Determination of functional aerobic capacity using the microcomputer

Comput. Bid. Med. Vol. 19. No. 5, pp. D-335. Pnntcd in Great Britain.

1989

OOIO-4825/89 $3.00 + 00 cp 1989 Pergamon Press plc

DETERMINATION OF FUNCTIONAL AEROBIC CAPACITY USING THE MICROCOMPUTER KENNON

FRANCIS

Division of Physical Therapy, Room B41 SHRP Building, University of Alabama at Birmingham, Birmingham, Alabama 35294, U.S.A. (Received

22 August

1988; in revised form

16 December

1988; received for publication

5 January

1989)

Abstract-The general interest in the application of exercise testing to evaluate the work capacity or change in the functional ability of individuals has resulted in the development of a variety of non-invasive tests and test protocols such as the Bruce Treadmill Protocol. This paper presents a computer program written in BASIC that determines an individuals functional work capacity and functional age from minutes completed during a Bruce Treadmill incremental exercise test. Results are summarized in tabular as well as graphical format. The use of the Bruce Treadmill protocol in conjunction with this computer program provides another means of efficiently determining the exercise capability and functional status of a patient. Microcomputers

Functional work capacity

Exercise test

Work capacity

The physiologic stress of exercise is a simple, yet efficient means to elicit abnormalities that may exist with the cardiopulmonary system. These abnormalities are typically evidenced by compromises of work capacity and work performance. Because of this, functional work capacity tests have gained widespread application in clinical practice as valuable measurement and diagnostic tools for the discernment of cardiopulmonary abnormalities by determining a patients’ ability to complete a work task successfully. The determination of functional capacity is useful not only for the diagnosis of abnormalities but the development of therapeutic strategies or valuation of the response to therapy. Moreover, functional work capacity test can be utilized in health promotion by utilizing the results of the test in the development of guidelines for work or recreational activities. Of the numerous diagnostic parameters collected from a test, direct assessment of maximal oxygen uptake (1/02max) is considered to be the best indicator of functional aerobic capacity and functional cardiorespiratory fitness. 1/02max expresses the ability of the cardiorespiratory system to transport oxygen to active tissues and of those tissues to use it [l]. Cardiopulmonary fitness depends on the coordinated function of the cardiovascular, pulmonary, blood and muscle systems, which collectively is called the “oxygen transport system”. To perform effectively during exercise, the heart must pump large quantities of blood to the working muscles with optimal oxygen extraction and utilization. When the pulmonary function is normal, oxygen transport capacity is limited mainly by cardiac output (CO) and the arterial-venous (AV) oxygen difference. 1/02max can be expressed mathematically using the following formula: VOZmax (ml/min) = Cardiac output (ml/min) x AV 02 difference (ml/liter of blood). As a result, the measurement of CO and AV oxygen difference is considered the most inclusive measure of V02max and hence the oxygen transport system [l, 23. Whereas these two parameters may be the “best” measure of the oxygen transport system, they are not the most “practical” measurements that can be obtained in a clinical environment. The complexity of the equipment required for such measurements precludes their normal use as measurements in most clinical settings. As an alternative to this direct measurement of 331

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KENNONFRANCIS

V02max, researchers have developed indirect methodologies based on physiological parameters that can easily be obtained in a clinical setting. For example, the response of the heart, reflected in the heart rate, to increasing work rates is similar to the response of the body to its requirement for oxygen. The maximum oxygen uptake and the maximum heart rate are reached at approximately the same level of work. If the slope of the relationship between change in oxygen uptake with increasing work rate and change in heart rate with increasing work rate is known, the V02max may be estimated rather precisely from the singular measure of heart rate response. Therefore, the utilization of a standardized test in which oxygen consumption and work load have been established, heart rate response to standardized work is an ideal alternative for assessing the functioning of the oxygen transport system and hence cardiorespiratory fitness when more complex means are unavailable or impracticable. BRUCE

PROTOCOL

FOR

THE DETERMINATION CAPACITY

OF WORK

Indirect determination of V02max requires appropriate multistage exercise testing of ambulatory persons who are not acutely ill. This may be achieved in healthy individuals, whether habitually sedentary or physically active as well as in symptomatic but stable patients with chronic disease such as coronary heart disease. The committee on exercise of the American Heart Association [3] and the American College of Sports Medicine [2] agree that multistage exercise testing (with successively increasing work loads) accompanied by continuous ECG monitoring and periodic blood pressure determination is the most informative type of testing to ascertain an individual’s functional work capacity. Of the various treadmill protocols, the Bruce Multistage Treadmill Test is one of the most widely used [4,5]. The Bruce protocol is popular because reasonable appraisals of VO2max can be derived not only from heart rate response but from “duration” of exercise as well. Duration of exercise using the Bruce protocol has been shown to be highly correlated with directly measured V02max (r = 0.906 in healthy men and women and r = 0.865 in men with hypertension or heart disease [6,7]). When the Bruce protocol is properly followed as outlined in Table 1, VO2 in ml/kg of body weight per min for healthy men equals 3.88 + (0.056 x duration of exercise) (ins), and for healthy women it equals 1.06 + (0.056 x duration) (ins) [8,9]. Table 1. Oxygen consumption for men and women at various treadmill grades and speeds of the Bruce Multistage Treadmill Test VO2 (ml/kg/min) Stage

Speed (m.p.h.)

% Grade

Men

Women

I II III IV V

1.7 2.5 3.4 4.2 5.0

10 12 14 16 18

13.96 24.04 34.12 44.20 54.28

11.14 21.22 31.30 41.38 51.46

The Bruce test protocol involves walking or running on a motor driven treadmill in 5 incremental stages starting at 1.7 miles/h at a 10% slope. The slope increases 2% and the speed increases at 0.8-0.9 m.p.h. at 3 min intervals [9]. Heart rate and blood pressures are recorded during the last 20s of each min of work. The initial workload of stage I exercise raises the oxygen consumption values to about 10.5-14ml of oxygen consumption per kg of body weight per min [2,9, lo]). Even though the speed and gradient are increased every 3 min, as subsequent exercise stages are entered, the weight-adjusted oxygen uptake increases almost linearly with time. FUNCTIONAL

AEROBIC

IMPAIRMENT

Bruce and his colleagues have also examined the relationship of age and physical activity status to V02max and have derived average values of “expected” oxygen consumption in

333

Functional aerobic capacity

healthy men and women [lo]. The regression equations based on sex and activity are shown in Table 2. Table 2 Activity status Active men: Sedentary men: Active women: Sedentary women:

Maximal oxygen consumption (ml/kg/min) VOZmax YOZmax VOZmax VOZmax

= = = =

69.7 57.8 42.9 42.3 -

0.612 (years 0.445 (years 0.312 (years 0.356 (years

of of of of

age) age) age) age)

Individuals who regularly each week engage in physical activity sufficient to generate an endogenous heart load and to perspire are classified as “active”; those who do not are considered “sedentary”.

The percentage deviation of VO2max estimated from duration of exercise in relation to age-predicted normal values for gender and habitual activity status has been designated “functional aerobic impairment” (FAI) [6,7] and can be expressed by the following equation: FAI = predicted V02max - observed V02max x 100. predicted V02max A normal value of FAI of 0% indicates that VOZmax is 100% of the average normal expected value. A FAI greater than 20% is considered abnormal. Additional categories of severity include those shown in Table 3. Table 3 Category

% Impairment

Mild Moderate Marked Extreme

20-40 41-54 55-68 >68

FUNCTIONAL

AGE

Another useful implication that can be derived from a functional work capacity test is the concept of “functional aerobic age”. Functional aerobic age represents the equivalent age of healthy persons of the same gender and activity status who have a normal expected VO2max [9-121. Functional aerobic age is easily derived from the observed VOZmax recorded during the Bruce Test protocol by determining the equivalent age of an individual based on their activity level and assuming their functional aerobic impairment is 0% (Table 4). Table 4 Activity status Active men: Sedentary men: Active women: Sedentary women:

Functional age Functional Functional Functional Functional

age age age age

= = = =

-((VOZmax -((V02max -(( VOZmax - ((VOZmax

-

69.7)/0.612) 42.9)/0.312) 57.8)/O&U) 42.3)/0.356)

Therefore when duration is subnormal, functional age exceeds chronological age; conversely, when duration is longer than expected, functional age is less than chronological age [lo, 121. Repeated aerobic exercise and conditioning lowers the functional aerobic age whereas sedentary living results in a relative greater functional age. MICROCOMPUTER

APPLICATIONS

In recent years the microcomputer has been applied to many areas of clinical medicine. The medical community is becoming increasingly sophisticated and aware of the need for assistance in analyzing and reducing the wealth of data that may be obtained from a single diagnostic test. Although nomograms are available for predicting normal values against

KENNON FRANCIS

334

which a given subject’s results can then be compared, these charts can be cumbersome. Similarly a series of simple ratios must be conducted on a small calculator in order to compare actual against predicted values. While this is not a difficult task, it is time consuming and errors are frequently introduced. Today the wide spread incorporation of the microcomputer into the clinical environment in combination with automation of the treadmill procedures provides an attractive alternative to the traditional methods of data acquisition, reduction and summary for functional work capacity tests. For example, with the aid of a microcomputer data can be displayed, organized, analyzed and printed while the patient is returning to steady-state following an exercise period. The results can be made readily available for immediate consultation and feedback with the patient without the need for a follow-up consultation visit. Data reduction and analysis can be accomplished with timely precision and without undue fatigue and boredom for the clinician. COMPUTER

PROGRAM

The following program demonstrates the use of a microcomputer in collating and organizing the data associated with a functional capacity work test. The computer program performs the related calculations and data reduction associated with optimizing the clinical evaluations associated with the Bruce Test. The program is written in Microsoft BASIC for IBM/compatible type machines. The following is a summary of the essentials of the program by line number. The graphing routine in lines 1760-2200 was adapted from a plotting routine developed by Spain [13]. Lines 20-80: initialization of the computer and program variables. Lines 90-220: input of subject characteristics and activity status. Lines 240-280: calculation of expected V02max values. Lines 290-570: Input of work capacity data derived from the treadmill test. Lines 580-630: determination of maximal values for heart rate and blood pressure; calculation of actual V02max. Lines 640-690: calculation of functional aerobic impairment. Lines 710-760: calculation of functional age. EXAMPLE

PRINTOUT

OF

FUNCTIONAL

WORK

’

’

’

8

6

8

MINUTES

Fig. 1.

TEST

HEARTRATE RESPOWSE

2w -I

50’

CAPACITY

OF

EXERClSE

1

12

Functional aerobic capacity

Lines Lines Lines Lines

780-1740: data summary and 1340-1920: bar graph plotting 1760-2200: heart rate plotting 2210-2260: exit from program

335

final printout. subroutine for V02max and functional age. subroutine. subroutine.

A printout of sample data derived from a functional work capacity test is shown in Fig. 1. SUMMARY Because of the attractiveness of the incremental exercise test in assisting the clinician in the diagnosis of disorders of the cardiorespiratory system as well as in evaluating exercise therapies and overall work capacity, the inclusion of the functional work capacity test as a routine test parameter is becoming more popular. The wide spread use of the standardized Bruce Treadmill protocol enhances the objectivity of this test parameter. The accompanying program utilizes this test for the rapid determination of functional work capacity and functional age. The use of the Bruce Treadmill protocol for incremental exercise in conjunction with this computer program provides a means of assessing the functioning of the oxygen transport system and hence cardiorespiratory fitness when more complex means are unavailable or impractible. REFERENCES 1. P. 0. Astrand, Quantification of exercise capability and evaluation of physical capacity in man, Prog. Cardiooas. Dis. 19, 51 (1976). 2. Guidelines for Exercise Testing and Prescription, Edn 3, Lea & Febiger, Philadelphia, PA (1986). 3. Guidelines for exercise testing: a report of the American College of Cardiology/American Heart Association Task Force on Assessment of Cardiovascular Procedures, J. Am. Coil. Cardiol. 8, 725 (1986). 4. R. J. Stuart and M. H. Ellestad, National survey of exercise stress testing, clinical application and predictive capacity, Chest 77, 94 (1980). 5. T. Jopke, Choosing and exercise testing protocol, Physician Sports Med. 9, 141 (1981). 6. R. A. Bruce, F. Kusumi and D. Hosmer, Maximal oxygen intake and nomographic assessment of functional aerobic impairment in cardiovascular disease, Am. Heart J. 85, 546 (1973). 7. R. A. Bruce, Principles of exercise testing, Exercise Testing and Exercise Training in Coronary Heart Disease, J. P. Naughton and H. K. Hellerstein, Eds, pp. 45-63. Academic Press, New York (1973). 8. R. A. Bruce, Methods of exercise testing, A. J. Cardiol. 33, 715 (1974). 9. R. A. Bruce, Functional aerobic capacity, exercise and aging, Principles of Geriatric Medicine, R. Andres. E. L. Bierman and W. R. Haxxard, Eds, pp. 87-103. McGraw-Hill, New York (1985). 10. R. A. Bruce, Maximal exercise testing: prognostic values for assessment of coronary heart disease risk, Postgrad. Med. 70, 161 (1981). 11. R. A. Bruce, Progress in exercise cardiology. Progress in Cardiology, P. N. Ye and J. F. Goodwin. Eds, pp. 113-l 15. Lea & Febiger, Philadelphia (1974). 12. E. B. Larson, R. A. Bruce, Health benefits of exercise in an aging society, Arch. Intern. Med. 147, 353 (1987). 13. J. D. Spain, Basic Microcomputer Models in Biology, p. 345. Addison-Wesley Publishing, Reading, MA (1982). T. FRANCIS received his Ph.D. in biochemistry from Auburn University in 1973, and has taught at the University of Alabama at Birmingham since 1974. He is currently a full professor in the Division of Physical Therapy, where he teaches physiology and pursues his primary research in exercise physiology. He devotes a great deal of his time to developing clinical applications of exercise science for computers and computer based interactive technology. He is the author of a number of scientific articles as well as a textbook on the clinical applications of the computer in physical therapy. About the hIthOr-KENNON