Pulmonary Ventilation during Exercise

Ventilation in Steady Rate Exercise During light & moderate steady rate exercise, VE:VO2 linear relationship. Ventilatory equivalent for oxygen (VE:VO2): ratio of minute ventilation to oxygen uptake. Usually 25 : 1 during submaximal exercise up to 55% max.

Ventilation in Steady Rate Exercise Ventilatory response to fixed level of submaximal exercise can be divided into 4 phases. Sudden increase at onset. Ventilation gradually increases to higher steady-rate level. Steady state level of ventilation maintained. Recovery period gradual return to resting levels. Phase IV higher than resting levels coincide with EPOC.

Ventilation in Steady Rate Exercise Ventilatory equivalent for carbon dioxide (VE:VO2): ratio of minute ventilation to carbon dioxide produced. Remains constant during steady rate exercise because pulmonary ventilation eliminates CO2 .

Ventilation in Non-Steady-Rate Exercise Minute ventilation (VE) increases in proportion to oxygen consumption over range from rest to moderate exercise. VE increases dispropor-tionately to oxygen consumption over range from moderate to strenuous.

Ventilation in Non-Steady-Rate Exercise The point at which ventilation increases disproportionately with oxygen uptake during incremental exercise is termed: ventilatory threshold (VT).

Ventilation in Non-Steady-Rate Exercise Lactic acid generated during anaerobic glycolysis is buffered in blood by sodium bicarbonate. Lactic acid + NaHCO3 → Na Lactate + H2CO3 → H20 + CO2

Ventilation in Non-Steady-Rate Exercise The excess, non-metabolic CO2 stimulates ventilation. Recall that metabolic CO2 is produced in Krebs Cycle in oxidation of acetyl CoA.

Ventilation in Non-Steady-Rate Exercise The non-metabolic CO2 from buffering HLa drives increased VE to eliminate it, so VE: VCO2 remains constant. The increased in VE exceeds increase in VO2 disproportionately. The point at which VEO2 breaks with linearity is the ventilatory threshold. RER = 1 where two lines intersect. R values > 1 indicate CO2 production exceeds O2 consumption, evidence of non-metabolic CO2 production.

Ventilation in Non-Steady-Rate Exercise As exercise intensity increases, blood lactate begins to systematically increase over a baseline value of 4 mM/L termed onset of blood lactate. Blood lactate accumulation associated with changes in CO2 production, blood pH, bicarbonate, [H+], RER.

Ventilation in Non-Steady-Rate Exercise Although variables (CO2 production, blood pH, bicarbonate, [H+], RER) are related to OBLA, doubtful that VT can be used to denote onset of anaerobic metabolism. OBLA directly assessed by measuring lactate level in blood.

Ventilation in Non-Steady-Rate Exercise Common practice to use “bloodless” techniques e.g. R >1, or break in ventilatory equivalent for oxygen to denote anaerobic threshold.

Does Ventilation Limit Aerobic Capacity for Average Person? If inadequate breathing capacity limited aerobic capacity, ventilatory equivalent for oxygen would decrease. Actually, healthy person tends to over-breathe in relation to VO2. In strenuous exercise, decreases arterial PCO2 & increase Alveolar PO2.

Work of Breathing Two major factors determine energy requirements of breathing Compliance of lungs Resistance of airways to smooth flow of air As rate & depth of breathing increase during exercise, energy cost of breathing increases too. At maximal exercise when VE= 100 L/m, oxygen cost of breathing represents 10-20% of total VO2.

Work of Breathing Acute effects of 15 puffs on a cigarette during a 5-minute period 3 fold increase in airway resistance Lasts an average 35 minutes Smokers exercising at 80% Energy requirement of breathing after smoking was 14% of oxygen uptake Energy requirement of breathing no cigarettes was only 9%.

References Axen and Axen. 2001. Illustrated Principles of Exercise Physiology. Prentice Hall. Kapit, Macey, Meisami. 1987. Physiology Coloring Book. Harper & Row. McArdle, Katch, Katch. 2006. Image Collection Essentials of Exercise Physiology, 3rd ed. Lippincott William & Wilkens.