August 1, 2018
From the Appeals Desk: Acute Respiratory Failure (Part 2/2)
In part 1 of this column, we discussed how “airway protection” is not a diagnosis and that it is essential that we understand the proper definition of hypoxia in order to document and accurately capture the diagnosis of acute respiratory failure. I continue to insist that a low pO2 does not always mean hypoxia. If the saturation is above 90%, regardless of the pO2, then there is no hypoxia. The key caveat, however, is that this is only is true on room air. If there is any supplemental oxygen on board, my previous “dictum” does not apply. And when there is a substantial amount of supplemental oxygen, then the pO2 does matter. In this case, we need to calculate the P/F ratio. The P/F ratio is used to quantify the degree of hypoxia and is calculated by dividing the pO2 by the FI02. Remember the FIO2 is the “fraction of inspired oxygen,” and it is a percentage. Room air is an FIO2 of 21% (.21). So, for example, if I get an arterial blood gas on a patient and the pO2 is 85 with an FIO2 of 85%, the P/F ratio will be 85/0.85 or 100. This is severe hypoxia, even though the oxygen saturation on this arterial blood gas may be well above 90%. This all stems from the alveolar gas equation: FIO2 * (713) – 50 At an FIO2 of 85%, the expected pO2 would be 556 (.85*713 – 50). Thus, a pO2 of 100 on an FIO2 of 85% is quite bad. The definitions of the degree of hypoxia comes from the ARDS literature. The most recent Berlin definition of ARDS lists the following P/F ratios as mild, moderate, and severe hypoxia:
- Mild: 200 mm Hg < PaO2/FIO2 ≤ 300 mm Hg
- Moderate: 100 mm Hg < PaO2/FIO2 ≤ 200 mm Hg
- Severe PaO2/FIO2 ≤ 100 mm Hg
Consequently, documenting the P/F ratio in your note will add robustness to your diagnosis of hypoxia and will make denial of your diagnosis of respiratory failure by a third party much more difficult.
The A-a Gradient
Another way to look at the P/F ratio is the A-a gradient. This is the difference between the expected pO2 and the measured pO2 on the arterial blood gas. Thus, in our above example, the A-a gradient would be 471 (556-86). The normal A-a gradient is Age/4 + 4. If the A-a gradient is greater than expected, then a cogent argument can be made for the presence of hypoxia. Please note that I made a lot of assumptions for my calculations above because I wanted to spare you the pulmonary pathophysiology. The last thing I want to do is to give you nausea and vomiting while reading this. This entire discussion relies on measuring an arterial blood gas. While I recognize that obtaining an arterial blood gas sample may be difficult, the multiple assumptions I have made above do not apply when a venous blood gas sample is used. Another thing I frequently see in charts that I review is that true hypoxia is present and supported in the labs, but the diagnosis of acute hypoxemic respiratory failure is not documented. This is selling yourself and our hospital partners short. Acute hypoxemic respiratory failure dramatically increases the severity of illness, and it increases the case mix index, which is very important to hospital partners. Further, a higher severity of illness means the risk to the patient is much higher, which is an important component of the documentation necessary to adequately support the appropriate E/M code for the hard work you have done. Therefore, if acute respiratory failure is present, it should be properly documented. Contrariwise, if the diagnosis of acute respiratory failure cannot be supported (as discussed throughout this post), then it should not be documented.
In summary, the following points are essential when documenting acute respiratory failure and hypoxia:
- There is no diagnosis called “airway protection.” You need to clearly document why a patient needs intubation, which is an invasive procedure, and mechanical ventilation, which is fraught with serious possible complications. Documenting “airway protection” greatly increases the risk of denial, which can put tens, if not hundreds, of thousands of dollars at risk for our hospital partners.
- Just because the pO2 is marked as “low” on an arterial blood gas, it does not necessarily mean that the patient is hypoxic. If the oxygen saturation is above 90%, then a diagnosis of hypoxia is not supported. This, however, is only applicable if the arterial blood gas sample was measured on room air.
- If the arterial blood gas sample was measured on supplemental oxygen, then we should get into the habit of documenting the P/F ratio or A-a gradient, which will strengthen the support for acute hypoxemic respiratory failure in our documentation.
- If there is a patient with hypoxia, remember to document “acute hypoxemic respiratory failure,” which increases the severity of illness and give you credit for all the hard work you do in terms of case mix index and relative value units.
We work very hard each and every day taking care of our patients and doing everything we can to heal and rehabilitate them. It takes a tremendous amount of time and effort to do this, and it is part of the reason why our work can sometimes be very difficult. It is true that the many documentation requirements can seem onerous and overwhelming. At the same time, it is also essential that we support the wonderful work we do by proper documentation. It only takes a few seconds longer, and it can make all the difference.