Hypoxemia and dyspnoea

The relationship between hypoxemia and dyspnoea (which is a sensation of uncomfortable, difficult or laboured breathing, or the experience of breathing discomfort) can be quite complex [1]. Hypoxemia per se does not necessarily elicit dyspnoea in healthy condition, despite an increased ventilatory drive and an increased minute ventilation.

The maximal sustainable ventilation for an average adult varies according to the time frame. The maximal voluntary ventilation (MVV) for 15 seconds is 130 L/min, and 75% of MVV can be maintained for 15 minutes at 100 L/min, whilst 50% of MVV can be maintained for many hours at 65 L/min. Furthermore, 30 L/min can be maintained for an almost indefinite period of time, which does not necessarily elicit dyspnoea. The ventilatory response to hypoxemia can be blunted by a number of factors such as age and diabetes, both imposing a 50% reduction in ventilatory response to hypoxemia. Similarly, fever causes a right shift in the dissociation curve which results in a lower SaO₂ with preserved PaO₂ (no chemoreceptor response to the low SaO₂) [2]. Hypoxemia and concomitant hypocapnia favour maintaining an acceptable SaO₂ despite low PaO₂. The most common reasons for dyspnoea include hypercapnia, increased work of breathing (reduced compliance), and neuromechanical dissociation. In fact, the aetiology of dyspnoea is often multifactorial and hypoxemia can be a contributing factor [3].

1. Parshall MB, et al. Am J Respir Crit Care Med. 2012 Feb 15;185(4):435-52.
2. Tobin MJ, et al. Am J Respir Crit Care Med. 2020 Jun 1;201(11):1319-1320.
3. Frausing Hansen E. Dyspnea and hypoxemia – what is the correlation (including silent hypoxemia). Nordic Lung Congress 2022, 01–03 June, Copenhagen, Denmark.

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