Chapter 5

Oxygen transfer

OUTLINE

• Clinical questions
• Lung oxygen transfer
• Oxygen diffusion and diffusing capacity
• Alveolar oxygen pressure
• Arterial oxygen pressure
• Alveolar arterial oxygen pressure difference
• PaO₂/PAO₂ and PaO₂/FIO₂
• Causes of low PaO₂
• Nonrespiratory causes of low PaO₂
• Respiratory causes of low PaO₂
• Ventilation­perfusion imbalance
• Shunts and venous admixture
• How V/Q imbalance reduces PaO₂ and increases PaCO₂
• Low mixed venous oxygen - effect on PaO₂
• V/Q imbalance in patients­­variable effect on oxygen and carbon dioxide

CLINICAL QUESTIONS

There are two broad clinical questions concerning the physiology of oxygenation. One question asks if oxygenation is adequate for the patient; it encompasses the patient's history and physical examination, plus measurement of his blood oxygen content, cardiac output, and oxygen delivery and Uptake. This question is discussed in Chapter 6.

The other question asks if the lungs are properly transferring oxygen into the blood; it is answered simply by comparing the patient's alveolar­arterial PO₂ difference with the expected difference.^* If this difference is elevated, the answer is no. Understanding why this is so is the subject of this chapter.

LUNG OXYGEN TRANSFER

In Chapter 4 one gas exchange function of the respiratory system was examined: elimination of carbon dioxide. The other component of gas exchange is the delivery of oxygen from the atmosphere, through the lungs, and into the blood. Gas exchange is a passive process; there is no active transport of oxygen by the lungs.

Gas exchange is accomplished solely by diffusion of gas from a region of higher, to one of lower, gas pressure. Fig. 5­1 shows the change in oxygen pressure as oxygen moves from the atmosphere to the arterial blood (when breathing ambient air at sea level). Note the following (see Fig. 5­1):

1. The partial pressure of oxygen in dry air is the fraction of inspired oxygen (FIO₂) times the barometric pressure; at sea level this is 0.21(760) = 160 mm Hg. With increasing altitude, the barometric pressure falls and FIO₂ remains constant.

Fig. 5­1. Changes in PO₂ and PCO₂ as oxygen moves from the atmosphere to the arterial blood. All pressures are in mm Hg. The PO₂ and PCO₂ are calculated using an FIO₂, of 0.21 and a barometric pressure of 760 mm Hg. (RA, right atrium; RV, right ventricle; PA, pulmonary arteries; PV, pulmonary veins; LA, left atrium; LV, left ventricle.)

2. In the upper airways (nose, larynx, trachea), water vapor is added to the inspired air. Water vapor pressure is 47 mm Hg at normal body temperature; this pressure affects all dry (nonvapor) gas pressures (oxygen, nitrogen, carbon dioxide). Thus tracheal PO₂ = 0.21(760­47) = 150 mm Hg.

3. Inhaled carbon dioxide is negligible and can be considered zero for clinical purposes. As air travels toward the alveoli, carbon dioxide increases; PCO₂ at the alveolar level = arterial PCO₂ = 40 mm Hg (normal alveolar ventilation).

4. Since the lungs are an open system in continuous contact with the atmosphere, total alveolar pressure equals barometric pressure. But since inspired PCO₂ is zero and alveolar PCO₂ equals 40 mm Hg, the partial pressure of some other gas must fall. Water vapor does not change since it is a function of body temperature. What about nitrogen and oxygen? Nitrogen is inert­­it is not metabolized, and there is no net uptake or excretion. Oxygen, on the other hand, is transported into the pulmonary capillary blood at the same time carbon dioxide is entering the alveoli. The result is a fall in inspired oxygen pressure commensurate with the addition of carbon dioxide to alveolar air.

5. The fall in PO₂ from the trachea to the alveoli is almost all accounted for by the alveolar PCO₂ (PACO₂). However, the change in PO₂ does not quite equal PACO₂ since more oxygen is taken up as carbon dioxide is eliminated. Under normal steady state conditions, approximately 250 ml of oxygen are added to the pulmonary circulation per minute (the VO₂), while 200 ml of carbon dioxide are removed (the VCO₂). The ratio of VCO₂/VO₂ is the respiratory quotient (R or RQ), so the normal R is approximately 0.8. Thus, as air moves from the trachea to the alveoli, PO₂ will fall 1.2 mm Hg for every I mm Hg increase in PaCO₂. If tracheal PO₂ is 150 mm Hg and if PACO₂ is 40 mm Hg, alveolar partial pressure of oxygen (PaO₂) is 102 mm Hg.

6. The initial diffusing gradient for oxygen is PAO₂ minus mixed venous PO₂ (PVO₂) = 102 - 40 = 62 mm Hg. This large gradient, plus the rapidity with which oxygen diffuses across the alveolar­capillary membrane, assures that end­capillary PO₂ is almost equal to PAO₂ (Fig. 5­2).

7. If the lungs were ideal organs, arterial PO₂ would equal end­capillary PO₂ and PAO₂. Normally, however, a small fraction of the cardiac output bypasses the alveoli, either anatomically or physiologically. This shunted blood, unoxygenated, mixes with the oxygenated blood so that arterial oxygen pressure is always lower than end­capillary PO₂. In Fig. 5­ l, PaO₂ is 8 mm Hg lower than end­capillary PO₂.

Fig. 5­2. Alveolar­capillary PO₂ gradient. Normal transit time for a red blood cell in the pulmonary capillary is approximately 0.75 seconds. Blood PO₂ when entering the pulmonary capillary is 40 mm Hg. Diffusion of oxygen is rapid so that capillary PO₂ reaches near equilibrium with alveolar PO₂ (PAO₂) well before the end of capillary transit time. Although there remains a very small difference between end­capillary PO₂ (PCO₂) and PAO₂, for practical purposes, such as when calculating shunt percentage, one can assume PCO₂ = PAO₂.

Based on the physiologic changes in oxygen pressure outlined above, the following observation can be made: the adequacy of overall lung oxygen transfer can be determined by comparing the oxygen pressure both in the alveolar air and in the blood entering the left atrium to normal oxygen pressure values. Since oxygen enters the blood only through the lungs, an increase in the alveolar­left atrial PO₂ difference must indicate either an impairment of oxygen diffusion or an increase in pulmonary venous admixture.

It is not practical to measure oxygen pressure in blood entering the left atrium. Instead, the difference between alveolar (PAO₂) and arterial oxygen pressure (PaO₂) must be used to assess the adequacy of oxygen transfer. Except for the situation where some unoxygenated blood enters the left side of the heart without passing through the lungs, the PAO₂­PaO₂ difference will indicate the adequacy of pulmonary oxygen transfer. This difference is notated P(A­a)O₂. Since PaO₂ is a basic blood gas measurement and PAO₂ can be calculated from the alveolar air equation (discussed later in this chapter), P(A­a)O₂ is easily determined. If P(A­a)O₂ is elevated above the normal range, the lungs are not transferring oxygen properly.^*

OXYGEN DIFFUSION AND DIFFUSING CAPACITY

Before discussing alveolar and arterial partial pressure of oxygen, it will be helpful to clarify the concept of oxygen diffusing capacity. Oxygen enters the blood by diffusion from the alveoli into the pulmonary capillaries. Diffusing capacity of the lung for oxygen (DLO₂) is the quantitative amount of oxygen the lungs can transfer from the alveoli into the blood; units for DLO₂ are ml 02/min/mm Hg driving pressure. If 250 ml of oxygen are transferred per minute and if the average alveolar­capillary PO₂ difference is 25 mm Hg, then DLO₂ is 10 ml O₂/min/mm Hg.

Measurement of DLO₂ is technically difficult since the gradient for diffusion changes constantly along the capillary. Using the values presented in the previous section, the initial gradient for oxygen diffusion is 62 mm Hg and the final gradient is almost zero. The decrease in diffusion gradient along the capillary is nonlinear, and the average gradient can be determined only by a complicated method employing calculus.

In practice the pulmonary diffusing capacity is measured by having the patient inhale carbon monoxide (CO) in a small enough amount that the test is harmless. The normal diffusing capacity of the lung for carbon monoxide (DLCO) is age­, sex­, and height­dependent and ranges from approximately 20 to 30 ml CO/min/mm Hg. The reason for using carbon monoxide is based on its properties as a gas. It is taken up so readily by hemoglobin that it is not perfusion limited; that is, no matter how fast blood flows past the lungs, carbon monoxide will combine with the available hemoglobin. On the other hand, carbon monoxide is diffusion limited so that a thickened alveolar membrane will delay its transfer into the blood.

An additional reason for using inhaled carbon monoxide is that normally very little of it is in the blood (less than 1.5% carboxyhemoglobin). Since virtually all inhaled carbon monoxide is taken up by hemoglobin, the plasma concentration and hence partial pressure of carbon monoxide remain very low (once a gas chemically combines with hemoglobin it no longer exerts a partial pressure).

For test purposes the partial pressure of plasma carbon monoxide along the entire capillary is considered zero so that CO alveolar pressure is, in effect, the pressure gradient for CO diffusion. By knowing the amount of inhaled and exhaled carbon monoxide, the mean alveolar CO pressure and the amount of carbon monoxide transferred into the blood can be calculated. Since the mean alveolar CO pressure is the gradient for diffusion, it becomes relatively easy to calculate the diffusing capacity of the lung for carbon monoxide.

There are several problems and potential pitfalls in interpretation of the DLCO test:

1. The diffusing capacity for carbon monoxide is not the same thing as DLO₂, and one cannot infer any specific value for DLO₂ from measurement of DLCO.

2. The diffusing capacity for carbon monoxide can be reduced by conditions other than a thickened alveolar membrane. Anemia will lower a patient's DLCO since carbon monoxide needs hemoglobin for maximal diffusion; without hemoglobin the plasma concentration would rise quickly and prevent further diffusion. Loss of alveolar membrane will also lower DLCO, and for this reason the DLCO is often reduced in emphysematous patients. Excess blood carbon monoxide, common in smokers, will also inhibit CO diffusion. Finally, ventilation­perfusion imbalance can lead to a reduced DLCO.

3. A reduced DLCO does not account for clinically significant hypoxemia at rest. The diffusion reserve of the lungs is so great that DLCO (or DLO₂) can be markedly reduced without lowering PaO₂ very much. Diseases leading to low DLCO, such as interstitial fibrosis, cause hypoxemia by the mechanism of ventilation­perfusion imbalance, not by impaired diffusion.

In practice, DLCO is helpful if it is normal, since a normal DLCO implies a normal diffusing capacity for oxygen. If the DLCO is reduced, the possible reasons mentioned previously must be considered, and one must avoid any premature conclusion about the lungs' intrinsic diffusing capacity for oxygen.

Since reduced pulmonary diffusing capacity does not cause significant resting hypoxemia, the DLCO test is not helpful in evaluating most abnormalities of gas exchange. There is simply no useful correlation between DLCO and resting arterial blood gas values. Nonetheless, the DLCO test is widely performed in hospital pulmonary function laboratories and is considered routine when ''complete pulmonary function tests'' are requested. Properly performed and interpreted, the DLCO test can provide useful information. The test is useful when treating patients with interstitial lung disease, either as a sign of disease progression or as a sign of response to treatment. Also, DLCO correlates with oxygen desaturation during exercise, an application discussed in Chapter 12.

ALVEOLAR OXYGEN PRESSURE

The partial pressure of oxygen in alveolar gas (PAO₂) represents an average of all the individual alveolar oxygen pressures throughout the lungs. It depends on the fraction of inspired oxygen (FIO₂), barometric pressure (PB), respiratory quotient (R), and alveolar carbon dioxide pressure (PACO₂). By calculating PAO₂, one can obtain a good idea of what the resulting arterial PO₂ should be if the lungs were functioning normally. If the difference between calculated PAO₂ and measured PaO₂ is increased, the lungs are not transferring oxygen properly.

Alveolar oxygen pressure is calculated using the alveolar air equation:

(Eqn 5-1)

where PIO₂ is the FIO₂ times the barometric pressure after water vapor pressure (47 mm Hg) has been subtracted; PACO, is alveolar partial pressure of carbon dioxide (assumed equal to Paco,); and R is the lung respiratory quotient (normally 0.8). Breathing ambient air (FIO₂ = 0.21), Equation I can be abbreviated to

(Eqn 5-2)

Physicians make several assumptions in the clinical application of Equation I (see box below). For this reason, plus the fact that there are no precise normal values for P(A­a)O₂ when FIO₂ is above 0.21, Equation 2 is sufficient for clinical purposes (Martin, 1986).

The assumptions listed in the box, which are routinely made in clinical practice, render the PAO₂ calculation no more than a useful approximation; reporting PAO₂ as more than a simple integer is false precision.^*

ARTERIAL OXYGEN PRESSURE

Arterial PO₂ can be looked at both for what it effects or causes, and what it reflects or results from. It effects the percent saturation of hemoglobin with oxygen and thus is a major determinant of oxygen content (Chapter 6). But to look at PaO₂ only in this way is to miss other important information.

Arterial oxygen pressure is a result of, and therefore reflects, alveolar oxygen pressure (PAO₂) and the alveolar air­pulmonary capillary interface. This interface is determined by the ratios of ventilation to perfusion in the millions of alveolarcapillary units. Oxygen enters the blood only by passive diffusion across a pressure gradient; hence, if the distribution of alveolar air and capillary perfusion is unchanged, then as PAO₂ increases, so will PaO₂; conversely, as PAO₂ decreases, so will PaO₂. PaO₂ alone is insufficient to indicate the adequacy of oxygen transfer. Without reference to PAO₂ it is not known if PaO₂ reflects an abnormal ratio of alveolar ventilation to capillary perfusion or simply a reduction in PAO,.

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