BACKGROUNDDyspnoea is a common and disabling symptom in patients with cardiopulmonary disease. Unfortunately the mechanisms that produce dyspnoea are still poorly understood. The relationship between dyspnoea and the load on the ventilatory muscles, chemical drive, and ventilatory indices was therefore assessed in patients with obstructive pulmonary disease during an incremental exercise test.
METHODSFifty patients with a wide range of obstructive pulmonary disease (mean forced expiratory volume in one second (FEV₁) 66.1 (28.8)% predicted) performed an incremental cycle ergometer test. A subdivision was made between subjects with CO₂ retention (PaCO₂ 0, n = 22) and subjects without CO₂ retention (PaCO₂ <0, n = 28) during exercise. During the test dyspnoea (Borg score), oesophageal pressures (mechanical load on the ventilatory muscles (time tension index (TTI)), blood gas tensions, and minute ventilation were measured. Correlations for changes in mechanical and chemical factors with changes in dyspnoea score were calculated to assess relevant factors. An analysis of covariance was used to examine whether there was a relationship between dyspnoea score and each of these factors and whether this relationship was different between the subgroups with and without CO₂ retention. Multiple regression analysis was used to assess the independent effect of each parameter on dyspnoea sensation. Furthermore, the amplitude of pleural pressure swing ((Pi + Pe)act) generated at maximal work load (Ptot, an indication of the load on all respiratory muscles) was calculated. Analysis of covariance was used to assess whether there was a relationship between tidal volume (VT) and Ptot and whether this relationship was different between the groups (slopes are an expression of the length-tension inappropriateness, LTI).
RESULTSIn the total group and the group without CO₂ retention a significant correlation between dyspnoea and the increase in the inspiratory time tension index (TTIi) was present. In the group with CO₂ retention a significant correlation was seen between dyspnoea and PaCO₂. The factors PaO₂, VE%MVV and (VT/Ti) showed a correlation with a p value of 0.10 both in the total group and in those without CO₂ retention. In an analysis of covariance the relationship between dyspnoea score and PaCO₂ appeared to be significantly different between the two subgroups, being more pronounced in the group with CO₂ retention. No other relationships with change in dyspnoea score were found. There was no significant relationship between VT and Ptot in the total group nor in the two subgroups, indicating some length-tension inappropriateness in both groups.
CONCLUSIONIn patients with distinctive pulmonary disease who are normocapnic or hypocapnic the mechanical load (TTIi) and length-tension inappropriateness (LTI) on ventilatory muscles seem to be the main determinant of exertional dyspnoea. As soon as hypercapnia occurs, this seems to override all other inputs for dyspnoea.

Keywords: dyspnoea; exercise; hypercapnia; respiratory muscles