Applications of a novel radio-telemetry method for the measurement of intrathoracic pressures and physiological rhythms in freely behaving mice.

Techniques to comprehensively evaluate pulmonary function carry a variety of limitations, including the ability to continuously record intrathoracic pressures (ITP), acutely and chronically, in a natural state of freely behaving animals. Measurement of ITP can be used to derive other respiratory parameters, which provide insight to lung health. Our aim was to develop a surgical approach for the placement of a telemetry pressure sensor to measure ITP, providing the ability to chronically measure peak pressure, breath frequency, and timing of the respiratory cycle to facilitate circadian analyses related to breathing patterns. Applications of this technique are shown using a moderate hypoxic challenge. Male C57Bl6 mice were implanted with radio-telemetry devices to record heart rate, temperature, activity, and ITP during 24 h normoxia, 24 h hypoxia (FIO2 = 0.15), and return to 48 h normoxia. Radio-telemetry of ITP permitted the detection of hypoxia-induced increases in 'the ITP-equivalent' of ventilation, which were driven by increases in breathing frequency and ITP on a short-term timescale. Respiratory frequency, derived from pressure waveforms, was increased by a decrease in expiratory time without changes in inspiratory time. Chronically, telemetric recording allowed for circadian analyses of respiratory drive, as assessed by inspiratory pressure divided by inspiratory time, which was increased by hypoxia and remained elevated for 48 h of recovery. Further, respiratory frequency demonstrated a circadian rhythm, which was disrupted through the recovery period. Radio telemetry of ITP is a viable, long-term, chronic methodology that extends traditional methods to evaluate respiratory function in mice.