Using an Ingestible Telemetric Temperature Pill to Assess Gastrointestinal Temperature During Exercise

Summary

This study describes an accurate, reliable and non-invasive technique to continuously measure gastrointestinal temperature during exercise. The ingestible telemetric temperature pill is suitable to measure gastrointestinal temperature in laboratory settings as well as in field based settings.

Abstract

Exercise results in an increase in core body temperature (Tc), which may reduce exercise performance and eventually can lead to the development of heat-related disorders. Therefore, accurate measurement of Tc during exercise is of great importance, especially in athletes who have to perform in challenging ambient conditions. In the current literature a number of methods have been described to measure the Tc (esophageal, external tympanic membrane, mouth or rectum). However, these methods are suboptimal to measure Tc during exercise since they are invasive, have a slow response or are influenced by environmental conditions. Studies described the use of an ingestible telemetric temperature pill as a reliable and valid method to assess gastrointestinal temperature (Tgi), which is a representative measurement of Tc. Therefore, the goal of this study was to provide a detailed description of the measurement of Tgi using an ingestible telemetric temperature pill. This study addresses important methodological factors that must be taken into account for an accurate measurement. It is recommended to read the instructions carefully in order to ensure that the ingestible telemetric temperature pill is a reliable method to assess Tgi at rest and during exercise.

Introduction

The oxidation of substrates during muscle contractions, necessary to perform exercise and physical activity, importantly impacts our thermoregulatory system as only 20% is used for muscle power¹, whilst the majority of the energy is released as heat (80%)^2,3. As a consequence, the elevated metabolic heat production during physical activity and exercise typically exceeds the heat dissipation capacity^4,5, resulting in an increase in core body temperature (Tc). Accordingly, Tc rises above the hypothalamic set point, which is defined as hyperthermia⁶, and may even result in an attenuated exercise performance^5,7,8 and/or the development of heat-related disorders^4,6. For this reason it is important to accurately measure Tc during prolonged exercise and in particular in strenuous ambient conditions.

Literature describes that an ideal method to measure Tc should: 1) be easy applicable, 2) not be biased by environmental conditions, 3) have a high temporal resolution to rapidly monitor changes in Tc, and 4) have the capacity to detect small changes (Δ0.1°C) in core body temperature^9,10. An overview of the different methods to measure the Tc was given by the International Organization of Standardization (ISO 9886)¹¹. It was stated that the esophageal temperature at the level of the left atrium provides the closest agreement with central blood temperature, while this measure is able to rapidly detect (minor) changes in temperature¹². Although esophageal temperature measurements are generally accepted as the gold standard to record Tc, its invasive nature limits the practical use of this method. Alternative measures to monitor Tc rely on temperature recordings of external tympanic membrane, mouth, or rectum¹². These measurement sites are not optimal to measure the Tc, given their invasive character, methodological difficulties and/or the potential bias by environmental conditions^9,12-14 (Table 1). This highlights the need to explore alternative strategies to monitor (changes in) Tc.

Previous studies have described the use of an ingestible telemetric temperature pill as an easily applicable, reliable and valid method to measure the Tgi, which is a representative estimation of Tc^9,15. Another, important, advantage of the temperature pill is the suitability in field-based situations, which is of great importance since exercise-induced elevations in Tc are generally higher in field than in laboratory settings¹⁶. Currently, the temperature pill is able to measure the Tgi every 10 sec with an accuracy of ±0.1°C, which makes this technique very suitable to measure the Tgi during an exercise event or an important match. Furthermore, in a study by Stevens et al.¹⁷ it is demonstrated that the telemetric temperature pill may also be used to monitor intragastric temperature. The ingestible temperature pill is first described in 1961¹⁸, and further developed at the Johns Hopkins University (Baltimore, USA) in collaboration with the Applied Physics Laboratory of the NASA. The result is a 20 x 10 mm capsule with a telemetry system, micro battery and a quartz crystal temperature sensor. The crystal sensor vibrates at a frequency relative to the temperature of the surrounding substance. This temperature radio signal is transmitted through the body, which can be measured by an external recorder (Figure 1). Each temperature pill has a unique serial and calibration number, which can be used by the recorder to convert the radio signal and measure the corresponding Tgi.

A small magnetic strip is attached to the outside of the temperature pill, which deactivates the battery. When this magnetic strip is removed, the pill is activated immediately and starts measuring Tc (Figure 2). Casa and colleagues,¹⁹ used six different techniques (gastrointestinal, rectal, aural, temporal, axial and forehead) to measure Tc, with the rectal temperature set as the reference value. They demonstrated that the gastrointestinal measurement of Tc with the temperature pill is the only technique that shows good agreement with the reference Tc. Others investigated the relation between Tgi and rectal temperature and have shown a small but significant bias ranging from 0.07°C to 0.20°C ^9,15,20,21. Although the direction and magnitude of the bias differed between studies, the Bland and Altman 95% limits of agreement were ±0.4°C, which is acceptable^9,22. Additionally, in a review by Byrne et al.⁹ the Tgi is compared with the rectal and esophageal temperature (gold standard) as a measure for the Tc. They demonstrate that the Tgi measured with the temperature pill is a valid measure for Tc based on the good agreement between intestinal and esophageal temperature. Furthermore, the 95% Bland and Altman limits of agreement were limited to ±0.4°C ²², while no significant bias was found between the two measurements^9,20,21. These results suggest that the Tgi is a valid measure for Tc.

Another important aspect of a good Tc/Tgi measurement technique is a high temporal resolution to rapidly monitor changes in Tc. Previous studies have demonstrated that the Tgi measured with the temperature pill responds more slowly on changes in Tc compared to the esophageal measurement^15,20,23, which can be explained due to the low heat capacity of the esophagus and the proximity to the heart¹⁰. In the esophageal temperature measurement, the thermistor is placed at the level of the left atrium¹⁰. At this level, the pulmonary artery and the esophagus are in contact and isothermal²⁴, which stimulates a fast response time on changes in temperature of the esophageal measurement. In contrast, the intestines and rectum are less perfused compared to the esophagus, resulting in a delay in measuring temperature changes at these anatomical locations. However, the ingestible telemetric temperature pill has an accuracy of ±0.1°C and is able to measure Tgi every 10 sec. A previous study reported that core body temperature can rise at a maximum of 1°C every 5 min if no heat is removed during exercise²⁵. Therefore, the temporal resolution of the temperature pill is suitable to measure changes in Tgi during exercise. Based on these findings, it can be concluded that the temperature pill is a reliable and valid technique to measure Tgi. Despite the use of the telemetric temperature pill in a large number of studies, a clear description about how to use the temperature pill is missing.

Therefore, the purpose of this study is to provide a detailed description of the measurement protocol using an ingestible telemetric temperature pill. Secondly, the application of the telemetric temperature pill in two different study protocols are described, in which a cross-sectional design (measurement every 5 km with a different recorder) and a protocol that continuously records Tgi in individuals are used.

Protocol

The steps described in the following section are in line with and accepted by the medical ethical committee of the Radboud University Medical Center in Nijmegen, The Netherlands. To our knowledge, 3 different commercial systems of ingestible temperature pills are currently available for researchers. The user manual of the ingestible temperature pills is brand-specific (Table of Specific Materials), but all systems are suitable for measurements during exercise and under resting conditions. 1. Exc…

Representative Results

Representative results from our previous work demonstrating the methods are presented in the next section, in which an example of a cross sectional (Figure 3A) and a continuous measurement (Figure 3B) are given. Cross sectional measurement of Tgi An example of data from a cross sectional measurement is shown in Figure 3A. After obtaining baseline Tgi, subjects walked 30 km at a self-selected pace. During exercise th…

Discussion

The ingestible telemetric temperature pill has the ability to provide a continuous, valid and non-invasive measurement of the Tgi. Furthermore, an advantage of the temperature pill is the fact that once ingested, the subjects are unaware of the presence of the pill in the body or that the measurements are performed. Therefore, this method is easily applicable under resting conditions as well as during exercise, a minimal burden for study participants, and can therefore be used in field and laboratory settings. Another ad…

Materials

CorTemp data recorder	CorTemp system, HQ Inc., Florida, USA	Not applicable	http://www.hqinc.net/cortemp-data-recorder/
Cortemp ingestible telemetric temperature pill	CorTemp system, HQ Inc., Florida, USA	HT150002	http://www.hqinc.net/cortemp-sensor-2/
CorTrack II software (Data processing for a PC only)	CorTemp system, HQ Inc., Florida, USA	Not applicable	http://www.hqinc.net/cortrack-ii-data-graphing-software/