Arterial carbon dioxide estimates during upper body exercise

Summary The purpose of this investigation was to develop a regression equation to predict arterial carbon dioxide tensions (PaCO₂) from end-tidal carbon dioxide tensions (PETCO₂) during upper body exercise. A secondary purpose was to examine the ability of an existing regression equation developed for lower body exercise (Jones et al. 1979) to predict PaCO₂ during upper body exercise. Nine male subjects completed a progressive intensity, discontinuous test on an arm crank ergometer with submaximal power output (PO) levels of 25, 74, and 98 W. During each submaximal PO level, steady state oxygen uptake was achieved and appropriate physiological responses were measured. PETCO₂ was found to be significantly (p<0.05) higher than PaCO₂ at both the 74 and 98 W levels. In addition, when PETCO₂'s were regressed against their corresponding PaCO₂ tensions a correlation coefficient of r=0.66 was found. Based upon our subjects' PETCO₂ and tidal volume (V_T) responses, a multiple regression equation was constructed to predict their PaCO₂ (SPaCO₂), this being SPaCO₂=24.7+0.43 PETCO₂–3.3 V_T. When SPaCO₂ was regressed against the corresponding PaCO₂ tension, a correlation coefficient of r=0,84 was found. When the equation of Jones et al. (1979) was used to predict PaCO₂ a correlation coefficient of r=0.77 was found. No significant differences were found between tensions predicted by either equation and the actual PaCO₂ at any PO level. These results indicate that either prediction equation can be satisfactorily used to estimate PaCO₂ tensions during upper body exercise.Supported by the Medical Research Services of the Veterans Administration, NIH Grant No. 7RO1NS16003-01 and by Air Force Contract No. F33615-78-0501     

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.)     

Ventilatory dynamics in children and adults during sinusoidal exercise

Summary The ventilatory response to sinusoidally varying exercise was studied in five adults and seven prepubertal children to determine whether the faster kinetics of ventilation observed in children during abrupt changes in exercise intensity remained more rapid when exercise intensity varied continuously. Each subject exercised on a cycle ergometer first against a constant load and then against a load fluctuating over six different periods ranging from 0.75 to 10 min. The pedal rate was kept constant for all loads. The inspiratory minute ventilation was determined breath-by-breath. Amplitude (A) and phase angle () of the fundamental component and the first harmonics of the ventilatory response were calculated by Fourier analysis for an integer number of waves for each period. From the relationship between A, and frequency, dynamic parameters of a first order model with and without delay were compared between adults and children. Firstly we found that the ventilatory time constant was significantly faster in children: 49.7 (SD 9.1) s vs 74.6 (SD 11.1) s (P<0.01). Secondly, the change in A and with the frequency was not however characteristic of a first order system without delay in most of the subjects > 90° for the shorter periods). Thirdly, even when the ventilatory control system was described as a first order model with a positive delay, time constants remained significantly shorter in children: 45.6 (SD 5.7) s vs 67.4 (SD 13) s (P<0.01). The ability to increase ventilation faster in children appeared to be a characteristic of the ventilatory control system during exercise independent of the type of drive used.     

Thermoregulation at rest and during exercise in prepubertal boys

Summary Thermal balance was studied in 11 boys, aged 10–12 years, with various values for maximal oxygen uptake ( ), during two standardized sweating tests performed in a climatic chamber in randomized order. One of the tests consisted in a 90-min passive heat exposure [dry bulb temperature (T _db) 45° C] at rest. The second test was represented by a 60-min ergocycle exercise at 60% of individual (T _db 20° C). At rest, rectal temperature increased during heat exposure similar to observations made in adults, but the combined heat transfer coefficient reached higher values, reflecting greater radiative and convective heat gains in the children. Children also exhibited a greater increase in mean skin temperature, and a greater heat dissipation through sweating. Conversely, during the exercise sweating-test, although the increase in rectal temperature did not differ from that of adults for similar levels of exercise, evaporative heat loss was much lower in children, suggesting a greater radiative and convective heat loss due to the relatively greater body surface area. Thermophysiological reactions were not related to in children, in contrast to adults.     

CO2 dissociation curves of oxygenated whole blood obtained at rest and in exercise

Summary In vitro CO₂ dissociation curves for oxygenated whole blood were determined in 19 healthy male subjects at rest and during submaximal and maximal bicycle work. Hemoglobin concentration and blood lactate increased with increasing work load and accordingly buffer value of the whole blood increased while bicarbonate and Base Excess (BE) decreased, resulting in a downward shift of the CO₂ dissociation curve during exercise. Despite the marked increase in buffer values of the blood, the slopes of the CO₂ dissociation curves during exercise were found to be about the same as those obtained at rest. It was inferred that the increasing effect of increased buffer value, on the dissociation slope, was essentially compensated by the decreasing effect of diminished bicarbonate content. The advantages of this relatively constant CO₂ dissociation slope for the indirect measurement of cardiac output by the Fick principle are discussed.     

Hyperpnoea and CO2 sensitivity of the respiratory centres during exercise

Summary The aim of this study was to specify whether exercise hyperpnoea was related to the CO₂ sensitivity of the respiratory centres measured during steady-state exercise of mild intensity. Thus, ventilation , breathing pattern [tidal volume (V _T), respiratory frequency (f), inspiratory time (T _I), total time of the respiratory cycle (T _TOT),V _T/T _I,T _I/T _TOT] and CO₂ sensitivity of the respiratory centres determined by the rebreathing method were measured at rest (SCO_{2 re}) and during steady-state exercise (SCO_{2 ex}) of mild intensity [CO₂ output =20 ml·kg⁻¹·min⁻¹] in 11 sedentary male subjects (aged 20–34 years). The results showed that SCO_{2 re} and SCO_{2 ex} were not significantly different. During exercise, there was no correlation between and SCO_{2 ex} and, for the same , all subjects had very close values normalized for body mass (bm), regardless of their SCO_{2 ex} ( =1.44 l·min⁻¹·kg⁻¹ SD 0.10). A highly significant positive correlation between SCO_{2 ex} andV _T (normalised for bm) (r=0.80,P<0.01),T _I (r=0.77,P<0.01) andT _TOT (r=0.77,P<0.01) existed, as well as a highly significant negative correlation between SCO_{2 ex} and (normalised for bm^−0.25) (r=−0.73,P<0.01). We conclude that the hyperpnoea during steady-state exercise of mild intensity is not related to the SCO_{2 ex}. The relationship between breathing pattern and SCO_{2 ex} suggests that the breathing pattern could influence the determination of the SCO_{2 ex}. This finding needs further investigation.     

Glucose and free fatty acid utilization during prolonged exercise in prepubertal boys in relation to catecholamine responses

Summary Ten prepubertal boys performed 60-min cycle exercise at about 60% of their maximal oxygen uptake as previously measured. To measure packed cell volume, plasma glucose, free fatty acids (FFA), glycerol and catecholamines, blood samples were drawn at rest using a heparinized cathether and at the 15th, 30th and 60th min of the exercise and after 30 min of recovery. At rest, the blood glucose concentrations were at the lowest values for normal. Exercise induced a small decrease of blood glucose which was combined with an abrupt increase of the noradrenaline concentration during the first 15 min. The FFA and glycerol concentrations increased throughout the exercise linearly with that of adrenaline. Compared to adults, the FFA uptake expressed per minute and per litre of oxygen uptake was greater in children. These results suggested that it is difficult for children to maintain a constant blood glucose concentration and that prolonged exercise provided a real stimulus to hypoglycaemia. An immediate and large increase in noradrenaline concentration during exercise and a greater utilization of FFA was probably used by children to prevent hypoglycaemia.     

The influence of CSF calcium and magnesium on the ventilatory response to carbon dioxide during hyperoxia

Summary Respiratory responses to inhaled carbon dioxide were measured in anaesthetized cats during perfusion of the ventriculocisternal system with artificial cerebrospinal fluid. A study was performed to evaluate the effect of changes in the magnesium and/or calcium concentration of the CSF on the CO₂ response curve which was described as .A decrease ofS was observed when the magnesium concentration of the perfusion fluid was increased; theB-value remaining the same. The reverse was true down to magnesium concentrations of 0.6 mmol · l^–1. Below this concentrationS remained the same or decreased; theB-value was lowered.When both the calcium and magnesium concentrations of the CSF were changed, the relation betweenS and these concentrations could be described as to be proportional toC _Ca ^a ·C _Mg ^b . The effect of changes in the calcium concentration was much more pronounced than comparable changes of the magnesium concentration as reflected by the magnitude of the exponentsa andb which were found to be –2.80 (S.D. 0.11) and –0.60 (S.D. 0.03) respectively.     

Ventilatory responses to exercise and carbon dioxide in elderly and younger humans

Summary The present investigation examined the relationship between CO₂ sensitivity [at rest (S _R) and during exercise (S _E)] and the ventilatory response to exercise in ten elderly (61–79 years) and ten younger (17–26 years) subjects. The gradient of the relationship between minute ventilation and CO₂ production ( _E/ CO₂) of the elderly subjects was greater than that of the younger subjects [mean (SEM); 32.8 (1.6) vs 27.3 (0.4); P<0.01]. At rest, S _R was lower for the elderly than for the younger group [10.77 (1.72) vs 16.95 (2.13) 1 · min^–1 · kPa^–1; 1.44 (0.23) vs 2.26 (0.28) 1 · min^–1 · mmHg^–1; P<0.05], but S _E was not significantly different between the two groups [17.85 (2.49) vs 19.17 (1.62) l · min^–1 · kPa^–1; 2.38 (0.33) vs 2.56 (0.21) 1 · min^–1 · mmHg^–1]. There were significant correlations between both S _R and S _E, and _E/ CO₂ (P<0.05; P<0.001) for the younger group, bot none for the elderly. The absence of a correlation for the elderly supports the suggestion that _E/ CO₂ is not an appropriate index of the ventilatory response to exercise for elderly humans.     

Ventilatory response to oscillating and non-oscillatingPa CO 2 in the anesthetized cat

Summary In 11 adult cats, lightly anesthetized with chloralose-urethane, blood from both common carotid arteries was led into a plastic chamber of 15–20 ml and returned to the carotids at a point 1.5 cm more cranial. By doing so arterial blood was assumed to pool within the chamber and lose itsP _{CO 2} oscillations which are normally known to exist as a result of the respiratory cycle. In control periods blood bypassed the chamber, thus maintaining respiratoryP _{CO 2} oscillations. Spontaneous ventilation was measured spirometrically. The animals were breathing pure O₂.Results. 1. When the sinus (carotid) nerves were intact or sectioned there was no significant difference in ventilation before or after switching from non-oscillating to oscillatingPa _{CO 2}. 2. When the vertebral arteries were ligated a drop in ventilation occurred after turning to oscillatingPa _{CO 2} which was followed by a slight rise above control values after 30–50 sec. This phenomenon was independent of sinus nerve integrity. Thus in hyperoxie condition the smallPa _{CO 2} oscillations known to occur in phase with respiration do not seem to provide a respiratory stimulus to resting ventilation above that generated by the mean level ofPa _{CO 2}. The ventilatory depression after vertebral artery ligation must at this time remain unexplained.     

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.     

Ventilatory CO2 response in endurance-trained rats

Summary A CO₂ rebreathing technique was used to assess possible changes in the ventilatory response to CO₂ in rats following a 14-week swim training program. Over the final 9 weeks, the rats swam 1 h per day with a weight of 2.5% of the body weight attached to the tail. Ventilation was measured by a barometric method in awake, restrained rats in a total body plethysmograph at CO₂ concentrations of 0, 2, 4, 6, and 8%, with an initial O₂ concentration of approximately 100%. Ventilation increased in the trained rats with increasing CO₂ from 775 ml·min^–1·kg^–1 at 0% CO₂ to 1,387 ml·min^–1·kg^–1 at 8% CO₂. This increase was a consequence of a 34% increase in tidal volume and a 32% increase in breathing frequency. In comparison with a group of sedentary control rats, there was a significantly higher ventilation and tidal volume at 0% CO₂; however, this difference disappeared with increasing levels of CO₂. A significantly lower resting heart rate was observed in the exercised (296±44 beats·min^–1, mean±SD) compared to the sedentary control rats (380±42). It was concluded that, while the normal training response of resting bradycardia was observed following this duration and intensity of training, endurance swimming had no significant effect on the ventilatory response to CO₂ in the rat.This research was funded in part by grants from the University of Waterloo Research Foundation, the Ontario Heart Foundation, and the Medical Research Council of Canada     

Transcutaneous,noninvasiveP O 2 monitoring in adults during exercise and hypoxemia

A new, commercially available, transcutaneous (tc)P _{O 2} monitor was tested in adult females and in laboratory animals to assess its applicability in measuring arterial oxygen tension during physiological stress. Observed values on dogs correlated well with direct measurements of arterialP _{O 2} and with previous data obtained from measurements of arterial blood during exercise and hypoxemia. In our female subjects the unit responded rapidly to changes in inspired ambient oxygen and electrical stability was excellent during maximal exercise tests. TranscutaneousP _{O 2} decreased to an average of 87.8 Torr during maximum exercise breathing 20.9% O₂, and to 32 Torr while breathing 12.6% O₂ at maximum work. Two distinct patterns of response in tcP _{O 2} were observed during hypoxic and normoxic exercise. The technique appears to have substantial future application both in clinical and physiological investigation involving adult subjects.     

Non-invasive assessment of cardiac output and stroke volume in patients during exercise

Summary A one-step CO₂ rebreathing method for the determination of cardiac output and stroke volume (SV) has been evaluated by comparison with the direct Fick technique during recumbent exercise (10–90 W) in 13 patients. In an initial analysis, the influence of different rebreathing times and of correction for haemoglobin concentration was studied. The best correlation with the direct Fick technique was obtained with the longest analysis time, i. e. 21 s, and correction for variations in haemoglobin concentration further improved the correlation. Consequently, an analysis time of 21 s and correction for haemoglobin have been used. At low cardiac outputs, the CO₂-rebreathing method overestimated the flow compared to the Fick technique. The correlation between the methods, however, was so good that a valid estimate of cardiac output could be obtained from the CO₂ rebreathing method with appropriate corrections (Cardiac output, CO₂ method=2.7+0.77. Cardiac output, Fick; r=0.91; Residual Standard deviation (SD res) =0.77 l · min⁻¹). Stroke volumes measured with the CO₂ rebreathing method did not differ significantly from those obtained with the direct Fick technique, although there was a tendency to overestimate stroke volume with the CO₂ rebreathing method (SV, CO₂ method=12+0.89 · SV, Fick; r=0.82; SD res=11 ml).     

Stroke volume measurement during supine and upright cycle exercise by impedance cardiography

This study evaluated impedance cardiography (ZCG) estimates of stroke volume (SV) during exercise. Seven subjects were studied at rest and during progressive cycle exercise in supine and upright positions. SV was determined by ZCG (SVzcg) during exercise and for the first 5 cardiac cycles following exercise. SVzcg was compared with separate measurements of SV by CO₂ rebreathing (SVco ₂). Static blood resistivity (ρ) was measured at each level of exercise. No significant differences were found between supine exercise and immediate post-exercise values for the peak of the first derivative of the impedance change (dZ/dt _max), left ventricular ejection time (LVET), or SVzcg. Small differences indZ/dt _max and SVzcg, but not LVET, were found in exercise to post-exercise cycling in the upright position. Intra-individual SVzcg and SVco ₂ were moderately correlated (upright meanr=0.64, supiner=0.42) from rest to 70% of peak . Similar correlations were found between Pulse-O₂ ( /heart rate, used as an index to SV) and both SVzcg (uprightr=0.73, supiner=0.57) and SVco ₂ (uprightr=0.8, supiner=0.65). The ZCG parametersdZ/dt _max and LVET correlated better with Pulse-O₂ (dZ/dt _max: uprightr=0.92, supiner=0.73; LVET: uprightr=−0.9, supiner=−0.9). SVzcg calculated with the Kubicek equation performed as well as SVco ₂. ZCG might be a superior method if the inversely correlated parameters,dZ/dt _max and LVET, were not expressed as a product to calculate SV.     

Does the threshold of transcutaneous partial pressure of carbon dioxide represent the respiratory compensation point or anaerobic threshold?

On reaching the respiratory compensation point (RCP) during rapidly increasing incremental exercise, the ratio of minute ventilation (V_E) to CO₂ output (VCO₂) rises, which coincides with changes of arterial partial pressure of carbon dioxide (P _aCO₂). Since P _aCO₂ changes can be monitored by transcutaneous partial pressure of carbon dioxide (PCO_2,tc) RCP may be estimated by PCO_2,tc measurement. Few available studies, however, have dealt with comparisons between PCO_2,tc threshold (T _AT) and lactic, ventilatory or gas exchange threshold (V _AT), and the results have been conflicting. This study was designed to examine whether this threshold represents RCP rather than V _AT. A group of 11 male athletes performed incremental excercise (25 W · min^–1) on a cycle ergometer. The PCO_2,tc at (44°C) was continuously measured. Gas exchange was computed breath-by-breath, and hyperaemized capillary blood for lactate concentration ([la^–]_b) and P _aCO₂ measurements was sampled each 2 min. The T _AT was determined at the deflection point of PCO_2,tc curve where PCO_2,tc began to decrease continuously. The V _AT and RCP were evaluated with VCO₂ compared with oxygen uptake (VO₂) and V_E compared with the VCO₂ method, respectively. The PCO_2,tc correlated with P _aCO₂ and end-tidal PCO₂. At T _AT, power output [P, 294 (SD 40) W], VO₂ [4.18 (SD 0.57)l · min^–1] and [la^–] [4.40 (SD 0.64) mmol · l^–1] were significantly higher than those at V _AT[P 242 (SD 26) W, VO₂ 3.56 (SD 0.53) l · min^–1 and [la^–]_b 3.52 (SD 0.75), mmol · l^–1 respectively], but close to those at RCP [P 289 (SD 37) W; VO₂ 3.97 (SD 0.43) l · min^– and [la^–]_b 4.19 (SD 0.62) mmol · l^–1, respectively]. Accordingly, linear correlation and regression analyses showed that P, VO₂ and [la^–]_b at T _AT were closer to those at RCP than at V _AT. In conclusion, the T _AT reflected the RCP rather than V _AT during rapidly increasing incremental exercise.     

The effects of ventilation pattern on carbon dioxide transfer in three computer models of the airways

We investigate the effects on arterial P(CO(2)) and on arterial-end tidal P(CO(2)) difference of six different ventilation patterns of equal tidal volume, and also of various combinations of tidal volume and respiratory rate that maintain a constant alveolar ventilation. We use predictions from three different mathematical models. Models 1 (distributed) and 2 (compartmental) include combined convection and diffusion effects. Model 3 incorporates a single well-mixed alveolar compartment and an anatomical dead-space in which plug flow occurs. We found that: (i) breathing patterns with longer inspiratory times yield lower arterial P(CO(2)); (ii) varying tidal volume and respiratory rate so that alveolar ventilation is kept constant may change both PA(CO(2)) and the PA(CO(2))-PET(CO(2)) difference; (iii) the distributed model predicts higher end-tidal and arterial P(CO(2)) than the compartmental models under similar conditions; and (iv) P(CO(2)) capnograms predicted by the distributed model exhibit longer phase I and steeper phase II than other models.     

Ventilatory and occlusion-pressure responses to exercise in trained and untrained children

Summary Pattern of breathing and mouth occlusion pressure were investigated during an incremental and exhaustive ergocycle test in untrained and trained 11 to 13 year old boys. At each level of exercise, the trained group had lower ventilation, a lower respiratory equivalent, and a lower respiratory rate. These results suggest that trained subjects have more efficient ventilation. Lower ventilation coincided with a smaller mean inspiratory flow (V_T/T_I), while the ratio of inspiratory to total breath (T_I/T_TOT) was unchanged. In contrast, mouth occlusion pressure and the index of neuromuscular inspiratory drive were the same up to 60 W for the two groups, and tended to be slightly lower in the trained boys above this level.     

Effect of increased blood fluidity through hemodilution on coronary circulation at rest and during exercise in dogs

Summary Coronary flow and myocardial oxygen consumption were measured in conscious dogs at rest and during two levels of submaximal treadmill exercise (3 and 7 km/h at 15% grade, respectively) during adaptation to progressive hemodilution with dextran 60. At rest coronary flow increased to more than seven-fold with diminishing hematocrit to 12.5% in order to cover myocardial oxygen consumption which increased from 6.5±0.3 ml/min· 100 g at hematocrit 47.5% to 13.5±0.8 ml/min· 100 g at hematocrit 12.5%. The dilatory capacity of the coronary vessels, estimated from the reactive hyperemia after a 12 sec occlusion of the left circumflex coronary artery, dropped from 602% at control to 45% at lowest hematocrit levels.During the superimposed stress of exercise coronary flow and myocardial oxygen consumption increased further, so that the dilatory capacity of the coronaries was exhausted at hematocrit levels between 16 and 22%.Myocardial oxygen consumption per unit of oxygen delivered to peripheral tissues increased substantially with progressive hemodilution. In the presence of the reduced arterial oxygen content the augmented myocardial oxygen demand limits the overall adaptability to hemodilution by an exhaustion of the coronary dilatory capacity.Supported by Deutsche Forschungsgemeinschaft.     

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.