Prediction of Hypoxemic Changes in COPD. Is it Currently Measurable and Possible?

Keywords:

• acute hypoxemic respiratory failure
• chronic obstructive pulmonary disease
• hypoxemia
• outcome

Dear Editor:

We read with interest the article of Saure et al. (1) on predictors for a potential change in partial pressure of oxygen (PaO2) over time. The strength of the study is the consideration of clinical and functional respiratory parameters, inflammatory markers, and physical characteristics as predictors for developing hypoxemia in (COPD).

We agree that some parameters, such as Forced Expiratory Volume in 1 second (FEV1), Total Lung Capacity (TLC), Functional Residual Capacity (FRC), resting Heart Rate (HR), and fat mass index, correlate with the values of PaO₂, even though it is difficult to identify variables that predict changes in PaO₂ over time in these patients. An interesting data from the study is the finding as predictors for episodic hypoxemia (FRC, resting HR) (1).

As the authors reported, the low Ventilation/Perfusion (V/Q) ratio is one of the most common reasons for hypoxemia in COPD. The detection of parameters that correlate over time with the development of respiratory failure in COPD may be particularly important in monitoring the progression of the disease.

Hyperinflation could be a key point to identify patients at risk of future hypoxemia, even though mechanisms that may contribute to acute and chronic ­respiratory failure are different: exacerbations, comorbidities, loss of muscle mass.

In a recent study, diffusing lung capacity of carbon monoxide (DLCO) <50%, FEV₁ <45%, baseline-saturation of peripheral oxygen (SpO₂) <95%, and female sex were the strongest determinants of exercise-induced desaturation in non-hypoxemic COPD patients. (2). However, we consider some interesting aspects on which to comment.

First, it would be interesting to know the role of DLCO in the study ­population of Saure et al. In fact, the characteristic values of FEV₁, Residual Volume (RV), TLC, and FRC suggest the relevant role of alveolar ventilation and DLCO. Hypoxemia due to impaired diffusing capacity occurs during exercise, when cardiac output is increased, or when the pulmonary parenchyma is destroyed more than 50%. Fortunately, the lung has physiologically a large capacity of diffusive compensation and the gradual change of DLCO over time could be used as monitoring of risk for hypoxemia.

Second, the role of the 6-minute walk test (6mWT) as predictor of hypoxemia is to be investigated in these patients: a relevant factor of hypoxemia during exercise in COPD may be the decrease in mixed venous P_O2 caused by right heart failure (3).

Third, the role of pulmonary artery pressure as predictor for hypoxemia in COPD seems to be limited. The mean pulmonary artery pressure showed a good correlation with PaO₂ only in patients with severe hypoxemia (4).

Finally, Saure et al. have already pointed out the possibility of identifying predictors for hypercapnic or normocapnic respiratory failure in COPD patients. They found that HR and fat mass index could be predictors of PaO₂ and AaO₂, while heart failure and current smoking status show an association with PaCO₂ (5). These interesting data need to be confirmed by future studies in order to identify COPD patients at risk of hypoxemia and non-invasive ventilation, so as to implement prevention in these patients.

Declaration of interest

The authors declare no conflict of interest. The authors alone are responsible for the content and writing of the paper.