Cardiac output during exercise: a comparison of four methods

Area of Science:

• Cardiovascular Physiology
• Exercise Physiology
• Biomedical Engineering

Background:

• Accurate assessment of cardiac output (Q) during exercise is crucial for understanding cardiovascular function.
• Multiple non-invasive and invasive techniques exist for measuring Q, but their comparative validity during dynamic exercise remains unclear.

Purpose of the Study:

• To simultaneously quantify cardiac output (Q) using four distinct methods during incremental cycling exercise.
• To compare the Q/oxygen uptake (VO2) relationship across different measurement techniques under normoxic and hypoxic conditions.

Main Methods:

• Simultaneous measurement of Q using the modified Fick method (Q(Fick-M)), Innocor (inert gas rebreathing; Q(Inn)), Physioflow (impedance cardiography; Q(Phys)), and Nexfin (pulse contour analysis; Q(Pulse)).
• 12 healthy male subjects performed incremental cycling exercise to exhaustion in both normoxia and hypoxia (12% FiO2).
• Analysis focused on the slope of the Q/VO2 relationship for each method.

Main Results:

• All methods showed a progressive increase in Q with exercise intensity.
• Significant differences in the Q/VO2 slope were observed between methods in both normoxia (P=0.001) and hypoxia (P=0.04), with variations up to 50%.
• Nexfin failed to detect the increased Q/VO2 slope in hypoxia, and Innocor yielded lower values potentially due to gas recirculation.

Conclusions:

• The determination of cardiac output during exercise is highly dependent on the specific measurement technique employed.
• Physiological responses, particularly under hypoxic conditions, may be inaccurately represented by certain Q measurement devices.
• Further research is needed to establish the most reliable methods for assessing cardiac output across diverse exercise scenarios.