The acute effects of oxygen and carbon dioxide on renal vascular resistance in patients with an acute exacerbation of COPD.

OBJECTIVE Changes in renal hemodynamics occur in patients with severe COPD, especially during an acute exacerbation. Renal hemodynamics are affected by changes in oxygen and carbon dioxide levels, but these changes have not been well defined, particularly in the acute clinical situation. We wished to determine whether oxygen or carbon dioxide levels have the predominant effect on renal hemodynamics in patients with an acute exacerbation of COPD. DESIGN Fourteen patients with an acute exacerbation of COPD and a PaO2 < 64 mm Hg were studied. Initially, the patients breathed room air (hypoxemia). Then their arterial oxygen saturation was raised to approximately 95% (normoxemia) and then to 98 to 99% (hyperoxemia). Finally, 1 L/min of carbon dioxide was added to the circuit (hyperoxemic hypercapnia). Using duplex ultrasonography, the pulsatility index (PI) of an intrarenal artery was measured after 10 min at each level of oxygenation. The PI is an index of distal renovascular resistance. RESULTS The PI fell significantly from room-air values on inducing hyperoxemia (p < 0.05). This suggests decreased renovascular resistance and increased renal blood flow. When hyperoxemic hypercapnia was induced, the PI rose significantly from the hyperoxemia level (p < 0.001). CONCLUSIONS In hypoxemic patients, renovascular resistance decreased when hyperoxemia was induced. This fall in renovascular resistance was reversed with the addition of carbon dioxide. This suggests that acute changes in carbon dioxide levels might have a more dominant role than oxygen levels in determining renovascular resistance.