Tilt-table Results

Before starting this page, be sure you work through the "Regulation of Cardiac Output" page, which shows how standing changes the distribution of blood in the veins and thus the stroke volume.

In viewing the results from lab section AE, shown to the right, we first want to look at the pulse pressure, as shown on the bottom graph. The red and blue squares at each time show the systolic and diastolic pressures. The pulse pressure is the difference between the two.

Notice how the pulse pressure is less in the initial vertical position. Then as the subject is moved horizontally, the pulse pressure increases.

Why is the pulse pressure increased as the tilt table is moved from the vertical position to the horizontal position? (In your explanation, be sure to use central venous pressure and end-diastolic volume. ) What will this do to the cardiac output and hence (at least briefly) to the mean arterial pressure?

(The subject was also briefly tilted slightly head down from about 11 to 13 minutes.)

Now observe what happens as the subject moves from the horizontal position back to the vertical position. Again, we see the pulse pressure is smaller in the vertical position.

Hopefully by now you have realized that we are changing the stroke volume via the Frank-Starling mechanism with the tilt table. When this happens, the cardiac output necessarily changes. This in turn would change the mean arterial pressure.

But if the mean arterial pressure changes, the carotid baroreceptor reflex immediately steps in to bring the pressure back to the original value. We observe the effect of the carotid baroreceptor reflex by following the heart rate. Notice how the heart rate changes throughout the experiment to compensate for the change in stroke volume. It is higher in the vertical position and lower in the horizontal postion.

In other words, the tilt-table is used to change the stroke volume (as indicated by the pulse pressure), and then the carotid baroreceptor reflex changes the heart rate in order to keep the blood pressure constant.

Note that walking has a similar effect on the stroke volume as moving to the horizontal position. This is because muscle pumping forces blood centrally, increasing the central venous pressure.

Notice how much scatter there is in the heart rate data. That is not measurement error -- the results are quite accurate. The heart rate of healthy young people actually fluctuates around quite a bit as the vagus nerve constantly regulates the heart rate.

Quick Quiz: Tilt-table

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Correct Answer

Does the pulse pressure increase or decrease as the subject moves from the vertical to the horizontal position?

What factor changes to cause the observed change in the pulse pressure?

What is the second factor that determines the pulse pressure?

Following a decrease in the stroke volume, what happens to the heart rate due to the carotid baroreceptor reflex?

Does the central venous pressure increase or decrease as the subject moves from the horizontal to the vertical position?

(Spelling must be correct)