The purpose of this labwas to see the effect of exercise onduration of ECG components and we hypothesized that increasing the activitylevel would decrease P-R interval, T-P segment, and R-R interval duration andmake no changes in QRS complex duration and QRS complex amplitude.

Afterrunning this experiment, we found out that there was a small decrease in QRScomplex duration which opposed our hypothesis (but we found out later that ourhypothesis was right and there was a problem in our experiment results) and thenwe conclude that as the activity level increases, ECG intervals and segments durationwould decrease and there would be no changes in QRS complex duration. We also observedan increase in heart rate after exercise.Aswe learned in class, T-P segments represents the time from the end ofventricular repolarization to the onset of atrial depolarization and R-Rinterval represents the time between two ventricular depolarizations which iscalled one cardiac cycle. The sympathetic nervous system which is the subgroup ofautonomic nervous system, releases the hormone Norepinephrine which acceleratesheart rate by increasing the slope of rising portion of the cardiac myocyte actionpotential during phase 0 and by activating the ?1 adrenergic receptors in the heartand decreasing K+ permeability (Gordan, Gwathmey and Xie, 2015). Thenautorhythmic cells start to depolarize because of the reduced efflux ofpotassium and the interior membrane potential becomes less negative, depolarizereally slow and generate the pacemaker potential. Pacemaker currents of SA nodecells (action potential initiators) bring action potential to threshold fasterthan AV node cells increase the firing rate and make the heart beats faster perminute. In another hand the parasympathetic nervous system releases thehormone Acetylcholine to slow down the heart rate by increasing K+ permeability.

Wedid learn in class that P-R segments represents the time interval between theAV node and the ventricular myocardium. Sympathetic stimulation of the AV node decreasesthe regular delay of conduction through the AV node which decreases the time intervalfrom atrial to ventricular contraction. As AV nodal conduction velocity increases,the P-R interval of ECG decreases (Carruthers et al.

, 1987). As we noticed inour experiment, there was not any changes in QRS complex. QRS complexrepresents depolarization of the right and left ventricles and also atrialrepolarization. There two bundle branches that come to a point to make a lot ofPurkinje fibers, which stimulate individual groups of myocardial cells tocontract. The movement of action potential through the ventricular myocardium makesthe QRS complex on the ECG.

Purkinje fibers and ventricular myocardium getactivated together and they do not depend on heart rate during our experiment,so there would be no change in QRS complex if heart rate increases. Someof the limitations in this experiment could be problems with the machine, beingsmoker, fitness level, health, gender, age, subject’s energy level for the day,sudden weakness during the experiment, etc.


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