ECG Questions

Exercise 1: ECG and Volume Pulse at Rest and After Exercise 1.

What happens to the R-R interval and the heart rate after exercise, which of the following components of the R-R interval became shorter AFTER the heart rate increased? What accounts for this change?

The R component of the QRS complex is the peak of depolarization of the right and left ventricles of the human heart. Therefore, the R-R interval represents the time between ventricular depolarizations, right before ventricular contractions occur (heartbeats). This interval is thus heart rate dependent. When heart rate increases, such as during exercise, the R-R interval decreases. This decrease occurs because the number of ventricular contractions increases in given period of time; because there are more contractions per unit time, there has to be a greater number of depolarization of the ventricles per unit time, which decreases the time between each ventricular depolarization, decreasing the R-R interval. After exercise, the R-R interval increases and the heart rate begins to decrease. The components of the R-R interval that become shorter after heart rate increases are the individual P wave and the T wave, which represent the depolarization of the myocardial cells in the atria and the and the repolarization of the myocardial cells in the ventricles. The distance between the P wave and the T wave also decreases after the heart rate increases. These changes occurs as result of the heart rate increasing. In order to pump blood at a faster rate, the atria and ventricles must synchronously contract at a faster rate, which requires them to depolarize and depolarize at a faster rate, so that more contractions can occur. The shortened length of depolarization of the atria allows them to quickly move blood into the ventricles. The shortened length of repolarization of the ventricles allows them to quickly achieve a resting membrane potential, which allows them to be depolarized more times during a given time frame. 2. During exercise, blood flow in the skin and finger pulp is usually reduced by sympathetic vasoconstrictor nerve activity. After exercise, however, skin and finger blood flow may be increased. Why is the blood flow to extremities reduced during exercise? Why does it increase to higher than normal levels during recovery? What other factors may influence the supply of blood to the skin and fingers during and after exercise? The blood flow to extremities is reduced during exercise because during a sympathetic response, the body is going to want to deliver oxygenated blood to larger skeletal muscle groups, such as the biceps, triceps, back, quadriceps, etc. whose collective sarcomere contraction will produce the greatest amount of force during exercise. The sympathetic response will cause vasodilation in blood vessels that lead to these larger muscle groups, to maximize blood flow to these areas, and minimize blood flow to areas such as the finger pulp, where the muscle within that given area doesn’t produce a great deal of contractions to sustain rigorous periods of activity. The blood flow increases to higher than

normal levels during recovery, because the parasympathetic nervous system will increase its function, causing the vasodilation of the blood vessels leading to these areas, increasing the blood flow. There is higher blood flow because the body is trying to return to a resting, homeostatic state by ensuring that these smaller areas receive sufficient blood flow. There could also be higher blood flow because the previous vasoconstriction of the arteries would perhaps have caused an increase in blood pressure for blood trying to enter these constricted arteries. Upon vasodilation, the blood, have been pushing against these previously constricted areas would enter these areas with greater velocities. Exercise 2: Blood Pressure Analysis 1. What is meant by pulse pressure? What happened during light and heavy exercise to the pulse pressure? Was this what you expected to happen? Pulse pressure is the difference between the systolic and diastolic pressure readings. It represents the force that the heart generates each time it contracts. During exercise periods, the pulse pressure will increase because the stroke volume, or the amount of ventricular blood pumped with a given contraction increases to fuel muscular contractions. This increase in stroke volume will cause an increase in systolic pressure, while the diastolic pressure will remain relatively constant, increasing the value of pulse pressure. Based on needs for muscular activity, the stroke volume will adjust accordingly, so lighter exercise will have a smaller increase in ventricular contraction and heavier exercise will have a greater increase in ventricular contraction. This will result in a smaller increase in systolic pressure for light activity and greater increase in systolic pressure for heavy activity. I expected this to happen because with an increasing demand for oxygenated blood from muscles engaging in sarcomeric contraction, the heart will contract with greater force (systolic pressure) in order to maximize stroke volume, ensuring that sufficient oxygen/nutrients is given to working muscles. 2.What happened during light and heavy exercise to the mean arterial blood pressure? Briefly describe your findings. The mean arterial blood pressure is the average blood pressure in an individual, and is measured as the average blood pressure of one complete cardiac cycle. It can be calculated by multiplying the total peripheral resistance of systemic circulation by the cardiac output. During light exercise, the mean arterial blood pressure increased a bit, but not as significantly as during heavy exercise, because the stroke volume of the heart will be greater during periods of heavier exercise. This increase in stroke volume with either no in the systemic vascular resistance, or even a decrease in systemic vascular resistance due to vasodilation of areas of muscle undergoing rapid contractions will cause the Mean Arterial Blood Pressure to increase.