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Effects of gas humidification tion on mechanical ventilation: An experimental study
Abstract
Introduction: Natural warming and humidification of the inspired air tend to be decreased in mechanical ventilation (MV) patients. The goal of this work was to evaluate the effect of humidity on lung mechanics (compliance, functional residual capacity (FRC), and airway resistance (RAW) in a canine experimental model with long-term mechanical ventilation. Materials and methods: Ten dogs were subjected to 24-hours mechanical ventilation as follows: Group I (n = 5): mechanical ventilation with dry air. Group II (n = 5): mechanical ventilation with air from a humidifier chamber. Results: Lung compliance was found to decrease in a higher rate for Group I than for Group II (ANOVA p = NS), while FRC decreased in both groups (ANOVA p = NS), with a lower rate for Group I. RAW values increased in both groups, with a lower rate for Group II (ANOVA p = NS). Conclusions: There were no significant differences in ventilatory mechanics with dry air compared to air from a humidifier chamber.
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Inhaled gas humidification and heating are necessary in patients under tracheal intubation or tracheostomy to prevent damage to respiratory system resulting from the contact of cold and dry gas with the airways. This study aimed at evaluating the effect of respiratory circle systems with carbon dioxide absorbers from Dräger's Cicero anesthesia machine (Germany) as to inhaled gases heating and humidification ability using low fresh gases flow (1 L min(-1)) or minimum flow (0.5 L min(-1)).
Participated in this study, 24 patients, physical status ASA I, aged 18-65 years, submitted to general anesthesia using Dräger's Cicero workstation (Germany) for abdominal surgery, who were randomly distributed in two groups: low flow group (LF) received 0.5 L min(-1) oxygen and 0.5 L min(-1) nitrous oxide, and minimum flow group (MF) received 0.5 L min(-1) oxygen only. Evaluated attributes were temperature, relative and absolute humidity of the operating room and of respiratory circuit gas.
There were no significant differences in inhaled gas temperature, relative and absolute humidity between groups, but they have increased along time in both groups, with influence of operating room temperature on inhaled gas temperature for both groups. Near optimal levels of humidity and temperature were reached as from 90 minutes in both groups.
There have been no significant differences in inhaled gas humidity and temperature with fresh gases low flow or minimum flow.
The variation in respiratory water loss (RWL) over time, expressed as the mass of water vapor lost per liter (body temperature and pressure, saturated) of ventilation (MH2O), was investigated in two groups: (1) children with exercise-induced asthma; and (2) healthy children. Children were matched for age and sex and went without medication for at least 12 hours before each experiment. The children breathed dry warm air (TI = 28.4 degrees C +/- 0.3 degree C) for 15 minutes while bicycling at constant and moderate work load (50 W). The MH2O was measured by collecting and weighing the expired water vapor (1) at rest breathing in warm conditions of inspired gas (control values), (2) every five minutes during exercise while breathing dry warm air, and (3) four minutes after the end of exercise. Pulmonary function tests were performed before and six minutes after exercise. The results were abnormal only in children with exercise-induced asthma. During exercise, RWL significantly fell (compared to control value) at the tenth and 15th minute in both groups. Whereas normal subjects recovered their initial values for MH2O four minutes after stopping exercise, asthmatic children still had a reduction in respiratory water loss. During exercise, MH2O decreased a little more in healthy than in asthmatic children. The decrease in MH2O in both groups suggests that the means to fully humidify expired gas are overwhelmed by thermal stress. The lack of increase in MH2O in asthmatic children on stopping exercise suggests that the airway mucosa is unable to produce enough water vapor and is thus dehydrated and probably hyperosmotic.
Because dry gases can damage respiratory mucosa, lung structure and function, gases need to be humidified for mechanically ventilated patients. Heat and moisture exchangers (HMEs) are the most commonly used humidification devices. They are inexpensive and simple, with additional bacterial and viral filtration properties. Although different brands of HMEs can seem to be similar, the humidification efficiency varies widely. The best performing, achieving absolute humidity of more than 30 mg/l, are often composite hygroscopic HMEs. These are appropriate devices for many patients on critical care units. The limited humidification efficiency of many HMEs makes them appropriate only for short-term use in anaesthesia. Hot water humidifiers (HWHs) are capable of delivering inspired gases at 37°C or higher, with humidity at saturation. The disadvantages of HWHs are that they are bulky, complex pieces of equipment with high maintenance costs. There is limited evidence of any significant difference in mortality, duration of mechanical ventilation, length of stay, incidence of ventilator-associated pneumonia or incidence of airway occlusion between the different categories of humidification device. In patients who have very viscous secretions, HWHs with a set temperature greater than 37.0°C should be given preference.
The influence of varying humidity levels in the air surrounding the mucous membrane of rabbit tracheae has been investigated in vitro at 34 degrees C, 37 degrees C and 40 degrees C. The following results were obtained: (1) The average mucociliary wave frequency was linearly reduced following decreasing levels of relative humidity (r.h.) from 90% to 20%. (2) A temperature of 40 degrees C was more detrimental to the mucociliary function than that of 34 degrees C and 37 degrees C (body temperature) at decreasing levels of r.h. (3) At decreasing levels of r.h. the duration of experiments with preservation in recordable activity ("mucociliary survival time") was significantly shorter (p smaller than 0.05) between 50% and 40% r.h. than between 60% and 50% r.h.
To compare the effects on canine ciliary and bronchial mucosal function of varying periods of ventilation with dry vs. humidified gas mixtures, 29 anesthetized dogs underwent 2, 4, and 6 hours of ventilation through a Carlen's double-lumen tube. Six dogs were evaluated by clearance studies, nine by differential bronchospirometry, six by surfactant studies, six by electron microscopy, and two by ventilation scanning. By means of two separate respiratory systems, dry gas (DG) was used to ventilate the right lung, and humidified gas (HG) ventilated the left lung in each dog. Serial chest roentgenograms showed more rapid clearance of inspired tantalum dust from the HG-ventilated lung in each of the six dogs, the disparity in clearance between the two lungs being more pronounced after longer periods of ventilation. The surface tension in DG-ventilated lungs increased fourfold, whereas in HG-ventilated lungs it increased only twofold as compared to preoperative values. Longer periods of ventilation did not change the surface tension appreciably in either DG- or HG-ventilated lungs. Scanning electron micrographs of bronchial mucosa from DG-ventilated lungs showed tangling and matting of cilia with a granular and stringy material attached to most cilia; these changes were much less pronounced in HG-ventilated lungs. Bronchospirometric studies showed an increase in ventilatory function in each of the lungs ventilated with the HG mixture (percent Vo2 on 100 percent oxygen increased 27.7 percent) to compensate for the decreased gas exchange provided by the contralateral DG-ventilated lung. Function in each of the lungs returned to normal within 24 hours. Ventilation scans with Xenon133 showed no apparent change in isotope uptake in the HG-ventilated lungs as compared to the DG lungs during the first 24 hours after ventilation. The observations from the present study suggest that ventilation of canine lungs with DG for 4 or more hours dries mucus and transiently retards mucociliary clearance and gas exchange. These changes may be minimized by ventilation with a humidified gas mixture. Application of these findings to patients undergoing prolonged general anesthesia and to lung preservation studies is suggested.
A laboratory study was conducted in rabbits to compare histologic changes in the lungs and bronchi after prolonged dry anesthesia and humidified anesthesia. Dry anesthesia caused severe damage to the respiratory epithelium, which was not observed with humidified controls.
The function of the nose in respiration is poorly understood. This article reports on two studies of the effects of nasal obstruction on pulmonary function. First, blood gases were studied in 71 patients undergoing intranasal surgery. Patients with bilateral nasal packing had an average PO2 drop of 6.9 mm Hg. Lung disease and polyps were more prevalent in patients who showed a significant drop in PO2, and those who became most hypoxic complained of a dry throat. The second study tested the influence of humidification. Pulmonary function and blood gases were measured in 10 subjects before and after nasal occlusion. Those breathing only dry air had a drop in compliance after nasal clipping. When air was humidified, nasal occlusion had no significant effect. Humidification may be an important respiratory function of the nose. Any future studies of the nasopulmonary relationship should adequately control for humidity and bronchial reactivity.