Not a PICU nurse but I had a couple thoughts. I am an anesthesia student with experience mostly in adult CVICU and now anesthesia FYI but I think the concept is relevant in pediatrics as well. I agree with the previous poster regarding the pulsus paradoxus. To go a little further you will see it anytime positive intrathoracic pressure causes significant decrease in venous return. If the pt is intubated the decrease in the pulse ox waveform or art line tracing will occur during inspiration and if spontaneously breathing it will occur during expiration due to positive pressure collapsing the vena cava and thus decreasing venous return/pre-load, thus decreasing stroke volume and blood pressure for those brief periods. With adults (and for the life of me I cant think why it wouldnt be the same for peds) this is a sign that someone is VOLUME responsive. Is the patient hypotensive with significant variability in the aline and/or pulse ox waveform with risk factors for hypovolemia: the BP will likely respond to a fluid challenge. This is the concept that the vigileo/flowtrack monitors use to calculate stroke volume variance (SVV). Other terms are systolic pressure variation, pulse pressure variation, SVV and they all refer to this phenomenon. So all the other causes like superior vena cava syndrome, tamponade, etc are all things you have to think about but I would say that the most common cause for this (at least in adults and surgical patients) is hypovolemia. Final caveat: seeing this on the monitor is not necessarily a bad thing, if everything else checks out OK then just note it and move on, maybe patient is just a tad on the dry side but still perfusing well, if so just leave it alone.