must be removed to maintain arterial carbon dioxide levels within the normal range This causes a significant rise in production of carbon dioxide out of proportion to the rise in consumption of oxygen, resulting in the respiratory exchange ratio increasing to greater than 1.0 The ratio in adults at peak exercise may be as high as 1.2 to 1.4 but is often lower in children.6,7,10–12 The increase in production of carbon dioxide associated with the buffering of lactic acid allows for measurement of a noninvasive surrogate of the lactate threshold This surrogate, known as the ventilatory anaerobic threshold, is defined as the point where production of carbon dioxide, and minute ventilation, begin to rise out of proportion to the consumption of oxygen Like the lactate threshold, the ventilatory anaerobic threshold occurs at approximately 50% to 60% of the maximal consumption of oxygen It may be significantly lower or higher in deconditioned or highly trained athletes, respectively Like maximal consumption of oxygen, the ventilatory anaerobic threshold is a physiologic limit and has been used as a marker of aerobic fitness Unlike maximal consumption of oxygen, the ventilatory anaerobic threshold has the virtue of being effort independent, is a level of exercise that can be sustained over a prolonged period of time, and is easily affected by training or sedentary behavior Unfortunately, the ventilatory anaerobic threshold can be especially difficult to measure accurately in smaller children, who tend to have erratic patterns of breathing.6,10–13 Heart and Lung as Service Organs As previously mentioned, consumption of oxygen rises in near linear response to increasing work rate The consumption can be derived from the Fick equation as follows: Where VO is consumption, Q is cardiac output, and a − vO2 diff is the difference in content of oxygen is arterial and mixed venous blood Q is defined as: where SV is stroke volume and HR is heart rate Rearranging the equations, VO can be calculated as: Therefore the rise in consumption of oxygen during exercise is dependent on the increases in stroke volume and heart rate, and widening of the difference in the content of oxygen in arterial and mixed venous blood During strenuous exercise, cardiac output may rise as much as fivefold over resting levels.5,14–17 Both stroke volume and heart rate contribute to this increase, but the relative contributions of each are different.5 At rest, stroke volume is approximately 60% of its maximal value At the onset of exercise, a combination of increased venous return and sympathetic tone causes stroke volume to increase This occurs as consequence of two mechanisms First, increased preload stretches the myocyte and increases tension, resulting in changes in the Frank-Starling forces Second, the increase in sympathetic tone results in an increase in the inotropic state of the myocardium This increase in inotropy improves the active tension developed for any given preload, thus further augmenting stroke volume Most of the increase in stroke volume takes place at less than 30% to 40% of the maximal consumption of oxygen (Fig 23.4) At higher heart rates, diastolic filling time is decreased, which limits any further augmentation in stroke volume irrespective of any increase in the inotropic state Therefore, at higher heart rates and naturally at higher workloads, the relative contribution of stroke volume to the overall increase in cardiac output is small FIG 23.4 Relationship of heart rate and stroke volume to increasing consumption of oxygen during cycle ergometry in 23 male and female subjects Note that stroke volume reaches its maximal value at approximately 30% to 40% of the maximal uptake of oxygen Heart rate continues to rise in a linear fashion throughout exercise (From Astrand P, Rodahl K The Muscle and Its Contraction Textbook of Work Physiology, Physiological Bases of Exercise 3rd ed McGraw-Hill; 1986:12–53.) It should be noted that cardiac output response to exercise varies quite strikingly with position The pattern described earlier applies to upright exercise There is even less stroke volume augmentation when exercise is performed in the supine position because preload has already been redistributed from the lower extremities to central circulation at rest without further effect due to the muscle pump Stroke volume and cardiac output at both rest and peak exercise are higher during supine exercise, whereas heart rate is higher during upright