Das Verhalten des Kohlenstoffdioxid-Partialdruckes während des 6-Minuten-Gehtests und in der Nacht bei Patienten mit COPD IV

COPD patients are compromised by hypercapnia and life threatening CO2 narcosis especially at the advanced stage of disease. An increase of pCO2 can particularly occur under exertion due to required enhancement of respiration and during sleep because of lower depth of ventilation. The behaviour of carbon dioxide in blood (pCO2) under exertion has been evaluated only in very few studies. Diaz et al (2010) examined in COPD patients (FEV1 = 64%) the CO2 production during the 6-minute-walk test through mobile spirometry. They found a development of a CO2 plateau between the third and sixth minute of the walking test. But no factors contributing to an especially high or fast pCO2 increase were mentioned. In this trial the pCO2 behaviour was – for the first time – examined in the 6-minute walk test by continuous transcutaneous measurement of pCO2. Further the relation between the maximum pCO2 increase under exertion and the main outcome measures FEV1 (level of obstruction) and residual volume (level of emphysema) was investigated. Additionally, potential predictors for the pCO2 behaviour under exertion (fat free mass index (FFMI), BMI, energy expenditure, further parameters of lung function, age) were explored in an orientating manner. Moreover the link between the behaviour of pCO2 under exertion and in resting condition respectively in sleeping condition was examined. 32 patients of COPD IV (GOLD classification) were included in this trial. After admission diagnostics (bodyplethysmograph with mouth occlusion pressure, resting blood gas analysis, body impedance analysis for body composition) the patients completed a 6-minute walk test with continuous transcutaneous pCO2, SpO2 and pulse measurement and capillary blood gas analysis directly before and after walking. Moreover transcutaneous pCO2 measurements during sleep and resting were performed. By the SenseWear™ armband activity and energy expenditure were analysed. The walking test, the measurements during sleep and rest and the measurement with the SenseWear™ armband were repeated after 1 week. The graphic representation of the course of the tcpCO2 graphics in the walking test showed a fanlike distribution of the graphics. An inclining, constant and declining course of the graphics could be roughly distinguished. In the few other trials mainly an inclining pCO2 behaviour under exertion was described. Significant correlations were found between the maximum pCO2 increase under exertion and the main outcome parameters FEV1 (level of obstruction) and residual volume (level of emphysema). One possible reason for this could be the level of dynamic hyperinflation during the walking test which is depending on the air flow limitation and the level of emphysema. Besides a further possible reason could be the insufficiency of respiratory pump because of increased load due to obstruction or emphysema. The overstrained respiratory pump is not able to ensure the essential increase of respiration under physical exertion. Moreover in an orientating manner the relation between pCO2 behaviour under exertion and further secondary outcome measures as possible predictors were examined. Significant correlations were seen with the energy expenditure (especially MET basal metabolic rate) the FFMI, the BMI, age and further parameters of lung function like intrathoracic gas volume (ITGV) and MEF25. There was no significant correlation found to the respiratory capacity as an indicator of respiratory pump insufficiency . In the walking test significantly negative correlations between pCO2 and pO2 behaviour were found which is most likely due to a global respiratory insufficiency as part of the dynamic hyperinflation or exhaustion of the respiratory pump (pCO2 increase, pO2 decline) respectively due to demand hyperventilation because of a mismatch of ventilation and perfusion (constant pCO2, pCO2 decline). It has been shown that the resting pCO2 does not provide information about the pCO2 behaviour under exertion. In resting conditions a distinct mismatch of ventilation and perfusion can be well obscured by compensating mechanisms which decompensate under exertion sothat pCO2 increases. Moreover no significant correlation between the pCO2 behaviour under exertion and the pCO2 behaviour in sleep has been seen. This might be attributed to the different pathomechanisms which cause a pCO2 increase under exertion and in sleep. Under exertion the dynamic hyperinflation plays an important role whereas in sleeping conditions especially the imbalance of capacity and load of the respiratory pump with consecutive respiratory pump insufficiency causes a pCO2 increase. If in further studies the here assumed correlation between the dynamic hyperinflation and pCO2 increase during the 6-minute walk test in COPD patients can be approved, the pCO2 increase and the associated risks under daily exertion could be reduced by intensified therapy of dynamic hyperinflation.