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Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas 75235
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Wu, Eugene Y., Khalid W. Barazanji, and Robert L. Johnson,
Jr. Sources of error in A-aDO2
calculated from blood stored in plastic and glass syringes.
J. Appl. Physiol. 82(1):
196-202, 1997.
We studied the effects of time delay on blood
gases, pH, and base excess in blood stored in glass and plastic
syringes on ice and the effects of resulting errors on calculated
alveolar-to-arterial PO2 difference
(A-aDO2).
Matched samples of dog whole blood were tonometered with gas
mixtures of 5% CO2-12%
O2-83% N2 (mixture
A), 10% CO2-5%
O2-85%
N2 (mixture
B), and 2.88%
CO2-4% O2-93.12%
N2 (mixture
C). Tonometered blood samples were transferred to
5-ml glass (5G), 5-ml plastic (5P), and 3-ml plastic (3P) syringes and
stored on ice. Blood gases were measured every 1 h up to 6 h. In 5G,
PO2 progressively decreased in blood
tonometered with mixture A but rose in
blood tonometered with mixtures B and C.
O2 saturation progressively fell
in all cases. In 5G, blood PCO2
progressively rose regardless of which gas mixture was used, and pH as
well as base excess progressively fell. The rise in
PO2 was faster in plastic than in
glass syringes, and O2 saturation
always rose in plastic syringes. Differences between storage in plastic
and glass syringes on PO2 change were
greatest when initial blood PO2 was
highest (mixture A). At the highest
PO2,
O2 exchange was faster in 3P than
in 5P. The rise of PCO2 was just as
fast in plastic as in glass syringes, but in both the rise in
PCO2 was faster at a higher initial
PCO2 (mixture
B) than at lower initial
PCO2 (mixtures
B and C). Rates of
PO2 and
PCO2 change in matched samples were
significantly faster in 3P than in 5P. Errors due to rises in
PCO2 and
PO2 cause additive errors in
calculated
A-aDO2,
and when blood is stored in plastic syringes for >1 h significant errors result. Errors are greater in normoxic blood, in which estimated
A-aDO2
decreased by >10 Torr after 6 h on ice in plastic syringes, than in
hypoxic blood.
pH; base excess; glass syringe; plastic syringe; alveolar-to-arterial O2 pressure
difference
INTRODUCTION DIFFUSIVE GAS EXCHANGE between air and blood stored in
glass syringes is negligible even over 24 h (3); however, there is a
progressive fall of PO2 and rise of
PCO2 due to metabolism by leukocytes
and erythrocytes (3, 5). Reduction of the temperature of storage to
1°C reduces metabolic rate to ~10% of that at 37°C (3);
hence, storage on ice between collection and measurement has been
standard in most clinical and research laboratories. Plastic syringes
have progressively supplanted glass for measuring arterial blood gases
for both clinical and research use because of convenience, low cost,
and resistance to breakage. However, plastic syringes are significantly
more permeable to both O2 and
CO2 than glass syringes (11, 13). Thus in plastic syringes changes in blood-gas tensions during storage
must reflect a balance between metabolic rate in the blood and gas
exchange with the atmosphere. Most studies of gas exchange between
blood and air in plastic syringes have measured blood PO2 or O2 content, and results from
different laboratories are consistent: when
PO2 in stored blood is higher than
ambient PO2, blood
PO2 falls with time; and when
PO2 in stored blood is lower than
ambient PO2, blood
PO2 rises, but the rate of rise is
buffered by the concentration of reduced hemoglobin that is available
(10, 13). Effects of blood storage in plastic syringes have been
measured for both PO2 and
PCO2, but results have been variable (4, 7, 8, 15); furthermore, combined effects of errors in
PO2 and
PCO2 on estimates of
alveolar-to-arterial PO2 difference
(A-aDO2)
have never been considered. Some of the sources of variability in the
effects of storage on PO2 and
PCO2 are
1) size of the syringe (i.e.,
surface-to-volume ratio) and wall thickness,
2) initial blood-gas tensions,
3) shapes of the
O2 and
CO2 dissociation curves, and
4) effects of the Haldane and Bohr
shifts as blood-gas tensions change. Thus our objectives in the present
study were to investigate 1) changes
of PO2, PCO2, and pH in glass and plastic
syringes with respect to time of storage on ice;
2) effects of initial blood-gas
tensions; 3) effects of syringe
size; and 4) potential effects of
the combined errors in PO2 and
PCO2 on calculating
A-aDO2.
MATERIALS AND METHODS Three types of syringes were tested: 3-ml plastic syringes
(n = 5; mean syringe ID of 8.5 mm and
wall thickness of 0.86 mm), 5-ml plastic syringes
(n = 5; mean syringe ID of 11.8 mm and
wall thickness of 0.89 mm), and 5-ml glass syringes
(n = 5) (Becton Dickinson, Rutherford,
NJ). Two different sizes of plastic syringes were used to determine
whether the lower surface-to-volume ratio of the larger syringe reduces
error. The relative permeability of the 3-ml plastic syringes to the
5-ml plastic syringes based on diameters and wall thickness is 1.44 [(11.8/8.5) × (0.89/0.86)].
Five dogs were used as blood source, and each dog was used once a week
for 3 wk. Each type of syringe was randomly chosen to be tested during
1 of the 3 wk. Each dog was ran- domly assigned to a certain day of the
week and was brought into the laboratory accordingly for blood
collection. A 12-ml plastic syringe was heparinized by filling the
syringe dead space with heparin solution (heparin sodium injection,
United States Pharmacopeia, 1,000 units/ml, Elkins-Sinn, Cherry Hill,
NJ), and blood was collected from external jugular veins of the dog
with the heparinized 12-ml plastic syringe. The 12-ml volume of blood
was then transferred to a chemistry laboratory where it was divided
into three 4-ml portions, and each 4-ml portion was randomly chosen to
be tonometered with one of the three standard calibration gas mixtures
in an IL-237 tonometer (Instrumentation Laboratory,
Lexington, MA). The standard calibration gas mixtures were 12%
O2-5%
CO2-83%
N2 for mixture
A, 5% O2-10% CO2-85%
N2 for mixture
B, and 4%
O2-2.88%
CO2-93.12%
N2 for mixture C.
Tonometered blood samples were transferred into test syringes, which
were appropriately capped and stored on ice in an insulated ice bucket.
An ABL3 acid-base laboratory (Radiometer Copenhagen, Copenhagen,
Denmark) was used for blood-gas analysis. Calibrations of the ABL3
acid-base laboratory were programmed to perform automatically every 2 h
for one-point calibrations and every 4 h for two-point calibrations.
The ABL3 acid-base laboratory was also checked for accuracy by the
American Thoracic Society Instrumentation Laboratory (ATS Proficiency
Program) every 3 mo. Syringes with blood sample were
rolled between palms for ~1 min before each analysis to ensure a
proper mixing before injection, and good mixing was confirmed by
reproducibility of hemoglobin concentrations with successive measurements. Care was always taken to avoid introducing air bubbles into the syringe during measurement (2). Baseline data were acquired by
immediate analysis of blood samples after tonometry. PO2,
PCO2, and pH were measured, from
which O2 saturation
(SO2) and
base excess (SBE) were calculated.
SO2 values
were calculated with modified Kelman subroutine (6, 12), and SBE values
were calculated with the equations used by the ABL3 acid-base
laboratory. Measurements were repeated on the same blood sample every
hour for up to 6 h.
Data were grouped by syringe type and gas mixture used for tonometry.
Means and variances of the pooled data of each hourly change from the
baseline value of PO2,
PCO2, SO2, pH,
and SBE ( RESULTS
Table 1.
Slopes of
PO2,
PCO2,
SO2, pH, and SBE) were calculated. Comparisons were made among the syringe types and among the three different initial paired values of
PO2 and
PCO2. Statistical significance of changes in blood gases with time and of differences among groups was
tested by repeated-measures analysis of variance (ANOVA) by using
StatView version 4.01 (Abacus Concept). Linear regression equations
were derived by the least squares deviations with the same computer
program. Significance between any two of the three syringe types and
between any two of the three groups with different initial
PO2 and
PCO2 values was tested with post hoc
tests (multiple comparison) by using Fisher's protected least
significant differences for the between-factors analysis. Slopes of the
regression lines of the grouped data of
PO2, PCO2,
SO2,
pH, and SBE vs. time were tested for significance among syringe
types as well as among different initial PO2 and
PCO2 by the standard method of
comparing simple linear regression equations (16).
P < 0.05 was considered statistically significant. Both statistical methods (ANOVA and slopes
comparison) yielded the same results. Rate of change in PCO2 and
SO2 of
matched blood samples tonometered with the same gas mixtures was
compared between 5- and 3-ml plastic syringes by using a nonparametric
sign test (16).
PO2
PO2 increased in both 5- and
3-ml plastic syringes whereas it decreased in 5-ml glass syringes
(P < 0.01). Also, the rate of
increase in PO2 was significantly
higher in the 3-ml than in the 5-ml plastic syringes
(P < 0.05). In blood samples
tonometered with gas mixtures B and
C (Table 1
and Fig. 1, B and
C),
PO2 increased with time in all
blood samples regardless of syringe types. However, the rate of
PO2 increase was significantly
lower in the 5-ml glass syringes than in either type of plastic
syringes (P < 0.01).
Fig. 1.
Hourly change in mean blood PO2
(PO2) with time at different
initial blood O2 and
CO2 concentrations in syringes.
Data are means ± SE. A:
blood tonometered with 5% CO2-12%
O2-83%
N2.
B: blood tonometered with 10%
CO2-5%
O2-85% N2.
C: blood tonometered with 2.88%
CO2-4%
O2-93.12%
N2.
[View Larger Version of this Image (11K GIF file)]
PO2,
PCO2,
SO2, pH, and
SBE in blood samples tonometered with three gas
mixtures and stored in three types of syringes
Syringe Type
Gas Mixture
Slope
PO2 Slope
PCO2 Slope
SO2 Slope
pH Slope
SBE
5G
A
0.251 0.563
0.079
0.005
0.002
5P
A
1.177a
0.507
0.038a
0.005
0.036
3P
A
1.499a,c
0.422c
0.131a,c
0.004
0.066
5G
B
0.161d
0.805d
0.045
0.005
0.075
5P
B
0.314a,d
0.974d
0.260a
0.005
0.067
3P
B
0.332a,d
0.644c,d
0.320a,c
0.004
0.114
5G
C
0.198d
0.401e
0.084
0.007d,e
0.101
5P
C
0.360a,d
0.397e
0.202
0.008
0.108
3P
C
0.335a,d
0.350c,e
0.391b,c
0.006d,e
0.085
Gas mixtures: A = 12% O2-5%
CO2-83% N2; B = 5%
O2-10% CO2-85% N2 ; C = 4% O2-2.88% CO2-93.12% N2.
, change in; PO2, O2 pressure; PCO2, CO2 tension;
SO2, O2 saturation; SBE, base
excess; 5G, 5-ml glass syringe; 5P, 5-ml plastic syringe; 3P, 3-ml
plastic syringe. Significantly different compared with glass syringes
of same gas mixture at:
a
P < 0.01;
b
P < 0.05.
c
Significantly
different compared with 5P for same gas mixture, P < 0.05.
d
Significantly different compared with gas mixture
A, P < 0.01.
e
Significantly different
compared with gas mixture B, P < 0.01.
PCO2
PCO2 increased with time in
samples stored in both glass and plastic syringes regardless of the gas
mixture (Table 1 and Fig. 2). The rate of
increase was not significantly different among syringe types. In 5-ml
glass syringes, the rate of PCO2
increase was significantly higher in blood samples tonometered with gas
mixture B (high
PCO2) than in blood samples
tonometered with mixture C (low
PCO2) (P < 0.01). Similarly, in both 5- and 3-ml plastic syringes the rate of
PCO2 increase was significantly
higher in blood samples tonometered with gas mixture
B (high PCO2) than in
blood samples tonometered with mixtures
A and C (low
PCO2 values)
(P < 0.01). The rise of
PCO2 vs. time is significantly
lower in 3-ml than in 5-ml plastic syringes regardless of the
tonometered gases (P < 0.05). This
is consistent with the relative permeability of 3-ml vs. 5-ml plastic
syringes based on surface-to-volume ratio (1.44).
PCO2) with time at different
initial blood-gas tensions in syringes. Data are means ± SE.
A: blood tonometered with 5%
CO2-12%
O2-83% N2.
B: blood tonometered with 10%
CO2-5%
O2-85%
N2.
C: blood tonometered with 2.88%
CO2-4%
O2-93.12%
N2.
SO2
SO2
decreased in all blood samples stored in glass syringes independent of
the tonometered gas mixtures (Table 1 and Fig.
3). However, blood samples stored in 3- and
5-ml plastic syringes show an increase in mean blood
SO2
independent of the gas mixtures. The difference in mean blood
SO2
between blood samples stored in glass and plastic syringes was
statistically significant (P < 0.01 and P < 0.05, respectively) except
between blood samples stored in 5-ml plastic and 5-ml glass syringes
and tonometered with gas mixture C due
to a high SD value. The rise of
SO2 vs.
time is higher in 3-ml than in 5-ml plastic syringes (P < 0.05), which is consistent with
the relative permeability of 3-ml vs. 5-ml plastic syringes (1.44).
SO2)
with time at different initial blood-gas tensions in syringes. Data are
means ± SE. A: blood tonometered
with 5% CO2-12%
O2-83%
N2.
B: blood tonometered with 10%
CO2-5%
O2-85%
N2.
C: blood tonometered with 2.88%
CO2-4%
O2-93.12%
N2.
pH and SBE
pH decreased with time in all the blood samples tonometered
with the three gas mixtures. There was no difference in the rate of
decrease among syringe types (Table 1). In 5-ml glass syringes and 3-ml
plastic syringes, the rate of decrease in pH was higher in the blood
samples tonometered with gas mixture C (lowest PCO2) than in the blood
samples tonometered with mixtures A
and B (intermediate and high
PCO2 values, respectively)
(P < 0.01). Mean SBE decreased
with time in all blood samples. There was no difference among syringe
types or among gas mixtures.
DISCUSSION
The data show how variable the changes in blood-gas tensions can be during storage in both glass and plastic syringes and the importance of limiting the time of storage even when the syringes are kept on ice. Permeability of glass syringes to O2 and CO2 is low, and diffusive gas exchange with the atmosphere is negligible over many hours; measured changes result from metabolism, which can be reduced but not eliminated by storage of the syringes on ice.
Comparison With Previous Studies and Sources of Variability Due to Syringe Size
Several studies, two in the early 1970's (4, 15) and one as late as 1989 (7) reported no significant differences between blood-gas changes during storage of arterial blood in plastic or glass syringes for up to 4 h. Evers et al. (4) and Winkle et al. (15) both used 10-ml plastic (polypropylene) syringes; the syringe size was not indicated in the study by Mathur (7). The importance of syringe size is indicated in the present study by the significantly higher rise inPO2 in 3-ml plastic syringes
compared with that in 5-ml plastic syringes and the greater slope of
increase in
SO2 and
lower slope of increase in PCO2 of
blood stored in 3-ml compared with 5-ml plastic syringes. The smaller
syringes have a larger surface-to-volume ratio as well as a slightly
thinner wall; both will increase the rate of diffusive gas exchange
[permeability of 3-ml plastic syringes/permeability of 5-ml
plastic syringes (1.44)]. Our results agree with those reported
by Scott et al. (13) and Restall et al. (10) on changes in
PO2 with the use of 2- and 5-ml
plastic syringes. They showed significant diffusive
O2 transport into or out of blood
(depending on the direction of the
O2 gradient) stored in plastic
syringes on ice for as short an interval as 1 h. The study of
Müller-Plathe and Heyduck (8) on both
O2 and
CO2 changes during storage in 1- to 2.5-ml plastic syringes for 45 min showed much greater rates of
increase in PO2 and lower rates of
increase in PCO2 than we measured.
Differences in O2 and
CO2 exchange can both be
attributed to increased diffusive exchange in the smaller polypropylene
syringes. The lower rate of rise in
PCO2 in the latter study compared
with our findings can be attributed to a greater rate of
CO2 transfer out of the smaller
syringes relative to a fixed rate of
CO2 production than indicated by
our data for larger plastic syringes. Thus available data suggest
significantly larger errors due to
O2 transfer between atmosphere and
blood stored in smaller plastic syringes than that in larger plastic
syringes.
Effects of Temperature on Differences Between Gas Exchange in Glass and Plastic Syringes
Storage on ice will reduce metabolic rate of blood to ~10% of that at 37°C and to ~23% of that at 22-24°C (3). Because metabolic rate is the primary source of changes in blood-gas tensions and the changes in pH due to lactic acid accumulation during storage in glass syringes, storage on ice clearly reduces the rates of change in blood-gas tensions and pH in glass syringes. Even so, changes are not completely prevented by icing the blood as shown in the study of Eldridge and Fretwell (3) and in the present study.The source of changes in blood-gas tensions in plastic syringes is a
balance between changes due to metabolism and changes due to diffusive
gas exchange. Rates and direction of diffusive gas exchange depend on
the solubility of the gas, permeability of the container walls to the
gas, the difference in gas tension between the blood and atmosphere,
and the surface-to-volume ratio of the syringe. Diffusive gas exchange
will tend to raise PO2 and lower
PCO2 with time. The following example
illustrates the expected effect of reducing the temperature of stored
blood in plastic syringes from 37 to 0°C on both metabolic and
diffusive changes in PO2. With the
reduction in temperature, the metabolic rate is reduced so that
metabolic utilization of O2 is
decreased (3), O2 tension at which
hemoglobin is half saturated with
O2
(P50) is decreased, and
solubility of O2 in plasma is increased (1). The net effect in blood that is 95% saturated with
O2 is a reduction of
PO2 from ~84 Torr (at 37°C) to
11 Torr (at 0°C) (9); thus the diffusion gradient for driving O2 into blood from the atmosphere
(O2 tension = 150 Torr) is
increased from ~66 Torr (150 84 Torr) to 139 Torr
(150 11 Torr), i.e., about a twofold increase. If we assume a
1% reduction in the Krogh diffusion constant per 1°C (1), the
Krogh diffusion constant for O2 in
blood is reduced to ~63% (100 37%) of that at 37°C. The
net effect of the reduction in temperature on the
PO2 gradient driving diffusion and
the Krogh diffusion constant is a 26% increase (2.0 × 0.63 × 100 = 126%, thus a 26% increase) in the rate of diffusive
O2 transfer into blood. Thus the
combined fall in metabolic utilization of
O2 and rise in diffusive transfer of O2 into blood caused by the
reduction in temperature will conspire to actually increase the rate of
rise in PO2 in blood stored in
plastic syringes.
Similar estimates can be made for CO2. One would expect that the high Krogh diffusion constant for CO2 will counterbalance the CO2 production in blood stored in plastic syringes. CO2 production continues during storage on ice as indicated by the rise in PCO2 in the glass syringes even though it occurs at a lower rate than at 37°C. Solubility of CO2 increases at the lower temperature so that PCO2 would drop to ~20% of that at 37°C, e.g., from 35 to 7 Torr (9); hence the gradient for CO2 diffusing out of blood is correspondingly reduced. If we assume that the Krogh diffusion constant at 0°C is reduced to 63% of that at 37°C, the rate of diffusive transfer of CO2 out of blood would be reduced to only 12.6% (0.2 × 0.63 × 100 = 12.6%) of that at 37°C. This could explain why there is little difference in the rate of rise of PCO2 in plastic and glass syringes during storage on ice.
Sources of Variability Caused by Differences in Blood-Gas Tensions and Acid-Base Status
Major sources of variability within a given type of syringe during storage on ice can result from 1) differences in slope of the oxyhemoglobin and CO2 dissociation curves in the range over which changes in blood-gas tensions are occurring and 2) changes in acid-base status during storage that will induce a Bohr shift in position of the O2 dissociation curve. Effects of slope of O2 and CO2 dissociation curves in blood. The characteristic of the shapes of the O2 and CO2 dissociation curves are such thatSO2/PO2
(slope of O2 dissociation curve)
at low PO2 is greater than that at
high PO2 (Fig.
4) and change in
CO2
content/PCO2 (slope of CO2 dissociation curve) at low
PCO2 is greater than that at high
PCO2 (Fig.
5). With a higher initial
SO2, the rate of PO2 increase is higher
because of lower slope at the higher region of the oxyhemoglobin
dissociation curve that has lower slope at higher
SO2. Figure
4, B and
C, shows the blown-up view of the
selected two regions of the curve in Fig.
4A. At the lower region of the curve
(Fig. 4C), a 1% increase of
SO2
resulted in an increase in PO2 of
~0.5 Torr. However, at the higher region of the curve (Fig.
4B), a 1% increase of
SO2
resulted in an increase in PO2 of
~10 Torr. Figure 5 shows that at a higher initial blood
CO2 concentration, the rate of
PCO2 change is greater than that at a
lower initial blood CO2
concentration. This is due to a lower slope at the higher region of the
curve and a higher slope at the lower region of the curve. This
explains why at high levels of blood
PO2 in the plastic syringes (mixture A) the slope of the
increase in PO2 with storage time
is much higher than that for the lower levels of blood
PO2 (mixtures
B and C) (Table 1
and Fig. 1) despite the higher PO2
gradient for diffusion from the atmosphere into blood when blood
PO2 is low. It also explains why at
high PCO2 (mixture
B) the slope of the increase in PCO2 with respect to storage time
is much higher than that at low PCO2
(mixtures A and
C) (Table 1 and Fig. 2) despite the
higher PCO2 gradient at the high PCO2 for diffusing out of the blood.
PCO2 to be
different at low and high blood PCO2
values. Arrows indicate direction and extent of
PCO2.
SO2
causes increase in PO2 of <1
Torr. However, in plastic syringes
(B) rightward shift with increased
SO2
causes increase in PO2 of ~1.5
Torr. Curve with solid line is original curve
(P50 = 29), and curve with dashed
line is curve after Bohr shift
(P50 = 30). Arrows indicate
direction of changes in
SO2 and
PO2.
Effects of Bohr shift. Through aerobic metabolism, white cells in blood continuously produce CO2 and consume O2, causing PCO2 to rise and PO2 and pH to fall. Erythrocytes, which do not contain mitochondria, maintain energy requirements by anaerobic glycolysis with the production of lactic acid and a progressive fall of SBE. The latter will further increase PCO2 by decomposing bicarbonate (14). Both of these effects on acid-base status during blood storage will cause an increase in P50 (i.e., a shift of the O2 dissociation curve to the right), causing PO2 to rise at any given fixed level of O2 content in blood. This Bohr shift explains how PO2 in the glass syringes can rise in hypoxic blood (mixtures B and C) when in fact SO2 and O2 content are falling owing to metabolic utilization of O2 (Fig. 6).
Effects of Delayed Measurements on Calculation of A-aDO2
The error effects of delayed measurements of blood in syringes can be illustrated by the effects on A-aDO2 estimation. The following standard equations were used
|
|
(1) |
|
(2) |
|
|
(3) |
Thus errors in
A-aDO2
caused by PO2 and
PCO2 with time of storage are
additive when breathing air or low
O2 mixtures. Because
PO2 rises significantly faster during
storage in plastic than in glass syringes and
PCO2 rises at about the same rate in
both, errors are significantly greater for plastic than for glass
syringes. Because PO2 rises much
slower in hypoxic blood, errors in
A-aDO2
should be much greater in normoxic blood. Table
2 shows the calculated A-aDO2
in glass and plastic syringes with time delays of 0, 2, and 6 h at
normoxic (BP = 750 mmHg,
FIO2 = 0.21) and hypoxic (BP = 750 mmHg, FIO2 = 0.10) conditions assuming R = 0.85. Changes in PO2 and
PCO2 with time were calculated by
using the regression equations of
PO2 and
PCO2 vs. time in this study. As
shown in Table 2, calculated
A-aDO2
decreased with time, with greater errors in plastic syringes.
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||
Blood-gas measurements change with time in both glass and plastic syringes, and errors become most apparent when used to estimate A-aDO2, since the combined errors in PCO2 and PO2 are additive. Net errors in A-aDO2 are in the same direction for glass and plastic syringes, but the magnitude of error with time between collection and measurement is much larger for the plastic syringes. Accurate corrections are difficult to make because of the multiple variables that affect the magnitude of error, including the initial blood-gas tensions, acid-base status that determines shape and position of the CO2 and O2 dissociation curves, temperature of storage, and metabolic rate of the blood. Storage of glass syringes on ice tends to minimize error by reducing metabolic rate in the blood, which is the major source of error from storage in glass syringes. Temperature of storage is a more complex issue with plastic syringes. Although metabolic rate is reduced by storage of plastic syringes on ice, errors due to diffusive gas exchange are exaggerated by storage on ice. Because error due to diffusive gas exchange predominates with plastic syringes, storage at room temperature may be significantly better than on ice, but measurements should be done as rapidly as possible.
ACKNOWLEDGEMENTS
The authors thank David Treakle and the staff of the Animal Resource Center for technical assistance and excellent care of the animals.
FOOTNOTES
Address for reprint requests: E. Y. Wu, Dept. of Internal Medicine, Univ. of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75235-9034.
Received 20 November 1995; accepted in final form 30 August 1996.
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| 5. | Greenbaum, R., J. F. Nunn, C. Prys-Roberts, and G. R. Kelman. Metabolic changes in whole human blood (in vitro) at 37°C. Respir. Physiol. 2: 274-282, 1967. [Medline] |
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