Dead space/tidal volume ratio (VDNT) evaluation is currently performed in patients with respiratory and cardiac disorders, and includes measurement of arterial C02 partial pressure (Paco 2 ). Paco 2 is generally derived from either Petco 2 (end-expiratory CO 2 pressure) or Pjco 2 (calculated as Pjco 2 = 5.5 + 0.9 Petco 2 − 2.1 V T ). The applicability of these methods may be questionable in chronic heart failure due to its frequent association with lung dysfunction. In 63 patients with congestive heart failure, the authors compared Paco 2 versus Petco 2 and PJCO 2 and VDNT measured with Paco2 versus VDNT estimated with Petco 2 (estimation 1) or Paco2 (estimation 2). Comparisons were made at rest, at submaximal exercise, and at peak exercise. Considering all 326 measurements, there was a strong correlation, but not an identity, between Paco 2 and Petco 2 (Paco 2 = 7.25 + 0.80 Petco 2 , r = .84, P < .0001) and between Paco 2 and Pjco 2 (Paco 2 = 6.18 + 0.84 Pjco 2 , r = .85, P < .0001). Results were comparable concerning Paco 2 versus Pjco 2 . Measured VDNNs also strongly correlated with estimated V D /V Ts (V D /V T measured = −0.03 + 1.11 VDNT [estimation 1], r = .90, P < .0001, and V D /VT measured = 0.03 + 0.92 V D /V T [estimation 2], r = .90, P < .0001). However, only at rest and, solely for estimation 1, at submaximal exercise were the slopes and y intercepts of measured versus estimated VDNT not different from 1 and 0, respectively; in this regard, lung dysfunction was more influential than the severity of cardiac failure. Although Paco 2 strongly correlates with Petco 2 and Pjco 2 , these measurements may not be reliable for a noninvasive calculation of V D /N T in chronic congestive heart failure.