Exercise-induced oxygen desaturation (EID) occurs in up to 20% of the patients with chronic obstructive pulmonary disease (COPD).1 However, despite being a relatively frequent condition, its definition has not yet been agreed upon.2–4 The most widely accepted criterion in common practice comprises a drop in oxygen saturation (SatO2) of more than 4% with a nadir of at least 88%. Although the pathophysiology of EID is a matter of debate, aspects such as the severity of airflow obstruction, a low CO diffusion capacity (DLCO), a high degree of emphysema, obesity, and a SatO2 at rest ≤93% have been related to it.5–8

Studies carried out by our group show that the decrease in the SatO2 curve during the 6-minute walk test (6MWT) has prognostic implications. Patients with such an event before the end of the first minute of the test, termed ‘early desaturators’ (EDs), had a high probability of also experiencing them during their daily life activities and of additionally developing hypoxaemic respiratory failure at rest within the subsequent 5 years compared to subjects who desaturated after 3min.9,10 Given the occurrence of the two desaturation profiles in patients with the same disease, the assessment of possible differential factors associated with this behaviour is imperative.

The purpose of this study was to characterise COPD patients with early desaturation versus non-early desaturation as well as determine potential markers that point to events of EID before the end of the first minute.

EID is the result of a mismatch between ventilation and perfusion during physical effort. Airflow limitation and loss of lung elasticity lead to increased air trapping and dynamic hyperinflation, increasing patient inspiratory work and thereby increasing oxygen consumption in COPD patients.3,13,14 Until a few years ago, this phenomenon observed during the 6MWT, did not provide any further information than that of the change in SatO2. It was considered a mere curiosity, which appeared in a group of COPD patients but did not translate into a change in medical attitude, perception of poor prognosis or treatment modification. Takigawa et al.15 demonstrated that, in addition to the distance walked in the 6MWT, the variation in SatO2 also had prognostic implications. Later, the phenomenon came to be no longer regarded as an unimportant fact, but one to be taken into account together with parameters such as the metres covered in the 6MWT, the FEV1, and the oxygen partial pressure in blood. A few years later, these findings were corroborated by the ECLIPSE8 group, who demonstrated that both desaturation and the frequency of stops during the walk test were prognostic factors. However, the possible different types of desaturation have not yet been well characterised, neither in terms of their definition, which remains ambiguous, nor addressing the different forms as to the distinction between basal and minimal desaturation, its quality and nature. We defined a category, termed ED, to differentiate it from late and intermediate desaturation. This was the result of a study where we compared desaturation in two different settings, the 6MWT and daily life activities. Only the patients who desaturated within the first minute of the test also did so outside the hospital in their daily activities, something that was not true of those who desaturated only after 3.5min.9 Therefore, early desaturation in the 6MWT is a relevant aspect, because it implies desaturation during everyday activities and under low levels of physical strain. In addition to this aspect, the ED patient group is more likely to develop respiratory insufficiency over time than the NEDs. Hence, the elapsed time to desaturation in the 6MWT is a useful variable, which provides information of clinical interest. Additional potential markers to consider are the depth and persistence of desaturation. Of note, early desaturation also tends to be deep; according to our observations in this study, the nadir is very low. We were interested in defining a profile of patients who desaturate early and analyse clinical and functional characteristics to identify potential differential parameters between EDs and patients who desaturate after the first minute of the 6MWT. Like other authors,2,6 we found that desaturation is typical of patients with low FEV1 and relatively low baseline DLCO and SatO2 values. But it remained to be determined whether there was a defined profile for ED patients. We did not observe differences in either age or FEV1 values, and although DLCO was somewhat lower in EDs, this was not statistically significant. From a functional point of view, basal SatO2 and the distance walked in the 6MWT were significantly lower in EDs compared to the NED group, and it should be emphasised that the desaturation in the ED group was more profound, which may be considered as another feature of severity. However, BMI was the most outstanding parameter in the ED group. These patients were more obese, although this was not associated with comorbidities such as ischaemic heart disease, cardiac arrhythmia, or AHT. Recently, Andrianopoulos et al.8 analysed patients from the ECLIPSE cohort and described obesity as a factor involved in EID (OR 1.57, 95% CI 1.15–2.14) along with additional parameters, such as a high degree of airflow obstruction and low DLCO values. While obesity has been linked to low baseline SatO2 and EID,8,16 our study adds that this feature appears to be limited to ED COPD patients.

COPD patients may exhibit very diverse characteristics, which depend on the association with other conditions such as bronchial asthma, sleep apnoea, and fibrosis. EID occurs preferentially in severe patients with low DLCO values; although, unexpectedly, it is not observed in lean but in obese patients. Early desaturation, which we think is of great importance because it translates to increased severity, is preferentially observed in subjects with a BMI ≥30. Alterations in respiratory mechanics or higher levels of proinflammatory cytokines such as leptin may possibly explain this phenomenon.17

There are some limitations; the study is retrospective, the number of patients is not very high, the proportion of women is very low, we did not select non-smoker patients and the study only took place in hospital.

In summary, we would like to draw special attention to COPD patients with severe to very severe obstruction, low DLCO, and overt obesity, because they may experience early and profound desaturation during exercise and, together with the obesity, suffer from very harmful long-term effects.

Menarini SA Corp Foundation.

The authors have no conflicts of interest to declare.