Over the past 20 years, research scientists have attempted to non-invasively measure mice and rat blood pressure (BP) with varying degrees of success.
The ability to accurately and non-invasively measure the systolic and diastolic blood pressure, in addition to the heart pulse rate and other blood flow parameters in rodents, is of great clinical value to the researcher.
The ability to accurately and non-invasively measure the systolic and diastolic blood pressure, in addition to the heart pulse rate and other blood flow parameters in rodents, is of great clinical value to the researcher.
Invasive Blood Pressure, Rat and Mouse Measurement
Direct blood pressure, an invasive surgical procedure, is the gold standard to compare the accuracy of non-invasive blood pressure (NIBP) technologies. Direct blood pressure should be obtained on the rodent's carotid artery when comparing to NIBP. "Validation in Awake Rats of a Tail Cuff Method for Measuring Systolic Pressure", Bunag, R.D., Journal of Applied Physiology, Vol 34, Pgs 279-282, 1973.
Radiotelemetry, a highly invasive surgical procedure, is a very reliable blood pressure technology and is also utilized to compare the accuracy of NIBP technologies. Telemetry involves the implantation of radio transmitters in the rodent's body. This technique is well validated and has excellent correlation with direct blood pressure.
The advantage of implantable radio telemetry is the ability to continuously measure rat and mouse blood pressure in free moving laboratory animals.
The disadvantages of radiotelemetry are: (1) morbidity associated with the initial surgical implantation of the transmitter; (2) morbidity associated with surgery required to replace the battery, which has a short battery life; (3) increase in the animal's level of stress, especially mice, in relationship to the large, heavy transmitters (2004, ATLA, 4th World Congress, Einstein, Billing, Singh and Chin); (4) abnormal behavior since the animal cannot have social interaction due to the current technology requiring the implanted animal to be isolated, one animal per cage; (5) inability to perform high throughput screening; (6) high cost of the initial equipment set-up and the expensive transmitters that require frequent factory maintenance; (7) cost of material and human resources relating to ongoing surgeries; and (8) the lack of a competitive market resulting in high product and servicing costs.
Non-Invasive Blood Pressure, Rat and Mouse Measurement
The NIBP methodology consists of utilizing a tail cuff placed on the tail to occlude the blood flow. Upon deflation, one of several types of NIBP sensors, placed distal to the occlusion cuff, can be utilized to monitor the rat BP. There are three (3) types of NIBP sensor technologies: photoplethysmography, piezoplethysmography and Volume Pressure Recording. Each method will utilize an occlusion tail-cuff as part of the procedure.
1. Photoplethysmography
The first and oldest sensor type is Photoplethysmography (PPG), a light-based technology. The purpose is to record the first appearance of the pulse while deflating the occlusion cuff or the disappearance of pulses upon inflation of the occlusion cuff. Photoplethysmography utilizes an incandescent or LED light source to record the pulse signal wave. As such, this light-based plethysmographic method uses the light source to illuminate a small spot on the tail and attempts to record the pulse.
Photoplethysmography (PPG) is relatively inaccurate since the readings are based solely on the amplitude of a single pulse and can only imprecisely measure the systolic blood pressure and the heart beat. There are many limitations to a light-based technology, such as: (1) over-saturation of the BP signal by ambient light; (2) extreme sensitivity to the rodent's movement (motion artifact); and (3) the difficulty in obtaining adequate mice blood pressure signals in dark skinned rodents (Pigmentation Differentiation). Light-based sensors also cause tail burns from close contact and prolonged exposure.
Diastolic blood pressure cannot be measured by photoplethysmography since the technology records only the first appearance of the pulse. If the diastolic BP is displayed on the photoplethysmographic instrumentation, it is only an estimation that is calculated by a software algorithm rather than a true measurement.
Additional variability and inaccuracy occurs in PPG devices that rely on obtaining readings during occlusion cuff inflation.
Occlusion cuff length is also another source of variability and inaccuracy. Occlusion cuff length is inversely related to the accuracy of the blood pressure. Long cuffs, predominantly in most photoplethysmographic devices, record lower than the actual blood pressure measurements.
These limitations severely compromise the consistency, dependability and accuracy of the NIBP measurements obtained by devices that utilize light-based/LED photoplethysmographic technology.
The photoplethysmography method correlates poorly with direct blood pressure measurements and is the least recommended sensor technology for NIBPe in rodents, especially mice.
2. Piezoplethysmography
The second NIBP sensor technology is piezoplethys-mography. Piezoplethysmography and photoplethysmography require the same first appearance of a pulse in the tail to record the systolic blood pressure and heart rate.
Both plethysmographic methods have similar clinical limitations. Whereas photoplethysmography uses a light source to attempt to record the pulse signal, piezoplethysmography utilizes piezoelectric ceramic crystals to do the same. From a technical point of view, piezoplethysmography is far more sensitive than photoplethysmography since the signal from the sensor is the rate of change of the pulse rather than just the pulse amplitude. Therefore, even extremely small mice with high velocity pulses will generate a sufficient signal to be detected with simple amplifiers.
Piezoelectric sensors are more accurate than light-based/LED sensors but the same plethysmographic limitations continue to produce inaccuracies in blood pressure measurements. On a positive note, the skin pigment of the rodent is not a measurement issue with piezoplethysmography as with photoplethysmography.
Although piezoplethysmography is better than photoplethysmography, both non-invasive tail-cuff blood pressure technologies correlate poorly with direct blood pressure measurements.
3. Volume Pressure Recording
The third sensor technology is Volume Pressure Recording (VPR). The Volume Pressure Recording sensor utilizes a specially designed differential pressure transducer to non-invasively measure the blood volume in the tail. Volume Pressure Recording will actually measure six (6) blood pressure parameters simultaneously: systolic, diastolic, mean, heart pulse rate, tail blood volume and tail blood flow.
Since Volume Pressure Recording utilizes a volumetric method to measure the blood flow and blood volume in the tail, there are no measurement artifacts related to ambient light; movement artifact is also greatly reduced. In addition, Volume Pressure Recording is not dependent on the animal's skin pigmentation. Dark-skinned animals have no negative effect on Volume Pressure Recording measurements. Very small, 10-gram C57/BL6 black mice are easily measured by the Volume Pressure Recording method.
Special attention is afforded to the length of the occlusion cuff with Volume Pressure Recording in order to derive the most accurate blood pressure readings.
Volume Pressure Recording is the most reliable, consistent and accurate method to non-invasively measure the blood pressure in mice as small as 10 grams to rats greater than 950 grams.
In an independent clinical validation study conducted in 2003 at Yale University, New Haven, Connecticut, Volume Pressure Recording correlated 99 percent with direct blood pressure:
"Volume Pressure Recording is excellent. It is very accurate and dependable. We performed experiments on temperature-controlled, adult rats and the non-invasive blood pressure measurements showed almost perfect correlation with invasive blood pressure measurements. We are very pleased with the results."
Numerous published research papers are available validating the accuracy, reliability and consistency of Volume Pressure Recording. See the Clinical Bibliography section.
