Lower O2 sat parameters in babies with cardiac defects

by candee9909 (New) New

Alright y’all. This one really has me stumped. In the NICU when we have a baby with a congenital heart defect we usually keep their sat parameters lower than normal. 85-90% for example. A long time ago someone told me that the reason we do this is because oxygen is a vasoconstrictor and it would cause a shunt of unoxygenated blood to the left side of the heart. But now I’m reading in several places that oxygen is a vasodilator and everything I thought I knew has gone out the window! Please help with an explanation as to why we keep these kids sat parameters low!

adventure_rn, BSN

Specializes in NICU, PICU. 1 Article; 1,515 Posts

I've got a background in NICU as well as Peds Cardiac ICU, so I can take a stab at explaining this concept; honestly, it's so complicated that I could talk about it for hours and it would still be a bit confusing, so bear with me. (Pardon my subtitles, I hope they help clarify the post...)

Oxygen is a Vasodilator

First of all, oxygen is always, always a vasodilator. Here's a completely NICU, non-cardiac example: That's why we set the O2 parameters for PPHN kids higher, and why we're ok with them high-sating. PPHN is, by definition, pulmonary vasoconstriction. The first-line treatment for PPHN is cranking up a kid's FiO2 because the O2 itself is a vasodilator (and has fewer adverse effects than other vasodilators like nitric oxide).

Different Conditions Have Different Sat Goals

Second, different conditions will have different sat goals; the cardiology team should recommend sat goals based on the baby's anatomy. Some conditions (i.e. critical coarctations) will have normal sat goals; some (i.e. AV canals, certain Tets) may have goals of > 80%; some (i.e. the ductal-dependent and/or 'single ventricle' lesions--HLHS, tricuspid atresia, aortic atresia) will have goals of 75-85%.

Why Cardiac Kids Have Lower O2 Sats

Many of these kids have lesions which allow or require mixing between the blood of the right heart (desaturated blood returning from the body to the lungs) and the blood of the left heart (oxygenated blood going from the lungs out to the body). The mixing can come from an ASD, a VSD, or at the level of the PDA.

The oxygenated blood in your left heart (the blood being pumped out to your body) has an O2 sat of 98-100% (i.e. a normal O2 sat). The O2 sat of your blood in your right heart (returning from your body to your lungs) isn't 0%--rather, it's got a sat of around 60-70%. Therefore, when kids have cardiac defects, the blood from their left heart (100%) and right heart (60%) mixes to create sats in the 80%-ish range. In the cardiac world, we call these kids 'mixers.'

This is where it gets super-confusing, so bear with me:

Blood Follows the Path of Least Resistance...

When you've got a cardiac lesion that allows mixing (i.e. an ASD, VSD, or PDA), your blood has two options--it can go towards the 'left' (out to your body and systemic circulation) or to the 'right' (to your lungs and pulmonary vasculature). A really good example is a Truncus: in this defect, the baby has a huge VSD (so there's mixing between the right and left heart) and there's a common artery that exists the heart. Instead of an aorta going to the body and pulmonary arteries going to the lungs, there's a huge super-vessel (the 'truncus') which does both.


So in this case, when the blood exists the heart through this huge, super-vessel, it has two options: it can go to the 'left' out of the aorta and into the body, or it can go to the 'right' into pulmonary circulation in the lungs.

In a perfect world, blood should go to the body and lungs in a 1:1 ratio, meaning that exactly as a much blood goes to the lungs as to the heart. (FYI, this is called the Qp:Qs ratio, i.e. p is pulmonary and s is systemic blood flow).

This makes sense, because you need balance: If you send too much blood to the lungs and not enough to the body, you end up with awful pulmonary edema (and in extreme cases, even relative hypotension). If you send too much blood to the body and not enough blood to the lungs, you end up with kids who are desaturated and poorly ventilated.

...Which is Why it Matters that Oxygen is a Vasodilator

The main reason we care so much about O2 sat goals in cardiac kids has to do with where their blood goes.

Let's say you've got an HLHS kid whose goal O2 sat is 75-85%, but you've cranked up their FiO2 and now they're sating 100%.

This can be FATAL, and I've seen cardiac kids die from hypotensive shock because of it.

Why? Because oxygen is a pulmonary vasodilator, so if you flood a cardiac baby's lungs with oxygen, then their pulmonary vascular resistance is going to rapidly drop. Therefore, the blood in their heart is going to preferentially go toward their lungs instead of their body.

This is a core concept in peds cardiac care, and it's called 'overcirculation.'

Recall how we said that the flow of blood to the lungs vs. body in a perfect world should be 1:1. If you give a cardiac baby too much oxygen, that ratio can quickly become 5:1, where 5 times as much blood is going to the lungs as is going to the body.

This is dangerous for two reasons: The first is more obvious, but less dangerous: it will cause severe pulmonary edema (which we all realize can be very problematic).

The second is less obvious, but far more dangerous: if 80% of the baby's blood volume is going to the lungs and only 20% is going to the body, the baby will present as though they have distributive hypovolemic shock.

The scary thing is that you literally can't fix it. Yes, this presents like 'hypovolemic shock,' but but unlike real hypovolemic shock, you can't treat it with volume because it will simply flood their lungs. The only way to treat this complication is by turning their FiO2 down to 21% (or in extreme circumstances, you can use subambient oxygen therapy where you literally give them less than 21%). However, that therapy takes a long time to work, and isn't helpful when a kid is already coding with a MAP of 15.

Goal Sats Matter

I don't mean to get on a soap box, but having seen both NICU and peds cardiac, this topic is really important to me.

In preemies, high-sating can cause ROP (which is important, but not always taken seriously in the moment). In certain cardiac kids, high-sating can literally cause cardiac arrest. My Peds Cardiac ICU would get transfers from NICUs that allowed their kids to sat too high, and like I said, I've seen kids die because of it. So if cardiology says that a kids goal should be 75-85, please don't let them sit in the 90s (or conveniently forget to change your alarm parameters because they're alarming too much).

This concludes my Peds Cardiac Nursing TED Talk.

Clear as mud?


6 Posts

Wow! Thank you! I couldn’t have asked for a better answer. This is so helpful.


Specializes in NICU. Has 36 years experience. 5 Posts

To Adventure RN ....

Thanks so much for the detailed explanation about O2 sats and cardiac babies. ..... I’ve been NICU connected for 30 + years. ... Cardiac has always been my uncomfortable / weak area. ... I know for our cardiac infants, the peds cardiologists always want lower O2 sats, but I attributed it to the cardiologists just accepting “mixing” is going on, and the lower sats are the best we’ll get in these infants. ... I’m old school, so I struggle with wanting to see higher O2 sats, (even though I always keep the sats where the docs order them). This even translated to the revised NRP guidelines from a few years back. Starting a neonatal resuscitation at 21%, felt so counterintuitive to me. But you just gave me that “Ah hah!” moment. Now I get it! You’ve even helped me understand & have less fear of the cardiac patients I encounter. That was the best explanation I have ever had. You have a knack for explaining things. If you are not an Instructor, you should be! Your explanations are spot on! ... Thanks so much for all of your insight!

adventure_rn, BSN

Specializes in NICU, PICU. 1 Article; 1,515 Posts

@PJG RN thank you for taking the time to write such a thoughtful response! I love NICU bedside, but I've been toying with the idea of moving into an educator role; fortunately my NICU offers opportunities to split your time and do both.

I honestly didn't understand cardiac very well until I was fully immersed in it in peds cards ICU. My hospital is a high-volume congenital cardiac center, and many of the cardiac preemies live with us for several months until they're ready for repairs. Now that I have a firm grasp on cardiac, I'm hoping that I can help our nurses better understand the ins and outs of the more complex defects that we see.


Specializes in ICU, Agency, Travel, Pediatric Home Care, LTAC, Su. 305 Posts

Adult ICU Nurse here. Thank you for the very enlightening education on how very different NICU & PICU Cardiac kiddos are compared to adults. As others have said, u genuinely have a teaching gift, and I hope you get a chance to utilize it to the fullest. You will save a lot of lives by educating other nurses & obviously at the bedside too♡ Your hospital and patients are blessed to have you☆