Figure 3.

Reduced steady-state ventilation (V̇E/V̇CO₂) and breathing frequency (fR), and the resultant CO₂ retention, resulting from type III–IV muscle afferent blockade via intrathecal fentanyl in healthy humans at mild to heavy exercise intensities. Fentanyl had no effect on mean SaO₂ except at the 327 W work rate where SaO₂ was 97.7% in placebo and 95% with fentanyl. Note the persistence of the hypoventilatory response in the presence of type III–IV afferent blockade – especially during mild and moderate intensity exercise – despite the presence of increased CO₂-induced chemoreceptor stimulation. Plasma lactate levels were within 0.5 mmol l⁻¹ of resting values (0.9 ± 0.1 mmol l⁻¹) during 50–150 W exercise and rose to 7-fold > rest during exercise at 325 W in both the placebo and fentanyl trials. Data from Amann et al. [3].