Comparison of the End-Tidal Arterial Pco₂ Gradient During Exercise in Normal Subjects and in Patients With Severe COPD

¹ From the Department of Medicine of the Veterans Affairs Medical Center, Long Beach, and the University of California, Irvine

We undertook the present study with the following objectives: (1) to compare the difference between the end-tidal and the arterial carbondioxide concentration (P[ETa] CO₂) gradients at rest and during exercise in normal subjects and patients with COPD; and (2) to analyze the factors contributing to this gradient. We studied seven normal subjects and seven patients with COPD using a symptom-limited exercise test on a cycle ergometer. Our results show that the P(ET-a)CO₂ increased progressively as the individuals went from rest to higher workloads in both the normal group and in the COPD group. The P(ET-a)CO₂ in patients with COPD both at rest (3.24±2.78 mm Hg) and during exercise (1.03±2.23 mm Hg) is significantly lower than that in normal individuals at rest (1.84±3.68 mm Hg) and during exercise (10.3±6.5 mm Hg) (p<0.01). However, the slope for the relationship between the P(ET-a)CO₂ and the workload is actually significantly steeper in the patients with COPD. Although the P(ET-a)CO₂ correlated significantly with the workload in both normal subjects (r=0.63, p<0.001) and patients (r=0.55, p<0.005), the P(ET-a)CO₂ was much more closely correlated with the ratio of dead space to tidal volume (VD/VT) (r values of 0.86 and 0.77, respectively). Moreover, when multiple regression analysis was performed, addition of any other physiologic measure (eg, oxygen consumption [VO₂], carbon dioxide production [VCO₂], minute ventilation [VE], or workload) as a second independent variable after the VD/VT did not improve the correlation. This indicates that the correlation between the P(ET-a)CO₂ and the workload is probably related to the dependence of the VD/VT on the workload. The PaCO₂ in normal subjects and in the COPD group correlated significantly with the partial pressure of end-tidal carbon dioxide (PETCO₂). Using multiple regression analysis, with the PaCO₂ as the dependent variable and the PETCO₂ (along with other physiologic measures) as the independent variables, we found that the standard error of the estimate was still above 2.1 mm Hg in normal subjects and in patients with COPD. We conclude that (1) during exercise, the P(ET-a)CO₂ in normal subjects and in patients with COPD increases significantly, (2) the P(ET-a)CO₂ gradient is more closely correlated with the VD/VT than any other physiologic variable, and (3) changes in the PETCO₂ during exercise are not correlated closely with changes in the PaCO₂.

Key Words: COPD • end-tidal carbon dioxide • exercise

Submitted on May 9, 1994
Accepted on September 13, 2007

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