Tetralogy of Fallot and Squatting

[aRnonymous]

I have a question about the mechanism between tetralogy of fallot, and specifically how squatting helps this.

Tetralogy of Fallot of course has a VSD, which together with the pulmonary stenosis results in the right to left shunt. Unlike uncorrected left to right shunts (such as isolated VSD, ASD, or PDA) which will become right to left over time and lead to symptoms, my understanding is that in TOF the shunt will ALWAYS remain right to left due to the right ventricular hypertrophy as a result of the pulmonary valve stenosis, which will overcome the pressure from the left side of the heart. Am I right so far on this?

Now, squatting. Squatting will increase TPR, and as a result increase pressure in the venous system, which will therefore increase venous return to the right ventricle. This will therefore increase the amount of blood going to the lungs, which will help counteract a "cyanotic spell".

Can someone confirm that my reasoning is right here? I was having a little trouble with understanding this and am not completely confident in my answer. Thanks!

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[Syncity]

You're on the right track, but....

If squatting were to increase venous return, this would simply worsen the problem as the additional venous return is simply going to be shunted to the left anyway. The answer relates to pressure. In ToF, the RV pressure exceeds the LV pressure, resulting in a right to left shunt. Squatting increases systemic vascular resistance which causes LV pressure to temporarily rise above RV pressure, thereby producing a temporary reversal of the shunt. Therefore, when the pt squats during the cyanotic spells, more blood enters the lungs for oxygenation.

 

[aRnonymous]

Ahh, OK, that makes a lot of sense. I was thinking that part of that increased venous return would go towards the lungs even though the rest would go through the VSD, but I do see how that wouldn't help much. Thanks for explaining that!

 

[JohnGT]

Great response. As a related note, if you suspected a child was undergoing a tet spell, which one of the below would be your best intervention?
1.) increase FiO2 on facemask from 60% to 100%
2.) immediate intubation with endotracheal tube
3.) administer appropriate phenylephrine dosage
4.) administer appropriate epinephrine dosage

 

[Syncity]

I don't know where you got this question, because at least three of the options are potentially correct.

Phenylephrine and epinephrine would have the above effect of increasing systemic vascular resistance and reversing the shunt. 100% oxygen would produce pulmonary vasodilation and systemic vasoconstriction, thereby increasing pulmonary blood flow.

In practice, I would go with a continuous IV infusion of phenylephrine as it increases systemic vascular resistance and has been shown to increase pulmonary blood flow.

 

[aRnonymous]

I don't know where you got this question, because at least three of the options are potentially correct.

Phenylephrine and epinephrine would have the above effect of increasing systemic vascular resistance and reversing the shunt. 100% oxygen would produce pulmonary vasodilation and systemic vasoconstriction, thereby increasing pulmonary blood flow.

In practice, I would go with a continuous IV infusion of phenylephrine as it increases systemic vascular resistance and has been shown to increase pulmonary blood flow.

That was my thought as well, I would have thought that phenylephrine is the best option.

 

[JohnGT]

Phenylephrine is the only correct answer.

1.) Increase FiO2 through facemask is incorrect for two reasons. First, the maximal FiO2 that one can receive through facemask is about 60%. You can never get 100% of fractional inspired O2 through facemask. If you want to deliver more supplemental O2, you need to use a rebreather or a face tent.

Also, TOF is mostly a problem of shunting. De oxygenated blood gets shunted from right to left, thereby decreasing systemic blood flow. The blood from right side of body sees very little oxygen because there is limited pulmonary blood flow oxygen exchange. Therefore, increasing inspired oxygen to generate more oxygen in alveoli does not really help the systemic oxygen saturation. Always remember, problems caused by shunting can not be resolved by delivering more supplemental Fi02

2.) Immediate intubation is not necessary unless the child is showing severe retractions or signs of instability such as a decreased heart rate or hypotension.

3.) Phenylephrine is the best choice because it serves largely to increase SVR. Increasing SVR will allow SVR to exceed the high PVR of ToF, thereby, decreasing the right to left shunt gradient. Since SVR is higher, the blood is able to flow into the pulmonary circulation, receive oxygen exchange, and return to systemic flow to alleviate systemic cyanosis.

4.) Epinephrine is incorrect because it has beta 2 effects which serve to increase heart rate, and possibly increase spasm of infundibular cardiac muscle. Noteably, spasm of infundibular cardiac muscles is a cause of tet spells; and conversely, beta blockers such as propanolol, are used for acute treatment of tet spell cyanosis.

Of note, I'm a pediatric anesthesiologist and care for some of these kids in the operative setting when acute stress such as pain, anxiety, hypercarbia, and blood loss can cause acute increases in PVR with resultant cyanosis. I'm also a founder of the GunnerTraining, and include this information in our test prep. Keep up the good discussion!

 

[Syncity]

Is there a Gunner Training program for Step 2? PM me if so.

 

[Rothbard]

nm