Journal of applied physiology
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A dual direct/indirect room-sized calorimeter is used at the Beltsville Human Nutrition Research Center to measure heat emission and energy expenditure in humans. Because the response times of a gradient layer direct calorimeter and an indirect calorimeter are not equivalent, the respective rate of heat emission and energy expenditure cannot be directly compared. A system of equations has been developed and tested that can correct the respective outputs of the direct gradient layer calorimeter and indirect calorimeter for delays due to the response times of the measurement systems. ⋯ However, heat emission measured during sleep was significantly greater (14%) than energy expenditure, suggesting a change in the energy stored as heat in the body. This difference was reversed during the day. These results illustrate how the simultaneous measurement of heat emission and energy expenditure provides insights into heat regulation.
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Clinically, a noninvasive measure of diaphragm function is needed. The purpose of this study is to determine whether ultrasonography can be used to 1) quantify diaphragm function and 2) identify fatigue in a piglet model. Five piglets were anesthetized with pentobarbital sodium and halothane and studied during the following conditions: 1) baseline (spontaneous breathing); 2) baseline + CO2 [inhaled CO2 to increase arterial PCO2 to 50-60 Torr (6.6-8 kPa)]; 3) fatigue + CO2 (fatigue induced with 30 min of phrenic nerve pacing); and 4) recovery + CO2 (recovery after 1 h of mechanical ventilation). ⋯ Mean inspiratory velocity also decreased from 13 +/- 2 to 8 +/- 1 cm/s during these conditions. All variables returned to baseline during recovery + CO2. Ultrasonography can be used to quantify diaphragm function and identify piglet diaphragm fatigue.
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Clinical Trial
Hemodynamics, cerebral circulation, and oxygen saturation in Cheyne-Stokes respiration.
Because cardiovascular disorders and stroke may induce Cheyne-Stokes respiration, our purpose was to study the interaction among cerebral activity, cerebral circulation, blood pressure, and blood gases during Cheyne-Stokes respiration. Ten patients with heart failure or a previous stroke were investigated during Cheyne-Stokes respiration with recordings of daytime polysomnography, cerebral blood flow velocity, intra-arterial blood pressure, and intra-arterial oxygen saturation with and without oxygen administration. There were simultaneous changes in wakefulness, cerebral blood flow velocity, and respiration with accompanying changes in blood pressure and heart rate approximately 10 s later. ⋯ Oxygen desaturations were more severe and occurred earlier according to intra-arterial measurements than with finger oximetry. It is not possible to explain Cheyne-Stokes respiration by alterations in blood gases and circulatory time alone. Cheyne-Stokes respiration may be characterized as a state of phase-linked cyclic changes in cerebral, respiratory, and cardiovascular functions probably generated by variations in central nervous activity.
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The possible role of pulmonary C fibers in the hypoxia-induced concomitant increases in end-expiratory lung volume (EELV) and in the activity of the diaphragm at the end of expiration (DE) were evaluated by measuring the effects of hypoxia (10% O2) on ventilation, EELV, and DE in eight chloralose-urethan anesthetized rats. Recordings were made before and after blocking vagal C fibers and after bilateral vagotomy. C-fiber conduction was blocked by applying capsaicin perineurally to the cervical vagi. ⋯ Vagotomy performed during hypoxia resulted in large decreases in DE and EELV. Hypoxia increased DE and EELV in vagotomized rats but less than in intact rats. We conclude that the hypoxia-induced increases in EELV and diaphragmatic activity are probably not mediated by vagal C fibers and that vagal afferents are involved but not fully responsible for this phenomenon.
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A simple mathematical model of intracranial pressure (ICP) dynamics oriented to clinical practice is presented. It includes the hemodynamics of the arterial-arteriolar cerebrovascular bed, cerebrospinal fluid (CSF) production and reabsorption processes, the nonlinear pressure-volume relationship of the craniospinal compartment, and a Starling resistor mechanism for the cerebral veins. Moreover, arterioles are controlled by cerebral autoregulation mechanisms, which are simulated by means of a time constant and a sigmoidal static characteristic. ⋯ If physiological compensatory mechanisms (CSF circulation and intracranial storage capacity) are efficient, acute hypotension has only negligible effects on ICP and cerebral blood flow (CBF). If these compensatory mechanisms are poor, even modest hypotension may induce a large transient increase in ICP and a significant transient reduction in CBF, with risks of secondary brain damage. 3) The ICP response to a bolus injection (PVI test) is sharply affected, via cerebral blood volume changes, by cerebral hemodynamics and autoregulation. We suggest that PVI tests may be used to extract information not only on intracranial compliance and CSF circulation, but also on the status of mechanisms controlling CBF.