What should be monitored if there is a greater than 10% difference in saturation between pre-ductal and post-ductal readings?

Monitoring a greater than 10% difference in saturation between pre-ductal and post-ductal readings is crucial because this indicates the possibility of a right-to-left (R-L) shunt. In a normal physiological state, pre-ductal saturation (usually measured from the right hand) is often higher than post-ductal saturation (measured from the lower limbs), primarily due to the mixing of oxygenated and deoxygenated blood.

When there's a significant difference in these readings, it raises the suspicion of conditions such as congenital heart defects, where blood may be shunted from the right to the left side of the heart, bypassing the lungs. This can lead to poorly oxygenated blood circulating in the body, which is a critical issue needing immediate attention.

In contrast, the other options do not directly address the implications of a significant saturation difference. Monitoring heart function, oxygen levels, or blood volume is important in their own right, but they do not specifically point to the potential for a R-L shunt and the urgent implications that a significant difference in saturation signifies in a clinical setting.