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# End diastolic pressure-volume relationship (EDPVR)

Find out what happens when the left ventricle is not allowed to contract, and instead you simply add and take away blood from it. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

## Want to join the conversation?

• If the atmospheric pressure is 760 mmHg, how is our heart only 120 mmHg during systole? I would imagine that a closed bag of air (atmosphere) would have less pressure than a closed bag being contracted without air being let out (LV). To me, the 2 numbers seem disproportionate, could someone please explain? Thx. :D
• Blood pressure is what is called a gauge pressure. Meaning it is taken in relation to a set zero value, that is not actually equal to zero, rather it is a number we measure and say is zero. In this case it is atmospheric pressure, which is 760mmHg ASL. Now, at sea level (ASL) there is 760mmHg of pressure on everything, thus any movement must first over come this pressure and BP (blood pressure) is no different. This means that 120mmHg is a gauge pressure and it can be translated from gauge pressure to an absolute pressure simply by adding 120 and the absolute pressure it must first overcome in-order to move. So at sea level, BP is 120 plus the 760 it must overcome and this gives us 880 mmHg.

We ignore the absolute pressure because it is a constant that affects everything equally and will throw-off calculations and ratios because it can make things look more similar than they actually are if it is larger, (basically it is a constant that is too large). When we look for cause and effect we need to look at how things are different, many times in isolation from how they are the same. The diff in 880 and 900 of a patient's BP gives us a .97 ratio of normal, but if we remove the constant pressure, we get 120 and 140, which yields a ratio of .85. The first ratio of .97 implies the pt has an insignificant deviation from normal (3%) due to randomness or other small deviations, and the second ratio shows a deviation of 15% from what is considered normal. This deviation is clearly significant and warrants investigation and can only be appreciated if we look at how things are different in isolation of constants.

Hope this helps!
• Is this diastolic pressure true?
• I believe you are questioning why ventricular end diastolic pressure seems very low, relative to diastolic pressure values for typical blood pressure. This is because at the end of systole, when ventricular contraction ends and the ventricle relaxes, left ventricular pressure drops below that of the pressure in the aorta, and blood begins to backflow from aorta to L ventricle. However, this backflow causes the aortic valve to be pushed shut, thus no longer allowing the pressure in the aorta to be transferred to the ventricle - the blood in the aorta is discontinuous from the blood in the ventricles once the aortic valves close. Arterial diastolic pressure is normally maintained at around 70-80 mmHg due to the elasticity of the arterial vessel walls pushing inward, however it is normal for the pressure in the ventricle to drop to 0-10 mmHg. So, short answer is yes, those diastolic values are normal.
• Is pressure and volume in the left ventricle a indirect relationship (one goes up the other goes down) or direct one goes up the other goes up (direct)?
• I would say it is not direct. However, it is not an inverse relalationship. If volume increases, pressure increases, but a direct relationship implies a linear relationship, which it is not. The greater the initial volume of blood, the greater the pressure increase for a given amount of blood entering the ventricle. For example, the pressure increase from a volume increase from 70 to 80 mL would be less than that from a a change from 90 to 100 mL
• He says several time that it is a line. Well it is not a line. It is an exponential function right? I mean more volume, even more pressure than the last time volume went up and the inverse for less volume means that pressure and volume, assuming something is in a constant state(that is relaxed or contracted) are exponentially related and so the elastance is never really a line even though the systolic and arterial get really close to linear right?
• It looks like an exponential function the way he drew it but it is not. He drew x/y or p/v not x^2 or p^2 or v^2. It is a linear relationship not an exponential one.
• How do you measure something like this clinically?