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Use of silastic tube and capillary sampling technic in the measurement of tissue PO2 and PCO2
Use of Silastic Tube and Capillary Sampling Technic in the Measurement of Tissue PO, and PCO,
Jaakko Kivisaari, CM, Turku, Finland Juha Niinikoski, MD, Turku, Finland
The tensions of oxygen and carbon dioxide gases in human tissues can be measured by implanting a Silastic@ tube into the target organ. The tube is perfused with anoxic saline that equilibrates to the average PO2 and PC02 of the surrounding tissue medium because Silastic is highly permeable to respiratory gases [I]. The method has been applied for determinations in the subcutaneous tissue and healing bone [2,3], and has proved to be a useful tool for assessing the viability of skin flaps [4]. The values obtained with this method, however, do not. represent those of an intact tissue. The tissue to be measured is injured during implantation and thus the sequence of healing is reflected in the determinations [1,2]. In the present study, the method was modified so as to omit the external conducting apparatus used for perfusion. The tonometer tube is filled with anoxic sahne that equilibrates to the average PO2 and PCO:! of the surrounding tissue within two minutes. The equilibrated fluid is collected in an Astrup glass capillary tube that is then emptied into a cuvette containing either an oxygen or carbon dioxide electrode. Since both gas tensions of a tissue are obtained within ten minutes, considerable time is saved, and the clinical applicability of the method is increased.
____ From the Department of Surgery and the Department of Medical Chemistry, University of Turku, Turk& Finland. This work was supported in part by Contract No. DAJA37-72-C-1573 for the US Army through its European Research Office. Requests for reprints should be addressed to Dr Niinikoski. Department of Medical Chemistry, University of Turku. 20520 Turku 52. Finland.
Material and Methods The tonometer was made from a sterile Silastic tube (Atria1 Catheter-J, A 190, Extracorporeal Medical Specialties Inc, Mount Laurel, New Jersey) measuring 16 cm long and impregnated with silver (OD 1.35 mm; ID 1.10 mm). Five male volunteers were the experimental subjects. With the patients under local anesthesia with 1 per cent lidocaine, the tube was implanted in the lateral side of the upper part of the arm by means of a widebore needle so that 14 cm of the tube remained under the skin. The puncture wounds were sealed with Nobecutanm (Bofors, Nobel-Pharma, Sweden) dressing and each end. of the tube was fixed with a Steri-Strip@ (3M Company, St. Paul, Minnesota). The tensions of oxygen and carbon dioxide were measured by filling the Silastic tube with anoxic saline for two minutes. During this period an equilibration of 90 to 95 per cent was achieved in the gas tensions between saline and the tissue in contact with Silastic. After the two minute period the equilibrated Ruid was sampled into an Astrup glass capillary tube by filling the tonometer with another dose of anoxic saline from a 1 ml glass syringe. (Figure 1.) The volumes of the glass capillary and Silastic tubes were equal (90 ~1). The Astrup capillary tube was then inserted into a microsample injector (Radiometer, Copenhagen, Denmark) and the sample emptied into a thermostatted 70 ~1 cuvette containing either a Clark oxygen electrode or a Severinghaus carbon dioxide electrode. The electrodes were connected to a gas monitor (Radiometer, Type PHM 71, Copenhagen, Denmark). Zero adjustment of the oxygen electrode was obtained with gaseous nitrogen and the calibration took place with aerated saline (PO2 150 mm Hg) using the capillary sampling technic. The carbon dioxide electrode was calibrated with two moistened gas mixtures of carbon dioxide tensions of 26 and 57 mm Hg.
623
Kivisaari and Niinikoski
GLASS CAPILLARY TUBE
Figure 1. The experimental design for measurement of tissue gas tensions with an implanted Silastic tube and the capillary sampling technic. 002 mmHg
A
PLAIN SILASTIC
OL 1
0
L I
L
I
/
I
0
1
2
3
EQUILIBRATION
I
,
4
5
1
0
TIME (mm)
Figure 2. The relative eMciencies of various Silastic tubes used to measure fluid oxygen tension. One hundred per cent eHiciency is a PO2 level of 150 or 550 mm fig. Temperature is 37°C.
10
5
20
15
35
30
25
I
40 nl,”
Figure 4. POz decay curve illustrating the extraction of oxygen by the perfused Silastic tube from the surrounding fluid space. Flow rate is 0.17 ml per minute. For details, see Material and Methods.
mmHg 100 -
o Ag - SILASTIC 0 Ba - SILASTIC
80 -
60 -
6 z
5 E
O PO,
8
x PC02
il0
608
O
40 40-
w
20-
o”? 20 -
OL
OL I
0
I
I
I
I
,
1
2
3
4
5
EQUILIBRATION
TIME (m(n)
Figure 3. The relative efficiencies of various Silastic tubes for measurement of fluid carbon dioxide tension. One hundred per cent efticiency is a PO2 level of 57 mm Hg. Temperature is 37°C.
624
I
I
I
I
I
I
I
0
2
4
6
8
10
12
Days after implantation Figure 5. Tissue gas tensions in the arms of five volunleers as measured with an implanted Silastic tube and capillary sampling technic. Each value represents one determination.
The American Jmrn=t
n* a.---
Measurement
of Tissue PO2 and PC02
mmtlg Air started
Figure 6. Response of gas tensions in tissue to breathing of pure oxygen or a mixture of 96 per cent 02 and 4 per cent C02. Tonometry with Silastic tube one day after implantation. Each value represents one determination made by the capillary sampiing technic. Gas concentrations inside the mask were checked with a Radiometer gas monitor.
OL
1 0
The relative efficiencies of various Silastic tubes were tested in vitro by bathing the tubes in normal saline of known PO2 (150 and 550 mm Hg) and PC02 (57 mm Hg) at 37°C. Plain Silastic tubes (OD 1.15 mm; ID 0.95 mm; Dow Corning, Midland, Michigan) and tubes inpregnated with silver (OD 1.35 mm; ID 1.10 mm) or barium (OD 1.50 mm; ID 1.20 mm; Atria1 Catheter-J, B 190, Extracorporeal Medical Specialties, Mount Laurel, New Jersey) were tested. In both oxygen tensions, a 90 to 95 per cent efficiency was achieved in two minutes using silver and barium Silastic whereas plain Silastic with a thinner wall was less permeable to oxygen. (Figure 2.) When the COs permeability was tested, silver Silastic was the most permeable with an 85 to 90 per cent efficiency shown at two minutes. (Figure 3.) Barium Silastic was less efficient probably because of the chemical affinity of COs to barium and a greater wall thickness. Increase in the equilibration time added very little to the relative efficiencies and thus a two minute equilibration period and silver Silastic were chosen for the experiments. During each measurement several capillaries were filled for the assay of tissue POs and PCOs. In some experiments the response of tissue gas tensions to breathing of pure oxygen or to a mixture of 96 per cent 0s and 4 per cent COs was tested, and the capillary blood PO2 was assessed serially in samples taken from fingertips. Prior to the measurement of tissue gas tensions, the Silastic tube was thoroughly rinsed with anoxic saline to avoid contamination by air. If the samples were not measured immediately, the glass capillaries were sealed with wax and stored on crushed ice. Continuous perfusion of the tonometer with anoxic saline used previously [I-31 was omitted because of a marked escape of tissue oxygen in the efflux. To test the amount of tissue oxygen lost during continuous perfusion, the following experiment was performed. A silver Silastic tonometer (15.2 cm long; OD 1.35 mm) was fed into the lumen of a glass tube (15.2 cm long; ID 3.40 mm) that was filled with aerated saline. The Silastic tube was then perfused with anoxic saline at a constant flow rate and the
Volume 125, May 1973
20
40
60
60
100
mm
oxygen extraction of the tonometer from the surrounding saline was recorded by measuring the rate of fall in the efflux PO2 and converting this to milliliters of oxygen. (Figure 4.) An infusion pump (Model 1100, Harvard Apparatus Co, Inc, Millis, Massachusetts), a 2 ml glass syringe, and oxygen-impermeable nylon tubes (OD 1.34 mm; ID 1.10 mm; Portex, Portland Plastics Ltd, Hythe, Kent, England) were used for perfusion. The decay of POz was detected on a chart recorder (Servogor, Goerz Electra GmbH, Vienna, Austria) that was linked to the gas monitor. The different perfusion rates of 0.11 and 0.065 ml/min were selected on the basis of earlier studies [l-3]. Results
Immediately after implantation the oxygen tension (PO,) varied between 60 and 80 mm Hg (Figure 5), then gradually declined over the next seven days until a minimal value of 40 to 45 mm Hg was reached. Between days seven and eleven, the POa remained essentially unchanged. Tissue PC02 values were between 30 and 40 mm Hg throughout the period of observation. (Figure 5.) The high initial POn was not caused by airborne contamination during implantation, since the highest responses of tissue PO2 to breathing of pure oxygen were recorded in the early phase. (Figures 6 and 7.) The addition of c&bon dioxide to pure oxygen in the breathing gas almost doubled the respiratory frequency. It had no effect on the maximal response of tissue Poe, and the tissue PC02 showed no change during breathing of pure oxygen or increased COz tensions. (Figure 6.) Alterations in the capillary blood POs were clearly reflected in the tissue POz. (Figure 7.) Studies of the oxygen extracted by the perfused tonometer showed that the higher the external POs, the greater the oxygen consumption by the tonomet-
625
Kivisaari and Niinikoski
medium during continuous perfusion with anoxic saline. Niinikoski, Heughan, and Hunt [1-31 have shown that the foreign body reaction that occurs around the Silastic tube consists of only two to four cell layers at one week post implantation. On the basis of our results, the Silastic tubing perfused with anoxic saline measures and consumes oxygen from a distance of at least 1.0 mm. (Tables I and II.) This distance clearly exceeds the thickness of the reactive area around the tonometer and, therefore, most of the oxygen in the sample probably derives from normal subcutaneous tissue after the initial trauma phase and capillary bleeding are over. Reported “average” tissue PO2 values have varied with the technic used. Niinikoski and Hunt reported a mean of 20 to 25 mm Hg in the subcutaneous tissue of rabbits [I] and 40 mm Hg in the subcutis of human subjects [2]. In the skin flaps of pigs, the average tissue PO2 varied between 21 and 53 mm Hg and the PC02 varied between 41 and 53 mm Hg [4]. All these results were obtained with the use of Silastic tonometers. Waring and Pearce [5] recorded a mean PO2 of 53 mm Hg in subcutaneous gas pockets in infants. On the dther hand, investigators using semi-micro needle electrodes often give only the change in arbitrary units, not in absolute values. The tissue PC02 recorded in the present study varied between 30 and 40 mm Hg, which is the range of capillary blood PCOz. (Figure 5.) The finding is somewhat unexpected, but can be attributed to the relatively low metabolism of the subcutaneous tissue [4]. This explanation is consistent with the fact that no changes occurred in the tissue PC02 during one hour of exposure to an increased CO2 concentration or pure oxygen. (Figures 6 and 7.) Some of the advantages of the capillary sampling technic over continuous perfusion of the tonometer are as follows: (1) The shorter duration of the mea-
mmHg
Figure 7. Response of capillary blood PO2, tissue PO2, and PCOl to breathing of pure oxygen. Tonometry with Silastic tube nine days after implantation. Each value represents one determination by the capillary sampling technic. Oxygen concentration inside the mask was checked with a Radiometer gas monitor.
er. (Table I.) Also, the higher the perfusion rate, the greater the consumption per minute and the smaller the consumption per milliliter of perfusate. (Table II.) Comments
Our results are comparable to those of earlier studies in man [2] in which tissue gas tensions were measured by continuous perfusion .of the implanted Silastic t,onometer. The normal sequences of change in PO2 (Figure 5) are identical and the responses to changes in ambient PO2 are generally similar to the findings of the earlier method. However, in the present work, the range of normal PO2 values is slightly higher between days three and seven, at least in the five persons tested. This difference might be caused by the extraction by the tonometer of a relatively large amount of oxygen from the surrounding tissue
TABLE
I
Oxygen Consumption
of Silastic Tonometer
in Various Oxygen Tensions* --Oxygen Consumption (ml 02)
PO* in the Surrounding Fluid (mm Hg) 105 80 60 40
--Per Minute 22.4 14.0 10.9 7.5
X 1O-5 x 10-S x 10-5 x 10-5
Per Minute and 1 Cm of Tube Length 14.7 9.2 7.2 5.0
x x X x
IO-6 10-e 1O-6 10-e
Per Milliliter of Perfusate 20.3 12.7 9.9 6.8
X X x x
1O-4 1O-4 10-e 10-d
Per Milliliter of Perfusate and 1 Cm of Tube Length 13.4 8.4 6.5 4.5
x X X x
10-b 1O-s lo-> 10-3
* Continuous perfusion of Silastic tonometer with anoxic saline at 2O”C, flow 0.11 ml per minute. Silastic tube (15.2 cm long; OD 1.35 mm) fed into the lumen of a glass tube (15.2 cm long; ID 3.40 mm) filled with aerated saline. Extraction of oxygen followed from 120 to 30 mm Hg PO2. (See Figure 4.)
626
The American Journal of Surgery
Measurement
TABLE II
Effect of Perfusion Rate on Oxygen Consumption of Silastic Tonometer* Oxygen
Perfusion Rate (ml/mini
PO2 in the Surrounding Fluid (mm Hg)
Per Minute
80 a0
14.0 X 10-S 10.9 x 10-S
9.2 x 10-e 7.2 X 1O-6
perfusion
of Silastic
tonometer
-
Consumption(ml02)
Per Minute and 1 Cm of Tube Length
0.110 0.065 *Continuous
of Tissue PO2 and PC02
Per Milliliter of Perfusate
Per Milliliter of Perfusate and 1 Cm of Tube Length
12.7.X 1O-4 16.8 x 10-d
a.4 x lo-5 11.0 x 10-S
with anoxic saline at 2O”C, flow 0.11 or 0.065 ml per minute.
Silastic tube (15.2 cm long;
‘OD 1..35 mm) fed into the lumen of a glass tube (15.2 cm long; ID 3.40 mm) filled with aerated saline. Extraction 110t060mmHgP02.
surement increases the clinical applicability of the method. Samples for the assay of tissue gas tensions can be collected in four minutes and measured in five by the capillary sampling technic whereas production of the same data using continuous perfusion takes one and a half hours. (2) The external conducting apparatus used for perfusion is exposed to a possibility of leakage and occlusion. These possibilities are excluded from the capillary sampling technic. (3) Each laboratory ,that has Astrup machinery for measuring acid-base equilibrium and blood gases can use the present method with minimal additional investments. It can be concluded that one of the main problems in measuring tissue gases is the wide variation that normally occurs. At the site of a capillary the PO2 is of the order of 90 mm Hg while only a few tens of microns away, the PO2 level can be almost zero [6,71. As Myers, Cherry, and Milton [4] have stated, it is not clinically feasible to locate a sampling site microscopically. Therefore, a method that gives an average reading is needed. We believe that the present method can be developed further to gain requirements for an ideal clinical assay. Even the new and costly mass spectrometer [S] has the same main disadvantage as the Silastic tubing, that is, the oxygen sensor has to be implanted into the target organ which produces trauma and interferes with the supply of oxygen. Summary
A new method of measuring tissue gases using an implanted Silastic tube has been developed. The
Volume 125, May 1973
of oxygen followed
from
Silastic tonometer is filled with anoxic saline which equilibrates to the average POZ and PC02 of the surrounding tissue within a few minutes. The equilibrated fluid is collected in an Astrup glass capillary tube that is then emptied into a microcuvette containing either an oxygen or carbon dioxide electrode. Because of the chemical inertness of Silastic, inflammatory reaction around the tonometer is minimal. Acknowledgment: We would like to thank Dr Thomas K. Hunt, San Francisco, California, Dr M. Bert Myers, New Orleans, Louisiana, and Prdfessor Eino Kulonen, Turku, Finland, for their great help and constructive criticism during this work. Our thanks are also due Mrs Heidi Pakarinen for skillful technical assistance. References 1. Niinikoski J, Hunt TK: Measurement of wound oxygen with implanted Silastic tube. Surgery 71: 22, 1972. 2. Niinikoski J, Heughan C, Hunt TK: Oxygen tensions in human wounds. J Surg Res 12: 77,1972. 3. Niinikoski J, Hunt TK: Dxygen tensions in healing bone. Surg Gynec Obstet 134: 746, 1972. 4. Myers MB, Cherry G, Milton S: Tissue gas levels as an index of the adequacy of circulation: the relation between ischemia and the development of collateral circulation (delay phenomenon). Surgery 71: 15, 1972. 5. Waring WW. Pearce MB: Gas exchange ratios of neonatal subcutaneous tissue. Biochem C/in 4: 31, 1964. 6. Silver IA: The measurement of oxygen tension in healing tissue. Progr Resp Res 3: 124,1969. 7. Niinikoski J, Hunt TK, Dunphy JE: Oxygen supply in healing tissue. Amer J Sorg 123: 247, 1972. 6. Gardner TJ, Brantigan JW, Perna AM, Bender HW, Brawley RK, Gott VL: lntramyocardial gas tensions in the human heart during coronary artery-saphenous vein bypass. J
Jhorac Cardiovasc Sqrg 62: 644, 1971.
627
Jaakko Kivisaari, CM, Turku, Finland Juha Niinikoski, MD, Turku, Finland
The tensions of oxygen and carbon dioxide gases in human tissues can be measured by implanting a Silastic@ tube into the target organ. The tube is perfused with anoxic saline that equilibrates to the average PO2 and PC02 of the surrounding tissue medium because Silastic is highly permeable to respiratory gases [I]. The method has been applied for determinations in the subcutaneous tissue and healing bone [2,3], and has proved to be a useful tool for assessing the viability of skin flaps [4]. The values obtained with this method, however, do not. represent those of an intact tissue. The tissue to be measured is injured during implantation and thus the sequence of healing is reflected in the determinations [1,2]. In the present study, the method was modified so as to omit the external conducting apparatus used for perfusion. The tonometer tube is filled with anoxic sahne that equilibrates to the average PO2 and PCO:! of the surrounding tissue within two minutes. The equilibrated fluid is collected in an Astrup glass capillary tube that is then emptied into a cuvette containing either an oxygen or carbon dioxide electrode. Since both gas tensions of a tissue are obtained within ten minutes, considerable time is saved, and the clinical applicability of the method is increased.
____ From the Department of Surgery and the Department of Medical Chemistry, University of Turku, Turk& Finland. This work was supported in part by Contract No. DAJA37-72-C-1573 for the US Army through its European Research Office. Requests for reprints should be addressed to Dr Niinikoski. Department of Medical Chemistry, University of Turku. 20520 Turku 52. Finland.
Material and Methods The tonometer was made from a sterile Silastic tube (Atria1 Catheter-J, A 190, Extracorporeal Medical Specialties Inc, Mount Laurel, New Jersey) measuring 16 cm long and impregnated with silver (OD 1.35 mm; ID 1.10 mm). Five male volunteers were the experimental subjects. With the patients under local anesthesia with 1 per cent lidocaine, the tube was implanted in the lateral side of the upper part of the arm by means of a widebore needle so that 14 cm of the tube remained under the skin. The puncture wounds were sealed with Nobecutanm (Bofors, Nobel-Pharma, Sweden) dressing and each end. of the tube was fixed with a Steri-Strip@ (3M Company, St. Paul, Minnesota). The tensions of oxygen and carbon dioxide were measured by filling the Silastic tube with anoxic saline for two minutes. During this period an equilibration of 90 to 95 per cent was achieved in the gas tensions between saline and the tissue in contact with Silastic. After the two minute period the equilibrated Ruid was sampled into an Astrup glass capillary tube by filling the tonometer with another dose of anoxic saline from a 1 ml glass syringe. (Figure 1.) The volumes of the glass capillary and Silastic tubes were equal (90 ~1). The Astrup capillary tube was then inserted into a microsample injector (Radiometer, Copenhagen, Denmark) and the sample emptied into a thermostatted 70 ~1 cuvette containing either a Clark oxygen electrode or a Severinghaus carbon dioxide electrode. The electrodes were connected to a gas monitor (Radiometer, Type PHM 71, Copenhagen, Denmark). Zero adjustment of the oxygen electrode was obtained with gaseous nitrogen and the calibration took place with aerated saline (PO2 150 mm Hg) using the capillary sampling technic. The carbon dioxide electrode was calibrated with two moistened gas mixtures of carbon dioxide tensions of 26 and 57 mm Hg.
623
Kivisaari and Niinikoski
GLASS CAPILLARY TUBE
Figure 1. The experimental design for measurement of tissue gas tensions with an implanted Silastic tube and the capillary sampling technic. 002 mmHg
A
PLAIN SILASTIC
OL 1
0
L I
L
I
/
I
0
1
2
3
EQUILIBRATION
I
,
4
5
1
0
TIME (mm)
Figure 2. The relative eMciencies of various Silastic tubes used to measure fluid oxygen tension. One hundred per cent eHiciency is a PO2 level of 150 or 550 mm fig. Temperature is 37°C.
10
5
20
15
35
30
25
I
40 nl,”
Figure 4. POz decay curve illustrating the extraction of oxygen by the perfused Silastic tube from the surrounding fluid space. Flow rate is 0.17 ml per minute. For details, see Material and Methods.
mmHg 100 -
o Ag - SILASTIC 0 Ba - SILASTIC
80 -
60 -
6 z
5 E
O PO,
8
x PC02
il0
608
O
40 40-
w
20-
o”? 20 -
OL
OL I
0
I
I
I
I
,
1
2
3
4
5
EQUILIBRATION
TIME (m(n)
Figure 3. The relative efficiencies of various Silastic tubes for measurement of fluid carbon dioxide tension. One hundred per cent efticiency is a PO2 level of 57 mm Hg. Temperature is 37°C.
624
I
I
I
I
I
I
I
0
2
4
6
8
10
12
Days after implantation Figure 5. Tissue gas tensions in the arms of five volunleers as measured with an implanted Silastic tube and capillary sampling technic. Each value represents one determination.
The American Jmrn=t
n* a.---
Measurement
of Tissue PO2 and PC02
mmtlg Air started
Figure 6. Response of gas tensions in tissue to breathing of pure oxygen or a mixture of 96 per cent 02 and 4 per cent C02. Tonometry with Silastic tube one day after implantation. Each value represents one determination made by the capillary sampiing technic. Gas concentrations inside the mask were checked with a Radiometer gas monitor.
OL
1 0
The relative efficiencies of various Silastic tubes were tested in vitro by bathing the tubes in normal saline of known PO2 (150 and 550 mm Hg) and PC02 (57 mm Hg) at 37°C. Plain Silastic tubes (OD 1.15 mm; ID 0.95 mm; Dow Corning, Midland, Michigan) and tubes inpregnated with silver (OD 1.35 mm; ID 1.10 mm) or barium (OD 1.50 mm; ID 1.20 mm; Atria1 Catheter-J, B 190, Extracorporeal Medical Specialties, Mount Laurel, New Jersey) were tested. In both oxygen tensions, a 90 to 95 per cent efficiency was achieved in two minutes using silver and barium Silastic whereas plain Silastic with a thinner wall was less permeable to oxygen. (Figure 2.) When the COs permeability was tested, silver Silastic was the most permeable with an 85 to 90 per cent efficiency shown at two minutes. (Figure 3.) Barium Silastic was less efficient probably because of the chemical affinity of COs to barium and a greater wall thickness. Increase in the equilibration time added very little to the relative efficiencies and thus a two minute equilibration period and silver Silastic were chosen for the experiments. During each measurement several capillaries were filled for the assay of tissue POs and PCOs. In some experiments the response of tissue gas tensions to breathing of pure oxygen or to a mixture of 96 per cent 0s and 4 per cent COs was tested, and the capillary blood PO2 was assessed serially in samples taken from fingertips. Prior to the measurement of tissue gas tensions, the Silastic tube was thoroughly rinsed with anoxic saline to avoid contamination by air. If the samples were not measured immediately, the glass capillaries were sealed with wax and stored on crushed ice. Continuous perfusion of the tonometer with anoxic saline used previously [I-31 was omitted because of a marked escape of tissue oxygen in the efflux. To test the amount of tissue oxygen lost during continuous perfusion, the following experiment was performed. A silver Silastic tonometer (15.2 cm long; OD 1.35 mm) was fed into the lumen of a glass tube (15.2 cm long; ID 3.40 mm) that was filled with aerated saline. The Silastic tube was then perfused with anoxic saline at a constant flow rate and the
Volume 125, May 1973
20
40
60
60
100
mm
oxygen extraction of the tonometer from the surrounding saline was recorded by measuring the rate of fall in the efflux PO2 and converting this to milliliters of oxygen. (Figure 4.) An infusion pump (Model 1100, Harvard Apparatus Co, Inc, Millis, Massachusetts), a 2 ml glass syringe, and oxygen-impermeable nylon tubes (OD 1.34 mm; ID 1.10 mm; Portex, Portland Plastics Ltd, Hythe, Kent, England) were used for perfusion. The decay of POz was detected on a chart recorder (Servogor, Goerz Electra GmbH, Vienna, Austria) that was linked to the gas monitor. The different perfusion rates of 0.11 and 0.065 ml/min were selected on the basis of earlier studies [l-3]. Results
Immediately after implantation the oxygen tension (PO,) varied between 60 and 80 mm Hg (Figure 5), then gradually declined over the next seven days until a minimal value of 40 to 45 mm Hg was reached. Between days seven and eleven, the POa remained essentially unchanged. Tissue PC02 values were between 30 and 40 mm Hg throughout the period of observation. (Figure 5.) The high initial POn was not caused by airborne contamination during implantation, since the highest responses of tissue PO2 to breathing of pure oxygen were recorded in the early phase. (Figures 6 and 7.) The addition of c&bon dioxide to pure oxygen in the breathing gas almost doubled the respiratory frequency. It had no effect on the maximal response of tissue Poe, and the tissue PC02 showed no change during breathing of pure oxygen or increased COz tensions. (Figure 6.) Alterations in the capillary blood POs were clearly reflected in the tissue POz. (Figure 7.) Studies of the oxygen extracted by the perfused tonometer showed that the higher the external POs, the greater the oxygen consumption by the tonomet-
625
Kivisaari and Niinikoski
medium during continuous perfusion with anoxic saline. Niinikoski, Heughan, and Hunt [1-31 have shown that the foreign body reaction that occurs around the Silastic tube consists of only two to four cell layers at one week post implantation. On the basis of our results, the Silastic tubing perfused with anoxic saline measures and consumes oxygen from a distance of at least 1.0 mm. (Tables I and II.) This distance clearly exceeds the thickness of the reactive area around the tonometer and, therefore, most of the oxygen in the sample probably derives from normal subcutaneous tissue after the initial trauma phase and capillary bleeding are over. Reported “average” tissue PO2 values have varied with the technic used. Niinikoski and Hunt reported a mean of 20 to 25 mm Hg in the subcutaneous tissue of rabbits [I] and 40 mm Hg in the subcutis of human subjects [2]. In the skin flaps of pigs, the average tissue PO2 varied between 21 and 53 mm Hg and the PC02 varied between 41 and 53 mm Hg [4]. All these results were obtained with the use of Silastic tonometers. Waring and Pearce [5] recorded a mean PO2 of 53 mm Hg in subcutaneous gas pockets in infants. On the dther hand, investigators using semi-micro needle electrodes often give only the change in arbitrary units, not in absolute values. The tissue PC02 recorded in the present study varied between 30 and 40 mm Hg, which is the range of capillary blood PCOz. (Figure 5.) The finding is somewhat unexpected, but can be attributed to the relatively low metabolism of the subcutaneous tissue [4]. This explanation is consistent with the fact that no changes occurred in the tissue PC02 during one hour of exposure to an increased CO2 concentration or pure oxygen. (Figures 6 and 7.) Some of the advantages of the capillary sampling technic over continuous perfusion of the tonometer are as follows: (1) The shorter duration of the mea-
mmHg
Figure 7. Response of capillary blood PO2, tissue PO2, and PCOl to breathing of pure oxygen. Tonometry with Silastic tube nine days after implantation. Each value represents one determination by the capillary sampling technic. Oxygen concentration inside the mask was checked with a Radiometer gas monitor.
er. (Table I.) Also, the higher the perfusion rate, the greater the consumption per minute and the smaller the consumption per milliliter of perfusate. (Table II.) Comments
Our results are comparable to those of earlier studies in man [2] in which tissue gas tensions were measured by continuous perfusion .of the implanted Silastic t,onometer. The normal sequences of change in PO2 (Figure 5) are identical and the responses to changes in ambient PO2 are generally similar to the findings of the earlier method. However, in the present work, the range of normal PO2 values is slightly higher between days three and seven, at least in the five persons tested. This difference might be caused by the extraction by the tonometer of a relatively large amount of oxygen from the surrounding tissue
TABLE
I
Oxygen Consumption
of Silastic Tonometer
in Various Oxygen Tensions* --Oxygen Consumption (ml 02)
PO* in the Surrounding Fluid (mm Hg) 105 80 60 40
--Per Minute 22.4 14.0 10.9 7.5
X 1O-5 x 10-S x 10-5 x 10-5
Per Minute and 1 Cm of Tube Length 14.7 9.2 7.2 5.0
x x X x
IO-6 10-e 1O-6 10-e
Per Milliliter of Perfusate 20.3 12.7 9.9 6.8
X X x x
1O-4 1O-4 10-e 10-d
Per Milliliter of Perfusate and 1 Cm of Tube Length 13.4 8.4 6.5 4.5
x X X x
10-b 1O-s lo-> 10-3
* Continuous perfusion of Silastic tonometer with anoxic saline at 2O”C, flow 0.11 ml per minute. Silastic tube (15.2 cm long; OD 1.35 mm) fed into the lumen of a glass tube (15.2 cm long; ID 3.40 mm) filled with aerated saline. Extraction of oxygen followed from 120 to 30 mm Hg PO2. (See Figure 4.)
626
The American Journal of Surgery
Measurement
TABLE II
Effect of Perfusion Rate on Oxygen Consumption of Silastic Tonometer* Oxygen
Perfusion Rate (ml/mini
PO2 in the Surrounding Fluid (mm Hg)
Per Minute
80 a0
14.0 X 10-S 10.9 x 10-S
9.2 x 10-e 7.2 X 1O-6
perfusion
of Silastic
tonometer
-
Consumption(ml02)
Per Minute and 1 Cm of Tube Length
0.110 0.065 *Continuous
of Tissue PO2 and PC02
Per Milliliter of Perfusate
Per Milliliter of Perfusate and 1 Cm of Tube Length
12.7.X 1O-4 16.8 x 10-d
a.4 x lo-5 11.0 x 10-S
with anoxic saline at 2O”C, flow 0.11 or 0.065 ml per minute.
Silastic tube (15.2 cm long;
‘OD 1..35 mm) fed into the lumen of a glass tube (15.2 cm long; ID 3.40 mm) filled with aerated saline. Extraction 110t060mmHgP02.
surement increases the clinical applicability of the method. Samples for the assay of tissue gas tensions can be collected in four minutes and measured in five by the capillary sampling technic whereas production of the same data using continuous perfusion takes one and a half hours. (2) The external conducting apparatus used for perfusion is exposed to a possibility of leakage and occlusion. These possibilities are excluded from the capillary sampling technic. (3) Each laboratory ,that has Astrup machinery for measuring acid-base equilibrium and blood gases can use the present method with minimal additional investments. It can be concluded that one of the main problems in measuring tissue gases is the wide variation that normally occurs. At the site of a capillary the PO2 is of the order of 90 mm Hg while only a few tens of microns away, the PO2 level can be almost zero [6,71. As Myers, Cherry, and Milton [4] have stated, it is not clinically feasible to locate a sampling site microscopically. Therefore, a method that gives an average reading is needed. We believe that the present method can be developed further to gain requirements for an ideal clinical assay. Even the new and costly mass spectrometer [S] has the same main disadvantage as the Silastic tubing, that is, the oxygen sensor has to be implanted into the target organ which produces trauma and interferes with the supply of oxygen. Summary
A new method of measuring tissue gases using an implanted Silastic tube has been developed. The
Volume 125, May 1973
of oxygen followed
from
Silastic tonometer is filled with anoxic saline which equilibrates to the average POZ and PC02 of the surrounding tissue within a few minutes. The equilibrated fluid is collected in an Astrup glass capillary tube that is then emptied into a microcuvette containing either an oxygen or carbon dioxide electrode. Because of the chemical inertness of Silastic, inflammatory reaction around the tonometer is minimal. Acknowledgment: We would like to thank Dr Thomas K. Hunt, San Francisco, California, Dr M. Bert Myers, New Orleans, Louisiana, and Prdfessor Eino Kulonen, Turku, Finland, for their great help and constructive criticism during this work. Our thanks are also due Mrs Heidi Pakarinen for skillful technical assistance. References 1. Niinikoski J, Hunt TK: Measurement of wound oxygen with implanted Silastic tube. Surgery 71: 22, 1972. 2. Niinikoski J, Heughan C, Hunt TK: Oxygen tensions in human wounds. J Surg Res 12: 77,1972. 3. Niinikoski J, Hunt TK: Dxygen tensions in healing bone. Surg Gynec Obstet 134: 746, 1972. 4. Myers MB, Cherry G, Milton S: Tissue gas levels as an index of the adequacy of circulation: the relation between ischemia and the development of collateral circulation (delay phenomenon). Surgery 71: 15, 1972. 5. Waring WW. Pearce MB: Gas exchange ratios of neonatal subcutaneous tissue. Biochem C/in 4: 31, 1964. 6. Silver IA: The measurement of oxygen tension in healing tissue. Progr Resp Res 3: 124,1969. 7. Niinikoski J, Hunt TK, Dunphy JE: Oxygen supply in healing tissue. Amer J Sorg 123: 247, 1972. 6. Gardner TJ, Brantigan JW, Perna AM, Bender HW, Brawley RK, Gott VL: lntramyocardial gas tensions in the human heart during coronary artery-saphenous vein bypass. J
Jhorac Cardiovasc Sqrg 62: 644, 1971.
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