of end-tidal carbon dioxide (ETco2) levels is a useful measure of the adequacy of ventilation. In patients with normal lungs, ET co2 is approximately 3 mmHg lower than PAco2, but in patients with obstructive airways disease, typically asthma, there can be a large difference between ET co2 and PAco2, since dead space air is never fully exhaled and thus neither is pure alveolar air. The use of ET co2 monitors is responsible for the reduction in undetected esophageal intubations.