Apparatus to treat an eye with an ophthalmic laser system comprises a patient interface having an annular retention structure to couple to an anterior surface of the eye. The retention structure is coupled to a suction line to couple the retention structure to the eye with suction. Liquid is added above the eye to act as a transmissive medium. A coupling sensor is coupled to the suction line to determine coupling of the retention structure to the eye. A separate pressure monitoring circuit having a much smaller volume than the suction line is connected to the annular retention structure to measure suction pressure therein. A system processor coupled to the monitoring pressure sensor includes instructions to interrupt firing of a laser when the pressure measured with a monitoring pressure sensor rises above a threshold amount.