A comparison of three pulmonary artery oximetry catheters in intensive care unit patients

DOI 10.1378/chest.102.3.896 1992;102;896-905Chest

P E Scuderi, D L Bowton, J W Meredith, L C Harris, J B Evans and R L Anderson catheters in intensive care unit patients.A comparison of three pulmonary artery oximetry

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896 Comparison of Three Pulmonary Artery Oximetnj Catheters (Scuderi at a!)

A Comparison of Three Pulmonary ArteryOximetry Catheters in Intensive Care UnitPatients*Phillip E. Scuderi, M.D., F.C.C.P;t David L. Bowton, M.D., F.C.C.P;�

j Wayne Meredith, M.D., F.C.C.P;� Lynne C. Harris, R.N.;�l

Joni Brockschmidt Evans, M.S.;II and Randy L. Anderson, Ph.D.#

Objective: To compare the clinical performance of three

pulmonary artery oximetry catheters (Oximetrix 3, SAT-2,and HEMOPRO2) in intensive care unit (ICU) patients.

Design: Unblinded comparison of performance over 24 husing an IL-282 CO-oximeter as a criterion standard.Setting: Multispecialty adult ICU at a university teachinghospital.

Patients: Thirty critically ill patients selected from those

requiring hemodynamic monitoring for medical manage-

ment.Measurements and Main Results: By all measures, perform-ance of the Oximetrix 3 and SAT-2 systems were compara-ble; bias±precision were - 1.98±3.07 and + 1.80±3.49,

respectively, vs - 2.28 ± 5.24 for the HEMOPRO,. TheOximetrix 3 and SAT-2 systems demonstrated consistent

performance over the range of saturations tested, thoughOximetrix 3 tended to underestimate and SAT-2 tended tooverestimate the CO-oximeter value. The HEMOPRO,underestimated the CO-oximetry-derived saturation, al-though this was not constant across the range of valuestested. The 95 percent confidence limits based on intrasub-

ject variability were similar(± 4.59, ±5.66, and ±6.56 for

the Oximetrix 3, SAT-2, and HEMOPRO5, respectively);

however, the 95 percent confidence limits based on totalvariability, while similar for Oximetrix 3 (± 6.03) and SAT-2 (± 6.86), were larger for the HEMOPRO, (±10.30). The

expected SD was similar for the three systems (2.03, 2.50,

and 2.90 for the Oximetrix 3, SAT-2, and HEMOPRO2systems, respectively). None of the systems equaled or

exceeded (p>O.O5) the manufacturers’ published specifica-

tions, which, in all cases, are listed as ±2 percent (satura-tion; 1 SD) when compared with bench oximetr�

Conclusions: Although each system measures mixed venousoxygen saturation, the Oximetrix 3 and SAT-2 systems

demonstrate closer agreement with CO-oximetry. However,

none of these catheters provided statistically significant

evidence that they would perform within ± 2 percent of

CO-oximetry. As a continuous monitor used to detect

changes or trends, any of the three may be acceptable.(Chest 1992; 102:896-905)

ESD = expected standard deviation; PA pulmonary artery;PAOP=pulmonary artery occlusion pressures; RMSE=rootmean squared error

C onsiderable controversy exists as to the clinical

utility of pulmonary artery (PA) mixed venous

oxygen saturation (Sv02) catheters as a continuous

monitor for patient treatment. Significant impedi-

ments to the clinical use of Sv02 oximetry systems

include both the reported inaccuracy of some

systems”2 and the absence of clinical data defining

acceptable levels of agreement when compared with

a criterion standard.

The published “accuracy specifications” contained

*From the Departments of Anesthesia (Section on Critical Care),

Medicine (Section on Pulmonary and Critical Care Medicine),Surgery, and Public Health Sciences, the Bowman Gray School ofMedicine of Wake Forest University Winston-Salem, NC.

tAssistant Professor, Department of Anesthesia.�Associate Professor, Departments of Anesthesia and Medicine.§Associate Professor, Department of Surgery.#{182}ResearchNurse, Department of Anesthesia.

IlBiostatistician, Department of Public Health Sciences.#Assistant Professor, Department of Public Health Sciences.

Presented in part at the annual meeting, American Society ofAnesthesiologists, Las Vegas, Nev, October 19-23, 1990.This study was supported in part by Edwards Critical CareDivision, Baxter Healthcare Corp., and Viggo-Spectrasned.

Manuscript received October 21; revision accepted March 12.Reprint requests: Dr. Scudeni, Department of Anesthesia, Bowman-Gray School of Medicine, Winston-Salem, NC 27157-1009

in the product information for all three instruments

supplied by their respective manufacturers are ± 2

percent saturation (± 2 SDs) when compared with a

bench oximeter such as a CO-oximeter. These figures

are presumably based on manufacturers’ studies using

flow bench preparations, animal data, and clinical

trials. Studies to date, however, have failed to confirm

these levels of performance clinically. Nevertheless,

Sv02 oximetry is a widely used clinical measurement

that has been advocated as a means of providing

information about the adequacy of oxygen supply

relative to demand and of assessing the adequacy of

cardiorespiratory function in certain disease states.

Few studies, however, have systematically compared

the performance of the three mixed venous oximetry

catheters available, and the analyses of catheter per-

formance have frequently been flawed by incomplete

or inappropriate statistical analysis. Further, recent

changes by the manufacturers in catheter design,

computer algorithms, and optical modules have led to

potentially improved clinical accuracy and perform-

ance of these devices. This study was designed to



CHEST / 102 I 3 I SEPTEMBER, 1992 897

ascertain the accuracy of the three available oximetry

PA catheters compared with conventional bench ox-

imetry (CO-oximetry). The study population consisted

of patients in the intensive care unit (ICU) who

required hemodynamic monitoring for medical man-

agement.

Equipment

METHODS AND MATERIALS

The most recent versions of the three commercially available,

FDA-approved in vivo oximetry systems (catheter, optical module,

and computer) were obtained from their respective manufacturers.

Three oximetry PA catheters were studied: (1) Oximetrix 3 P7110-

EP-H 7.5 Fr; (2) Edwards Critical Care SAT-2 93-A-770H 7.5 Fr;

and (3) Viggo-Spectrained HEMOPRO, 7.5 Fr. The Oximetrix 3

system (Abbott Critical Care Systems, Hospital Products Division,

Abbott Laboratories, North Chicago, Ill) uses a dual fiberoptic

bundle in the PA catheter. Three reference wavelengths are

produced by the optical module and transmitted down one fiber-

optic bundle. The other fiberoptic bundle is used for detection of

backscatter of light from the red blood cells. The SAT-2 system

(Edwards Critical Care Division, Baxter Healthcare Corporation,

Irvine, Cali!) also uses a dual fiberoptic bundle in the PA catheter.

Two reference wavelengths are used. The HEMOPRO, system

(Viggo-Spectramed, Oxnard, Calil) uses a triple fiberoptic bundle

in the PA catheter. One bundle is used for transmission of two

reference wavelengths; the other two bundles are used for detection

of backscatter.

Each manufacturer supplied the saturation computer, optical

module, and oximetry PA catheters that were used in the study.

Representatives from each company instructed two of the authors

(P.E.S. and L.C.H.) in the proper use of the instruments. Several

catheters from each manufacturer were placed in patients before

initiation of the comparative study. Proper insertion technique and

use of the equipment were confirmed by the company representa-

tives during these prestudy trials.

The performance of the three instruments was evaluated by

comparing the SvO, obtained using the PA oximetry system (in vivo

SvO,) with that from a blood sample drawn from the distal port of

the catheter (in vitro SvO,) analyzed using an Instrumentation

Laboratory IL-282 CO-oximeter (Lexington, Mass) as the criterion

standard. This device determines the relative concentrations of

oxyhemoglobin, deoxyhemoglobin, methemoglobin, and carboxy-

hemoglobin using a four�reference wavelength spectrophotometric

technique. The instrument was maintained and calibrated according

to the manufacturer�s specifications. Yearly maintenance was per-

formed by a factory representative before the start of the study.

Ihtients

A total of 30 patients were enrolled in the study, which was

approved by the Clinical Research Practices Committee of the

Bowman Gray School of Medicine/North Carolina Baptist Hospital.

Informed consent was obtained from patients’ next of kin beforestudy initiation. All patients enrolled in the study were admitted to

the Multidisciplinary Intensive Care Unit of North Carolina Baptist

Hospital. The primary admitting service of each potential study

patient had previously determined that the patient would benefit

from hemodynamic monitoring. Patients were divided equally into

three groups corresponding to the three PA oximetry systems being

evaluated. The three PA oximetry systems were evaluated sequen-

tially based on availability of equipment from the manufacturers.

The order of testing was Oximetrix 3, SAT-2, and HEMOPRO,. In

addition to PA catheters, all patients had arterial lines placed for

continuous blood pressure monitoring. All patients had PA occlusion

pressures (PAOP) and thermodilution cardiac outputs measured.

During the course of the study, no patient received either intrave-

nous fat embolism or propofol, both of which have been reported

to confound the measurement of SvO,. No patient with a total

bilirubin level greater than 2 mg/dl was enrolled in the study.

Procedure

All of the study catheters were inserted percutaneously by one

of the investigators (P.E.S.) using a modified Seldinger technique

to place an 8.5-Fr introducer sheath. The catheters were calibrated

according to manufacturers’ instructions. The Oximetrix 3 was

calibrated in vitro as was the HEMOPRO,. The SAT-2 system was

calibrated in vivo, as this was the method recommended by the

manufacturer for assuring best agreement with CO-oximetry. Under

continuous electrocardiographic and pressure waveform monitor-

ing, the catheters were passed through the introducer sheath until

intrathoracic location was confirmed by venous waveforms and

respiratory fluctuation. The catheter balloons were inflated to tbe

recommended volume (1.5 ml) and advanced until a PAOP wasobtained after characteristic waveform progression. The position of

the catheter was then adjusted so that a balloon inflation volume of

1.5 ml was required to obtain a PAOP The position of each catheter

in the proximal PA was confirmed by chest roentgenogram (portable

supine anteroposterior [AP] projection) before beginning the Sv02

comparison, and catheter tip location was defined using the method

of Johnston et al .� The distance in centimeters of the catheter tip

from a zero point, defined as the intersection of the descending

portion of the PA catheter in the superior vena cava with the PAcatheter in the pulmonary artery, was determined. Position of the

catheter tip to the left of the zero point was defined as positive and

to the right as negative. Distances were recorded in centimeters.

After proper location of the catheter was confirmed, the SAT-2

oximetry system was calibrated to CO-oximetry (in vitro Sv02) and

the current hematocrit (Hct) and hemoglobin (Hgb) values were

entered into the saturation computer. The HEMOPRO, system also

had the current Hct and 11gb values entered into its saturation

computer as recommended by the manufacturer. The Oumetrix 3

system does not require entry of Hgb or Hct values into the

saturation computer.

The study began immediately after calibration of each catheter.

Blood samples for comparison were obtained every 15 mm for 3 h,

and hourly for an additional 9 h, with a finalsample obtained at 24h. The in vivo SvO, was recorded immediately before and after

each sample was drawn. This pair of in vivo values was thenaveraged and compared with the corresponding in vitro SvO,. The

blood samples were withdrawn from the distal port of the PA

catheter after an initial volume of approximately 10 ml was

withdrawn to clear the flush solution from the lumen of the PA

catheter and stopcock. The sample for analysis was then collected

anaerobically into a 3-mI heparinized syringe. Each sample was

withdrawn over approximately 1 mm to minimize the possibility of

obtaining a retrograde, arterialized sample. Samples were processed

immediately on an IL-282 CO-oximeter for total hemoglobin,

oxyhemoglobmn, deoxyhemoglobmn, methemoglobin, and carboxy-

hemoglobin determinations. No recalibrations were done duringthe study period in order to establish whether there were tendencies

for in moo SvO, to drift over time relative to in vitro SvO,.

All patients enrolled in the study were considered to require

hemodynamic monitoring for optimal clinical care. The physicianscaring for each patient were given access to the data collected and

the information was available to guide therapeutic interventions.

Statistical Analysis

Measurement error was calculated as the difference between In

vivo and in vitro SvO,. Correlation coefficients of in vivo vs In vitroSvO, were also calculated in order to compare current findings with

previous studies. Bias (Eq 1), defined as the mean difference

between in vivo and in vitro determinations of SvO,, and precision



F 1

2

0

3

5

10

3

3

0

2

5

9

6

2

2

2

4

9

Table 1-Patient and Catheter Characteristics*

898 Comparison of Three Pulmonary Artery Oxirnetry Catheters (Scuden eta!)

(Eq 2). defined as the SD of the bias, were determined for each

patient.5

Let i index the n = 10 patients within each group and let j index

the n = 21 observations made on each patient. Let SvO2 (in vitro5 - in

vivo5) denote the difference between the spectrophotometric (CO-

oximetry) measure and the oximetry PA catheter measure for the

j” measurement in the i” patient.

Bias for the i5’ patient is the average difference between CO-

oximeter and PA catheter values.

�S�O,(in vitro,, - in vivo5)

Bias=’’Eqin

where n = the number of repeated measurements

Precision for the jth patient is defined as the SD of the differences.

It reflects the replicability of the bias assessment within a given

patient.

Precision = /�[S�JO,(in vitro�- in vivo�) - Bias] 2 Eq2

Bias and precision were also computed for each device within each

measurement time. For any given device bias at the j” measurement

time is defined as follows:

�S�O2(in vitro5- in vivo,,)

Bias,= ‘�‘ N

For any given device, precision at the j” measurement time is

defined as follows:

�[SVO,(in vitro5- in vivo5) - Bias,]�Precision,= / -� Eq4

N-i

Ambiguity (Eq 5), proposed as a measure of overall within-patient

operational performance or agreement when comparing instrumen-

tation,’ is calculated as the precision plus the absolute value of the

bias.

Ambiguity, = Precision + Bias Eq 5

Root mean squared error (RMSE) (Eq 6), which is a group summary

measure of PA catheter efficiency at approximating the in vitroSvO2, is calculated as the square root of the average squared bias.�

/��S�O,(in vitro5 - in vivo5)2RMSE= I

N(n-1)

This equation can also he expressed as follows:

RMSE = �J�Precision2+ Bias2 Eq 7

The expected standard deviation (ESD; Eq 8) of repeated measure-

ments within individuals was calculated as the square root of the

average intrasubject variance.

vitro�- in vivo,1) - Bias]2

ESD n-i Eq8

N

where N = number of patients

Equation 8 can also be rewritten as follows:

/�precision2ESD f-i Eq9

N

Catheter

Oximetrix 3 SAT-2 HEMOPRO,

Characteristics (n = 10) (n = 10) (n = 1O�

Age, yr mean ± SD 60.1 ± 17.2 48.6 ± 20.2 49.2 ± 18.9

M 9 7 4

Insertion site

1W

LU

RSC

LSC

Mechanical ventilation

Distance to zero point -0.7±2.6 0.6±2.0 - 1.0±2.8

5SD= standard deviation; RIJ=right internal jugular vein;

LIJ = left internal jugular vein; RSC = right subclavian vein;

LSC = left subclavian vein. Distance to zero point is in centimeters

measured on a portable anteroposterior (AP) chest roentgenogram.

A negative number indicates that the catheter tip has not crossed

over the descending catheter.

A 95 percent confidence interval was determined for each catheter’s

Eq3 average bias using two methods. The first method used the SD, a,of all differences over all patients and the standard normal distri-

bution (mean bias± 1.965 * a). This method assumes that the

measurement error is independent of the patient measured. The

second method used the ESD as the variance estimate, thereby

factoring out the intersubject variability. A Student�s t distribution

with (10-1) degrees of freedom was used to construct this interval

(mean bias ± 2.262 * ESD). Trends over time were assessed using a

univariate repeated measures of analysis of variance for the differ-

ence.

Similarity of catheter performance to the manufacturers’ pub-

lished specifications, ± 2 percent (± 1 SD), was tested using the

null hypothesis that each catheter (in vivo) differs from CO-

Oximetry (in vitro) by more than ± 2 percent. Statistical inference

procedures for comparing groups detail the use of sample informa-

tion to infer information about population comparisons within

certain probabalistic limits. In a usual t test of group differences,

the null hypothesis of “no difference” between groups is tested.

Eq 6 Substantial sample evidence of a difference is needed to concludewith high probability that there is a nonzero population difference.

In studies intended to examine group similarity or agreement, such

as generic drug bioequivalence studies or instrument comparison

studies, the null hypothesis states that the groups differ by more

Table 2-Physiologic Variables for the Three StudyGroups (Mean ± SD)

Physiologic

Variables

Catheter

‘�

Oximetrix 3 SAT-2 HEMOPRO2

Blood pressure, mm Hg

Systolic 138±27 137±25 134±23

Diastolic 64 ± 13 68 ± 15 66 ± 12

Heartrate,beats�minute�’ 114±16 112±21 111±22

Pulmonary artery pressure,

mm Hg

Systolic 42±12 46±14 38±10

Diastolic 18±8 23±8 21±6

Wedge 14.19 ± 5.62 16.52 ± 5.7 17.43± 5.8

Cardiac output, L’min 7.6±2.6 6.6±2.4 7.1±2.5



CHEST / 102 / 3 / SEPTEMBER. 1992 899

than some predetermined meaningful amount. To reject this null

hypothesis, the sample evidence of dissimilarity must be consider-

ably smaller in magnitude than the predetermined limits. For the

group comparison herein, the sample difference must he consider-

ably less than 2 percent of the SvO, mean (see below) to conclude

that the population difference is 2 percent or less. Thus, the

rejection criterion is more rigorous than the 2 percent criterion,

hut the conclusion upon rejection is much stronger. The hypothesis

was tested using the t statistic”:

i’i,,,., �(1,= II S

where u denotes the respective sample meansand s is the within-patient standard error of the differences. The t test usesa rejectioncriterion of the following:

0.02#{252},I � I I I -c

for a paired t test where c is chosen from a central T distributionwith n-i degreesof freedom such that:

� �Eq12

5 SAll testswere performed with a =0.05.

RESULTS

Patient demographics, catheter insertion site, andcatheter tip location for each of the three groups aresummarized in Table 1. The characteristics of eachgroup were similar except for the relative proportionof men vs women in each group. Physiologic variablesfor each of the three groups during the course of thestudy are shown in Table 2. There were no significantdifferences among the groups for any of the variablesrecorded. Table 3 shows the mean, SD, and range ofin vitro Sv02 and hemoglobin for each patient in eachgroup. The group values for Sv02 are also shown inthis table.

The scatter plots of in vivo vs in vitro Sv02 for eachmeasurement time in all patients are shown in Figure1. Figure la shows the difference (in vivo - in vitro)in Sv02 vs the in vitro Sv02 as a criterion standard.Figure lb shows the difference (in vivo - in vitro) in

EqlO

Sv02 vs the mean Sv02 ([in vivo + in vitm]/2) in

recognition that the in vitro Sv02, as determined by

CO-oximetry, is not free of measurement error. The

“accuracy” of the IL-282 CO-oximeter, as reported by

the manufacturer, is ± 1 percent (saturation). Although

previous studies have all assumed that the in vitro

Sv02 value obtained by bench oximetry represents a

true criterion standard for comparison, no independ-

ent assessment of the “accuracy” of CO-oximetry

exists (see appendix). It has been suggested that a plot

of the difference between methods against their means

may be more informative� and eliminates the statistical

Eq 11 artifact caused by plotting the difference against eithervalue separately.b0 While this concept has been dis-

cussed in statistical publications,”’� it is not widely

recognized by medical users of statistics.

The Oximetrix 3 and HEMOPRO2 systems tended

to underestimate Sv02 compared with CO-oximetry

with average biases (Eq 3) of - 1.98 and -2.28,

respectively. The SAT-2 system, on the other hand,

tended to overestimate Sv02 compared with CO-

oximetry, with an average bias of + 1.86. The precision

(Eq 4) of the three instruments based on the pooled

data across patients was ± 3.07, ± 3.49, and ± 5.24

for the Oximetnx 3, SAT-2, and HEMOPRO2, respec-

lively. The ESD (Eq 7) for the Oximetrix 3, SAT-2,

and HEMOPRO2 systems was ± 2.03, ± 2.50, and

± 2.90, respectively. The 95 percent confidence limits

for the three systems are shown in Figure 1. The

confidence limits based on total variability and on

intrasubject variability are shown and are plotted

relative to the overall bias exhibited by each instrti-

ment. It should be noted that the 95 percent confi-

dence limits are not affected by the manner in which

the data are plotted (iv, [in vivo - in vitro] against in

vitro, as in Figure la, or [in vivo - in vitro] against [in

vivo + in vitro]/2, as in Figure ib). The 95 percent

Table 3-CO-Oximetry Derived 5v05 and Hemoglobin for Patients in the Three Study Groups*


SvO, Hemoglobin Sv02 Hemoglobin SvO2 hemoglobin

Patient Mean±SD Range Mean±SD Range Mean±SD Range Mean±SD Range Mean±SD Range Mean±SD Range

1 74.7±2.8 70.9-83.3 8.5±0.4 7.6-9.5 72.6±3.0 65.6-78.5 10.6± 1.1 8.9-12.5 75.2±3.3 71.3-85.1 9.0±0.3 8.4-9.5

2 81.8±1.8 77.3-85.2 8.7±0.4 7.7-9.3 76.9±1.4 74.6-81.0 7.7±0.9 6.3-9.6 77.9±1.4 74.6-81.7 10.4±0.3 9.7-11.0

3 66.0±5.9 56.0-75.6 9.1±0.6 7.5-10.0 58.8±9.4 40.9-73.8 11.3±0.7 10.0-12.3 70.9±1.7 67.1-74.0 9.1±0.9 8.1-12.5

4 75.2±7.5 54.6-84.7 9.5±0.5 8.7-10.3 75.9±6.1 42.2-73.2 10.4±0.6 8.7-11.4 50.5±2.6 45.4-57.0 8.7±0.7 7.6-10.3

5 77.9±7.6 68.7-88.4 9.2±0.6 8.4-10.8 54.1±3.4 47.6-58.7 11.5± 1.2 9.8-13.4 57.6±4.5 45.7-63.4 8.9±0.8 7.5-10.6

6 75.5± 1.9 72.2-80.3 7.9±0.6 7.0-9.8 61.3±5.3 44.0-66.0 11.1±0.8 9.3-12.9 56.0±3.4 49.2-63.9 9.9±0.5 9.4-11.1

7 65.5±2.7 58.0-72.6 8.7±0.7 7.1-10.5 71.9±5.1 60.6-78.8 9.6±0.9 8.3-12.6 69.6±3.7 60.8-78.2 7.9±0.3 7.3-8.7

8 64.5±4.1 53.3-71.5 7.1±0.4 6.5-7.8 75.3± 1.8 71.8-79.6 8.2± 1.5 6.3-11.3 81.1± 1.3 77.9-83.2 9.9± 1.1 8.5-13.2

9 71.7±2.8 67.0-76.9 10.6±0.6 9.3-11.5 69.4± 1.8 66.0-72.7 11.9± 1.4 9.6-13.8 71.4±3.0 65.2-74.9 9.6±0.5 8.8-10.5

10 66.5±4.8 55.3-78.6 8.0±0.4 7.4-8.9 67.8±3.5 62.2-77.3 9.4±1.0 7.8-11.7 78.7±2.1 75.7-82.6 9.1±0.5 8.4-10.1

Group 71.9±4.7 53.3-88.4 8.7±0.5 6.5-11.5 68.4±7.6 40.9-81.0 10.2± 1.0 6.3-14.0 68.9±2.9 45.4-85.1 9.3±0.8 7.3-13.2

*SD = standard deviation. Values for SvO, are expressed in percent saturation. Values for hemoglobin are expressed in grams per deciliter.

Group means are expressed ± ESD.



Oxlmetrix 3

S10

5

0

-5

-10

-15

-20

Oximetrix 3

.10.

5

0-

-5,

-10-

-15-

-20-

15-2

� 10-

C

5.

H 0-

‘SI -5-

-10-UC� -15-a

� -20-

10-

5.

0-

-5.

-10-

#{149}.

I #{149}S

HEMOPRO2

_.�__!.�_�

#{149}: #{149}S#{149}SI4�

v.

..#{149}.

15-

10-

5-

0-

.5.

-10-

-15-

.

30 40 50 60 70 80 90 100

CO-Oximeter Sv02(in vitro)

30 40 50 60 70 80 90 100

Mean S�O2[(In vivo + in vitro) / 2]

Ficune 1. Performance of each of the three groups across the range of values tested shown as the difference

between in vivo and in vitro Sv02 vs CO-oximetry-derived (in vitro) SvO, (Fig La, left) and as the difference

between in vivo and in vitro Sv02 vs the mean of in vivo plus in vitro (Fig ib, right). Dashed lines=95percent confidence limits based on total variability; dotted lines = 95 percent confidence limits based on

intrasubject variability; solid lines = bias.

15 15

900 Comparison of Three Pulmonary Artery Oximetry Catheters (Scuderi eta!)

15.

15.

-15-

SAT2 #{149}#{149} #{149}.C #{149} ‘� 10-

#{149} #{149} .-� -Ia-.

C� -15-a

� -20-

HEMOPRO2

.j!

#{149}.I#{149}s #{149}

#{149} S

#{149} #{149}.i#{149}�

� I

II-TIIJIT!t

SAT 2

confidence limits are determined by the scatter along

the y-axis, which is a function of the (in vivo - in vitro)

values. Whether values are plotted against in vitro

values or against (in vivo + in vitro/2) determines the

position along the x-axis, and does not affect y-axis

scatter of 95 percent confidence limits.

The data by time for the 24-h study period are

shown in Figure 2. Bias and precision for each

oximetry system relative to CO-oximetry are shown

at each study period. Neither the Oximetrix 3 nor the

SAT-2 systems demonstrated a significant change in

bias (p = 0.882 and p = 0.15, respectively) over time.

The HEMOPRO2, however, did exhibit a statistically

significant change in bias (p<O.OOl) during the 24-h

study period.

Performance (bias and precision) of each of the in

vivo oximeters by individual patient is shown in Figure

3. As noted above, each of the systems had an overall

tendency to either overestimate or underestimate the

CO-oximetry Sv02 (ie, positive or negative bias).

However, some patients within each group had bias

values different in direction from the group average.



Oximetrlx 3

10 -

SAT 2

5.

0�

-5 -

.10

-15 -

20

15

10

Oc

0

-5,I,a.

0

-15

.20 -

15 -

10

5

0

.5

.10

I

5

0

-5

-10

-15

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15

10

C0-’.

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fi -:

-15

-20

15

10

5

0

-5

-10

-15

-20

Ficuisa 2. Bias and precision for each of the three groups for the

24-h study period. Each plot represents the pooled data for the ten

patients in each respective group at each time point.

HEMOPRO2

1 2 3 4 5 6 7 8 9 10 11 1224

TIme in Hours

Table 4-Overall Performance of Three OrimetryPUlmonary Artery Catheters Compared with CO-Oximetry

eRegression equation is expressed as y = ax + b, where y = in vltsv

SvO,, x=in vivo SvO,, and a and b represent the slope and

intercept of the equation. All units are percentage saturation

except r’. RMSE=root mean squared error; ESD�=expected

standard deviation.

CHEST / 102 / 3 / SEPTEMBER, 1992 901

15 Oxlmetrlx 3

10

For example, patient 2 of the Oximetrix 3 group had a

bias for the study period of + 2, while the average

bias for all patients in that group was -1.98. Similarly,

the individual precision for some patients was consid-

erably better than the precision for the group as a

whole. When similarity of catheter performance to

manufacturers’ specifications was tested (Eq 10 to 12)

(± 2 percent saturation, ± 1SD), none of the systems

rejected the null hypothesis to prove similarity at ± 2

percent relative to CO-oximetry.

The performance characteristics for each of the

systems are summarized in Table 4. The slope of the

regression analysis for each group was similar; how-

ever, there were striking differences in the intercept

values. The r2 values showed remarkable consistency

for each of the three groups. As noted above, Oximetrix

3 and SAT-2 had comparable performance as measured

.15

- T - � - -

J�-�- - -. -j

HEMOPRO2

-

H’I I I I I I I I

0 1 2 3 4 5 6 7 8 9 10

Patient Number

FICURE 3. Performance by patient for the 24-h study period. Eachplot represents all data collected for each patient across the 24-h

study period.

Descriptive Statistic

Catheter


Regression 1.OOx-2.11 0.99x+2.67 1.08x-7.75

r2 0.85 0.86 0.83

Bias ± precision - 1.98± 3.07 1.86±3.49 -2.28 ± 5.24

Ambiguity 5.05 5.29 7.52

RMSE 3.64 3.92 5.71ESD ±2.03 ±2.50 ±2.90

95% confidence limits ±6.03 ±6.86 ± 10.30(total variability)

95% confidence limits ±4.59 ±5.66 ±6.56

(intrasubject

variability)



902 Comparison of Three Pulmonary Artery Oximetry Catheters (Scuiieri eta!)

by average bias and precision, while HEMOPRO2

showed poorer precision. Ambiguity and RMSE, both

proposed as measures of overall performance, were

similar for the Oximetrix 3 and SAT-2 groups. The

HEMOPRO2 group again demonstrated poorer per-

fi)rmance by these measures. The ESD of repeated

measurements within subjects, calculated as the

square root of the average intrasubject variance (Eq

5), excludes intersubject variability. This measure of

performance of the three catheter systems showed the

greatest agreement among the three groups. The ESD

for each group is also shown in Table 4. The 95 percent

confidence limits based both on total variability and

iiitrasubject variability are shown. The 95 percent

confidence limits based on intrasubject variability are

similar for the Oximetrix 3, SAT-2, and HEMOPRO2

(±4.59, ±5.66, and ±6.56, respectively) while the

95 percent confidence limits based on total variability

are more disparate (±6.03, ±6.86, and ± 10.30,

respectively).

DISCUSSION

Techniques for monitoring mixed venous oxygen

saturation (Sv02) were first described in 1962.13,14

Technologic advancements in the design and manufac-

ture of fiberoptics and light-emitting diodes have led

to the development of PA catheters capable of moni-

toring Sv02 continuously, using the principles of the

Beer Lambert Law, in vivo measurement of blood

oxygen saturation is possible because of the differential

reflection and absorption of various wavelengths of red

asid infrared light by oxyhemoglobin and reduced

hemoglobin. In theory, a solution containing a mixture

of oxyhemoglobin and deoxyhemoglobin has absorp-

tion or reflection characteristics that can be quantified

to yield the relative percentage of each species pres-

ent. Light of appropriate wavelength(s) is generated

by light-emitting diodes and delivered to the PA by a

fiberoptic bundle contained in the catheter. The light

is hackscattered off the hemoglobin in the red blood

cells present in the circulation, and the reflected light

is collected by a detecting fiberoptic bundle also

contained in the PA catheter. Analysis of the reflected

light and the application of appropriate algorithms

permit the calculation of the relative proportions of

)xyhemoglobin and deoxyhemoglobin.

Previous studies have evaluated the clinical per-

formance of PA oximetry catheters in various settings,

including animal models’ arid human studies.2”�’8

Those studies, which did not employ periodic recali-

bration of the PA oximetry catheters,2’15-’7’9 showed

that the Oximetrix system produced closer agreement

to bench oximetry than did the Edwards Critical Care

device; however, these studies all evaluated the older

SAT-i version of the Edwards system. Only one of

these studies’5 undertook a three-way comparison of

the available oximetry systems. This study also showed

that the Oximetrix system produced closer agreement

to bench oximetry than did the Edwards Critical Care

device or the Spectramed device. It has l)een hypoth-

esized that the previously reported differences in

performance were attributable to inherent differences

in two- vs three-wavelength catheter systems.’-”

The purpose of this study was to compare the

clinical performance of in vivo vs in vitro SvO2

determinations in ICU patients using the three most

current commercially available in vivo oximetry sys-

tems (Oximetrix 3, SAT-2, and HEMOPRO2). Evalu-

ation of a monitor requires that both “accuracy” and

reproducibility are quantified.�#{176}Previous studies of

the performance of PA oximetry catheters have relied

heavily on the use of regression analysis and correlation

coefficients as a method of comparison. Clearly, each

of these systems correlate with CO-oximetry and all

are capable of reflecting Sv02. It has been suggested,

however, that correlation coefficients constitute an

inappropriate statistical tool for assessing agreement.5

A high level of correlation requires only that a plot of

paired data points lie along a straight line, while

agreement requires that the data points lie along a line

of identity. In addition, the apparent strength of a

correlation can be increased by increasing the range

of the values tested. Indeed, the correlation coeffi-

cients of the three systems compared with CO-

oximetry are remarkably similar (0.85, 0.86, and 0.83

for Oximetrix 3, SAT-2, and HEMOPRO2, respec-

tively; Table 4). Alternatively, performance can be

defined by bias (see Eq 1 and 3) as a measure of the

constant or proportional error that may occur across a

range of measurements. Likewise, reproducibility can

be defined by precision (see Eq 2 and 4), which

measures the nonsystematic or random variation that

occurs with repeated measurements.2’

Comparison studies in clinical medicine are typi-

cally carried out in one of two settings. In one, an

accepted criterion standard exists and is available for

comparison; in the other, no independently estab-

lished criterion standard for comparison exists and a

new test or method must be evaluated by comparison

to an already established, though possibly inaccurate,

technique. For the purposes of this study, we have

considered multiwavelength spectrophotometry (eg,

CO-Oximetry) as a criterion standard for the deter-

mination of hemoglobin oxygen saturation. The statis-

tical methods that we have used to assess agreement

between in vivo and in vitro oximetry are based on

this assumption. However, for completeness, we have

included a “difference versus mean” plot (Fig ib) as

recommended by Bland and AItman.�

Statistical analysis of performance comparison data

is further complicated by two prol)lems. First, two

statistical approaches are used to evaluate measure-



CHEST / 102 / 3 / SEPTEMBER, 1992 903

ments for agreement. The first approach uses a tradi-

tional null hypothesis stating that there is no difference

between the two measures, and a traditional alterna-

tive hypothesis of a nonzero difference. This null

hypothesis is not rejected unless the measures are

different with high probability. However, in measure-

ment agreement studies, the goal of testing is to

discern whether the measures are functionally equiv-

alent. Thus, it is more appropriate to test a null

hypothesis of difference, with an alternative hypoth-

esis of similarity. When this more direct null hypoth-

esis is rejected, we conclude that the measures are

similar with high probability. The second approach

presented herein uses a null hypothesis that each

catheter (in vivo) mean differs from CO-oximetry (in

vitro) by more than 2 percent of the in vitro mean.

The corresponding alternative hypothesis states that

the measures are similar within these limits. These

tests for equality require that the level of agreement

to be considered as “equality” be established before

statistical testing. No data in the literature establish

an acceptable level of agreement between PA oximetry

catheters and bench oximeters. Consequently, we are

unable to test whether the performance of catheters

in question differs significantly from bench oximetry

clinically. However, the performance of these devices

does not match, with high probability, the performance

specifications published by their respective manufac-

turers.

Second, statistical analysis for most tests requires

independent observations. Limitations of clinical com-

parison studies such as these require multiple obser-

vations in individual patients with subsequent pooling

of data, violating the rule of independent observation.

All of the comparison studies noted to date also suffer

from this flaw.1,a.ls.16.le A more appropriate analysis

should account for the covariance among repeated

measures within patients. The tests for agreement (95

percent confidence limits based on intrasubject vari-

ability) used in this study accounted for the repeated

measures by using the within-subject standard error

of the differences.

Despite the limitations noted, there are differences

in the performance of the three systems. The differ-

ence plots of in vivo vs in vitro Sv02 (Fig 1) demon-

strate that both Oximetrix 3 and SAT-2 have a system-

atic error or inaccuracy (bias) that is consistent. The

Oximetrix 3 group consistently underestimated in vitro

Sv02 by approximately two percentage units

(bias = -1.98 percent saturation), while the SAT-2

group consistently overestimated in vitro Sv02

(bias = + 1.80 percent saturation). The respective un-

derestimation and overestimation within these two

groups were constant over the range of values tested.

The HEMOPRO2 group also demonstrated an overall

tendency to underestimate in vitro Sv02 (bias = -2.28

percent saturation); however, this systematic error was

not constant over the range of values tested. The bias

demonstrated by the HEMOPRO2 group appears

proportional to the value tested, ranging from nearly

-5 percent saturation at low target Sv02 values (40

percent to 50 percent) to approximately - 1 percent

at the higher target saturation values (80 percent to

90 percent).

More disturbing than either the relatively constant

negative or positive bias exhibited by the Oximetrix 3

and SAT-2 or the nonconstant bias of the HEMOPRO2

are the wide 95 percent confidence intervals relative

to CO-oximetry. A constant bias, or even a nonconstant

bias that is a function of the Sv02 target value, can be

corrected by modifications in the computer algorithm

used to calculate saturation. Large random measure-

ment errors (differences between in vivo and in vitro

Sv02) resulting in wide 95 percent confidence inter-

vals cannot be corrected by simple software manipu-

lation. We have presented both 95 percent confidence

limits for both total variability and intrasubject varia-

bility. The 95 percent confidence interval based on

total variability defines the limits of agreement present

in circumstances when oximetry PA catheters are used

to guide therapeutic interventions when the goal is to

identify the patient’s actual Sv02. For instance, if the

therapeutic goal is to ensure that a patient’s actual

Sv02 is 70 percent, it would be necessary for the

Oximetrix 3 catheter to read 76 percent to ensure,

with 95 percent confidence, that the patient’s actual

Sv02 was at least 70 percent. In the case of the

HEMOPRO2, it would be necessary for the catheter

to read 80 percent to ensure the same level of certainty

regarding the target Sv02 of 70 percent. Similar

analogies apply in cases where the decision to treat

may be triggered if certain low Sv02 values are

reached. Conversely, if the oximetry PA catheters are

used as trending devices, a smaller change can be

used to predict an actual trend in the patient’s Sv02.

For instance, all three systems will allow the detection,

with 95 percent confidence, of a directional trend with

a change in readings of approximately ±7 percent.

Thus, a decline in the oximeter system reading from

70 percent to 64 percent likely represents an actual

change in the patient’s condition. It does not predict

the actual value of the patient’s Sv02.

The assessment of agreement between clinical

measurements may also be influenced by the passage

of time.as Previous studies’’5,’6 have demonstrated that

the performance of in vivo oximetry systems may

deteriorate with the passage of time. During our 24-h

study period, there was no statistically significant

change in the bias observed in either Oximetrix 3 or

SAT-2. The HEMOPRO2 group did exhibit a significant

(p<O.O5) change in bias over the course of the study,

paradoxically showing improved performance as the



904 Comparison of Three Pulmonary Artery Oximetry Catheters (Scuden eta!)

study progressed (Fig 2).

The differences in reproducibility (precision) among

the three groups are less clear than the differences in

offset (bias). The overall precision for the Oximetrix 3

and SAT-2 groups was remarkably similar (± 3.07

percent and ± 3.49 percent, respectively), while the

HEMOPRO2 group showed more variation (± 5.24

percent); however, none of the three groups showed a

statistically significant change in precision over the

course of the study.

If one considers the variability between in vivo and

in vitro determinations in individual patients, the

differences in performance are less clear. The square

root of the average intrapatient variance, referred to

here as ESD, is a measure of reproducibility within a

given patient. As shown in Figure 3, the intrapatient

reproducibility of in vivo vs in vitro Sv02 is better

than would be expected if one looked only at the

precision for the entire group (see Table 4). The

comparison of ESD of the three systems shows better

intrapatient reproducibility than would be expected

from group performance. Clinically, therefore, if in

vivo Sv02 is to be used only as a trending monitor,

each of these in vivo systems might be expected to

perform similarly.

Many factors are capable of influencing the perform-

ance of in vivo oximetry, including hemoglobin con-

centration, changes in hemoglobin concentration, ar-

tifact as a result of backscatter from the blood vessel

wall, blood flow velocity, and changes in blood pH.’7”8

Each manufacturer has developed proprietary tech-

nology and computer algorithms in an attempt to deal

with potential error introduction from these and other

factors. Since there were no clinically relevant differ-

ences among the groups for any of the patient variables

shown in Tables 1 through 3, any differences in

performance should be related to differences in the

capabilities of the three systems.

Because therapy may be either instituted or with-

held on the basis of information obtained from this

type of monitor, it is essential to understand the

performance characteristics of available systems. The

overall performance of Oximetrix 3 and SAT-2 com-

pared with CO-oximetry was nearly identical (Table

4). Differences in performance of oximetry PA catheter

systems, therefore, do not appear to be caused by

inherent limitations of two-wavelength systems as

previously thought.’-” Regression analysis and corre-

lation coefficients for Oximetnx 3 and SAT-2 are

similar. Bias and precision are also quite similar in

magnitude; however, there is a directional difference

in bias between the Oximetrix 3 group (negative bias)

and the SAT-2 group (positive bias). While a similar

correlation coefficient and bias were obtained in the

HEMOPROZ group, precision appears worse. Both

ambiguity and RMSE have been proposed as global

indicators of performance relative to a criterion stan-

dard. The Oximetrix 3 group and SAT-2 group have

virtually identical ambiguity (5.05 vs 5.29) and RMSE

values (3.64 vs 3.92). The performance achieved by

the HEMOPRO2 group is apparently worse when

compared by these two tests (ambiguity= 7.52,

RMSE = 5.71).

In conclusion, if knowledge of the correct value for

Sv02 is necessary for clinical decision-making, the

differences in ambiguity, RMSE, and 95 percent

confidence limits based on total variability are impor-

tant. If, however, these devices are used primarily as

trending monitors, where identification of directional

changes in saturation is of paramount importance,

then ESD and 95 percent confidence limits based on

intrasubject variability are most relevant and the

differences in performance are less clear. Without

clinical studies to establish acceptable levels of agree-

ment between PA oximetry catheters and bench

oximeters, no true statisticalconclusions can be drawn

regarding the clinical performance of these devices;

however, the lack of agreement with manufacturers’

published specifications (± 2 percent compared with

CO-oximetry) demonstrated herein and the wide 95

percent confidence limits for total variability indicate

that these devices are probably most useful as trend

indicators. Determination of a patient’s actual Sv02

requires the use of a conventional bench oximeter

such as a CO-oximeter.

APPENDIX

Bench oximetry for the analysis of hemoglobin

oxygen saturation performed on instruments such as

the IL CO-oximeter has become the dc facto criterion

standard. Because of their ease of use and apparent

reliability formal calibrations are seldom performed.

Techniques for determining the oxygen content of

blood directlyu.as (and from that oxygen hemoglobin

saturation) are also subject to inaccuracy.25 Even newer

methods25’� cannot be assumed to be both accurate

and precise. While it would thus appear to be impos-

sible to test the “accuracy” of bench oximetry since

no universally acceptable criterion standard exists, the

reproducibility of repeated measures on a sample with

constant saturation can be performed.

To define the level of reproducibility of repeated

measurements on blood samples with the same oxygen

saturations, we performed the following experiment

in our laboratory. Using an IL-237 tonometer, samples

of fresh whole human blood were tonometered against

analytic-grade gas mixtures composed of 5 percent

carbon dioxide; sufficient oxygen to yield hemoglobin

saturations of approximately 40 percent, 50 percent,

65 percent, and 75 percent; and the balance, nitrogen.

Blood samples were equilibrated for 45 mm at 37#{176}C

at each oxygen value. Ten samples tonometered at



CHEST / 102 I 3 I SEPTEMBER, 1992 905

each oxygen level were analyzed on the IL-282 CO-

oximeter used in our study. The mean ± SD at each

levelwere as follows: 41.4 ±0.4, 52.8± 0.7, 65.1±0.3,

and 75.7 ± 0.4. While this experiment does not deter-

mine whether CO-oximetry has a systematic error (ie,

bias) in determining blood oxygen saturation, the level

of reproducibility is consistent with the manufacturer’s

reported “accuracy” specification of ± 1 percent.

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DOI 10.1378/chest.102.3.896 1992;102; 896-905Chest

P E Scuderi, D L Bowton, J W Meredith, L C Harris, J B Evans and R L Andersonunit patients.

A comparison of three pulmonary artery oximetry catheters in intensive care

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A comparison of three pulmonary artery oximetry catheters in intensive care unit patients