Reply : Relationship between hydrodynamic parameters and endothelial cell loss after phacoemulsification

Abstract

ber depth measurement with a-scan ultrasonography, Orbscan II, and IOLMaster. Optom Vis Sci 2005; 82:900-904 Relationship between hydrodynamic parameters and endothelial cell loss after phacoemulsificationThe article by sets out to determine whether any relationship exists between fluid usage and/or phaco power expended during cataract surgery and subsequent endothelial cell loss. Two sets of fluidics parameters are compared; one is low flow (20 cc/min) and low vacuum (200 mm Hg), the other high flow (40 cc/min) and high vacuum (400 mm Hg). The same ultrasound (US) power settings are used in both groups.First, there is a glaring proofreading error on the part of the authors and reviewers (eg, US power modulations quoted for the AMO Sovereign [Abbott Medical Optics, Inc.] were in seconds of on and off time, not milliseconds). What is more concerning, however, is the basic lack of understanding of how changes in phacodynamic parameters will influence phacoemulsification. In the final paragraph it is stated, ''For example, choosing a low flow rate with high vacuum might induce a surge phenomenon in the anterior chamber, which could cause more endothelial cell loss by making mechanical changes in corneal shape. That is why we increased the flow rate from 20.0 to 40.0 cc at the chop stage in the high-vacuum group to preserve the integrity of the anterior chamber.'' The opposite is of course true; a higher aspiration flow rate will lead to a less stable anterior chamber at occlusion break, particularly here, where a fixed flow rate is used.In the discussion section, it is noted that there appears to be a higher mean power usage in the high vacuum group than in the low-vacuum group and stated that this might be due to ''.the longer chop stage in the low-vacuum group. Low-vacuum status reduces nuclear engagement in the phaco tip, which indirectly prolongs total phaco time. High-vacuum status increases nuclear engagement in the phaco tip, which indirectly shortens total phaco time.'' What is not clear is whether total phaco time is the time spent in foot position 3 or the time taken to remove the nucleus. One would expect that less power would be used when chopping with high vacuum and flow as the pieces are held more firmly and can be drawn through the phaco needle with less energy. It is unfortunate that the energy used and the time taken for the separate sculpt and chop stages were not recorded. These data are available in the printout from the Sovereign machine.In the same paragraph, it is stated that there was no significant difference in the dissipated energy between the 2 groups but then stated ''[w]e found total US energy to be a significant risk factor in corneal endothelial cell loss.'' This is presumably because a similar amount of fluid was used in both groups and it was thought that this alone would exclude it from blame. However, the same amount of fluid was used but for different times. The phaco time was 50% longer in the low-vacuum group than in the high-vacuum group. One might infer from this that b...