The Multi-modal Automated Sensory Testing (MAST) System was used to assess pressure pain sensitivity. The MAST device is a small, portable device designed for research, as well as potential point-of-care applications.14 (link); 24 (link); 25 It applies discrete pressure stimuli to the thumbnail bed. Our group has extensive experience using thumbnail pressure as an evoked pain stimulus and its validity in the measurement of pain sensitivity has been discussed extensively.26 (link)27 (link); 28 (link)The MAST System features a control computer that coordinates testing protocols and program execution. A second computer displays pain rating scales and captures participant feedback at predetermined times on a touch screen monitor. A wireless thumbnail stimulator serves as an actuator device to evoke pressure pain. Both the operator and the participant have a stop button and are able to stop the testing at any point. The thumbnail stimulator applies blunt force, delivered by a 1 cm2 rubber probe, to the thumbnail bed. The probe is attached to a cylindrical transducer housed inside a plastic joystick designed to be held comfortably in either hand. The transducer is driven by a miniature servo-motor and a digital load-cell measures the exact pressure applied to the thumb to ensure accurate and repeatable testing. After the intravenous line was placed, participants received training on how to use the MAST device including a series a practice pressures before starting data collection. QST was conducted three times in the following order: before IV infusions (baseline), after saline placebo administration, and after fentanyl administration (see below). The MAST system delivered an ascending series of discrete 5-s duration stimuli to the patient's dominant thumbnail beginning at 0.50 kg/cm2 and increasing 0.50 kg/cm2 steps up to the patient's pain tolerance or to a maximum of 10 kg/cm2. This test was designed to be brief for implementation in clinical settings and is typically completed within 5-6 minutes. The pain intensity evoked by each pressure was rated on a 0-100 numerical rating scale (0 = no sensation, 20 = just noticeable pain, and 100 = worst pain imaginable) displayed on the touch screen monitor. Participants used a stylist or their finger to tap the touch screen and select their pain rating. They then hit a “confirm” button to proceed to the next pressure. Subsequent pressures were delivered 20 seconds after each rating confirmation. Stimulus-response curves were constructed from each test and three measures were derived: pain (1) Threshold - first pressure sensation rated as painful (i.e., first rating ≥ 20/100); (2) Pain50-pressure that evoked an intermediate suprathreshold pain intensity rating halfway between pain threshold and tolerance and (3) Tolerance - the maximum pressure tolerated, or rated ≥ 80/100, or a maximum of 10 kg/cm2. The primary outcome of the study was differences in pressure pain sensitivity between suspected OIH patients and healthy controls.