Obesity affects lung function and gas exchange and imposes mechanical ventilatory limitations during exercise that could disrupt the predictability of PaCO2 from end-tidal PCO2 (PETCO2), an important clinical tool for assessing gas exchange efficiency during exercise testing. PaCO2 has been estimated during exercise with good accuracy in normal-weight individuals by using a correction equation developed by Jones and colleagues (PJCO2 = 5.5 + 0.9 × PETCO2 – 2.1 × tidal volume). The purpose of this project was to determine the accuracy of PaCO2 estimations from PETCO2 and PJCO2 values at rest and at submaximal and peak exercise in morbidly obese adults.
PaCO2 and PETCO2 values from 37 obese adults (22 women, 15 men; age, 39 ± 9 y; BMI, 49 ± 7; [mean ± SD]) were evaluated. Subjects underwent ramped cardiopulmonary exercise testing to volitional exhaustion. PETCO2 was determined from expired gases simultaneously with temperature-corrected arterial blood gases (radial arterial catheter) at rest, every minute during exercise, and at peak exercise. Data were analyzed using paired t tests.
PETCO2 was not significantly different from PaCO2 at rest (PETCO2 = 37 ± 3 mm Hg vs PaCO2 = 38 ± 3 mm Hg, P = .14). However, during exercise, PETCO2 was significantly higher than PaCO2 (submaximal: 42 ± 4 vs 40 ± 3, P < .001; peak: 40 ± 4 vs 37 ± 4, P < .001, respectively). Jones’ equation successfully corrected PETCO2, such that PJCO2 was not significantly different from PaCO2 (submax: PJCO2 = 40 ± 3, P = .650; peak: 37 ± 4, P = .065).
PJCO2 provides a better estimate of PaCO2 than PETCO2 during submaximal exercise and at peak exercise, whereas at rest both yield reasonable estimates in morbidly obese individuals. Clinicians and physiologists can obtain accurate estimations of PaCO2 in morbidly obese individuals by using P jco 2 .