Cerebrovascular and cardiorespiratory responses to conditions of local ischaemia and systemic hypoxaemia

Conditions of local ischaemia and systemic hypoxemia trigger necessary cardiorespiratory and cerebrovascular responses to ensure nutrients and oxygen (O2) are delivered to vital organs such as the brain and the heart. The aim of this thesis is to determine cardiorespiratory and cerebrovascular responses to conditions of local ischaemia and systemic hypoxemia. Four experimental studies investigated cardiorespiratory and cerebrovascular responses during diving (simulated by facial cooling), the ventilatory response during hypoxia and the interaction between hypoxia and metaboreflex during exercise. The first experimental study aimed to uncouple the effects of trigeminal nerve stimulation on cerebral blood flow (CBF), from other modifiers associated with the diving response, such as apnoea and changes in arterial carbon dioxide tension. It was observed that the CBF responses to diving are attributable to factors related with breath-hold and not facial cooling (i.e., trigeminal nerve stimulation). The second experimental study determined whether moderate acute systemic hypoxia increases cerebral perfusion via the hypoxic ventilatory response, independently of changes in arterial oxygenation. Increases in cerebral perfusion during hypoxia were observed to be mainly influenced by reduced arterial oxygen saturation (SpO2) and not the hypoxic ventilatory response. The final experimental studies investigated the influence of hypoxia on cardiorespiratory and cerebrovascular responses to metaboreflex activation. Normobaric poikilocapnic hypoxia (FiO2=13.5%) failed to augment the cardiorespiratory and cerebrovascular responses to metaboreflex activation. However, a more robust normobaric isocapnic hypoxia (FiO2=10.5%) caused augmented heart rate (HR) and minute ventilation (VE) to metaboreflex activation. Collectively, these studies reinforce the notion that conditions of local ischaemia and systemic hypoxaemia evoke a highly integrated cardiorespiratory and cerebrovascular response