Search results
Found 12048 matches for
Minimum fresh gas flow requirements of anaesthetic breathing systems during spontaneous ventilation: a graphical approach.
A general solution is presented to the problem of finding the minimum fresh gas flow requirements, during spontaneous ventilation, of anaesthetic breathing systems in the Mapleson classification. The solution is applicable to any pattern of breathing, dead space volume and tidal volume. The method is graphical and its use and understanding require no mathematical skills. However, if an analytical form of the respiratory waveform is known, the method is easily extended by use of calculus to obtain a precise analytical solution.
Active transport in the alveolar epithelium of the adult lung: vestigial or vital?
Active secretion by mammalian fetal pulmonary alveolar epithelium is well recognized, as is the role of the adult epithelium in the secretion of surfactant. Recent studies have demonstrated active absorption by adult epithelium involving two sodium-dependent pathways. This finding has focused attention on how poorly we understand both the disposition of alveolar liquid and the physiological role of surfactant. In this paper we review the evidence that the adult mammalian alveolar epithelium absorbs solutes by active transport, and we assess the physiological importance of the resulting liquid movements.
Femoral arteriovenous extracorporeal carbon dioxide elimination using low blood flow.
BACKGROUND AND METHODS: Conventional extracorporeal CO2 removal systems require blood flow rates of 1 to 2.5 L/min in the extracorporeal circuit. We hypothesized that standard hemofiltration equipment can be combined with a high-performance extracorporeal lung to achieve high rates of CO2 removal at lower blood flow rates. To test this hypothesis, we performed experiments on nine sheep to examine the extent to which CO2 elimination can be achieved at blood flow rates less than 600 mL/min using a 5-m2 hollow fiber membrane lung with countercurrent gas flow, combined with a hemofiltration blood pump, and connected to femoral arterial and venous hemodialysis catheters. RESULTS: CO2 eliminations of 130 to 180 mL/min at standard temperature and pressure were achieved with blood flow rates in the range 470 to 600 mL/min. With a pumpless artery-to-vein shunt, up to 90 mL/min of CO2 at standard temperature and pressure was eliminated. However, in this mode, the resistance of the access catheters and tubing was the main factor limiting CO2 elimination. CONCLUSIONS: Standard hemofiltration equipment may be combined with a hollow fiber membrane lung to remove the equivalent of a high proportion of the basal metabolic CO2 production of an adult human at low blood flow rates. Use of this technology would bring extracorporeal CO2 removal within the budget and capability of more ICUs.
Rebreathing during spontaneous and controlled ventilation with T-piece breathing systems: a general solution.
A general solution is presented to the problem of finding the degree of rebreathing generated by T-piece breathing systems. The solution is applicable to any ventilatory waveform, dead space volume and tidal volume and is identical for spontaneous and controlled ventilation for any given ventilatory waveform. The method is graphical and its use and understanding require no mathematical skills. However, if an analytical form of the ventilatory waveform is known, the method is easily extended by use of calculus to obtain a precise analytical solution.
Oxygen and CO2 transfer of a polypropylene dimpled membrane lung with variable secondary flows.
Gas transfer performance is presented for one form of the Oxford membrane lung in which vortex mixing is induced in blood flow across a dimpled polypropylene membrane. Dimensional analysis has been used to define the parameters characterizing mass transfer in the device, and of three fluid mechanical parameters: Reynolds number based on peak pulsation velocity, Strouhal number, and ratio of mean to oscillatory flows, only the first has been found to affect mass transfer significantly in the ranges studied. For oxygenation, rated flows in excess of 5 l min-1 m-2 are measured. A new definition is presented for a rated flow for extracorporeal CO2 removal and values in excess of 1.2 l min-1 m-2 are obtained.
Anticoagulation by ancrod for carbon dioxide removal by extracorporeal membrane lung in the dog.
Ten dogs were subjected to defibrinogenation with an intravenous perfusion of ancrod (1 unit/kg) (Arvin, Knoll AG, Ludwigshafen, Federal Republic of Germany) over a 2 1/2 hour period. Six of them were subjected to extracorporeal elimination of carbon dioxide with a polypropylene membrane lung by means of veno-venous bypass. The remaining four dogs did not undergo extracorporeal circulation and served as control subjects. In both groups, ancrod administration itself resulted in a marked drop in alpha 2-antiplasmin (33% and 67%, respectively, of the baseline values) and in slight but significant decreases in factor II and plasminogen activities of 25% and 20%, respectively (p less than 0.05), in the group subjected to carbon dioxide removal. There were no significant changes in platelet number or factor V and antithrombin III activities. During the 6-hour bypass period, platelet count and antithrombin III and factor II and V levels decreased significantly. No bleeding was observed. Histologic examination of lung biopsy tissue showed no pathologic features. Analysis of the membrane of the artificial lungs revealed no fibrin deposits. In the control group, except for a drop in alpha 2-antiplasmin levels (54%), no significant changes in hemostatic parameters occurred during the corresponding 6 hours. We conclude that, despite the drop in coagulation factors and in alpha 2-antiplasmin activity during bypass, ancrod can be considered as a valuable alternative anticoagulant for extracorporeal carbon dioxide removal.
Time course of hypoxic pulmonary vasoconstriction: a rabbit model of regional hypoxia.
There is disagreement in the literature about the time required for hypoxic constriction of pulmonary vessels to reach its full intensity. Some studies suggest that only minutes are required, others that several hours are needed. We examined the time course over 6 h of changes in pulmonary shunt (as a fraction of cardiac output) following induction of unilateral hypoxia by collapse or liquid filling of the left lung in 47 anesthetized rabbits. The time course was examined at four degrees of lung inflation: during collapse and at airway pressures of 0.3 kPa, 0.6 kPa, and 0.9 kPa. The respective volumes (mean +/- SD) of the liquid-filled lung were estimated to be 6.4 +/- 1.0, 12.8 +/- 2.5, and 15.8 +/- 1.6 ml/kg body weight (BW). During sustained hypoxia (the period from 150 to 360 min after inducing hypoxia), shunt declined at a slow linear rate of 2.37 x 10(-4)/min, which was independent of lung inflation (p = 0.65 analysis of variance [ANOVA]) and significantly different from zero (p < 0.001). The stability of cardiac output in this animal model, as measured sequentially by thermodilution, was confirmed in a further 20 animals. The experiments provide evidence for a slow intensification of blood-flow diversion at a rate that does not depend upon the degree of lung inflation. Whether this change is a feature of hypoxic constriction itself, or some modulation of it, remains unclear.
Effect of potassium on ventilation in the rhesus monkey.
Increasing the concentration of arterial plasma K+ to 6-8 mM increased ventilation in two sedated analgesic-treated rhesus monkeys who had their end-tidal CO2 held constant during euoxia (arterial oxygen pressure, Pa,O2, ca 100 Torr) and hypoxia (Pa,O2, ca 40 Torr). During euoxia and hypoxia, hyperkalaemia increased ventilation up to 40 and 250%, respectively. This effect was reduced in euoxia and virtually abolished in hypoxia following an abrupt switch to 100% oxygen. Thus the ventilatory response of this primate to hyperkalaemia is at least as sensitive as that of the cat and if hypoxia is added the two stimuli generate a powerful drive to breathing.
Physiological profile during venovenous perfusion in dogs using a polypropylene membrane lung with secondary flows.
Venovenous perfusion has been conducted in 12 healthy dogs to examine carbon dioxide (CO2) transfer and haemocompatibility over 9 h during total extracorporeal CO2 removal using a microporous polypropylene membrane lung with secondary flows in the blood channel. The anaesthetized animals were maintained normocapnic by including CO2 in the inspired gases. The CO2 removal was achieved using 0.631 m2 of active membrane, at a pulsatile Reynolds number of 50, and a CO2 extraction from blood of 17.8 ml (STP) dl-1. Gas exchange remained constant during the perfusions. Several aspects of our results suggest that the haemocompatibility of a system of the kind used here is at least as favourable as that of a steady flow device using a continuous silicone rubber membrane of equivalent gas transfer capability.