Effect of rising body temperature on respiratory chemosensitivity to CO2
© Hayashi et al.; 2015
Published: 14 September 2015
A rise in body temperature (Tb) is known to cause minute ventilation (VE) to increase. However, the mechanism of the ventilatory response to rising Tb is still unclear. In the context of the relationship between VE and Tb, it is known that respiratory chemosensitivity is influenced by Tb, and that a rise in Tb of more than 0.7 °C enhances respiratory chemosensitivity . It is not known, however, whether increases in Tb less than 0.7 °C also influence respiratory chemosensitivity. The aim of this study was to clarify the effect of mild hyperthermia (0.3 °C and 0.7 °C) on respiratory chemosensitivity.
Eight persons (five males and three females, mean (SD) age 25 (10) years, height 171.5 (8.9) cm, weight 66.9 (8.7) kg) participated in the study. All were lowlanders and had not been exposed to altitude above 1,000 m within the 6 months prior the study. We measured sublingual temperature (Tsl) as an index of Tb, and measured respiratory chemosensitivity to CO2 using a rebreathing method . The subjects wore a mask connected to a closed one-way circuit with a rubber bag containing the test gas (7 % CO2, 43 % O2, 50 % N2). Rebreathing was terminated when the inspired CO2 fraction reached 9.2 %. This test was performed before heating (ΔTsl = 0 °C) and during heating (ΔTsl = 0.3 °C and 0.7 °C). Measurements were made twice with a 15-min interval between tests at ΔTsl = 0°C, 0.3°C and 0.7°C. During the experiment subjects wore a water-perfused suit. The initial water temperature was 35 °C and was increased to 45 °C.
Before heating mean (SD) Tsl was 36.15 (0.22) °C (ΔTsl = 0 °C) and rose to 36.47 (0.21) °C at ΔTsl = 0.3 °C and then to 36.87 (0.21) °C at ΔTsl = 0.7 °C during heating. While subjects breathed the CO2-rich mixture, VE was 1.49 (0.68) L.min-1.mmHg-1 (ΔTsl = 0 °C), 1.52 (0.75) L.min-1.mmHg-1 (ΔTsl = 0.3°C) and 1.75 ± 0.98 L.min-1.mmHg-1 (ΔTsl = 0.7°C). The tidal volume was 44.7 (12.4) mL.mmHg-1 (ΔTsl = 0°C), 55.4 (24.9) mL.mmHg-1 (ΔTsl = 0.3 °C) and 61.9 (19.5) mL.mmHg-1 (ΔTsl = 0.7 °C) (P < 0.06). The respiratory frequency was 0.47 (0.38) breaths.min-1.mmHg-1 (ΔTsl = 0 °C), 0.40 (0.42) breaths.min-1.mmHg-1 (ΔTsl = 0.3 °C) and 0.37 (0.41) breaths.min-1.mmHg-1 (ΔTsl = 0.7 °C).
These results suggest that increases in Tsl less than 0.7 °C do not influence respiratory chemosensitivity to CO2, though the respiratory pattern did tend to change. The ventilatory response to rising Tb has a threshold around 38 °C (esophageal temperature) in the resting state . Moreover, we suggest that increasing the inspired CO2 fraction did not reduce that threshold to the temperatures reached in the present study (Tsl around 37 °C).
Our findings suggest that respiratory chemosensitivity is not affected by mild hyperthermia (~0.7 °C rise in body temperature). It is possible that there is a Tb threshold for changes in respiratory chemosensitivity that is greater than around 37 °C.
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