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FIG 23.7 (A) Relationship of minute ventilation (VE) to rate of work in the same subject as shown in Fig 23.3 Note there is a steady rise in minute ventilation as rate of work increases (B) The ventilatory equivalents of oxygen (VE/VO2) and carbon dioxide (VE/VCO2) for the same subject The onset of the ventilatory anaerobic threshold (VAT) and terminal hyperventilation (TV) are marked (see text for discussion) Minute ventilation is defined as: where VT is tidal volume and F is respiratory rate At the onset of exercise, minute ventilation initially increases primarily by an increase in tidal volume rather than respiratory rate Tidal volume includes both alveolar ventilation and the physiologic dead space Physiologic dead space is made up of both anatomic components, the trachea and bronchi, and the functional dead space, namely the ventilation of hypoperfused or nonperfused pulmonary components At rest the ratio of dead space to tidal volume is approximately 30% to 35% in adults and adolescents With the increase in tidal volume at the onset of exercise, the ratio falls This is due to a larger tidal volume relative to the fixed anatomic dead space In addition, there is an improvement in ventilation-to-perfusion matching as the increased negative thoracic pressure causes recruitment of additional capillary beds, resulting in a fall in the physiologic dead space At peak exercise, the ratio of dead space to tidal volume may fall to approximately 5% to 15% Children typically have less efficient ventilation than either adolescents or adults and tend to have a higher respiratory rate for any given minute ventilation, resulting in a higher ratio at any level of exercise.23–27 At higher levels of exercise, minute ventilation increases due to increases in both tidal volume and respiratory rate Much of the improvement in the ratio occurs at lower to moderate levels of work The rapid fall in the ratio is responsible for the initial steep fall in the ventilatory equivalents for oxygen and carbon dioxide that occurs at the onset of exercise (see Fig 23.7B) Although both consumption of oxygen and production of carbon dioxide are increasing in early exercise, this is more than compensated for by the improved efficiency of the lungs, resulting in a less than proportional rise in minute ventilation In healthy children and adults, ventilation is not the rate-limiting step in exercise performance.4,22 This is because the cardiovascular system usually reaches its limit of delivery of oxygen before the pulmonary system is exhausted At the maximal level of exercise, most subjects have not maximally stressed their pulmonary system, and there is pulmonary reserve, which is described by exercise physiologists as the breathing reserve This is the theoretical ability to increase minute ventilation that remains untapped at maximal minute consumption of oxygen This value is in the range of 20% to 50% in healthy children and adults.22 Breathing reserve is defined as: and is expressed as a percentage, where VE is the maximally achieved VE at peak exercise and MVV is maximal voluntary ventilation Maximal voluntary ventilation is obtained by having the subject hyperventilate for 10 seconds as vigorously as possible prior to exercise testing Much longer periods of rapid and deep breathing may result in fainting and should be avoided The volume achieved is then multiplied by 6, theoretically to give a maximally achievable minute ventilation A low breathing reserve, of less than 20%, may indicate that a primary pulmonary abnormality is limiting exercise performance Because maximal voluntary ventilation is a highly effort-dependent measurement, care must be taken when interpreting breathing reserve Other confirmatory data, such as an abnormal response of the ventilatory equivalents or abnormal resting spirometry, should be sought to confirm pulmonary abnormalities
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