the inventor has discovered that the changes in irrigation and aspiration pressure during the priming cycle of a surgical system can be used to estimate the flow resistance in the system . once the approximate flow resistance is known , that information can be used to estimate the pressure within the system and , as a consequence , the pressure at the operative site . as best seen in fig1 , system 10 of the present invention generally includes pressurized source of irrigation fluid 12 , irrigation line 14 running from source 12 to handpiece 16 , irrigation pressure sensor (“ ips ”) 18 , aspiration line 20 running from handpiece 16 , pump 22 for providing a vacuum to aspiration line 20 and aspiration pressure ( vacuum ) sensor 24 . during surgery , tip 26 on handpiece 16 is held within eye 28 so that there is continuous fluid communication from irrigation source 12 to drainage bag 30 through irrigation line 14 , handpiece 16 , aspiration line 20 and pump 22 . the fluid resistance in this continuous path is unknown , because of the various combinations of sleeves , tips , handpieces and tubings that can be used . therefore , in order to estimate the iop in eye 28 , the fluidic resistance must be determined . as best seen in fig2 , system 10 ′ is identical to system 10 illustrated in fig1 except that fig2 represents a surgical system during pre - surgical set - up , or priming , so that test chamber 32 is substituted for eye 28 . during the priming operation , a sequence of steps is added to the existing priming steps . test chamber 32 is evacuated by running pump 22 ′ with irrigation valve 34 ′ closed so that approximately 2 - 3 cc of fluid is evacuated without pulling a high vacuum in system 10 ′. at the end of the step test chamber 32 should be collapsed and vacuum in system 10 ′ is approximate 100 - 150 mm hg . pump 22 ′ is stopped and irrigation valve 34 ′ is opened to refill test chamber 32 with irrigation fluid from source 12 ′. irrigation and / or aspiration pressure sensors 18 ′ and 24 ′ readings are monitored . when a flat ( i . e . test chamber 32 refill ) segment is detected in the pressure reading ips 18 ′ pressure is recorded . fig3 illustrates the pressure curves for both aspiration pressure ( vacuum ) and irrigation pressure during this step . flat segment 36 in fig3 illustrated test chamber 32 refill . flat segment 36 indicates that test chamber 32 pressure ( which is the pressure at the handpiece tip 26 ′ as well ) is approximately equal to ambient , i . e . 0 mm hg . irrigation line 14 ′ resistance ratio ( k r ) based on the pressure readings is estimated as follows : k r = r admin r irr = δ ⁢ ⁢ p admin δ ⁢ ⁢ p irr = p source 0 - p ips 0 p ips 0 - p tip 0 = p source 0 - p ips 0 p ips 0 - 0 = p source 0 p ips 0 - 1 where ⁢ : p source 0 ⁢ - ⁢ irrigation ⁢ ⁢ source ⁢ ⁢ pressure ⁢ ⁢ during ⁢ ⁢ the ⁢ ⁢ test , ⁢ p ips 0 ⁢ - ⁢ ips ⁢ ⁢ reading ⁢ ⁢ during ⁢ ⁢ the ⁢ ⁢ test ⁢ ⁢ chamber ⁢ ⁢ refill , ⁢ p tip 0 ⁢ - ⁢ pressure ⁢ ⁢ at ⁢ ⁢ the ⁢ ⁢ handpiece ⁢ ⁢ tip ⁢ ⁢ during ⁢ ⁢ test ⁢ ⁢ chamber ⁢ ⁢ refill , during surgery the iop is estimated based on the current bottle height , current ips 18 reading , and previously estimated resistance ratio ( k r ) as follows : in general , the method utilizes compliance of the test chamber to instrument a transient condition in which the pressure at the handpiece is known , despite the absence of the direct measurement means . the compliance of the test chamber determines the refill pressure at the handpiece during the instrumented condition . for currently used test chambers ( highly compliant ) the pressure can be accurately assumed to be at ambient ( i . e . 0 mmhg ). for a lower compliance test chamber , the typical refill pressure value can be accurately established in lab testing . this description is given for purposes of illustration and explanation . it will be apparent to those skilled in the relevant art that changes and modifications may be made to the invention described above without departing from its scope or spirit .
a method for direct indication of the iop level without requiring an additional pressure transducer being introduced into the irrigation path . the method of the present invention estimates resistance ratio for a particular setup , and thus does not assume a typical value ; the estimation is performed in a pre - operational configuration that is the closest possible to the surgical configuration .