[pediRNteacher]

Can someone explain the significance of which limb the pulse oximeter probe is placed on in a child with a cyanotic heart defect with respect to shunting?

Thanks!

[janfrn]

Placing the probe on the right hand will give you a preductal O2 sat, meaning before the blood has crossed the ductus arteriosus (remember your cardiac anatomy?), which in kids with cyanotic heart disease is usually open. It will be largely deoxygenated blood. Placing the probe on the left hand will give you a postductal O2 sat, measuring the degree of mixing at the ductus. Blood under pressure will flow from an area of higher pressure to lower, typically left to right. If there are septal defects in addition to a patent ductus, there will be more shunting. Oxygenated blood from the left ventricle will flood the right ventricle and result in both mixing of oxygenated blood and deoxygenated blood, plus pulmonary overcirculation. This shunting is sometimes all that keeps a child alive before their heart is repaired, but survival comes at a price. The child may develop pulmonary hypertension to protect the lungs from overcirculation, which later will be problematic when the heart is repaired and the volume decreases to normal. And they usually have some degree of congestive heart failure, often severe.

If you need to assess cerebral perfusion the probe can be placed on the earlobe. Sometimes these numbers will be higher than those on the limb. Does this help?

[pediRNteacher]

Thanks!

[NancyRN]

jan, thanks for the fascinating explanation! I was finally able to "see" in my mind why left ventricular hypertrophy causes pulmonary hypertension!

What about pulse oximetry in infants with congenital hearts? We use the toes. Does it matter?

[janfrn]

There may be a slight difference from right to left with the toes, but it usually isn't significant. If the CV surgeon, cardiologist or intensivist wants specific information about pre- or post-ductal sats they'll ask you to move the probe.

[NancyRN]

Thank you jan!

[janfrn]

No prob, Nancy.