At the conclusion of this session, students will be expected understand an approach to each of the following concepts of applied pulmonary physiology:
The A-a Gradient helps describe the efficiency with which oxygen is transferred from the atmosphere into the pulmonary capillaries. In an ideal alveolus-capillary unit, the partial pressure of oxygen in the alveolus equilibrates completely with that of the blood by the end of the pulmonary capillary. In such an idealized situation the gradient between the partial pressure of oxygen in the alveolus (P A O 2 ) and that of the arterial blood (P a O 2 ) would be 0. Even in perfect health, however, this does not occur, as there is always some amount of intra-pulmonary shunting resulting in a P a O 2 that is slightly lower than the P A O 2 ; this results in a gradient between the P A O 2 and the P a O 2 of approximately 10 mm Hg in young healthy individuals. In disease this gradient increases. While one can directly measure the P a O 2 , the P A O 2 can only be estimated based upon the following equation:
P A O 2 = [(P B - P H20 ) x F I O 2 ] - P a CO 2 /RQ.
By calculating the A-a Gradient, one can assess the degree to which oxygen transfer from the atmosphere to the blood is impaired.
- Calculate the expected P A O 2 for a normal person at sea level (P B = 760 mmHg), with fully humidified air at body temperature (P H2O = 47 mmHg), breathing room air (F I O 2 = 21 %), with a normal P a CO 2 (40 mmHg), and a normal respiratory quotient (RQ = 0.8).
The effect of variations in P B , F I O 2 , P a CO 2 , and RQ should be apparent.
- For this same individual, what would the A-a gradient be if the measured P a O 2 = 95 mm Hg? What if the P a O 2 = 60 mm Hg?
- Assuming no other changes, what is the A-a gradient if the P a O 2 = 60 mm Hg but the P a CO 2 = 60 mm Hg?
Note that hypoventilation, with resulting increases in P a CO 2 will result in some degree of hypoxemia with minimal if any increase in the A-a Gradient.
- Describe situations in which changes in each of the following parameters might affect the predicted P A O 2 and hence the P a O 2 :
- Increased P B
- Decreased P B
- Increased F I O 2
- Decreased F I O 2
- What is the effect of increased age on the predicted A-a Gradient?
- Finally, can you name any clinical situation in which calculating the A-a Gradient affects specific patient management?
Arterial Oxygen Content
The arterial oxygen content equation quantifies how much oxygen is carried in each deciliter of blood and illustrates the relative contribution of hemoglobin-bound versus dissolved oxygen.
- What is the equation for arterial oxygen content (C a O 2 )? |
The relative amount of oxygen bound to hemoglobin versus dissolved in the blood under normal circumstances should be apparent.
- What is the normal C a O 2 ?
The term Oxygen Delivery refers to the amount of oxygen delivered to the tissues per minute. Oxygen Delivery (D a O 2 ) is the product of Cardiac Output (CO) times Oxygen Content (C a O 2 ):
D a O 2 = CO x C a O 2
- Assuming a normal CO of 5 lpm, what is the normal D a O 2 (in ml/O 2 /min)?
- The term “Extraction Fraction” refers to the percent of delivered oxygen that is extracted (or consumed) by the tissues. Assuming that a normal person at rest has an oxygen consumption of 250 mL/O 2 /min, what is the normal extraction fraction?
Mixed Venous Oxygen Content
The amount of oxygen contained per deciliter of mixed venous blood is referred to as the C MV O 2 . The formula for the C MV O 2 is identical to that for the CaO 2 except that the oxygen saturation refers to the mixed venous saturation.
- What is the normal C MV O 2 ?
- What are the two general explanations for a reduced C MV O 2 ?
Overall, the C MV O 2 allows one to assess the adequacy of oxygen delivery to the tissues relative to oxygen consumption.
Mechanisms of Hypoxemia
Remembering the general mechanisms of hypoxemia (and how to distinguish one from another) helps one determine the specific cause of hypoxemia.
- What are the mechanisms of hypoxemia?
- For each mechanism, is the resulting hypoxemia generally mild or severe?
- For which mechanisms is the resulting hypoxemia generally responsive to supplemental oxygen?
Acid-Base (Covered in Another Setting)
- Qualitative Recognition of Metabolic/Respiratory Acidosis/Alkalosis
- Quantitative Equations
?? How does one identify obstruction and/or restriction on PFT's ??
The main reason to obtain PFT's is to determine whether there is obstruction or restriction as an explanation for a patient's dyspnea. Every medical student/physician should be able interpret PFT's for these two findings without difficulty.a. Obstruction
1. Flow Volume Loop Interpretation
2. FEV 1 , FVC, FEV 1 /FVC
1. Total Lung Capacity
a. (Covered in Another Setting)
?? What is the normal limiting factor to one's maximum exercise capacity ??
?? What is meant by the term “Anaerobic Threshold” ??
a. Normal Limitation to Exercise.......Heart vs Lungs vs Peripheral Muscle
b. Resting VO 2 , VCO 2 , RQ
?? What is meant by the term “Dead Space” ??a. Normal Values at Rest and During Exercise
?? What are the implications of having higher than normal Dead Space ??