Chapter 5, cont… (Page 2)
Oxygen transfer
ALVEOLARARTERIAL OXYGEN PRESSURE DIFFERENCE
The normal PaO2 is agedependent, declining slightly over the years ( Fig. 53). This decline reflects shifts in ventilation/perfusion (V/Q) ratios in the aging lung. In contrast, alveolar partial pressure of oxygen (PAO2) depends only on the pressure of inspired oxygen (PIO2), respiratory quotient (R), and alveolar partial pressure of carbon dioxide (PACO2). Since none of these values is agedependent, PAO2 does not change as we age. The horizontal line in Fig. 53 shows normal PAO2 breathing ambient air (FIO2 = 0.21) at sea level; the difference between the diagonal band and the horizontal PAO2 line is the alveolararterial oxygen pressure difference (P(Aa)O2). Note that normal P(Aa)O2 breathing ambient air can reach approximately 30 mm Hg in elderly people.
Studies defining normal P(Aa)O2
for increments of FIO2 above 0.21 show that it increases
as FIO2 increases, at least up to an FIO2
near 0.6 ( Fig. 54). Thus both age and FIO2 affect
the normal P(Aa)O2. Whatever the normal value,
with the exception of a righttoleft extrapulmonary
shunt, an elevated P(Aa)O2 indicates either lung
disease or an abnormal lung condition interfering with oxygen
transfer. A P(Aa)O2 determination is most useful
when FIO2, is 0.21 (Fig. 54).
Fig. 53.
Changes in PaO2 and P(Aa)O2 with age.
The line for PaO2 is based on the regression equation,
PaO2 = 109 0.43 (age in years), for Pao, measured
at PB 760 mm Hg. (From Sorbini, C.A., Grassi, V., Solinas, E.,
et al.: Respiration 25:3, 1968.)
| Clinical problem 1 |
| A 35yearold patient has a PaO2 of 90 mm Hg. Are his lungs working properly to transfer oxygen? Is more information needed'? |
Fig. 54.
Normal range of P(Aa)O2 from FIO2
of 0.21 to 1.00, based on data obtained from 16 healthy subjects
aged 40 to 50 years. Lines represent mean values and + or
2 SD (standard deviations). The P(Aa)O2 increases
up to 0.6 and then plateaus with increasing FIO2. Note
that P(Aa)O2 may normally exceed 100 mm Hg on
an FIO2 of 1.00. (Redrawn based on data from Harris,
E.A., et al. Reprinted by permission from Clin. Sci. Mol. Med.,
vol. 46, pp. 89104, copyright (c) 1974, The Biochemical
Society, London.)
| Clinical problem 2 |
| A 35yearold patient has an arterial PO2 at sea level of 85 mm Hg. For each of the following FIO2 and PaCO2 values, state if his lungs are transferring oxygen properly. Assume R = 0.8.
a. FIO2 = 0.21, PaCO2 = 25 mm Hg b. FIO2 = 0.21, PaCO2 = 40 mm Hg c. FIO2 = 0.21, PaCO2 = 50 mm Hg d. FIO2 = 0.40, PaCO2 = 30 mm Hg |
| Clinical problem 3 |
| Which of the following are potential causes of increased P(Aa)O2 in a patient who is healthy except for the indicated physiologic condition? a. Thickening of alveolarcapillary membrane b. Elevated PaCO2 c. Ventilationperfusion imbalance d. Anemia e. Righttoleft intrapulmonary shunting f. Righttoleft intracardiac shunting g. High altitude h. Carbon monoxide inhalation |
| Clinical problem 4 |
| For each of the following patients calculate P(Aa)O2, assuming an R of 0.8 and barometric pressure of 760 mm Hg. Which of these patients is most likely to have lung disease? Do any of the values represent a measurement or recording error? a. A 35yearold patient with PaCO2 of 50 mm Hg, PaO2 of 150 mm Hg, and FIO2 of 0.40 b. A 44yearold patient with PaCO2 of 75 mm Hg, PaO2 of 95 mm Hg, and FIO2 of 0.28 c. A young, anxious patient with PaO2 of 120 mm Hg, PaCO2 of 15 mm Hg, and FIO2 of 0.21 d. A patient in the intensive care unit with PaO2 of 350 mm Hg, PaCO2 of 40 mm Hg, and FIO2 of 0.80 e. A patient with PaO2 of 80 mm Hg, PaCO2 of 72 mm Hg, and FIO2 of 0.21 |
Fig. 55. Changes
in P(Aa)O2 and PaO2/PAO2
with increasing FIO2 in a healthy subject and in a
patient with respiratory failure. With increasing FIO2,
PaO2./PAO2 varies much less than P(Aa)O2.
PaO2/PAO2 AND PaO2/FIO2
Since the normal P(Aa)O2 increases with increasing FIO2, it is not a useful test to follow a patient's course when FIO2 changes. A more stable parameter is PaO2/PAO2, the ratio of arterial to alveolar partial pressure of oxygen (Gilbert and Keighley, 1974). In contrast to P(Aa)O2, PaO2/ PAO2 remains fairly constant with increasing FIO2, as long as the underlying lung condition is stable; the reason is that both PaO2 and PAO2 increase by the same factor when FIO2 is raised.
Normal PaO2/PAO2 ranges from 0.74 in elderly people to approximately 0.9 in young subjects. If FIO2 changes for a given patient, PaO2/PAO2 should not change much unless there is a change in V/Q relationships. The effect of an increasing FIO2 on P(Aa)O2 and on PaO2/PAO2 is shown in Fig. 55.
Recently some investigators have used PaO2/
FIO2 as an index of oxygenation (Hess and Maxwell,
1985). This ratio has the advantage of avoiding the calculation
of PAO2, but it also ignores changes in arterial carbon
dioxide pressure. There is no evidence that PaO2/FIO2
is any better than PaO2/PAO2 as an index
of oxygenation.
CAUSES OF LOW PaO2
Table 51 lists the physiologic causes, which are classified as respiratory or nonrespiratory, of a low arterial PO2. For completeness, artifactual causes are also listed, and these will be discussed before the nonrespiratory and respiratory causes.
| Table 51. Physiologic causes of a low PaO2 | |
| Increased Normal Normal Increased
Increased
Increased |
| *Only in presence of increased venous admixture | |
When the white blood cell count (WBC) is over 100,000 (e.g., in leukemia), the excess cells in the blood may consume enough oxygen between the time the blood is drawn and the PaO2 is measured to cause a very low PaO2 measurement. The true (in vivo) PaO2 is not affected by the high WBC count since oxygen is continuously replenished from the atmosphere.
In patients with hyperthermia, the in vivo PaO2
is higher than what is measured. Hyperthermia raises the in vivo
PaO2, but the blood sample is always measured using
a water bath that is maintained at normal body temperature, 37°
C. For example, if a patient's temperature is 39°
C, the measured PaO2 (at 37°
C) will be approximately 8 mm Hg lower than the in vivo PaO2.
For this reason, some laboratories correct for body temperature
when reporting blood gas results.
NONRESPIRATORY CAUSES OF LOW PaO2
(Table 51)
Righttoleft intracardiac shunt is rare in adult medical practice; its presence is usually suggested by the patient's history, physical examination, and chest xray, although its confirmation may require cardiac catheterization.
Decreased inspired oxygen pressure (PIO2) is rarely, if ever, a cause of hypoxemia in hospitalized patients. The PIO2 is a function of both fraction of inspired oxygen ( FIO2) and barometric pressure. A decrease in FIO2 may occur during administration of an anesthetic (as from improper installation of oxygen supply lines) or from being in a sealed chamber cut off from adequate ventilation. Low barometric pressure is the cause of decreased PIO2 at a high altitude, and low alveolar PO2 has been implicated in highaltitude pulmonary edema. Note that FIO2 does not change with increasing altitude (Table 52).
| Table 52. Inspired and alveolar oxygen pressure at various altitudes* | ||||
(feet) | (mm Hg) | (mm Hg) | (mm Hg) | |
| Miami Denver Airplanepassenger cabin Mexico City Leadville, Colorado Top of Mt. Everest† Air outside plane while flying | 5,280 6,000 7,347 10,200 29,028 35,000 | 629 608 578 517 253 160 | 132 128 121 109 53 34 | 74 70 64 51 -5 24 |
| *At all altitudes, FIO2, 0.21; PB, barometric pressure; PIO2, partial pressure of oxygen in atmosphere (dry air); PAO2, alveolar PO2. | ||||
| §PaO2 is calculated using the formula PAO2 = FIO2(PB 47) 1.2(PCO2), where PCO2 = 40 mm Hg. Normally there is compensatory hyperventilation with increasing altitude so that the actual PAO2 will be higher than is shown, depending on the degree of hyperventilation. | ||||
| †Calculations for Mt. Everest are based on data from West, J.B., Hackett, P.H., and Maret, K.H., et al.: J. Appl. Physiol. 55:678, 1983. On the top of Mt. Everest alveolar PCO2 was measured as 7.5 mm Hg, and PAO2 measured as 35 mm Hg; the calculated PaO2 without supplemental oxygen is 28 mm Hg. | ||||
| Clinical problem 5 |
| An unacclimatized, 40yearold man went hiking in the Colorado mountains. After 6 hours he became acutely short of breath and was taken to a hospital in Leadville, Colorado (see Table 52). The patient's initial arterial blood gas revealed PaCO2 27 mm Hg, PaO2 40 mm Hg, and pH 7.52 (room air). How should these values be interpreted? Is P(Aa)O2 increased? |
Decreased respiratory quotient (R) (VCO2/VO2) has been described in patients undergoing hemodialysis and is a result of the diffusion of carbon dioxide from the patient's blood directly into the dialysis solution (Aurigemma, Feldman, Gottlieb, et al., 1977; Martin, 1980; Hunt, Chappell, Henrich, et al., 1984; Quebbeman, Maierhofer, and Piering, 1984). This carbon dioxide diffusion occurs when the dialysis is performed against an acetate bath but not when bicarbonate dialysate is used. Because some carbon dioxide diffuses from the patient's blood into the acetate bath, less carbon dioxide is delivered to the lungs for excretion; as a result alveolar ventilation declines. However, arterial partial pressure of carbon dioxide does not change (alveolar ventilation is still matched to the amount of carbon dioxide brought to the lungs for elimination). The alveolararterial oxygen pressure difference (P(Aa)O2) also does not change.
The metabolic exchange of carbon dioxide and oxygen
does not change during dialysis; thus the lungs must bring in
the normal (predialysis) oxygen consumption (VO2).
However, because the amount of carbon dioxide brought to the lungs
for elimination (the VCO2) is reduced, the R value
decreases. The result is a reduced alveolar (and hence arterial)
PO2. Calculate the new R value in the following problem
to see how this reduction occurs.
| Clinical problem 6 |
| A patient is being dialyzed against an acetate dialysis solution. Before dialysis his PaO2 is 84 mm Hg. PaCO2 is 38 mm Hg. and pH is 7.36. If predialysis carbon dioxide production and oxygen consumption are 200 and 250 ml/min, respectively, and if 50 ml of carbon dioxide enters the dialysate per minute, what is his Pao, during dialysis? Assume the patient is breathing room air ( FIO2 = 0.21) and that PaCO2, pH, and P(Aa)O2, are unchanged during dialysis. |
Low mixed venous oxygen content will aggravate hypoxemia
only in the presence of venous admixture. This is of some importance
in patients with pulmonary edema or with other causes of large
venous admixture. The reserve for oxygen diffusion is so great
that mixed venous blood with a PO2 of zero would be
fully oxygenated after one pass through normally ventilating alveoli.
This important cause of hypoxemia is discussed on p. 106.
RESPIRATORY CAUSES OF LOW PaO2
Compared to nonrespiratory causes, respiratory causes of low PaO2 are far more common. Of the four physiologic respiratory causes (Table 51), diffusion barrier is rarely, if ever, a cause of low Pao, in patients at rest. It can, however, account for hypoxemia during exercise (see Chapter 12). This leaves ventilationperfusion imbalance, rightto-left pulmonary shunt, and hypoventilation as the physiologic respiratory causes of low PaO2 at rest.
Hypoventilation (elevated PaCO2) does
not, per se, lead to an increase in the P(Aa)O2.
This is because increased PaCO2 lowers PAO2
and PaO2 together. Hence one can always determine whether
or not hypoventilation is a cause of low Pao.-simply check
the PaCO2; if it is elevated, hypoventilation is one
reason for the decrease in PaO2. If P(Aa)O2,
is normal when PaCO2 is elevated, then hypoventilation
is the sole cause of any reduction in PaO2. This hypoventilation
can occur from nonpulmonary conditions such as brainstem depression
or chest muscle paralysis. On the other hand, hypercapnia accompanied
by an elevated P(Aa)O2, invariably indicates
lung disease or an abnormal lung condition affecting oxygen transfer.
| Clinical problem 7 |
| A 73yearold woman is brought to the emergency room in a comatose state. The family states she had become confused and had swallowed an excess number of sleeping pills. Blood gas analysis obtained with the patient breathing room air (FIO2 = 0.21) shows the Pao, is 42 mm Hg, the Paco, is 74 mm Hg, and the pH is 7.10. Why is her PaO2 reduced? |
A righttoleft pulmonary shunt can occur
from an alteration in normal lung parenchyma or through abnormal
vascular channels within the lungs. In either case the shunt represents
one extreme of ventilationperfusion imbalance. Such areas
of the lung have a ventilation to perfusion ratio of 0.
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