The Effects of Caffeine on Ventilation and Pulmonary Function During Exercise - Caffeine: Chemistry, Analysis, Function and Effects - page 343

space ventilation. By comparison at 90% of VO 2 max, where SaO 2 was
significantly improved with caffeine, a much larger 68% of the increase in VE
went to increasing alveolar ventilation. In the end, it appears that the
stimulatory effects of caffeine on exercise ventilation may be helpful in
increasing the alveolar partial pressure and SaO 2 during exercise at
submaximal workloads in individuals with impaired pulmonary gas exchange
- either physiologic or altitude induced.

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18.4 Potential Negative Consequences of Increased
Ventilation During Exercise
The above section reads as if increases in exercise ventilation at any given
workload with caffeine ingestion may be positive for endurance exercise
performance. However, the reality may just be the opposite. One of the
potential negative consequences of increased minute ventilation during exercise
with caffeine is the increased work of breathing done by the ventilatory
muscles. The oxygen consumption of the diaphragm and respiratory
musculature is estimated to be 3-6% of whole body VO 2 during moderate
exercise, rising to as high as 15% of whole body VO 2 during maximal exercise
in highly trained athletes (Figure 18.3; Aaron et al 1992). Using previously
published data on the metabolic cost of ventilation (Aaron et al 1992) the 4-7%
increase in VE observed during submaximal and maximal exercise with
caffeine (Chapman and Stager 2008) would result in an estimated increase in
O 2 consumption by the respiratory muscles of y1-2% of whole body VO 2 . For
athletes in endurance events, where economy is a significant factor affecting
performance, this increase in the metabolic cost of breathing is not trivial.
Perhaps more significantly, an increased work of breathing during exercise
holds negative consequences for leg blood flow during endurance exercise. In a
study by Harms et al (1997), the authors measured blood flow to the legs
utilizing a cold saline thermodilution technique in cyclists exercising at
VO 2 max. In trials where inspiratory resistance was added, increasing the work
of breathing, blood flow to the leg was significantly reduced compared to
normal breathing trials. Similarly, when ventilatory work was reduced by
y60% utilizing a pulmonary assist ventilator, blood flow to the legs increased.
Some of the reduced blood flow to the legs with increased ventilatory work is a
result of a decreased cardiac output, secondary to a change in pleural
pressures. However, it appears that a large portion of the blood flow change is
a ''metaboreflex'' response, where sympathetic outflow to the legs is increased
in response to increased ventilatory work (Dempsey et al 2008). In other
words, during periods of heavy exercise and high ventilatory work, the body
responds by ''protecting'' the diaphragm and accessory respiratory muscu-
lature by redirecting blood flow away from locomotor muscles. How much the
analeptic effects of caffeine on exercise ventilation increases the overall work of
breathing or affects blood flow to the exercising locomotor muscles is not
known. During heavy exercise, is a potential gain in SaO 2 and arterial oxygen