Model to determine resistance and leakage depending on flow performance of insufflators to predict gas flow rate of cannulas

Efficient and reliable insufflation technique isessential for laparoscopy. All insufflation componentshave different gas flow properties because of differ-

ent specific resistances.1,2 The limiting factor for per-formance of the entire insufflation system is the small-est diameter between insufflator and patient, in

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August 2000, Vol. 7, No. 3 The Journal of the American Association of Gynecologic Laparoscopists

From the Department of Obstetrics and Gynecology, Frauenklinik, Christian-Albrechts-University, Kiel, Germany (Drs. Jacobs, Mundhenke andGolombeck, and Professor Jonat); Department of Surgery, Fayette Medical Center, Fayette, Alabama (Drs. Jacobs and Morrison); and Institute for Med-ical Physics and Biophysics, Georg-August University, Göttingen, Germany (Professor Harder).

Address reprint requests to Volker R. Jacobs, M.D., Department of Obstetrics and Gynecology, Christian-Albrechts University, Michaelisstrasse 16,24105 Kiel, Germany; fax 49 431 597 2146.

Accepted for publication May 28, 2000.

Presented at the 28th annual meeting of the American Association of Gynecologic Laparoscopists, Las Vegas, Nevada, November 8–11, 1999.

Abstract

Study Objective. To characterize insufflator CO2 gas flow performance to predict gas flow rate with standardcannulas.Design. Prospective, observational study (Canadian Task Force classification II-2).Setting. Laboratory of university clinic.Patients. None.Intervention. Gas flow (L/min) and average pressure (mm Hg) inside an abdomen model were measured at 12 mmHg nominal pressure during steady state.Measurements and Main Results. An abdomen box model for laboratory measurements was designed with dif-ferent entrance and exit diameters simulated with hole disks from 0.5 to 7.6 mm. With a computer-based data-acquisition model, five insufflators (Olympus 9L and 16L, Storz 10L and 30L, HiTec 16L) were evaluated with 150disk combinations. Flow performance in three-dimensional profiles showed different flow rates for all insufflatorsdepending on resistance and leakage combination, maximum flow rate, and insufflation principle. Maximum flowwas reached without resistance only in the insufflation system at high leakage rates. Low-pressure principle is moreaffected by resistance. Cannula flow rates at 12 mm Hg and 15 L/minute leakage ranged from 4.8 (Origin) to6.0 L/minute (Storz HiCap) for Olympus 9-L insufflators and from 5.4 (Origin) to 15.10 L/minute (Storz HiCap) forStorz 30-L Thermoflator. Reusable cannulas have more flow efficacy than disposable ones, especially with high-flow insufflators, because of larger diameter at insufflation supply.Conclusion. Gas flow depends not only on maximum flow of insufflators but also on resistance of cannulas andleakage rate. With this model it is possible to predict the real, available flow of insufflator-cannula combinationsfor the first time. Improved resistance of all components can save insufflation time.

(J Am Assoc Gynecol Laparosc 7(3):331–337, 2000)

Model to Determine Resistance andLeakage-Dependent Flow on FlowPerformance of Laparoscopic Insufflators to Predict Gas Flow Rate of CannulasVolker R. Jacobs, M.D., John E. Morrison, Jr., M.D., Christoph Mundhenke, M.D., Kirstin Golombeck, M.D.,Walter Jonat, M.D., and Dietrich Harder, Ph.D.

Reprinted from the JOURNAL OF THE AMERICAN ASSOCIATION OF GYNECOLOGIC LAPAROSCOPISTS, August 2000, Vol. 7 No. 3© 2000 The American Association of Gynecologic Laparoscopists. All rights reserved. This work may not be reproduced in any form or by any means without written permission fromthe AAGL. This includes but is not limited to, the posting of electronic files on the Internet, transferring electronic files to other persons, distributing printed output, and photocopying.To order multiple reprints of an individual article or request authorization to make photocopies, please contact the AAGL.

general, the diameter at the insufflation supply (Fig-ure 1). An important point is that gas flow through theinsufflation system is turbulent and does not thereforeobey Hagen-Poiseuille’s law for laminar gas flow.1,2

Despite this fundamental difference, turbulent gasflow also shows strong dependence on the radii of can-nulas, valves, and connection tubing.

So far the literature has paid little attention to thesignificance of this effect. Only one publication todate attempted to evaluate the flow performance of anentire insufflation system.3 That group found differ-ent insufflation properties at gas flow of 1 to 2L/minute, too low for current laparoscopic procedures.Up to now, no reliable model has been described toevaluate gas flow performance of insufflators depend-ing on flow-limiting factors and consequences forcannula gas flow.

With the background of previous researchresults,1,2,4 three questions were formulated:

1. How do resistance and leakage size affectinsufflator performance?

2. Can influencing factors be graphically pre-sented with a measurement model?

3. Is the gas flow rate of a cannula-insufflatorcombination predictable?

During this continuing independent project eval-uating insufflation techniques,1,2,4 a model is to bedeveloped that should be able to characterize for thefirst time gas flow-, resistance- and leakage-dependentinsufflation performance of random standard insuf-flators. These insufflators represent the variety of olderand newer models with a flow range from 10 to30 L/minute and based on both low- and high-pressure

principles. Once insufflation diameter is known, itshould be possible to determine the flow rate of stan-dard disposable and reusable cannulas from three-dimensional graphics.

Materials and Methods

A computer-based data-acquisition laboratorymeasurement model was developed that consisted ofa laptop, a data-acquisition card PCI-20089-W-1, andthe measurement program Labtech Notebook version5.01 (both Intelligent Instrumentation, Tucson, AZ),two electronic manometers (Digima premo, one forpressure, one for gas flow), and a laminar flow ele-ment (LFE, type 1; Special Instruments, Nördlingen,Germany). Measurement accuracy was ±0.1%. Theinstruments were connected as shown in Figure 2.

A rigid, air-sealed metal box of about 5 L contentswith entrance (equals insufflation diameter at can-nula) and exit (for leakage size) was chosen as anabdomen model (Figure 3). Flexible and even semi-flexible abdomen models were too inconstant for themeasurements. Different disks with holes from 0.5 to7.6 mm (0.5, 1.0, 1.2, 1.5, 1.7, 2.0, 2.2, 2.5, 2.7, 3.0,3.5, 4.0, 4.5, 5.0, 7.6 mm; Figure 4) simulated in anycombination the variety of entrance and exit diame-ter sizes. Gas flow in the insufflation hose and pres-sure inside the box model were measured with fivestandard insufflators (Figure 5, Table 1) from Olym-pus (9 and 16 L), Storz (10 and 30 L), and HiTec (16L) with 150 disk combinations and displayed in three-dimensional gas flow diagrams (Figures 6–10).

All insufflators except HiTec’s are based on anover-pressure insufflation principle, starting insuffla-tion with an initial short pressure overshoot (up to

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Model to Predict Gas Flow Rate of Cannulas Jacobs et al

FIGURE 1. Comparison of diameters at insufflation supply:Left: Storz HiCap reusable cannula with 8.0 mm. Right:Ethicon Endopath 512-mm disposable cannula with 2.5 mm. FIGURE 2. Measurement scheme.

several times nominal pressure) to reach the intendedpressure in the abdomen faster. HiTec’s is the onlyinsufflator in this study that operates on the low-pressure principle, a security feature that preventsnominal pressure being exceeded.

For measuring purposes, the insufflator wasstarted and we waited until steady state was reached.Steady state was defined as well-balanced gas flow inthe entire system. Data acquisition of gas flow (L/min)and pressure (mm Hg) at 3 Hz began after continuousuniform insufflator function was reached and averagebox pressure values were stable for at least 15 seconds.The insufflator setting was maximum flow (flowmax)

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FIGURE 3. Abdomen model box with (left) entrance, (back)exit, and (closed) additional supplies.

FIGURE 7. Flow rate of the Olympus 16-L insufflator.

FIGURE 4. Disks with holes in different sizes from 0.5 to 20.0mm (for measurements only disks from 0.5–7.6 mm wereincluded).

FIGURE 5. Insufflators. Top: HiTec Laserpneu 1100 16 L/min.Middle: Storz Thermoflator 30 L/min (left), Storz ElectronicLaparoflator 10 L/min (right). Bottom: Olympus SurgicalCO2 Insufflator 9 L/min (left), abdominal CO2 insufflator16 L/min (right).

FIGURE 6. Flow rate of the Olympus 9-L insufflator.

and a nominal box model pressure of 12 mm Hg waschosen. This should be the maximum intraabdominalpressurebecause at 15 mm Hg the back-flow of bloodto the heart through the vena cava is reduced.

Although millimeters of mercury is not an Inter-national System (SI) unit, it is a standard pressureunit in medicine. For practical daily use and meaning(pressure at insufflator display is shown in mm Hg)this unit was used. The SI unit is Pascal (1 mm Hg =133,322 N/m2).

Insufflation diameter at the cannula is the small-est diameter left for insufflation, usually at the insuf-flation port.1,2 The radius is measured in millimeters.

Measurement results in three-dimensional graph-ics for all insufflators were calculated and displayed

with Origin version 3.5 scientific graphic program(Microcal, Inc., Northampton, MA).

Results

The results are presented in three-dimensionalgraphics with leakage diameter (x axis, right), insuf-flation diameter (y axis, left), and insufflator gas flowrate (z axis, upward). All diagrams show differentflow rates depending on insufflation diameter, leak-age rate, maximum flow performance, and insuffla-tion principle. Flow rates increase from front to back.The maximum measured flow rate can be less than thatquoted in the manual specifications; for example,Olympus 6.41 L instead of 9 L (Table 2) and can be

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FIGURE 8. Flow rate of the Storz Laparoflator 10-L. FIGURE 9. Flow rate of the Storz Thermoflator 30-L.

TABLE 1. Insufflators Evaluated

Flowmax MeasuredAccording to Manual Flowmax

Manufacturer Type Model No. (L/min) (L/min)

Olympus CO2 insufflator 9 L A5643.1 9.0 6.41Winter & Ibe GmbH, Hamburg, Germany

Olympus Abdominal CO2 insufflator 16 L 16.0 12.24Winter & Ibe GmbH, Hamburg, Germany A5850

Karl Storz GmbH, Tuttlingen, Germany Electronic laparoflator 26430020 10.0 9.89Karl Storz GmbH, Tuttlingen, Germany Thermoflator 26432020 30.0 28.93HiTec Medical Vertriebs GmbH, Laserpneu 1100 16.0 12.72

Tuttlingen, Germany

reached only with low resistance in the insufflationsystem and maximum leakage rate. The gas flow rateincreased for all insufflators with increasing insuf-flation diameter and leakage rate. A flow plateau(Figure 11) is seen in insufflators with low gas flow.The maximum flow rate does not occur at the begin-ning of insufflation but after a certain insufflationdiameter-leakage combination, the earliest flowmax,and has to be determined for each insufflator.

Increasing gas flow rates can only be used withlarge insufflation diameters and cannulas with lowresistance. The best example is Storz 30-L Ther-moflator. With 1.5-mm insufflation diameter, only5.43 L/minute is available, but more than 15 L/minutewith 8 mm, in contrast to the Olympus 9-L insuffla-tor, which has only 4.84 and 5.97 L/minute, respec-tively (Table 2). Although HiTec’s insufflator has a

good flow rate performance with up to 16 L accord-ing to the manual, the increase in flow rate in com-parison with other insufflators is delayed. Because ofits low pressure insufflation principle this insufflatoris more affected by resistance than any other one eval-uated. Only cannulas with a large insufflation diam-eter can be used to reach maximum flow performance.

Short over-pressure peaks with 75 mm Hg or lesswere measured in the insufflation hose during start ofinsufflation but were not visible at steady state. Theyexceeded by far the nominal pressure of 12 mm Hgand are therefore a potential threat to patient safety.

With three-dimensional diagrams it is possibleto determine maximum gas flow rate at 12 mm Hgnominal intraabdominal pressure for standarddisposable cannulas (Origin, Richard Allan, Ethicon,Auto Suture) as well as reusable cannulas (Storz, Storz

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FIGURE 10. Flow rate of the HiTec insufflator 16-L. FIGURE 11. Limitations of insufflator gas flow performance.

TABLE 2. Maximum Gas Flow Rate (L/min) for Different Cannula-Insufflator Combinations at Nominal Pressure of 12 mmHg Insufflator Setting and 3.0 mm Disk Hole Diameter

Disk Hole InsufflationDiameter Diameter Olympus Olympus Storz Storz HiTec(mm) Examples of Cannulas (mm) (9 L) (16 L) (10 L) (30 L) (16 L)

1.5 Origin,a Richard Allan 1.5 4.84 4.67 4.77 5.43 3.141.7 Ethicon (old) 1.7 5.33 5.10 5.28 6.29 3.772.0 Auto Suture 2.0 5.49 6.14 6.21 8.34 4.412.5 Ethicon (new) 2.5 5.63 7.78 7.43 10.33 5.833.5 Storzb 3.4 5.90 7.03 8.67 12.93 7.497.6 Storz HiCapb 8.0 5.97 6.94 9.01 15.10 8.60

aEstimated, because insufflation diameter is restricted through the valve.bEstimated, because disk hole and insufflation diameters are similar but not identical.

HiCap) if the insufflation diameter is known. (Tech-nical details about insufflation properties, flow per-formance, and resistance of cannulas and trocars arepublished elsewhere.1,2)

Discussion

Gas flow properties of insufflators depend on typeand resistance of cannulas. The limitation of the entireinsufflation system’s performance is the smallest diam-eter left for insufflation, which is usually at the insuf-flation supply of the cannula. Insufflator function islimited by resistance, leakage amount, and maximumflow performance. Maximum gas flow is availableonly with a certain combination of insufflation diam-eter and leakage rate (earliest flowmax). The higherthe insufflator’s gas flow performance, the later thispoint is reached, and more efficient higher flowdepends on cannulas with less resistance. Reusablecannulas have better flow performance than disposableones because of larger insufflation diameter. High-flowinsufflation (>10 L/min) is not possible with standarddisposable cannulas at nominal pressure (12 mm Hg).

Although high CO2 flow is often intended in lap-aroscopy to improve insufflation, a side effect cannotbe overlooked. If gas is directed at high flow througha small diameter against tissue, it can dry out the tis-sue, causing laparoscopic hypothermia and potentialtissue damage.5,6 Although significant damage isunlikely with today’s cannulas and how and wherethey are placed, hypothermia can and should be pre-vented with adequate gas hydration and warmingdevices.4,7

So far neither an adequate model nor a com-prehensive method has been described for charac-terization and objective standard measurements oflaparoscopic insufflator-cannula combinations. Pub-lications neglect the meaning of resistance for insuf-flation gas flow performance of insufflators. Trialsby Borten et al, who first found flow variations amonginsufflators, were not continued.3 Their method toevaluate an entire insufflation system and compare dif-ferent components was insufficient to assess phys-ical properties, such as resistance and its meaning forthe system’s performance. Their flow rates of only 1to 2 L/minute are too low for today’s laparoscopystandards.

The Emergency Care Research Institute (ECRI),a nonprofit organization, published two comprehen-

sive evaluations of laparoscopic insufflators.8–10 How-ever no mention is made of the insufflation diameterof cannula used for the evaluation or specific resis-tance. Therefore these results are neither transferablenor repeatable. The two-dimensional graphics used byECRI are unable to describe sufficiently insufflator gasflow performance with its dependence on resistanceand leakage rate as shown in this study.

Not enough attention has been paid to the mean-ing of flow resistance for insufflation devices, althoughit has a large influence on the system’s performance.Resistance is the weak point in the system. Measure-ments also show that insufflation systems with all theircomponents have to be seen as entire units. For highflow efficiency they cannot be combined randomly. Inan improved system all single components have to bedesigned to optimize resistance and be adjusted toeach other. Improved resistance-reduced componentsare more efficient and faster, and save time and money.Manufacturers should publish standard comparablereference data about specific insufflation properties oftheir products, especially for insufflator-cannula com-binations, so that users can ensure optimum perform-ance of systems to obtain a more efficient and reliabletechnique. Purchase of expensive equipment such ashigh-flow insufflators can be postponed if cannulaswith optimum resistance increase the performance ofalready available insufflators.

Conclusion

With this model, flow performance of insufflatorsbased on resistance and leakage were characterized forthe first time in three-dimensional graphics. With theresults, gas flow rate of cannula-insufflator combina-tions can be predicted and thus the flow performanceof each insufflation system can be improved and timecan be saved.

References

1. Jacobs VR: Experimentelle Untersuchung zu denInsufflationseigenschaften verschiedener Veressnadeln,Trokare und Insufflatoren zum Aufbau eines Pneumo-peritoneums in der Laparoskopie. (Experimental studyon insufflation properties of different Veress needles,trocars and insufflators to establish pneumoperitoneumin laparoscopy.) Doctoral dissertation, University ofGöttingen, Germany, 1996

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2. Jacobs VR, Morrison JE Jr, Mettler L, et al: Specificresistance of Veress needles, disposable and reusabletrocars limiting CO2 gas flow performance in pelvis-copy and laparoscopy. Minimal Invas Ther Allied Tech-nol 8(1):37–47, 1999

3. Borten M, Walsh AK, Friedman EA: Variations in gasflow of laparoscopic insufflators. Obstet Gynecol68:522–526, 1986

4. Jacobs VR, Morrison JE Jr, Mettler L, et al: Measure-ments of hypothermia in laparoscopy and pelviscopy:How cold it gets and how to prevent it. J Am AssocGynecol Laparosc 6(3):289–295, 1999

5. Ott DE: “Wind chill effect” at laparoscopy. J Am AssocGynecol Laparosc 5(3 suppl):S37, 1998

6. Gray RI, Ott DE, Henderson AC, et al: Severe localhypothermia from laparoscopic gas evaporative jet cool-ing: Amechanism to explain clinical observations. J SocLaparoendosc Surg 3(3):171–177, 1999

7. Ott DE, Reich H, Love B, et al: Reduction oflaparoscopic-induced hypothermia, postoperative painand recovery room length of stay by preconditioning gaswith the Insuflow device: A prospective randomizedcontrolled multi-center study. J Soc Laparoendosc Surg2(4):321–329, 1998

8. Emergency Care Research Institute: Laparoscopic insuf-flators. Health Devices 21:143–179, 1992

9. Emergency Care Research Institute: Laparoscopic insuf-flators. Evaluation updates. Health Devices 21:443–446,1992

10. Emergency Care Research Institute: High-flow laparo-scopic insufflators. Health Devices 24:252–285, 1995

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Model to determine resistance and leakage depending on flow performance of insufflators to predict gas flow rate of cannulas

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