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Calculation of the A-a Gradient

PAO2 = PIO 2 – PaCO 2. R. Calculation of the A-a Gradient. 1. Calculate the P A O 2. Where. P I O 2 = (F I O2) * (P B -P H 2 O ). At sea level: FIO 2 = 0.21, PB = 760 mmHg, PH 2 O = 47 mmHg  PIO 2 = 150 mmHg R = 0.8 (alternatively you can multiple by 1.25)

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Calculation of the A-a Gradient

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  1. PAO2 = PIO2 – PaCO2 R Calculation of the A-a Gradient 1. Calculate the PAO2 Where PIO2 = (FIO2) * (PB-PH2O) At sea level: FIO2 = 0.21, PB = 760 mmHg, PH2O = 47 mmHg  PIO2 = 150 mmHg R = 0.8 (alternatively you can multiple by 1.25) For a normal PaCO2 = 40 mmHg: PAO2 = 150 – (40*1.25) = 100 mmHg

  2. 2. Calculate the A-a Gradient (A-a)gradient = PAO2 – PaO2 Calculation of the A-a Gradient If the PaO2 = 75 mmHg: (A-a)gradient = 25 mmHg

  3. Schema for interpreting primary acid-base disturbances pH <7.35 >7.45 7.35-7.45 CO2 CO2 <35 >45 CO2 Metabolic Acidosis HCO3- <23 Metabolic Alkalosis HCO3- >26 >45 <35 35-45 HCO3- HCO3- Normal 23-26 >27 <23 23-26 Acute Respiratory Acidosis Chronic Respiratory Acidosis Chronic Respiratory Alkalosis Acute Respiratory Alkalosis

  4. Case I • A 22 year old male presents to the emergency room difficulty breathing. He states he has had a productive cough with thick mucoid sputum for 3-4 days. He has noted dyspnea on exertion, especially when running in the cold for about the past year. In the past few days, he has begun to have difficulty in breathing which wakes him at night. Today he is having difficulty performing his routine activities due to shortness of breath. His past medical history is positive for hay fever and eczema as a child. He is a life long nonsmoker.

  5. Case I • A 22 year old male presents to the emergency room difficulty breathing. He states he has had a productive cough with thick mucoid sputum for 3-4 days. He has noted dyspnea on exertion, especially when running in the cold for about the past year. In the past few days, he has begun to have difficulty in breathing which wakes him atnight. Today he is having difficulty performing his routine activities due to shortness of breath. His past medical history is positive for hay fever and eczema as a child. He is a life long nonsmoker.

  6. Case 1 • Exam: He is entirely normal except for tachypnea (RR = 32) and bilateral expiratory wheezing. • Labs: ABGs: pH 7.26 PCO2 56 PO2 65 • CXR: Normal.

  7. Case 2 • A 51 year old female is admitted with severe shortness of breath for the past 24 hours. Three days prior to admission, she developed nasal congestions, sore throat and generalized myalgias. The following day, she developed a cough productive of white mucoid sputum. She admits to a 4 year history of progressively worsening dyspnea on exertion. For the past several months she has only been able to climb ½ a flight of stairs before becoming short of breath. She is not bothered by chronic cough. She has smoked one pack of cigarettes a day for the past 20 years, and has one sister with breathing problems.

  8. Case 2 • Exam: Thin, tachypnea 51 year old female in moderate respiratory distress. BP 140/80; RR 30; P 120; a febrile. She is not cyanotic but is breathing with pursed lips and using accessory muscles. Breath sounds are diminished over the entire chest, but diffuse expiratory wheezing is heard. Heart sounds are also diminished, being heard best in the epigastrium. The remainder of the physical exam is unremarkable. • LAB: ABGs on room air at rest: pH 7.42 PCO2 32.3 PO2 70 SaO2 91%

  9. Case 3 • History: A 28 year old male complains of chills, fever, pleuritic chest pain and cough productive of rusty sputum for the past 2 days. The remainder of the history is unremarkable.

  10. Case 3 • Exam: Alert, tachypneic 28 year old male in moderate respiratory distress. BP 100/60, RR 20, P 120, T 102 F. His lips are cyanotic. The chest reveals percussion dullness, bronchial breath sounds and increased fremitus and egophony over the right lower chest posteriorly. The remainder of the physical exam in normal. • LAB: Hb 14, WBC 15,000 with a marked left shift. ABGs: pH 7.48, PCO2 30, PO2 50.

  11. Case 4 • History: A 35 year old male alcoholic complains of night sweats, weight loss and cough productive of yellowish sputum for the past 2 months. • Exam: Cachectic 35 year old male who is weak but in no respiratory distress. BP 110/70, P 100, RR 16, afebrile. Scattered rhonchi are noted throughout the chest bilaterally and bronchial breath sounds are heard over the left upper chest posteriorly. The remainder of the exam is unremarkable. • Lab: Hb 9, WBC 10,000, with a normal differential. ABGs: pH 7.40, PCO2 40’ PO2 75, SaO2 93%. PPD skin test is negative.

  12. Case 5 • H & P: A 55 year old male presents with no complaints but had a abnormal CXR taken during a routine physical exam. Other than a 40 pack year smoking history, he has an entirely normal history and examination. • Lab: normal, including ABGs.

  13. Case 6 • History: A 33 year old black female is referred to you because of an abnormal CXR which was taken as part of a pre-employment physical. On questioning, she is found to have had dry cough and dyspnea on exertion for about one year. These symptoms have been getting progressively worse. The remainder of the history is unremarkable.

  14. Case 6 • Exam: Alert, oriented, well developed, well nourished black female in no distress. Vital signs are normal. Physical exam is normal. • ABGs: pH 7.47, PCO2 33 PO2 89

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