In some additional aspects, an apparatus can include a chamber having an inlet valve for receiving a reactant gas and an outlet valve for delivering a product gas, a piston positioned inside the chamber and configured to move along a length of the chamber for adjusting pressure in the chamber, a sensor for collecting information related to one or more conditions of a respiratory system associated with a patient, a controller for determining one or more control parameters based on the collected information, and one or more pairs of electrodes positioned inside the chamber for initiating a series of electric arcs external to the patient to generate nitric oxide based on the determined control parameters.