Predicting infusion pressure during pars plana vitrectomy: a physically based model

Purpose: Intraocular pressure (IOP) during pars plana vitrectomy (PPV) decreases as aspiration generates flow, a phenomenon known as head loss. Since direct measurement of the IOP during surgery is impractical, currently, available compensating systems infer IOP by measuring infusion flow rate and estimating corresponding pressure drop. The purpose of the present paper is to propose and validate a physically based algorithm of the infusion pressure drop as a function of flow. Methods: Complete infusion lines (20G, 23G, 25G and 27G) were set up and primed. The infusion bottle was set at incremental heights and flow rate measured 10 times and recorded as mean Å} SD. Overall head loss (OHL) was defined, according to hydraulics laws, as the sum of frictional head loss (FHL; i.e., pressure drop due to friction along tubing) and exit head loss (EHL). The latter is equal to the kinetic energy of the exiting flow through the trocar (FKE = V2/2g). A 2nd degree polynomial equation (i.e., ΔP = aQ2 + bQ, where ΔP is the pressure drop, or OHL, and Q is the volumetric flow) was derived for each gauge and compared to experimental data 2nd order polynomial best-fit curve. Results: Ninety-seven percent of the pressure values for all gauges predicted using the derived equation fell within 2 SD of the mean difference yielding a Bland-Altman statistical significance when compared to 91% of best fit curve. Conclusion: The derived equations accurately predicted the head loss for each given infusion line gauge and can help infer IOP during PPV.