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Discussion

Any CHD experts in the house?

i am an rt studying to take my nps (neonatal-pediatric specialist) exam, and i'm confused about a cardiac defect question i got wrong on a "mock" examination. this will be my "achilles heel" on the exam because the nicu i work in does not provide care for neonates with cardiac defects (we turf to children's hospital because our hospital has no pediatric cardiology team on staff).

anyway, here is the question (correct answer in red):

a ptco2 monitor placed on the right thorax of a 2-day-old neonate is reading 80 torr. a second ptco2 monitor placed on the left thigh is reading 50 torr. this would most likely be the result of

a. transposition of the great vessels

b. ventricular septal defect

c. atrial septal defect

d. congenital diaphragmatic hernia

i automatically rulled-out "a" (the correct answer) because, according to all the literature i've read, a tga will always present with reverse differential cyanosis...which on page 40 of the textbook "pediatric cardiac intensive care" by anthony c. chang is defined as "cyanosis of the upper extremities associated with normal saturation of the lower extremities" this, i assume, is why tga is the only defect where the post-ductal (lower limb) spo2 reading will be higher than the pre-ductal (right hand).

if i'm understanding the "reverse" part of the differential cyanosis correctly, the lower extremities would be higher in o2 content than the upper...whether you're using spo2 monitors or ptco2 monitors shouldn't matter. yet the above scenario gives a thigh (lower limb) reading that is lower than the upper reading.

i understand with any cyanotic chd they'll be de-oxygenated blood being sent to the body...but its the whole reverse differential cyanosis-thing (which is specific to transposition) that is confusing me. normally i would just commit this answer to memory and move on, but there were two errors on the answer key to the pre-test in this review course...so i just want to make sure the answer they provided really is correct.

if anyone here can clarifiy this for me, it would be greatly appreciated.

Featured Replies

I'm confused too. In TGA the post ductal should be higher. :uhoh21:

  • Expert

Well, let's look at it this way:

B: VSD = mixing acyanotic lesion. You wouldn't see different sats. Wrong.

C: ASD = mixing acyanotic lesion. Same as above. Wrong.

D. CDH = I guess you "could" argue that PPHN could cause the difference in sats, though you'd see that before 2 days....

I'm wondering if the pTC02 actually means transcutaneous CO2 measurement. It would then make perfect sense that your "preductal" Co2 levels would be higher than post. Plus, we don't typically measure pulse oximetry on the thorax or thigh. I'm pretty sure you can't measure O2 saturations in torr (although I may be unfamiliar with it, being American) I have never heard of a higher Spo2 preductally in TGAs, except when they are ballooned.

  • Author

I'm wondering if the pTC02 actually means transcutaneous CO2 measurement.

Plus, we don't typically measure pulse oximetry on the thorax or thigh.

You're correct, Steve...in fact, you'll never measure "pulse oximetry" on the chest or thigh. However, unlike transcutaneous monitors, pulse oximeters measure the amount of oxygen "saturated" hemoglobin (using UV light and "Beer's" law of absorption) and the calculation is reported as a percentage ( % ). Transcutaneous monitors use the same polarographic electrodes found in blood gas analyzers (Clark electrode for O2, Severinghaus electrode for CO2) and provide a direct measurement of the oxygen that diffuses to the skin surface from the dermal capillary bed beneath. PtcO2 stands for "transcutaneous oxygen tension" (partial pressure of oxygen reported in mmHg or "torr")...the lower case "tc" being the "transcutaneous" portion (duh, I know). If the monitor in the question were measuring carbon dioxide (which it can), it would have read "PtcCO2".

Blah, blah, blah...sorry. I was just trying to clear up any confusion about the measurement being reported in torr.

P.S. I went ahead and sat for the exam WITHOUT ever knowing the answer to my question. Low and behold, there wasn't even a referrence to TGA in any of the 140 questions!! I'm still curious to know why TGA was the answer (other than the fact that none of the other answers made sense)...but either way, I'm now "RRT-NPS, NREMT-P"...and $500 richer for adding another set of initials (that no one really gives a rats-ass about) to my uniform!!

Next up...C-NPT exam.

  • Expert

haha. Ok, thanks for clearing that up. I am used to working with a device called a TCoM, which measures transcutaneous Co2 levels. We don't have equipment to measure transcutaneous O2 levels here in the US. At least that I'm aware of.

  • Author
We don't have equipment to measure transcutaneous O2 levels here in the US. At least that I'm aware of.

Children's Los Angeles NICU was evaluating one while I was there...I believe it was manufactured by Radiometer America (the same company that made most every T-CO2 monitor I've ever used). But, like you, I have yet to work in a NICU that utilizes them.

P.S. Is this website based outside of U.S.?

Children's Los Angeles NICU was evaluating one while I was there...I believe it was manufactured by Radiometer America (the same company that made most every T-CO2 monitor I've ever used). But, like you, I have yet to work in a NICU that utilizes them.

P.S. Is this website based outside of U.S.?

Have they figured out a way to make it stick better than the TCOM? :rolleyes:

  • Author
Have they figured out a way to make it stick better than the TCOM? :rolleyes:

Yeah...remove vernix first!! :D

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