Dear Jennifer, if you can measure one, probably sleeping rat, I can imagine getting heart rate from cage vibrations. 

Hello Jennifer,

there is two option:

-If it is acute  heart rate measurement , you can cannulate  animal femoral artery and calculate heart rate using Labchart .

- For chronic  measurement of heart rate ,might Physio-suite from Kent  be good option.

https://www.kentscientific.com/products/productView.asp?productID=6437&Mouse_Rat=PhysioSuite%AE&products=PhysioSuite%AE

Hi Jennifer, I assume that you want to measure the heart rates in freely moving conscious rats. For long term monitoring for more than a few days, telemetry is the better option, in my opinion. You may be able to find a receiver on Ebay, but you will probably have to acquire the implantable transmitters from the vendor. Some transmitters do not require battery and are reusable, such as Emitters. For short term, I think you should be able to route your subcutaneous electrodes in the chest muscles to the connector plug that is cemented at the top of the skull, and connected the plug to a recording cable after animal recovered from the surgery. The movements of cable tethering on top of the head tends to generate some artifacts during wakefulness, and sometimes the animal can still chew it up despite the effort of shielding the cable with metal spring- but in general it should work. 

Dear Jennifer,

depending on your experiment the golden standard for long-term measurement is telemetry. You will get good signal quality and most important no stress "artefacts", but it is also the most expensive option. Consider a collaboration by identifing research institutes nearby using radio-telemetry in rodents.

For short, acute measurements you may be able to use the method described in "Noninvasive method for electrocardiogram recording in conscious rats: feasibility for heart rate variability analysis".

See: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0001-37652010000200019

But be aware of stress artefacts due to the use of the "jacket" and the restrainer.

Another, commercial option is https://mousespecifics.com/heart-monitoring/ecgenie/

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0001-37652010000200019

https://mousespecifics.com/heart-monitoring/ecgenie/

HI,

I suggest using optical detection using a LED light source / photo detector such as the OSRAM 7050 ($0.75).  It can be digitally controlled using a TI AFE 4404 ($2.65), or driven using analog circuits.  The TI AFE 4044 may be driven using an I2C interface, for example from an Arduino for prototype.  Smaller systems may be developed using microcontrollers such as the NRF51822 from Nordic Semiconductor (or other similar microcontrollers that include BlueTooth LE) if you need the rat to be mobile.

Shave the rat.  Adhere the 7050 to the skin surface.  Set the Green LED to illuminate for about 100 uS every 1 ms.  You could use also the IR and the Red LED, but green seems to provide the best pulsatile information.   Read the output of the photodetector near the middle of the 100 us.  The little bumps are pulses.  Take the first derivative of the data and the bumps are larger & easier to detect.

This works on people with white and dark skin, not sure about rats.  

The TI AFE 4044 has an evaluation module that is $199.  

Alternately, you can build a basic ECG circuit using an instrumentation amplifier, or a "real" system using something like a TI ADS 1291.  As with the AFE 4044, there is an evaluation module available for $199.

http://www.osram-os.com/osram_os/en/press/press-releases/ir-devices-and-laser-diodes/2014/compact-osram-sensor-improves-fitness-monitoring/index.jsp

http://www.ti.com/tool/afe4404evm

http://www.ti.com/product/AFE4404/description

https://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF51822

The osram 7050 is no longer in production. Another option is the SFH 7050. It only has green LEDs, but for that wavelength, It provides a better signal than the 7050.