Breath-by-breath assessment of alveolar gas stores and exchange

The volume of O-2 exchanged at the mouth during a breath (VO2,m) is equal to that taken up by pulmonary capillaries (V-O2,V-A) only if lung O-2 stores are constant. The latter change if either end-expiratory lung volume (EELV), or alveolar O-2 fraction (F-AO2) change. Measuring this requires breath-by-breath (BbB) measurement of absolute EELV, for which we used optoelectronic plethysmography combined with measurement of O-2 fraction at the mouth to measure V-O2,V-A = V-O2,V-m - (DeltaEELV.F-AO2 + EELV.DeltaF(AO2)), and divided by respiratory cycle time to obtain BbB O-2 consumption ((V)overdot O-2) in seven healthy men during incremental exercise and recovery. To synchronize O-2 and volume signals, we measured gas transit time from mouthpiece to O-2 meter and compared (V)over dotO(2) measured during steady-state exercise by using expired gas collection with the mean BbB measurement over the same time period. In one subject, we adjusted the instrumental response time by 20-ms increments to maximize the agreement between the two (V)over dot O-2 measurements. We then applied the same total time delay (transit time plus instrumental delay = 660 ms) to all other subjects. The comparison of pooled data from all subjects revealed r(2) = 0.990, percent error = 0.039 +/- 1.61 SE, and slope = 1.02 +/- 0.015 ( SE). During recovery, increases in EELV introduced systematic errors in (V)over dot O-2 if measured without taking DeltaEELV.F-AO2 + EELV.DeltaF(AO2) into account. We conclude that optoelectronic plethysmography can be used to measure BbB (V)over dot O-2 accurately when studying BbB gas exchange in conditions when EELV changes, as during on- and off-transients.