Friday, October 29, 2010

Evaluating hypoxia - not that tough!

This is one of the most common finding for ICU transfer/admission. And there are many ways to evaluate hypoxia. Lets try and do  a simple way which would cover the common problems causing hypoxia in clinical practice. 
OK……the usual situation will be that you are called to see a patient with reduced O2 saturations on the wards….and an ABG, chest xray has been done and pt is on O2. (If not done, you need these to make a good decision ). While waiting for these to be done ….we can do the most important thing..taking a SHORT focused history and exam (many times this will give us the clue and we can use the test to confirm them). If not..then lets go through the tests

First thing to assess is that whether this hypoxia is due to pulmonary or extrapulmonary causes( sedatives, neuromuscular weakness, central hypoventilation). The A – a gradient will help. It can be  calculated by the formula  PAO2 (  713 x FiO2 ) – PaO2 – PaCo2 – PaCo2/4 .  Normal gradient is <12 and differs with age…so use age/ 4 +4 to correct for age. Ok..back to the evaluation- If A-a gradient is normal then its likely extrapulmonary cause(usually they have resp acidosis). If it is normal then look at PaCo2. If high..that will suggest airway obstructive disease ie. COPD/asthma. If PaCo2 id normal then have a look at the Chest Xray(CXR). If the CXR is normal then its likely to be a Pulmonary embolism (if the history correlates…..give a shot of heparin(Fondaparinux or Enoxaparin) and send the patient for CTA of chest).  If CXR is abnormal and shows a focal infiltrate then its likely to be pneumonia or atelectasis. If  infiltrates are diffuse then there are 2 possibilities – cardiogenic pulmonary edema or non cardiogenic pulmonary edema(ARDS). The ideal way to distinguish them is to do a Pulmonary artery wedge pressure. But this is not done in recent days as it does not seem to improve mortality. So …we have to rely on factors like ECHO(if done already), EKG, CVP and History.  We can try giving a bolus of Frusemide  to see if it helps. Both may get better, but usually patients with CHF might find a quicker relief.  One other  point here….is to always compare this CXR to a previous one(esp when pt. was stable)

Throughout this evaluation ..history will be helpful. I have found it useful go back to the history(mainly the presenting symptoms) when Iam stuck in the process of evaluating these numbers( any numbers!).                      
So to summarise…the order with which we can look at the numbers ..
1.      1. A-a gradient   2. PaCo2   3. Chest Xray  4. EKG,ECHO,JVD,CVP etc
     Whenever  we are stuck….always get back to the history!


  1. i use this, PAO2 = 150 - PaCO2/.8 assuming we are not in space or doing deep sea diving and in room air and ofcourse have a Pa02 from just simplified.

  2. You are absolutely right. I didn't write 150 as it applies for only room air. And also ..I prefer to work without the decimals.mine is just a derivation from what you ve mentioned.
    PaCo2/0.8 =5PaCo2/4 = 1.25 PaCo2 = PaCo2 +PaCo2/4.

    but you are right....if its room air ...PAO2 is taken as 150 ...(760 - 47 which is water vapour pressure)x FiO2(0.21 for breathing room air)
    Thanks da

  3. How did you arrive at 713.Is it not 760 the atmospheric pressure.
    I have always used 150 for PA02, which is straightforward and easy.

  4. 150 applies if the patient is on room air not otherwise. Do we not use 760 mm Hg at sea level?

  5. 713 mmHg is after removing the vapor pressure of water from air

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