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BOOK EXCERPT Bottle Height: Relationship to Flow Basically, adjusting the bottle height proportionately adjusts the anterior chamber depth. More specifically, the function of proper bottle height is to produce an adequate IOP, which will maintain the anterior chamber despite aspiration outflow as well as any surges or incisional drainage. Higher flow rates require higher bottle heights to maintain the same IOP as when using lower flow rates. Conversely, lower bottle heights (ie, useful for lowering IOP for such conditions as a small posterior capsule tear) require a correspondingly lower flow rate in order to prevent anterior chamber shallowing and potential collapse. Therefore, bottle height needs to be adjusted dynamically (ie, with an unoccluded aspiration port and active pump function) in foot pedal position 2 or 3. When adjusting bottle height for a given flow rate setting, it is helpful to know if the height adjustment itself affects flow rate. The effect of bottle height on flow is entirely dependent on the type of pump in question. Because flow is constrained at the point where the aspiration line is first interdigitated by a peristaltic pump roller or a scroll element, changing bottle height has no effect on flow with these flow pump machines. In essence, the pump head is acting as a flow regulator; despite increased pressure from an increased bottle height, flow cannot proceed any faster than the speed of the pump element traversing the fluid in the aspiration tubing. On a vacuum pump with an unoccluded aspiration port, however, there is open communication without restriction between the irrigation bottle and the drainage chamber in positions 2 and 3. Therefore, increasing bottle height with resultant increased IOP produces higher flow rates by pushing fluid harder through the aspiration line and into the drainage chamber. By corollary, a given increase in bottle height on a vacuum pump will not produce as much of an increase in IOP as with a flow pump because part of the increased pressure head is dissipated in the faster flow rate with the vacuum pump. In Figure 1-43, both the flow and vacuum machines were set up to produce a measured 30 cc/min outflow at 24 inches of bottle height; aspiration and irrigation tubing was simply connected without a handpiece. Outflow was then measured at bottle heights of 12 and 48 inches. It can be seen that the peristaltic machine’s flow rate was unaffected by the bottle height changes, whereas the venturi machine’s flow rate changed proportionately to the bottle height change. Clinically, it is important to realize when flow rate is being increased so that you can anticipate the resultant faster anterior chamber current and stronger attraction of intraocular material to the aspiration port.
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