The Effects of Exercise on Variations in Cardiac Output

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The Effects of Exercise on Variations in Cardiac OutputSarah Buchanan Biology 493 09 November 2001


Review of LiteratureHeart rate variability (HRV) is a qualified predictor of mortality (Liao et al. 1997). Heart rate is a risk factor for mortality from coronary disease and all cardiovascular diseases (Greenland et al. 1997). Low HRV is an indicator of the potential for early death in middle-aged and elderly men (Dekker et al. 1997). Low HRV is associated with increased risk of coronary heart disease and death from cardiovascular causes (Dekker et al. 2000). Low HRV is a marker of less favorable health (Dekker et al. 2000).


Literature (Cont.)Exercise is a factor associated with reduced morbidity (Cashion et al. 2000). Exercise improves health by modulating heart function through the autonomic nervous system (Gandevia & Hobbs 1990). Exercises and other interventions have been designed to modulate heart rate in persons with low HRV in an effort to reduce mortality risks (Sherman et al. 1994)(Stein et al. 1999). Exercise increases cardiac output (Jenkins & Quigley 1992).


Cardiac output is influenced by both stroke volume (SV) and heart rate (HR) (Goodwill 1998) Exercise increases SV by preloading the heart with blood (Spina at al. 1992) Exercise impacts HR by modulating the autonomic nervous system (Cashion et al. 2000). Literature (Cont.)


This study was designed with the intent of determining whether the importance of exercise in reducing morbidity is due to an increase in heart rate variability or the result of some other unrelated factor or factors. Purpose


This study involved 14 women (ages 18-28). Six electrodes were placed on each subject as follows: (1) mid forehead (2) at the base of neck (2) at level of xiphoid in mid-axillary line (1) at level of upper thigh in mid-axillary lineMethods


Electrode PlacementHeart monitor electrodes mid forehead at base of neck at level of xiphoid in mid-axillary line at level upper thigh in mid-axillary lineECG electrodes At base of neck at level of xiphiod just outside mid-axillary line


Methods (Cont.)The distances between electrodes were then carefully measured. The CO, SV, and HR of each subject was measured in 15 second intervals over a period of five minutes using a Sorba cardiac output monitor.


Methods (Cont.)Each subject then pedaled a bicycle ergo meter at 70% her heart rate reserve maximum for 8-10 minutes. Immediately following the period of exercise, each subject’s CO, SV, and HR were measured again. Data collected were analyzed through coefficients of variation for HR, SV, and CO measured at rest and following exercise.


ResultsData obtained through the experiment indicated that, in most cases, stroke volume exhibited higher variability than heart rate in response to exercise. Although heart rate variability increased in response to exercise in some test subjects, on average HRV decreased one percent following exercise.


Results (Cont.)Stroke volume increased in nearly every subject, increasing an average of nearly seven percent. Stroke volume was found to increase more than the heart rate in response to exercise in all but two of the subjects.


Results (Cont.)


DiscussionSince heart rate increases with an increase in cardiac output, exercise may reduce morbidity by increasing heart rate variability. The increase of cardiac output after exercise does not increase heart rate variability. Increases in cardiac output have been found to have a more significant effect on stroke volume.


Conclusion While exercise has been shown to reduce morbidity, the decrease is probably not due to and increase in heart rate variability as previously assumed.


Acknowledgments Dr. Randy Day MSC Faculty & Staff My parents My wonderful, handsome, friendly, intelligent, and all-around amazing husband

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