Both subjects were deceased individuals who had expired < 5 h prior to the beginning of the study. Ethics committee approval was not required as Institutional Review Board approval is not a requisite for cadaver studies. Appropriate consent was obtained for research purposes..
To obtain direct measurements from the intraocular, intra-goggle and retrobulbar space, the retrobulbar space, posterior segment and intra-goggle space were cannulated. From each space, individual lines were connected with three separate, disposable pressure transducers, and each line was aligned at eye level to maximize precision and accuracy of measurements. The three pressures (intraocular, goggle and retrobulbar) of interest in this study are shown diagrammatically in Fig. 1.
IOP values were directly measured by cannulating the posterior segment (e.g. vitreous) with a 16-gauge needle on the distal end of a fluid-filled line via a transconjunctival approach 4 mm posterior to the limbus. To obtain direct retrobulbar pressure (RBP) measurements, the retrobulbar space was cannulated with an 18-gauge needle on the distal end of a fluid-filled line via a transcutaneous approach. An additional line was air-filled and connected to the goggle to directly measure pressure within the MPD goggles. The positioning and setup of the ports for pressure measurement are shown in Fig. 2.
The measurement system employed three disposable pressure transducers to provide direct pressure measurements. To permit digital data acquisition from the sensors, the transducers were connected to a data acquisition (DAQ) device that was connected to a computer to allow for real-time monitoring of the IOP, RBP and intra-goggle pressure.
Prior to the application of negative pressure, the MPD goggles were fastened to the skin and anterior orbital rim using Dermatac® tissue sealant (3M Company, Saint Paul, MN, USA) to ensure an airtight seal. After a proper seal was confirmed, test runs were performed with continuous pressure measurements from each of the three transducers. The data were recorded on a computer connected to the tranducers, and the individual test runs were separated for analysis. The aim of each test run was to apply negative pressure three times at each of three negative pressure levels (− 5, − 10 and − 20 mmHg) with the MPD.
IOP and RBP pressure tracings relative to application of negative pressure in the goggles were available in real time, allowing simultaneous comparison of RBP, IOP and intra-goggle pressure. The data from the sensors were plotted against time and presented for each run. Since pressure measurements were continuously obtained with the DAQ system, a series of measurements was averaged to generate a value for comparison and evaluation. For example, across a test run, the last 20 measurement samples prior to each event (e.g. the 20 sample measurements prior to the application of negative pressure or the 20 sample measurements prior to the release of negative pressure) were averaged to assess the magnitude of response for IOP and RBP.
Subject 1 underwent a total of four test runs. During the first test run, a gradual diminution in IOP and RBP was noted as time progressed. Over the course of the first test run, the baseline IOP (without negative pressure) gradually decreased from 19.69 to 17.31 mmHg over the course of the test run. In addition, the baseline RBP (without negative pressure) measurement was 1.27 mmHg and gradually decreased to 0.42 mmHg. To attempt to compensate for the pressure decay, the IOP and RBP were increased prior to test run 2 to re-establish a baseline setting by injecting a balanced salt solution into the intraocular and retrobulbar space. Despite this effort to re-pressurize the intraocular and retrobulbar space, the small drift in IOP and RBP was still observed in between applications of negative pressure across the second and third test runs. In subject 1, the IOP eventually stabilized at a baseline IOP of approximately 13 mmHg prior to the fourth test run.
Subject 2 also underwent a total of four test runs. Similar to subject 1, there was a gradual drift in IOP during the test runs in between applications of negative pressure, but no artificial pressurization was applied prior to any test runs. The baseline IOP was 15 mmHg prior to the first test run and 13 mmHg prior to the second test run, and then stabilized around 10 mmHg for the third and fourth test runs. Following the second test run, an additional 1-h test run was performed to evaluate the behavior and response of the IOP and RBP when negative pressure was applied for an extended duration. The fourth test run was thus performed following the 1-hour-long application of negative pressure.
This video demonstrates that with application of negative pressure with the MPD device, there is an instantaneous reduction in IOP without any changes in RBP (MP4 84734 KB)
The MPD is made up of two components: a pair of pressure-sensing goggles that is connected by a specialized tubing system on each side to a programmable pump. The current version of the MPD is shown in Fig. 3. The MPD goggles include an adjustable head strap for support and are designed with an anthropometric fit that enables extended wear length with negative pressure application. The programmable pump is connected to the goggles by crush-resistant tubing with separate lumens that enable independent negative pressure settings for each eye. The tubing system has separate vacuum- and pressure-sensing lines to allow a closed-loop control of the vacuum. This design permits real-time titratability of the negative pressure in addition to negative pressure sensors directly in the goggle chambers to ensure the programmed negative pressure setting is achieved.
The MPD goggles used in this present study were modified to create sealed ports used for cannulation and direct measurement of the intra-goggle, intraocular and retrobulbar space.
Pressure Measurement System
The method used for retrobulbar pressure measurement is similar to the one described in a prior study measuring orbital pressure in which a compartment pressure transducer (Compass; Mirador Biomedical Inc., Seattle, WA, USA) was utilized . The pressure transducer used in the present study was also similar, but was connected to a fluid-filled line, as opposed to a fluid-filled syringe. This modification enabled continuous pressure monitoring and subsequent logging of data.
The system included multiple components. Three disposable pressure transducers (Deltran® II series, model 6199; Utah Medical Products Inc., Midvale, UT, USA) were separately connected to each space to obtain pressure measurements. Voltage was supplied to the sensors using a voltage reference component (LJTick-VRef-41 [LJTVR-41] module; LabJack Corporation, Lakewood, CO, USA), and the voltage was amplified using a signal-conditioning module (LJTick-AMP; LabJack Corporation). The entire system was connected to a USB DAQ board (model U3-HV; LabJack Corporation). The DAQ board enabled continuous sampling of the data, and the data was recorded via a connected computer. This system enabled the assessment of time-based responses relative to negative pressure application in real time.
Prior to initiation of the study, the linearity and sensitivity of the system was evaluated to ensure the pressure measurements were accurate and representative. The linearity was tested by evaluating the sensor output against a calibrated differential pressure manometer (model HD755; Extech Instruments, Nashuah, NH, USA). The sensitivity of the system was assessed using a water column at three different pressure ranges. In addition, a “pinch test” was performed after each needle was positioned at the target location by compressing the fluid tubing and confirming a spike in pressure reading, as confirmation that the system was actively measuring pressure. The “pinch test” was repeated during the second test run of subject 1 to verify the system was patent and correctly obtaining pressure measurements, as shown in Fig. 4.