In North Carolina - a fair bit, actually.
This is a 14-hospital registry of cardiac catheterization activations for which the authors retrospectively evaluated how many were subsequently cancelled after activation. They don't delve into a great deal of detail regarding specific findings that accounted for the cancellation - they simply observe the broad categories of cancellation.
Of all cath lab activations, it was judged that 15% were "inappropriate", with the gold standard being the consulting cardiologist opinion. Of the cancellations, 40% were based on the EMS ECG, 31% were ED ECG, and the remainder were "not cath lab candidates". The author's main focus in their conclusion is on the difference between EMS ECG cancellation and ED ECG cancellation due to ECG reinterpretation following activation.
What's more interesting from the paper, however, is when they break it down to the precise cohorts of activation and arrival - and note that 24.7% of EMS activations were subsequently judged inappropriate. It is also interesting that 13% of non-PCI center activations were inappropriate vs 8% of PCI center activations. Reading between the lines, there's probably some experiential component to the differences in activation rates, but this study doesn't specifically look at volume and training.
"Rates of Cardiac Catheterization Cancelation for ST Elevation Myocardial Infarction after Activation by Emergency Medical Services or Emergency Physicians: Results from the North Carolina Catheterization Laboratory Activation Registry (CLAR)"
http://www.ncbi.nlm.nih.gov/pubmed/22147904
