Search PubMed⌕ Search

Biomedical subjects

C S Poon

Publications and source records attributed to C S Poon.

At least 73 records · Page 4Linked to original sources
PubMed1987

Influence of inspiratory assistance on ventilatory control during moderate exercise.

In five healthy subjects, we studied the effects of controlled mechanical unloading of the respiratory system on ventilatory control during moderate exercise, utilizing a modified positive-pressure ventilator (IEEE Trans. Biomed. Eng. BME-33: 361-365, 1986). We were especially interested in whether isocapnia was maintained when a portion of the normal ventilatory response to constant-load cycling was subserved by the ventilator. The mechanical unloading was achieved by "assisting" airflow throughout inspiration in a constant proportion to instantaneous flow. Two modest degrees of assistance (A1 = 1.5 and A2 = 3.0 cmH2O X l-1 X s) were imposed. The assistance caused minute ventilation (VE) to increase immediately (inspiratory time shortening and tidal volume rising) and end-tidal PCO2 (PETCO2) to fall. Some 10-15 s later, inspiratory occlusion pressure (P100) decreased, and in the new steady-state VE and PETCO2 were virtually restored to their control exercise levels. The modest residual hyperventilation [delta PETCO2 = -0.9 Torr (A1) and -1.6 Torr (A2)], which was not significant statistically, contrasted markedly with the much larger increase predicted for VE had there been no compensatory reduction in ventilatory drive (as evidenced by the fall in P100). Consistent with earlier studies utilizing resistive loading (J. Appl. Physiol. 35: 361-366, 1973 and Acta Physiol. Scand. 120: 557-565, 1984), these observations suggest that ventilatory drive during moderate exercise is controlled to compensate for modest changes in respiratory-mechanical load, so that VE is preserved at a level appropriate to metabolic rate or nearly so.

Adult↗
PubMed1987

Ventilatory control in hypercapnia and exercise: optimization hypothesis.

A model of the respiratory control system incorporating both chemical and respiratory neuromechanical feedbacks is proposed to describe the steady-state ventilatory responses to CO2 inhalation and exercise. It is postulated that ventilatory output (VE) is set by the respiratory center to minimize a net operating cost representing the conflicting challenges of arterial chemical imbalance and respiratory-mechanical discomfort (intolerance of effort), given, respectively, by a quadratic function of arterial PCO2 and a logarithmic function of VE. In addition, the system is assumed to be mechanically limited at maximum VE (Vmax). The predicted responses in VE during moderate hypercapnia, exercise, and ventilatory loading closely mimic those normally observed, even though no separate signal unique to exercise is assumed. As a quantitative validation, the model yielded good fits to ventilatory response data obtained in eight healthy subjects during eucapnic and hypercapnic exercise; the predicted Vmax averaged approximately 77% of the maximum voluntary ventilation in all subjects. The results demonstrate the plausibility of the proposed optimization mechanism and suggest an important role for respiratory-mechanical factors in the control of VE.

Carbon Dioxide↗
PubMed1987

Effects of expiratory resistive load on respiratory motor output in conscious humans.

We examined, in five conscious human subjects, the steady-state effects of expiratory resistive loading (ERL; R = 8 cmH2O.l-1.s) on the time course of inspiratory and postinspiratory muscle activities (IA and PIA, respectively) and ventilatory pattern during quiet breathing. Driving pressure (DP) was calculated by means of a respiratory neuromechanical model (J. Appl. Physiol. 51: 963-989, 1981) that permitted the derivation, from tidal volume and flow, of the occlusion pressure equivalent (at functional residual capacity) of respiratory neural output throughout the breath. ERL caused a prolongation of both neural inspiratory duration (12.2 +/- 6.9% SD) and expiratory duration (25.0 +/- 10.1%) and an increase in the amplitude of DP (16.5 +/- 10.2%) without any changes in the waveshape of IA and in end-expiratory level. The relative time course of PIA was not altered by ERL. Minute ventilation was depressed (-6.75 +/- 2.88%) during ERL with little change in alveolar PCO2. The results indicate that pulmonary gas exchange may be improved during ERL through increased tidal volume as well as delayed expiratory lung emptying secondary to sustained PIA.

Adult↗
PubMed1986

Estimation of response curves in closed-loop physiological control.

Several recent reports have addressed the problem of estimating the response slope from repeated measurements of paired data when both stimulus and response variables are subject to biological variability. These earlier approaches suffer from several drawbacks: useful information about the relationships between the error components in a closed-loop system is not fully utilized; the response intercept cannot be directly estimated; and the normalization procedure required in some methods may fail under certain circumstances. This paper proposes a new, general method of simultaneously estimating the response slope and intercept from corrupted stimulus-response data when the errors in both variables are specifically related by the system structure. A direct extension of the least-squares approach, this method [directed least squares (DLS)] reduces to ordinary least-squares methods when either of the measured variables is error free and to the reduced-major-axis (RMA) method of Kermack and Haldane (Biometrics 37: 30-41, 1950) when the magnitudes of the normalized errors are equal. The DLS estimators are scale invariant, statistically unbiased and always assume the minimum variance. With simple modifications, the method is also applicable to paired data. If, however, the relation between error components is uncertain, then the RMA method is optimal, i.e., having the least possible asymptotic bias and variance. These results are illustrated by using various types of closed-loop respiratory response data.

Animals↗
PubMed1985

Characteristics of VA/Q distributions recovered from inert gas elimination data.

The resolving powers of the enforced smoothing and log-normal parametric estimation techniques in recovering ventilation/perfusion ratio (VA/Q) distributions were evaluated using noisy inert gas elimination data simulated from hypothetical distribution functions representing various degrees of heterogeneity. The resolving powers were assessed in terms of the statistical recoverabilities of the shunt, dead space, modality, and modal moments characterizing the perfusion distribution. For all distributions tested, both modal mean and shunt were estimated by either technique with sufficient accuracies. Modal dispersions (sigma) were consistently overestimated by up to 0.15 decade for narrow distributions, but the mean errors became negligible for sigma greater than 0.2 decade. As compared with the shunt, the dead space estimates were more variable and biased, probably due to their indirect estimation from the perfusion distribution, which was imperfectly recovered. Both broad unimodal and widely separated bimodal or trimodal distributions (sigma greater than 0.6 decade) were recovered as bimodal distributions of similar forms, so that detection of modality was difficult. The recoveries by both techniques were comparable in most cases studied, except that parametric estimation generally tended to be more sensitive to measurement errors and was computationally less efficient. These results provide a useful basis for the interpretation of VA/Q distributions obtained from empirical inert gas data.

Humans↗
PubMed1985

Control of exercise hyperpnea during hypercapnia in humans.

Previous studies have yielded conflicting results on the ventilatory response to CO2 during muscular exercise. To obviate possible experimental errors contributing to such variability, we have examined the CO2-exercise interaction in terms of the ventilatory response to exercise under conditions of controlled hypercapnia. Eight healthy male volunteers underwent a sequence of 5-min incremental treadmill exercise runs from rest up to a maximum CO2 output (VCO2) of approximately 1.5 l . min-1 in four successive steps. The arterial PCO2 (PaCO2) at rest was stabilized at the control level or up to 14 Torr above control by adding 0-6% CO2 to the inspired air. Arterial isocapnia (SD = 1.2 Torr) throughout each exercise run was maintained by continual adjustment of the inspired PCO2. At all PaCO2 levels the response in total ventilation (VE) was linearly related to exercise VCO2. Hypercapnia resulted in corresponding increases in both the slope (S) and zero intercept (V0) of the VE-VCO2 curve; these being directly proportional to the rise in PaCO2 (means +/- SE: delta S/ delta PaCO2, 2.73 +/- 0.28 Torr-1; delta V0/ delta PaCO2, 1.67 +/- 0.18 l . min-1 . Torr-1). Thus the ventilatory response to concomitant hypercapnia and exercise was characterized by a synergistic (additive plus multiplicative) effect, suggesting a positive interaction between these stimuli. The increased exercise sensitivity in hypercapnia is qualitatively consistent with the hypothesis that VE is controlled to minimize the conflicting challenges due to chemical drive and the mechanical work of breathing (Poon, C. S. In: Modelling and Control of Breathing, New York: Elsevier, 1983, p. 189-196).

Adult↗
PubMed1982

Density-dependent airflow and ventilatory control during exercise.

The influence of respired gas density on ventilatory control during cycle-ergometer exercise was investigated in six healthy subjects. They underwent constant-load exercise for 10 min both at 50% and 90% of the anaerobic threshold, inhaling air for the first 5 min followed abruptly by 80% helium-20% oxygen (He-O2) for the remaining 5 min (and vice versa). The He-O2 breathing elicited no discernible effect on ventilation (VI) or mean alveolar PCO2 (PACO2) at rest or at the lower work rate. However, at the higher work rate, He-O2 breathing resulted in a clear and sustained hyperventilation in all subjects. A compensatory response to the hypocapnia, consequent to the helium-induced hyperventilation, was not evident even though all subjects demonstrated a normal ventilatory responsiveness to inhaled CO2 while in this condition. These observations suggest that turbulent airflow normally imposes a constraint on the magnitude of the hyperpnea of high-intensity exercise.

Carbon Dioxide↗
PubMed1981

Frequency dispersions of human skin dielectrics.

The electrical properties of many biological materials are known to exhibit frequency dispersions. In the human skin, the impedance measured at various frequencies closely describes a circular locus of the Cole-Cole type in the complex impedance plane. In this report, the formative mechanisms responsible for the anomalous circular-arc behavior of skin impedance were investigated, using data from impedance measurements taken after successive strippings of the skin. The data were analyzed with respect to changes in the parameters of the equivalent Cole-Cole model after each stripping. For an exponential resistivity profile (Tregear, 1966, Physical Functions of Skin; Yamamoto and Yamamoto, 1976, Med. Biol. Eng., 14:151--158), the profile of the dielectric constant was shown to be uniform across the epidermis. Based on these results, a structural model has been formulated in terms of the relaxation theory of Maxwell and Wagner for inhomogeneous dielectric materials. The impedance locus obtained from the model approximates a circular are with phase constant alpha = 0.82, which compares favorably with experimental data. At higher frequencies a constant-phase, frequency-dependent component having the same phase constant alpha is also demonstrated. It is suggested that an approximately rectangular distribution of the relaxation time over the epidermal dielectric sheath is adequate to account for the anomalous frequency characteristics of human skin impedance.

Electrophysiology↗
PubMed1981

Breath-to-breath monitoring of inspiratory occlusion pressures in humans.

A pneumatically drive occlusion device has been designed to implement brief (ca. 130 ms) inspiratory airway occlusion and to monitor the associated inspiratory occlusion pressure (P100) on a breath-to-breath basis in humans. The device, which has a low resistance when patent, provides no discernible modulation of ventilation during steady and nonsteady states of moderate intensity cycle-ergometer exercise. With this technique, it is possible to undertake investigations of dynamic as well as steady-state P100 responses to stimuli such as extensive, inhaled CO2, and hypoxia.

Airway Resistance↗