I worked as a paramedic for ten years and just about every patient we had needed an IV. We kept pressure bags on the ambulance but didn’t have to use them very much. If the patient was not critical, we would start an IV and set it at TKO (to keep open), also called KVO (keep vein open). This just barely lets the fluids run through the vein. It’s just enough to keep the vein open and provide intravenous fluids during the transport to the emergency room.
If we had a patient that had a traumatic accident, such as a bad car wreck, we would often start two IV’s. We would either set them as “bolus” which basically means wide open. Depending on the status of the patient, we would use pressure bags. If the patient was severely injured and lost a large volume of blood, we would use pressure bags to keep fluids steadily running.