Oxygen debt in aerobic work

Systemic oxygen extraction can be improved during repeated episodes of cardiac tamponade

We used a tamponade model to study the relationship between oxygen uptake (VO2) and oxygen delivery (DO2) during successive, reversible decreases in blood flow. In 7 pentobarbital-anesthetized and mechanically ventilated dogs, a catheter was introduced via a left thoracotomy into the pericardium to inject and to withdraw saline. Each experiment consisted of three steps. First, cardiac output was reduced by successive pericardial fluid injections until 4 to 6 data points were obtained in the dependent region of the VO2/DO2 plot (step 1). Second, cardiac output was restored by progressive withdrawal of pericardial fluid (step 2). Third, cardiac output was lowered again by reinjection of fluid into the pericardium until death (step 3). Expired gases were collected for determination of VO2. In each animal, critical DO2 (DO2crit), below which VO2 became DO2 dependent, was determined from a plot of VO2 versus DO2. When releasing tamponade, VO2 was restored to baseline. For the 3 steps, DO2crit were 10.5 +/- 2.2 mL/kg/min in step 1, 9.8 +/- 1.8 mL/kg/min in step 2, and 8.3 +/- 1.9 mL/kg/min in step 3 (P < .01 v step 1; P < .05 v step 2, respectively). There was no significant difference in VO2 at DO2crit for the three steps. Hence, critical oxygen extraction ratio (ERO2crit) increased from 60% +/- 12% in step 1 to 64% +/- 11% in step 2 (not significant) and to 73% +/- 12% in step 3 (P < .01). The VO2/DO2 dependency slope was also steeper in step 3 than in step 1 (0.77 +/- 0.31 v 0.54 +/- 0.20, P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of exercise intensity and duration on the oxygen deficit and excess post-exercise oxygen consumption

Nine males with mean maximal oxygen consumption (VO2max) = 63.0 ml.kg-1.min-1, SD 5.7 and mean body fat = 10.6%, SD 3.1 each completed nine counterbalanced treatments comprising 20, 50 and 80 min of treadmill exercise at 30, 50 and 70% VO2max. The O2 deficit, 8 h excess post-exercise oxygen consumption (EPOC) and EPOC:O2 deficit ratio were calculated for all subjects relative to mean values obtained from 2 control days each lasting 9.3 h. The O2 deficit, which was essentially independent of exercise duration, increased significantly (P less than 0.05) with intensity such that the overall mean values for the three 30%, 50% and 70% VO2max workloads were 0.83, 1.89 and 3.09 l, respectively. While there were no significant differences (P greater than 0.05) between the three EPOCs after walking at 30% VO2max for 20 (1.01 l), 50 (1.43 l) and 80 min (1.04 l), respectively, the EPOC thereafter increased (P less than 0.05) with both intensity and duration such that the increments were much greater for the three 70% VO2max workloads (EPOC: 20 min = 5.68 l; 50 min = 10.04 l; 80 min = 14.59 l) than for the three 50% VO2max workloads (EPOC: 20 min = 3.14 l; 50 min = 5.19 l; 80 min = 6.10 l). An analysis of variance indicated that exercise intensity was the major determinant of the EPOC since it explained five times more of the EPOC variance than either exercise duration or the intensity times duration interaction. The mean EPOC:O2 deficit ratio ranged from 0.8 to 4.5 and generally increased with both exercise intensity and duration.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen deficit is not affected by the rate of transition from rest to submaximal exercise

Five subjects cycled on an ergometer at power outputs corresponding to 20, 40, 60 and 80% of their maximal oxygen uptake (VO2 max). On one occasion the transition from rest to work was direct (D), while on the other occasion the power output was increased slowly (S) in a stepwise manner for 6-15 min prior to exercise at the predetermined intensity. Oxygen uptake (VO2) was measured, and O2 deficit and O2 debt were calculated. Oxygen deficit increased with the exercise intensities, the peak values being 2.1 +/- 0.2 and 1.9 +/- 0.1 litres (mean +/- SEM) at 80% of VO2 max after D and S respectively. No significant difference was observed in O2 deficit or O2 debt between D and S at any exercise intensity (P less than 0.05). The O2 debt was similar to the O2 deficit at 20, 40 and 60% of VO2 max but lower than the O2 deficit (P less than 0.05) at 80% of VO2 max. Femoral venous blood lactate remained unchanged at 20% of VO2 max but increased at the higher exercise intensities, reaching peak values of 7.6 +/- 0.6 and 7.4 +/- 1.1 mmol l-1 at 80% of VO2 max after D and S respectively. Blood lactate was not significantly different between D and S at any exercise intensity (P greater than 0.05). It is concluded that O2 deficit, O2 debt and blood lactate are not affected by the rate of transition from rest to submaximal exercise. The data contradict the hypothesis that O2 deficit is caused by an inadequate O2 transport at the onset of exercise.

Gas exchanges during exercise in normoxia and hyperoxia

Exercises of constant workload (90 watt) have been carried out during normoxia or hyperoxia FIO2 = 0.45). It has been shown that, in spite of a significant dispersion in the values of O2 deficit and O2 debt calculated, these values are related to the increased blood lactate level which contributes to the marked acidosis observed in both conditions of oxygenation. Hyperoxia reduces lactate level as well as the O2 debt. In addition to the significant increase in arterial [H+] and PCO2, exercise provokes a slight decrease in PO2. It is suggested that the significant variations of these humoral factors might contribute to the control of ventilation during exercise in both conditions of oxygenation.

Oxygen deficit and repayment in submaximal exercise

Oxygen deficit and repayment ratios were investigated at various work loads, intensities and durations. An active baseline was used (walking at 60 m/min) from which deficit and repayment values were calculated. Oxygen uptake (VO2) and core temperatures were measured in 30 males at baseline and during treadmill running (140 m/min) for randomly assigned durations (0.5 ...20 min). Measurements were also made during a 30-min recovery period at baseline work. Results indicated: 1) No difference in O2-repayment between steady-state work and work prior to steady state (P greater than 0.10). 2) O2-repayment was independent of work duration (P greater than 0.10). 3) When workload and intensity were controlled, O2-deficit was not significant factor in O2-repayment (P greater than 0.10). 4) Work intensity (work VO2/VO2 max) was the most significant factor in O2-repayment accounting for 69% of the variance (r equals 0.83, P less than 0.001). Small increments in core temperature and ventilation were not significant factors in O2-repayment. When a working baseline is used, the magnitude of O2-repayment after exercise is independent of the work duration or the attainment of steady state. The extent of O2-repayment after exercise is mainly dependent upon the physiological intensity of the work and the absolute workload (R=0.89, P less than 0.001).

Cardiorespiratory and metabolic costs of continuous and intermittent exercise in man

1. Cardiorespiratory and metabolic responses to paired patterns of continuous and intermittent exercise with the same average power output were studied in eight men. Heart rate, ventilation and pulmonary gas exchange were measured during the different patterns of exercise performed on a cycle ergometer. The recovery oxygen volume was measured over 30 min of loadless pedalling. Needle biopsy samples of the vastus lateralis muscle were taken before, during and after completion of the exercise for measurement of muscle metabolites.2. Heart rate, ventilation, oxygen intake, respiratory exchange ratio, and blood lactate concentration were generally higher with intermittent compared with continuous exercise as were the accumulated totals for heart beats, ventilation and oxygen intake. Muscle biopsy samples tended to have higher lactate and lower phosphocreatine contents in intermittent exercise. The lactate concentration in muscle and blood water was the same during loadless pedalling before exercise but was significantly higher in muscle than blood during exercise. This concentration gradient was larger in intermittent than in continuous exercise.3. Work efficiency, calculated from the total oxygen cost of work in excess of a loadless pedalling control, was significantly lower in intermittent exercise. The explanation is thought to be connected with the observation that when the work was performed at a high rate in short bursts a large part of the oxidative recovery took place after the contraction during the rest periods, whereas in the low intensity continuous exercise the oxygen was mainly utilized while the work was being performed. This indicates that for part of the time in the intermittent exercise the muscle was working under anaerobic conditions. Although the possibility exists that the efficiency of resynthesis of phosphagen may be reduced in this form of activity, it is more likely that the result described is due to the greater amount of lactate formed in the intermittent exercise.