Breathing During Exercise

Exercise and Control of Breathing

stresses evoked by exercise are primarily cellular—muscle use of O₂, CO₂ production, heat generation

systemic responses to optimize gas exchange without changing pCO₂ or pH:

triggered by:

central and peripheral chemoreceptors
activation of muscle afferents by metabolic substances released in working muscle
vascular responses resulting from the local release of metabolites from working muscle

conflicting demands may arise (e.g., increase blood flow to muscle but keep blood pressure constant)

Characteristics of the Cardiopulmonary Adjustments to Sustained Whole Body Exercise

below anaerobic threshold:

increase in ventilation, with ventilation rising in proportion to oxygen consumption
increase in CO (cardiac output), first by increasing SV, later by increasing HR
reduction in pulmonary vascular resistance (increased CO does not cause increase in pulm. pressure)
increase in the percentage of ventilation that can provide for gas exchange (alveolar ventilation) and a maintenance of a normal alveolar-arterial oxygen tension gradient
increase in skin blood flow, maintenance of coronary blood flow, reduction in splanchnic blood flow
a dilatation of muscle vascular beds

above anaerobic threshold:

lactic acid is produced within muscle, producing additional CO₂ (lactic acid buffered by bicarbonate)
total expired CO₂ rises out of proportion to oxygen uptake by the body
thus ventilation and oxygen consumption are no longer linearly related to one another
anaerobic threshold = point at which VCO₂ (carbon dioxide production) rises out of proportion toVO₂

below: ventilation, VCO₂, and VO₂ rise proportionally; above: ventilation increase > VCO₂ > VO₂
thus above anaerobic threshold, R (respiratory exchange quotient) > 1

Methods of Exercise Testing

bicycle—direct measure of work output

treadmills—less reliable measurement of work output, but higher maximum VO₂ (more muscle groups)

quantification of exercise:

by measuring O₂ consumption ("met" = resting uptake = 3.5 ml O₂ uptake/minute/kg body weight)
by measuring the actual rate of work accomplished (watts)

common assessments made during exercise:

measurement of ventilation
the rapid analysis of expired gas to determine VO₂ and VCO₂
assessment of oxygen saturation
assessment of the EKG to measure heart rate
determination of blood gas tensions to determine arterial-end tidal O₂ and CO₂ gradients

Incremental Exercise to Maximal Levels – point of exhaustion

maximum VO₂ measurement (predicted from height and age)
anaerobic threshold that is greater than 40% of the predicted VO₂max
maximum ventilation, generally less than 75% of MVV (maximal voluntary ventilation)
normal people never exhaust their ventilatory reserve, even when exercising at maximal levels
cardiac or pulmonary disease—reduction in exercise capacity, shortness of breath at low exertion
VO₂max may be appreciably lower than the predicted values
cardiac disease—reach the anaerobic threshold prematurely (heart cannot deliver sufficient O₂)
pulmonary disease—V_E/MVV ratio greater than 75%—exhaust ventilatory reserve
cardiopulmonary adaptations resulting from training
an increase in maximum cardiac output in response to exercise, in comparison to normal individuals
increase in muscle capillary and mitochondrial densities, so more O₂ extraction from blood