Assessment of breath-by-breath alveolar gas exchange: an alternative view of the respiratory cycle.

PURPOSE: Breath-by-breath (BbB) determination of the O2 flux at alveolar level implies the identification of the start and end points of each respiratory cycle; Grønlund defined them as the times in two successive breaths showing equal expiratory gas fractions. Alternatively, the start and end points of each breath might be linked to the ratio between the exchangeable and non-exchangeable gases. The alternative algorithm is described and evaluated with respect to the algorithm proposed by Grønlund. METHODS: Oxygen and carbon dioxide fractions, and ventilatory flow at the mouth were continuatively recorded in 20 subjects over 6 min at rest and during a cycloergometer exercise including 4 increasing intensities lasting 6 min each. Alveolar BbB oxygen uptake was calculated from the gas and flow traces by means of the two methods at stake. RESULTS: Total number of analysed breaths was 14,257. The data obtained with the two methods were close to the identity line (average slope 0.998 ± 0.004; R > 0.994; n > 334 in all subjects). Average difference between the O2 uptake data obtained by the two methods amounted to -0.27 ± 1.29 mL/min, whilst the standard deviation of the differences was 11.5 ± 4.6 mL/min. The relative percentage difference was independent from the O2 uptake and showed an average bias amongst subjects close to zero (-0.06 ± 0.15 %). CONCLUSIONS: The alternative timing of the respiratory cycle provided congruent O2 uptake data and made the identification of the start and end points of each breath more robust without introducing systematic errors.

Assembling more O2 uptake responses: is it possible to merely stack the repeated transitions?

Kinetic parameters for pulmonary O2 uptake at exercise onset are estimated by non-linear regression on repeated responses assembled together. The native data contain the information, thus the "stacking" of the responses should provide correct values and uncertainties. Kinetic parameters and corresponding uncertainties (computed from the asymptotic standard errors; ASE) were estimated on 10(4) simulated noisy responses (with time constant τ=25s), repeated 10 times and assembled over an increasing number of repetitions (Nr) by "stacking" or ensemble averaging the responses processed to obtain 1s bins ("1-s-bins"). Independent of the assembling method, the average estimated τ amounted to ∼25.05 s. Independent of Nr, the "1-s-bins" and the "stacking" yielded an ASE/SD ratio for τ amounting to ∼0.52 and ∼0.98, respectively, resulting in a probability of including τ=25s within the estimated uncertainty from an individual kinetics amounting to ∼70% and >94% for the "1-s-bins" and the "stacking", respectively. In conclusion, the "stacking" allowed obtaining, also for individual kinetics, coherent estimated parameters and associated uncertainties.

Confidence intervals for the parameters estimated from simulated O2 uptake kinetics: effects of different data treatments.

The behaviour of pulmonary O2 uptake following a moderate-intensity step exercise increment is usually described by a first brief increase, followed by a second exponential time course reaching the new steady state (phase II). The parameters describing the phase II kinetics are investigated by applying different data treatments to the acquired O2 uptake data to reduce the effects of their noise before running a non-linear regression procedure. The effects of different data treatments (nothing, resampling at various time intervals or averaging of more repetitions) on the precision and/or accuracy of the kinetics parameters estimated by non-linear regression with a simple mono-exponential model were investigated by artificially generating 10(5) simulated responses with average breath duration of 3.5 s. The simulations showed that, whatever the explored data treatment, the average estimated parameters were close to the theoretical ones. Nevertheless, in all the explored conditions, the non-linear regression provided narrower asymptotic confidence intervals than the real ones. In particular, when the responses were resampled at 1 s time intervals, the width of the asymptotic confidence interval for the time constant was 50% of the real one, even after the averaging of more repetitions. The reasons for this discrepancy were investigated further, allowing us to identify some methods to obtain the correct confidence interval of the O2 uptake kinetics parameters. The simplest method to obtain an asymptotic confidence interval close to the real one is to average more responses resampled to a time interval slightly longer than the average breath duration.

Oxygen uptake kinetics at work onset: role of cardiac output and of phosphocreatine breakdown.

The hypothesis that variability in individual's cardiac output response affects the kinetics of pulmonary O2 uptake (VO2) was tested by investigating the time constants of cardiac output (Q) adjustment (τ(Q)), of PCr splitting (τ(PCr)), and of phase II pulmonary O2 uptake (τ(VO2)) in eight volunteers. VO2, Q, and gastrocnemius [PCr] (by (31)P-MRS) were measured at rest and during low intensity two-legged exercise. Steady state VO2 and Q increased (ΔVO2(s) = 182 ± 58 mL min⁻¹; ΔQ = 1.3 ± 0.4 L min⁻¹), whereas [PCr] decreased significantly (21 ± 8%). τ(VO2), τ(PCr) and τ(Q) were significantly different from each other (38.3 ± 4.0, 23.9 ± 2.5, 11.6 ± 4.6 s, respectively; p<0.001). τ(PCr) assumed to be equal to the time constant of VO2 at the muscle level (τ(mVO2)), was not related to τ(Q), whereas τ(VO2) and τ(Q) were significantly related (p<0.05) as were τ(VO2) and τ(PCr) (p<0.05). Venous blood O2 stores changes, as determined from arterio-to-mixed-venous O2 content, were essentially equal to those estimated as (τ(VO2)-τ(PCr))·ΔVO2(s). This suggests that cardiac output responses affect O2 stores utilization and hence τ(VO2) : thus τ(VO2) is not necessarily a good estimate of τ(mVO2).

Mitochondrial coupling in humans: assessment of the P/O2 ratio at the onset of calf exercise.

Coupling of oxidation to ATP synthesis (P/O2 ratio) is a critical step in the conversion of carbon substrates to fuel (ATP) for cellular activity. The ability to quantitatively assess mitochondrial coupling in vivo can be a valuable tool for basic research and clinical purposes. At the onset of a square wave moderate exercise, the ratio between absolute amount of phosphocreatine split and O2 deficit (corrected for the amount of O2 released from the body O2 stores and in the absence of lactate production), is the mirror image of the P/O2 ratio. To calculate this value, cardiac output (Q), whole body O2 uptake (VO2), O2 deficit (O2(def)) and high-energy phosphates concentration (by 31P-NMR spectroscopy) in the calf muscles were measured on nine healthy volunteers at rest and during moderate intensity plantar flexion exercise (3.44 +/- 0.73 W per unit active muscle mass). Q and VO2 increased (from 4.68 +/- 1.56 to 5.83 +/- 1.59 l min(-1) and from 0.28 +/- 0.05 to 0.48 +/- 0.09 l min(-1), respectively), while phosphocreatine (PCr) concentration decreased significantly (22 +/- 6%) from rest to steady-state exercise. For each volunteer, "gross" O2(def) was corrected for the individual changes in the venous blood O2 stores (representing 49.9 +/- 9.5% of the gross O2(def)) yielding the "net" O2(def). Resting PCr concentration was estimated from the appropriate spectroscopy data. The so calculated P/O2 ratio amounted on average to 4.24 +/- 0.13 and was, in all nine subjects, very close to the literature values obtained directly on intact skeletal muscle. This unfolds the prospect of a non-invasive tool to quantitatively study mitochondrial coupling in vivo.

Estimation of the phosphocreatine T1 time constant using a clinical NMR scanner.

AIM: The aim of this study was to determine the "spin-lattice relaxation" or "T1" time constant of phosphocreatine in the human gastrocnemius muscle, both at rest and during aerobic exercise, when a clinical nuclear magnetic resonance (NMR) scanner is available. MATERIALS AND METHODS: Using the multipoint saturation recovery technique, we tested four acquisition protocols differing in number and duration of repetition times. Moreover, two mathematical models describing the phenomenon were also evaluated. Protocols and models were tested on a phantom containing 1 kg of distilled water in which were dissolved 5.88 g sodium chloride, 1 cm3 phosphoric acid and 7.58 g sodium phosphate dodecahydrate. One protocol only was used on a group of four healthy volunteers both at rest and during exercise. Each volunteer repeated the rest-exercise sequence three times. RESULTS AND CONCLUSIONS: On the phantom, the average T1 values for the various protocols and mathematical models used proved to differ widely, ranging from 0.61 s to 7.20 s. On volunteers, the T1 values obtained at rest and during exercise were not significantly different--0.91 s on average. Correction of this T1 value with the results obtained using the phantom provides a T1 value of 5.73 s, which is comparable with the value reported in the literature for resting conditions only.

T(1) measurement of (31)P metabolites at rest and during steady-state dynamic exercise using a clinical nuclear magnetic resonance scanner.

This article illustrates some problems and possible solutions to determine the apparent spin-lattice relaxation time (T(1)) of the muscular (31)P metabolites at rest and during dynamic steady-state exercise using a clinical 1.5 T NMR scanner and a surface coil. T(1) was first estimated on a phosphates solution (phantom) using four different acquisition protocols, all based on the multiple-point "progressive saturation" method, and by fitting each data set with two different mathematical models. Subsequently, two of the four protocols and both models were used to estimate T(1) both at rest and during exercise on the calf muscles of 10 healthy volunteers. Experimental results obtained on the phantom showed that T(1) is greatly affected by the longest nominal explored repetition time (P<0.001) and by the mathematical model (P<0.001), ranging from 0.65+/-0.10 to 8.4+/-0.8 s. The two acquisition protocols applied on volunteers yielded significantly different T(1) (P<0.001), which were also rather different from the literature values for the same metabolites. Nevertheless, independently of the acquisition protocol and/or the fitting procedure, T(1) of all muscular phosphagens did not change statistically from rest to steady-state aerobic exercise.

Energetics of muscular exercise at work onset: the steady-state approach.

The energetics of muscular exercise at steady state is tightly coupled to, and dependent on, the events that occurred during the transient phase, of which the steady-state is therefore the "memory". The aim of the present study is to show that it is possible to utilize data obtained at exercise steady state to gain information on variables traditionally assessed during exercise transients. A theoretical model based on the steady-state relationships between mechanical power, O(2) uptake (VO(2)) and phosphocreatine (PC) split allows us to highlight three interdependent parameters: the time constant of the VO(2) on response at the muscular level (tau), the mechanical equivalent of PC splitting and the P/O(2) ratio. The model was applied to experimental data obtained during moderate calf exercise in humans inside an MR unit. For a P/O(2) from 5.6 to 6.2, the obtained tau values range from 10.6 to 24.9 s (for PC concentrations from 17.8 to 37.7 mmol/kg fresh muscle), and the corresponding mechanical equivalents of PC from 9.6 to 10.6 J/mmol. The analysis also shows that the time constant of the O(2) uptake kinetics is strictly dependent on the PC concentration at rest, whereas the mechanical equivalent of PC is unaffected by its concentration.

Relationships between mechanical power, O(2) consumption, O(2) deficit and high-energy phosphates during calf exercise in humans.

Whole-body O(2) uptake ( VO(2)), O(2) deficit and the concentration of high-energy phosphates (determined by (31)P spectroscopy) in human calf muscle were measured during moderate aerobic square-wave exercise of increasing intensity in ten volunteers. Net VO(2) (above resting) increased linearly with mechanical power, yielding a delta efficiency of 13.1%. "Gross" O(2) deficit increased linearly with net VO(2). The fraction of phosphocreatine (PC) split at steady state increased linearly with the mechanical power and with the O(2) deficit. If the [PC] in resting muscle is known, the slope of the regression between PC split and O(2) deficit (in millimoles) yields the P/O(2) ratio. To calculate this, the O(2) deficit was corrected for the amount of O(2) derived from the body stores, as obtained from literature data. The value so obtained, for a resting [PC] of 30 mM was 5.9, consistent with canonical textbook values. Furthermore, the ratio of "true" O(2) deficit to steady-state VO(2) is a measure of the time constant of VO(2) kinetics at work onset at the muscle level: assuming a monoexponential time course without time delays it amounted to about 17 s, close to the value that can be expected in mammalian muscle at 37 degrees C.

Two-pedal ergometer for in vivo MRS studies of human calf muscles.

This article describes an ergometer that enables human subjects to exercise one or both limbs while (31)P NMR spectra are obtained. Two independent pedals, equipped with position and force transducers, were mounted on the ergometer in order to calculate mechanical work performance. First, the effect of the magnetic field upon the signal coming from the transducers was investigated. Then the ergometer was tested by performing a series of steady-state exercises of increasing intensity. Experimental data showed that actual mechanical power ranged from 1.5 +/- 0.2 W to 11.0 +/- 1.6 W, while the corresponding oxygen consumption increased from 0.28 +/- 0.04 l/min at rest to 0.48 +/- 0.10 l/min at the highest load, and the PCr/(PCr+P(i)) ratio, as calculated from the (31)P spectra, decreased from 0.94 +/- 0.01 at rest to 0.73 +/- 0.04. These results are consistent with the values reported in the literature and show that this ergometer, which is easy to use, is suitable for in vivo spectroscopy research when metabolic steady-state conditions are required.

Ipsilateral involvement of primary motor cortex during motor imagery.

To investigate whether motor imagery involves ipsilateral cortical regions, we studied haemodynamic changes in portions of the motor cortex of 14 right-handed volunteers during actual motor performance (MP) and kinesthetic motor imagery (MI) of simple sequences of unilateral left or right finger movements, using functional magnetic resonance imaging (fMRI). Increases in mean normalized fMRI signal intensities over values obtained during the control (visual imagery) task were found during both MP and MI in the posterior part of the precentral gyrus and supplementary motor area, both on the contralateral and ipsilateral hemispheres. In the left lateral premotor cortex, fMRI signals were increased during imagery of either left or right finger movements. Ipsilateral cortical clusters displaying fMRI signal changes during both MP and MI were identified by correlation analyses in 10 out of 14 subjects; their extent was larger in the left hemisphere. A larger cortical population involved during both contralateral MP and MI was found in all subjects. The overall spatial extent of both the contralateral and the ipsilateral MP + MI clusters was approximately 90% of the whole cortical volume activated during MP. These results suggest that overlapping neural networks in motor and premotor cortex of the contralateral and ipsilateral hemispheres are involved during imagery and execution of simple motor tasks.

Temporal and intensity coding of pain in human cortex.

Temporal and intensity coding of pain in human cortex. J. Neurophysiol. 80:3312-3320, 1998. We used a high-resolution functional magnetic resonance imaging (fMRI) technique in healthy right-handed volunteers to demonstrate cortical areas displaying changes of activity significantly related to the time profile of the perceived intensity of experimental somatic pain over the course of several minutes. Twenty-four subjects (ascorbic acid group) received a subcutaneous injection of a dilute ascorbic acid solution into the dorsum of one foot, inducing prolonged burning pain (peak pain intensity on a 0-100 scale: 48 +/- 3, mean +/- SE; duration: 11.9 +/- 0.8 min). fMRI data sets were continuously acquired for approximately 20 min, beginning 5 min before and lasting 15 min after the onset of stimulation, from two sagittal planes on the medial hemispheric wall contralateral to the stimulated site, including the cingulate cortex and the putative foot representation area of the primary somatosensory cortex (SI). Neural clusters whose fMRI signal time courses were positively or negatively correlated (P < 0.0005) with the individual pain intensity curve were identified by cross-correlation statistics in all 24 volunteers. The spatial extent of the identified clusters was linearly related (P < 0.0001) to peak pain intensity. Regional analyses showed that positively correlated clusters were present in the majority of subjects in SI, cingulate, motor, and premotor cortex. Negative correlations were found predominantly in medial parietal, perigenual cingulate, and medial prefrontal regions. To test whether these neural changes were due to aspecific arousal or emotional reactions, related either to anticipation or presence of pain, fMRI experiments were performed with the same protocol in two additional groups of volunteers, subjected either to subcutaneous saline injection (saline: n = 16), inducing mild short-lasting pain (peak pain intensity 23 +/- 4; duration 2.8 +/- 0.6 min) or to nonnoxious mechanical stimulation of the skin (controls: n = 16) at the same body site. Subjects did not know in advance which stimulus would occur. The spatial extent of neural clusters whose signal time courses were positively or negatively correlated with the mean pain intensity curve of subjects injected with ascorbic acid was significantly larger (P < 0.001) in the ascorbic acid group than both saline and controls, suggesting that the observed responses were specifically related to pain intensity and duration. These findings reveal distributed cortical systems, including parietal areas as well as cingulate and frontal regions, involved in dynamic encoding of pain intensity over time, a process of great biological and clinical relevance.

Primary motor and sensory cortex activation during motor performance and motor imagery: a functional magnetic resonance imaging study.

The intensity and spatial distribution of functional activation in the left precentral and postcentral gyri during actual motor performance (MP) and mental representation [motor imagery (MI)] of self-paced finger-to-thumb opposition movements of the dominant hand were investigated in fourteen right-handed volunteers by functional magnetic resonance imaging (fMRI) techniques. Significant increases in mean normalized fMRI signal intensities over values obtained during the control (visual imagery) tasks were found in a region including the anterior bank and crown of the central sulcus, the presumed site of the primary motor cortex, during both MP (mean percentage increase, 2.1%) and MI (0.8%). In the anterior portion of the precentral gyrus and the postcentral gyrus, mean functional activity levels were also increased during both conditions (MP, 1.7 and 1.2%; MI, 0.6 and 0.4%, respectively). To locate activated foci during MI, MP, or both conditions, the time course of the signal intensities of pixels lying in the precentral or postcentral gyrus was plotted against single-step or double-step waveforms, where the steps of the waveform corresponded to different tasks. Pixels significantly (r > 0.7) activated during both MP and MI were identified in each region in the majority of subjects; percentage increases in signal intensity during MI were on average 30% as great as increases during MP. The pixels activated during both MP and MI appear to represent a large fraction of the whole population activated during MP. These results support the hypothesis that MI and MP involve overlapping neural networks in perirolandic cortical areas.

[Functional mapping of the motor and primary sensorial cortex using magnetic resonance techniques. I. Preliminary data]. / Mappa funzionale della corteccia motoria e sensoriale primaria con tecniche di Risonanza Magnetica. I. Dati preliminari.

Functional Magnetic Resonance Imaging (fMRI) techniques sensitive to the local changes in blood flow, volume and oxygenation accompanying neuronal activation are powerful tools to investigate the human brain function. Experiments were performed on 10 right-handed healthy volunteers (age range: 20-39 years), using a 1.5 T whole-body MRI system. Two oblique contiguous planes were investigated along the central sulcus of the left hemisphere. Functional images were acquired using a Gradient Echo sequence while the subjects repetitively performed sequential finger to thumb opposition movements of the right hand or mental imagery of a visual scene. Twelve images for each task were obtained over a 6-min experimental period; they were then analyzed with the software provided by the manufacturer. In all the subjects small areas were activated in both the precentral and postcentral gyrus, mean percentage signal increases during finger movement being 10.7% and 3.8%, respectively. These values are fairly higher than literature ones. However several factors, such as voxel volume, are involved in determining the measured signal increase during activation. Moreover, in most cases the software procedures provided with the MR equipment to analyze the functional images imply subjective choices. It is thus necessary to implement new software packages for the analysis of fMRI images to apply more appropriate statistical procedures and to obtain more homogeneous and objective final information.

[Functional mapping of the motor and primary sensorial cortex using magnetic resonance techniques. II. Image analysis techniques]. / Mappa funzionale della corteccia motoria e sensoriale primaria con tecniche di Risonanza Magnetica. II. Tecniche di analisi dell'immagine.

Functional Magnetic Resonance Imaging (fMRI) techniques to investigate brain function are now available on clinical MR systems. However, the software packages provided with the MR equipment to analyze the functional images are often inadequate. In the present study, two registration algorithms for correcting motion artifacts and three procedures of statistical analysis (t-test, correlation analysis, Kolmogorov-Smirnov test) were compared using programs implemented on a graphic workstation. For both registration algorithms, transformation parameters for in plane translations and rotation of images were significantly affected by the task, being higher during sequential finger movements than during the control (visual imagery) condition. Regions of interest were identified on the anatomical images and their boundaries automatically projected on functional images. The number of significantly activated pixels in the pre- and postcentral areas was not significantly different after the registration with the two procedures. The percentage of pixels of the pre- and postcentral areas whose signal intensity was significantly different between the two tasks decreased with respect to the adopted threshold of significance as a power function. For an area identified outside the brain, the same relation was linear: no activated pixel was found for p < 0.001. The application of the t-test or of the correlation analysis yielded similar results. The analysis of the profile of mean normalized signal intensity showed higher increases in signal intensities during the motor task in the precentral gyrus than in the postcentral gyrus. This appears to be due to a greater number of activated pixels during motor performance. The application of registration procedures, the identification of the regions of interest on the basis of the anatomical images and appropriate statistical analyses allow a more detailed characterization of task-related activation.