Pulmonary Vascular Resistance in Emphysema: Technical Factors

Pulmonary Vascular Resistance in Emphysema: Technical FactorsAlthough a reduced DcoSB is a useful clinical predictor of the presence of emphysema, morphologic studies do not support a direct correlation between emphysema score and DcoSB. This is particularly true for mild emphysema. The diminution of DcoSB characteristically observed in emphysema has been ascribed to microvascular disruption. The question remains as to how precise a marker of the extent of the pulmonary microcirculation DcoSB affords. This variable appears to provide an acceptable index in patients with ILD in whom we previously demonstrated a tight, linear relationship between DcoSB and pulmonary blood volume (r = 0.821, p < 0.001). In such subjects, however, the distribution of ventilation is relatively well preserved. This is not the case in emphysema. Maldistribution of inspired CO might lead to an underestimate of the extent of the microcirculation. Calculation of DcoSB requires an estimate of the alveolar volume (Va) attained during the inspiratory maneuver required for DcoSB measurement more asthma inhalers online. Two different methods are generally utilized for measuring Va. The first method, originally proposed by Ogilvie et al and utilized in the present study, requires Va to be calculated as the sum of the inspired volume and a separately determined residual volume (RV), usually by helium rebreathing. The second method measures Va from the dilution of a single breath of helium during the diffusing capacity maneuver. The difference between multiple and single breath helium dilution TLC measurements can be greater than 20 percent in patients with moderate or severe airflow obstruction. One would, therefore, expect the relative difference between the two methods of calculating DcoSB to be similar to that found between the difference in TLC measurements. (In ILD there are trivial differences between the two methods of calculating DcoSB.)
In a study designed to examine the effects of chronic airways obstruction on the difference between these two common methods of calculating Dco, we found a mean difference of 16 ± 11 percent in patients with moderate obstruction and 21 ± 14 percent in patients with severe obstruction. Previous reports of a smaller discrepancy may relate to patients consistently failing to exhale to their true RV before inhaling the gas mixture, producing a systematic underestimation of Va in the rebreathing method. The single-breath measurement of Va does not include gas exchange in poorly ventilated lung compartments and excludes this portion of the microcirculation from the estimation. In contrast to the underestimate utilizing the singlebreath Va, the rebreathing method appears to yield a value of DcoSB that may provide a more faithful index of the extent of the microcirculation.

This entry was posted in Emphysema and tagged airflow obstruction, emphysema, pulmonary function, pulmonary vascular resistance, resistance pulmonary blood.