Anaerobic threshold was determined by two methods: (1) modification of the V-slope method of Beaver et al; and (2) the Vo* at which there was a fall in plasma standard bicarbonate. The latter was determined for all tests by one observer who estimated the Vo* at which log standard HC03~ was seen to decrease from its baseline value (Fig 1A) in a way similar to that previously described. The Vo2 at this point was taken as the AT shown by development of metabolic acidosis. The modified V-slope method is shown in Figure IB, in which carbon dioxide output (Vco*) is plotted against oxygen uptake (Vo*). A line parallel to the line of identity is drawn through Vco* vs Vo* points during the incremental phase of the exercise test The point at which the Vco* departs from the line (begins to increase more rapidly than Vo*) is taken as the “V-slope” AT.
The basis for drawing a line with a slope of 1 through the points below the AT was established empirically because, in our patients and normal subjects, and in the subjects described by Beaver et al,5 the slope of a line drawn through points below the AT closely approximated 1, conforming to the finding that the muscle uses primarily glycogen for energy in normally fed people.13 Note that the Vco* vs Vo* points drawn on a line with slope = 1 do not indicate that the VccVVoj (gas exchange ratio, R) is equal to 1, but rather that the ratio of increase in VcoJ increase in Vo* is close to 1. Any additional increase in Vco* out of proportion to the increase in Vo* with increasing work must be attributed to CO* evolved from the buffering of metabolic (lactic) acid because (a) subjects do not hyperventilate at this point in exercise; and (b) the CO* evolved is too great to be attributed to hyperventilation.
For the normal subjects, comparison of the AT determined by the modified V-slope and log standard HC03~-log Vo* methods was made by the paired t test and the correlation coefficient The COPD patients were divided into two groups on the basis of the decrease in standard HC03_ in response to exercise between rest and 2 min into recovery: group 1, in which the fall in standard HC03~ was minimal (^2 mEq/L), and group 2, in which standard HC03~ fell by 2.5 mEq/L or more. These groups were compared using the t test for unrelated measures. The relationships between a number of variables and the degree of metabolic acidosis were examined using the correlation coefficient.
Figure 1(A). Log (standard HC03~) vs log (oxygen Uptake (Vo*)) during cycle incremental exercise for single subject Point at intersection of lines drawn id best fit points before and after decline in log standard HC03~ is taken as anaerobic threshold. In Figure 1(B), carbon dioxide output (VcoJ is plotted against oxygen uptake (Vos). Using this method, increase in Vco* is more rapid than increase in Vo, when anaerobic threshold is reached.
Category: Pulmonary Disease
Tags: lung disease, metabolic acidosis, Pulmonary function