Mixed venous oxygenation, exercise, body posture, and v/q ratio in chronic obstructive pulmonary disease

Mixed venous oxygenation (partial pressure, PVO2; saturation, SVO2) and its impact on pulmonary function at rest in the supine position (SUP), at rest in the standing position (STAND), and during maximal treadmill exercise (Emax), were studied in 41 stable patients with chronic obstructive pulmonary disease, using arterial and right heart (Swan-Ganz) catheterization. The patients represented a broad spectrum of disease severity (VC, 3.1 +/- 0.9 L; FEF 25-75, 1.0 +/- 0.7 L). It was found that: (a) venous oxygenation was uniform among different patients in the supine position at rest (PVO2, 33.8 +/- 2.4 mmHg' SVO2, 76.2 +/- 3.6%), regardless of the heterogeneity in airway obstruction, as previously reported by others; (b) venous oxygenation was also uniform in STAND (PVO2, 29.5 +/- 2.5 mmHg; SVO2, 58 +/- 3.9%) and treadmill Emax (PVO2, 22.4 +/- 3.3 mmHg; SVO2, 34.4 +/- 6%), an observation not previously reported; (c) the orthostatic mixed venous desaturation at rest was pronounced, and was nearly as great as that from STAND to Emax; (d) the potential effect of mixed venous desaturation on alveolar O2 partial pressure (PAO2) and end-capillary O2 saturation (SCO2) was not obvious from SUP to STAND, and from STAND to Emax conditions, being masked by an appropriate increase in overall V/Q ratio; (e) because of constant SCO2 and low interindividual variations of SVO2, there was a high correlation of arterial saturation (SaO2) to venous admixture (Qva/Qt, sum of true shunt and shuntlike components) for each of the conditions studied. Hence, because of high correlation of the equations relating SaO2 to Qva/Qt (r greater than 0.9), these relationships might be used for noninvasive prediction of Qva/Qt from SaO2 in patients with COPD.