JOURNAL
OF SURGICAL
21, 277-M (1976)
RESEARCH
Reliability
of Mass Tissue
STEPHEN
H. MILLER, GRAHAM,
Spectrometric
Techniques
for
Gas Analysis’
M.D.,’ ARTHUR CALABRETTA, M.D., WILLIAM III, M.D., AND THOMAS S. DAVIS, M.D.
P.
The Department of Surgery, The Division of Plastic Surgery, The Milton S. Hershey Medical Center of The Pennsylvania State University, Hershey, Pennsylvania 17033 Submitted for publication February 19, 1967
The Medspect system was designed to continuously measure respiratory, blood, or tissue gas levels in vivo [16]. In the tissue mode, gas is sampled via a Teflon membrane, 0.75 or 1 in. in length, incorporated into the terminal end of a catheter (Fig. 1). The opposite end of the catheter is connected to a vacuum system within the console of the Medspect. Small amounts of gases in proportion to their partial pressure in tissue diffuse into the membrane at a slow rate, 5 x lop6 ml/set, thus avoiding gas depletion at the sampling site. Quantitative analysis according to the molecular weight of the gas is performed by the mass spectrometer and measurements represent the average partial pressure of the gas present in the tissue along the length of the diffusion membrane. Teflon membranes are flow dependent and calibration adjustments are necessary to obtain accurate measurements of tissue gases in static interstitial fluid. Calibration drifts during 24 hr are said to be less than 4 percent [l]. As with all invasive monitoring devices, insertion in tissue may produce artifacts resulting in erroneous measurements [2]. Bleeding, which results in a hematoma forming about the catheter, causes a lower p0, reading than normal. Air contamination is a ‘Presented in part at the Surgical Forum of the American College of Surgeons, San Francisco, California, October 14, 1975. This study is supported in part by the Irvin Zubar Memorial Fund. ‘Address for reprints: Stephen H. Miller, M.D., Associate Professor of Surgery, M. S. Hershey Medical Center, Hershey, Pennsylvania 17033.
more common artifact and occurs when there is exposure of the Teflon membrane to air or when a subcutaneous air pocket is created during catheter insertion. It results in a higher PO, reading that is usual for normal tissue. Aside from minor adjustable calibration drifts and early temporary measuring errors, the system is thought to reliably analyze tissue gases during the acute monitoring conditions [l-3, 12, 131. To our knowledge, no studies have been reported to document the reliability of this system during chronic monitoring conditions. Prior to utilizing the Medspect for chronic experiments, we felt it necessary to determine whether the in vitro calibration drift exceeded the 4% previously reported for 24 hr, and if chronic tissue implantation of the catheters altered either calibration of the system or analysis of tissue gas tensions. METHOD Experiment 1. All catheters were calibrated on the day of insertion and following removal from the tissue using a known dry gas mixture, 5% CO, and 95% air, heated to 37°C. Oxygen readings were set 22% higher and carbon dioxide readings were set 1% higher than their true values in the known gas in order to calibrate them for accurate interstitial tissue readings. [I]. Male Sprague-Dawley rats weighing 500-600 g were anesthetized with intraperitoneal pentobarbital and the dorsal skin in the site of proposed catheter insertion
277 Copyright o 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
278
JOURNAL
OF SURGICAL
RESEARCH:
VOL. 21, NO. 4, OCTOBER
1976
FIG. I. The Teflon diffusion membrane 0.75 in. long and 0.055 in. in diameter. Note the solid Teflon core distal to the diffusion membrane to allow the catheter to act as its own obturator during tissue replacement.
was shaved and prepared with iodine and alcohol. A No. 14 angiocath was used to puncture the skin, the needle was withdrawn, and the angiocath was inserted for a distance of 1 in. The Teflon membrane was then inserted through, and 1 in. beyond, the tip of the plastic angiocath, into the subcutaneous tissue. Both were secured to the skin and each other with a pursestring suture and adhesive tape. Continuous PO, and pC0, recordings were performed for 1 hr after insertion, to be certain that no significant air contamination or hematoma had occurred. All animals were dressed with circumferential Kling gauze and a plaster cast. Two days after implantation of the catheters, 10 animals were anesthetized and a new catheter was placed on the opposite side of the back in a similar position with reference to the midline and head of the animal. Continuous and simultaneous recordings of ~0, and pC0, were begun from both catheters 30-40 min. after the acute
catheter had been inserted and were continued for 1 hr. The same procedure was performed on the remaining nine animals, 7 days after the original catheters had been inserted. Following gas analysis, tissue samples were removed from the region of the midportion of the Teflon diffusion membrane for histologic study. The plastic angiocath was used as a guide to prevent injury to the membrane and the latter was periodically removed and recalibrated. Data from Experiment I were subjected to analysis by the Student’s t test for statistical significance. Experiment II. Six pairs of unused catheters were calibrated in a heated (37”C), known gas mixture on Day 1 and then were recalibrated on Days 4, 7, 14, and 21. No compensation for tissue use was necessary as the catheters were to be recalibrated in a stream of gas. During these experiments, Oagain settings were kept constant, but the emission-control settings and CO,-gain set-
MILLER
ET AL.: RELIABILITY
OF MASS
tings were varied to achieve the correct p0, and pC0, of the gas mixture. No attempt was made to standardize the pN, readings. During recalibration, emission-control and COz-gain settings were recorded for each catheter. Relative CO, gain was then calculated according to the formula: Relative gain = 1 + (gain control setting/500), as suggested by the manufacturer [8]. Mean values obtained for the emission control, CO, dial setting, and relative CO, gain were then expressed as a percentage, either positive or negative, of the mean values obtained on Day 1. RESULTS Experiment 1. The mean p0, reading from catheters implanted in tissue for 2 days was 38.8 mm Hg. Simultaneously recorded p0, readings for acute catheters in the same animals average 47.3 mm Hg. These means were not significantly different. The mean ~0, value from catheters implanted in tissue for 7 days was 3 1.8 mm Hg, while simultaneously recorded values from the acutely placed catheters had a mean of 53 mm Hg. The difference between the two catheters in the 7-day group is significant with a P value of less than 0.05 (Table 1). Mean pC0, values recorded from chronic catheters on Days 2 and 7 were not significantly different than the mean pC0, values recorded simultaneously from the acutely placed catheters (Table 1). Histologic examination of the specimens removed from the animals who had catheters implanted for 2 days revealed very little inflammatory reaction and no evidence of hematoma (Figs. 2A and 2B). Specimens from the 7-day study group had a 2-4-cell
SPECTROMETRIC
279
TECHNIQUES
layer, 30-50 pm thick, fibrous reaction around each catheter (Figs. 3A and 3B). Catheters which had been implanted for 2 days averaged less than 3% drift in emissioncontrol settings, but averaged 11% drift in CO,-gai-, settings. The 7-day catheters averaged 4% drift in emission-control calibration, but averaged 26.5% in CO,-gain settings. PO,-gain settings remained constant throughout the experiment. Experiment II. Mean emission-control settings varied from the baseline mean as little as +2.5% on Day 21 to as much as -12.7% on Day 7. Mean CO,-gain settings varied from their baseline mean over an even larger range, but the calculated relative CO,gain variation was slightly less throughout the %I-day experiment. There was no consistent pattern of variation noted in any of the calibration settings, as compared to the original baseline means, and when compared to one another on any given day (Fig. 4). DISCUSSION Many techniques have been developed to measure tissue gas tension, including polarographic electrodes [3, 11, 141, tonometers [5, 10, 1l] subcutaneous gas pockets [15], and most recently, mass spectrometry [2, 91. Each has its disadvantages and all share the difficulties of maintaining calibration of the system and errors in measurement caused by tissue trauma and air contamination. Preliminary studies with the Medspect in our laboratory have confirmed that calibration drifts over 24 hr average less than 4%, and appreciable hemorrhage rarely accompanies insertion of the Teflon diffusion membrane. Air contamination to some degree almost always occurs
TABLE I Tissue Gas Tensions Recorded from Acute YSChronic Catheters on Days 2 and 7 PO*
PCO*
Day
Acute
Chronic
P*
Acute
Chronic
P*
2 7
47.3 f 1.75’ 53.0 f 3.21
38.8 z!z 1.67 31.8 zt 2.30
NS
OF SURGICAL
21, 277-M (1976)
RESEARCH
Reliability
of Mass Tissue
STEPHEN
H. MILLER, GRAHAM,
Spectrometric
Techniques
for
Gas Analysis’
M.D.,’ ARTHUR CALABRETTA, M.D., WILLIAM III, M.D., AND THOMAS S. DAVIS, M.D.
P.
The Department of Surgery, The Division of Plastic Surgery, The Milton S. Hershey Medical Center of The Pennsylvania State University, Hershey, Pennsylvania 17033 Submitted for publication February 19, 1967
The Medspect system was designed to continuously measure respiratory, blood, or tissue gas levels in vivo [16]. In the tissue mode, gas is sampled via a Teflon membrane, 0.75 or 1 in. in length, incorporated into the terminal end of a catheter (Fig. 1). The opposite end of the catheter is connected to a vacuum system within the console of the Medspect. Small amounts of gases in proportion to their partial pressure in tissue diffuse into the membrane at a slow rate, 5 x lop6 ml/set, thus avoiding gas depletion at the sampling site. Quantitative analysis according to the molecular weight of the gas is performed by the mass spectrometer and measurements represent the average partial pressure of the gas present in the tissue along the length of the diffusion membrane. Teflon membranes are flow dependent and calibration adjustments are necessary to obtain accurate measurements of tissue gases in static interstitial fluid. Calibration drifts during 24 hr are said to be less than 4 percent [l]. As with all invasive monitoring devices, insertion in tissue may produce artifacts resulting in erroneous measurements [2]. Bleeding, which results in a hematoma forming about the catheter, causes a lower p0, reading than normal. Air contamination is a ‘Presented in part at the Surgical Forum of the American College of Surgeons, San Francisco, California, October 14, 1975. This study is supported in part by the Irvin Zubar Memorial Fund. ‘Address for reprints: Stephen H. Miller, M.D., Associate Professor of Surgery, M. S. Hershey Medical Center, Hershey, Pennsylvania 17033.
more common artifact and occurs when there is exposure of the Teflon membrane to air or when a subcutaneous air pocket is created during catheter insertion. It results in a higher PO, reading that is usual for normal tissue. Aside from minor adjustable calibration drifts and early temporary measuring errors, the system is thought to reliably analyze tissue gases during the acute monitoring conditions [l-3, 12, 131. To our knowledge, no studies have been reported to document the reliability of this system during chronic monitoring conditions. Prior to utilizing the Medspect for chronic experiments, we felt it necessary to determine whether the in vitro calibration drift exceeded the 4% previously reported for 24 hr, and if chronic tissue implantation of the catheters altered either calibration of the system or analysis of tissue gas tensions. METHOD Experiment 1. All catheters were calibrated on the day of insertion and following removal from the tissue using a known dry gas mixture, 5% CO, and 95% air, heated to 37°C. Oxygen readings were set 22% higher and carbon dioxide readings were set 1% higher than their true values in the known gas in order to calibrate them for accurate interstitial tissue readings. [I]. Male Sprague-Dawley rats weighing 500-600 g were anesthetized with intraperitoneal pentobarbital and the dorsal skin in the site of proposed catheter insertion
277 Copyright o 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
278
JOURNAL
OF SURGICAL
RESEARCH:
VOL. 21, NO. 4, OCTOBER
1976
FIG. I. The Teflon diffusion membrane 0.75 in. long and 0.055 in. in diameter. Note the solid Teflon core distal to the diffusion membrane to allow the catheter to act as its own obturator during tissue replacement.
was shaved and prepared with iodine and alcohol. A No. 14 angiocath was used to puncture the skin, the needle was withdrawn, and the angiocath was inserted for a distance of 1 in. The Teflon membrane was then inserted through, and 1 in. beyond, the tip of the plastic angiocath, into the subcutaneous tissue. Both were secured to the skin and each other with a pursestring suture and adhesive tape. Continuous PO, and pC0, recordings were performed for 1 hr after insertion, to be certain that no significant air contamination or hematoma had occurred. All animals were dressed with circumferential Kling gauze and a plaster cast. Two days after implantation of the catheters, 10 animals were anesthetized and a new catheter was placed on the opposite side of the back in a similar position with reference to the midline and head of the animal. Continuous and simultaneous recordings of ~0, and pC0, were begun from both catheters 30-40 min. after the acute
catheter had been inserted and were continued for 1 hr. The same procedure was performed on the remaining nine animals, 7 days after the original catheters had been inserted. Following gas analysis, tissue samples were removed from the region of the midportion of the Teflon diffusion membrane for histologic study. The plastic angiocath was used as a guide to prevent injury to the membrane and the latter was periodically removed and recalibrated. Data from Experiment I were subjected to analysis by the Student’s t test for statistical significance. Experiment II. Six pairs of unused catheters were calibrated in a heated (37”C), known gas mixture on Day 1 and then were recalibrated on Days 4, 7, 14, and 21. No compensation for tissue use was necessary as the catheters were to be recalibrated in a stream of gas. During these experiments, Oagain settings were kept constant, but the emission-control settings and CO,-gain set-
MILLER
ET AL.: RELIABILITY
OF MASS
tings were varied to achieve the correct p0, and pC0, of the gas mixture. No attempt was made to standardize the pN, readings. During recalibration, emission-control and COz-gain settings were recorded for each catheter. Relative CO, gain was then calculated according to the formula: Relative gain = 1 + (gain control setting/500), as suggested by the manufacturer [8]. Mean values obtained for the emission control, CO, dial setting, and relative CO, gain were then expressed as a percentage, either positive or negative, of the mean values obtained on Day 1. RESULTS Experiment 1. The mean p0, reading from catheters implanted in tissue for 2 days was 38.8 mm Hg. Simultaneously recorded p0, readings for acute catheters in the same animals average 47.3 mm Hg. These means were not significantly different. The mean ~0, value from catheters implanted in tissue for 7 days was 3 1.8 mm Hg, while simultaneously recorded values from the acutely placed catheters had a mean of 53 mm Hg. The difference between the two catheters in the 7-day group is significant with a P value of less than 0.05 (Table 1). Mean pC0, values recorded from chronic catheters on Days 2 and 7 were not significantly different than the mean pC0, values recorded simultaneously from the acutely placed catheters (Table 1). Histologic examination of the specimens removed from the animals who had catheters implanted for 2 days revealed very little inflammatory reaction and no evidence of hematoma (Figs. 2A and 2B). Specimens from the 7-day study group had a 2-4-cell
SPECTROMETRIC
279
TECHNIQUES
layer, 30-50 pm thick, fibrous reaction around each catheter (Figs. 3A and 3B). Catheters which had been implanted for 2 days averaged less than 3% drift in emissioncontrol settings, but averaged 11% drift in CO,-gai-, settings. The 7-day catheters averaged 4% drift in emission-control calibration, but averaged 26.5% in CO,-gain settings. PO,-gain settings remained constant throughout the experiment. Experiment II. Mean emission-control settings varied from the baseline mean as little as +2.5% on Day 21 to as much as -12.7% on Day 7. Mean CO,-gain settings varied from their baseline mean over an even larger range, but the calculated relative CO,gain variation was slightly less throughout the %I-day experiment. There was no consistent pattern of variation noted in any of the calibration settings, as compared to the original baseline means, and when compared to one another on any given day (Fig. 4). DISCUSSION Many techniques have been developed to measure tissue gas tension, including polarographic electrodes [3, 11, 141, tonometers [5, 10, 1l] subcutaneous gas pockets [15], and most recently, mass spectrometry [2, 91. Each has its disadvantages and all share the difficulties of maintaining calibration of the system and errors in measurement caused by tissue trauma and air contamination. Preliminary studies with the Medspect in our laboratory have confirmed that calibration drifts over 24 hr average less than 4%, and appreciable hemorrhage rarely accompanies insertion of the Teflon diffusion membrane. Air contamination to some degree almost always occurs
TABLE I Tissue Gas Tensions Recorded from Acute YSChronic Catheters on Days 2 and 7 PO*
PCO*
Day
Acute
Chronic
P*
Acute
Chronic
P*
2 7
47.3 f 1.75’ 53.0 f 3.21
38.8 z!z 1.67 31.8 zt 2.30
NS
