VO2 and cardiac output during rest-exercise and exercise-exercise transients

Area of Science:

• Exercise Physiology
• Cardiovascular Physiology
• Respiratory Physiology

Background:

• External gas exchange dynamics during exercise involve complex interactions of pulmonary, cardiovascular, and metabolic systems.
• Rest-exercise transitions present unique challenges in understanding these physiological interactions compared to exercise-exercise transients.

Purpose of the Study:

• To directly compare the kinetics of oxygen uptake (VO2) and cardiac output (CO) during rest-exercise versus exercise-exercise transitions.
• To investigate the specific early features of VO2 and CO responses during rest-exercise transitions.

Main Methods:

• Nine students performed step changes in exercise intensity on a bicycle ergometer in an upright position.
• Responses included rest-20W, 20W-80W, and 20W-140W.
• Oxygen uptake (VO2) was measured breath-by-breath, and cardiac output (CO) was determined beat-by-beat using Doppler ultrasound.

Main Results:

• VO2 responses from a 20W baseline exhibited an initial cardiovascular component followed by a metabolic component.
• Rest-exercise transitions showed a rapid initial VO2 increase and CO reaching steady-state within approximately 10 seconds, suggesting increased venous return.
• Both VO2 and CO kinetics were faster when exercise commenced from rest compared to transitions between exercise intensities.

Conclusions:

• The kinetics of oxygen uptake and cardiac output are accelerated when initiating exercise from rest in an upright position.
• Early physiological events in the lungs or pulmonary circulation contribute to the rapid VO2 increase at the onset of exercise.
• The rapid attainment of steady-state CO from rest implies a prompt increase in venous return.