Objectives: To describe the continuous pattern of dead space during exercise and recovery using noninvasive monitoring of transcutaneous carbon dioxide pressure (PtcCO₂).
Methods: During routine exercise testing across various conditions in 132 subjects, including healthy individuals and patients, PtcCO₂ was validated against arterial sampling of carbon dioxide pressure (PaCO₂), and continuous dead-space recording was performed using a transcutaneous probe.
Results: During hyperventilation, a lag of 76 ± 12 seconds was observed between arterial and transcutaneous measurements. Comparison of PtcCO₂ and PaCO₂ values showed good accuracy, with 83% to 85% of samples within 4 mm Hg, with or without an 80-second time delay. During exercise, PtcCO₂ exhibited a biphasic pattern, initially increasing and then continuously decreasing beyond peak exercise during the 5-minute recovery period. End-tidal carbon dioxide pressure (PETCO₂) was an unreliable surrogate. A decrease in dead space was observed throughout exercise, with a significant proportion occurring during the warm-up phase. The nadir occurred shortly after peak exercise, with minimum values/relative decreases ranging from 0.12/68% in healthy volunteers to 0.39/16% in patients with chronic obstructive pulmonary disease (COPD). Dead space began to increase again during the second minute of recovery, driven mainly by the mixed expired carbon dioxide fraction (PECO₂), and remained below resting values. The proportion of recovery ranged from 50% in healthy volunteers to 94% in patients with COPD. Dead-space dynamics were not meaningfully affected by the 80-second lag time. The PaCO₂–PETCO₂ gradient decreased to negative values for most of the test in healthy volunteers and transiently in patients.
Conclusions: PtcCO₂ monitoring enables continuous dead-space assessment with good accuracy. PaCO₂ follows a biphasic pattern independent of exercise peak and is driven by ventilation, whereas dead-space modifications closely follow exercise onset and cessation. Exercise-induced ventilatory and metabolic/vascular adjustments require more than 5 minutes for full recovery.
