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Transmural surgical gown pressure measurements remits in the operating theater
K. W. Attman, PhD J. H. McElhaney, PhD J. A. Moylan, MD K. T. Fitzpatrick PA Durham, North Carolina
Transmural gown pressures encountered when the surgeon comes into contact with a patient were measured in the operating theater. The surgical gown industry has assumed these pressures to be less than 5 psi in testing the efficacy of the gown and drape barrier material to impede bacterial transmission through its pores, In this study, pressure-sensitive contact film and resistive strain gauge recordings made from the surgeon’s abdominal region and forearms indicated peak contact pressures in excess of 60 psi. This report indicates a need to reassess the basis of test utilization in evaluating barrier materials used in gowns and drapes. (AM J INFECT CONTROL
1991;19:147-155)
Wound infection after surgery remains a problem despite major advancements in sterile operating technique. ’ This risk has been reported to be from 1% to 5% and as high as 8.9% in “clean wound” surgery.’ Although penetration through wet barriers remains a leading theory, bacteria may go through dry surgical gowns as a result of pressures encountered when the surgeon comes into contact with the patient, and this may be equally or more important. (A similar biomechanical situation may be that of latex condoms as a barrier material to protect against viral penetration.‘) The standard industry test used to evaluate the effectiveness of gown and drape barriers to impede transmission relies on hydrostatic head
From the Departmentsof Biomedical Engineering and Surgery, Duke University. This work was supported in part by a contract with Surgikos, a Johnson 8 Johnson company. Reprint requests: Dr. J. A. Moylan, Box 3947, Duke University Medical Center, Durham, NC 27710. 17146mo2s
pressure generated from a liquid column, resulting in a “positive pressure differential” across the barrier material. Little attention has been directed to pressures without liquid since the pressure is assumed to be on the order of 5 psi. Surgeon/patient contact pressures have not been studied in clinical situations. This article reports distributed pressure measurements made during actual operating time by means of (1) pressure-sensitive contact film and (2) a specially fabricated resistive strain gauge transducer pad. Results from each device are presented for two possible surgeon contact sites with the patient (the forearm and the abdomen) and indicate transmural pressures encountered in excess of 60 psi. Analysis of the surgeon/patient contact suggests that a pressure differential is not directly analogous to the situation encountered in the operating room. Additional theoretical requirements for small surgical gown pore size relative to bacterial size and viscous properties are discussed and indicate the importance of adequate surgical gown standards to reduce the incidence of patient infection. 147
Amewan
140
Altman et al.
INFECTION
Journai
of
CONTROL
SUPER LOW PpIEsiiERE TYPE P cwpllm
PI@. 1. Pressure chart for Fuji prescale film, showing color density increase with pressure increase. Note that (5-20 kg f/cm2)(2.2 lb/kg f)(2.54 cm/in)* = 70-280 psi.
Pressure-sensitive contact film (Fuji prescale film, C. Itoh & Co., America, Inc., New York, N .Y .) with a “super low” range of 5 to 20 kg f/cm2 (70 to 280 psi) was cut into 3 x 10 inch strips and sterilized in ethylene oxide. Fig. 1 shows the pressure chart, indicating an increase in the film’s color density with an increase in contact pressure. Film samples were tested to control for possible effects of sterilization and moisture on the registered color density, which were not noticeably affected (although Et0 sterilization did seem to change the resulting color slightly). The film formally requires a contact time of 2 minutes, with the density of the film to be read within 9’2 to 5 hours. However, a test of the time-varying response of the film does
not show a noticeable difference. Similarly, the color density gradation in response to different pressures does not change noticeably overnight. For the eight surgical procedures studied (all involving the same surgeon), the film was applied to the abdominal region of the surgeon’s scrub suit, as well as the bare scrubbed left and right forearms, with the use of sterile Ioban 2 antimicrobial film (Medical-Surgical f)ivision, 3M Co., St. Paul, Minn.). Fig. 2 displays films recorded during thoracic surgery on which color density indicates pressures well in excess of 120 psi, although the appearance of dark ridges where the paper folds may re&ct stres7ses in the paper. With seemingly h&h registered pressures, an “in house” calibration was performed to compare the relative &f&s of static
Volume June
19 Number
3
Transmural
1991
surgical
gown pressure
measurements
149
Fig. 2. Pressure-sensitive contact film from forearms and abdominal region of surgeon taken during abdominal aneurismectomy. Color density indicates pressures in excess of 120 psi, although the appearance of dark ridges where the paper folds may reflect stresses in the paper.
versus shear forces (shown in Fig. 3). It is found that dragging the object over the prescale film tends to increase the registered pressure (compare coloration of the 35.2 psi shear test with the 35.2 psi static test, for example). Color density decreased (to slightly greater than 70 psi) after the surgeon was requested to minimize shear when contacting the patient (as evidenced in Fig. 4). These contact-sensitive film studies determined that physically demanding surgical procedures (e.g., Nissen fundoplication) encounter greater pressures between the surgeon and the patient than more delicate procedures (e.g., hernia repair). Also, the forearm is a site of somewhat greater pressure than the abdominal area; this is to be expected since the downward force of a surgeon’s arm more closely reflects body weight than does the horizontal force of the surgeon’s abdomen coming into contact with the patient. Measured pressures are similar to other
biomechanical contact film, walking.4 STRAIN
GAUGE
uses of the pressure-sensitive such as gait analysis during MEASUREMPNTS
For more formal documentation of peak local pressures encountered, a device that consisted of a 2 X 8 array of resistive strain gauges (0 to 30 psi range, Precision Measurement Co., Ann Arbor, Mich.) encased in a 2 x 6 x 0.25 inch mold of silicone elastomer, (Sylgard 186, Dow Corning Corp., Midland, Mich.) was constructed, as shown in Fig. 5. Individual resistive strain gauges have a linear output voltage response in contrast to piezoelectric devices, which have a temporal current “bleed-off” into the accompanying circuitry’ Each strain gauge channel is fed through model 2 120/2 110 Vishey amplification (Instrument Division, Raleigh, N.C.) at a gain of 8000, which avoids amplifier saturation while maintaining an adequate sig-
American
150 Statk
Altman et al.
INFECTION
lhrat 1‘
aik-‘--* c
-
:
35 p.s.i.
x . .
,” a. 82 p.s.i.
128 pai.
177 p.s.i.
sbffwrr ttblilt
17.5 p.s.i.
35 pa.i.
* _.
C-------.5
I
d
cm/stx-4
P@. 9. “In-house” calibration of prescale film shows tendency toward stress concentrations at edge of surface whose contact with film registers pressure (static test). Shear test: Dragging the object over the prescale film tends to increase the registered pressure. (Compare coloration of the 35.2 psi shear test with the 35.2 psi static test, for example.)
nal/noise ratio. A magnetic tape data recorder was used to capture the initial signals ( 17/8 ips), which were digitized (140 msec sample rate) and evaluated on an IBM personal computer with the use of a Computerscope (RC Electronics Inc., Santa Barbara, Calif.). The resistive strain gauge package was calibrated to determine the linear response of each channel for increasing loads (*calibration factors”): these factors are necessarily affected by encasement of the strain gauges in the silicone elastomer (Silastic) package. An “area effect” was also evaluated by application of the same
Journal
of
CONTROL
pressure to the Silastic surface above each gauge with differing contact areas; this exercise determines the specificity for a channel response dependent on the contact area of the applied force. The area effect is different from a “location effect,” where the response depends on forces concentrated at sites not directly above a particular strain gauge; no location effect was observed. Finally, amplifier imbalance at higher gains merely corresponds to a direct current offset in the output voltage of an individual channel. Although the strain gauges used were rated to 30 psi, there is a significant safety factor that allows for possible nonlinear output voltage at pressures greater than 30 psi but not great enough to cause (irreversible) plastic deformation of the transducer. The energydispersing property of the Silastic encasement” is also expected to increase the viable transducer range. Calibration of individual transducer channel strain gauges was performed to measure output voltage at pressures up to 120 psi. We found that first-order linear regression curves for this pressure range produced relatively low correlation coefficients (R’) of 0.799 to 0.948 and suggested possible nonlinear response to increasing loads. Fig. 6 shows the output voltage versus pressure for six transducer channels with a second-order polynomial curve fit, revealing that the nonlinear behavior is predominant at pressures greater than 100 psi. We then decided to rely on first-order linear regression calibration factors that include pressures only up to 58 psi, producing correlation coefficients that range from 0.949 to 0.990. The strain gauge device was placed within a sterile intestinal bag and pinned to the abdominal region of the scrub suit or fixed to the surgeon’s forearm by means of antimicrobial film. Motion artifact in the cabling and transducer pad was evident in only one channel under rigorous conditions at 171 mV, which corresponded to a maximum error of 10 psi. Fig. 7 shows six transducer channel output voltage traces recorded from contacts made with the surgeon’s forearm and either the patient or the operating table, taken during (a) a 73-minute segment of a resection and grafting of abdominal aortic aneurysm and (b) a 34-
Volume June
19 Number
3
Transmural
1991
surgical
gown pressure
measurements
151
Fig. 4. Pressure-sensitive contact film from forearms of surgeon during colostomy takedown, after surgeon was requested to minimize shear when contacting patient. Color density indicates slightly more than 70 psi
minute segment of a thyroidectomy. Additional procedures included hernia repairs, excisional biopsies, femoral/popliteal bypass graft surgery, a cholecystectomy, and a blood clot evacuation. Although peak pressures varied with the procedure, Fig. 7 is representative of the results in general: (1) output voltages that correspond to peak pressures well in excess of 60 psi (the linear range of the transducers calibrated in Fig. 6) have been repeatedly recorded, (2) low pressures measured during relatively inactive periods of surgery reflect pressures of less than 10 psi, and (3) peak pressures above 60 psi have been encountered, whether the transducer pad was on the surgeon’s left or right forearm or on his abdomen. DlSCUSSlON
Although apparently large, peak pressures are encountered
the measured for only brief
contact times. Fig. 7c and d indicate that peak pressure envelopes (described with brackets in Figs. 7a and 7c, respectively) actually contain multiple peak pressure “spikes” of shortduration activity, so that the total energy of each contact is reduced. Furthermore, typical peak pressures in excess of 60 psi are consistent with the ability of human skin to withstand such impacts. (McElhaney et al.’ describe several studies in which it is concluded that the elastic deformation compressive strength of skin is in excess of 300 psi.) To understand the consequence of the measurements reported in this article, we first discuss possible mechanisms that may account for bacterial transmission through a surgical gown’s pores. The transmissibility of microspheres and red blood cells through singlepore membranes has been investigated from a fluid mechanics perspective by Frank and
American
152
Altman et al.
INFECTION
Vishey 212Ot2110 Amplifiers
of
Personal Computer Computerscope
HP 3966A
Instrumentation
Journal CONTROL
Recorder
Fig. 5. Block diagram of pressure-measurement system. The transducer pad shown on the left is amplified; signals are stored on the instrumentation recorder and later digitized on a personal computer for analysis
2
50
100
El
Channel2
l
Channel3
m
Channel4
6
Channel5
n
Channel7
0
Channel8
150
p.s.1.
Fig. 6. Output voltage versus pressure for six transducer channels, with second-order polynomial curve fit. Note the nonlinear behavior at pressures greater than 100 psi.
Hochmuth*; although concerned with capillary blood flow, this study performs theoretical analysis and experimental verification of an idealized particle as well as viscoelastic cell penetration through a pore. For the idealized case of rigid spherical bacteria whose size is smaller than the pore, Stokes’ radius describes the force necessary for transmission, as derived from the drag coefficient of fluid flow over the surface of that sphere.’ Cells that are larger than the pore necessarily need to deform in order to
penetrate the idealized single-layer pore, as evaluated with the red cell.’ With either situation there is also the propensity for bacterial adhesion to the surgical gown material, increasing the force necessary for adequate penetration. This factor is probably more important with the streptococcal and staphylococcal bacteria that significantly cause the surgical u&ction, considering the complicated bacterial geometries and glycocalyx.‘O A theoretical description of bacterial penetration through a
Volume
19 Number
June 1991
3
Tram-mural
surgical
gown pressure
measurements
Fig. 7. a, Six transducer channel voltage traces displayed from 73-minute segment of a resection and grafting of an abdominal aortic aneurysm (pad on surgeon’s left forearm), b, Six transducer channel voltage traces displayed from 34-minute segment of thyroidectomy (pad on surgeon’s right forearm). c, Time-expanded view of bracketed section in Fig. 7a (3.7 minutes); d, time-expanded view of the bracketed section in c (17.5 seconds) (transducer pad on forearm).
153
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INFECTKIN
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of
CONTROL
b)
4
Positive Pressure
Differential
Sandwich
Pressure
F&j, 8. Schematic drawings of the positive pressure testing situation on a sheet of surgical gown material (a) and the “sandwich pressure” actually encountered across the gown when the surgeon contacts the patient (b).
barrier material is further complicated by the prospect that the pore itself may deform to accommodate the bacteria. Other factors such as relative moisture, hydrophilic/hydrophobic interactions, shape, and composition of the bacterial or viral particle of interest may also be important in the actual pass-through of microorganisms. The aforementioned description relies on the assumption that the environment suitable for bacteria transmission through the surgical gown pore is moist and that there is a pressure differential across the gown, providing the analogy to fluid mechanics. (This is also in accordance with the industry norm of setting up a positive pressure differential across the gown to test transmissibility.) However, a recent study of the barrier protection of surgical gowns found that the “soak-through point” of hydrostatic pressure is not an exclusive determinant of “viral penetration point.“” In other words, there may be particle penetration across the gown at pressures below the material’s soakthrough point. The more relevant (and inappropriate) assumption here is that a pressure differential is set up across the gown. Consider the idealized situation of an object placed on a table; although the object exerts a pressure on the table (object weight divided by its contact area), the table exerts a normal force on the object to
maintain equilibrium. In the more complicated (but analogous) situation of a surgeon coming into contact with a patient, both the surgeon’s force and an equal and opposite force are encountered by the gown. Fig. 8, a shows a schematic diagram of the positive pressure differential test (where forces applied across the gown are countered at the periphery of the material); conversely, Fig. 8, b demonstrates that contact “sandwich” pressures are countered by forces at the same location as the source. We measured contact pressures, rather than pressure differentiak; it is therefore imperative that a mechanism for bacterial transmission be determined before the consequences of pressures encountered across the gown under surgical conditions are evaluated. Pressure measurements reported in this article open the arena to further studies that would more precisely quantify peak pressures encountered between the surgeon and the patient. To delineate various contact surface shapes (such as the edge of the surgical table as opposed to the patient), a future study may take advantage of large arrays of conductive polymer pressure sensors’* that need to be multiplexed to counter the aforementioned current “bleed-off.” Transducers capable of accurately measuring pressures greater than 60 psi should also be used in accordance with our initial measurements. Statistical numbers of procedures,
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1991
surgeons, and their assistants may also be done to determine conditions that pose a greater risk of encountering high pressures. Transmural surgical gown pressure measurements encountered in the operating theater are only a first step in determining the conditions necessary for bacterial or viral particle penetration. This article also raises the concern that “positive pressure” testing of the surgical gown may not adequately duplicate the surgical theater environment and leads us to recommend experimental verification. This could be done by applying a pressure to the gowns by dropping weights from a fixed distance’ or by other means of applying forces that squeeze the fabric rather than force air through it. CONCLUSIONS
Measurements of the distributed pressures encountered between a surgeon and a patient in the operating theater were made as a first step in quantifying the conditions with which infecting bacteria may penetrate the surgical gowns. This study required the fabrication of a multisite-resistive strain gauge device. Although relatively high pressures were recorded (in excess of 60 psi from the forearm during thoracic surgery), it is uncertain whether favorable conditions exist for bacterial transmission (such as a warm, moist contact environment) .
Transmural surgical gown pressure measurements
I SS
Reforonces 1. Fitzgerald RH, Washington JA II. Contamination of the operative wound. Orthop Clin North Am 1975;6:110514. 2. Beatty D, Robinson GV, Zaia JA, et al. A prospective analysis of nosocomial wound infection after mastectomy. Arch Surg 1983;118:1421-4. 3. Regnault W, Sisler M. FDA HIMA Conference on Latex as a Barrier Material. University of Maryland, April 67, 1989, College Park, Md. 4. Aritomi H, Morita M, Yonemoto K. A simple method of measuring the footsole pressure of normal subjects using prescale pressure-detecting sheets. J Biomech 1983;16:157-65. 5. Webster JG. Tactile sensors for robotics and medicine. New York: John Wiley, 1988. 6. Carmines DV. The quasilinear viscoelastic modeling of human and porcine coronary arteries tested in vitro [Ph.D. dissertation]. Durham, NC: Duke University, 1989. 7. McElhaney JH, Roberts VL, Hilyard JF. Handbook of Human Tolerance. Tokyo: Japan Automobile Research Institute, 1976. 8. Frank RS, Hochmuth RM. An investigation of particle flow through capillary models with the resistive pulse technique. J Biomech Eng 1987; 109: 103-9. 9. Fox RW, McDonald AT. Introduction to fluid mechanics. 3rd ed. New York: John Wiley, 1985. 10. Costerton JW, Geesey GG, Cheng KJ. How bacteria stick. Sci Am 1978;238:86-95. 11. Tyler DS, Lyerly HK, Nastala CL, et al. Barrier protection against the human immunodeficiency virus. 31st Annual University Surgical Residents’ Conference, February 11, 1989, Baltimore, Md. 12. Maalej N, Bhat S, Webster JG, Tompkins WJ, Wertsc JJ, Bach-y-Rita P. A conductive polymer pressure sensor. Proc 10th Ann IEEE EMBS Conference. 1988;07700771.
Transmural gown pressures encountered when the surgeon comes into contact with a patient were measured in the operating theater. The surgical gown industry has assumed these pressures to be less than 5 psi in testing the efficacy of the gown and drape barrier material to impede bacterial transmission through its pores, In this study, pressure-sensitive contact film and resistive strain gauge recordings made from the surgeon’s abdominal region and forearms indicated peak contact pressures in excess of 60 psi. This report indicates a need to reassess the basis of test utilization in evaluating barrier materials used in gowns and drapes. (AM J INFECT CONTROL
1991;19:147-155)
Wound infection after surgery remains a problem despite major advancements in sterile operating technique. ’ This risk has been reported to be from 1% to 5% and as high as 8.9% in “clean wound” surgery.’ Although penetration through wet barriers remains a leading theory, bacteria may go through dry surgical gowns as a result of pressures encountered when the surgeon comes into contact with the patient, and this may be equally or more important. (A similar biomechanical situation may be that of latex condoms as a barrier material to protect against viral penetration.‘) The standard industry test used to evaluate the effectiveness of gown and drape barriers to impede transmission relies on hydrostatic head
From the Departmentsof Biomedical Engineering and Surgery, Duke University. This work was supported in part by a contract with Surgikos, a Johnson 8 Johnson company. Reprint requests: Dr. J. A. Moylan, Box 3947, Duke University Medical Center, Durham, NC 27710. 17146mo2s
pressure generated from a liquid column, resulting in a “positive pressure differential” across the barrier material. Little attention has been directed to pressures without liquid since the pressure is assumed to be on the order of 5 psi. Surgeon/patient contact pressures have not been studied in clinical situations. This article reports distributed pressure measurements made during actual operating time by means of (1) pressure-sensitive contact film and (2) a specially fabricated resistive strain gauge transducer pad. Results from each device are presented for two possible surgeon contact sites with the patient (the forearm and the abdomen) and indicate transmural pressures encountered in excess of 60 psi. Analysis of the surgeon/patient contact suggests that a pressure differential is not directly analogous to the situation encountered in the operating room. Additional theoretical requirements for small surgical gown pore size relative to bacterial size and viscous properties are discussed and indicate the importance of adequate surgical gown standards to reduce the incidence of patient infection. 147
Amewan
140
Altman et al.
INFECTION
Journai
of
CONTROL
SUPER LOW PpIEsiiERE TYPE P cwpllm
PI@. 1. Pressure chart for Fuji prescale film, showing color density increase with pressure increase. Note that (5-20 kg f/cm2)(2.2 lb/kg f)(2.54 cm/in)* = 70-280 psi.
Pressure-sensitive contact film (Fuji prescale film, C. Itoh & Co., America, Inc., New York, N .Y .) with a “super low” range of 5 to 20 kg f/cm2 (70 to 280 psi) was cut into 3 x 10 inch strips and sterilized in ethylene oxide. Fig. 1 shows the pressure chart, indicating an increase in the film’s color density with an increase in contact pressure. Film samples were tested to control for possible effects of sterilization and moisture on the registered color density, which were not noticeably affected (although Et0 sterilization did seem to change the resulting color slightly). The film formally requires a contact time of 2 minutes, with the density of the film to be read within 9’2 to 5 hours. However, a test of the time-varying response of the film does
not show a noticeable difference. Similarly, the color density gradation in response to different pressures does not change noticeably overnight. For the eight surgical procedures studied (all involving the same surgeon), the film was applied to the abdominal region of the surgeon’s scrub suit, as well as the bare scrubbed left and right forearms, with the use of sterile Ioban 2 antimicrobial film (Medical-Surgical f)ivision, 3M Co., St. Paul, Minn.). Fig. 2 displays films recorded during thoracic surgery on which color density indicates pressures well in excess of 120 psi, although the appearance of dark ridges where the paper folds may re&ct stres7ses in the paper. With seemingly h&h registered pressures, an “in house” calibration was performed to compare the relative &f&s of static
Volume June
19 Number
3
Transmural
1991
surgical
gown pressure
measurements
149
Fig. 2. Pressure-sensitive contact film from forearms and abdominal region of surgeon taken during abdominal aneurismectomy. Color density indicates pressures in excess of 120 psi, although the appearance of dark ridges where the paper folds may reflect stresses in the paper.
versus shear forces (shown in Fig. 3). It is found that dragging the object over the prescale film tends to increase the registered pressure (compare coloration of the 35.2 psi shear test with the 35.2 psi static test, for example). Color density decreased (to slightly greater than 70 psi) after the surgeon was requested to minimize shear when contacting the patient (as evidenced in Fig. 4). These contact-sensitive film studies determined that physically demanding surgical procedures (e.g., Nissen fundoplication) encounter greater pressures between the surgeon and the patient than more delicate procedures (e.g., hernia repair). Also, the forearm is a site of somewhat greater pressure than the abdominal area; this is to be expected since the downward force of a surgeon’s arm more closely reflects body weight than does the horizontal force of the surgeon’s abdomen coming into contact with the patient. Measured pressures are similar to other
biomechanical contact film, walking.4 STRAIN
GAUGE
uses of the pressure-sensitive such as gait analysis during MEASUREMPNTS
For more formal documentation of peak local pressures encountered, a device that consisted of a 2 X 8 array of resistive strain gauges (0 to 30 psi range, Precision Measurement Co., Ann Arbor, Mich.) encased in a 2 x 6 x 0.25 inch mold of silicone elastomer, (Sylgard 186, Dow Corning Corp., Midland, Mich.) was constructed, as shown in Fig. 5. Individual resistive strain gauges have a linear output voltage response in contrast to piezoelectric devices, which have a temporal current “bleed-off” into the accompanying circuitry’ Each strain gauge channel is fed through model 2 120/2 110 Vishey amplification (Instrument Division, Raleigh, N.C.) at a gain of 8000, which avoids amplifier saturation while maintaining an adequate sig-
American
150 Statk
Altman et al.
INFECTION
lhrat 1‘
aik-‘--* c
-
:
35 p.s.i.
x . .
,” a. 82 p.s.i.
128 pai.
177 p.s.i.
sbffwrr ttblilt
17.5 p.s.i.
35 pa.i.
* _.
C-------.5
I
d
cm/stx-4
P@. 9. “In-house” calibration of prescale film shows tendency toward stress concentrations at edge of surface whose contact with film registers pressure (static test). Shear test: Dragging the object over the prescale film tends to increase the registered pressure. (Compare coloration of the 35.2 psi shear test with the 35.2 psi static test, for example.)
nal/noise ratio. A magnetic tape data recorder was used to capture the initial signals ( 17/8 ips), which were digitized (140 msec sample rate) and evaluated on an IBM personal computer with the use of a Computerscope (RC Electronics Inc., Santa Barbara, Calif.). The resistive strain gauge package was calibrated to determine the linear response of each channel for increasing loads (*calibration factors”): these factors are necessarily affected by encasement of the strain gauges in the silicone elastomer (Silastic) package. An “area effect” was also evaluated by application of the same
Journal
of
CONTROL
pressure to the Silastic surface above each gauge with differing contact areas; this exercise determines the specificity for a channel response dependent on the contact area of the applied force. The area effect is different from a “location effect,” where the response depends on forces concentrated at sites not directly above a particular strain gauge; no location effect was observed. Finally, amplifier imbalance at higher gains merely corresponds to a direct current offset in the output voltage of an individual channel. Although the strain gauges used were rated to 30 psi, there is a significant safety factor that allows for possible nonlinear output voltage at pressures greater than 30 psi but not great enough to cause (irreversible) plastic deformation of the transducer. The energydispersing property of the Silastic encasement” is also expected to increase the viable transducer range. Calibration of individual transducer channel strain gauges was performed to measure output voltage at pressures up to 120 psi. We found that first-order linear regression curves for this pressure range produced relatively low correlation coefficients (R’) of 0.799 to 0.948 and suggested possible nonlinear response to increasing loads. Fig. 6 shows the output voltage versus pressure for six transducer channels with a second-order polynomial curve fit, revealing that the nonlinear behavior is predominant at pressures greater than 100 psi. We then decided to rely on first-order linear regression calibration factors that include pressures only up to 58 psi, producing correlation coefficients that range from 0.949 to 0.990. The strain gauge device was placed within a sterile intestinal bag and pinned to the abdominal region of the scrub suit or fixed to the surgeon’s forearm by means of antimicrobial film. Motion artifact in the cabling and transducer pad was evident in only one channel under rigorous conditions at 171 mV, which corresponded to a maximum error of 10 psi. Fig. 7 shows six transducer channel output voltage traces recorded from contacts made with the surgeon’s forearm and either the patient or the operating table, taken during (a) a 73-minute segment of a resection and grafting of abdominal aortic aneurysm and (b) a 34-
Volume June
19 Number
3
Transmural
1991
surgical
gown pressure
measurements
151
Fig. 4. Pressure-sensitive contact film from forearms of surgeon during colostomy takedown, after surgeon was requested to minimize shear when contacting patient. Color density indicates slightly more than 70 psi
minute segment of a thyroidectomy. Additional procedures included hernia repairs, excisional biopsies, femoral/popliteal bypass graft surgery, a cholecystectomy, and a blood clot evacuation. Although peak pressures varied with the procedure, Fig. 7 is representative of the results in general: (1) output voltages that correspond to peak pressures well in excess of 60 psi (the linear range of the transducers calibrated in Fig. 6) have been repeatedly recorded, (2) low pressures measured during relatively inactive periods of surgery reflect pressures of less than 10 psi, and (3) peak pressures above 60 psi have been encountered, whether the transducer pad was on the surgeon’s left or right forearm or on his abdomen. DlSCUSSlON
Although apparently large, peak pressures are encountered
the measured for only brief
contact times. Fig. 7c and d indicate that peak pressure envelopes (described with brackets in Figs. 7a and 7c, respectively) actually contain multiple peak pressure “spikes” of shortduration activity, so that the total energy of each contact is reduced. Furthermore, typical peak pressures in excess of 60 psi are consistent with the ability of human skin to withstand such impacts. (McElhaney et al.’ describe several studies in which it is concluded that the elastic deformation compressive strength of skin is in excess of 300 psi.) To understand the consequence of the measurements reported in this article, we first discuss possible mechanisms that may account for bacterial transmission through a surgical gown’s pores. The transmissibility of microspheres and red blood cells through singlepore membranes has been investigated from a fluid mechanics perspective by Frank and
American
152
Altman et al.
INFECTION
Vishey 212Ot2110 Amplifiers
of
Personal Computer Computerscope
HP 3966A
Instrumentation
Journal CONTROL
Recorder
Fig. 5. Block diagram of pressure-measurement system. The transducer pad shown on the left is amplified; signals are stored on the instrumentation recorder and later digitized on a personal computer for analysis
2
50
100
El
Channel2
l
Channel3
m
Channel4
6
Channel5
n
Channel7
0
Channel8
150
p.s.1.
Fig. 6. Output voltage versus pressure for six transducer channels, with second-order polynomial curve fit. Note the nonlinear behavior at pressures greater than 100 psi.
Hochmuth*; although concerned with capillary blood flow, this study performs theoretical analysis and experimental verification of an idealized particle as well as viscoelastic cell penetration through a pore. For the idealized case of rigid spherical bacteria whose size is smaller than the pore, Stokes’ radius describes the force necessary for transmission, as derived from the drag coefficient of fluid flow over the surface of that sphere.’ Cells that are larger than the pore necessarily need to deform in order to
penetrate the idealized single-layer pore, as evaluated with the red cell.’ With either situation there is also the propensity for bacterial adhesion to the surgical gown material, increasing the force necessary for adequate penetration. This factor is probably more important with the streptococcal and staphylococcal bacteria that significantly cause the surgical u&ction, considering the complicated bacterial geometries and glycocalyx.‘O A theoretical description of bacterial penetration through a
Volume
19 Number
June 1991
3
Tram-mural
surgical
gown pressure
measurements
Fig. 7. a, Six transducer channel voltage traces displayed from 73-minute segment of a resection and grafting of an abdominal aortic aneurysm (pad on surgeon’s left forearm), b, Six transducer channel voltage traces displayed from 34-minute segment of thyroidectomy (pad on surgeon’s right forearm). c, Time-expanded view of bracketed section in Fig. 7a (3.7 minutes); d, time-expanded view of the bracketed section in c (17.5 seconds) (transducer pad on forearm).
153
Amewan
154
Altman et al.
INFECTKIN
Journal
of
CONTROL
b)
4
Positive Pressure
Differential
Sandwich
Pressure
F&j, 8. Schematic drawings of the positive pressure testing situation on a sheet of surgical gown material (a) and the “sandwich pressure” actually encountered across the gown when the surgeon contacts the patient (b).
barrier material is further complicated by the prospect that the pore itself may deform to accommodate the bacteria. Other factors such as relative moisture, hydrophilic/hydrophobic interactions, shape, and composition of the bacterial or viral particle of interest may also be important in the actual pass-through of microorganisms. The aforementioned description relies on the assumption that the environment suitable for bacteria transmission through the surgical gown pore is moist and that there is a pressure differential across the gown, providing the analogy to fluid mechanics. (This is also in accordance with the industry norm of setting up a positive pressure differential across the gown to test transmissibility.) However, a recent study of the barrier protection of surgical gowns found that the “soak-through point” of hydrostatic pressure is not an exclusive determinant of “viral penetration point.“” In other words, there may be particle penetration across the gown at pressures below the material’s soakthrough point. The more relevant (and inappropriate) assumption here is that a pressure differential is set up across the gown. Consider the idealized situation of an object placed on a table; although the object exerts a pressure on the table (object weight divided by its contact area), the table exerts a normal force on the object to
maintain equilibrium. In the more complicated (but analogous) situation of a surgeon coming into contact with a patient, both the surgeon’s force and an equal and opposite force are encountered by the gown. Fig. 8, a shows a schematic diagram of the positive pressure differential test (where forces applied across the gown are countered at the periphery of the material); conversely, Fig. 8, b demonstrates that contact “sandwich” pressures are countered by forces at the same location as the source. We measured contact pressures, rather than pressure differentiak; it is therefore imperative that a mechanism for bacterial transmission be determined before the consequences of pressures encountered across the gown under surgical conditions are evaluated. Pressure measurements reported in this article open the arena to further studies that would more precisely quantify peak pressures encountered between the surgeon and the patient. To delineate various contact surface shapes (such as the edge of the surgical table as opposed to the patient), a future study may take advantage of large arrays of conductive polymer pressure sensors’* that need to be multiplexed to counter the aforementioned current “bleed-off.” Transducers capable of accurately measuring pressures greater than 60 psi should also be used in accordance with our initial measurements. Statistical numbers of procedures,
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1991
surgeons, and their assistants may also be done to determine conditions that pose a greater risk of encountering high pressures. Transmural surgical gown pressure measurements encountered in the operating theater are only a first step in determining the conditions necessary for bacterial or viral particle penetration. This article also raises the concern that “positive pressure” testing of the surgical gown may not adequately duplicate the surgical theater environment and leads us to recommend experimental verification. This could be done by applying a pressure to the gowns by dropping weights from a fixed distance’ or by other means of applying forces that squeeze the fabric rather than force air through it. CONCLUSIONS
Measurements of the distributed pressures encountered between a surgeon and a patient in the operating theater were made as a first step in quantifying the conditions with which infecting bacteria may penetrate the surgical gowns. This study required the fabrication of a multisite-resistive strain gauge device. Although relatively high pressures were recorded (in excess of 60 psi from the forearm during thoracic surgery), it is uncertain whether favorable conditions exist for bacterial transmission (such as a warm, moist contact environment) .
Transmural surgical gown pressure measurements
I SS
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