Do peripheral and/or central chemoreflexes influence skin blood flow in humans?

Carotid chemoreceptors have a limited role in mediating the hyperthermia-induced hyperventilation in exercising humans

Hyperthermia causes hyperventilation at rest and during exercise. We previously reported that carotid chemoreceptors partly contribute to the hyperthermia-induced hyperventilation at rest. However, given that a hyperthermia-induced hyperventilation markedly differs between rest and exercise, the results obtained at rest may not be representative of the response in exercise. Therefore, we evaluated whether carotid chemoreceptors contribute to hyperthermia-induced hyperventilation in exercising humans. Eleven healthy young men (23 ± 2 yr) cycled in the heat (37°C) at a fixed submaximal workload equal to ~55% of the individual's predetermined peak oxygen uptake (moderate intensity). To suppress carotid chemoreceptor activity, 30-s hyperoxia breathing (100% O₂) was performed at rest (before exercise) and during exercise at increasing levels of hyperthermia as defined by an increase in esophageal temperature of 0.5°C (low), 1.0°C (moderate), 1.5°C (high), and 2.0°C (severe) above resting levels. Ventilation during exercise gradually increased as esophageal temperature increased (all P ≤ 0.05), indicating that hyperthermia-induced hyperventilation occurred. Hyperoxia breathing suppressed ventilation in a greater manner during exercise (-9 to -13 l/min) than at rest (-2 ± 1 l/min); however, the magnitude of reduction during exercise did not differ at low (0.5°C) to severe (2.0°C) increases in esophageal temperature (all P > 0.05). Similarly, hyperoxia-induced changes in ventilation during exercise as assessed by percent change from prehyperoxic levels were not different at all levels of hyperthermia (~15-20%, all P > 0.05). We show that in young men carotid chemoreceptor contribution to hyperthermia-induced hyperventilation is relatively small at low-to-severe increases in body core temperature induced by moderate-intensity exercise in the heat. NEW & NOTEWORTHY Exercise-induced increases in hyperthermia cause a progressive increase in ventilation in humans. However, the mechanisms underpinning this response remain unresolved. We showed that in young men hyperventilation associated with exercise-induced hyperthermia is not predominantly mediated by carotid chemoreceptors. This study provides important new insights into the mechanism(s) underpinning the regulation of hyperthermia-induced hyperventilation in humans and suggests that factor(s) other than carotid chemoreceptors play a more important role in mediating this response.

Left ventricular strain rate is reduced during voluntary apnea in healthy humans

During an apneic event, sympathetic nerve activity increases resulting in subsequent increases in left ventricular (LV) afterload and myocardial work. It is unknown how cardiac mechanics are acutely impacted by the increased myocardial work during an apneic event. Ten healthy individuals (23 ± 3 yr) performed multiple voluntary end-expiratory apnea (VEEA) maneuvers exposed to room air, while a subset ( n = 7) completed multiple VEEA exposed to hyperoxic air (100% [Formula: see text]). Beat-by-beat blood pressure, heart rate, and stroke volume were measured continuously. Effective arterial elastance (EA) was calculated as an index of cardiac afterload, and myocardial work was calculated as the rate pressure product (RPP). Tissue Doppler echocardiography was used to measure LV tissue velocities, deformation via strain, and strain rate (SR). Systolic blood pressure (Δ18 ± 13 mmHg, P < 0.01), EA (Δ0.13 ± 0.10 mmHg/ml, P < 0.01), and RPP (Δ9 ± 10 beats/min × mmHg 10^-2, P < 0.01) significantly increased with room air VEEA. This occurred in parallel with decreases in peak longitudinal systolic (Δ-0.62 ± 0.41 cm/s, P < 0.01) and early LV filling (Δ-2.81 ± 1.99 cm/s, P < 0.01) myocardial velocities. Longitudinal SR (Δ-0.30 ± 0.32 1/s, P = 0.01) was significantly decreased during room air VEEA. VEEA with hyperoxia did not alter ( P > 0.18) EA or RPP and attenuated the systolic blood pressure response compared with room air. Myocardial velocities and LV strain rate response to VEEA were unchanged ( P = 0.30) with hyperoxia. Consistent with our hypotheses, VEEA-induced increases in EA and myocardial work impact LV mechanics, which may depend, in part, on stimulation of peripheral chemoreceptors. NEW & NOTEWORTHY Transient increases in arterial blood pressure and systemic vascular resistance occur during sleep apnea events and may contribute to the associated daytime hypertension and risk of overt cardiovascular disease. To date, the link between this apnea pressor response and acute changes in left ventricular function remains poorly understood. We demonstrate that in parallel to increases in cardiac afterload a depressed left ventricular systolic function occurs at end apnea.