Factors affecting the components of the alveolar CO2 output‐O2 uptake relationship during incremental exercise in man

Abstract

SUMMARYThe V VO (alveolar CO2 output-alveolar 02 uptake) relationship ( V-slope) during increasing work rate (r-amp) cycle ergometer exercise has two approximately linear components: a lower component slope (S,) with a value of about 0-95 and a steeper, upper component (S2). We examined the effect of muscle glycogen depletion (protocol 1) and the rate of increase in work rate (ramp rate) without muscle glycogen depletion (protocol 2) on S1 and S2. In protocol 1, ten healthy men with a mean age of 31 4 years (S.D. 6-2) were studied on each of 3 days (days I and 3 were control days). They performed a ramp exercise test to maximum tolerance and steady-state tests at rest, during unloaded pedalling and at two constant work rates below their anaerobic threshold (AT). To deplete muscle glycogen before the test on day 2, the subjects performed 2 h of very heavy cycle exercise on the preceding day and fasted overnight. Si was reduced on day 2 (0 79 compared with 0 95, P < 0 001), as was the VC0 -VO slope derived from steady-state measurements (0 81 compared with 0 99, P < 0 001), but AT and the slope difference (S2 -S5) were unchanged. In protocol 2, seven healthy men with a mean age of 20 6 years (S.D. 2-4) performed ramp tests at three different rates of increasing work rate (1 5, 30 and 60 W min'), each ramp rate being performed twice in random sequence. The ramp rate did not affect Si but S2 was steeper with the faster rates of work rate increase (127, 143 and 163, respectively, P < 0 01). Our findings support the concept that the lower component of the Vslope plot (below AT) represents muscle substrate respiratory quotient (RQ) while the difference between Si and S2 reflects 'excess CO2' derived from bicarbonate buffering of lactic acid.

Effect of Altered Body CO2 Stores on Pulmonary Gas Exchange Dynamics During Incremental Exercise in Humans

“…This is because the bicarbonate-related increase in ýCOµ is a function of the rate of bicarbonate decrease rather than the magnitude of the decrease (Douglas, 1927). However, unlike the results of Cooper et al (1992), Ward & Whipp (1992) have demonstrated that the work rate also has an important effect on the ýCOµ−ýOµ relationship below èL, i.e. S1 was found to be shallower when the work rate was incremented rapidly.…”

“…The intersection of the S1 and Sµ slopes was closely associated with the onset of metabolic acidosis, as shown in Fig. 3 and previously demonstrated (Beaver et al 1986;Sue et al 1988;Cooper et al 1992). This occurred at a ýOµ which averaged 57% of the subjects' peak ýOµ, i.e.…”

“…In this study, it was less than 0·7, compared with expected values of 0·95-1·00 Cooper et al 1992;Wasserman et al 1994) or the average of 0·95 in our control conditions (Table 3). Low S1 values can also be manifest in glycogen-depleted subjects, however ).…”

Respiratory Compensation Point during Incremental Exercise as Related to Hypoxic Ventilatory Chemosensitivity and Lactate Increase in Man.

“…Cooper et al [15] divided the CO 2 output-O 2 uptake (V CO 2 -V O 2 ) relationship up to RCP during incremental exercise into two linear components. The intersection point of the two linear components per se has been described as V-slope AT (AT: anaerobic threshold, expressed in terms of V O 2 level) and a more reliable, non-invasive estimate of AT [7].…”

“…The intersection point of the two linear components per se has been described as V-slope AT (AT: anaerobic threshold, expressed in terms of V O 2 level) and a more reliable, non-invasive estimate of AT [7]. It has been argued that the lower linear component with a lower slope (S 1 ) in the V CO 2 -V O 2 relationship reflects the aerobic metabolism (respiratory quotient) of the working muscles and the upper linear component with a higher slope (S 2 ), aerobic metabolism plus "excess CO 2 " derived from bicarbonate buffering of lactic acid taking place intramuscularly and extracellularly [16] if the body CO 2 stores are stable [15,17]. Therefore, it would be assumed that the difference between S 2 and S 1 reflects an "approximate" rate of lactic acid increase occurring intramuscularly and extracellularly, although it does not reflect the "total" amount of lactic acid increase in the body fluids, a small portion of which is buffered by the non-bicarbonate buffer system [18].…”