Usb x series

Invasive recordings of AP, JVP, and IOP were digitized at a sampling frequency of 1000 Hz via an analogue to digital converter (USB X Series, National Instruments). The recording protocol involved calibrating the pressure sensors before each experiment and visually confirming the digital output via a custom LabView program before recording for periods of 5–20 seconds.
Regular calibration of the transducer was conducted with an electronic pressure calibrator (Halstrup Walcher KAL84, Kirchzarten, Germany). The maximum pressure calibration point (100 mm Hg) was also checked with a standard mercury manometer. The pressure transducer was mounted at eye level to avoid offsets due to height differences in the fluid filled lines connected to the anterior chamber. As an additional check for offset problems, the eye was incised at the limbus at the end of the experiment to reduce IOP to zero and this was confirmed by the direct IOP measurement.
Each complete pulse wave was analyzed to extract diastolic (minimum) and systolic (maximum) pressure (Figure 1). Each pulse, therefore, was considered to be an experimental unit. The area was calculated by Simpson’s rule, which fits parabolas to successive points to estimate the definitive integral.²⁷ The cardiac cycle duration was calculated as the time between diastolic pressures (the lowest pressure during the timeframe of that cardiac cycle).