Revisiting cardiac output estimated noninvasively from oxygen uptake during exercise: an exploratory...

Area of Science:

• Exercise Physiology
• Cardiovascular Physiology
• Respiratory Physiology

Background:

• Stringer et al. (1997) proposed a linear relationship between arterio-mixed venous oxygen content difference and maximal oxygen consumption during incremental exercise.
• This linear model is crucial for estimating cardiac output using Fick's principle during exercise testing.

Purpose of the Study:

• To re-evaluate the relationship between arterio-mixed venous oxygen content difference and maximal oxygen consumption.
• To investigate if an S-shaped curve better models this relationship and to identify its inflection point.
• To determine if the inflection point correlates with physiological markers like the first ventilatory threshold (VT1).

Main Methods:

• Analysis of published data from Stringer et al. (1997) and Åstrand et al. (1964).
• Comparison of a linear model with a third-order polynomial model using partial F-tests.
• Calculation of the inflection point of the fitted S-shaped curve and the standard oxygen dissociation curve.

Main Results:

• A third-order polynomial (S-shaped) model provided a significantly better fit to the data than a linear model (P < 0.001).
• The inflection point of the S-shaped curve occurred at 56% of maximal oxygen consumption, closely matching VT1 (50%).
• The inflection point of the oxygen dissociation curve (21.5 mmHg, 36% saturation) aligned with Stringer et al.'s "critical capillary Po2" (21.2 mmHg).

Conclusions:

• The arterio-mixed venous oxygen content difference increases in an S-shaped, not linear, manner with increasing maximal oxygen consumption.
• The inflection point of this S-shaped curve may correspond to VT1 and the inflection point of the in vivo oxygen dissociation curve.
• These findings suggest a potential refinement for estimating cardiac output during exercise testing.