Integrative aspects of cardio-respiratory control in humans

Several aspects of cardio-respiratory control during exercise in humans are not yet understood. One such question is how the autonomic outflow to the circulation is calibrated to provide accurate regulation of arterial pressure (AP) during dynamic exercise. One hypothesis is that the calibration can be 'learnt' through feedback from the arterial baroreceptors arising over multiple trials of exercise. ro test this hypothesis, we paired bouts of submaximal exercise with neck suction to overstimulate the carotid baroreceptors during the course of a 7-day training protocol (study presented in chapter three). The study showed a reduction in systolic AP (but not mean AP) following the training protocol which suggests the presence of some plasticity within the autonomic response consistent with this hypothesis. Eight hours of exposure to hypoxia result in a progressive elevation of ventilation, a lowering of end-tidal Peo2 (PETeo2)' a progressive pulmonary vasoconstriction and an elevation of pulmonary arterial pressure. These effects persist for some time after return to air-breathing conditions. However, little has been known of the effect of preconditioning with 8 hours of hypoxia on the ventilatory and pulmonary vascular response to air-breathing exercise. The study presented in chapter four demonstrated that preconditioning with 8 hours of hypoxia (but not euoxic control) significantly reduced air-breathing PETeo2 at rest, and that this reduction was maintained nearly constant throughout an air-breathing incremental exercise test. This indicates that the increment in ventilation following 8 hours of hypoxia is proportional to the rise in metabolic rate associated with exercise, thus defending the lowered PETe02. The maximum pressure gradient during systole across the tricuspid valve (~Pmax, measured by Doppler ultrasound) can be used as an index of pulmonary arterial pressure. The study presented in c~apter five found that preconditioning with 8 hours of hypoxia (but not euoxic control) significantly increased ~Pmax both at rest and during exercise and increased the sensitivity of ~Pmax to air-breathing exercise. This suggests that the pulmonary vasculature remains in' a somewhat constricted state during air-breathing exercise following 8 hours of hypoxia.EThOS - Electronic Theses Online ServiceGBUnited Kingdo