A tilt table test is used to verify if a patient has low pressure drops associated with syncope. The heart, of a horizontal patient, pumps blood at a mean arterial pressure of 90 mmHg (12 kPa). The patient faints as soon as the tilt table goes vertical, this means the blood acceleration equal the gravitational acceleration. If the heart rate does not increase to compensate for the change in angle, at what angle would the blood be pumping one half its horizontal pressure.
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Correct Answer: B. 30o.
Pressure is force over area. The cross-sectional area of the of the circulatory system is not changing. The direction of the force appears to be changing. But the blood is still flowing orthogonal to the area, creating the same pressure due to the heart. The only additional element is that gravitational force is being applied.
This additional gravitation force opposes the heart force by a factor of sin(theta) times its weight. We are told that the blood acceleration is equal to g. All we need to find now is when the sin is equal to one half. This will yield a pressure of one half of the horizontal value. This occurs at 30o.
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