FIO2
PAO2
PaO2
PaCO2
A-a O2
difference
FIO2
helpful?
The Patient: a 49-year-old man
Principal Complaint: breathlessness during even moderate activity
History: The patient reported shortness of breath that had become progressively more severe over the last five years. He is unable to work and becomes dyspneic after walking a single block. He has a history of several chest colds each year and has had a productive cough for more than half his life. The patient has smoked two or three packs of cigarettes a day since his late teen-age years.
Clinical Examination: The patient’s arterial pressure was 130/80 mm Hg, and his heart rate was 80 beats/min. His nail beds were blue, but no clubbing of the fingers was evident; 2+ ankle edema was present. His breathing was labored, and he used his accessory muscles for breathing. The anterior-posterior diameter of his chest was increased, and the chest tended to remain in the inspiratory position during the respiratory cycle. Radiographic examination revealed flattened diaphragms (intense contraction) and minimal excursions with ventilation. The heart sounds were loudest over the epigastrium but were otherwise normal. Electrocardiographic examination revealed right ventricular hypertrophy. The hemoglobin concentration was 17 gm/dl, the hematocrit was 50.5%, and the bicarbonate concentration was 30 mEq/l, PaO2 was 44 mm Hg, PaCO2 was 60 mm Hg, and the hydrogen ion concentration was 44 nEq/l. When the patient was given 100% oxygen to breathe, his PaO2 increased to 290 mm Hg, PaCO2 increased to 70 mm Hg, and the hydrogen ion concentration increased to 56 nEq/l. The results of pulmonary function tests are as follows: TLC, 7200 ml (131% of predicted); RV, 5100 ml (364% of predicted); FRC, 5700 ml; RV/TLC, 0.72 (288% of predicted); FVC, 1500 ml (35.8% of predicted); FEV1.0, 1100 ml (34.4% of predicted); (FEV1.0, 1200 ml after bronchodilator); FEV1.0/FVC, 0.41; MVV, 27 l/min (21 % of predicted); CO diffusion capacity, 8 ml/min/mm Hg (33% of predicted).
|
FIO2 |
PAO2 |
PaO2 |
PaCO2 |
A-a O2 |
|
| 21% | 75 | 44 | 60 | 31 |
-- |
| 100% | 626 | 290 | 70 | 336 |
Yes |
STUDY QUESTIONS:
1. From the history and from the results of the clinical examination and pulmonary function tests, what is the most likely explanation for this patient’s breathlessness? Support your answer with data from the REFERENCE CASE. The FEV1.0/FVC= 0.41 indicates an obstructive disease probably emphysema/COPD. The person has to work harder because of this making him "breathless".
2. What is the most likely cause of the low carbon monoxide diffusion capacity in this patient? Because of the destruction of alveolar walls, there is a decrease in surface area for gas diffusion.
3. What does the value for arterial PO2 during breathing with 100% oxygen reveal about gas exchange in this patient? What would be the expected value for PaO2 while breathing pure oxygen and what might cause the large discrepancy between the expected and observed values? A normal A-a gradient for a 49 year old should be around 21 so a PaO2 of 605 could be expected. Because of the decreased diffusion area, the A-a gradient is large.
4. What is this patient’s calculated alveolar PO2 while breathing room air? While breathing 100% oxygen? See table above.
5. What effect does emphysema have on physiological dead space? Why? It increases it. Gas will only diffuse a short distance in a short period of time (during breaths). Emphysema collapses normal (small) alveolar walls making BIG alveolar. Much of the gas that is inspired just sits in the middle of these large spaces without diffusing (dead space).
6. What effect does emphysema have on physiological shunting? Why? It increases it by creating a shunt like area. The PAO2 is normal but the PaO2 is low.
7. Of the five [5] possible causes of low PaO2 discussed in class, which are present in this patient? Hypoventilation. Shunt. Diffusion. V/Q mismatch.
Last Updated 04/10/00 12:27:10 PM
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