Can heart rate skew RMR/BF% tests - Science Q&A

CoderGal

QUESTIONS:

Can someone help me fill in the gaps?

Anyone know why doctors want your heart rate and blood pressure differences when they do RMR readings?

I'm wondering if someone has an unusual heart rate, how this will affect the accuracy of RMR and body fat readings?

LOGIC?:

Fact: If a doctor is doing a body fat reading they take heart rate and blood pressure differences.
Fact: heart rates and blood pressures and body fat have equations linked to voltages in the body

Therefore somewhere out there there's an equation linking heart rate/blood pressure to body fat

So, can an unusually high heart rate skew body fat reading?

SCIENCE CLASS:

BIA body fat analyzers use voltage to get your body fat %
V=Voltage=IR=Current*Resistance=Current*|Impedance|^(Imaginary#*phase)=Complicated

Therefore body fat % is linked to voltages in the body

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Average Blood Pressure (millimeters of mercury/mmHg) = mean arterial pressure (MAP) = CO*SVR+CVP
Cardiac Output (CO) = Heart rate * Stroke volume
systemic vascular resistance (SVR) in mm of merc/mmHg = difference in pressure/CO
central venous pressure (CVP)

so combining the above gives MAP=CO*resistance+CVP = (diff in pressure)+CVP = heart rate * stroke * SVR + CVP

heart rate = something to do with voltage since heart rate monitors can read heart rates

Therefore Heart rate and blood pressure relate to voltages in the body

yarwell

Body fat measurement by impedance imposes a voltage to measure the impedance, so I don't really follow "Therefore body fat % is linked to voltages in the body" - it's not a passive measurement it's like an ohmmeter (has batteries in to provide the current for the measurement). Water especially with sodium / potassium in it is a good conductor, so I suspect impedance is measuring water content as opposed to fat, then expressing it as fat (but I haven;t looked into that).

Doctors tend to record BP & HR at every opportunity, it gives them time to think and provides a historical record.

ETA: Heat rate monitor - http://en.wikipedia.org/wiki/EKG - "The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat. At rest, each heart muscle cell has a negative charge, called the membrane potential, across its cell membrane. Decreasing this negative charge towards zero, via the influx of the positive cations, Na+ and Ca++, is called depolarization, which activates the mechanisms in the cell that cause it to contract."

ETA2: BIA - http://www.tanita.co.uk/index.php?id=86 - "Tanita uses BIA to make an indirect measurement of body composition. A safe electrical signal (50Khz, 800µA) is sent through the body via the patented pressure-contact electrodes, housed in a single stand-alone unit. The only direct measurements that Tanita BIA makes are weight and impedance; all other values such as body fat percentage, Fat Free Mass, Total Body Water, etc., are calculated using an equation based on these and other values such as height, age, gender and body type."

CoderGal

so I suspect impedance is measuring water content as opposed to fat, then expressing it as fat (but I haven;t looked into that).

Correct. Most weight scales that use BIA will look at your weight, calculate your water via BIA, then figure out your lean body mass, fat mass, percentage ratios of both, and bone mass. So all these calculations can fluctuate and the accuracy can be messed up for various reasons...such as your to sweaty, dehydrated, ate to much salt, produced to much glycogen for other reasons and jazz etc.

It's not the voltage that screws things up, it's the other variables that I can imagine causing problems.

Drastiic

Water especially with sodium / potassium in it is a good conductor, so I suspect impedance is measuring water content as opposed to fat, then expressing it as fat (but I haven;t looked into that).

Doctors tend to record BP & HR at every opportunity, it gives them time to think and provides a historical record.

If you want an equation and explanation, see this link: http://www.brianmac.co.uk/fatbia.htm

As for doctors measuring BP & HR, I would agree with yarwell and add that the readings would give them a good baseline to suggest improvements depending if it's high/low and give proper instruction in conjunction with the body fat and RMR tests.

ETA: HR could in fact skew the readings INDIRECTLY. For example: if your heart rate is elevated, this could be due to recent exercise which could also mean loss of water due to sweat. If a BF impedance test was taken right after exercise, the results would reflect a lower water content.

Another example would be RMR. When testing RMR, they tell you to remain still as much as possible, which will keep your HR steady. If you start jumping up and down, your HR will increase, but so will the amount of oxygen being used.

CoderGal

Another example would be RMR. When testing RMR, they tell you to remain still as much as possible, which will keep your HR steady. If you start jumping up and down, your HR will increase, but so will the amount of oxygen being used.

At the same time one can remain perfectly still and increase and decrease their heart rate just by thought. I'm pretty good at it. It's a party trick when there's heart rate monitors lying around.

Thank you for that calculator btw I was wondering the math.