A simple breathing circuit to maintain isocapnia during measurements of the hypoxic ventilatory response

We report the development and testing of a simple breathing circuit that maintains isocapnia in human subjects during hypoxic hyperpnea. In addition, the circuit permits rapid switching between two gas mixtures with different partial pressures of oxygen. Eleven volunteers breathed repeated cycles of exposure to air (2 min of 21% O(2), balance N(2)) and hypoxia (2 min of 8.3+/-0.1% O(2), balance N(2)). Hypoxia induced significant increases in minute ventilation, breathing frequency and tidal volume (P < 0.05) that were consistent over repeated cycles of hypoxia (P > 0.1, one-way ANOVA). The system successfully maintained isocapnia in all subjects, with an average change in end-tidal CO(2) of only -0.2 mmHg during hyperventilation in hypoxia (range 0.4 to -0.8 mmHg). This system may be suitable for repeated tests of the hypoxic ventilatory response (HVR) and may prove useful for exploring intra- and inter-individual variability of HVR in humans.     

A Dynamic Morphometric Model of the Normal Lung for Studying Expiratory Flow Limitation in Mechanical Ventilation

A nonlinear dynamic morphometric model of breathing mechanics during artificial ventilation is described. On the basis of the Weibel symmetrical representation of the tracheobronchial tree, the model accurately accounts for the geometrical and mechanical characteristics of the conductive zone and packs the respiratory zone into a viscoelastic Voigt body. The model also accounts for the main mechanisms limiting expiratory flow (wave speed limitation and viscous flow limitation), in order to reproduce satisfactorily, under dynamic conditions, the expiratory flow limitation phenomenon occurring in normal subjects when the difference between alveolar pressure and tracheal pressure (driving pressure) is high. Several expirations characterized by different levels of driving pressure are simulated and expiratory flow limitation is detected by plotting the isovolume pressure–flow curves. The model is used to study the time course of resistance and total cross-sectional area as well as the ratio of fluid velocity to wave speed (speed index), in conductive airway generations. The results highlight that the coupling between dissipative pressure losses and airway compliance leads to onset of expiratory flow limitation in normal lungs when driving pressure is increased significantly by applying a subatmospheric pressure to the outlet of the ventilator expiratory channel; wave speed limitation becomes predominant at still higher driving pressures.     

Development of multielectrode impedance plethysmography

The feasibility of simultaneous independent measurements of impedance variations in the right and left apex and base of the lungs using the technique of multielectrode impedance plethysmography (MIPG) was investigated. To obtain independent impedance measurements in each region, high impedance sensitivity areas must be localised by weighting the impedance sensitivity distribution. 12 planar coaxial-type electrodes were attached on the right and left upper, middle and lower sites of the anterior and posterior chest walls. Currents of identical absolute values and differing polarities were simultaneously applied to neightbouring electrodes and voltage measurements were carried out sagittally at the right and left upper and lower sites of the chest walls. The effect of weighting the impedance sensitivity distribution was verified through experimental studies on mongrel dogs. The methods utilised for the induction of regional physiological conductivity changes were selective ventilation and selective indicator infusion into the pulmonary vasculature. The detected impedance variation showed reasonably indenpendent responses which were consistent with our expectations from the results of the computer simulation.     

Patient-ventilator interaction: A general model for nonpassive mechanical ventilation

A general mathematical model for the dynamic behaviour of asingle-compartment respiratory system in response to an arbitraryapplied inspiratory airway pressure and arbitrary respiratorymuscle activity is investigated. The model is used to computeexplicit expressions for ventilation and pressure variablesof clinical interest for clinician-selected and impedance-determinedinputs. The outcome variables include tidal volume, end-expiratorypressure, minute ventilation, mean alveolar pressure, averagepleural pressure, as well as the work performed by the ventilatorand the respiratory muscles. It is also demonstrated that undersuitable conditions, there is a flow reversal that can occurduring inspiration.     

Clinical efficacy of reduction in house-dust mite exposure in specially designed, mechanically ventilated "healthy" homes

In temperate climates, energy-conserving measures may increase indoor humidity, enhancing house-dust mite (HDM) growth. Movement of families to "healthy" homes with mechanical ventilation systems reduced HDM exposure. The effect on asthma control of moving to the "healthy" homes was studied in 14 asthmatic patients allergic to HDM. Base-line evaluations of lung function, asthma symptoms, and medication requirements were made before moving and again after 5 and 15 months' residence. A control group of 11 mite-sensitive asthmatic patients who did not move were examined contemporaneously with the study group at base line and at the first follow-up investigation. After 5 months, the residents of the "healthy" homes improved in forced expiratory volume in 1 s (FEV₁), medicine score, and serum IgE. These changes were significantly different from control group measurements. After 15 months, statistically significant improvements from base line were found in FEV₁, average daily peak expiratory flow values, medicine score, symptom score, and serum IgE. Insignificant trends toward improvement were seen in provocation concentration of histamine and blood eosinophils. A significant relation was found between reduction in medicine score and fall in HDM exposure. The present study shows that a specific HDM-avoidance procedure can result in an overall, clinical improvement in HDM-sensitive asthmatic patients.     

Branchial ventilation rates of cataleptic goldfish (Carassius auratus) and the effects of light and electric shock

Inverted, cataleptic goldfish showed an increase in ventilation rate over a 20 min period. The time-dependent increase in respiratory movement was initially suppressed when a 10 sec light or a 0.1 sec shock was presented at 1.5 min intervals. Contingent presentation of the light and shock, in a classical conditioning paradigm, resulted in reduced ventilation rates during the session. Physiological changes accompanying conditioning may thus alter the cataleptic state.     

利用磁共振成像实现肿瘤热疗中实时无创测温的方法

肿瘤热疗特别是高强度聚焦超声（HIFU）技术，已成为一种重要且非常具有前景的肿瘤治疗方法，温度是直接决定热疗效果的重要参数，实时准确的进行深部无创测温是目前制约肿瘤热疗进一步发展的一项关键技术。近来利用磁共振成像（MRI）进行无创测温，正受到越来越多的研究者的重视。本文简述了MRI无创测温的三种方法，比较了不同测温原理的特点和适用条件，报道了已有研究发展状况，并介绍了尚待解决的问题。     

The acute effects of insulin on the cardiorespiratory responses to hypoxia in streptozotocin‐induced diabetic rats

Aims: We examined whether or not streptozotocin (STZ)‐induced diabetic rats, which have a lower heart rate (HR, beats min^?1) than control rats, could maintain hypoxic ventilatory response. Methods: Twenty‐six Wistar rats, which had been injected with STZ (60 mg kg^?1, EXP) or vehicle (0.1 m citrate buffer, CONT) intraperitoneally at 9 weeks of age, had their cardiorespiratory responses to normoxia and 12%O₂ examined after 5 weeks. Results: Compared with CONT rats, EXP rats had a higher blood glucose [24 ± 3 vs. 5 ± 1 (mean ± SD) mmol L^?1], a lower body weight (320 ± 23 vs. 432 ± 24 g), lower HR (303 ± 49 vs. 380 ± 44 in normoxia, and 343 ± 56 vs. 443 ± 60 in hypoxia) and a lower mean arterial blood pressure (MAP) (89 ± 6 vs. 102 ± 10 mmHg in hypoxia). In contrast, both groups had similar values in ventilation (), –metabolic rate (MR) ratio and arterial blood gases (ABGs). In EXP rats, with an acute insulin supplement (i.v., 0.75 U h^?1 for 1.5–2 h), not only blood glucose, but also HR, and MAP were normalized as those obtained in CONT rats, and in hypoxia further increased without affecting –MR ratio and ABGs. Such acute cardiorespiratory stimulating effects of insulin could not be obtained in non‐diabetic rats (n = 7, 355 ± 24 g), in which euglycaemia (mean 6.4 mmol L^?1) was maintained during the measurements. Conclusions: Our results suggest that, in STZ‐induced diabetic rats: (1) ventilation is hardly suppressed by hyperglycaemia, (2) cardiorespiratory responses can be acutely stimulated by short insulin injection, and (3) the effects, including those through acute blood glucose normalization, are possibly specific for the diabetic impairments.     

Role of potassium in the regulation of systemic physiological function during exercise

In exercise, potassium (K⁺) is released from contracting muscle predominately through K⁺ channels associated with the repolarization phase of the action potential. Increases in extracellular K⁺ are directly related to increases in metabolic rate and may reach concentrations as high as 8–9 mm in the arterial blood during exhaustive work. Exercise-induced hyperkalaemia has been implicated in several physiological processes, in particular skeletal muscle fatigue, hyperaemia, pressor reflex, arterial chemosensitivity and myocardial stability. There is no direct evidence to show that hyperkalaemia causes muscle fatigue, although raised extracellular [K⁺] may contribute to fatigue during prolonged tetani by depressing the propagation of the action potential down the t-tubule system, thus impairing the release of Ca²⁺ from the sarcoplasmic reticulum. The vasodilating properties of K⁺ may transiently contribute to the early phase of exercise hyperaemia and interact synergistically with other vasoactive substances to cause relaxation by hyperpolarizing K⁺ channels in vascular smooth muscle. Hyperkalaemia has been implicated in the regulation of arterial blood pressure through activation of the muscle afferent reflex where potassium-depolarized C fibres may contribute to a reflex increase in arterial blood pressure. K⁺ can also increase ventilation and the sensitivity of the ventilatory response to hypoxia through direct excitation of the arterial chemoreceptors. Finally, to maintain myocardial electrical stability in exercise, there is a beneficial interaction between raised K⁺ and catecholamines on the heart, so that when they combine, each offsets the other's deleterious effects.     

Near‐infrared spectroscopy determined brain and muscle oxygenation during exercise with normal and resistive breathing

To elevate effects of carbon dioxide (CO₂) retention by way of an increased respiratory load during submaximal exercise (150 W), the concentration changes of oxy‐ (ΔHbO₂) and deoxy‐haemoglobin (ΔHb) of active muscles and the brain were determined by near‐infrared spectroscopy (NIRS) in eight healthy males. During exercise, pulmonary ventilation increased to 33 (28–40) L min^–1 (median with range) with no effect of a moderate breathing resistance (reduction of the pneumotach diameter from 30 to 14 and 10 mm). The end‐tidal CO₂ pressure (P_ETCO ₂) increased from 45 (42–48) to 48 (46–58) mmHg with a reduction of only 1% in the arterial haemoglobin O₂ saturation (S_aO ₂). During control exercise (normal breathing resistance), muscle and brain ΔHbO₂ were not different from the resting levels, and only the leg muscle ΔHb increased (4 (–2–10) μM , P < 0.05). Moderate resistive breathing increased ΔHbO₂ of the intercostal and vastus lateralis muscles to 6 ± (–5–14) and 1 (–7–9) μM (P < 0.05), respectively, while muscle ΔHb was not affected. Cerebral ΔHbO₂ and ΔHb became elevated to 6 (1–15) and 1 (–1–6) μM by resistive breathing (P < 0.05). Resistive breathing caused an increased concentration of oxygenated haemoglobin in active muscles and in the brain. The results indicate that CO₂ influences blood flow to active skeletal muscle although its effect appears to be smaller than for the brain.