Refinement of Telemetry for Measuring Blood Pressure in Conscious Rats

Abstract

Although considered the ‘gold standard’ for measuring blood pressure in laboratory animals, telemetry would benefit from refinement. In the present study, we tested the hypothesis that the small telemetric device used for blood pressure recording in mice would work for rats as well and would serve as an alternative for those studies where abdominal cavity space is quite limited (such as in young animals and pregnant females). Here we report that the use of a smaller and lighter telemetric device implanted in the abdominal aorta of rats led to acquisition of stable and high-quality blood pressure and heart rate data, similar to those obtained by using a larger telemetric device developed for rats. The use of smaller transmitters represents an alternative telemetry technique, especially for those cases in which space in the abdominal cavity is particularly limited such as during pregnancy.

Radiotelemetry is the state-of-the-art method for monitoring physiologic functions in awake and freely moving laboratory animals while minimizing stress-associated artifacts.^2,3,10 Currently, radiotelemetry systems are used widely for measurements of blood pressure, heart rate, blood flow, electrocardiogram, respiratory rate, body temperature, and many other biologic signals. Although radiotelemetry technology has existed for at least 50 y,^8,9 affordable, reliable, user-friendly commercial products have become available for monitoring physiologic signals in laboratory animals only during the last 10 y. In particular, the use of telemetry for measuring blood pressure in mice and rats has helped researchers to unravel several mechanisms involved in the physiology and pathophysiology of diseases such as hypertension, heart failure, and preeclampsia.^4,6,7,12,15

Telemetry has become the ‘gold standard’ for measuring blood pressure in laboratory animals,^10,11 mainly because this technology reduces or eliminates discomfort caused to the animals (for example, there is no need for restraints). However, like many other techniques for noninvasive measurements, telemetry procedures require refinement.

In the present study, we tested the hypothesis that a small telemetry device used for blood pressure recording in mice could be used for rats. This adapted use would provide an alternative for future studies in models in which space in the abdominal cavity is particularly limited such as during pregnancy. Here we propose a new telemetry approach that could be applied to rats implanted with radiotelemeters for blood pressure recordings. We replaced a telemetric device recommended for use in rats (device weight, 7.6 g; volume, 4.4 mL) with a considerably smaller device designed for mice (device weight, 1.4 g; volume, 1.1 mL). Using the smaller device, we were able to record high-quality blood pressure signals in very accurately, comparable to the data obtained by using the larger telemetry device designed for rats.

Materials and Methods

Animals and housing.

We obtained 12 male Sprague–Dawley rats (weight, 200 to 220 g) for this study from a commercial supplier (Harlan, Indianapolis, IN). Because we were studying blood pressure in adult rats, we chose to individually house the animals to give them sufficient cage space (39.3 cm × 28.5 cm × 19.4 cm). The cages had corncob bedding and were changed once each week on Mondays. Animals were checked and their drinking water refreshed daily in the morning, typically between 0900 and 1000 h. A 12:12-h light:dark cycle was maintained (light on, 0600 h) in the animal room, with constant temperature (21 ± 1 °C) and relative humidity (50% ± 2%). Rats had free access to tap water in drinking bottles and were fed a standard pelleted rodent chow ad libitum (containing 18% protein, 5% fat, and 5% fiber: Harlan Teklad, Madison, WI). All the procedures met or exceeded the guidelines set forth by the National Institutes of Health and were approved by the University of Chicago Animal Care and Use Committee.

Transmitter implantation.

Animals were anaesthetized with pentobarbital (50 mg/kg IP; Nembutal, Abbott Laboratories, North Chicago, IL) and surgically prepared according to AAALAC-recommended aseptic techniques. Telemetric devices originally developed for use in mice (weight, 1.4 g; volume, 1.1 mL; model PAC10; Data Sciences International, St Paul, MN) were implanted in 6 rats according to the standard procedure as previously described.² For comparison purposes, 6 rats were implanted with the commonly used rat telemetric device (weight, 7.6 g; volume, 4.4 mL; model PAC40, Data Sciences International). Briefly, a midline abdominal incision was made, and the abdominal aorta was exposed by using sterile cotton swabs. The catheter of the telemetric device was inserted into the abdominal aorta and guided upstream. Tissue adhesive (Vetbond, 3M Animal Care Products, St Paul, MN) was used to secure the catheter and obtain hemostasis. The body of the telemetric device was placed in the abdominal cavity and sutured to the abdominal musculature. The abdominal musculature and skin were closed individually. Rats were kept on a heating pad (38 °C) until fully recovered from anesthesia. Analgesia was achieved by the administration of buprenorphine (0.5 mg/kg SC) every 12 h for 2 d. Animals were allowed 1 wk for recovery before being used in experiments.

Telemetry and data acquisition.

Blood pressure (mm Hg) was recorded (Dataquest ART 2.1 Gold Telemetry System, Data Sciences International, with RPC1 receivers) continuously for 1 h twice daily (between 0800 and 0900 and between 2000 and 2100) on 1 d each week for 3 wk. The transmitters were turned on by using a magnetic device, which was briefly positioned close to the animal from the outside of the cage, 20 min before starting the recordings. The same procedure was performed to turn the transmitter off after the recordings. Heart rate data were derived from the blood pressure signal offline. Data from each animal were exported (Dataquest ART analysis software; Data Sciences International) to a spreadsheet program (Excel 2003, Microsoft Corporation, Seattle, WA) and plotted (GraphPad Prism 4, GraphPad Software, La Jolla, CA).

X-ray imaging.

To compare the 2 different-sized telemetric devices implanted, a microfocal X-ray source and a CsI-coupled complementary metal-oxide semiconductor detector were used for imaging. The microfocal X-ray system (model MX20, Faxitron, Wheeling, IL), which comprised a tungsten anode with beryllium exit window, was operated at 32 KV, with a tube current of 0.3 mA and exposure time of 3.3 s. The distance from the source to the detector was 57.1 cm. Under pentobarbital anesthesia (50 mg/kg IP), rats were placed on top of the detector; therefore, the magnification effect was minimal. Two sets of images (craniocaudal and lateral views) were obtained. All images were acquired by a 14-bit digital camera (Bioptics, Tucson, AZ), transferred to a computer and analyzed by using the manufacturer-supplied software (Faxitron).

Statistical analysis.

All values are presented as mean ± SEM. The reproducibility of the parameters recorded was tested by 1-way ANOVA followed by the Tukey posttest (GraphPad Prism 4, GraphPad Software) when necessary. A P value of less than 0.05 was considered statistically significant.

Results

Blood pressure and heart rate measurements.

The blood pressure signals obtained by both telemeter types during the 3 wk of the study are illustrated in Figure 1. Of note, 21 d after the implantation of the telemetric devices, blood pressure signals remained consistently stable in all animals, regardless of whether they were implanted with the rat device (PAC40) and that originally developed for mice (PAC10). In addition, the values for mean arterial pressure (Figure 2 A) and heart rate (Figure 2 B) on days 7, 14, and 21 d telemeter implantation were comparable between both groups animals during the inactive period (0800 to 0900) as well as the active period (2000 to 2100). Taken together, the data from the inactive and active periods suggest that the small telemetric device can be used without compromising the reliability of cardiovascular data acquired from rats as shown in Table 1.

Figure 1.

Typical blood pressure tracings of 2 different rats implanted with a telemetric device designed for rats (PAC40) or mice (PAC10). Animals were sampled once every 7 d for 21 d. Tracings are shown in 3 different time scales (30, 2.5, and 0.5 s).

Figure 2.

Group data illustrating the values for (A) mean arterial pressure (MAP, mm Hg) and (B) heart rate (HR, bpm) recorded during the day (white background) or night (gray background) at 7, 14, and 21 d after rats were implanted with a telemetric device designed for either mice (PAC10) or rats (PAC40). Data are expressed as mean ± SEM (n = 6 rats per group); P > 0.05 between groups.

Table 1.

Cardiovascular parameters in conscious rats implanted with telemetry devices designed for rats (PAC40) or mice (PAC10).

		PAC40		PAC10
Days after implantation	Time of recording	Mean arterial pressure (mm Hg)	Heart rate (bpm)	Mean arterial pressure (mm Hg)	Heart rate (bpm)
7	0800–0900	107 ± 2	360 ± 5	110 ± 2	358 ± 7
7	2000–2100	110 ± 2	361 ± 4	107 ± 3	360 ± 5
14	0800–0900	107 ± 1	367 ± 5	109 ± 2	359 ± 6
14	2000–2100	107 ± 2	367 ± 4	108 ± 2	359 ± 6
21	0800–0900	109 ± 1	357 ± 7	110 ± 2	368 ± 4
21	2000–2100	110 ± 1	365 ± 7	108 ± 3	368 ± 4

X-ray imaging.

Radiographs of 2 different rats implanted with either a mouse (PAC10, Figure 3 A, B) or rat (PAC40, Figure 3 C, D) telemetric device reveal that the rat device occupies considerably more space than does the smaller telemetric device designed for mice. These observations highlight the importance and refinement of the proposed method.

Figure 3.

Radiographs of 2 rats each implanted with a telemetric device designed for either (A and B) mice (PAC10) or (C and D) rats (PAC40).

Discussion

Minimizing pain and distress and promoting animal wellbeing are important factors to address when acquiring reliable results from experimental rodent models. Compared with conventional cardiovascular measurement methods, telemetry offers several advantages including 1) the elimination of the confounding effects of stress introduced by handling, restraint, and anesthesia; 2) reduction in the number of animals used by 60% to 70%;^8,14 and (c) unrestricted continuous data collection for months without the need for any additional manipulation. However, like many other procedures involving animals, even telemetry techniques would benefit from improvement.¹¹ In the current study, we refined the technique for blood pressure recording in conscious rats by reducing the size and weight of the telemetric device used. To this end, we implanted rats with telemetric devices originally developed for mice, which are markedly smaller and lighter than the devices developed specifically for rats. Our results demonstrate that we were able to collect reliable blood pressure and heart rate data from conscious rats by using the miniaturized telemetric device.

The effect of telemetry on an animal's welfare depends on the nature of the technique chosen for implantation and the mass and volume of the device.¹¹ Adding additional mass to animals’ bodies can have a significant physiologic effect and can cause distress and discomfort, particularly in small species such as rodents. For instance, increasing the mass in abdominal viscera can compromise diaphragmatic movement and alter breathing pattern.¹³ In the short term, changes in body mass and behavior after implantation surgery in mice indicate that their wellbeing is impaired during the first days after surgery.¹ An animal may require 5 to 7 d to regain its circadian rhythm.³ Recovery of wellbeing follows the same time course in rats implanted with transmitters.⁵ Therefore, any device should be as light as possible, but it may be difficult to establish general principles on establishing the appropriate device size and mass.

Considering that the size and shape of the device must avoid or minimize any compromise of the normal physiologic function or welfare of the animal, the telemetric device designed for mice in rats affords some remarkable advantages. For example, implantation of the smaller telemeter is easier because the diameter of the catheter tip is smaller than that for the device designed for rats, thus facilitating insertion into the abdominal aorta. Another important advantage is the size of the animal that can be implanted: the mouse telemeter can be implanted in animals as small as 17 g whereas the rat telemeter is only appropriate for animals weighing at least 175 g. Therefore, our proposed approach would be useful for blood pressure studies in neonates and young rats. In addition, the mouse device occupies much less space in the abdominal cavity (Figure 3), allowing better accommodation of the internal organs around the transmitter. This feature would be particularly relevant for researchers investigating cardiovascular physiology and physiopathology during pregnancy (such as preeclampsia). As important disadvantages, the transmitters designed for mice are more expensive and have a considerably shorter battery life than do the rat transmitter. The costs for a long-term experiment might be increased by 70% to 100% when using the mice transmitter. The main points that must be taken into account for designing such studies are the battery life of the mice transmitter, which is about 6 wk of continuous recording (compared with 17 wk for the rat transmitter), and the costs for refurbishing the transmitter (that is, replacing catheters and recharging batteries) are considerably higher. All of these advantages and disadvantages should be taken into consideration when designing chronic experiments using different transmitters.

In conclusion, we have devised an alternative telemetry technique in which rats are implanted with telemetric transmitters designed for mice. This refinement allows the recording of reliable blood pressure and heart rate data similar to those acquired by using transmitters developed for rats. Reducing the size of the transmitter, while maintaining accuracy in data acquisition, will allow researchers to obtain reliable cardiovascular data that will help to elucidate complex diseases such as hypertension, heart failure, and preeclampsia while contributing to the welfare of the rats used in their research.