Cardiovascular Response to Isometric Exercisein Normal Adolescents

W. PENNOCK LAIRD, M.D., DAVID E. FIXLER, M.D., AND F. DOUG HUFFINES

SUMMARY The purpose of this study was to document the cardiovascular response to submaximalisometric handgrip (IHG) exercise in 32 normal adolescents. Left ventricular (LV) dimensions and systolictime intervals were recorded using echocardiography; blood pressure was measured by sphygmomanometryboth at rest and during IHG exercise at 25% maximum contraction. This level of isometric exercise producedsignificant (p < 0.001) increases in mean heart rate, systolic, diastolic and mean blood pressures. Despite thisresponse LV diastolic and systolic dimensions remained unchanged during exercise; hence, stroke volumeremained constant. Cardiac index increased by 22% (p < 0.001) due to the increase in heart rate. Systemicvascular resistance did not change significantly. LV ejection indices, including shortening fraction, mean Vcfand systolic time intervals also remained unchanged, except for an increase in LV ejection time index(p < 0.025). These data indicate that the cardiovascular response to submaximal isometric exercise in normaladolescents is similar to that reported in adults. This study demonstrates that sustained isometric stress test-ing in adolescents is safe and provides normal hemodynamic values.

EVALUATION OF CIRCULATORY ALTERA-TIONS during sustained isometric muscle contrac-tion is a useful method to assess cardiac function. Thehemodynamic responses of this provocative test havebeen well-documented in adults. Characteristically,there is an increase in cardiac output and bloodpressure, but little change in total peripheralresistance.'`4 Among adults, there appear to be age-related differences in the response, whether evoked atsubmaximal5 or near-maximal6 exertion.No information is available describing the cardio-

vascular effects of static muscular contraction in nor-mal adolescents. We undertook a noninvasive studyusing echocardiographic determinations of left ven-tricular (LV) function and systolic time intervals todocument in normal, young adolescents the cardiovas-cular responses to submaximal isometric exercise.

Materials and Methods

The study population included 32 healthyvolunteers (10 female and 22 male) ages 14-16 years,with an average age of 14.7 years. All were considerednormal on the basis of history, physical examination,ECG and chest x-ray. A handgrip dynamometer(Stoelting Co., Chicago, Illinois) was used to deter-mine maximum voluntary isometric contraction atleast 15 minutes before the study. The average max-imum contraction was calculated from three con-secutive maximal handgrip attempts. Subjects wereinstructed not to perform a Valsalva maneuver dur-ing the exercise period, and were carefully observedfor this during the isometric exercise.Data were obtained just before and in the third and

fourth minute of handgrip exercise at 25% maximumvoluntary contraction (MVC). Arterial blood pressure

From the Department of Pediatrics, University of Texas HealthScience Center, Southwestern Medical School, Dallas, Texas.

Supported by grant HL-18456-01 from the NHLBI.Address for reprints: W. Pennock Laird, M.D., Children's

Medical Center, 1935 Amelia Street, Dallas, Texas 75235.Received June 5, 1978; revision accepted November 8, 1978.Circulation 59, No. 4, 1979.

was measured in the non-exercising arm using a mer-cury sphygmomanometer. LV echocardiograms wererecorded during exercise at 3 minutes; systolic time in-tervals were determined from the aortic valve echo-cardiogram during the fourth minute.

All echocardiograms were obtained using anEkoline 20 ultrasonoscope interfaced to an Ekoline 21strip chart recorder. Studies were performed from thethird or fourth intercostal space at the left parasternaledge with the subject in the supine position. Oc-casionally a slight left lateral decubitus position wasrequired to record the ventricular septum clearly.

Dimensions of the left ventricle were obtained withthe ultrasound beam passing through the left ventricleat the tips of the mitral valve leaflets.7 Once this posi-tion was obtained and baseline recordings made withthe subject at rest, the transducer was not removedfrom the chest wall and we attempted to maintain thetransducer beam in the same direction during theisometric exercise. End-diastolic diameter (LVIDd)was measured at the onset of the QRS complex. End-systolic diameter (LVID8) was measured at the pointin late systole when the septum and LV posterior wallwere in closest apposition (fig. 1). Near the end of thethird minute of exercise, LV dimensions wererecorded. In the fourth minute, LV systolic time inter-vals were determined using the aortic valve echorecorded at 100 mm/sec paper speed (fig. 2).8 The aor-tic valve signal was visualized by angling the ultra-sound beam in a medial and cephalad direction fromthe mitral valve.7 For both LV dimensions and systolictime intervals, five consecutive complexes weremeasured and averaged. We obtained recordingssatisfactory for measuring systolic time intervals in 18of 32 subjects (56%).From these echocardiographic recordings, we deter-

mined LV end-diastolic and end-systolic volumes,9stroke volume and cardiac index,9 shortening fraction(SF),0'11 and mean velocity of circumferential fibershortening (Vcf).'0-12 The cardiac index and calculatedmean blood pressure'3 were used to estimate systemicvascular resistance.14 LV ejection time index (LVETI)was calculated using data from normal pediatric sub-

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jects.F1 Statistical analysis of the results was doneusing the t test for paired data.

Results

The results are summarized in tables 1-3. In theadolescents isometric handgrip contraction at 25%MVC produced a significant (p < 0.001) elevation inmean heart rate, systolic, diastolic, and mean bloodpressures. Despite this response, LV diastolic andsystolic dimensions were unchanged during exercise.Stroke volume remained constant. Therefore, theobserved increase (p < 0.001) in cardiac index wasdue to the increase in heart rate. Systemic vascularresistance index did not change significantly. With thisdegree of isometric exercise LV ejection indices in-cluding SF, mean Vcf, and systolic time intervals alsoremained unchanged, except for LVETI, which in-creased (p < 0.025).

-4~J E

B

- s, v

FIGURE 1. Echocardiogram showing method by which leftventricular (L V) dimensions were determined. End-diastolicdiameter (L VID5) was measured between the endocardialsurfaces of the interventricular septum (IVS) and the leftventricular posterior wall (L VPW) at the onset of the QRScomplex of the ECG. End-systolic diameter (L VIDj) wasmeasured at the point in late systole when the septum andposterior wall were in closest apposition. Portions of mitralvalve (MV) are recorded.

FIGURE 2. Aortic valve echocardiogram demonstratingmethod by which left ventricular systolic time intervals weredetermined. Paper speed 100 mm/see; time dots 0.I sec.PEP = preejection period; LVET left ventricular ejec-tion time; Ao aaorta; LA - left atrium.

Discussion

We chose adolescents as subjects to provide ayounger population distinct from adults. To some ex-tent, this relatively narrow age range may limit theapplicability of the findings.

For these adolescents we set the level of isometricexercise at 25% of a MVC held for 4 minutes. We hadseveral reasons for using this relatively low level of

TABLE 1. Hemodynamic Changes Daring Isometric Exercisein Normal Adolescents (n = 32)

IsometricRest exercise

Heart rate(beats/min) 70 9 (p <0.001) 88 11

Systolic pressure(mm Hg) 110 = 7 (p <0.001) 124 = 10

Diastolic pressure(mm Hg) 61 8 (p <0.001) 76 8

Mean pressure(mm Hg) 78 d4 7 (p <0.001) 92 d 7

Cardiac index(1/mmn/M2) 3.1 - 0.7 (p <0.001) 3.8 0.8

Systemic vascularresistance(mm Hg/min/1-V) 27 - 7 (NS) 25 i 5

Values are mean i SD.

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RESPONSE TO ISOMETRIC EXERCISE/Laird et al.

TABLE 2. Echographic Findings During Isometric Exercise inNormal Adolescents (n = 32)

IsometricRest exercise

LVIDd (cm) 4.72 0.34 (NS) 4.74 - 0.32

LVID, (cm) 3.06 0.23 (NS) 3.10 = 0.29LV shortening

fraction (%) 35 3 (NS) 34 - 4

Left atrialdiameter (cm) 2.7 0.4 (NS) 2.8 - 0.4

Values are mean = SD.Abbreviations: LVIDd and LVID. = end-diastolic and

end-systolic internal minor dimensions of the left ventricle.

exertion. Since our protocol called for high-qualityechocardiograms to first record ventricular dimen-sions and then systolic time intervals from the aorticvalve, sufficient recording time of 1-2 minutes duringexertion was required. We began to record after 2minutes of exercise, since that amount of time may benecessary before a significant effect is observed insome subjects.5 Accordingly, a test period of 4 minuteswas necessary.Our preliminary work with the adolescents in-

dicated that at 25% MVC, virtually all wouldcooperate and maintain the desired level of contrac-tion for 4 minutes without performing a Valsalvamaneuver, excessive body movements, or stopping torest. We felt that a 25% MVC effort would besufficient to induce hemodynamic effects in thechildren, since information in the literature indicatesthat in normal adults, a response will be produced aslong as a fatiguing load greater than 20% MVC ismaintained.1 However, it seems clear that higher worklevels will induce greater responses.16' 17The circulatory effects of sustained isometric con-

traction have been well studied in healthy adult sub-jects. Using either catheterization techniques13 ormore recently, noninvasive methods4 such as echo-cardiography, the findings have been consistent. Thenormal response includes an elevation of arterialblood pressure which is primarily due to an increase incardiac output, with little or no change in systemicvascular resistance. The augmented cardiac outputresults from an increase in heart rate, since strokevolume remains relatively unchanged.The results of our study indicate that the response in

adolescents is remarkably similar to that found inadults. The actual percent increases with exercise inmean heart rate (26%) and cardiac index (22%) amongour subjects are in agreement with published data inadults.2`4 Blood pressure elevations in normal adults(systolic, diastolic or mean) appear to vary in relationto the magnitude of the work load involved. At30-33% MVC a 14-25% increase in mean arterialblood pressure was recorded,2 8 while at 50% MVCsubstantially higher blood pressure elevations havebeen noted." 16 The 18% rise in mean blood pressure inthe adolescents seems in line with the adult response,considering the level of exertion. It has been suggested

TABLE 3. Indices of Left Ventricular Function During Iso-metric Exercise in Normal Adolescents (n = 18)

IsometricRest exercise

Ejection time(msec) 284 - 16 (NS) 275 - 15

Preejectionperiod (msec) 84 13 (NS) 82 11

PEP/ET 0.297 i 0.054 (NS) 0.299 - 0.045

Ejection timeindex (msec) 373 - 12 (p <0.025) 386 - 14

Mean Vcf(circ/sec) 1.29 i 0.12 (NS) 1.32 - 0.14

Values are mean i SD.

Abbreviations: PEP = preejection period; ET = ejectiontime; Vcf = velocity of circumferential fiber shortening.

of isometric exercise on systemic vascular resistancewith younger subjects possibly experiencing a fall inresistance.5 Our findings do not support such con-clusions. We found that in the adolescents systemicvascular resistance did not change significantly dur-ing the exercise period.

In recent years echocardiography has been shown tobe a reliable noninvasive method to assess LV perfor-mance both at rest and during exercise. The com-monly used indices include the percent shortening ofthe LV lateral minor axis during systole (SF),9-1' themean Vcf'0-12 and systolic time intervals.8The LV SF has been proven a useful measure of

ventricular function, since normal values are similarfor both children and adults despite differences in rest-ing heart rates." In the present study the restingvalues for SF were in agreement with published nor-mals."1 During isometric exercise the SF did notchange significantly, a finding similar to that reportedin adults.'The mean Vcf has been suggested as a more sensi-

tive index of LV function, since it adds an element oftime to the measurement of change in LV dimen-

12sions. However, this results in a direct relationbetween mean Vcf and heart rate. In our subjects,despite an increase in heart rate with isometric exer-cise, the average values for mean Vcf remained un-

changed, which is the response reported in adults.'8This failure for the Vcf to increase might have beendue to the acute rise in blood pressure.

Hirshliefer et al.19 found mean Vcf fell afterphenylephrine increased blood pressure in normaladults whose heart rates were controlled. In our sub-jects the tachycardia during isometric exercise mayhave enhanced myocardial contractility sufficiently tomaintain Vcf even with an increased afterload.

Systolic time intervals have been extensively studiedin both adults and children in various clinical situa-tions, and are generally felt to correlate well with ven-

tricular hemodynamics.8 20 Several studies in healthyadults indicate that systolic time intervals are sensitiveenough to detect changes in ventricular performanceduring isometric exercise. 7 21 The ratio of preejec-

that there may be age-related differences in the effect tion period (PEP) to LV ejection time (LVET) is par-

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ticularly useful as an expression of LV performance,since it appears to correlate closely with other indicesof LV function and is not influenced by a wide varia-tion in heart rate.22' 23 Furthermore, the ratio tends toreflect abnormalities in both intervals when neithermeasurement appears abnormal.

In the present investigation the average PEP/LVETat rest was similar to normal values previouslyreported in pediatric studies."' 15 With isometric exer-cise at 25% MVC, we found a tendency for LVET toshorten, with even less shortening of the PEP.However, the ratio PEP/LVET did not changesignificantly with isometric exercise. The results ofstudies in normal adults using similar isometric workloads have been variable. Although most reportsdescribe a shortening of LVET, changes inPEP/LVET have been inconsistent, either increas-ing 7 24 or showing no change'8 with submaximalisometric exercise.

In order to correct for the effect of heart rate onLVET, we calculated the LVETI on each subject bothat rest and with isometric exercise. The results in-dicate that the isometric exercise produced prolonga-tion of the LVETI in these adolescents. Since strokevolume was unchanged and correction was made forthe effect of heart rate, presumably this response was aresult of increased arterial pressure. However, Quarryand Spodick did not come to this conclusion in theirstudies on normal adults.'7 They found that in adults,moderate isometric handgrip exercise produced aprolonged LVETI only with upright posture, not inthe supine position. They theorized that stroke volumechanges were responsible for these findings, sincepressure changes- were similar in each group.

Further investigation will be necessary to determinethe significance of these apparently different responsesin adults and adolescents. It would also be appropriateto evaluate the effects on systolic time intervals inadolescents produced by higher levels of isometricexertion, since adult studies have demonstrated thatincreased exertion will produce greater alteration ofsystolic time intervals.'7

This study demonstrates that sustained isometricexercise is a safe technique which is well-tolerated byadolescents. Noninvasive evaluation of cardiac func-tion during isometric exercise can be accomplishedusing echocardiography. The data obtained in thesenormal subjects provide control values which may beused in studies of adolescents with known or suspectedcardiovascular disease. The technique would be es-pecially useful in assessing LV function in youngpatients with abnormalities of the left heart, such aspre- or postoperative aortic stenosis, coarctation ofthe aorta or systemic hypertension.

References

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W P Laird, D E Fixler and F D HuffinesCardiovascular response to isometric exercise in normal adolescents.

Print ISSN: 0009-7322. Online ISSN: 1524-4539 Copyright © 1979 American Heart Association, Inc. All rights reserved.

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