Bradycardia in response to hypoxemia in kids

[charles-thor]

I may be missing something completely obvious, but can someone give an IN-DEPTH physiologic explanation why kids immediately respond to hypoxemia with bradycardia? Thanks in advance.

[srnaKate]

I may be missing something completely obvious, but can someone give an IN-DEPTH physiologic explanation why kids immediately respond to hypoxemia with bradycardia? Thanks in advance.

Children have a higher cardiac output and oxygen consumption per kilogram than adults. They support this higher output with a higher baseline heart rate. Infants are heart rate dependent for their cardiac output. In other words, they have a fixed stroke volume, and must increase their heart rate to increase cardiac output. They may respond to stress, such as hypoxia, by becoming bradycardic, and therefore decreasing CO. This can make resuscitation quite difficult. Normal vital signs for children include higher heart rates and lower blood pressures than adults. http://www.anesthesia.wisc.edu/med3/Peds/pedshandout.html

It has to do with the SA node. Instead of firing faster it slows the rate due to hypoxia, and hypercapnia, to try to diffuse more gasses at the alveolar level, right? At least that's been my understanding of it thus far. It is taught that it just "is". I'll be interested to read the other responses!

[Brenna's Dad]

I can't remember if I've ever heard a really good explanation. Perhaps it has to do with their immature sympathetic nervous system.

[zacarias, ASN, RN]

It has to do with the SA node. Instead of firing faster it slows the rate due to hypoxia, and hypercapnia, to try to diffuse more gasses at the alveolar level, right? At least that's been my understanding of it thus far. It is taught that it just "is". I'll be interested to read the other responses!

I like the indepth explanations but can't the bradycardia be just like when adults have MIs. Often MI folks get bradycardia which if, unsymptomatic, can be great because it decreases myocardial oxygen requirements. Can't it be the same for kids when they get hypoxemic?

[smiling_ru]

Maintenance of adequate heart rate is the most important factor in ensuring adequate cardiac output. However, the dominance of the parasympathetic portion of the autonaumic nervous system tone makes neonates, infants, and young children particularly vulnerable to bradycardia when they are faced with physiologic stressors. http://66.102.7.104/search?q=cache:pXQEYkTGac8J:www.vh.org/pediatric/provider/anesthesia/proceduralsedation/pedscsa_p.html+physiology+bradycardia+children&hl=en

So the answer is high vagal tone.

[Brenna's Dad]

Adults with MI's become bradycardic because of the lack of blood flow to the SA and AV node, usually Inferior. This is completely different than what occurs in children.