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A. Acute Respiratory Failure

==Table On==
Cause History Physical Exam Chest XRay AadO2* ECG
Pulmonary Edema CHF Rales, Wheezes "Wet",Large Heart very high Tachy
Pneumonia Fever/Chill E to A, Bronchial Infiltrate high Tachy

P. Embolism Stasis, Cancer None Large pulm artery high or NL RAD**,S1Q3T3
Pneumothorax Trauma, COPD Decreased Breath Diagnostic NL or high Tachy, Axis
COPD/Asthma Chronic Dz Wheezes, Rhonchi Hyperexpanded NL or high Tachy, Block
Foreign Body Drugs, EtOH Breath Sounds Maybe Diagnostic NL Tachy
* AadO2=Alveolar-Arterial Oxygen Gradient (NL=normal is < ~20mmHg, age depedent)
**RAD=Right Axis Deviation; classically have S in I, Q in III, inverted T in III
In fact, sinus tachycardia is most common finding in pulmonary embolism
B. Hypoxemia (in decreasing order of occurrence)
  1. Hypoventilation
    1. Causes hypercapnia and hypoxia (no A-a gradient because of CO2 elevation )
    2. Common in drug overdose, stroke syndromes
  2. Ventilation/Perfusion Inequality
    1. "V/Q Mismatch"
    2. Most common cause for hypoxemia in chronic lung disease
    3. Seen in Chronic Bronchitis, Asthma, others
    4. Also seen in ARDS (mixed picture), congestive heart failure, others
  3. Shunted Blood
    1. Blood does not perfuse aerated lung - V/Q is infinity
    2. Pulmonary infarction, embolism, pneumothorax, others
  4. Impaired diffusion
    1. Relatively rare cause of hypoxia
    2. Does not cause hypercapnia (until very late stage)
    3. This is why patients with emphysema have normal CO2 levels until late stage

C. Hypercapnia

  1. Ventilation/perfusion inequality: mismatch causes impaired CO2 transfer
  2. Hypoventilation - stroke, brain injury, medications, respiratory muscle fatigue
  3. Forced (Permissive Hypercapnea) - mechanical ventialation technique

D. Impaired Diffusion

  1. Diffuse interstitial fibrosis (chronic)
  2. Sarcoidosis
  3. Asbestosis / silicosis
  4. Alveolar cell carcinoma
  5. Reduced membrane area
    1. Pneumonectomy
    2. Emphysema
    3. Pneumonia
  6. Edematous Membranes: cardiogenic and non-cardiogenic pulmonary edema, anasarca