Is the more (intricate) the better?

Toraldo and colleagues (December 2005)1 are to be congratulated on their endeavor to identify daytime variables that are predictive of nocturnal desaturation in COPD patients, a field in which many attempts have failed. However, we think that some points are worth addressing. First, despite the plethora of data presented by Toraldo et al,1 it does not greatly contribute to the evidence base of numerous previous articles that the probability for nonapneic desaturation is proportional to the severity of blood gas disturbances and lung function impairment. Second, we think that an analysis of the possible usefulness of the oxyhemoglobin dissociation curve2 (ODC) as a predictive tool for nonapneic desaturation in COPD patients could have been included in the discussion. A physiologic decrease of Pao2 during sleep in healthy subjects does not result in significant desaturation because of the plateau in this section of the lung. The decrease of Pao2 in COPD patients frequently combined with an increase in Paco2 and a decrease in pH, as well as with some specific changes in the biochemistry of hemoglobin (ie, an increase in 2,3-diphosphoglycerate and Po2 corresponding to 50% saturation of hemoglobin) causes a rightward shift of ODC and a displacement of the desaturation point to the steeper slope of the ODC.3 All of those conditions are prerequisites for significant nocturnal desaturation. We believe that the so-called capacitance coefficients that are a measure of the slope of different parts of the ODC3 and their “desaturation capacity” may be a promising avenue for the more accurate prediction of the nocturnal desaturation in COPD patients. Having in mind the major clinical implications of significant nocturnal desaturation, we believe that the authors should comment on these issues.