BREATHING PATTERN AND CARBON-DIOXIDE RETENTION IN SEVERE CHRONIC OBSTRUCTIVE PULMONARY-DISEASE

Background - The factors leading to chronic hypercapnia and rapid shal low breathing in patients with severe chronic obstructive pulmonary di sease (COPD) are not completely understood. In this study the interrel ations between chronic carbon dioxide retention, breathing pattern, dy spnoea, and the pressure required for breathing relative to inspirator y muscle strength in stable COPD patients with severe airflow obstruct ion were studied. Methods - Thirty patients with COPD in a clinically stable condition with forced expiratory volume in one second (FEV(1)) of <1 litre were studied. In each patient the following parameters wer e assessed: (1) dyspnoea scale rating, (2) inspiratory muscle strength by measuring minimal pleural pressure (PPLmin), and (3) tidal volume (VT), flow, pleural pressure swing (PPLsw), total lung resistance (RL) , dynamic lung elastance (ELdyn), and positive end expiratory alveolar pressure (PEEPi) during resting breathing. Results - Arterial carbon dioxide tension (PaCO2) related directly to RL/PPLmin, and ELdyn/PPLmi n, and inversely to VT and PPLmin. There was no relationship between P aCO2 and functional residual capacity (FRC), total lung capacity (TLC) , or minute ventilation. PEEPi was similar in eucapnic and hypercapnic patients. Expressing PaCO2 as a combined function of VT and PPLmin (s tepwise multiple regression analysis) explained 71% of the variance in PaCO2. Tidal volume was directly related to inspiratory time (TI), an d TI was inversely related to the pressure required for breathing rela tive to inspiratory muscle strength (PPLsw, %PPLmin). There was an ass ociation between the severity of dyspnoea and both the increase in PPL sw (%PPLmin) and the shortening in TI. Conclusions - The results indic ate that, in stable patients with COPD with severe airflow obstruction , hypercapnia is associated with shallow breathing and inspiratory mus cle weakness, and rapid and shallow breathing appears to be linked to both a marked increase in the pressure required for breathing relative to inspiratory muscle strength and to the severity of the breathlessn ess.