I am currently in the waiting room in the ambulatory surgery department waiting for Greg who is having cataract / lens replacement surgery because oxidation, despite weeks of prone positioning designed to prevent it, clouded his vision after multiple retinal tears and vitrectomies. This procedure should be the end of it, the last step in a nearly decade long process we’ve titled “Ageing sucks but it beats being dead.”
At least for now that seems true. But we’re getting old. And as we age, the gap between the pain of existing and the joy of being alive grows narrower procedure by procedure, prescription by prescription, diagnosis by diagnosis, injury by injury and limitation by limitation, until we are left in a second childhood facing oblivion sans teeth, sans eyes, sans taste, sans everything. Shakespeare. So cheery. And that is from one of his “comedies.”
Lung function peaks around age 20. This is commonly measured by what we call the A-a gradient, the partial pressure of alveolar oxygen minus the partial pressure of the arterial oxygen. Because O2 moves in one direction across the alveolar membrane to the capillaries, the alveolar pressure is always higher. How _much_ higher is a reflection of alveolar integrity or the diffusion capacity of the lungs. And that integrity and diffusion capacity, no matter how we might fight it, no matter what good of shape we are in, decreases with age. Now I sound as morbidly depressed as Jacques in As You Like It. This is distinctly NOT as I like it.
An estimation or expected A-a gradient is calculated using the equation G = (age/4) + 4. So at age 20, an average gradient would be 9 torr. At age 80 it would be 24 torr.
PaO2, arterial O2, requires a blood draw. This is common, and sometimes super easy if a patient has an ART line in and one has direct access to their arterial blood without the need for another needle stick. PAO2, alveolar O2, however, requires math. No one in medicine seems to like math but me. So the A-a isn’t often a topic of discussion or a measure of therapeutic impact.
We do have a simpler derived option. The P/F ration. In this equation P = PaO2, and F = FiO2 or Fraction of inspired O2. For people breathing room air, this is .21. But the majority of patients for which such numbers would even be of interest are being mechanically ventilated either invasively or non-invasively.
If you remember from previous posts, and you probably don’t because no one reads this blog and you’re probably not even reading this, the FiO2 is a major variable in the Alveolar Air Equation used to calculate PAO2. The other variable of note is PaCO2 but its impact is far less than that of FiO2 on the PAO2, so we can simply choose to ignore the requisite math and use FiO2 in its place and the ratio of PaO2/FiO2 as a measure for overall lung health. And tracking this ratio actually is a used and useful measure of therapeutic impact. So much so that this ratio is one of the main diagnostic measurements in the Berlin Definition of ARDS. Along with timing, chest imaging, and origin of edema, the P/F ration dictates the severity of the disease as shown below.
Tracking this ratio can indicate the progress of the disease and let us know if the patient is about to shuffle off this mortal coil or shuffle off to buffalo.
OK, Greg is out of surgery and I have to take him home now where we will drink and eat. Because he may be temporarily sans eyes, he is not sans teeth nor sans taste. Jacques was such a drama queen.