Cardiac output in paraplegic subjects at high exercise intensities

Summary The purpose of this investigation was to compare cardiac output ( _c ) in paraplegic subjects (P) with wheelchair-confined control subjects (C) at high intensities of arm exercise. At low and moderate exercise intensity _c was the same at a given oxygen uptake ( O₂) in P and C. A group of 11 athletic male P with complete spinal-cord lesions between T6 and T12 and a group of 5 well-matched athletic male C performed maximal arm-cranking exercise and submaximal exercise at 50%, 70% and 80% of each individual's maximal power output (W_max) . Maximal O₂ ( O_2max) was significantly lower, O_2max per kilogram body mass was equal and maximal heart rate (f _c) was significantly higher in P compared to C. At O₂ of 1.3, 1.5 and 1.7 1-min^–1, and for P 65%–90% of the O_2max, _c was not significantly different between the groups, although, _c in P was achieved with a significantly lower stroke volume (SV) and a significantly higherf _c. Although the SV was lower in P, it followed the same pattern as SV in C during incremental exercise, i.e. an increase in SV until about 45%W _max and thereafter a stable SV. The similar _c at a given O₂ in both groups indicated that, even at high exercise intensities, circulation in P can be considered isokinetic with a complete compensation byf _c for a lower SV.     

CO2-ventilatory response of the anesthetized rat by rebreathing technique

The ventilatory response to CO₂ in rats under sodium pentobarbital anesthesia has been measured using the rebreathing technique. The animal rebreathed through a tracheal cannula for a period of 4 min from an apparatus of 200–400 ml capacity, containing 5–6% CO₂ in O₂. in the rebreathing apparatus (PappCO₂), instantaneousV _T,f, and were monitored before, during, and after rebreathing. During the rebreathing run,PappCO₂ andPa _CO2 rose linearly from 35–40 to 65–70 mm Hg; there was no significant difference betweenPappCO₂ andPa _CO2 at any time during rebeathing.V _T and increased almost linearly with the rise inPappCO₂, whilef increased to a maximum within 2 min of rebreathing. In the rat,V _T regulation seemed to operate exclusively as a proportional control system in response to linearly increasing CO₂ stimulus. The slopes ofPappCO₂,V _T or response curves varied considerably during the time course of the experiment, depending upon the level of anesthesia, even though there was no large change in in the control periods which were under hyperoxic conditions. However, a significant linear relationship was seen betweenf in the respective control period and the slope ofPappCO₂-V _T response at various levels of anesthesia. We concluded that the rebreathing technique can be applied in small experimental animals and that changes in the sensitivity of the respiratory control system to a CO₂ stimulus by anesthesia can be easily monitored by repeating the rebreathing test.     

Ventilatory response of prepubertal boys and adults to carbon dioxide at rest and during exercise

Summary The aim of this study was to determine whether the greater ventilation in children at rest and during exercise is related to a greater CO₂ ventilatory response. The CO₂ ventilatory response was measured in nine prepubertal boys [10.3 years (SD 0.1)] and in 10 adults [24.9 years (SD 0.8)] at rest and during moderate exercise ( CO₂ = 20 ml·kg^–1·min^–1) using the CO₂-rebreathing method. Three criteria were measured in all subjects to assess the ventilatory response to CO₂: the CO₂ sensitivity threshold (Th), which was defined as the value of end titalPCO₂ (P _ETCO₂) where the ventilation increased above its steady-state level; the reactivity slope expressed per unit of body mass (SBM), which was the slope of the linear relation between minute ventilation ( _E) andP _ETCO₂ above Th; and the slope of the relationship between the quotient of tidal volume (V _T) and inspiration time (t _I) andP _ETCO₂ (V _T ·t _I ^–1 ·P _ETCO₂ ^–1) values above Th. The _E,V _T, breathing frequency (f _R), oxygen uptake ( O₂), and CO₂ production ( CO₂) were also measured before the CO₂-rebreathing test. The following results were obtained. First, children had greater ventilation per unit body weight than adults at rest (P<0.001) and during exercise (P<0.01). Second, at rest, onlyV _T ·t _I ^–1 ·P _ETCO₂ ^–1 was greater in children than in adults (P<0.001). Third, during exercise, children had a higher S_BM (P < 0.02) andV _T ·t _I ^–1 ·P _ETCO₂ ^–1 (P<0.001) while Th was lower (P<0.02). Finally, no correlation was found between _E/ CO₂ and Th while a significant correlation existed between _E/ CO₂ and S_BM (adults,r=0.79,P<0.01; children,r=0.73,P<0.05). We conclude that children have, mainly during exercise, a greater sensitivity of the respiratory centres than adult. This greater CO₂ sensitivity could partly explain their higher ventilation during exercise, though greater CO₂ production probably plays a role at rest.     

Reliability of the cardiac output measurement with the indirect Fick-principle for CO2 during exercise

Cardiac output measurements were performed during 50 exercise tests in 16 normal subjects employing the indirect Fick principle for CO₂. During sub-maximal steady state exercise the plateau CO₂ tension ( ) was estimated with a rebreathing procedure. The mixed venous CO₂ tension ( ) was calculated by subtracting the alveolocapillary CO₂ tension difference from the . Compared with data from the literature the most valid calculation of the cardiac output was obtained by using the . Cardiac output values, calculated via the turned out to be too low.The reproducibility was tested by repitition of 18 exercise tests at least after 5 days. The relative standard error of a single observation was 4.1% for the cardiac output, which was found to be as good as that of invasive measurements.     

Skeletal muscle determinants of maximum aerobic power in man

Summary The interrelationship between whole body maximum O₂ uptake capacity ( O_{2 max}), skeletal muscle respiratory capacity, and muscle fiber type were examined in 20 physically active men. The capacity of homogenates of vastus lateralis muscle biopsy specimes to oxidize pyruvate was significantly related to O_{2 max} (r=0.81). Correlations of 0.75 and 0.74 were found between % slow twitch fibers (%ST) and O_{2 max}, and between % ST fibers and muscle respiratory capacity, respectively (P<0.01). Multiple correlation analysis (R=0.85) indicated that 72% (R ²=0.72) of the variance in CO_{2 max} could be accounted for by the combined effect of muscle respiratory capacity and the % ST fibers. When the % ST fibers was correlated with O_{2 max}, with the effect of respiratory capacity statistically removed, the relationship became insignificant (r=0.38). These data suggest that muscle respiratory capacity plays an important role in determining O_{2 max}, and that the relationship between % ST fibers and O_{2 max} is due primarily to the high oxidative capacity of this muscle fiber type.This research was supported by NIH grant (HL 20408-02)     

Influence of lifting technique on perceptual and cardiovascular responses to submaximal repetitive lifting

Oxygen consumption ( O₂), heart rate, ventilation and central rating of perceived exertion (RPE) in repetitive lifting while executing squat and stoop techniques were investigated in ten male forestry workers. In all five mass/frequency combinations studied, O₂ was significantly higher for the squat than for the stoop technique. No differences were found in RPE between the techniques. The O₂ and RPE recordings were also related to those obtained during maximal repetitive lifting (same lifting technique) and maximal treadmill running. The O₂ expressed as a percentage of that obtained during maximal repetitive lifting with the same lifting technique was defined as relative aerobic intensity (% O_{2max, lifting}). The % O_{2max, lifting} was not significantly different between the techniques except for the lowest mass lifted (1 kg). This study therefore would support the hypothesis that RPE is more closely related to % O_{2max, lifting} than to absolute aerobic intensity. Related to maximal treadmill running, it was demonstrated for both lifting techniques that relative RPE (percentage of the RPE during maximal running) was more accurate than relative O₂ (percentage of maximal O₂ during maximal running) for determining the % O_{2max, lifting} in repetitive lifting. The study showed that the higher O₂ during squat. lifting compared to stoop lifting was caused by the O₂ expended in lifting and lowering the body rather than the O₂ expended lifting and lowering the external mass. It was concluded that the stoop technique was not superior to the squat technique in terms of central RPE. Based on % O_{2max, lifting}, there may be a rationale for choosing the stoop technique during repetitive lifting with light masses, but not with heavy masses.     

The validity of anaerobic threshold determination by a Douglas bag method compared with arterial blood lactate concentration

Summary Using an open circuit system (Douglas bag method), measurement of the anaerobic threshold (AT) was performed on ten healthy male college students during an incremental exercise test on a bicycle ergometer in an attempt to determine the validity of this method as compared with arterial blood lactate AT measurement.Blood samples were taken from either the radial or brachial artery through a Teflon catheter (3 ml/each time) every minute until the subject's maximal exercise tolerance was reached. Blood lactate was analyzed by the enzymatic method.Differences in work rate, O₂, % O_{2 max}, _E, HR, and R at AT _LA (AT determined by the increase in blood lactate) and at AT _GE (gas exchange AT based on the non-linear increases in _E, CO₂, and other respiratory parameters), respectively, were all found to be statistically insignificant. There was a significant correlation (r=0.866, p<0.01) between AT _LA and AT _GE when expressed in O₂ values (l/min). There was also a significant correlation between AT _LA and O_{2 max} (r=0.778, p<0.01). These results indicate that the commonly used Douglas bag method could provide a valid non-invasive measure of anaerobic threshold.     

A method for estimating bicarbonate buffering of lactic acid during constant work rate exercise

A method to estimate the CO₂ derived from buffering lactic acid by HCO₃ ^– during constant work rate exercise is described. It utilizes the simultaneous continuous measurement of O₂ uptake ( O₂) and CO₂ output ( CO₂), and the muscle respiratory quotient (RQ_m). The CO₂ generated from aerobic metabolism of the contracting skeletal muscles was estimated from the product of the exercise-induced increase in O₂ and RQ_m calculated from gas exchange. By starting exercise from unloaded cycling, the increase in CO₂ stores, not accompanied by a simultaneous decrease in O₂ stores, was minimized. The total CO₂ and aerobic CO₂ outputs and, by difference, the millimoles (mmol) of lactate buffered by HCO₃ ^– (corrected for hyperventilation) were estimated. To test this method, ten normal subjects performed cycling exercise at each of two work rates for 6 min, one below the lactic acidosis threshold (LAT) (50 W for all subjects), and the other above the LAT, midway between LAT and peak O₂ [mean (SD), 144 (48) W]. Hyperventilation had a small effect on the calculation of mmol lactate buffered by HCO₃ ^– [6.5 (2.3)% at 6 min in four subjects who hyperventilated]. The mmol of buffer CO₂ at 6 min of exercise was highly correlated (r = 0.925, P < 0.001) with the increase in venous blood lactate sampled 2 min into recovery (coefficient of variation = ±0.9 mmol·l^–1). The reproducibility between tests done on different days was good. We conclude that the rate of release of CO₂2 from HCO₃ ^– can be estimated from the continuous analysis of simultaneously measured CO₂, O₂, and an estimate of muscle substrate.     

Cardiac output during exercise in paraplegic subjects

Summary The purpose of this study was to measure the cardiac output using the CO₂ rebreathing method during submaximal and maximal arm cranking exercise in six male paraplegic subjects with a high level of spinal cord injury (HP). They were compared with eight able bodied subjects (AB) who were not trained in arm exercise. Maximal O₂ consumption ( O_2max) was lower in HP (1.1 1·min^–1, SD 0.1; 17.5 ml·min^–·kg, SD 4) than in AB (2.5 1·min^–1, SD 0.6; 36.7 ml·min^–1·kg, SD 10.7). Maximal cardiac output was similar in the groups (HP, 141·min^–1 SD 2.6; AB, 16.81·min^–1 SD 4). The same result was obtained for maximal heart rate (f _c,max (HP, 175 beats·min^–1, SD 18; AB, 187 beats·min^–, SD 16) and the maximal stroke volume (HP, 82 ml, SD 13; AB, 91 ml, SD 27). The slopes of the relationshipf _c/ O₂ were higher in HP than AB (P<0.025) but when expressed as a % O_2max there were no differences. The results suggests a major alteration of oxygen transport capacity to active muscle mass in paraplegics due to changes in vasomotor regulation below the level of the lesion.     

Ventilatory response to CO2 at rest and during positive and negative work in normoxia and hyperoxia

Summary Ventilation versus alveolar relationships were determined by the steady-state method in 6 normal male subjects at rest and during positive and negative work at one load in both normoxic and hyperoxic condition. In 5 subjects the slopes of the lines during positive and negative work increased in normoxia as compared with rest. This effect was less evident in hyperoxia. It was also found that the slopes of the lines in positive and in negative work were about the same in both normoxic and hyperoxic conditions. Oxygen uptake and CO₂ production during positive work is higher than during negative work.These results suggest that: 1) the disagreement between various authors on the change of the slope of the line may be due to the differences in the method of calculation of the slope or the method of the determination of lines; 2) the stimuli from the muscle spindles in the working muscle during exercise probably do not contribute to the increase in ventilatory response to CO₂; 3) the increased slope of the normoxic line during exercise may be due to the interaction of several factors such as impulses from working muscles, chemosensitivity of central or peripheral chemoreceptors, adrenal-sympathetic pathways or temperature; 4) respiratory oscillations of or do not seem to influence the respiratory response to CO₂.This study was supported in part by a grant from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.)     

Left atrial pressure and postprandial diuresis in conscious dogs on a high sodium intake

Summary 5 conscious, well trained, female dogs kept on a high sodium intake (14 meq Na/kg bw) were used to measureeft atrial pressure (LAP), urine volume ( ), sodium and potassium excretion (U_Na , U_K ) as well as plasma osmolality (P_osm) before and up to 180 minafter food intake. The dogs were fitted with a catheter in the left atrium (thoracotomy). In all experiments (n=23) LAP increased postprandially (pp) above fasting controls. The mean peak increase range from 4 to 6 cm H₂O and was observed as early as 61–80 and as late, as 161–180 min pp. Increase in LAP was closely correlated to V which rose from 36±28 to 160±51 ul/min·kg. pp was also correlated to pp U_Na which increased from 4.8±3.3 to 34.0±8.5 ueq/min·kg.The pp increase in LAP and its close relation to pp and pp U_Na emphasize the assumption that intrathoracic receptors are involved in the regulation of body fluids.     

Oxygen uptake during swimming in a hypobaric hypoxic environment

Summary The purpose of this study was to determine oxygen uptake O₂) at various water flow rates and maximal oxygen uptake ( O_2max) during swimming in a hypobaric hypoxic environment. Seven trained swimmers swam in normal [N; 751 mmHg (100.1 kPa)] and hypobaric hypoxic [H; 601 mmHg (80.27 kPa)] environments in a chamber where atmospheric pressure could be regulated. Water flow rate started at 0.80 m · s^–1 and was increased by 0.05 m· s^–1 every 2 min up to 1.00 m · s^–1 and then by 0.05 m · s^–1 every minute until exhaustion. At submaximal water flow rates, carbon dioxide production ( CO₂), pulmonary ventilation ( _E) and tidal volume (V _T) were significantly greater in H than in N. There were no significant differences in the response of submaximal O₂, heart rate (f _c) or respiratory frequency (f _R) between N and H. Maximal _E,f _R,V _T,f _c blood lactate concentration and water flow rate were not significantly different between N and H. However, VO_2max under H [3.65 (SD 0.11) l · min^–1] was significantly lower by 12.0% (SD 3.4) % than that in N [4.15 (SD 0.18) l · min^–1] . This decrease agrees well with previous investigations that have studied centrally limited exercise, such as running and cycling, under similar levels of hypoxia.     

The effects of continuous and interval training in women and men

Summary Fourteen Subjects (6 male, 8 female) participated in a training program upon a bicycle ergometer for 7 weeks. Group CT followed a continuous training regimen 4 days per week at 70% O₂ max. Group IT trained by an interval method at 100% O₂ max. The duration of each training session was assigned so that each subject would complete 10,000 kpm of work per session during the first week. Each subsequent week, the work load was increased 3000 kpm. Pretraining tests included O₂ max, standard 7 min tests at 80% O₂ and 90% O₂, an endurance test at 90%, and an intense anaerobic work bout at 2400 kpm. Variables assessed were O₂, HR, and blood lactic acid concentrations. The mean increase in O₂ max was 5.1 ml/kg min (15%) for both groups with a corresponding increase in maximal lactate of 20 mg-%. The response to the post-training tests was nearly identical for both groups: submaximal heart rate at the same absolute work load declined 17 beats/min (CT) and 15 beats/min (IT), submaximal lactate levels declined significantly, endurance ride duration increased 26 min. Continuous and interval training at 70% and 100% O₂ max respectively produce identical changes in heart rate response, blood lactic acid concentration and O₂ max when the total work load is equated per training session.     

Analysis of alveolar PCO 2 control during the menstrual cycle

We attempted to analyze how is regulated during progesterone-induced hyperventilation in the luteal phase. A model for the CO₂ control loop was constructed, in which the function of the CO₂ exchange system was described as and that of the CO₂ sensing system as . Using this model, we estimated (1) the primary increase in produced by progesterone stimulation and (2) the effectiveness (E) of the loop to regulateP _{A CO 2}, defined as P _{A CO 2} (op)/P _{A CO 2} (cl) in which op signifies open-loop and cl, closed-loop. These respiratory variables were investigated throughout the menstrual cycle in 8 healthy women. During the luteal phase, on average, increased by 9.4% andP _{A CO 2},B andH decreased by 0.33 kPa (2.5 mm Hg), 0.47 kPa (3.5 mm Hg) and 13.6%, respectively, whileS and did not change significantly. (op) increased progressively on successive days of the luteal phase whileE remained unchanged at a value of 7.9, thus there was a progressive decrease inP _{A CO 2}. The decrease inH was considered to lessen P _{A CO 2} (op) and so reduce the final deviation ofP _{A CO 2} (P _{A CO 2} (cl)) during the luteal phase. The decrease inB was found to be dependent on (op).     

Criteria for maximum oxygen uptake in progressive bicycle tests

Summary Different criteria for O₂ max in a progressive bicycle exercise were studied in 115 healthy subjects. In the repeated progressive tests performed on 16 men, aged 25–35 years, three types of O₂ response against work load were noticed: a linear increase, an unexpectedly high increase, and a plateau; the last two only appearing when O₂ max was achieved. The last three O₂ values at least were required to define the plateau. Most commonly, subjective exhaustion was achieved, respiratory quotient (R) was over 1.15 and maximal heart rate (HR) at the estimated level for age, though O₂ max was not achieved. No significant differences were found between peak O₂ in the first progressive test (mean=2.95 l/min), the second progressive test (mean=3.14 l/min), or the constant-load test (mean=3.05 l/min). In the progressive test performed once on 55 men and 44 women, aged 35–62 years, subjective exhaustion was achieved by most of the subjects, but the plateau in O₂ was shown only in 17 subjects, and the peak O₂ values were somewhat lower than expected. Moreover, R max did not correlate with peak O₂, and was over 1.15 only in 9 subjects, and HR max was often below the estimated level. Thus, the progressive test appeared to be convenient in testing the physical work capacity of the subjects, but the establishment of the physiological maximum was more difficult: the relatively uncommon plateau in O₂ was the only useful criterion for O₂ max, the value of other criteria being unacceptable.     

O2 Uptake in hyperthyroidism during constant work rate and incremental exercise

Summary To investigate the effect of hyperthyroidism on the pattern and time course of O₂ uptake ( O₂) following the transition from rest to exercise, six patients and six healthy subjects performed cycle exercise at an average work rate (WR) of 18 and 20 W respectively. Cardiorespiratory variables were measured breath-by-breath. The patients also performed a progressively increasing WR test (1-min increments) to the limit of tolerance. Two patients repeated the studies when euthyroid. Resting and exercise steady-state (SS) O₂ (ml·kg^–1·min^–1) were higher in the patients than control (5.8, SD 0.9 vs 4.0, SD 0.3 and 12.1, SD 1.5 vs 10.2, SD 1.0 respectively). The increase in O₂ during the first 20 s exercise (phase I) was lower in the patients (mean 89 ml·min^–, SD 30) compared to the control (265 ml·min^–1, SD 90), while the difference in half time of the subsequent (phase 11) increase to the SS O₂ (patient 26 s, SD 8; controls 17 s, SD 8) were not significant (P = 0.06). The OZ cost per WR increment ( O₂/WR) in ml·min^–1·^–1, measured during the incremental period (mean 10.9; range 8.3–12.2), was always within two standard deviations of the normal value (10.3, SD 1). In the two patients who repeated the tests, both the increment of O₂ from rest to SS during constant WR exercise and the O₂/WRs during the progressive exercise were higher in the hyperthyroid state than during the euthyroid state. While both resting and exercise O₂ are increased in the hyperthyroid patients, the O₂ cost of a given increment of WR is within the normal range. However, a small reduction in the O₂ requirement to perform exercise following treatment of the hyperthyroid state suggests a subtle change O₂ cost of muscle work in this disease.     

Respiratory gas exchange indices used to detect the blood lactate accumulation threshold during an incremental exercise test in young athletes

Summary The time course of changes in blood lactate concentration and ventilatory gas exchange was studied during an incremental exercise test on a cycle ergometer to determine if the lactate accumulation threshold (LT₂) could be accurately estimated by the use of respiratory indices (VT₂) in young athletes. LT₂ was defined as the starting point of accelerated lactate accumulation. VT₂ was identified by the second exponential increase in _E and the ventilatory equivalent for O₂ uptake with a concomitant nonlinear increase in the ventilatory equivalent for CO₂ output. Twelve trained subjects, aged 18–22 years, participated in this study. The initial power setting was 30 W for 3 min with successive increases of 30 W every minute except at the end of the test when the increase was reduced. Ventilatory flow ( _E), oxygen uptake ( O₂), carbon dioxide output ( CO₂), and ventilatory equivalents of O₂ and CO₂ were determined during the last 30 s of every minute. Venous blood samples were drawn at the end of each stage of effort and analysed enzymatically for lactate concentration. After each test, LT₂ and VT₂ were determined visually by two investigators from the graphic results using a double-blind procedure. The results [mean (SEM)] indicate no significant difference between LT₂ and VT₂ expressed as O₂ [43.98 (1.70) vs 44.93 (2.39) ml - min^– - kg^–], lactataemia [4.01 (0.28) vs 4.44 (0.37) mM - 1^–], or heart rate [171 (3.36) vs 173 (3.11) min^–]. In addition, strong correlations were noted between the two methods for O₂ (r=0.90,P<0.001), lactataemia (r=0.75,P<0.01), and heart rate (r=0.96,P<0.001). It is concluded that VT₂ coincides with LT₂ determination and that the ventilatory gas exchange method can thus satisfactorily evaluate the lactate accumulation threshold in young athletes.     

Influence of hepatic innervation on renal glomerular filtration rate

Electrical stimulation of perivascular portal nerves leads to rapid, transient increase of renal glomerular filtration rate (GFR) and of urinary flow rate ( ). In contrast, perivascular stimulation at the vena cava inferior does not significantly alter GFR and . Spinal transfection at the thoracocervical junction does not significantly modify the effect of periportal nerve stimulation. Infusion of the -adrenergic agonist phenylephrine (20 nmol/min) into the superior mesenteric vein increases GFR and , whereas infusion of identical amounts of phenylephrine (20 nmol/min) into the jugular vein does not significantly alter GFR or . The observations indicate that -adrenergic innervation of the liver modifies renal function.     

Pulmonary gas exchange and ventilatory responses to brief intense intermittent exercise in young trained and untrained adults

To investigate pulmonary gas exchange and ventilatory responses to brief intense intermittent exercise and to study the effects of physical fitness on thes responses, nine trained and nine untrained healthy male subjects aged 18–33 years performed the force-velocity (F-) exercise test. This test consisted of 6-s sprints against increasing braking forces (F) separated by 5-min recovery periods. Oxygen uptake ( ), carbon dioxide output ( CO₂), and ventilation _E) were continuously measured during the test and the magnitudes of their responses to the sprints were then calculated.For all subjects CO₂ increased rapidly after beginning the sprints, and the peaks of the responses (F = 13.4;P < 0.001), end of recovery values (F = 6.5;P < 0.01), and O₂ magnitudes of response (F = 12.4;P < 0.001) rose significantly with the repetition of the sprints. The O₂ magnitudes of response correlated with the corresponding sprint power outputs (r = 0.55;P < 0.001) and with the sprint repetitions (r = 0.51,P < 0.001). The CO₂ (F = 7.1;P < 0.01) and {ie442-8} (F = 5.0;P < 0.01) peaks of response increased with the initial load incrementation, then stabilized when the subjects attained peak power output. End of recovery CO₂ (F = 18.0;P < 0.001) and _E (F = 14.1;P < 0.001) values rose with increasingF. TheF- peak O₂, CO₂, _E, tidal volume and respiratory frequency responses attained 53%, 40%, 44%, 66%, and 82% of the peak values measured at exhaustion of maximal graded exercise, respectively.Trained and untrained subjects had the same first sprint power output and braking, force. Nevertheless, the trained subjects had higher O₂ peaks (F = 35.2;P < 0.001) and CO₂ magnitudes of response (F = 30.0;P < 0.001) than the untrained subjects for all sprints. The higher peak O₂ values represented similar percentages of maximal oxygen uptake in the trained and untrained subjects. In summary, the present study showed that in brief intense intermittent exercise, i.e. theF- test, the O₂, CO₂, and ventilatory responses in young subjects were submaximal with respect to the peak values attained at exhaustion of maximal graded exercise. The CO₂ magnitude of response increase was related to the power output rise in the corresponding sprints and to the repetition of sprints. Moreover, the trained subjects presented higher CO₂ peaks and magnitudes of response to the sprints than the untrained subjects.     

Cardiac output measured by acetylene rebreathing technique at rest and during exercise

Cardiac output ( ) was measured by a rebreathing technique, using acetylene and a mass-spectrometer for analyzing. In addition the rate of pulmonary uptake of O₂ ( ) during the rebreathing period and during a preceding steady-state period were determined. Measurements were made on 8 adult humans at rest and at different levels of exercise up to maximum at two occasions. The ratio ( ) during steady-state/ during rebreathing) was found to be significantly below 1 when the was below about 21·min^–1 and to be about 0.55 for subjects at rest. This indicates that , and hence is increased by the rebreathing procedure when this involves deeper and more frequent respirations than those of the preceding period. Accordingly, when was below about 21·min^–1, the value, calculated exclusively from acetylene concentrations recorded during rebreathing, was multiplied by the above-mentioned -ratio. It is shown that this correcting procedure gives more reasonable values than those obtained by acetylene data alone. It is pointed out in what respects this correcting procedure of calculation deviates from that originally used by Grollman, and it is shown that there are only moderate differences between the results obtained by the two procedures.List of Symbols Ac Acetylene - _Ac ^B Bunsen solubility coefficient for Ac in blood (0.700 ml·ml^–1·atm^–1, Chapman et al. 1950) - _PT ^Ac Solubility coefficient for Ac in pulmonary tissue (0.768 ml·ml^–1·atm^–1, Cander and Forster 1959) - Mixed end-capillary to mixed venous oxygen difference per ml blood - F _AC Fractional (dry) concentration of a gas (Ac) in the gas mixture during rebreathing (s) or in the initial mixture in the bag (b) - Alveolar oxygen tension (mm Hg) - pB Barometric pressure (mm Hg) - Pulmonary capillary blood flow (cardiac output), (ml·min^–1) - t Time (s) - V ^b Initial gas volume (ml STPD) in the rebreathing bag (b) - V ^L Initial gas volume in lungs and airways after a deep expiration - V ^PT Volume of pulmonary tissue and blood in the pulmonary capillaries - V _Ac ^s (t _O) Distribution compartment (system volume) of a gas (Ac) at timet _O - Oxygen uptake (ml·min^–1) during rebreathing (RB) or steady-state (SS)