Dynamics of Dead Space and its Components During Exercise Testing With Continuous Transcutaneous Carbon Dioxide...

To describe the continuous pattern of dead space during exercise and recovery using noninvasive monitoring of transcutaneous carbon dioxide pressure (PtcCO₂).

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.

During hyperventilation, a lag of 76±12s was observed between arterial and transcutaneous measurements. Comparison of PtcCO₂ and PaCO₂ values showed good accuracy, with 83% to 85% of samples within 4mm Hg, with or without an 80-s time delay. During exercise, PtcCO₂ exhibited a biphasic pattern, initially increasing and then continuously decreasing beyond peak exercise during the 5-min 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-s lag time. The PaCO₂-PETCO₂ gradient decreased to negative values for most of the test in healthy volunteers and transiently in patients.

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 5min for full recovery.