The aim of the present work was to test the efficiency of the EVA phacovitrectomy system (DORC) in maintaining baseline pressure values during the various phases of the surgical procedure.
In addition, flow rate experiments have been performed with the aim of assessing the performance of EVA high-flow connection for the infusion system.
In order to evaluate the pressure drop, we computed the average pressure jump, ΔP, defined as ΔP = Pb − Pa, where Pb and Pa are the time-averaged pressures during the infusion and aspiration phases. We found that the pressure drop was reduced in AIC mode compared to VGPC mode. The baseline infusion pressure was quite efficiently maintained with both 23- and 25-gauge TDC cutters, imposing a minimum and a maximum infusion pressures of 30 and 55 mm Hg, respectively. Generally, however, the 23-gauge TDC cutter resulted in better pressure compensation during aspiration with respect to the 25-gauge TDC cutter, using AV. Pressure variations were larger using the 27-gauge TDC cutter compared to those of larger size, demonstrating that the former was less efficient in IOP compensation (tested only with BSS).
In BSS, we found on average ΔP values larger than those obtained with AV, especially when using the 23-gauge TDC cutters or when we imposed large values of the infusion pressure.
In some cases, compensation led to a pressure increase in the eye model (Δ
P < 0), which is an undesirable effect. This was found, for example, when the 25-gauge TDC cutter was used in AV with a low value of the minimum infusion pressure. From a surgical point of view, it may be argued that imposing a low value of the minimum infusion pressure in the attempt to keep IOP at low levels can be a wrong operating strategy. This is particularly relevant in eyes where high intraoperative IOP and the consequent ocular hypoperfusion may be harmful, such as cases in which blood flow is already compromised (e.g., diabetic retinopathy, retinal vein occlusion, or advanced glaucoma).
20,44,45 This effect could be avoided by setting a lower value of the maximum infusion pressure in AIC mode. The ideal values range for the infusion pressure in AIC depends on the rheological properties of the vitreous humor and the aspiration pressure.
A pressure spike was always found when aspiration ceased at the end of phase 2. This behavior was likely enhanced by the fact that we operated in a rigid domain, and the only compliance was due to the pipes. Thus, such an effect is expected to be reduced in real eyes, which are compliant. We also note that Sugiura et al.
20 suggested that relatively large IOP spikes up to 10 to 20 mm Hg may not have clinical relevance when their duration is very limited. Despite the rigid material of the model, the present pressure measurements are in line with previous studies performed on porcine eye.
21,23,24 We also note that our main interest is in pressure values attained in the eye model during “steady” phases of the experiment, during which the aspiration pressure is maintained fixed over time. In this case, the pressure in the eye model is not expected to be significantly affected by the model material properties.
During vitrectomy, the cutter produces high-frequency pressure fluctuations that can be harmful.
11 In this study, dynamic variations of the pressure due to the motion of the cutter blade have been measured and always found to be small compared with the average pressure in the eye model. However, inspection of the pressure signals shows that the amplitude of pressure fluctuations is larger at the back pole than in the front region of the vitreous chamber model. This observation means that the posterior pressure probe is more affected by the flow than the anterior one, in agreement with previous studies.
16 Translated to the surgical practice, this last finding suggests that surgeons should be aware that pressure fluctuations due to the jet of the infusion line could lead to a focal chorioretinal damage.
46
Finally, we also carried out flow rate experiments in BSS to compare the performance of HF and SF connections for the infusion line. The purpose of the HF configuration is to maximize the flow rate, as the infusion line is connected to the external part of the trocar-cannula, thus exploiting the entire section of the pipe. Consistently with the intended use, the HF infusion system was found to be more efficient in terms of flow rate, with both 23- and 25-gauge TDC probes. In particular, using the 25-gauge TDC cutter, we obtained an increase in flow rate up to
≈20% and
≈70% higher than the SF configuration, in AIC and VGPC mode, respectively. Moreover, the flow rates achieved with the HF connection were significantly larger compared to those found in previous studies with a different vitrectomy device.
24 From the surgical point of view, this finding is important since within a closed system, such as the eye during vitrectomy, the use of instruments of progressively smaller diameter can result in the need to increase the infusion pressure in order to maintain a satisfactory flow rate.
47 However, a sustained increase of infusion pressure and intraoperative IOP is not advisable due to the negative effect on the mean ocular perfusion pressure and, thus, potential ischemic damage.
14 Therefore, HF can optimize vitrectomy fluidics, allowing the operator to obtain large flows and keeping the infusion pressure within a safe range, also with smaller gauge instruments. When the HF configuration was used, smaller values of Δ
P were found with respect to the SF connection for the same operating conditions, meaning that this infusion system also improves pressure stability during surgery.