Moral Uncertainty, Lobster, and ABGs

April 09, 2017

Hello everyone,

We're in Wildwood, NJ this weekend where the girls danced in a feis, which is an Irish Dance Festival. Gabrielle finished 6th and just missed winning a sash. Francesca needs to practice. We ended Saturday at the Lobster House, by the shore in Cape May. The dinner decisions were relatively straightforward- 3 lobsters for 3 adults (2 steamed, one broiled), and lobster bisque for the 11-year-old. After that, Francesca and I strolled on the beach, collecting shells as the sun set.

I listened to lots of podcasts on the five-hour drive to Wildwood, including an interview on moral uncertainty with the Oxford philosopher, William MacAskill. I won't trouble you with the details (unless you demand it) except to note that moral uncertainty acknowledges distinct and sometimes conflicting norms, and pushes us to choose between exclusive paths when we want to do the right thing.

For example, some ethicists think animals have a moral status; others don't. If animals have a moral status, how can we justify experimenting on them? Or eating them? If they don't, why do we protest factory farms? Or cry when the dog dies? A philosophy of moral uncertainty could guide us through vexing decisions on animal welfare- or abortion, physician assisted death, and other controversial issues.

But I'm sure most of you have less controversial topics on your mind today. So instead of dwelling on my morally conflicted love for both animals and lobster, let's consider an issue we should all agree upon. Let's discuss blood gases.

I'm no physical chemist, but I know ABGs. They've been on my mind recently, particularly after attending in the MICU. For years, I've been struck by the near philosophical deliberation that goes into interpreting acid-base disturbances- even without getting into anion gaps or delta-deltas. Let's call it "ABG uncertainty." But unlike moral uncertainty, there should be no ABG uncertainty, assuming your lab measurements are accurate, as they almost always are.

The pH is the negative logarithm of the proton concentration, [H+]. A low pH (acidosis) occurs when the [H+] is high; a high pH (alkalosis) occurs when the [H+] is low. Proton concentrations are determined by the following formula, which includes three mutually dependent variables:

[H+] = Ka [PCO2] / [HCO3]

As you can see, acidosis results from any combination of a high [PCO2] and low [HCO3]. Alkalosis results from any combination of low [PCO2] and high [HCO3]. On ABG machines, the [H+] and [PCO2] are measured directly and the [HCO3] is calculated with this formula (related to the Henderson Hasselbalch equation). Some people disparage the [HCO3] because it is calculated and presumed to be less accurate. This is unfair to the poor [HCO3]. If the [H+] and [PCO2] measurements are accurate, this equation tells you what the [HCO3] MUST be. If the simple calculation is done correctly, the [HCO3] MUST be accurate.

For now, let's not worry about measurement error. There are only five primary acid base interpretations:

1. Normal (normal pH, normal PCO2, and normal HCO3) 2. Metabolic acidosis (low pH, low HCO3) 3. Metabolic alkalosis (high pH, high HCO3) 4. Respiratory acidosis (low pH, high PCO2) 5. Respiratory alkalosis (high pH, low PCO2)

Patients can also have combined or mixed disturbances:

1. Combined respiratory alkalosis, metabolic alkalosis (high pH, low PCO2, high HCO3) 2. Combined respiratory acidosis, metabolic acidosis (low pH, high PCO2, low HCO3) 3. Mixed respiratory alkalosis, metabolic acidosis (low PCO2, low HCO3), with the pH being normal, low, or high depending on the magnitude of the PCO2 and HCO3 disturbances 4. Mixed respiratory acidosis, metabolic alkalosis (high PCO2, high HCO3), with the pH being normal, low, or high depending on the magnitude of the PCO2 and HCO3 disturbances

Finally, the kidneys can compensate for respiratory disturbances and the lungs can compensate for metabolic disturbances. Over a couple of days, the kidney can spill HCO3 to offset the effects of a respiratory alkalosis, or they can retain HCO3 to offset the effects of a respiratory acidosis. Almost immediately, the lungs can hypoventilate and retain CO2 to compensate for a metabolic alkalosis (usually only slightly, though, because you have to breathe!). Most importantly, normal lungs will immediately hyperventilate to compensate for a metabolic acidosis.

Your mission, if you choose to accept it (please do), is to determine if the kidneys and lungs are compensating appropriately, in which case the patient has a compensated disturbance. If patients overcompensate or undercompensate, you have a problem to address.

I'm getting too close to a producing a Sunday morning renal or critical care chapter, so let's just say that you need to memorize Winter's formula to recognize when your patient with a metabolic acidosis is compensating effectively:

PCO2 = 1.5 x [HCO3] (measured on the same blood gas) + 8 +/-2

With Winter's formula, you can distinguish between three scenarios:

1. Compensated metabolic acidosis (low pH, low HCO3, PCO2 appropriately low) 2. Combined metabolic acidosis, respiratory alkalosis (low pH, low HCO3, PCO2 lower than it should be) 3. Metabolic acidosis with inadequate respiratory compensation (low pH, low HCO3, PCO2 not as low as it should be).

When you recognize scenario #1, you can just focus on fixing the metabolic acidosis. When you have scenario #2, you have to determine why the patient has a respiratory alkalosis. When you have scenario #3, you have to respond immediately, because the patient's respiratory compensation is failing and ventilatory support may be needed. This happened on my MICU team last week when our Winter's calculation told us we had to start a patient on BiPAP.

The beauty and simplicity of blood gases is that you can figure out all you need to know about an acid base status by knowing the pH, PCO2, and HCO3. In fact, when you get really good at ABG interpretation, you just need to know two of the numbers because the third is restricted by the other two, according to Henderson Hasselbalch.

I encourage each of you to practice your ABGs over and over, so you can interpret any blood gas instantly. As a reward you won't ever struggle with ABGs again. And, unlike questions of moral uncertainty, or whether to broil or boil the lobster, you'll be able to make crucial decisions for patients accurately, confidently, and quickly.

With that, we're heading out to the boardwalk amusement park before the long drive back to Connecticut. I'll probably just listen to music.

Enjoy your Sunday, everyone,

Mark

Submitted by Mark David Siegel on April 10, 2017