Saturday, March 12, 2011

Intern Report Case Discussion 3.3

Case Presentation by Dr. Ryan Doss

This patient is suffering from acute chest syndrome, a complication of sickle cell disease and currently the leading cause of death in patients with SCD in the United States (with a total mortality in adults of 9%).  As we all know, in sickle cell disease the sickling of red blood cells is precipitated by hypoxia. Any condition which results in a relative hypoxia in the pulmonary microvasculature can thus cause an attack localized to the lungs (or of course distributed more diffusely). 

ACS is more common but less deadly in the pediatric age group, especially ages 2-4, and the incidence gradually decreases with age.  In younger patients an attack of acute chest syndrome is most often brought on by an infection with viruses or bacteria. Fat microembolization from marrow necrosis - itself a complication of a more generalized sickle cell crisis - is more common in adults. Some other causes include asthma, atelectasis from splinting secondary to rib pain, or iatrogenic fluid overload/opiate-induced respiratory depression.

Given his age we can speculate that this patient's ACS is caused by either a fat embolus from his days-long sickle cell crisis or atelectasis from his right lower rib cage infarction. Practically, however, it doesn't really matter what caused his current condition. The management of ACS from any cause involves supportive care, empiric antibiotics, and exchange transfusion.

Aggressive pain control and oxygenation serve to reduce the sickling of the patient's RBCs by decreasing splinting, improving tidal volume, and helping to reduce the localized hypoxemia. Pulse oximetry will probably be inaccurate but can be useful as an approximation. A reading on room air below 92% should be evaluated more fully with an ABG. Calculation of the A-a gradient helps guide the disposition. There are no agreed-upon standards, but in general an elevated A-a gradient or a rapid rise in the gradient should steer the ED physician toward intubation and ICU admission. Inhaled bronchodilator therapy may be helpful, especially if the patient has a history of asthma.

Empiric antibiotics should include coverage for Chlamydia pneumoniae, Mycoplasma pneumoniae, and Streptococcus pneumoniae, the three most commonly encountered bacteria in infectious ACS (with Chlamydia pnemoniae being most common). This would involve a third-generation cephalosporin paired with a macrolide or quinolone.

Simple transfusion of 2-4u PRBCs should be considered early after the diagnosis of ACS. This appears to be as effective as exchange transfusion unless the patient's clinical condition is more severe. It is important to maintain a Hb below 10 mg/dL because higher Hb levels can cause hyperviscosity in the microvasculature in sickle cell patients. This is not normally a concern with simple transfusion due to the sickle cell crisis patient's acute drop in Hb.


1. As discussed above, calculation of the A-a gradient is an important management tool for the patient with moderate-severe ACS. But nobody wants to do that, including me, because math is hard. Especially when you have a patient in front of you with chest pain struggling to breathe. So let's take this a step at a time. (Or skip ahead to the end of this question if you just want to memorize the equation which is totally cool but cowardly).

First: the A-a gradient is simply the difference between the oxygen in the alveoli and the oxygen in the artery. We know the pressure of oxygen in the artery from the ABG measurement (81). There we're halfway done! (Not really).

The second half involves calculating the pressure of oxygen in the alveoli. Let's start by assuming we're at about sea level and the barometric pressure is 760mmHg. Multiply that by the room air FIO2 (21%, duh) and we have our oxygen pressure (159.6 or 160).

Unfortunately, we can't just use the calculated pressure of oxygen in the surrounding atmosphere. First of all, the pressure of oxygen in the air we inspire changes on its way through our respiratory tract. This is because the atmosphere becomes more humid as the air enters our body. The water vapor pressure (47mmHg) needs to be subtracted from the atmosphere's barometric pressure. Wait...why would we subtract water vapor pressure from barometric pressure? It seems like we should be adding it. BUT the molecular mass of water vapor is actually lower than that of dry air. Ok, crap, rewind and subtract 47 from 760 to get 719. Now 719 x .21 = 150.99 or 151.

If you're still reading at this point, I love you. And I'm sorry because it's about to get a whole lot worse.

Ok so the alveolar pressure of oxygen is 151! Now let's just compare that to the oxygen in the arteries! NO, you FOOL. When we measure the arterial pressure of oxygen we are measuring the oxygen left over in the artery AFTER the tissues take what they need. We don't want to compare the oxygen in the alveoli to the oxygen in the artery AFTER the body uses what it needs. We want to compare the gradient at the exact moment they straddle the air/blood barrier. So we need to add the oxygen the body consumed back into the arterial oxygen reading. OR we need to subtract it from the PIO2. Either way. Whoever wrote down the alveolar gas equation did it the second way since it's less intuitive (hooray...).

The respiratory quotient is a number derived by experimentation (for completely unrelated reasons...namely to calculate BMR based on different types of caloric intake) which equates to (CO2 eliminated/O2 consumed). It is a constant (for our purposes) and this constant is 0.8. So we just need to know CO2 eliminated and we'll know our O2 consumed, which we can then add back into our PaO2. If you think about it we kind of already know our CO2 eliminated, because we measured the CO2 in our artery with the ABG and our body HAS to get rid of all of that or we'll just steadily accumulate CO2 until we die. The PaCO2 in our ABG is 31. Rearrange (R=CO2 eliminated/O2 consumed) and you get (O2 consumed = CO2 eliminated/R or 31/0.8 or 38.75 or 39).

Oh god we're getting so close. Now add our O2 consumed (39) back to our measured PaO2 (81) to get 120. Subtract 120 from our calculated PAO2 (151) to get 31! Normal is less than 10. This guy is screwed.

Or you could memorize (150 – PaCO2/0.8) – PaO2. Whichever you prefer.

Er...P.S. The alveolar gas equation is PAO2 = FIO2(Patm – PH2O) – (PaCO2/R)

Answer: B

2. I don't really have a long-winded explanation for this one. According to Principles of Critical Care, neurologic complications including seizure and stroke are the most common complications of acute chest syndrome. Cardiac, gastrointestinal, and renal complications are infrequent.

Answer: C

3. Again, no big explanation here. This is just experimentally determined as far as I know. If anybody has a hypothetical explanation I'd be very interested in hearing it.

Answer: A

4. So a lot of these would be useful in some way, and arguments could be made for each. I'll skip over A and D and E as they don't involve any treatment (well, not any scientifically proven treatment anyway) and presumably you'd be most interested in treatment.
Answer C could be useful to decrease splinting and increase oxygenation. Plus with a 9% mortality and no REAL effective medical treatment you might as well go out with a smile on your face.
Answer B is a devious misdirection. Phlogisticated (i.e. gaseous) laughing anesthetic is nitrous oxide (N2O). According to Tintinalli's, “Inhaled nitric oxide has shown benefit in the management of acute chest syndrome, apparently due to its vasodilatory effects that improves the coordination between ventilation and perfusion in the damaged lung regions with minimal systemic absorption. In addition nitric oxide reduces adhesion of RBCs and leukocytes to endothelial cells by decreasing the activity of vascular cell adhesion molecule-1.” But of course the chemical formula for nitric oxide is NO and not N2O! HAHAHA you fools! To my knowledge nitrous oxide does not have the useful vasodilatory effects nitric oxide does in the treatment of ACS. While it might be useful in the same sense that heroin would be (anesthesia) you would be decreasing your FIO2 by displacing O2 with another gas. Then again, chugging a bottle of baby heroin would presumably decrease your respiratory drive so I guess you can feel free to pick your poison.

In light of this snap reconsideration, I will accept both B and C.

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