TILIF #17: Forecasting an airway, ECMO hope, and lot's o' math

On an endotracheal tube (breathing tube for intubated patients) there is often an inflatable ring called a cuff that can be filled with air to help make a better seal in a patient's airway. Since you can't easily see the cuff to check how inflated it is, there is a little air-bladder at the other end of the tiny tube used to inflate it. Since the two soft bubbles of plastic are connected by the rigid little tube, they feel the same pressure. This means if you squeeze on the little balloon outside the body, you get instant intuitive feedback about the balloon inside the body. After intubating, you can also use this relationship to feel where the internal balloon is by placing your hand on the neck while squeezing the outer bubble and feeling the bounce of the inner balloon pressing out through the soft tissue of the neck. Nifty!

I never gave a second thought to what the little balloon would be called. Cuff pressure bladder, maybe? The anesthesiologist I worked with today scraped the bottom of the esoteric knowledge barrel by informing me that it is actually called a "pilot balloon." But that made me wonder: why?! What a weird term. The anesthesiologist hypothesized that it was like a pilot light for a furnace, because the little fire is indicative of the ability to start larger fires inside. I don't buy that.

If you Google "pilot balloon" you'll note that the first non-medical entry you find from meteorology. Pilot-balloon (or "pibal") observation is when you send a balloon up into the atmosphere to measure the direction of upper-level winds. This is shockingly close to a perfect metaphor for our little device - a balloon that measures the ability for air to move through a patient's upper airway. Coincidence?

I also recently learned the amazing story¹ of the first ECMO baby. Extracorporeal membrane oxygenation (ECMO) is basically medium-term heart-lung bypass. It is one of the most extreme, invasive and risky interventions that is done in all of medicine. It was first tried on adults in the '60s but the first study was a disaster with 90% of patients dying. The whole concept may have been scrapped where it not for a hail-mary intervention on the newborn baby of an illegal immigrant in Irvine, CA. The child's mother was told that the child was dying (which she almost certainly was) and, before fleeing the hospital due to immigration status concerns, she consented to allowing the doctors there to try a radical, unproven treatment.

The new technology that allowed this medical advancement was actually the development of silicone tubing, made from basically the same stuff used as a bathroom tile sealant. Unlike other plastics, this substance actually allows gases like oxygen to diffuse into blood flowing by without requiring the blood to be damaged with mixing devices. This baby's lungs were stuck in a high resistance fetal physiology and needed more time to mature. The surgeons placed catheters into the major veins and arteries to divert most of the baby's blood to this oxygenation device outside the body before allowing it back in to feed the tissues. It worked! The baby survived on the device for 6 days, enough time for the pulmonary pressures to lower and the lungs to oxygenate the blood on their own. One of the only complications was that a piece of the silicone tubing broke off while being disconnected, lodging in the little girl's lung.

The mother was never seen again. She had given her child birthright US citizenship and given the doctors permission to try for a miracle, but likely never knew the girl's fate. The baby was named Esperanza, Spanish for "hope," by the nurses and she was raised by loving foster parents. She later met the doctor that saved her life and has become a mild celebrity in the ECMO world.

Lastly, and this is basically just for me, I learned a bunch of hard-core physiology. And, to better understand the relationships between some variables, I did a long-form derivation. Nothing to see here!

  DO2 = oxygen delivery

         = CO x CaO2 x 10

         = (HR x SV) x (1.34 x [Hgb] x SaO2 + 0.003 x PaO2) x 10

         = (HR x SV) x (1.34 x [Hgb] x SaO2) x 10

  VO2 = oxygen consumption

         = CO x (CaO2 - CvO2)  x 10

         = CO x ((1.34 x [Hgb] x SaO2) - (1.34 x [Hgb] x SvO2))  x 10

         = CO x ((1.34 x [Hgb] x SaO2) - (1.34 x [Hgb] x SvO2))  x 13.4

VO2:DO2 =	10 x CO x (CaO2 - CvO2)
	10 x CO x CaO2

=	1.34 x [Hgb] (SaO2  -  SvO2)
	1.34 x [Hgb] SaO2

                    =  SaO2   -   SvO2
                        SaO2       SaO2

                    = 1 - SvO2
                            SaO2

  DO2:VO2 =          1         
                          1 - SvO2
                               SaO2

                 = typically 5:1, but stable down to 2:1

Comparable to mixed venous saturation: 5:1=80%, 2:1=50%, but only when FiO2 is 100%

ECMO is for getting back above 2:1 or limiting damaging interventions used to do so.

1. Bartlett, Robert H. Esperanza: The First Neonatal ECMO Patient, ASAIO Journal: November/December 2017 - Volume 63 - Issue 6 - p 832-843 doi: 10.1097/MAT.0000000000000697