Heart Rate & Mean Arterial Pressure

[Carlos Danger]

Your patient has a HR of 42 and a BP of 122/68.

A few minutes later, his HR increases to 77. You cycle the BP and it is still 122/68.

Has the Mean Arterial Pressure (MAP) changed? If so, in what way, and why?

 

[Summit]

HR isn't that high at 77 so if all I have are the numbers you gave, I'd say the MAP hasn't changed much, although I suspect it increased slightly. I could more confidently say that SV and/or SVR have decreased.

 

[Jason]

MAP = (CO × SVR) + CVP
Because CVP is usually at or near 0 mmHg, this relationship is often simplified to:
MAP approx = CO × SVR
Therefore, changes in either CO or SVR will affect MAP. If CO and SVR change reciprocally and proportionately, then MAP will not change. For example, if CO doubles and SVR decreases by one-half, MAP does not change (if CVP = 0). It is important to note that variables found in equation 1 are all interdependent. This means that changing one variable changes all of the others.

In practice, MAP is not determined by knowing the CO and SVR, but rather by direct or indirect measurements of arterial pressure. From the aortic pressure trace over time, the shape of the pressure trace yields a mean pressure value (geometric mean) that is less than the arithmetic average of the systolic and diastolic pressures as shown to the right.

For example, if systolic pressure is 120 mmHg and diastolic pressure is 80 mmHg, then the mean arterial pressure is approximately 93 mmHg using this calculation. At high heart rates, however, MAP is closer to the arithmetic average of systolic and diastolic pressure (therefore, almost 100 mmHg in this example) because of the change in shape of the arterial pressure pulse (it becomes narrower). To determine mean arterial pressure with absolute accuracy, analog electronic circuitry or digital techniques are used. In normal clinical practice, however, systolic and diastolic pressures are measured, not MAP. That measurement is only measured when SVR needs to be calculated.

http://www.cvphysiology.com/Blood Pressure/BP006.htm
The above site also has some diagrams that are useful for the visual learner.

 

[TXmed]

The MAP hasn't changed because it doesn't factor HR. "2x diastolic +systolic /3"

But SV and minute volume have

 

[TXmed]

MAP = (CO × SVR) + CVP
Because CVP is usually at or near 0 mmHg, this relationship is often simplified to:
MAP approx = CO × SVR
Therefore, changes in either CO or SVR will affect MAP. If CO and SVR change reciprocally and proportionately, then MAP will not change. For example, if CO doubles and SVR decreases by one-half, MAP does not change (if CVP = 0). It is important to note that variables found in equation 1 are all interdependent. This means that changing one variable changes all of the others.

In practice, MAP is not determined by knowing the CO and SVR, but rather by direct or indirect measurements of arterial pressure. From the aortic pressure trace over time, the shape of the pressure trace yields a mean pressure value (geometric mean) that is less than the arithmetic average of the systolic and diastolic pressures as shown to the right.

For example, if systolic pressure is 120 mmHg and diastolic pressure is 80 mmHg, then the mean arterial pressure is approximately 93 mmHg using this calculation. At high heart rates, however, MAP is closer to the arithmetic average of systolic and diastolic pressure (therefore, almost 100 mmHg in this example) because of the change in shape of the arterial pressure pulse (it becomes narrower). To determine mean arterial pressure with absolute accuracy, analog electronic circuitry or digital techniques are used. In normal clinical practice, however, systolic and diastolic pressures are measured, not MAP. That measurement is only measured when SVR needs to be calculated.

http://www.cvphysiology.com/Blood Pressure/BP006.htm
The above site also has some diagrams that are useful for the visual learner.

That is interesting. I've never seen or heard it put that way. Thanks for posting it. This is why I frequent this site

 

[jwk]

SImply based on the numbers given, it has not changed.

 

[Underoath87]

While the calculated MAP hasn't changed, the true Main (or average) Arterial Pressure has. The equation S+(Dx2)/3 is tailored to work with a normal heart rate, where the heart is in diastole for twice as long as it is in systole. But as the HR declines, then the heart will spend more time in diastole. In the case you presented, it would turn the equation into something like: S+(Dx4)/5, since a heart rate cut in half is in diastole twice as long. MAP of 86 for the first case (normal HR) vs 79 for the bradycardic one.

 

[jwk]

While the calculated MAP hasn't changed, the true Main (or average) Arterial Pressure has. The equation S+(Dx2)/3 is tailored to work with a normal heart rate, where the heart is in diastole for twice as long as it is in systole. But as the HR declines, then the heart will spend more time in diastole. In the case you presented, it would turn the equation into something like: S+(Dx4)/5, since a heart rate cut in half is in diastole twice as long. MAP of 86 for the first case (normal HR) vs 79 for the bradycardic one.

I haven't found a reference for that particular estimation, although I did find something similar on http://www.ncbi.nlm.nih.gov/pubmed/15558774 that does take heart rate into account. Regardless, they're all estimations, and a few mmHg difference is probably not that significant in most patients (although obviously the difference between 50 and 90 is quite significant regardless of method). The only way to get a true mean is with an arterial line and a monitor that will calculate the area under the pressure wave. I think most oscillometric monitors calculate MAP using the [(2 x diastolic)+systolic] / 3 method.

 

[Aprz]

I haven't found a reference for that particular estimation, although I did find something similar on http://www.ncbi.nlm.nih.gov/pubmed/15558774 that does take heart rate into account. Regardless, they're all estimations, and a few mmHg difference is probably not that significant in most patients (although obviously the difference between 50 and 90 is quite significant regardless of method). The only way to get a true mean is with an arterial line and a monitor that will calculate the area under the pressure wave. I think most oscillometric monitors calculate MAP using the [(2 x diastolic)+systolic] / 3 method.

"MAP = DP + [0.33 + (HR x 0.0012)] x [PP] where SP and DP are systolic and diastolic pressure and HR is heart rate" from the link. They defined DP and HR. I am assuming PP is pulse pressure? Seems right when I plug in HR of 72 and BP of 120/80. I got a MAP of about 96. I just didn't see them define PP anywhere.

 

[Brandon O]

I think most oscillometric monitors calculate MAP using the [(2 x diastolic)+systolic] / 3 method.

I think that's actually backwards. They directly measure area-under-the-curve (i.e. MAP) and then derive the systolic and diastolic.

 

[Carlos Danger]

I think that's actually backwards. They directly measure area-under-the-curve (i.e. MAP) and then derive the systolic and diastolic.

Not something I've spent any time looking into, but like JWK said, I was taught that invasive monitors use algorithms that allow for areal calculations - and they can do so because they have more data points that allow for a more accurate curve to be drawn than a NIBP can - and that oscillometric monitors use an equation based on SBP and DBP, and possibly HR.

The other day I was in a fairly long case and the patient's BP and HR were remarkably stable, but the displayed MAP seemed to change more than the other parameters. After watching it for a while and comparing the monitor readings to my own calculations, it seemed that HR was having more of an impact than I would have expected - subtle changes were causing noticeable differences in MAP - which I didn't think the monitor would take into account. I guess it either uses a more sophisticated equation than the [2(DBP)+SBP]/3 that we all know.....OR you are right and it actually draws a curve and measures beneath it, just like an invasive monitor.

Not the first time I've wondered about this, but this example was very vivid because the parameters were varying so little, yet still affecting MAP. I should have taken a pic of all the readings.

 

[MackTheKnife]

Your patient has a HR of 42 and a BP of 122/68.

A few minutes later, his HR increases to 77. You cycle the BP and it is still 122/68.

Has the Mean Arterial Pressure (MAP) changed? If so, in what way, and why?

Calculated MAP hasn't changed. 2X Diastolic + Systolic/3 would yield the same number.