Intraocular Goldmann tonometer, pneumatonometer, Tono-Pen ... · 671 Intraocular pressure measurementafter penetrating keratoplasty: minifiedGoldmannapplanation...

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Intraocular pressure measurement after penetratingkeratoplasty: minified Goldmann applanationtonometer, pneumatonometer, and Tono-Pen versusmanometry

Mitchel J Menage, Paul L Kaufman, Mary Ann Croft, Stuart P Landay

AbstractThe accuracy ofintraocular pressure measure-ment with the minified Goldmann applanationtonometer, the pneumatonometer, and theTono-Pen tonometer were compared in post-mortem human eyes which had undergonepenetrating keratoplasty. Enucleated post-mortem human eyes underwent same sized(7.75 mm) or 0 5 mm oversized (8.25 mm)autologous penetrating keratoplasty. Intra-ocular pressure was then set and measuredmanometricaily while being determined suc-

cessively with each tonometer over the range of0-65 mm Hg. Linear regression analysis com-paring tonometric and manometric readingsshowed: (1) minified Goldmann applanationtonometer - slope 0O985 and 0*944, intercept1*64 and 2*55 mm Hg, correlation coefficient0*99 and 0O99 in same sized and oversizedgrafted eyes respectively; (2) pneumatono-meter - slope 1*008 and 0*990, intercept 3-37and 3*69 mm Hg, correlation coefficient 0-99and 0*98; (3) Tono-Pen - slope 1*061 and 1*002,intercept 5-01 and 4*06 mm Hg, correlationcoefficient 0 97 and 0*98. We concluded thatthe minified Goldmann applanation tonometeris as accurate or more accurate than thepneumatonometer and the Tono-Pen in post-mortem post-keratoplasty human eyes, andmay be an economical, convenient alternativeto the latter two instruments in clinicalpractice.(Br_' Ophthalmol 1994; 78: 671-676)

Department ofOphthalmology andVisual Sciences,University of Wisconsin,Clinical Science Center,Madison, WI, USAM J Menage*P L KaufmanM A CroftS P Landay*Current address:General Infirmary at Leeds,Great George Street, LeedsLSI 3EX, UK.Correspondence to:Dr Paul L Kaufman,Department ofOphthalmology and VisualSciences, University ofWisconsin, Clinical ScienceCenter, 600 HighlandAvenue, Madison, WI53792-3220, USA.Accepted for publication26 April 1994

The measurement of intraocular pressure afterpenetrating keratoplasty- is reported to be diffi-cult with the Goldmann applanation tonometerbecause of inaccuracy induced by the suturesinterfering with the tip ofthe instrument, cornealsurface irregularity, and corneal oedema.'Several authors have shown that there is a

marked tendency to high intraocular pressure inthe immediate postoperative period, particularlyafter aphakic or pseudophakic graft surgery.`'3Some patients show a late postoperative chronicelevation ofintraocular pressure,"5 particularly ifthere is a preoperative history of glaucoma.3Goldmann applanation tonometry is most prob-lematic in the immediate postoperative periodand in some cases can remain a long termdifficulty. Since the advent of the Mackay-Margtonometer6 and its more modern version, theTono-Pen,7 measurement ofintraocular pressure

after penetrating keratoplasty is reported to bemore reliable."'0 Others have investigated the use

ofthe pneumatonometer" in irregular or recentlygrafted eyes.'2The accuracy of a minified version of the

Goldmann applanation tonometer in normalcynomolgus monkey and human eyes has beendescribed.'3 In this instrument the front 4 mm ofa standard Goldmann applanating prism isreduced in diameter from 7 0mm to 4 00 mm on

a lathe, without changing the endpoint of intra-ocular pressure measurement, which is theapplanation of a circle of cornea 3 06 mm indiameter. The authors suggested that theminified Goldmann applanation tonometerwould permit applanation of the central grafttissue, without interference from the sutures orirregular interface tissue, in eyes which haveundergone penetrating keratoplasty.The pneumatonometer, developed by

Langham and co-workers," consists of a hollowplunger in a porous sleeve propelled by gas andusing a gas bearing. The tip is 4 4 mm indiameter and covered with a thin silicone rubbermembrane. When the tip is applied to the corneathe membrane occludes multiple small orifices,impeding the outflow of gas and propelling theplunger toward the eye with greater force untilequilibrium is reached. At equilibrium the pres-

sure in the flow system is proportional to theintraocular pressure and is measured electron-ically and displayed either on a liquid crystalpanel (Modular One; Mentor 0 & 0, Inc,Norwell, MA, USA) or as a pen recording(Model 30 R; Mentor 0 & 0). The pneuma-

tonometer was claimed to be an applanationtonometer by Langham but this has been dis-puted by other investigators. 14

The Tono-Pen (Mentor 0 & 0) is a hand held,self-contained instrument which operates on thesame principle as the Mackay-Marg tonometer.It consists of a 1-2 mm central plunger attachedto a micro-strain gauge transducer, surroundedby a stationary, nearly flush 3-2 mm annulus. Adisposable tip cover is used to avoid contamina-tion. The force exterted on the plunger is trans-mitted as a voltage wave that is analysed by a

microprocessor for acceptability. Three to sixaccepted samples are averaged and the mean

shown on a liquid crystal display along with a

coefficient of variation (standard deviationdivided by the mean) indicator.Both the pneumatonometer and the Tono-Pen

are in current clinical use for intraocular pressure

measurement after penetrating keratoplasty.We investigated the accuracy of the minified

Goldmann applanation tonometer, pneumato-nometer and Tono-Pen in measuring intraocular

British3Journal ofOphthalmology 1994; 78: 671-676

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pressure characteristics in postmortem humaneyes which had received same sized and oversizedpenetrating corneal grafts.

Materials and methodsThe study used postportem human eyes con-sidered unsuitable for human transplant forvarious reasons. All experiments were carried outwithin 48 hours of death. Seven eyes underwentsame sized penetrating keratoplasty using a 7-75mm hand-held trephine, the button beingrotated 90° and sutured back into place. Sevenpairs of eyes underwent a 0 5 mm oversizedpenetrating keratoplasty using a 7-75 mmtrephine for the recipient eye and an 8 25 mmtrephine to cut the donor tissue from the pairedeye. In all cases the donor tissue was secured with16 10/0 deep stromal nylon sutures in the usualmanner. Sutures were not buried but the knotswere pulled to the recipient side of the interface.The anterior chambers were reformed usingbalanced salt solution and if necessary any inter-face leaks treated by replacement or additionalsutures.Each eye was mounted on a custom built

plastic eye holder and secured in position, withthe cornea facing horizontally, by pins passedthrough the optic nerve remnant. The anteriorchamber was cannulated through the limbuswith a branched 26 gauge needle, one are ofwhich was connected via rigid polyethylenetubing to a vertically adjustable test tube reser-voir of degassed mock aqueous humour(Barany's solution),'5 the other arm to a pressuretransducer, amplifier, and pen recorder. Thepressure drop across the needle has previouslybeen calculated to be 0- 17mm Hg at 24mm Hg. 16A small amount ofcyanoacrylate glue was used toseal the entry wound of the needle at the limbus.The transducer was calibrated to 0 mm Hg,30 mm Hg, and 60 mm Hg using test tubereservoirs of water before each experiment.The eye was placed at the calibrated 0 mm Hg

height, the test tube reservoir raised to a heightcorresponding to 20mm Hg, and the intraocularpressure allowed to stabilise. The central grafttissue thickness was measured using an opticalpachymeter (Haag-Streit) and the corneal curva-ture at the steepest and flattest meridiansmeasured using a keratometer (Bausch & Lomb,Inc, Rochester, NY, USA).The intraocular pressure was then measured

using the minified Goldmann applanationtonometer (mounted on a standard Goldmannapplanation apparatus on a Haag-Streit slit-lamp), the pneumatonometer (Modular One),and the Tono-Pen (XL). The instruments werealways used in that order and all measurements atall pressures were carried out with one instru-ment before using another. The manometricpressure was raised in approximately 12 arbitraryincrements between 0 mm Hg and 65 mm Hg,with measurements at each level after 60 secondsof stabilisation. The test tube reservoir was opento the eye throughout the experiment.

If the epithelium became oedematous andfriable during the experiment, it was carefullyscraped from the central graft tissue with a blade.This occurred in five of the seven same sized

grafts and three of the seven oversized grafts,always early in the minified Goldmann tonometermeasurement series.

Before each experiment, calibration of theminified Goldmann applanation tonometer wastested with the standard metal calibration rod inthe usual manner. The scale wheel was dialled to20 mm Hg before and after each intraocularpressure measurement and was masked with apaper strip before determination ofeach applana-tion end point. The planed 4 mm diameter tipwas placed precisely within the graft interface onthe surface of the graft tissue. To correct for anyinaccuracy caused by graft astigmatism, theintraocular pressure was measured with themires oriented both horizontally and verticallyfor each manometric pressure and the mean ofthe readings used for comparison.'7 'Half andHalf coffee creamer (Borden, Inc, Columbus,Ohio; milk, cream, non-fat dry milk, disodiumphosphate, sodium citrate) was used in the tearfilm instead of fluorescein to generate the mires,since the tendency of fluorescein to infiltrate thecorneal epithelium with repeated measure-ments'8 made end point determination very diffi-cult. The effect ofthis substitution on intraocularpressure readings with the Goldmann applana-tion tonometer should be minimal. The 'HalfandHalf was applied directly to the tip of the prismwith an eye dropper and cleaned offwith alcoholbetween each measurement.The pneumatonometer and the Tono-Pen

were both supplied by the manufacturer for ourexperiments. The pneumatonometer was cali-brated with the manufacturer's standard testapparatus, which artificially mimics an intra-ocular pressure of20mm Hg. The plastic tip wasplaced on the graft tissue within the interface andthe digital readout of the intraocular pressurerecorded. The surface of the pneumatonometertip was cleaned with alcohol and driedthoroughly after each measurement.The calibration procedure for the Tono-Pen

was carried out at the beginning of each experi-ment. The tip of the instrument was applied tothe centre of the graft tissue. The latex tip coverwas replaced after every second intraocularpressure measurement. The instrument willaccept a series of three to six readings andautomatically displays the mean intraocularpressure and a coefficient of variation indicator.We only accepted values with a coefficient ofvariation -5%.

ResultsDetails are summarised for the same sized graftsin Table 1 and for the oversized grafts in Table 2.As would be expected'9 the mean keratometerreadings from the same sized grafts were signifi-cantly flatter than the oversized grafts (p=0-002unpaired two tailed t test).The results for the minified Goldmann

applanation tonometer, pneumatonometer, andTono-Pen are summarised in Figures 1, 2, and 3respectively. There were a total of 81 intraocularpressure measurements in the seven same sizedpenetrating keratoplasty eyes and 87 in the sevenoversized penetrating keratoplasty eyes with theminified Goldmann applanation tonometer, 84

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Intraocular pressure measurement after penetrating keratoplasty

Table I Details ofsame sized penetrating keratoplasty eyes

Timefrom death Keratometry (dioptres)Donor Age to experiment Graft thicknessno (years) (hours) (mm) Flattest Steepest Mean

1 52 21 0-76 36-0 41-5 38-82 52 24 0-80 36-3 39 5 37-93 71 24 0 90 36 5 41-0 38-84 71 45 0 85 36-0 40 5 38-35 57 14 070 385 42-0 4036 57 34 075 395 42 5 41-07 73 29 0 75 39 5 48-0 43-8Mean 62 27 0 79 37-5 42-1 39-8SD 9 10 0 07 1-6 2-8 2-1

Table 2 Details ofoversized penetrating keratoplasty eyes

Timefrom death Keratometry (dioptres)Donor Age to experiment Graft thicknessno (years) (hours) (mm) Flattest Steepest Mean

1 75 33 0 85 39 5 49 0 44-32 55 30 075 405 42 5 41-53 76 7 0-80 42 5 48 5 45 54 67 25 070 41-5 46-0 43-85 70 9 0 70 390 45 5 42-36 64 29 0-80 41-5 45.3 43-47 81 33 070 41-0 445 42-8Mean 70 24 0-76 40-8 45 9 43-3SD 9 11 0-06 1-2 2-3 1-3

and 91 with the pneumatonometer, and 85 and 91with the Tono-Pen. Linear regression analyses ofthe relations between the measured intraocularpressures and actual manometric intraocularpressures are summarised in each figure.From Figure 1 it can be seen that the minified

Goldmann applanation tonometer gave a very

70A B

~60 0

50-~~~~~.E 5

0.. 400

3030

20 . Sope =0.985 (0.015) :Slope =0.944** (0.015)Intercept =1.64* (0.55) *. Intercept =2.55* (0.57)

10 r 0.99 r=O.99n 81 n=87

10 20 30 40 50 60 70 10 20 30 40 50 60 70

Manometric IOP (mm Hg)Figure I Manometric (x) versus minified Goldmann tonometric (MGAT;y) intraocularpressure (IOP) measurements in enucleated postmortem human eyesfollowing same size (A) or05 mm oversized (B) penetrating keratoplasty. Broken line, y=x; solid line, least squaresregression ofMGAT measurements on manometric IOP. **Slope significantly differentfrom1 0, p<OOO1. *Intercept significantly differentfrom 0.0, p<0004.

' AE 70A'

60

2 50

a 40E00 30c0- 20 Slope = 1.008 (0.019) *E 10 Intercept= 3.37** (0.71) Ir1 ronor0.99 rr

a) ~~~~n84 r

reliable measurement of the intraocular pressureafter penetrating keratoplasty in both the samesized and oversized penetrating keratoplastyeyes. The slopes are very close to one with a lowintercept and a high correlation coefficient.From Figures 2 and 3 it can be seen that

although both the pneumatonometer and theTono-Pen gave a reasonable estimate of themanometric intraocular pressure with highcorrelation coefficients, there was a tendency forboth instruments to overread in a linear fashionthroughout the range of measurements and thiswas most pronounced for the Tono-Pen measure-ments in the eyes with same sized penetratingkeratoplasty.

Because of the possibility that the overestima-tion of intraocular pressure of the pneuma-tonometer and Tono-Pen could be due to acalibration fault we also tested a second model ofeach instrument.The second pneumatonometer (Model 30 R)

had been serviced recently by the manufacturerand was used to measure the intraocular pressurein three of the same sized grafted eyes. Results ofa total of 38 separate measurements are graph-ically summarised in Figure 2C with linearregression analysis. Comparison of Figure 2Cwith Figure 2A shows that the second instrumentalso tended to overread, although this effectdiminished with increasing pressure.The second Tono-Pen was in clinical use and

was used to measure the intraocular pressure inthree of the oversized grafted eyes. The results of39 separate measurements are graphically sum-marised in Figure 3C with linear regressionanalysis. Comparison of Figure 3C with Figure3B shows a very similar overread of intraocularpressure throughout the range of measurements.Removal of the corneal epithelium changed

the slope or intercept in a few instances, but didnot alter the overall findings (Table 3).

DiscussionWe found the minified Goldmann applanationtonometer to be very accurate in the measure-ment of the intraocular pressure in both samesized and oversized grafted postmortem humaneyes. Minified Goldmann applanation tonometervalues reflected manometric intraocular pressuremore closely and uniformly across the entireexperimental range than the values of the

Manometric IOP (mm Hg)Figure 2 Manometric (x) versus pneumatonometer (y) intraocular pressure (IOP) measurements in enucleated postmortemhuman eyesfollowing same size (A, pneumatonometer 1; C, pneumatonometer 2) or 0-5 mm oversized (B, pneumatonometer 1)penetrating keratoplasty. Broken line, y=x; solid line, least squares linear regression ofpneumatonometric measurements onmanometric IOP. *Slope significantly differentfrom 1-0, p<O-005. **Intercept significantly differentfrom 0-0, p<0-0009.

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10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70

Manometric IOP (mm Hg)Figure 3 Manometric (x) versus Tono-Pen (y) intraocular pressure (IOP) measurements in enucleated postmortem humaneyesfollowing same size (A, Tono-Pen 1) or 0-5 mm oversized (B, Tono-Pen 1; C, Tono-Pen 2) penetrating keratoplasty.Broken line, y=x; solid line, least squares regression of Tono-Pen measurements on manometric IOP. *Slope significantlydifferentfrom 1-0, p<005. **Intercept significantly differentfrom 0-0, p<0-0009.

pneumatonometer or Tono-Pen. This was stilltrue when we tested second units of the otherinstruments.The much reduced diameter of the tip of the

minified Goldmann applanation tonometerallows accurate placement in the centre of thegraft tissue, where there is the least irregularityand very little interference with the mires fromthe sutures or the interface area. The graftdiameters we used were large and the minifiedGoldmann applanation tonometer might be moredifficult to use in very small grafts. The more

viscous 'Half and Half also provided slightlybroader and more regular mires than ordinaryfluorescein.The minified Goldmann applanation tono-

meter was certainly the most difficult of the threeinstruments to use, requiring considerablepractice by an operator already very experiencedin the use of a conventional Goldmann applana-tion tonometer. By trial and error we found thebest mires were formed by applying the 'HalfandHalf direct to the tip of the prism with an eyedropper and cleaning the tip with alcoholbetween each measurement.

Previous authors have suggested that in thepresence of corneal oedema Goldmann applana-tion tonometry is grossly inaccurate, markedlyunderstimating the true intraocular pres-sure.'8 2021 Moses'8 found an underestimation ofintraocular pressure in an enucleated human eye33 hours post mortem and attributed theinaccuracy to corneal thickening, although onlyeight measurements in one eye are shown.Goldmann and Schmidt,22 however, found itaccurate in postmortem human eyes with slightcorneal oedema. Kaufman2' postulated that theinaccuracy is caused by the oedematous corneal

epithelium being easier to applanate, giving a

falsely low estimate of the intraocular pressure.Several of the eyes used in our experiments were

33 or more hours post mortem and there was

unquestionably corneal oedema of the grafttissue as judged from the appearance and theelevated pachymetry measurements. Oneexplanation for our improved results was that weremoved the corneal epithelium if it becamefriable and tattered with repeated measurement,providing a smooth surface for the determinationof the applanation end point. This occurred ineight of the 14 eyes studied (five of seven same

sized and three of seven oversized grafts). Sinceepithelial removal was always done early in theminified Goldmann measurement series, andsince the Goldmann measurements always pre-ceded the pneumatonometer and Tono-Penmeasurements, it seemed unlikely that epithelialremoval would bias our measurements in favourofthe Goldmann apparatus in these eyes. Indeed,although there were some statistically significantdifferences between slopes or intercepts with andwithout the corneal epithelium present in a few ofthe tonometer/graft size subcategories, the over-

all conclusion was unchanged. A smooth cornealsurface is more important for precise measure-

ment with the Goldmann tonometer than withthe other two, and in a clinical situation theremoval of corneal epithelium after penetratingkeratoplasty would normally be precluded, as itwould be detrimental to healing. However, in a

clinical situation dozens of measurements wouldnot be made within a few minutes, so theepithelium would be far less likely to become so

oedematous, friable, or tattered. Although aseverely oedematous or irregular corneal graftmight compromise measurements with the

Table 3 Regression analysis oftonometry data in same sized and oversized corneal grafts with epithelium intact and removed

MGAT PneumatonometerNo I Tono-Pen No I

Same sized Oversized Same sized Oversized Same sized Oversized

Intact Removed Intact Removed Intact Removed Intact Removed Intact Removed Intact Removed

Slope 0996 0-983 0 974 o0_god 0o901d,g 1.049b 1-028f 0-942b 1-042 1.069b 1-017 0-998SEM 0 019 0-020 0-016 0-024 0-023 0-021 0-022 0-028 0-071 0-027 0-026 0 025Intercept 1-52b 1*65b 2.55d 2-38b 4-68d 2_90d 3-33d 4-17d 4-60a 5-17d 4-94d,e 2.59cSEM 0-63 0 73 0 59 0 90 0-83 0-76 0-80 1-05 2-58 1 00 0-93 0 95r 100 0 99 0.99 0 99 0 99 0.99 0.99 0-98 0-95 0.99 0 99 0.99n 23 58 52 35 25 59 52 39 25 60 51 40

Slope significantly different from 1-0 or intercept significantly different froth 0 0 by the two tailed paired t test: p<O_ 10; bp<0-05; cp<0_01; dp<0_001 .Significantly different from slope or intercept for eyes with corneal epithelium removed, by the two tailed two sample t test: 'p<O- 10; fp<o-05; gp<0-001.

Slope = 1.002 (0-020)Intercept = 4.06** (0-73)

* r= 0.98n = 91

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675Intraocular pressure measurement after penetrating keratoplasty

minified Goldmann tonometer, the instrument isprobably suitable for many, if not most, post-keratoplasty eyes.We performed and reported keratometry

measurements only to define our material. Themeasurements were not used to guide tonometerorientation or in data analysis, but could be usedfor the former. 7"

Since modern ophthalmic offices are alreadyequipped with conventional slit-lamp mountedGoldmann applanation tonometers, the lathedminified Goldmann applanation tonometerprism is easily the cheapest of the three instru-ments. When mounted on a slit-lamp, theminified Goldmann applanation tonometer is lessportable than the pneumatonometer and,particularly, the Tono-Pen, and unlike the latterinstruments would not allow the intraocularpressure measurement in any position other thanat the slit-lamp - for example, lying flat. Thisproblem would be overcome by mounting theminified Goldmann applanation tonometerprism on a Perkins hand held applanationtonometer23 or by using a Draeger unit withminified tip.24Both the pneumatonometer and the Tono-Pen

tended to overestimate the intraocular pressurein our grafted postmortem eyes, and this was

confirmed for a second unit of each instrument.The pneumatonometer can be difficult to use inrecently grafted living eyes'2 and overestimatesthe intraocular pressure in manometric experi-ments in postmortem eyes.25 In some previousstudies, the instrument overestimated low intra-ocular pressures and underestimated highpressures,'426 or showed a tendency to over-estimate intraocular pressure throughout therange tested, when compared with standardGoldmann applanation tonometry.27The Tono-Pen is a modem development of the

Mackay-Marg tonometer, which is no longercommercially available. The Tono-Pen isreported to be as accurate as the Mackay-Margtonometer in measuring intraocular pressureafter keratoplasty," and the Mackay-Marg hasbeen shown previously to be accurate afterpenetrating keratoplastyl"9 or in the presence ofcorneal irregularity or oedema.2" 2'2'

The Tono-Pen has been directly comparedwith the Goldmann applanation tonometer inpostmortem252629 and living human eyes,726 3032and tended to overestimate low intraocularpressures and underestimate high pressures in allbut two studies. A comparative study in post-mortem human eyes reported accurate measure-ment throughout the range of intraocularpressures.2' However, this was based on a meanof five separate pressure measurements at onlyfive manometric pressures between 10 and50 mm Hg. A recent study comparing the Tono-Pen with the Goldmann applanation tonometerin normal and grafted human eyes reportedsignificant overestimation of intraocular pressure

by the Tono-Pen throughout the range of intra-ocular pressures, when compared with theGoldmann applanation tonometer readings.32This was more pronounced in the eyes which hadundergone penetrating keratoplasty.Our study also showed a linear overestimation

of intraocular pressure after penetrating kerato-

plasty by both ofthe Tono-Pen units, throughoutthe range of manometric pressures tested. Itseems unlikely that both units of the instrumentwould be incorrectly calibrated.

Clinicians should be aware of the 3-6 mm Hgaverage overestimation of intraocular pressureafter penetrating keratoplasty by the Tono-Penand the pneumatonometer.We used an open stopcock manometric system

to evaluate the accuracy ofthe instruments, sinceintraocular pressure remains stable throughoutthe measurement. Closed stopcock manometrywould have mimicked in vivo conditions moreclosely but is technically more difficult, sinceintraocular pressure begins to fall as soon as thestopcock is closed, and rises when the tonometeris placed in the eye.'325 This is a particularproblem when repeated measurements at onemanometric pressure are required - for instance,two for the minified Goldmann applanationtonometer and as many as six for the Tono-Pen.These confounding effects can be managed withsome effort,'3 which did not seem warranted heresince the magnitudes and measurement conse-quences of aqueous humour displacement by allthree tonometers are already known.3 433

This study was supported by National Eye Institute grantEY02698. Dr Menage was supported in part by Bristol andWeston Health Authority and an Ethicon Award from the CollegeofOphthalmologists. Tono-Pen unit 1 and Pneumatonometer unit1 were graciously loaned to us by Mentor0 & 0, Inc.

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