Full transcript
0:05Hello everyone and welcome to our module
0:07on the cardiovascular response to
0:08exercise.
0:10In this video I'm going to go through
0:12the cardiovascular response to exercise.
0:14And by that I mean I'm going to discuss
0:16the physiologic changes that occur in
0:18the vascular system when you exercise.
0:20This is a very important application of
0:22basic principles of cardiac physiology.
0:25It's also commonly tested and very high
0:27yield for the USMLE step one exam. So
0:29when you exercise, the body's overall
0:31goal is to maximize the profusion to
0:33skeletal muscles in the heart and to
0:35minimize the profusion to all other
0:36areas of the body. The initiator for
0:39this response is muscle hypoxia. When
0:41you begin exercising, your tissue beds
0:43in your muscles become hypoxic and that
0:46triggers the body's response. And the
0:48mediator of that response is the
0:49sympathetic nervous system. It is what
0:51drives all the different changes that
0:53occur in the cardiovascular system. The
0:55process of the body's response to
0:57exercise begins with muscle contraction.
0:59When you start exercising, you contract
1:00your muscles and this leads to the
1:02consumption of ATP. When you consume
1:04ATP, you then consume oxygen because you
1:07need more ATP for your muscle tissue.
1:09The result is local hypoxia in muscle
1:11tissue. And this leads to vasoddilation,
1:13which is a very important intermediate
1:14step in the body's response to exercise.
1:17Vasoddilation results because of
1:19multiple mediators that are released
1:20into the plasma in muscle tissue, which
1:23is hypoxic. This includes adenazine
1:25generated from ATP consumption also
1:27lactate, carbon dioxide and potassium
1:29among other mediators. The net result of
1:32the vasoddilation in muscle tissue is a
1:34lowering of total peripheral resistance
1:36which is a very important physiologic
1:38change that occurs with exercise. The
1:40fall in peripheral resistance leads to
1:42activation of the sympathetic nervous
1:44system which leads to many of the
1:45physiologic changes associated with
1:47exercise. There's a rise in
1:49contractility and stroke volume. The
1:51heart rate goes up. The net result of
1:52both of these changes is a rise in
1:54cardiac output. When the cardiac output
1:56goes up, you push more blood into the
1:58arterial system. This leads to a rise in
2:00the systolic blood pressure. In
2:02addition, there's vasoc constriction to
2:04some areas that are not important during
2:06exercise, areas like the gut and the
2:07skin. This helps to redistribute blood
2:09to the important areas like the heart
2:11and the muscle. The blood pressure
2:13changes that occur with exercise are
2:15very high yield. So, let's review those
2:17on this slide. The systolic blood
2:19pressure usually rises with exercise. is
2:21this is because the cardiac output goes
2:22up that drives more blood into the
2:24arterial tree and that raises the
2:26systolic blood pressure. Always remember
2:28that the primary determinant of the
2:29systolic blood pressure is the cardiac
2:31output. When the cardiac output goes up
2:33the SBP usually rises and when the
2:35cardiac output falls the systolic
2:36pressure usually falls. The diastolic
2:39blood pressure with exercise decreases
2:41slightly or stays normal. Why does this
2:43happen? Well, as we discussed, there's
2:44local dilation of skeletal muscles. And
2:47remember that the primary determinant of
2:49the diastolic blood pressure is the
2:50peripheral resistance. So that dilation
2:52can lower the peripheral resistance and
2:54that can decrease the diastolic blood
2:56pressure. So because you've got systolic
2:58pressure going up and diastolic blood
3:00pressure staying flat or going down, the
3:02net result is that you will have a
3:04widening of the pulse pressure which is
3:06the difference between the two. So the
3:07pulse pressure goes up with exercise and
3:10as we discussed previously the total
3:11peripheral resistance goes down because
3:13of dilation of vessels in skeletal
3:15muscles.
3:17There are important changes to coronary
3:19artery profusion that occur as part of
3:21the response to exercise. This is
3:22because the faster heart rate shortens
3:24diastily. Recall that coronary arteries
3:26are profused during diastily, not cy
3:29like most arteries in the body. This
3:31means that when the faster heart rate
3:33shortens diastily, there is less
3:34coronary filling time. As a result, the
3:37coronary arteries must vasoddilate in
3:39order to increase their blood flow. This
3:41is the only way to get more oxygen to
3:43myioardial tissue. Myioardial tissue
3:45cannot extract more oxygen because it
3:47already extracts the maximum oxygen from
3:50red blood cells at baseline. Other
3:52tissues in the body can take more oxygen
3:54from red cells when they need it. But
3:55the heart cannot do this because it is
3:58always extracting the maximum amount.
3:59Thus, the only way it can increase the
4:02oxygen delivery is by increasing the
4:03blood flow. And the only way it can do
4:05that in the setting of a shorter
4:06diastily is to increase the amount of
4:08coronary vasoddilation. Your preload
4:10rises with exercise. This is because
4:12sympathetic stimulation leads to venus
4:14contraction. As we discussed previously,
4:16this will increase the preload and the
4:18end diastolic volume and this is another
4:20contributor to the rise in cardiac
4:22output seen with exercise along with the
4:24increased heart rate and the
4:25contractility. The left ventricular
4:27ejection fraction increases dramatically
4:29with exercise. If you look at the two
4:31moving images on the bottom of the
4:32screen, the left side is a patient
4:34before exercise, the right side is after
4:37exercise. You can see that before
4:39exercise, the left ventricle thickens
4:41and the walls come in, but there's still
4:43plenty of black left in the middle. On
4:45the right side, the heart is contracting
4:46so vigorously after exercise that almost
4:48all the black in the middle is
4:50eliminated. I show you these pictures
4:51because it's an easy way to remember
4:53that the left ventricular ejection
4:54fraction goes up with exercise. The
4:57reason this happens is because there's
4:58more vigorous contraction in the setting
5:00of exercise and sympathetic stimulation.
5:02The major impact is decreasing the ends
5:05systolic volume as a result of increased
5:07contractility. There's minor changes on
5:09the end diastolic volume. The reason is
5:11because there's more preload which tends
5:13to raise the end diastolic volume.
5:15However, there's less filling at fast
5:16heart rates. Thus, the change in the end
5:18diastolic volume tends to be relatively
5:20minor and somewhat variable. However,
5:22the ejection fraction if you look at the
5:23top right side of the screen is dictated
5:26by the end diastolic volume minus the
5:27ends systolic volume over the end
5:29diastolic volume. So if the ends
5:31systolic volume goes way down, it will
5:33make the EF go way up by this equation.
5:36There's one additional concept in
5:37cardiac physiology that we haven't
5:39discussed yet and it's particularly
5:40important in exercise and that's the
5:42concept of lucotropy. Lucotropy is
5:44myocardial relaxation. It's the opposite
5:46of contractility. Contractility is how
5:48vigorously the walls of the left
5:50ventricle come together to push blood
5:52out of the ventricle into the aorta in
5:54cy. Lucotropy is how vigorously the
5:56walls of the left ventricle come apart
5:58in diastily. When they come apart, they
6:01suck in blood and raise the preload,
6:03which raises the cardiac output. Thus,
6:05when the degree of lucotropy increases,
6:07the left ventricle relaxes more
6:08vigorously. This pulls in more blood and
6:10raises a preload and raises
6:12contractility. And just like
6:14contractility, lucotropy is increased
6:16with exercise. This contributes to the
6:18increased preload seen with exercise,
6:20which leads to the rise in cardiac
6:21output. The key regulatory protein of
6:23lucotropy in the heart is called
6:25phospholaman. This regulatory protein is
6:28an inhibitor of a very important enzyme
6:30called circa. Circa stands for the
6:32cycoplasmic reticulum calcium ATPAS. And
6:35this enzyme regulates lucotropy in
6:37ventricular myasytes. To understand
6:38this, imagine we have the cycloplasmic
6:41reticulum inside a ventricular myasy. In
6:43order for relaxation to occur, the
6:45cycloplasmic reticulum must pull calcium
6:48from the cytool so that the myasy can
6:50relax. Circa facilitates uptake of
6:53calcium from the cytool into the
6:54cycoplasmic reticulum and it is normally
6:56inhibited by phospholamand. However,
6:59betaadronurgic stimulation via the
7:01sympathetic nervous system can
7:03phosphorolate phospholamand. This makes
7:05it stop inhibiting circa and it allows
7:07circa to take up calcium more vigorously
7:10out of myioytes in the left ventricle.
7:12This leads to more rapid and more
7:13vigorous relaxation and an increase in
7:16lucotropy. Here's a picture to make this
7:18more clear. here circa in the membrane
7:19of the cycloplasmic reticulum. It is
7:21normally inhibited by phospholamban.
7:23However, sympathetic stimulation
7:25phosphorolates phospholamand via beta
7:27adinuric stimulation. This inactivates
7:30phospholamand. It relieves the
7:31inhibitory effect. It allows circa to
7:33take up more calcium and thus there is
7:35more vigorous relaxation of the left
7:37ventricle that leads to a rise in
7:39lucotropy and an increase in preload.
7:41I'll finish this video with this concept
7:43map which summarizes all the changes
7:45that occur in exercise that you need to
7:47know for the step one exam. So when
7:48exercise begins this leads to muscle
7:51hypoxia which causes vasoddilation.
7:53Vasoddilation lowers the TPR. It lowers
7:56the afterload and that activates the
7:58sympathetic nervous system. In addition
8:00the lowering of afterload can decrease
8:01the diastolic blood pressure. At this
8:04point, the sympathetic nervous system
8:05becomes the mediator for all the
8:07physiologic changes that occur with
8:09exercise. In the peripheral vessels,
8:11there will be constriction of arterials
8:13to unimportant areas like the skin and
8:15the gut. There will be venus
8:16constriction which will send blood back
8:18to the left ventricle, raising the
8:20preload and raising the end diastolic
8:21volume. In the heart, the sympathetic
8:24activation will increase contractility
8:26and heart rate. The increased
8:27contractility will lower the ends
8:29systolic volume and raise the ejection
8:31fraction as we saw in those echo
8:32cardiogram images earlier. The higher
8:34heart rate plus the increase in
8:36contractility plus the increase in
8:38lucidotropy all contribute to a rise in
8:40cardiac output and that raises the
8:41systolic blood pressure. And that
8:44concludes our video on the
8:45cardiovascular response to