Lung tissue volume and blood flow by rebreathing theory

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Abstract
Rebreathing determinations of pulmonary tissue volume (Vt) and pulmonary capillary blood flow (.ovrhdot.Qc) are supplanting breath-holding methods because of the shorter time for data collection and better reproducibility. However, rebreathing methods have errors due to dead space (VD), volume rebreathed (.ovrhdot.VE), rebreathing rate (RR), and from uneven distribution between Vt, .ovrhdot.Qc, .ovrhdot.VE and alveolar volume (VA). These errors were studied by calculating Vt and .ovrhdot.Qc from a normal lung model with dimensions: Vt = 600 ml, .ovrhdot.Qc = 6 1/min, .ovrhdot.VE = 2000 ml, residual volume = 2000 ml, VD = 200 ml and RR = 30/min. RR as slow as 10/min and VD of 200 ml caused less than a 4% error in Vt and .ovrhdot.Qc. Increases in VD to 1000 ml overestimated Vt by 31% and underestimated .ovrhdot.Qc by 19%, but a simple correction for the decreased effective VA with the 1st breath reduced these errors to 8% and 13%, respectively. More rapid RR and larger .ovrhdot.VE always decreased errors due to VD. The uneven distribution reported in normal man due to gravitational forces caused a 4% error in Vt and a 1% error in .ovrhdot.Qc. Lung models constructed from reported data for uneven distribution in severe obstructive lung diseases showed large errors in Vt (-13% to +46%) and smaller errors in .ovrhdot.Qc (-1% to -18%). Models of severe restrictive diseases generally gave marked underestimates of Vt (-3% to -64%). Estimates of Vt and .ovrhdot.Qc have acceptable accuracy in normal man and subjects with moderate degree of uneven distribution, but large errors develop with more severe forms of lung disease.

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