Effect of rising body temperature on respiratory chemosensitivity to CO₂

Introduction

A rise in body temperature (T_b) is known to cause minute ventilation (V_E) to increase. However, the mechanism of the ventilatory response to rising T_b is still unclear. In the context of the relationship between V_E and T_b, it is known that respiratory chemosensitivity is influenced by T_b, and that a rise in T_b of more than 0.7 °C enhances respiratory chemosensitivity [1]. It is not known, however, whether increases in T_b 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.

Methods

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 (T_sl) as an index of T_b, and measured respiratory chemosensitivity to CO₂ using a rebreathing method [2]. 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 (ΔT_sl = 0 °C) and during heating (ΔT_sl = 0.3 °C and 0.7 °C). Measurements were made twice with a 15-min interval between tests at ΔT_sl = 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.

Results

Before heating mean (SD) T_sl was 36.15 (0.22) °C (ΔT_sl = 0 °C) and rose to 36.47 (0.21) °C at ΔT_sl = 0.3 °C and then to 36.87 (0.21) °C at ΔT_sl = 0.7 °C during heating. While subjects breathed the CO₂-rich mixture, V_E was 1.49 (0.68) L.min^-1.mmHg^-1 (ΔT_sl = 0 °C), 1.52 (0.75) L.min^-1.mmHg^-1 (ΔT_sl = 0.3°C) and 1.75 ± 0.98 L.min^-1.mmHg^-1 (ΔT_sl = 0.7°C). The tidal volume was 44.7 (12.4) mL.mmHg^-1 (ΔT_sl = 0°C), 55.4 (24.9) mL.mmHg^-1 (ΔT_sl = 0.3 °C) and 61.9 (19.5) mL.mmHg^-1 (ΔT_sl = 0.7 °C) (P < 0.06). The respiratory frequency was 0.47 (0.38) breaths.min^-1.mmHg^-1 (ΔT_sl = 0 °C), 0.40 (0.42) breaths.min^-1.mmHg^-1 (ΔT_sl = 0.3 °C) and 0.37 (0.41) breaths.min^-1.mmHg^-1 (ΔT_sl = 0.7 °C).

Discussion

These results suggest that increases in T_sl 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 T_b has a threshold around 38 °C (esophageal temperature) in the resting state [3]. Moreover, we suggest that increasing the inspired CO₂ fraction did not reduce that threshold to the temperatures reached in the present study (T_sl around 37 °C).

Conclusion

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 T_b threshold for changes in respiratory chemosensitivity that is greater than around 37 °C.