Practical problems in assessing risk for coronary artery bypass grafting

J THoRAc

CARD10VASC SURG

89:673-682, 1985

Practical problems in assessing risk for coronary artery bypass grafting In order to put results of a surgical program in proper prospective, risk factors for the population being treated should be carefully assessed. This paper discltiSeS the practical problerm involved in determining the risk of surgical morta6ty for patients undergoing isolated coronary artery bypass grafting. A risk equation developed by the Collaborative Study in Coronary Artery Bypass was applied to a veterans hospital population. A simplified method of determining left ventricular function from clinical angiography reports was found to be a reasonable substitute for the more complex left ventricular scoring system used by the collaborative study. Results showed the veterans group to be at increased risk, primarily due to an older average age and higher incidence of left ventricular dysfunction.

Gordon L. Pierpont, M.D., Ph.D., Margaret Kruse, Susan Ewald, and E. Kenneth Weir, M.D., Minneapolis. Minn.

h e decision to perform an operative procedure requires careful assessment of the potential risks and benefits involved. Risk is usually assessed empirically by applying knowledge of results from both surgical series published in the literature and the personal experience of the physician or physicians who are about to perform the procedure. The overall mortality rate for a particular procedure is often quoted, and this figure is modified for an individual patient according to known factors he/she may possess that are likely to increase or decrease the expected risk. Well over a decade of experience with coronary artery bypass grafting has provided numerous reports of mortality rates for this operation. However, relatively few have identified specific risk factors for surgical mortality,':" and only one report'? provides a quantitative method for determining this risk. Using multivariate discriminant analysis, the investigators for the Collaborative Study in Coronary Artery Surgery (CASS) developed equations that can be used to predict the risk of operative mortality for patients undergoing coronary

artery bypass grafting. By calculating the risk for each patient undergoing the operation and adding the results for a group of patients, one can project the expected mortality rate for a patient population. Since CASS is a large multi-institutional cooperative study, this allows a good base with which to compare the results of a surgical program. In the veteran's hospitals, a somewhat select group of patients is treated. These patients are very predominantly male and are age grouped according to past periods of heavy military activity by the United States. In order to see if these patients represent a population with a different overall risk for coronary artery bypass grafting compared to a less select group, we applied the CASS equations for predicting operative mortality to a group of patients undergoing isolated coronary artery bypass grafting at the Minneapolis Veterans Administration Medical Center (VAMC). This report describes the practical problems involved in such an analysis and provides simplified methods for assessing overall risk of patients selected for coronary artery bypass. The predictive equations

From the Minneapolis Veterans Administration Medical Center and University of Minnesota Division of Cardiology, Minneapolis, Minn. Supported by the Veterans Administration Received for publication Jan. 10, 1984.

The estimated probability of in-hospital operative mortality (EM) for isolated coronary artery bypass grafting as originally published by CASSIO is calculated according to the equation:

Accepted for publication May 25, 1984. Address for reprints: Gordon L. Pierpont, M.D., Ph.D., Department of Cardiology 618/ll1C, VA Medical Center, 54th St. and 48th Ave. South Minneapolis, Minn. 55417.

EM

= exp(c + b, XI + b 2 X2 + b, X3 + b, x,)/ (1

+ exp[c + b, XI + ... b, x,D

where: 673

The Journalof Thoracic and Cardiovascular Surgery

6 7 4 Pierpont et al.

25

25

female age 50

rna Ie age 50

20

20

...>..J

...

< a:

\'1.

15

15

10

10

5

5

o

~

o

...o w

o

w a: Q.

,.

5

10

15

20

LV SCORE

r

i

5

I

10 LV

I 15

I 20

SCORE

Fig. 1. The effect of left ventricular (LV) score (see text) on predicted operative mortality for isolated coronary artery bypass grafting for a 50-year-old man and woman with two different left ventricular end-diastolic pressures (LVEDP in mm Hg) with or without severe left main coronary artery disease (LMCD).

Table L Coefficients for estimating probability of in-hospital death Original Men

c b, b, b, b.

-9.462 2.716 0.122 0.036 0.0649

XI

= 1

I

Revised Warnell

Men

-6.964 2.946 0.0508 0.0602 0.0411

-9.240 1.328 0.095 0.027 0.070

I

Women

-5.87 0.708 0.038 0.0484 0.0414

if the left main coronary disease is 2: 90%

o if the left main coronary disease is < 90%

X2 = Left ventricular (LV) score as described below x, = LV end-diastolic pressure (mm Hg) X4 = Age in years

The coefficients for this equation as originally published'? were subsequently found to give estimates of operative mortality that were too high, especially for those with left main disease." More accurate coefficients, estimated by logistic regression analysis,* are presented along with the originally published coefficients in Table I. A programmable calculator makes it possible to obtain the estimated surgical mortality for a patient relatively easily despite the complexity of the equation. *Fisher L: CASS Coordinating Center, University of Washington, Seattle, Washington, personal communication.

We used an HIP 97 calculator (Hewlett-Packard Co., Corvallis, Ore.) for this purpose, but many suitable programmable calculators would do as well. The relative importance of the various factors used in determining estimated mortality are not readily apparent by looking at the equation. Therefore, Figs. 1 and 2 were generated by the aforementioned equations with the revised coefficients to provide a better intuitive understanding of the relative weights of the risk factors involved. In attempting to apply this equation to our population, we easily obtained sex, age, presence of severe (::::::90%) left main coronary artery stenosis, and LV end-diastolic pressure. However, LV score was not available, as determining this score necessitates analysis of left ventriculograrns obtained in the right anterior oblique projection according to a specific system uniquely used by the CASS study. This involves dividing the left ventricle into five segments: anterobasal, anterolateral, apical, diaphragmatic, and posterobasal. The systolic motion of each of these segments is scored numerically as follows: 1 = normal, 2 = moderate hypokinesis, 3 = severe hypokinesis, 4 = akinesis, 5 = dyskinesis,and 6 = aneurysm. The numbers are summed for each segment to give an overall LV score, where 5 would be normal ventricular function and larger numbers represent successively greater LV dysfunction. To utilize the CASS equation in assessing the risk of our patient population would require reanalysis of all left

Volume 89

Assessing risk for coronary bypass grafting

Number 5 May. 1985

20

>

20

fern a Ie n o LMCD LVEDP 12

male

no LMCD LVEDP 12

age 70

15

675

15

IoJ 0(

lll:

o

age 60 10

age

70 10

:E

c

W IU

C

..----5

~ a g e 50

: : : : : : : : : : = - - - - e 9 e 40

60 ag e

5

W

_.~

ll: ll.

/1

5

,

10 LV

.______ege 50

~Z:::-age40

15

20

,. i i ' 5 10 15 LV

SCORE

,

20

SCORE

Fig. 2. The effect of LV score (see text) and age on predicted operative mortality for isolated coronary artery bypass grafting for a man and woman without severe left main coronary artery disease (LMCD) and having a normal left ventricular end-diastolic pressure (LVEDPj of 12 rnm Hg.

ventriculograms according to this method. This appeared to be a prohibitively time-consuming task to do retrospectively. Such an analysis could be done prospectively but would require the cooperation of a whole staff of cardiologists to adopt this system of analysis,a method more cumbersome and time-consuming than the normal verbal description of the ventriculographic findings. Such a prospective analysis might also result in a significant delay in being able to analyze the results. We therefore attempted to develop an alternate method of determining LV score for use in the risk equation.

Variability in LV score Using the LV score obtained as described herein allows a quantitative assessment of overall LV function. However, it is not totally objective in that judgment is still required in assessing the functional status of each segment of the left ventriculogram. Usefulness of such a scoring system will depend on several factors, including the degree of correlation of the LV score with other methods of assessing LV function (such as ventriculographic determination of LV ejection fraction), the ease of obtaining the score, and the amount of agreement/ disagreement between different observers reading the same ventriculogram. We therefore determined the interobserver variability in using the LV scoring system to test the reproducibility of the scores obtained. Two experienced cardiac angiographers independently generated LV scores by reading 100 consecutive left ventriculograms of patients undergoing isolated coronary artery bypass at the Minneapolis VAMC. Fig. 3

40

35

30

I-

z

w :E w w

ll:

e0(

25

20

'" 15

10

~f;.

5

0

.X

;I~I:

::::::::;:::

0

2

3

I ~4

INTER-OBSERVER DIFFERENCE IN LV SCORE

Fig. 3. Extent of agreement between two independent observers scoring 100 left ventriculograrns according to the CASS system. In 29% of cases, there was no difference in LV score, and the scores differed by 1 in 35% of cases.

diagrammatically presents the degree of agreement in these readings. The total LV score can range from a minimum of 5 to a theoretical maximum of 30. There was complete agreement between the two observers in

The Journal of

6 7 6 Pierpont et al.

Thoracic and Cardiovascular Surgery

70

80

50 I-

Z

III

2

III III

a:

40

CJ 0<

#-

o

1

2 3 4

0

1 2

3 4

0

1 2

3 4

0

1 2

3

4

0

1

2

3 4,

INTER-OB8ERVER DIFFERENCE IN LV ,8CORE

Fig. 4. Extent of agreement between two independent observers for each ventricular segment analyzed according to the CASS system.

29% of the readings, whereas they differed by one point in 35%, two points in 15%, etc. The interobserver correlation coefficient (r) was 0.81. To determine if the amount of agreement or disagreement was related to the segment analyzed, the percent agreement is presented in Fig. 4 for each ventricular segment. The segment analyzed was not important in affecting interobserver agreement. Fig. 5 shows the correlation between LV score by one observer and ejection fraction obtained by analyzing the left ventriculogram according to the method of Kennedy, Trenholme, and Kasser." Although, as expected, a correlation is clearly present, significant variability exists between the two methods of assessing LV function. Because of the interobserver variability and the variability between two different methods of assessing LV function, we considered the possibility that a more readily available assessment of LV function might be a reasonable substitute for the LV score or ejection fraction without serious loss of accuracy. Fig. 6 compares the LV scores in our patients with those entered into the CASS randomized trial." The results with this sample of 100 consecutive patients suggest that there is a greater incidence of LV dysfunction in the veterans.

Estimating LV score Written reports of left ventriculograms obtained in the right anterior oblique projection were obtained. However, as occurs in many institutions, these reports were dictated by several different cardiologists in a variety of formats. In order for a semiquantitative estimate of LV score to be generated, it was necessary to develop a summary of overall LV function that could be obtained uniformly from the dictated report. Moreover, it was desirable that this be done in such a way that it was not technically difficult and did not require a high level of expertise. Therefore, guidelines were developed for transforming the written report into a single overall assessment of LV function categorized as normal, mild dysfunction, moderate dysfunction, or severe dysfunction. The guidelines used in this transformation are presented in Table II. A cardiovascular technician used these guidelines to summarize the written catheterization reports for the 100 patients whose angiograms were also scored by the CASS system. For all patients in each category of LV function (normal, mild, etc.), the CASS LV scores were averaged and rounded to the nearest whole number. The numbers corresponding to a particular written description could subsequently be used in the equation to

Volume 89 Number 5

Assessing risk for coronary bypass grafting 6 7 7

May, 1985

-.. o

7

c:

C

c: o

.

•

IL

-•

8

5

c:

o

..

4

w

3

2

10

o

5

10

15

LV Scor.

Fig. S. Correlation between total LV score obtained according to the CASS system and angiographically derived ejection fraction.

approximate LV score and generate an estimated probability of in-hospital mortality for each patient without reanalysis of the ventriculogram. The numbers used in the transformation are as follows: normal = LV score of 6; mild dysfunction = LV score of 10; moderate dysfunction = LV score of 12; severe dysfunction = LV score of 14. Using this transformation, we determined the distribution of LV scores approximated from the written report and compared it to the distribution of LV scores obtained according to the CASS method. Distribution of LV scores for the 100 patients analyzed by both methods is presented in Fig. 7. The distribution obtained with the approximated LV scores is a reasonable representation of that obtained by the CASS system. Interobserver variation in predicting operative mortality by estimated LV score In order to determine the effect of interobserver variation in LV score on overall predicted operative mortality, we calculated the operative mortality for 100 patients using seven different methods. Operative mortality was calculated from the LV scores generated by

the two independent readings of the left ventriculograms done according to the CASS system, by the technician's assessment of LV score using the transformation from the catheterization report described above, by separate assessment of two cardiologists using the transformation from the catheterization report, and by two cardiologists using the transformation system applied to independent clinical readings of the left ventriculograms provided by a cardiovascular radiologist. The results of this analysis are presented in Table III for both the original and revised coefficients. According to analysis of variance for repeated measures on the same elements," there is no significant difference among the seven different methods of assessing expected operative mortality for this group of patients with the originally published set of coefficients. However, the decreased range of values obtained with the revised coefficients decreases variance in the statistical analysis, and the differences between observers becomes significant even though the magnitude of the differences remains small. The lower estimated operative mortality obtained with the revised coefficients is readily apparent. The relative consistency of these results suggests the

The Journal of Thoracic and Cardiovascular Surgery

6 7 8 Pierpont et al.

L::::::::I

50

MPLS. VAMC (n:l00) mean:8.8-"3.4

~

.

I

40

c

-'"

0-

~i

30

"0

;

~

'"

. 0

; ;;

;:

20

;;

f1

::

; ;;

i.

;;;; ; ;; ;

; ;;; ;;

LV function CASS (n:763)

:~

:;: :;;;

;

;;

Normal

mean:7.5±3.0

;

;;:

Table II. Assessment of LVfunction from angiographic reports

Mild

~

0-

10

;;

:;: .;.; ;

;;;;

5

j~j

~~~

6-10 LV

Moderate

11-15

16-20

Score

Fig. 6. Comparison of LV scores obtained in 100 consecutive patients at the Minneapolis Veterans Administration Medical Center with LV scores of patients studied by CASSo

I IJ

Approximation From Report

Anglographle Seora

c

•

30

a.

'0 ;; .0

E

: 20

:0

:

Z

10

0

1. Normal 2.' "Slight or minimal hypokinesis" if EF>60%. l , One area of definite hypokinesis (not "slight"), or 2. Localized "slight" hypokinesis if EF < 60%, or 3. Generalized hypokinesis if EF 500/0-60%, or 4. Verbal summary of mild dysfunction I. Two areas of hypokinesis. or 2. Generalized hypokinesis if EF < 50% but >40%, or 3. Localized dyskinesis or akinesis if EF>45%, or 4. Verbal summary of moderate LV dysfunction 1. Dyskinesis or akinesis if EF < 45%, or 2. Generalized hypokinesis if EF < 40%, or 3. Verbal summary of severe LV dysfunction

Legend: LV, Left ventricular. EF, Ejection fraction.

50

= •

Severe

Angiographic data

: ::

:

5 LV Score

Fig. 7. Comparison of the distribution of LV scores obtained by the CASS scoring system to LV scores approximated from angiographic reports.

method devised for approximating LV score is valid, or at least is unlikely to significantly alter the results of predicting operative mortality for a patient population. We therefore applied this method to a broader patient population. Applying the methods to a larger population Using the simplified method of assessing LV score from clinical reports, we calculated the expected opera-

tive mortality for a larger group of patients. From January, 1980, through December, 1982, 420 patients underwent cardiac operations at the Minneapolis VAMC. Of these, 319 had isolated coronary artery bypass grafting and were analyzed for estimated operative mortality. For 22 patients (7%) complete data could not be obtained, either because their records were transferred elsewhere or the records were incomplete. Incomplete records were usually due to the patients receivingtheir cardiac catheterization elsewherewithout an official report being retained on file. The final analysis included 297 patients who had isolated coronary artery bypass grafting. All were men with a mean age of 57.5 ± 7.3 years. This compares to a mean age of 52.6 ± 9.3 years for men entering into the CASS registry and 51 ± 7 years for those who underwent operation in the CASS randomized trial. 15 The age distributions are compared in Fig. 8. The ages of all patients entered in the CASS registry are used here rather than only those who underwent coronary artery bypass, as the latter data are not presented with the risk equations. to The average LV end-diastolic pressure for our patients was 18.4 ± 7.4 mID Hg, and the distribution is shown in Fig. 9. The average LV end-diastolic pressure for the CASS study was not available in their report," so that comparison is difficult. The average LV wall motion score was 7.6 ± 3.1 for surgical patients in the CASS randomized trial, 7.5 ±

Volume 89

Assessing risk for coronary bypass grafting

Number 5

679

May, 1985

E::::::::::::I MPLS. VAMC (n=297) mean=57.3 ~ 7.28 (S.D.)

30

'"....z

•

CASS (n=18,894) me a ne

w ....-

ea.e

.t 9.3

0(

Q.

u.

0

....

20

z

w

o a: w

Q.

10

oL--"'""'T---r"-....,---~ 10 20 30 40

50

70

60

80

AGE (years)

Fig. 8. Comparison of the age distribution in 297 consecutive patients at the Minneapolis Veterans Administration Medical Center undergoing isolated coronary artery bypass grafting to the age of similar patients in the CASS study. .

Table m. Comparison of different methods of predicting operative mortality* Cardial. #1 estimate

Cardial. #2 estimate

Cardiol. #1 calculated

Cardial. #2 calculated

Radial. #1 estimate

Radial. #2 estimate

Mean

3.13 ±4.47

Using original coefficients: grand mean = 3.20, F = 1.07 (not significant) 3.21 3.13 3.09 3.21 ±5.27 ±4.50 ±4.12 ±4.24

3.27 ±4.54

3.33 ±4.55

Mean

2.70 ±2.01

Using revised coefficients: grand mean = 2.76, F = 2.45 tp < 0.05) 2.74 2.71 2.69 2.80 ±2.26 ±2.04 ±2.09 ±2.20

2.85 ±2.13

2.89 ±2.l7

SD SD

Legend: Tech.• Technician. Cardiol., Cardiologist. Radiol., Radiologist. SD, Standard deviation. 'See text for details of different methods.

3.0 for medical and surgical patients combined in the randomized trial, and 7.8 ± 3.8 for all patients in the CASS registry. The LV scores for the group of 6,176 patients used in deriving the risk equation were not reported." Using the transformation from catheterization reports to estimate LV score, our population had a mean LV score of 8.7 ± 3.0. The calculated estimate of operative mortality for our 297 patients undergoing isolated coronary artery bypass grafting was 2.9% according to the original coefficients and 2.7% according to the revised coefficients. These figures are comparable to the estimated mortality in our subpopulation of 100 patients analyzed by multiple methods of obtaining LV score (Table III). The actual

mortality for the total group of 297 patients, defined as any death within 30 days of operation, was 5.1%. This figure contrasts with an actual mortality of 1.8% for men undergoing the same operation in the CASS study. When women were included in the analysis, the CASS mortality was 2.3%.10 Discussion

The equation generated by CASS and used in this analysis was chosen as the simplest approach to obtaining a reasonably objective comparison of the Veterans Administration population undergoing cardiac operations with a similar population from other institutions. The variables predictive of operative mortality in the

The Journal of Thoracic and Cardiovascular Surgery

6 8 0 Pierpont et al.

.....

30

z w 0: -e

...... o

a: w

20

ID

:::E

:)

z

o

10

80

20

40

UNKNOWN

LVEDP

Fig. 9. Distribution of left ventricular end-diastolic pressure (LVEDP) for patients undergoing isolated coronary artery bypass grafting at the Minneapolis Veterans Administration Medical Center.

CASS study depended on the subpopulation examined. Indeed, it may well have been more appropriate to use the additional variables and coefficients included by CASS in their analysis of number of the deaths at a clinic (Table B of appendix in reference 10). However, this analysis becomes more cumbersome to use as it also requires knowing the presence or absence of rales in the chest and the functional class of each patient's angina before the operation. Parenthetically, it should be noted that a typographical error is present in this table. As published in Table B, the presence of rales in the chest would reduce rather than increase expected mortality. This can be corrected by altering the codes in that table such that 1 = rales present and 2 = rales absent. An analysis such as ours becomes still further complicated by recent revision of the predictive variables and coefficients from the CASS data." This analysis not only changed some of the coefficients previously reported but also included several other variables to a total of 11. Included in these 11 variables was a congestive heart failure score and a myocardial jeopardy score, and all of the coefficients were different depending on whether the coronary circulation was left dominant or right dominant. Clearly, in order for this equation to actually be

used in analyzing any patient population not participating in the CASS study would require a herculean effort, a prohibitive amount of time, and extensive resources, whether done retrospectively or prospectively. In analyzing our patient population, we attempted to balance the predictive accuracy of the equations generated by CASS and the practical problems of utilizing the equation. Even choosing the equation with the least number of variables required further simplification of the input data. This simplification (estimating LV score from the catheterization reports rather than directly from the angiograms) could potentially alter the results obtained. However, the interobserver variability present when the angiographic readings obtained according to the CASS system are used suggests that attempts to use a scoring system may actually be no better than the more simple judgment of LV function classifying it as normal, mild, moderate, or severely compromised. The interobserver variability between two cardiologists that we observed using the CASS system is consistent with that obtained in the CASS study when LV motion scores obtained at the quality control center were compared to those at the clinical sites. I? Their interobserver correlation coefficient (r) was 0.83 (n = 869)

Volume 89 Number 5 May, 1985

compared to our value of 0.81 (n = 100). Interestingly, the interobserver variation did not vary in our study depending on the particular LV segment analyzed. This is in contrast to the fmdings of Sheehan and colleagues,18 who found variability to be highest when analyzing the apex. Operative risk factors have changed over the past decade" and are likely to continue to change with new developments in surgical technique and technology as well as evolution in demographic patterns within the patient population. It is likely, for example, that more and more patients with previous coronary artery bypass grafting will develop recurrent severe symptoms over time, and previous operation will likely appear as an important risk factor for bypass grafting. These changes in risk factors over time limit the usefulness of equations such as those of the CASS study to time periods relatively close to the period during which the data were obtained. Our population was examined over the 3 years, 1980 to 1982, whereas the CASS data were obtained on patients undergoing their operations from 1975 through 1978. It is likely that changes between these two time periods are not significant, but that cannot be said with absolute certainty. As previously mentioned, the risk factors identified by CASS depended on the subpopulation analyzed. Similarly, it is quite possiblethat a patient population such as seen at a large Veterans Administration hospital may have characteristics that include risk factors not identifiable in a different population. For example, if a low incidence of severe chronic obstructive pulmonary disease or of renal impairment were found in the CASS population,death in these patients may be so infrequent that presence of these diseases would not become identifiable as a risk factor. On the other hand, if concomitant chronic obstructive pulmonary disease or renal impairment was present with a high incidence in a veterans group, either disease could contribute to surgical complications enough to carry important prognostic significance. Moreover, the judgment of the physicians selecting candidates for operation could affect which factors evolve as carrying significant risk. A patient with severe chronic obstructive pulmonary disease who retains carbon dioxide and requires bronchodilators and long-term oxygen therapy may clearly not be a surgical candidate. However, just how bad chronic obstructive pulmonary disease has to become before cardiac surgery is contraindicated is not always clear, and the cutoff point may differ among physicians. More aggressive physicians might operate on higher risk patients, and their population may demonstrate different risk factors when analyzed statistically.

Assessing risk for coronary bypass grafting 6 8 I

Our analysis suggests that surgical mortality for patients undergoing isolated coronary artery bypass grafting at a veterans hospital, such as the Minneapolis VAMC, can be expected to be higher than that at many other institutions because of the prevalance of risk factors identified by CASSo Of the 15 institutions participating in the CASS study, only one was a veterans hospital, and this could account for the differences in populations we observed. Nonetheless, our actual mortality was still higher than predicted based on the CASS equations. Factors such as those mentioned earlier could explain this difference between expected and observed mortality. Alternatively, factors unique to our institution could account for this difference, much as variation from institution to institution was important in the CASS study.'? In the CASS study one of the most powerful predictors of operative mortality was surgical priority.'? Need for urgent or emergency operation resulted in a higher mortality rate. We did not include this variable in our analysis, as it was not included in the formulas used for estimating operative risk. We believe it is unlikely that adjustments for surgical priority would dramatically alter the results of analyzing our patient population, as a very small percent of our patients are operated upon under emergency conditions. Despite these drawbacks, the analysis we performed probably represents the most objective way presently available to assess risk in a patient population undergoing an operative procedure. The results of our analysis suggest that the population of veterans at the Minneapolis VAMC represent a group at higher risk for coronary artery bypass grafting than those in the CASS study. The reasons for this are clear by examining the independent risk factors in the two populations. The veterans were older and had a greater incidence of LV dysfunction. We gratefully acknowledge the secretarial assistance of Camille Quaglio. REFERENCES Oldham HN Jr, Kong Y, Bartel AG, Morris 11 Jr, Behar YS, Peter RH, Rosati RA, Young WG Jr, Sabiston DC Jr: Risk factors in coronary artery bypass surgery. Arch Surg 105:918-923, 1972 2 Hammermeister KE, Kennedy JW: Predictors of surgical mortality in patients undergoing direct myocardial revascularization. Circulation 49,50:Suppl 2:112-115, 1974 3 Loop FD, Berrettoni IN, Pichard A, Siegel W, Razavi M, Effier DB: Selection of the candidate for myocardial revascularization. A profile of high risk base on multivariate analysis. J THORAC CARDIOVASC SURG 69:40-51, 1975

The Journal of Thoracic and Cardiovascular Surgery

6 8 2 Pierpont et al.

4 Jones EL, Craver JM, Kaplan JA, King SB III, Douglas JS, Morgan EA, Hatcher CR Jr: Criteria for operability and reduction of surgical mortality in patients with severe left ventricular ischemia and dysfunction. Ann Thorac Surg 25:413-424, 1978 5 Brawley RK, Merrill W, Gott VL, Donahoo JS, Watkins L Jr, Gardner TJ: Unstable angina pectoris. Factors influencing operative risk. Ann Surg 191:745-750, 1980 6 Pelletier C, Cossette R, Dontigny L, Mercier C, Page A, Verdant A: Determinants of mortality following coronary bypass surgery. Can J Surg 23: 199-204, 1980 7 Jones EL, Craver JM, King SB III, Douglas JS, Bradford JM, Brown CM, Bone DK, Hatcher CR Jr: Clinical, anatomic and functional descriptors influencing morbidity, survival and adequacy of revascularization followingcoronary bypass. Ann Surg 192:390-402, 1980 8 Miller DC, Stinson EB, Oyer PE, Jamieson SW, Mitchell RS, Reitz BA, Baumgartner WA, Shumway NE: Discriminant analysis of the changing risks of coronary artery operations: 1971-1979. J THORAC CARDIOVASC SURG 85:197-213, 1983 9 Chaitman BR, Rogers WJ, Davis K, Tyras DH, Berger R, Bourassa MG, Fisher L, StoverHertzberg V, Judkins MP, Mock MB, Killip T: Operative risk factors in patients with left main coronary-artery disease. N Engl J Med 303:953957, 1980 10 Kennedy JW, Kaiser GC, Fisher LD, Maynard C, Fritz JK, Myers W, Mudd JG, Ryan TJ, Coggin J: Multivariate discriminant analysis of the clinical and angiographic predictors of operative mortality from the Collaborative Study in Coronary Artery Surgery (CASS). J THORAC CARDIOVASC SURG 80:876-887, 1980. II Kennedy JW, Kaiser GC, Fisher LD, Fritz JK, Myers W,

12

13

14

15

16

17

18

Mudd JG, Ryan TJ: Clinical and angiographic predictors of operative mortality from the Collaborative Study in Coronary Artery Surgery (CASS). Circulation 63:793802, 1981 Fisher LD, Kennedy JW, Davis KB, Maynarad C, Fritz JK, Kaiser G, Myers WO, and the Participating CASS Clinics: Association of sex, physical size, and operative mortality after coronary artery bypass in the coronary artery surgery study (CASS). J THORAC CARDIOVASC SURG 84:334-341, 1982 Fisher L, Kennedy JW: Operative mortality in coronary bypass grafting. J THORAC CARDIOVASC SURG 85:146-147, 1983 Kennedy JW, Trenholme SE, Kasser IS: Left ventricular volume and mass from single-plane cineangiocardiogram. A comparison of anteroposterior and right anterior oblique methods. Am Heart J 80:343-352, 1970 The Principal Investigators of CASS and Their Associates: The National Heart, Lung, and Blood Institute Coronary Artery Surgery Study (CASS). Circulation Monograph 79, 63:Suppl 1:1-81, 1981 Winer BJ: Statistical Principles in Experimental Design, ed 2, New York, 1971, McGraw-Hill Book Company, Inc., pp 261-308 Wexler LF, Lesperance J, Ryan TJ, Bourassa MG, Fisher LD, Maynard C, Kemp HG, Cameron A, Gosselin AJ, Judkins MP: Interobserver variability in interpreting contrast left ventriculograms (CASS). Cathet Cardiovasc Diagn 8:341-355, 1982 Sheehan FH, Stewart DK, Dodge HT, Mitten S, Bolson EL: Variability in measuring regional left ventricular wall motion from contrast angiograms. Am Rev Diagn 3:7982, 1983

Bound volumes available to subscribers Bound volumes of THE JOURNAL OFTHORACIC AND CARDIOVASCULAR SURGERY are available to subscribers (only) for the 1985 issues from the Publisher, at a cost of $61.45 ($77.10 international) for Vol. 89 (January-June) and Vol. 90 (July-December). Shipping charges are included. Each bound volume contains a subject and author index and all advertising is removed. Copies are shipped within 30 days after publication of the last issue of the volume. The binding is durable buckram with the JOURNAL name, volume number, and year stamped in gold on the spine. Payment must accompany all orders. Contact The C. V. Mosby Company, Circulation Department, 11830 Westline Industrial Drive, St. Louis, Missouri 63146, USA; phone (800) 325-4177, ext. 351. Subscriptions must be in force to qualify. Bound volumes are not available in place of a regular JOURNAL subscription,