Chapter 5, cont
V/Q IMBALANCE IN PATIENTS - VARIABLE EFFECT ON
OXYGEN AND CARBON DIOXIDE
Fig. 512 demonstrates that increasing degrees of V/Q imbalance should cause both low PaO2 and elevated PaCO2. Clinically, however, most patients who are hypoxemic from V/Q imbalance have low PaO2 but also low or normal PaCO2. Why this apparent contradiction?
The reason is not, as some often think, the
greater diffusibility of carbon dioxide as compared to oxygen.
(Diffusion impairment is not a limiting factor for transfer of
carbon dioxide or oxygen, at least with the patient at rest.)
The main reason is the physiologic difference between the
oxygen and carbon dioxide dissociation curves ( Fig. 513).
|Table 53. Effect of anemia on PaO2 in the presence of venous admixture*|
|Hemoglobin content |
|Mixed venous oxygen content |
(ml O2/100 ml)
|Mixed venous PO2 |
|Arterial PO2 |
|*The above values are kept constant for these calculations: percent venous admixture, 15%; FIO2, 0.21; PaCO2, 40 mm Hg; cardiac output, 5 L/min; oxygen consumption, 250 ml/min; arterialvenous oxygen content difference, 5 ml/100 ml blood.|
Fig. 513. V/Q
imbalance and the dissociation curves for carbon dioxide and oxygen.
v/Q represents low V/Q units and V/Q represents high V/Q units.
See text for discussion.
The different shape and position of the two curves allows increased alveolar ventilation to lower partial pressure of carbon dioxide PCO2), but not raise partial pressure of oxygen PO2).
With V/Q imbalance, some alveolarcapillary units are relatively overventilated and others relatively underventilated. Blood leaving the various units mixes in the pulmonary veins. As already pointed out, hyperventilation of some units does not add enough oxygen to balance the low oxygen contents from low V/Q areas.
Fig. 513 shows the oxygen and carbon dioxide dissociation curves plotted on the same scale. The upper curve is for carbon dioxide; note that it is diagonal in the physiologic range. The lower curve is for oxygen; it is almost flat in the physiologic range. (This is the same oxygen dissociation curve shown in Fig. 511.) On the abscissa is the partial pressure of either oxygen or carbon dioxide; on the ordinate is the content of either oxygen or carbon dioxide. Point a on each curve is the normal arterial point for content and partial pressure.
To the right of the graph are two lung units, one representing low V/Q and one high V/Q. The content of oxygen and carbon dioxide in the blood from each type of unit is represented on the dissociation curves.
Note that the effect of low V/Q units is to lower PO2 and raise PCO2; the shape of the dissociation curves dictates that the respective contents will also change in the same direction. The effect of high V/Q units is to raise Po, and lower Pco.; the shape of the dissociation curves dictates that this high V/Q unit can reverse the high PCO2 but not the low PO2. Thus any elevation in PCO2 from low V/Q units can be compensated for by PCO2 reduction in high V/Q units. These high V/Q units cannot compensate for the reduction in oxygen content since the oxygen dissociation curve is nearly flat in the range of high PO2 values.
The final carbon dioxide content in Fig. 513
is point a on the carbon dioxide dissociation curve, arrived
at by averaging the high and low V/Q points on the carbon dioxide
curve. The final oxygen content is point x, arrived at by averaging
the high and low V/Q points on the oxygen curve. Point x is between
the low V/Q and arterial points on the oxygen curve and represents
a PO2 considerably reduced from normal.
Fig. 514. Changes
in PaO2 and PaCO2 from V/Q imbalance. All
values are mm Hg. See text for discussion.
The important point is that hyperventilation in some units can compensate for the tendency of hypoventilated units to increase the PCO2. Thus if PCO2 is increased in the blood draining the hypoventilated units, hyperventilation of other units can compensate because the carbon dioxide dissociation curve (carbon dioxide content vs. PCO2) is almost linear and diagonal in the range of physiologic PCO2 values. To take advantage of this mechanism, the patient has to augment his minute ventilation and alveolar ventilation (Fig. 514). Patients who are able to increase their minute ventilation in the face of V/Q imbalance will usually manifest hypoxemia along with normal or low PaCO2.
Although the majority of patients with V/Q imbalance
do not manifest hypercapnia, some do, particularly when V/Q imbalance
is severe and chronic. Such patients are unable to sustain the
necessary increase in minute ventilation. The most common example
clinically is the patient with severe chronic obstructive pulmonary
disease whose V/Q imbalance has resulted in large amounts of alveolar
dead space. Such a patient must maintain an increased minute ventilation
to maintain a normal PaCO2. However, if the work of
breathing that is required to keep a high minute ventilation is
too great, the patient will opt for less work and hence an elevated
PaCO2; this occurs in many patients with severe chronic
lung disease. Even so, in such cases the basic cause of hypercapnia
is increased dead space from V/Q imbalance. V/Q imbalance is
the most common cause, not only of reduced PaO2, but,
by the mechanism just explained, also of elevated PaCO2.
The physiologic process of oxygenation involves (I) delivering oxygen from the atmosphere to the lungs where it is taken up by the pulmonary capillaries, and (2) transporting oxygen from the pulmonary circulation to all the body's tissues. Understanding the basic physiology of these two steps helps answer two important clinical questions: (I) Are the lungs properly transferring oxygen into the blood? and (2) is the patient adequately oxygenated?
The first question can be answered yes or no by determining the alveolararterial oxygen pressure difference (P(Aa)O2), also referred to as the "A-a gradient." The P(Aa)O2 is determined by subtracting the measured arterial partial pressure of oxygen PaO2) from the calculated alveolar partial pressure of oxygen (PAO2), with the latter obtained from the equation PAO2 = PIO2 (1.2 x PaCO2). For a patient breathing room air, normal P(Aa)O2 is agedependent, ranging from approximately 5 to 30 mm Hg. Normal P(Aa)O2 increases as the fraction of inspired oxygen FIO2) increases and can go up to approximately 100 mm Hg on 100% oxygen. If P(Aa)O2 is elevated above normal, the lungs are not transferring oxygen properly. An elevated P(Aa)O2 represents a parenchymal lung problem and is almost always caused by ventilationperfusion (V/Q) imbalance.
V/Q imbalance is an imbalance of the normal distribution of ventilation to perfusion among the millions of alveolar capillary units. Gas exchange (the transfer of oxygen and carbon dioxide between the alveolar air and the pulmonary capillary blood) is always disturbed in the presence of V/Q imbalance. Hypoxemia results when areas of the lung are relatively overperfused (or underventilated), a common occurrence in parenchymal lung diseases.
Graphic representation of V/Q relationships helps
explain the pattern of gas exchange abnormality seen clinically.
For example, patients with predominant chronic bronchitis are
hypoxemic because they have relatively large areas of underventilated
(overperfused) lung. Patients with predominant emphysema have
large areas of overventilated (underperfused) lung representing
extra dead space that they must ventilate with each breath; as
long as they can augment their total ventilation to satisfy the
extra dead space, hypoxemia does not occur.
State whether each of the following is true or false.
1. The most common physiologic cause of hypoxemia is diffusion barrier.
2. An increase in the alveolararterial PO2 difference most commonly arises from a ventilation/perfusion imbalance within the lungs.
3. Thebesian vessels supply the myocardium and drain the venous blood directly into the ventricles.
4. Alveolar PO2 is a function of, among other factors, PCO2, respiratory quotient, and altitude.
5. The alveolararterial PO2 difference may normally be as high as 50 mm Hg when breathing 100% oxygen.
6. Both the PaO2 and PaCO2 are agedependent, the former decreasing and the latter slightly increasing with age.
7. The reason why patients with ventilation/perfusion imbalance commonly manifest hypoxemia but not hypercapnia is the greater diffusibility of carbon dioxide compared to oxygen.
8. While breathing 100% oxygen, a patient's respiratory quotient becomes unity.
9. Hemodialysis can lead to hypoxemia by reducing alveolar ventilation.
10. Alveolar units that receive ventilation but no
perfusion have a ventilationperfusion ratio of infinity.
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See also General References (Physiology) in Appendix
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