Differences between estimates and measured Pa(CO₂) during rest and exercise in older subjects

Arterial P(CO₂) (Pa(CO₂)) has been estimated during exercise with good accuracy in younger individuals by using the Jones equation (P(J)(CO₂)) (J. Appl. Physiol. 47: 954-960, 1979). The purpose of this project was to determine the utility of estimating Pa(CO2) from end-tidal P(CO2) (PET(CO₂)) or P(JCO₂) at rest, ventilatory threshold (V̇Th), and maximal exercise (Max) in older subjects. PET(CO₂) was determined from respired gases simultaneously (MGA 1100) with arterial blood gases (radial arterial catheter) in 12 older and 11 younger subjects at rest and during exercise. Mean differences were analyzed with paired t-tests, and relationships between the estimated Pa(CO₂) values and the actual values of Pa(CO₂) were determined with correlation coefficients. In the older subjects, PET(CO₂) was not significantly different from Pa(CO₂) at rest (- 1.2 + 4.3 Torr), v̇Th (0.4 ± 2.5), or Max (-0.8 ± 2.7), and the two were significantly (P < 0.05) correlated at V̇Th (r = 0.84) and Max (r = 0.87) but not at rest (r = 0.47). P(J)(CO₂) was similar to Pa(CO₂) at rest (- 1.0 ± 3.9) and V̇th (-1.3 ± 2.3) but significantly lower at Max.(-3.0 ± 2.6), and the two were significantly correlated at V̇th (r = 0.86) and Max (r = 0.80) but not at rest (r = 0.54). PET(CO₂) was significantly higher than Pa(CO₂) during exercise in the younger subjects but similar to Pa(CO₂) at rest. P(J)(CO₂) was similar to Pa(CO₂) at rest and V̇th but significantly lower at Max in younger subjects. In conclusion, our data demonstrate that Pa(CO₂) during exercise is better estimated by PET(CO₂) than by P(J)(CO₂) in older subjects, contrary to what is observed in younger subjects. This appears to be related to the finding that PET(CO₂) does not exceed Pa(CO₂) during exercise in older subjects, as occurs in the younger subjects. However, Pa(CO₂) at rest is best estimated by P(J)(CO₂) in both younger and older subjects.