Brain Blood Flow Responses at Rest and During Exercise

Tight regulation of brain blood flow is integral for delivery of oxygen and energy for survival. During childhood, the brain has a twofold higher metabolic requirement, thus requires greater blood flow to match this. Despite this knowledge, brain blood flow responses and the mechanisms of regulation during maturation are largely unknown. Thus, we are trying to understand what happens to blood vessel function and the mechanisms of regulation at rest and during handgrip exercise. This will give us valuable information on brain blood vessel responses, which will help future interventions aimed at improving blood vessel function in youth, for future disease prevention. Utilizing the pubertal transition will provide insights into the influence of sex hormones on brain blood flow regulation.

The goal of this cross-sectional observational study is to examine the influence of age and maturation on cerebral blood flow regulation, achieved through exploring the responses to increases in carbon dioxide concentrations, and static handgrip exercise in children (7-10 years), adolescents (12-16 years) and young adults (19-35 years).

The main questions the study aims to answer are:

* Investigate the brain blood flow responses to increases in carbon dioxide concentrations in children, adolescents and adults.
* Investigate brain blood flow responses to handgrip exercise with and without increases in carbon dioxide concentrations in children, adolescents and adults.

During all protocols, participants will have their end-tidal gas concentrations measured and/or altered using prospective end-tidal gas targeting using a computer controlled gas blender system in which we have obtained Health Canada approval for.

The goal of this cross-sectional experimental study to determine the influence age, sex and pubertal status on cerebrovascular responses at rest and during isometric exercise.

The main objective is to investigate the regulatory mechanisms of cerebral blood flow during hypercapnia and isometric exercise, and determine whether any mechanistic differences in regulation are present with advancing age and maturation.

The following questions will be addressed in 20 healthy children (7-10 years), 20 healthy adolescents (12-16 years) and 20 young healthy adults (19-35 years), to compare the influence of age, sex and maturation on these responses, as follows:

1. Investigate the brain blood flow responses to isometric exercise with and without hypercapnia (+9 mmHg carbon dioxide) in children, adolescents and adults.
2. Investigate the brain blood flow responses to hypercapnia (+9 mmHg carbon dioxide) in children, adolescents and adults using a ramp incremental 4 minute protocol compared with a 30 second protocol, to identify which method is the most tolerable in children and adolescents.
3. Determine whether sex, and the sex-dependent influences of age and maturation influence any of the brain blood flow responses to hypercapnia or isometric handgrip exercise.

To control and manipulate breathing gases during the study protocols, the partial pressures of end-tidal carbon dioxide and oxygen will be sampled at the mouth using the Investigational Testing Authorization approved computer controlled gas blender system. To target specific end-tidal carbon dioxide and oxygen levels during the clamped breathing and hypercapnic trials respectively, prospective end-tidal gas targeting will be utilized. The system allows for breath-by-breath regulation of end-tidal gases, independently of ventilation. End-tidal concentrations are prospectively targeted by controlling the inspired concentration of carbon dioxide and oxygen according to the previous expired breath, allowing this to be precisely controlled throughout the duration of the protocol. Gas analysis, and spirometry measures of tidal volume and breathing frequency will be recorded using the computer controlled system. Prospective end-tidal gas targeting is safe, with all inspired gases containing oxygen, with rigorous control procedures and fail-safe mechanisms which do not allow inhaled gases to fall outside of safe thresholds.

Eligibility Criteria:
Inclusion Criteria:

* Are aged between 7-10 years, 12-16 years or 19-35 years
* Male or Female
* Have a normal blood pressure (\<160/110 mmHg) and are not taking any anti-hypertensive medications.
* No prior or current history of cardiorespiratory of cerebrovascular diseases.
* Can communicate in English

Exclusion Criteria:

* Inability to perform handgrip exercise
* Are currently taking any medications known to influence vascular function, heart rate or blood pressure
* Have epilepsy
* Are a current smoker
* Have known cardiometabolic abnormalities
* Have congenital cardiac abnormalities (e.g., tetralogy of Fallot)
* Unstable angina (frequent chest pain)
* Atrial Fibrillation
* Myocardial infarction (heart attack) within the previous 3 months
* Transient ischemic attack (TIA) within the previous 6 months
* Heart failure ≥ class 2
* Have a known respiratory disease (e.g., asthma)
* Have a known metabolic disease (e.g., Type 1 diabetes)
* Are unable to exercise
* Are pregnant
* Less than 6 months post childbirth or stopped breastfeeding within the last month