Der Anaesthesist
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Comparative Study
[Comparative study of the efficiency of bacterial filters in long-term mechanical ventilation].
Two commercially available bacterial filters to be used as part of the mechanical ventilation unit during anaesthesia were tested for hygienic criteria. Manufacturers claim that bacterial breathing filters have a filtration capacity of about 99.995%, so that there would be no need for thermal disinfection of tubing and ventilation circuits after each use. One filter is designed for a single use only, the other can be used up to 24 times after sterilisation. Both filters consist of hydrophobic glass fibres. ⋯ The use of bacterial filters during mechanical ventilation reduces the probability of bacterial contamination, but does not make sterilisation of the tubes and ventilation circuit unnecessary.
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Heated humidifiers (HH) as well as heat and moisture exchangers (HME) are commonly used in intubated patients as air-conditioning devices to raise the moisture content of the air, thus preventing mucosal damage and heat loss resulting from ventilation with dry inspired gases. In contrary to HME, HH are able to add heat and moisture to the inspired air in surplus, which is often stressed as an advantage in warming hypothermic patients or reducing major heat losses, e.g., during long operations. The impact of air conditioning on the energy balance of man was calculated comparing HME and HH. ⋯ The water content in the inspired and expired air is the most important parameter for estimating pulmonary heat loss in mechanically ventilated patients. In adults (minute volume approximately 71/min) the main fraction of pulmonary heat loss results from water evaporation from the airways (approximately 6 kcal/h), whereas the heat loss due to convection is negligible (approximately 1.2 kcal/h). In intubated patients ventilated with dry air, the heat loss increases to approximately 8 kcal/h due to greater water evaporation from the airways. Both HME and HH are able to reduce the pulmonary heat loss to 1-2 kcal/h. In normothermic as well as hypothermic patients, HH do not offer significant advantages in heat balance compared to effective HME. In conclusion, air conditioning in intubated patients is neither a powerful too for maintaining body temperature during long-lasting anaesthesia nor a sufficient method of warming hypothermic patients in intensive care units.
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Impaired pulmonary gas exchange can result from lung parenchymal failure inducing oxygenation deficiency and fatigue of the respiratory muscles, which is characterized by hypercapnia or a combination of both mechanisms. Contractility of and coordination between the diaphragm and the thoracoabdominal respiratory muscles predominantly determine the efficiency of spontaneous breathing. Sepsis, cardiac failure, malnutrition or acute changes of the load conditions may induce fatigue of the respiratory muscles. ⋯ As with APRV, alveolar ventilation is maintained even if the spontaneous breathing efforts of the patient cease, which improves the safety of both modes of respiratory therapy. The contribution of spontaneous breathing to total minute ventilation may be important, since a decreased shunt and improved VA/Q relationship have been observed in experimental non-cardiogenic lung oedema. These data give support to the concept that spontaneous breathing should be maintained and augmented in the setting of acute respiratory failure.
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We describe unexpected episodes of paced tachycardia in two patients with rate-responsive pacemakers during anaesthesia. Five months after a heart transplant and implantation of a pacemaker a 43-year-old patient suffered cardiac tamponade as a result of chronic pericarditis. The second case involved embolic occlusion of the femoral artery in a 33-year-old female patient previously operated on for tricuspid valve replacement and implantation of a pacemaker. In both cases induction of anaesthesia was performed with fentanyl, etomidate and vecuronium. Following intubation and mechanical ventilation, the heart rates (HR) of the two patients increased to 140 and 130 min-1 respectively. This was interpreted as a sign of inadequate anaesthesia, and therefore additional doses of fentanyl and etomidate were given, with no effect on the tachycardia. After exclusion of other possible reasons for this complication such as hypokalaemia, hypercapnia, hypoxaemia or allergic reactions, unexpected functioning of the rate-responsive pacemakers due to thoracic impedance changes was assumed. Minute ventilation was reduced, lowering paced HR in 3-5 min. ⋯ These case reports suggest that anaesthetic management affects the action of rate-responsive pacemakers, causing haemodynamic complications, and inadequate interventions by the anaesthesiologist. Thus, it is necessary for anaesthesiologists to make a preoperative evaluation of the underlying medical disease and the type of pacemaker in order to adjust anaesthetic management accordingly and to understand the haemodynamic responses that may occur during the perioperative period. Preoperative programming to exclude the rate-responsive function is advised.