Coates A L, Desmond K J, Demizio D, Allen P, Beaudry P H
Department of Pediatrics, McGill University, Canada.
Chest. 1988 Nov;94(5):976-82. doi: 10.1378/chest.94.5.976.
The popularity of the maximum expiratory flow-volume curve (FVC) is in part due to the effort independence of expiratory flow. Of interest are expiratory flow rates at specific lung volumes, usually 50 and 25 percent of vital capacity (VC); Vmax50 and Vmax25, which make accurate assessment of lung volumes essential. Changes in lung volume during the test are due to both the volume of gas expired and the volume change due to gas compression (Vcomp). In normal subjects, Vcomp is small but may be considerable in those with airflow obstruction. When the FVC is measured in a plethysmograph (FVCp), both expired volume and Vcomp are measured. When the volume of the FVC is derived from gas expired at the mouth (FVCm), Vcomp is not considered and differences in Vmax25 or Vmax50 may occur. The magnitude of these errors was assessed in 30 children and young adults: nine normal subjects, ten with cystic fibrosis (CF) and 11 with asthma. For Vmax50, use of FVCm instead of FVCp resulted in an error of 8 +/- 7 percent (mean +/- 1 SD) in the normal subjects compared to 32 +/- 23 in those with CF (p less than 0.01) and 24 +/- 18 for those with asthma (p less than 0.05). For Vmax25, the errors were similar. These errors were not predictable from FEV1 or RV/TLC but were related to a combination of expiratory effort, the shape of the FVCp, and the absolute volume of gas that was being compressed (p less than 0.0001). These findings suggest that expiratory flows in the FVCm are not effort-independent in the face of significant airflow obstruction and that comparisons of values derived from an FVCp with those from an FVCm may not be valid.
最大呼气流量 - 容积曲线（FVC）广受欢迎，部分原因在于呼气流量与用力程度无关。特定肺容积下的呼气流量，通常是肺活量（VC）的50%和25%时的呼气流量，即Vmax50和Vmax25，这使得准确评估肺容积至关重要。测试期间肺容积的变化既源于呼出气体的量，也源于气体压缩导致的容积变化（Vcomp）。在正常受试者中，Vcomp较小，但在气流受限的患者中可能相当可观。当在体积描记器中测量FVC（FVCp）时，呼出气体量和Vcomp都会被测量。当FVC的容积通过口腔呼出气体得出（FVCm）时，Vcomp未被考虑，可能会出现Vmax25或Vmax50的差异。在30名儿童和年轻人中评估了这些误差的大小：9名正常受试者、10名患有囊性纤维化（CF）的患者和11名患有哮喘的患者。对于Vmax50，使用FVCm而非FVCp时，正常受试者的误差为8±7%（平均值±1标准差），而CF患者为32±23%（p<0.01），哮喘患者为24±18%（p<0.05）。对于Vmax25，误差相似。这些误差无法从第一秒用力呼气容积（FEV1）或残气量/肺总量（RV/TLC）预测，但与呼气用力程度、FVCp的形状以及被压缩气体的绝对量有关（p<0.0001）。这些发现表明，在存在明显气流受限的情况下，FVCm中的呼气流量并非与用力程度无关，并且将FVCp得出的值与FVCm得出的值进行比较可能无效。
