probit analysis of burn mortality in 1,831 patients:

Probit Analysis of Burn Mortality in 1,831 Patients:Comparison with Other Large Series

MAx S. RrITrENBURY,* M.D., RHODA W. MADDOX, B.S., FiR H. ScHmIDT,** M.S.,WILLiAM T. HAM, JR., PH.D., BoYD W. HAYNES, JR., M.D.

From the Departments of Surgery and Biophysics, and the Strauss Surgical ResearchLaboratories, Medical College of Virginia, Richmond, Virginia

CASE RECORDS of 1,831 patients who wereconsecutively treated on the Burn Unit atthe Medical College of Virginia from 1949through 1962 are presently undergoing anextensive statistical review. The purpose ofthis report is to present the results of aprobit analysis of these records relatingthe age of the patient and the extent ofthe body surface area that was burned tothe mortality rate. Weidenfeld l showedover 60 years ago that the prognosis forthe survival of burn patients could be re-lated to both the extent of the burn andthe age of the patients, but it remained forBull and Squire,2 and later Bull and Fisher,3to define this relationship more fully byusing the probit transformation of theirmortality data to convert it into linearform and to improve the goodness of fit.This same method has since been used toreport mortality data by other authors,' 7the most recent report being that by Pruittet al.9 of a study of a large series of pa-

Submitted for publication August 8, 1965.* Present address: Department of Surgery,

Medical College of South Carolina, Charleston,South Carolina.

** Present address: Department of Biometry,Emory University, School of Medicine, Atlanta,Georgia.

Funds from these studies were made availablefrom the Department of Surgery Research andDevelopment Funds; the National Institutes ofHealth, Research Grant FROO016-02 supportingthe Clinical Computer Center at this institution;and the Defense Atomic Support Agency ContractDA-49-146-XZ-423.

tients (1,100) treated at the Army SurgicalResearch Unit, Brooke Army Hospital.

Material and MethodsThe Burn Unit of the Medical College

of Virginia is responsible for the care ofall patients admitted to this institution withthermal injuries. Most of the patients arefrom Richmond or are referred from thesurrounding urban areas. One thousand andforty-nine of the patients whose records arebeing reviewed were admitted within lessthan 5 hours after they had sustained theirinjury, and only 7 per cent of the patientswere referred from another hospital wheresome form of primary treatment had beengiven.

Until 1956 most patients were treatedby a "closed" method of wound care, butsince 1958 most burn wounds have beentreated by an "open" method. At present,patients are taken to the Burn Unit dressingroom immediately after arrival. Here theextent of the injury is determined and ex-pressed as the percentage of the body sur-face area involved by both second andthird degree burns, and immediate resus-citative therapy is started. This includesadministering intravenous fluids (modifiedEvan's formula), tetanus toxoid or anti-serum, and prophylactic antibiotics, andcleaning and debriding the burn wound.These resuscitative principles have beenunchanged during the time covered by thisstudy. Systemic infections continue to betreated with the "appropriate" antibiotic

123

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Annals of SurgeryJuly 1966

as determined by in vitro bacterial cultureand sensitivity tests. Every effort is madeto obtain a clean bum wound ready forgrafting at the earliest possible moment. Anincreasing number of selected wounds arebeing treated by primary excision andgrafting.The record of each patient was ab-

stracted at the time of their discharge(either from the hospital or the Out-patientClinic), and the information was placed onan IBM Source Document designed byDr. E. I. Evans' coworkers following hisdeath. This information was then trans-ferred to IBM tabulation cards, and thenecessary computations were performed bythe Clinical Computer Center. All pa-tients dying with a thermal injury havebeen included, although a certain numberprobably died due to associated injuries.The relationship between the extent of

the body surface area that was burned,the age of the patient, the race and sex ofthe patient, and the patient's time of entryinto the hospital to the mortality rate hasbeen determined using both standardstatistical methods and the probit analysisdescribed by Finney.4 5

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PROBIT ANALYSIS OF BURN MORTALITY IN 1,831 PATIENTS

100

80

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at

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20

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87

7

6

k

ci>5

QC

4

I W- EI I I 1 30 20 40 60 80 100

% AREA BURNED

FiG. 1. Probit transformation of MCV mortalitydata. See text for explanation.

analytical technic showed a change in theeffect of age on mortality at the age of 40years. The number of patients dying andthe per cent mortality is shown for eachgroup.

Three hundred and sixty-nine patientsdied, an overall mortality rate of 20.15%.The mortality rate increased both withthe patient's age and the percentage ofthe total body surface area that was

burned. No patients survived with a burninvolving more than 85% of the totalbody surface area, and only nine pa-tients survived with a total body surfacearea burn of 55% or greater. The 15- to 39-year-old patients were the most tolerantof severe burns. Mortality rate was very

low for patients less than 45 years of agewith 25% or less of body surface area

burned, but it became appreciable in olderpatients with the same amount of burn. Asreported by Moyer,8 these older patientstolerated a burn wound very poorly, andno patient 65 years or older lived with a

burn greater than 25% of the total bodysurface area. Eighteen per cent of thisoldest group of patients with 4% (or less)body burns died.

Probit Analysis. The data was next sub-jected to the probit transformation noted

6

K

0-

OS

4

3

2

yrs.

0 20 40 60 80 100O/* TOTAL AREA BURNED

FIG. 2. Mortality probit lines for different agegroups related to the total body surface areaburned.

earlier, and the overall results of this anal-ysis are shown in Figure 1. In this figurethe dashed line represents a rough plot ofmortality rate against per cent of the totalbody surface area that was burned. Thesolid sigmoid curve shows the "smoothed"data and the straight line shows the probittransformation of the sigmoid curve. Probitvalues are given on the right ordinate. Thepercentage mortality and the probit scaleare not linearly related because of dif-ferences in their true values. Thus a probitvalue of 5.0 represents a mortality rate of50%o, but probit values of 3.0 and 7.0, re-

spectively, represent mortality rates of 2%oand 98%o. The use of this transformationmakes it more convenient to fit the straightline to this raw data by the method ofleast squares in order to obtain an accurateequation to express its slope. This equationin turn can then be used both to verifythe fit of other data and to predict themortality for a given area of burn.

Effect of Age and Severity of BurnsMortality probit lines for the different

age groups of patients are shown in Fig-

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ure 2. The slope of each line representsthe tolerance of that group of patients tothe burn injury. It is obvious that the 15- to39-year-old age group tolerate the burnsomewhat better than any of the othergroups, and the 65+ year-old age groupshow the least tolerance.

It is frequently very difficult or impos-sible to delineate areas of second degreeburn from those with third degree injuries,and it is also very difficult to separate dif-fernnt effects upon the patient. The role ofinfection in converting areas of deepsecond-degree injury into full-thickness in-juries is also of importance,6 and, as yet,there is no data available on whether or notthe "converted" full-thickness injury has adifferent effect upon mortality. Most pa-tients have a combination of these twotypes of burns. Five hundred and twenty-four of 1,931 patients studied had no areasof third degree burn, and only 10 (1.98%o)died. Causes of death in these patients withsecond degree burns were delerium tremensin one patient with a 2%o surface area burn;respiratory smoke burn in one 51-year-oldpatient who died less than 24 hours follow-ing admission; pneumonia and acute myo-cardial failure in an 86-year-old man whohad a 4% surface burn and a body tem-perature of 26.10 C. on admission; aspira-tion from a feeding tube in a 3-year-old child with a 31% body burn, andbronchopneumonia and sepsis in 39- and73-year-old males with 45% and 46% deepsecond burns, respectively. Causes of deathin the remaining four patients could notbe determined.As reported earlier 10 this low mortality

rate associated with a partial thickness burnis striking, and therefore mortality datafor patients with any amount of third de-gree burn has been determined and ispresented in Table 2. The patient group-ings are the same as those used in Table 1,but the degree of surface area burn refersto the extent of third degree burn and notto the total body surface area burn. Two

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Volume 164Number 1

PROBIT ANALYSIS OF BURN MORTALITY IN 1,831 PATIENTS 127TABLE 3. Comparison of Probit Equations and LA5o's by Area Burned

Age (yr.) Total Area LA50 3rd Degree LA50

0-4 y = 1.824 + 0.0881 x 36.1% y = 2.944 + 0.0745 x 27.6%5-14 y = 2.638 + 0.0569 x 41.5 y = 3.044 + 0.0587 x 33.30-14 y = 2.131 + 0.0733 x 39.1 y = 2.924 + 0.0673 x 30.815-39 y = 2.346 + 0.0574 x 46.2 y = 2.849 + 0.0680 x 31.615-44 y = 2.422 + 0.0580 x 44.5 y = 2.876 + 0.0709 x 30.040-44 y = 3.131 + 0.0488 x 38.3 y = 2.705 + 0.1125 x 20.440-64 y = 2.963 + 0.0638 x 31.9 y = 3.209 + 0.0834 x 21.545-64 y = 2.945 + 0.0663 x 31.0 y = 3.310 + 0.0786 x 21.565+ y = 3.893 + 0.0864 x 12.8 y = 2.577 + 0.2834 x 8.5

Overall y = 2.894 + 0.0571 x 36.9 y = 3.344 + 0.0655 x 25.3

y = probit; x = % area burned.

groups of patients, aged 5-14 and 40-44,with smaller third degree burns were theonly ones with no mortality. If this table iscompared to Table 1 it is obvious that themortality rate increased for each of thegroups, and total overall mortality rose to27.5%. Mortality rates were very highwhen the area of third degree burn wasgreater than 25% for the 0-44-year-old agegroups, greater than 15% for the 45-64-year-old age groups, and more than 5%1for the 65+ year-old age group. Only onepatient older than 40 years of age surviveda third degree burn wound of greaterthan 24%.The severity of this type of wound is

again illustrated in Figure 3, showing themortality probit lines for the major agegroups. All lines are shifted to the left, andlines for the two older age groups aresteeper than those calculated on the basisof total body surface area burn.Equations for the mortality probit lines

shown in Figures 2 and 3 and the calcu-lated LA50 values * for both the amountof total body surface area burn and theamount of third degree burn for the dif-ferent age groups are given in Table 3.LA50 values for the 0-4-year-old age groupare lower than those for the 5-14-year-old

age group, but the difference in thesevalues is not statistically different at thep < .05 level.5 The effect of the age is moreapparent when the LA50 value for the pa-tients in the 15-39-year-old age group iscompared to that for the 40-44-year-oldage group. The LA50 value for the totalbody surface area burn is 46.2% (95%confidence limit range 41-53%) for the15-39-year-old age group, but this falls to38.3% for the 40-44-year-old age group.There were too few patients in the lattergroup to calculate meaningful statisticaldifferences. Difference in the LA50 valuesis even greater for patients in this agegroup with third degree burns. As expected,LA50 values decrease for older patients.The total body surface area burn LA50value range (95% confidence limits) forthe 40-64-year-old age group is 24-40%,and for the 45-64-year-old group 23-38%,again illustrating that patients between theages of 40-44 years tolerate the burn woundin a manner similar to that shown by olderpatients. It is notable that the differencebetween the LA50 values for both typesof bum wounds in the 65+ age group isless than for younger age groups, suggest-ing that other factors assume a relativelymore important role in these patients. De-spite the facts that LA50 values for the percent of the body surface area involved bythird degree bums were lower, and that

* The percentage of the total body surfacearea that produces a 50% mortality rate wheninvolved by a thermal injury.


8

7

6

K5

0k0k

5

4

4 _/

3r

a.0 20 40 60 80 100

%o 3rd DEGREE AREA BURNED

FIG. 3. Probit transformation of data for percent of body surface area involved by third degreewound.

patients with second degree injuries hada very low mortality rate, subsequent datathat will be presented is based on the percent of total body surface area burned

8

15-39 YRS.

7 ,40-44 YRS.

//w6/

5 ,/w CONFIDENCE BANDS

K_ X.S# *--.15-39 YRS.Q~~~44

3~~~~~~~4

0 20 40 60 80 100

% TOTAL AREA BURNED

FIG. 5. Mortality probit line with 95%o confi-dence limits for 15-39-year-old age group, andprobit lines for 40-44-year-old age group.

3

2


0-4 YRS.

CONFIDENCE BANDS__. 5-14 YRS.._ 0-4 YRS.

0 20 40 60 80 100

% TOTAL AREA BURNED

FiG. 4. Mortality probit lines for 0-4- and5-14-year-old age groups with 95% confidencelines.

because previous authors have calculatedtheir data on this basis.

Figure 4 shows the mortality probit linesfor the 0-4- and 5-14-year-old age groupswith their overlaping 95%o confidence limitlines, again showing that the response ofthese two age groups to the burn injury issimilar. Figure 5 shows the lines for the15-39- and 50-44-age groups. This again

100

A 80

% 60

40

at 20

0

MORTALITY

0 20 40 60 80

AGE IN YEARSFIG. 6. Mortality contours showing the mortality

rates for the different ages and percentage of totalbody surface burns.

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7

6

K-0~Q~ 5


FIG. 7. Mortality grid according to age and per cent of total body surface area burned.

illustrates the changing tolerance to theburn wound that occurs at the age of 40years. As noted earlier another analyticalmethod (discriminate function analysis)has also been used to determine the effectof age upon mortality, and the same effectwas apparent.10Equal mortality contours have been con-

structed from the mortality rates, and theyare shown in Figure 6. The age in years isshown on the abscissa and the per centof the total body surface area that was

burned is on the ordinate. Thirty-year-oldpatients with a 50% total body surfacearea burn would therefore be expected tohave a mortality rate of approximately65%o, but the same burn in 10-year-oldpatients would have a mortality rate ofapproximately 75%. This effect was notshown for younger age groups in data pre-

sented by Bull and Fisher, but it was pres-

ent in data presented by Winterscheid andMerendino 12 and Pruitt et al.9

Figure 7 is the mortality grid constructedfrom mortality curves shown in Figure 6.

Finer divisions for both age of the patientand per cent of the total body surface area

that was burned are used. The numbersin this grid are approximations only, andsome patients with a value of 0 would die

8 r-

7

6

k

I0 5

4- WHITE MALE--a WHITE FEMALE

NEGRO MALE___ NEGRO FEMALE

CONFIDENCE BANDS--------- WHITE MALE

0 20 40 60 80 100

% TOTAL AREA BURNED

FIG. 8. Mortality probit lines according to race, sex,and per cent of total body surface area bum.

Volume 164Number 1

GRID OF APPROXIMATE MORTALITY PROBABILITIES

129

% Body Age - Yrs.Area Burned

0-4 5-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 60-64 65+

68 or more 1 1 1 1 1 1 1 1 1 1 1 1 1 1

63 - 67 1 1 1 .9 .9 .9 .9 1 1 1 1 1 11158-62 1 1 .9 .8 .8 .8 .8 .9 1 1 1 1 1 1

53 - 57 .9 .9 .8 .8 .7 .7 .7 .8 .8 .9 l1 1 1 1

48-52 .8 .8 .7 .7 .6 .6 .6 .7 .8 .8 .9 1 1 1

43 - 47 .7 .7 .6 .5 .5 .5 .5 .6 .7 .7 .8 ,911 1

38 - 42 .6 .5 .5 .4 .4 .4 .4 .5 .5 .6 .7 .8 .9 1

33 -37 .5 .4 .3 .3 .3 .3 .3 .4 .4 .5 .6 .7 .9 1

28 - 32 .4 .3 .2 .2 . I .2 .2 .3 .3 .4 .5 .6 .7 .9

23 - 27 .2 .2 .1 .1 .1 .1 .1 .2 .2 .3 .3 .5 .6 .8

18_-22 .1 .1 I 0 D 0 0 -1 .1 .1 .2 .2 .3 .5 .7

13_- 17 0 0 0 0 0 0 0 0 0 -I. .1 .2 .3 .5

8- 12 0 0 0 0 0 0 0 0 0 0 0 .1 .2 .3

3-7 0 0 0 0 0 0 1) 0 0 0 0 0 .1 .2

0-2 0 0 0 0 0 0 0 0 0 0 0 0 0 .1

15.1i

RITTENBURY AND OTHERS Annals of SurgeryJuly 1966

TABLE 4. Nutmber of Patients and Afortality by Race and Sex

White Males Negro Males WNhite Females Negro FemalesAge(yr.) No. Pts. Deaths %70 Mort. No. Pts. Deaths %7 Mort. No. Pts. Deaths % Mort. No. Pts. Deaths % Mort

0-4 142 9 6.3 81 7 8.6 125 6 4.8 145 33 22.75-14 49 3 6.1 84 13 15.5 44 4 9.1 98 29 29.615-39 162 20 12.3 166 27 16.3 65 16 24.6 113 17 15.040-64 105 19 18.1 144 38 26.4 46 13 28.3 72 16 22.265 + 32 10 31.3 77 44 57.1 27 16 59.2 54 29 53.7Total 490 61 12.4 552 129 23.4 307 55 17.9 482 124 25.7

TABLE 5. LA5o's by Race and Sex

Age White Negro White Negro(yr.) Male Male Female Female

0-4 44.2% 35.9% 36.5% 31.2%0-14 48.8 37.4 36.4 33.515-39 48.8 39.5 46.0 53.740-64 39.2 25.2 29.7 25.665+ 17.4 9.6 16.6 11.9

Overall 44.8 32.5 39.3 35.1

while an occasional patient with a value of1 would survive. This grid differs from thatof Bull and Fisher because it shows a de-creased survival in the younger age groupalthough, as noted above, the difference isnot statistically significant. It is otherwiseremarkably similar to theirs.

Effect of Race and Sex

Seven hundred and ninety-seven (43.5%)of the patients were Caucasians, and 1,042(56.9%o) of the patients were males. Aneffort was made to determine whether ornot the race or sex or both had a significanteffect upon mortality rates. Results are sum-marized in Table 4, with patients dividedaccording to race, sex and age. If totalnumbers of white patients are comparedto total numbers of Negro patients, thereis a significant difference in mortality rates(p < .001),** but there is no significant

difference in overall mortality rates accord-ing to sex of the patient, disregarding race.Data in Table 4 shows that the overall

mortality rate for male patients in any agegroup except for 0-4-year-old white femaleswas less than that for the other patientclassifications. In the 0-14-year-old agegroups it is significantly lower than themortality rate for Negro females in thesame age groups. White females have thehighest mortality rates when the other agegroups are compared however.

Relationships between the effect of raceand sex of the patient on mortality are bestshown by the probit lines for over-all mor-tality rates for these patients (Fig. 8) andthe LA50 values for each age group (Table5). The numeric value for the 95% con-fidence limits for the white male LA.50 valueis 39 to 56%, widely overlapping the samevalues as those for the white female (33 to50%). This is also shown in Figure 8 wherethe superimposed confidence limits for thewhite male probit line encompass the probitline for the white female. The respectiveLA50 95% confidence limits for the probitline for Negro females are 30 to 43%o, andfor Negro males 31 to 40%. Although thereis some overlapping of these limits (andthese are not shown in Figure 8) the meanLA50 values of these patients are statis-tically different from those of white males.A statistical comparison of values shownin Table 5 for different age groups showsthat the Negro female LA50 value is statis-tically significantly lower than that of the

130

II Test for significance of difference in pro-portions.

PROBIT ANALYSIS OF BURN MORTALITY IN 1,831 PATIENTS 131TABLE 6. Time of Admission Post Burn

2 Hrs. 2-5 Hrs. 6-23 Hrs. 1D-6D 1 Wk.+

Area No. No. % No. No. % No. No. % No. No. % No. No. %Burned Pts. Deaths Mort. Pts. Deaths Mort. Pts. Deaths Mort. Pts. Deaths Mort. Pts. Deaths Mort.

0-24 503 22 4.4 238 17 7.1 104 7 6.7 156 9 5.8 320 11 3.4

25-64 123 77 62.5 112 73 65.2 17 15 88.2 13 8 61.5 2 2 100.065 + 48 48 100.0 25 23 92.0 2 2 100.0 0 0 0 0 0 0

Total 674 147 21.8 375 115 30.7 123 24 19.5 169 17 10.0 322 13 4.0

TABLE 7. LA5o's by Time of Admission Post Burn

Age(yr.) 2 Hr. 2-5 Hr. 6-23 Hr. 1-6 Da. 1 Wk.+

0-4 35.4 41.2 31.715-39 35.8 48.340-64 32.0 28.4 24.2 35.965 + 13.7 15.2 21.7* 11.9

Overall 35.2 37.4 28.9 34.1 36.8

* 50 + yrs.

white male in the 0- to 14-year-old agegroups. There is no significant difference invalues for other age groups.5There is insufficient data to determine

why this racial difference exists, althoughthe lower socio-economic status of theNegro in the population from which thesepatients were taken may explain this dif-ference.

Mortality Rates by Postburn Timeof Admission

The effect of the time of admission tothe hospital after the patient is burnedupon the mortality rate is shown in Tables6, 7 and 8. Table 6 shows the per centmortality for different times of admissionin relation to percentage of the total bodysurface area burned. Mortality rates were

low and very similar for patients with a

burn of 24%o or less of total body surfacearea. In addition there was very little dif-ference in mortality rates for patients withlarger burns, although 15 of 17 patientswith 25 to 64% total body surface area

burns that were admitted from 6 to 23

hours after injury died. Lowered mortalityrate for 491 patients admitted 24 or more

TABLE 8. Overall Probit Equations by Timeof Admission Post Burn

Time of Admission Equation LA50

2 Hr. y = 2.519 + 0.0705 x 35.2%2-5 Hr. y = 3.068 + 0.0517 x 37.46-23 Hr. y = 2.608 + 0.0828 x 28.91-6 Da. y = 2.941 + 0.0604x 34.11 Wk.+ y = 2.871 + 0.0578 x 36.8

y = probit; x = area burned.

hours after injury is due to the fact thatonly 3% of these patients had the moresevere burn wounds. This, plus the fact thatonly two of 75 patients admitted with burnsinvolving more than 65% of body surfacearea were admitted after one day, eitherreflects the immediate referral of moreseverely burned patients to the Burn Unitwithout a significant delay in surroundinghospitals or communities or death of thesepatients prior to admission.

Table 7 lists the LA50 values for the samegroups that contain sufficient numbers ofpatients for analysis and that were listed inTable 6. Changes in the LA50 values rela-tive to age of the patient are similar tothose noted earlier, but there is no sig-nificant difference relative to the time ofadmission to the hospital. The overall probitequations for admitting times are shown inTable 8, and LA50 values are very similar.This data shows that the transit time fromthe place of injury to the Burn Unit isshort for the majority of these patients, thatthe more severe burns are probably sentdirectly to this Unit, and that those pa-tients with larger burns who are not

Volume 164Number 1


TABLE 9. Survival Time of Fatal Cases 1949-1962

Mortality MeanProbability 0-2 3-7 8-21 22 or Total Survivalfrom Grid Period Days Days Days More Days Cases (Days)

1949-1952 1 1 7 5 14 32.10-0.4 1953-1957 5 7 8 23 43 31.9

1958-1962 2 8 21 19 50 21.9

1949-1952 3 8 13 4 28 11.50.5-0.9 1953-1957 6 11 14 6 37 12.2

1958-1962 9 16 27 13 65 14.6

1949-1952 16 10 2 1 29 3.91.0 1953-1957 11 22 8 3 44 6.7

1958-1962 11 16 14 4 45 9.7

brought to the unit shortly after burningprobably do not survive long enough to bebrought there later.

Change in Survival Timein Fatal Cases

An effort has been made to determinewhether or not there has been an increasein the survival time of patients dying ofthe burn wound within the period of timecovered by this study. Patients were di-vided according to mortality probability as

determined from Figure 5, and by threenearly equal time periods covered by thisstudy. Table 9 shows this division and thenumbers of patients that survived for vari-ous times in each of these groups. Onehundred and seven patients with a mor-

tality probability of 0 to 0.4 died, and theproportion of cases in each time periodstudied that survived for 8 or more daysis essentially the same. However the mean

survival time in this group of patients diddecrease from an average of 32.1 days dur-ing the first 4 years to 21.9 days during thelast 5 years of this study. These patientswould all have total body surface area

burns of less than 42%o, and therefore thechange could probably be due to associatedinjuries, although the information to proveor disprove this assumption is not yetavailable.

There were 130 patients who died thathad a high probability of dying (0.5 to0.9), and the mean survival time for thesepatients was essentially unchanged duringthe time periods covered by this study. Theproportion of patients that survived for 8or more days was also remarkably uniformfor each time period.One hundred and eighteen patients had

a mortality probability of 1.0, implying onlya minute statistical chance for survival. Inthe years 1949 through 1952, 55% of thesepatients died within the first 2 days follow-ing injury, but this percentage fell to 25%ofor the latter two periods. The survival timegradually increased, and whereas only 14%and 25% of the patients seen from 1949 to1952 and from 1953-1957, respectively, sur-

vived for 8 or more days, 40% of the pa-

tients seen in 1958-1962 survived for thistime. The mean survival time increasedfrom 3.9 days in the first time period to 9.7days in the latter.These data would show that, despite

other evidence that overall mortality hasnot decreased, progress has been made inprolonging life of patients with major burns.

Mortality Rate by Year of Admission

The mortality rate was also determinedfor patients according to year of admission,these being divided into the same time pe-

132

Volume 164Number 1

PROBIT ANALYSIS OF BURN MORTALIT1Y IN 1,831 PATIENTS 133TABLE 10. No. of Patients and Mortality by Year of Admission

1949-1952 1953-1957 1958-1962

Area Burned No. No. % No. No. %7 No. No. %%0 Pts. Deaths Mort. Pts. Deaths Mort. Pts. Deaths Mort.

0-4 101 0 0 211 8 3.8 137 6 4.4

5-14 167 6 3.6 238 13 5.5 276 16 5.8

15-24 72 5 6.9 97 13 13.4 114 17 14.9

25-34 31 15 48.4 48 21 43.7 61 29 47.5

35-44 19 6 31.6 27 17 63.0 38 30 78.945-54 15 9 60.0 21 17 80.9 18 15 83.355-64 19 15 78.9 9 9 100.0 20 18 90.0

65-74 9 7 77.8 8 7 87.5 12 12 100.075-84 6 6 100.0 9 9 100.0 5 5 100.0

85-94 3 3 100.0 9 9 100.0 9 9 100.095 + 5 5 100.0 5 5 100.0 5 5 100.0Total 447 77 17.2 682 128 18.8 695 162 23.3

TABLE 11. Probit Equations and LA5o'sby Year of Admission

Years Equation LA 5o

1949-1952 y = 2.773 + 0.0503 x 44.3%1953-1957 y = 2.954 + 0.0562 x 36.41958-1962 y = 2.898 + 0.0632 x 33.2

y = probit; x = area burned

riods used in Table 9. The number of pa-tients, per cent of total body surface areaburned, and overall percentage mortalityare shown in Table 10. Patients with 0 to4% total body surface area burns had nomortality until 1953, and since that timeonly 14 of 348 patients (about 4%) died.Mortality rates for patients with 15 to 24%oand 35 to 44%o body surface area burns hasdoubled in the last 10 years covered by thisstudy, and mortality rates for all of thelarger surface burns has increased. Theoverall mortality rate was only 17.2% forthe first 4 years covered by this study, butthis had risen to 23.3%o for the last 5 years.

Table 2 lists the probit equations and theLA50 values by the different periods of ad-mission. The LA50 value for the first 4 yearsof the study was 44.3%, but this decreasedsignificantly to 33.2%o for the last 5 years.5Probit lines and 95%o confidence limits forthese years are shown in Figure 9. Theprobit lines themselves lie almost com-

1958-628

I /t,t/--- 1949-52

0 20 40 60 8~~~~~~~~~~~0O

% CONIDNCBURNED

fiec iit 995 and 19496, or952 pe

4 AY ~ I

FaiG. 9at Mralit prbi linaesntdecreas con-

cetheftotleacbodypatientstoea giverbur

exet fordifiurnstoflstccont 20r ofi totalgboing thsurae are.pTeseodatitial anlshow tAtthereihastionodaa prefinitedicesin mor-1

usingl thesetypessentialytiestiameanlyes.ent

age (23.97o, 19.917o and 24.27o) of admis-


TABLE3 12. Comparison of Patients and Mortality

Bull and Fisher Brooke MCVAge(yr.) No. Pts. Deaths % Mort. No. Pts. Deaths % Mort. No. Pts. Deaths % Mort.

0-14 1366 46 3.4 238 43 18.1 769 104 13.515-44 967 28 2.9 806* 124 15.4 590 101 17.145-64 330 24 7.3 56** 22 39.3 282 65 23.065% 144 63 43.7 190 99 52.1Total 2807 161 5.7 1100 189 17.2 1831 369 20.1

* 15-49 yrs.** 50+ yrs.

sions during the three periods of time be-ing studied were patients with greater than25% total body surface area burned, al-though the mortality rate for these selectedpatients rose from 61.7%o to 73.2% duringthis time. If the same calculations are car-ried out relative to the numbers of patientsadmitted with 15% or more total body sur-

face area burns the same percentage of ad-missions had burns of this magnitude inboth the first 4 years of this study and thelast 5 years, although the mortality rateagain rose 10 per cent. Therefore the dif-ference in mortality cannot be due to anysignificant increase in numbers of patientsthat were admitted to this Burn Unit withlarger burns.However the incidence of the patients in

various age groups that were admitted inthese different time periods did vary sig-nificantly. Patients aged 0 to 14 years de-creased progressively from 1949-52 (46%)to 1958-62 (37%7o), while patients aged 45-64 years increased from 11% to 20% (p <.001).** The incidence of patients in theother age groups remained the same. Thisshift would therefore affect the overallprobit lines and mortality rates calculatedaccording to the year of admission. Differ-ences in these values from the first 4 yearsto the last 5 years of this study are there-fore probably due to a significant increasein the number of older patients and a de-crease in the number of younger patientsadmitted during the latter period. This di-

* Test for significance of difference in propor-tions.

rect relationship is presently being analyzedusing discriminate function analysis.

Discussion

The present study has used the same

method of analysis as that of Bull andFisher3 and Pruitt et ali9 in order to facili-tate comparing mortality rates in largenumbers of burn patients treated in threeseparate burn centers. Certain differencesexist in the patient populations, however,and these could account for certain appar-

ent differences in mortality rates. The seriesreported by Bull and Fisher and the pres-

ent one were composed of civilian patientstreated at different times. Bull and Fisherdo not present socio-economic data thatcould be important in view of the effect ofrace (or socio-economic status) shown inthe present series. Patients comprising theseries reported by Pruitt et al. (BrookeArmy Hospital) were highly selected inthat they were either military personnelor dependents, and were drawn from mili-tary bases scattered throughout the UnitedStates and in some instances foreign coun-

tries. In addition a significant number ofpatients treated at Brooke Army Hospitalare transferred (largely by air transport)from other military hospitals after resusci-tative therapy has begun. Those patientsthat died before they were referred, dueeither to the seriousness of their injury or a

delay in transportation facilities, have notbeen included in their series. These wouldbe further selective processes not in theseries of Bull and Fisher or this one.

134

Volume 164Number 1

PROBIT ANALYSIS OF BURN MORTALITY IN 1,831 PATIENTS 135TABLE 13. Comparison of Overall Mortality Excluding Area Groups with Less Thtan 12% ,IMortality

Bull and Fisher MCV

Remaining % Remaining %Age (yr.) Area Excluded Pts. Mortal. Pts. Mortal.

0-14 0-14% 187 22.4 285 35.415-44 0-14% 97 25.8 248 39.545-64 0-4% 87 25.3 282* 23.065+ None 144 43.7 190 52.1

Overall 515 29.3 1005 36.1

* No area group excluded.

Brooke Army Medical Center

Age (yr.) Area Excluded Remaining Pts. % Mortal.

0-14 0-10% 158 27.215-49 0-10% 605 20.550+ 0-20% 30 73.3

793 23.8

Data from these three series relative tomortality rate for different patient age

groups is presented in Table 12. The mor-

tality rate for each group reported by Bulland Fisher is much lower than that re-

ported by the Brooke Army Surgical Re-search Unit or the present series. Thistable, as noted, does not have strictly com-

parable age groupings for the Brooke se-

ries due to the method by which their datawere presented, and their mortality rate forthe older patients is therefore not strictlycomparable.Raw mortality rates are strongly influ-

enced by the number of patients within anyseries with relatively minor or extremelysevere burn injuries. Therefore, in order tocompare these rates more meaningfully, pa-tients with minor burns having little or no

mortality have been excluded, and resultsare shown in Table 13. It is obvious thata large number of patients reported by Bulland Fisher were admitted to the hospitalwith minor burns, and only 515, or 18.4%of the total, had burns involving more than

14% of the body surface. Seventy-two percent of the Brooke series and 55% of thisseries had more severe burn injuries. The

overall mortality rate for more severelyburned patients reported by Bull andFisher increases to 29.3%, somewhat closerto those reported from Brooke Army Hos-pital and the Medical College of Virginia(MCV). The percentage of patients agedO to 14 years and 15 to 44 or 49 years ineach series are fairly evenly distributed, andwithout determining the extent of the totalbody surface area burn in each series, theMCV mortality rates appear to be signifi-cantly higher.Table 14 compares the LA50 values for

total body surface area that was burned byage groups for the three series. Both Bulland Fisher and the Brooke Army Hospital

TABLE 14. Comparison of LA6o's for Total AreaBurned by Age Groups

Age (yr.) Bull and Fisher Brooke MCV0-14 49.4%7 48.5% 39.1%

0-14 49.4% 48.5% 39.1%15-44 46.4 55.8* 44.545-64 27.1 29.0** 31.065+ 10.1 12.8

* 15-49 yr.**50+ yr.


NO.of PTS.

B a F 1041BROOKE 171

MCV 493

% MORTAL.

UNKOWN17.5II.1

LA 50

49.4%45.1%35.5%

8

7

K

6

5

4

3

2

8

7

6

K

0z

0 20 40 60 80 100

% TOTAL AREA BURNED

FIG. 10. Mortality probit lines for age group0-4 years for present series and those reported byBull and Fisher and Brooke Army Hospital.

series LA50 values for the age groups 0

to 14 years were statistically significantlyhigher than MCV's.

Pruitt et al. showed that the mortalityrate for the 0- to 4-year-old patients ishigher than that for the 5- to 14-year-oldpatients. Figure 10 shows mortality probitlines for the total body surface area burnfor this age group from the three series nowbeing compared. It is obvious that for burnsof more than 25 to 30%o of the body sur-

face area there is a difference in mortality.As noted earlier in Tables 4 and 5 the 95%oconfidence limits for the LA50 values of pa-

tients treated at MCV in the 0 to 4 agegroup ranged from 31 to 44%o, the latterbeing close to the values reported in theother series. The significantly decreasedLA50 values for Negro females in this agegroup, and the overall decreased toleranceto the burn wound shown by Negro pa-tients in this series, could account for thedifference in the LA50 value. Figure 11 re-

5

4

3

2

MCV

0-14 YRS.

0 20 40 60 80 100

% TOTAL AREA BURNED

FIG. 11. Mortality probit lines for age group0-14 years for present series and those reportedby Bull and Fisher and Brooke Army Hospital.

produces mortality probit lines for the per

cent of total body surface area burned forthe 0- to 14-year-old age group, and herethe lines representing patients from Bulland Fisher and Brooke Army Hospital lievery close, with a significant decrease inthe LA50 value being shown again, and forthe same reason, for the MCV patients.Therefore patients reported in this seriesdo not show the significantly increased mor-

tality reported by Pruitt et al. for the 0- to4-year-old age group, although the totalmortality for these younger patients was

higher and the LA50 value was lower thanwith either of the other series.

In addition the Brooke Army Hospital15- to 49-year-old age group LA50 valuewas significantly higher than that for the15- to 44-year-old age group from bothMCV and Bull and Fisher. This age groupencompasses most of the active militarypersonnel. The remaining LA50 values are

essentially the same.

136 Anr

NO.of PTS. % MORTAL.

B a F 1366 3.4BROOKE 238 18.1

MCv 769 13.5

nals of SurgeryJuly 1966

LA 50

49.7%48.5%39.2%


One of the major conclusions that can bemade from these data is that the mortalityrate for burned patient has not significantlychanged during the 10 years prior to 1962,although there is data showing, as reportedby others, that survival time for fatal cases

is increasing significantly. The effect of pri-mary excision of the burn wound or newer

local and systemic antibiotic and chemo-therapeutic agents or both on mortality re-

mains unevaluated at the present time al-though preliminary reports have been en-

couraging.6The probit type of analysis has definite

limitations, and some of these have beenillustrated in the present series. The analy-sis is limited in the studies quoted by theuse of only two factors to determine mor-

tality, these being age of the patient andper cent of total body surface area that wasburned. It is obvious that total body sur-

face area burned is not as significant a fac-tor as area of third degree burn, but clini-cal difficulties in delineating second degreefrom third degree burns are well knownand limit the use of this factor. In additionthere are other factors which affect the pa-

tient's chance of living or dying. These in-clude pre-existing disease, associated in-juries and postinjury complications. Thistype of information is not available usingthe probit transformation of raw mortalitydata, even by grouping patients accordingto the different complications that occur.

A preliminary study of important factors indetermining lethality in burn patients has

previously been reported,10 and results haveled the authors to believe that a discrimi-nate function type of analysis is more use-

ful than probit analysis. It is hoped thatother authors will use this type of analysisso that a better understanding of these vari-ous factors can be obtained.

Summary

The present study presents results of a

probit analysis of the mortality rate seen in

1,831 burn patients treated between 1949and 1962 at the Medical College of Vir-ginia Hospital. Differences in mortalityrates are related both to age of the pa-

tient and to percentage of total body sur-

face area and the third degree surface area

burn. In addition an attempt is made toshow the effects of race and sex of the pa-

tient, time of admission to the hospital fol-lowing burn injury, and the year of admis-sion on mortality.

Results of this study confirm the relation-ship existing between expected mortalityand age and amount of body surface area

burned, but, as shown in Table 9, 107 pa-

tients or 29% of those dying had less than50%o chance of dying according to thisanalysis. This is a large degree of error andthe total error in predicting mortality bythis method would be higher if patientswith a probability of dying of greater than50%, but who lived, were included. Thisshows a definite need for a more accuratemeans to predict burn mortality. There was

no effect on mortality according to time ofadmission to the hospital following theburn. The race of the patient was found tobe important, but sex was not.

These data show that progress has beenmade in prolonging survival time of pa-

tients who eventually die, but the overall

true mortality rate is unchanged at thishospital and in other reported series.The LA50 value is a useful figure to ex-

press tolerance of a patient or a group ofpatients for burn injury. These values were

significantly lower for younger patientstreated at the Medical College of Virginiacompared to values reported by Bull andFisher and Pruitt et al. The best survivalrates for middle aged patients have beenreported by Pruitt et al. at Brooke ArmyHospital.These data show a striking difference be-

tween effects of second- and third-degreeburn injuries in that the partial-thicknessinjury is rarely lethal.

Volume 164Number 1 137

138 RITTENBURY AND OTHERS Annals of Surgery138 ~~~~~~~~~~~~~~~~~~~~~~~~~~~July 1966

References

1. Barnes, B. A.: Mortality of Burns at the Mas-sachusetts General Hospital, 1939-1954. Ann.Surg., 145:210, 1957.

2. Bull, M. P. and Squire, J. R.: A Study ofMortality in a Burns Unit. Ann. Surg., 130:160, 1949.

3. Bull, J. P. and Fisher, A. J.: A Study of Mor-tality in a Burns Unit: A Revised Estimate.Ann. Surg., 139:269, 1954.

4. Finney, D. J.: Probit Analysis: A StatisticalTreatment of the Sigmoid Response Curve.Ed. 2. Cambridge, University Press, 1952.

5. Finney, D. J.: The Statistical Method in Bio-logical Assay. Ed. 2. New York, Hafner Pub-lishing Company, 1964.

6. Lindberg, R. B., Moncrief, J. A., Switzer,W. E., Order, Stanley, E. and Mills, W., Jr.:The Successful Control of Burn WoundSepsis. J. Trauma, 5:601, 1965.

7. Moyer, C. A.: An Assessment of the Therapyof Bums: A Clinical Study. Ann. Surg., 137:628, 1953.

8. Moyer, C. A.: Aging and Mortality from Ther-mal Injury. J. Gerontol., 9:456, 1954.

9. Pruitt, B. A., Tumbusch, W. T., Mason, A. D.and Pearson, E.: Mortality in 1,100 Consecu-tive Bums Treated at a Bums Unit. Ann.Surg., 159:396, 1964.

10. Rittenbury, M. S., Schmidt, F., Maddox, R.,Beazeley, W., III, Ham, W. and Haynes,B. W., Jr.: Factors Significantly AffectingMortality in the Bumed Patient. J. Trauma,5:587, 1965.

11. Wiedenfeld, S.: Ober den Verbrennungstod.I. Abhangigkeit des Verbrennungstodes vonder Grosse der Verbrannter Hautflache. Arch.Dermat. Syph., 61:33, 1902.

12. Winterscheld, L. C. and Merendino, K. A.:Etiology of and Mortality from Bums at aGeneral Hospital. Amer. J. Surg., 100:375,1960.

(Continued from page 89)

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(Continued on page 144)

probit analysis of burn mortality in 1,831 patients:

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