Active Humidifier Research Articles

0953 A Novel Active Humidifier for Non-Invasive Ventilated Patients

Abstract Introduction The RespirAq® humidifier is a novel, active heated humidifier that humidifies without the need for additional water. Within milliseconds, the humidifier switches between a super-hydrophilic state to capture moisture during expiration, and a super-hydrophobic state, returning moisture during inspiration. The device delivers gases at 37°C/100% relative humidity to individuals requiring humidification, including those with tracheostomies. We demonstrated the device meets EN ISO 80601 2 74:2020 during bench testing. This study aimed to compare the performance of the novel humidifier with a heated humidifier - HH (MR850™ Fisher & Paykel Healthcare) and a heat and moisture exchanger - HME (DAR™ Covidien) in healthy volunteers during non-invasive ventilation (NIV) using a non-vented full face mask. Methods Ten healthy volunteers were recruited. NIV was performed at different settings (inspiratory positive airway pressure (IPAP) range 5-20cmH20, expiratory positive airway pressure (EPAP) = 5cmH20) over a one-hour period. Humidity and temperature were collected continuously during the study. Results Sixty per cent of participants were female. Mean age of 35.6 ± 5.89 years (M±SD, n=10). Absolute humidity inside the face mask increased from 22.55 ± 2.41 mg/L without humidification to 30.18 ± 2.07 mg/L with an HME, and 39.09 ± 1.05 mg/L and 38.23 ± 1.78 mg/L when using a water-based heated humidifier and the novel humidifier, respectively. When comparing the performance of the humidification devices head-to-head, the two heated humidifiers significantly outperformed the HME (HME vs Novel = P < 0.0001, HME vs HH = p < 0.0001) and there was no significant difference seen between the two active humidifiers (p =0.2047). Conclusion Results show that the device meets the 12 mg/L absolute humidity established by the EN ISO 80601-2-74:2020 standards and that there is no significant difference in performance compared to traditional heated humidification. This suggests that the novel device can deliver the performance of a heater humidifier at the size, reliability and ease of use of a heat and moisture exchanger. Due to the mechanism of action, droplet/aerosol transmission of COVID-19 may also be mitigated. Further studies are warranted to confirm these findings. Support (if any) This research was supported by Tier 2 KiwiNet funding.

High-Flow Nasal Oxygen Therapy in Acute Hypoxemic Respiratory Failure: Concise Review on Technology and Initial Methodology.

High-flow nasal cannula oxygen therapy (HFNCOT) system consists of an air/oxygen supply system capable of delivering up to 100% humidified and heated oxygen at a flow rate of up to 80 L/min. The system includes a blender, active humidifier, single heated tube, and nasal cannula. HFNCOT has many physiological advantages compared with other standard oxygen therapies, such as anatomical dead space washout, more constant fraction of inspired oxygen, positive end-expiratory (PEEP) effect, supplement of adequate humidification and maintenance of muco-ciliary function. HFNCOT is mostly used for hypoxemic acute respiratory failure, although it also has other indications. HFNCOT is a common choice of physicians as its technology makes it more silent and comfortable. Though HFNCOT is used in many clinical settings, there is a lack of publications addressing devices and initial settings. We present a review on HFNCOT, with focus on device and application methodology.

The addition of a humidifier device to a circuit and its impact on home ventilator performance: a bench study

Introduction and objectivesHumidification and non-invasive ventilation are frequently used together, despite the lack of precise recommendations regarding this practice. We aimed to analyse the impact of active external and built-in humidifiers on the performance of home ventilators, focusing on their pressurization efficacy and their behaviour under different inspiratory efforts. MethodsWe designed a bench study of a lung simulator programmed to emulate mechanical conditions similar to those experienced by real respiratory patients and to simulate three different levels of inspiratory effort: five different commonly used home NIV devices and active humidifiers attached to the latter (internal or “built-in”) or to the circuit (external). To test ventilator pressurization under different humidification and effort settings, pressure-time products in the first 300ms and 500ms of the respiratory cycle were calculated in the 45 situations simulated. Inferential statistical analysis was performed. ResultsA significant reduction of PTP 300 and PTP 500 was observed with the external humidifier in three of the devices. The same pattern was noted for another device with an internal humidifier, and only one device showed no significant changes. This impact on pressurization was commonly higher under high inspiratory effort. ConclusionsThese results indicate the need to monitor pressure changes in the use of external humidification devices in some home NIV ventilators.

Prospective pilot study for evaluation of high-flow oxygen therapy in dyspnoeic dogs: the HOT-DOG study.

To describe the use, effectiveness and tolerance of high-flow oxygen therapy in dyspnoeic dogs. Prospectively, dogs in acute respiratory distress admitted to an intensive care unit between January and May 2018 that failed to respond to nasal oxygen therapy and medical stabilisation after 30 minutes were transitioned to high-flow oxygen therapy. High-flow oxygen therapy, delivered an inspired oxygen fraction of 100% using an air/oxygen blender, active humidifier, single warmed tube and specific nasal cannula. Respiratory rate, pulse oximetry (SpO2 ), heart rate and a tolerance score were assessed every 15 minutes from T0 (under nasal oxygen) to 1 hour (T60 ), and PaO2 and PaCO2 at T0 and T60 . Complications were recorded for each dog. Eleven dogs were included. At T60 , PaO2 , flow rate and SpO2 were significantly greater than at T0 (171 ± 123 versus 73 ± 24 mmHg; P=0.015; 18 ±12 L/minute versus 3.2 ± 2.0 L/minute, P<0.01; 97.7 ±2.3% versus 91.6 ±7.2%, P=0.03, respectively). There was no significant difference in PaCO2 , respiratory rate or heart rate between T0 and T60 . Tolerance was excellent, and there were no complications. High-flow oxygen therapy improves markers of oxygenation in dyspnoeic dogs and is an effective means to deliver oxygen with comfort and minimal complications.

Aquecimento e umidificação dos gases inspirados na ventilação mecânica e sua relação com a secreção

Introdução: A ventilação mecânica (VM) requer cuidados especiais, pois em situações inadequadas torna-se deletéria. Objetivo: Verificar se os sistemas ativos de aquecimento e umidificação da Unidade de Terapia Intensiva de um Hospital Público estão sendo realizados de forma adequada e se eles possuem relação com as secreções. Métodos: Foram incluídos 30 pacientes que estavam em VM, com umidificadores ativos, sem restrições de idade e sexo. Observou-se os parâmetros da VM, características das secreções, nível de aquecimento e de água nos copos de umidificação, além da presença de água nos circuitos. Resultados: em 65,8% das vezes, o aquecimento estava abaixo do esperado; em 87,5% das ocasiões, a quantidade de água nos copos umidificadores foram inadequadas; em 53,3%, havia água nos circuitos. As secreções apresentaram relação com a umidificação (r=0,014) e a frequência respiratória com as secreções (p0,05). Conclusão: O aquecimento do ar inspirado e os sistemas ativos de umidificação não estão sendo realizados de forma adequada e eles possuem relação com a viscosidade das secreções.

Is high flow nasal cannula as an effective oxygen delivery alternative in intensive care unit?

Background: High-flow nasal cannula (HFNC) oxygen therapy is carried out using an air/oxygen blender, active humidifier, single heated tube, and nasal cannula. It is an oxygen delivery system which uses air blender to deliver accurate oxygen concentration to the patient from 21% to 100% at desired temperature. It can be administered via wide bore nasal cannula or to the tracheostomy tube via connector. It can give upto 60L/min flow hence can generate positive end expiratory pressure between 2 to 7 cmH20. By providing humidified oxygen along with the high flow rates it satisfies air hunger and reduces work of breathing for the patient.Methods: This is a retrospective observational study. Patients with persistent hypoxia in spite of conventional oxygen therapy were treated with HFNC. Patients with possible need for immediate invasive ventilator support were excluded. Clinical respiratory parameters and oxygenation were compared under conventional and HFNC oxygen therapy.Results: Thirty patients, aged more than 18 years admitted in intensive respiratory care unit with acute hypoxemic respiratory failure from June 2017 to January 2018 were included in the study. Study period was of 6 months. Etiology of acute respiratory failure (ARF) was mainly pneumonia (n = 17), interstitial lung disease (n = 5), bronchial asthma (n=3) and others (n = 5). There was statistically significant reduction in respiratory rate (29.40 before Vs 23.50 after; P- &lt;0.0001) and significant improvement in comfort level of the patient after HFNC therapy. Median duration of HFNC was 48 hrs (24-360) hours. Five patients were intubated later on and 4 died in the intensive care unit.Conclusions: Use of HFNC in patients with persistent ARF was associated with significant and sustained improvement of clinical parameters (respiratory rate). It can be used comfortably for prolonged periods.

Water content of delivered gases during Helmet Continuous Positive Airway Pressure in healthy subjects.

Introduction: During Continuous Positive Pressure Ventilation delivered through helmet, the patient inhales high flows of gas without adequate conditioning. However, the need to humidify the inspired gas during Helmet-CPAP, has not been sufficiently explored. Methods: Experimental design study. Six healthy individuals underwent High Flow Helmet CPAP with different gas flows (60 and 80 L/min) and FiO2 (0.35, 0.5, 0.7 and 1) generated by a Venturi system, with and without active humidification. The active humidifier setting was 26 ° at the humidification chamber and 28 ° at the helmet gas inlet. At each setting, measurements about temperature and relative humidity inside helmet were taken. Comfort level at each setting was evaluated using a visual analog scale rated from 0 to 10. Results: Without heated humidification, the mean value of absolute humidity in the eight combinations investigated was 5.9±2.1 mg H2O/L, with a mean temperature of 25.8±0.9°. With heated humidification mean absolute humidity was 15.0±3.5 mg H2O/L with mean temperature of 29.0±0.1°. The median comfort scale value was 6 (IQR: 5.25-6.75) during the phase without humidification vs 8 (7.25-8.0 - P<0.01) when active humidification was applied. Conclusions: In healthy subjects undergoing High Flow Helmet CPAP, heated humidifiers with heated wires tubes are necessary to avoid the under-humidification inside the helmet. To obtain patient’s comfort and airways mucosal humidification during continuous Helmet CPAP, the most desirable conditions are reached by heated humidifiers with a humidifying chamber temperature settled between 26-28°C. (www.actabiomedica.it)

High-Flow Nasal Cannula Oxygen Therapy in Adults: Physiological Benefits, Indication, Clinical Benefits, and Adverse Effects.

High-flow nasal cannula (HFNC) oxygen therapy is carried out using an air/oxygen blender, active humidifier, single heated tube, and nasal cannula. Able to deliver adequately heated and humidified medical gas at flows up to 60 L/min, it is considered to have a number of physiological advantages compared with other standard oxygen therapies, including reduced anatomical dead space, PEEP, constant F(IO2), and good humidification. Although few large randomized clinical trials have been performed, HFNC has been gaining attention as an alternative respiratory support for critically ill patients. Published data are mostly available for neonates. For critically ill adults, however, evidence is uneven because the reports cover various subjects with diverse underlying conditions, such as hypoxemic respiratory failure, exacerbation of COPD, postextubation, preintubation oxygenation, sleep apnea, acute heart failure, and conditions entailing do-not-intubate orders. Even so, across the diversity, many published reports suggest that HFNC decreases breathing frequency and work of breathing and reduces the need for respiratory support escalation. Some important issues remain to be resolved, such as definitive indications for HFNC and criteria for timing the starting and stopping of HFNC and for escalating treatment. Despite these issues, HFNC has emerged as an innovative and effective modality for early treatment of adults with respiratory failure with diverse underlying diseases.

Effect of High-Flow Nasal Oxygen on Pulmonary Complications and Outcomes After Adult Cardiothoracic Surgery: A Qualitative Review.

IGH-FLOW NASAL OXYGEN (HFNO) therapy delivers low-level, flow-dependent positive airway pressure and may be tolerated better than continuous positive airway pressure (CPAP) or noninvasive bi-level positive airway pressure (BiPAP) and enhance washout of nasopharyngeal deadspace, thus improving oxygenation. 1,2 The apparatus is composed of an air/oxygen blender, an active heated humidifier, a single heated circuit, and a nasal cannula. At the air/ oxygen blender, the inspiratory fraction of oxygen (FIO2) is set from 0.21 to 1.0 at a flow of up to 60 L/min. The gas is heated and humidified with the active humidifier and delivered through the heated circuit. Theoretically, HFNO has a number of advantages over other respiratory support systems, including conventional nasal cannulae, face masks, or CPAP/BiPAP. First, because gas generally is warmed to 371C and completely humidified in HFNO circuits, mucociliary function remains intact and patients report minimal discomfort. Mechanical splinting of the nasopharynx prevents supraglottic collapse and decreases nasopharyngeal resistance, and at 60 L/min HFNO can generate positive end-expiratory pressure (PEEP) of 4 to 7.4 cmH2O, which may counterbalance auto-PEEP and reduce the work of breathing. 3,4 HFNO therapy has been gaining attention as an innovative form of respiratory support in various clinical settings. It is being used increasingly in critically ill patients and gaining favor in the perioperative period. HFNO has been used for multiple indications, including hypoxemic respiratory failure and cardiogenic pulmonary edema, to counterbalance autoPEEP in patients with chronic obstructive airway disease and as prophylactic therapy or treatment of respiratory failure after surgery and extubation. 5 It has been shown to be both safe and noninferior to conventional CPAP in providing prophylactic support to very preterm neonates after extubation. 6 In this same study, the incidence of nasal trauma also was significantly lower in the nasal cannulae group than in the CPAP group. Little clinical experience has been reported on the use of HFNO in the cardiothoracic surgical patient population. Patients undergoing cardiothoracic surgery are at significant risk of postoperative pulmonary complications, and these complications may increase morbidity and mortality and lead to prolonged intensive care unit and hospital length of stay. 7 The incidence of pulmonary complications ranges from 8% to 79% after cardiac surgery and has been reported to be as high as 50% after lung resection surgery. 8 Postoperative pulmonary complications manifest early as arterial hypoxemia, during the later course as pneumonia, and in rare cases also as acute respiratory distress syndrome. 9 The pathogenesis of

High-flow nasal cannula oxygen therapy in adults.

High-flow nasal cannula (HFNC) oxygen therapy comprises an air/oxygen blender, an active humidifier, a single heated circuit, and a nasal cannula. It delivers adequately heated and humidified medical gas at up to 60 L/min of flow and is considered to have a number of physiological effects: reduction of anatomical dead space, PEEP effect, constant fraction of inspired oxygen, and good humidification. While there have been no big randomized clinical trials, it has been gaining attention as an innovative respiratory support for critically ill patients.Most of the available data has been published in the neonatal field. Evidence with critically ill adults are poor; however, physicians apply it to a variety of patients with diverse underlying diseases: hypoxemic respiratory failure, acute exacerbation of chronic obstructive pulmonary disease, post-extubation, pre-intubation oxygenation, sleep apnea, acute heart failure, patients with do-not-intubate order, and so on. Many published reports suggest that HFNC decreases breathing frequency and work of breathing and reduces needs of escalation of respiratory support in patients with diverse underlying diseases.Some important issues remain to be resolved, such as its indication, timing of starting and stopping HFNC, and escalating treatment. Despite these issues, HFNC oxygen therapy is an innovative and effective modality for the early treatment of adults with respiratory failure with diverse underlying diseases.

Measurement of heat and moisture exchanger efficiency

Deciding between a passive heat and moisture exchanger or active humidification depends upon the level of humidification that either will deliver. Published international standards dictate that active humidifiers should deliver a minimum humidity of 33 mg.l(-1); however, no such requirement exists, for heat and moisture exchangers. Anaesthetists instead have to rely on information provided by manufacturers, which may not allow comparison of different devices and their clinical effectiveness. I suggest that measurement of humidification efficiency, being the percentage moisture returned and determined by measuring the temperature of the respired gases, should be mandated, and report a modification of the standard method that will allow this to be easily measured. In this study, different types of heat and moisture exchangers for adults, children and patients with a tracheostomy were tested. Adult and paediatric models lost between 6.5 mg.l(-1) and 8.5 mg.l(-1) moisture (corresponding to an efficiency of around 80%); however, the models designed for patients with a tracheostomy lost between 16 mg.l(-1) and 18 mg.l(-1) (60% efficiency). I propose that all heat and moisture exchangers should be tested in this manner and percentage efficiency reported to allow an informed choice between different types and models.

Impact of passive humidification on clinical outcomes of mechanically ventilated patients: A meta-analysis of randomized controlled trials*

Previous meta-analyses reported advantages of passive (i.e., heat and moisture exchangers, or HMEs) compared with active (i.e., heated humidifiers, or HHs) humidifiers in reducing the incidence of ventilator-associated pneumonia, but they did not examine the effect of these devices on mortality, length of intensive care unit stay, and duration of mechanical ventilation. In addition, relevant data were recently published. Meta-analysis of randomized controlled trials comparing HMEs with HHs for the management of mechanically ventilated patients to determine the impact of these devices on clinical outcomes of such patients. We searched PubMed and the Cochrane Central Register of Controlled Trials as well as reference lists from publications, with no language restrictions. We estimated pooled odds ratios (ORs) and 95% confidence intervals (CIs), using a random effects model. Thirteen randomized controlled trials, studying 2,580 patients, were included. There was no difference in incidence of ventilator-associated pneumonia among patients managed with HMEs and HHs (OR 0.85, 95% CI 0.62-1.16). There was no difference between the compared groups regarding mortality (OR 0.98, 95% CI 0.80-1.20), length of intensive care unit stay (weighted mean differences, -0.68 days, 95% CI -3.65 to 2.30), duration of mechanical ventilation (weighted mean differences, 0.11 days, 95% CI -0.90 to 1.12), or episodes of airway occlusion (OR 2.26, 95% CI 0.55-9.28). HMEs were cheaper than HHs in each of the randomized controlled trials. The available evidence does not support the preferential performance of either passive or active humidifiers in mechanical ventilation patients in terms of ventilator-associated pneumonia incidence, mortality, or morbidity.

Impact of passive humidification on clinical outcomes of mechanically ventilated patients: A meta-analysis of randomized controlled trials *

Objective: Previous meta-analyses reported advantages of passive (i.e., heat and moisture exchangers, or HMEs) compared with active (i.e., heated humidifiers, or HHs) humidifiers in reducing the incidence of ventilator-associated pneumonia, but they did not examine the effect of these devices on mortality, length of intensive care unit stay, and duration of mechanical ventilation. In addition, relevant data were recently published. Design: Meta-analysis of randomized controlled trials comparing HMEs with HHs for the management of mechanically ventilated patients to determine the impact of these devices on clinical outcomes of such patients. Methods: We searched PubMed and the Cochrane Central Register of Controlled Trials as well as reference lists from publications, with no language restrictions. We estimated pooled odds ratios (ORs) and 95% confidence intervals (CIs), using a random effects model. Results: Thirteen randomized controlled trials, studying 2,580 patients, were included. There was no difference in incidence of ventilator-associated pneumonia among patients managed with HMEs and HHs (OR 0.85, 95% CI 0.62–1.16). There was no difference between the compared groups regarding mortality (OR 0.98, 95% CI 0.80–1.20), length of intensive care unit stay (weighted mean differences, -0.68 days, 95% CI -3.65 to 2.30), duration of mechanical ventilation (weighted mean differences, 0.11 days, 95% CI -0.90 to 1.12), or episodes of airway occlusion (OR 2.26, 95% CI 0.55–9.28). HMEs were cheaper than HHs in each of the randomized controlled trials. Conclusion: The available evidence does not support the preferential performance of either passive or active humidifiers in mechanical ventilation patients in terms of ventilator-associated pneumonia incidence, mortality, or morbidity.

Was gibt es Neues in der Intensivmedizin - eine Übersicht

This manuscript gives a review about important studies addressing problems in intensive care medicine that have been published in journals focussing on critical care medicine and surgery in 2005. Only clinical studies are included in this review, mostly meta-analyses, randomized controlled trials and a few important or interesting observational studies. In addition to describing major results a critical appraisal of each study is undertaken, which, however, is neither comprehensive nor complete. It is merely intended to address some important aspects for the reader who should be stimulated to go deeper into one or the other topic or study. The publication of the new CPR-guidelines of the American Heart Association and the European Resuscitation Council as well as the newly developed SAPS III score to predict intensive care unit outcome are among the outstanding topics. Several randomized trials and meta-analyses deal with aspects of drug therapy of septic patients. Some important and relevant findings have been reported with respect to the efficiency of the open-lung concept, non-invasive ventilation, the use of heat and moisture exchanger filters compared to active humidifiers and of closed systems for endotracheal suctioning. The role of immuno-nutrition in adults and children as well as of early enteral nutrition can be defined more clearly. Whether corticosteroids should be used in the treatment of severe traumatic brain injury can be definitely answered now. There are some new insights reported into the management of patients infected or contaminated with MRSA in the intensive care unit. Last but not least an impressive study shows that not only the newest therapeutic developments but the stringent use of the already known treatment options may result in dramatic improvements of patient outcome.

Relationship between absolute humidity of inspired and expired gases with an active humidifier

Relationship between absolute humidity of inspired and expired gases with an active humidifier

Humidity control of an incubator using the microcontroller-based active humidifier system employing an ultrasonic nebulizer

Relative humidity levels of an incubator were measured and controlled. An ultrasonic nebulizer system as an active humidifier was used to humidify the incubator environment. An integrated circuit-type humidity sensor was used to measure the humidity level of the incubator environment. Measurement and control processes were achieved by a PIC microcontroller. The high-performance and high-speed PIC provided the flexibility of the system. The developed system can be used effectively for the intensive care of newborns and/or premature babies. Since the humidifier generates an aerosol in ambient conditions, it is possible to provide the high relative humidity level for therapeutic and diagnostic purposes in medicine.

A new device for 100% humidification of inspired air

Passive heat/moisture ex-changers (HME) which are based on a hygroscopic condenser principle usually provide adequate humidity (up to 32 mgH2O/l air) of the inspired gas during ventilator treatment [1,2]. However, in about 5-10% of the patients with e.g. thick secretions [1,2] active humidifiers that can provide 100% humidity are needed. These devices cause free water condensation in the tubings [3] with risks of contamination and of compromising the ventilator function. To avoid this a new humidifier has been developed. It consists of a supply unit with a microprocessor and a water pump, and a humidification device, which is placed between the Y-piece and the endotracheal tube. The humidification device is based on a hygroscopic HME, which absorbs the expired heat and moisture and releases it to the inspired gas. External heat and water are then added to the patient side of the HME, so the inspired gas reaches 100% humidity at 37 oC (44 mgH2O/l air). The external water is delivered via a pump onto a porous membrane and then evaporated in the inspired air by an electrical heater. The microprocessor controls the water pump and the heater by an algorithm using the minute ventilation (which is fed into the microprocessor) and the airway temperature measured by a sensor mounted in the flex tube on the patient side of the humidification device. The aim of this study was to test the performance of this humidifier at different ventilator settings in a lung model.

Advantages of a new humidification technique

Recently, an active HME (AHME) (Humid-Heat, Gibeck) has been developed. The AHME combines a HME with a unit which adds humidity and heat to the patient-side of the HME. The supply of humidity and heat is automatically regulated, in order to achieve 100% humidity of inspired gases at 37°C. The operation of AHME requires only the user-set input of the patient minute ventilation. We evaluated the potential advantages of the AHME over a conventional active humidifier.

Is reduction of intraoperative heat loss and management of hypothermic patients with anesthetic gas climate control advisable? Heat and humidity exchangers vs. active humidifiers ina functional lung model

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 efficiency of a HME (Medisize Hygrovent) and a HH (Fisher & Paykel MR 730) was evaluated in a mechanically ventilated lung model simulating the physiological heat and humidity conditions of the upper airways. The gas flow from the central supply was dry; the model temperature varied between 32 and 40 degrees C. By using a HH in the inspiratory limb, a circle system was simulated with water-saturated inspired air at room temperature. The water content of the ventilated air was determined at the tracheal tube connection using a fast, high-resolution humidity meter and was compared with the moisture return of the HME. The energy balance was calculated according to thermodynamic laws. Both HME and HH were able to create physiological heat and humidity conditions in the airways. With the normothermic patient model, the moisture return of the HME was equal to that of the HH set at 34 degrees C. Increasing the heating temperature resulted only in reduced water loss from the lung; heat and water input in the normothermic model was not possible. This was only effective with almost negligible amounts under hypothermic patient model conditions. 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.

Ventilator Circuits, Humidification and Ventilator-Associated Pneumonia

Technical issues in the care of mechanically ventilated patients include those related to the ventilator circuit, humidification and ventilator-associated pneumonia. Principal issues related to ventilator circuits include leaks and compression volume. Circuit compression volume affects delivered tidal volume as well as measurements of auto-positive end-expiratory pressure and mixed expiredPCO2. Resistance through the ventilator circuit contributes to patient-ventilator dyssynchrony during assisted modes of mechanical ventilation. Adequate humidification of inspired gas is necessary to prevent heat and moisture loss. Common methods of humidification of inspired gas during mechanical ventilation include use of active heated humidifiers and passive artificial noses. Artificial noses are less effective than active humidifiers and are best suited to short term use. With active humidifiers, the circuit can be heated to avoid condensate formation. However, care must be exercised when heated circuits are used to avoid delivery of a low relative humidity and subsequent drying of secretions in the artificial airway. Although pneumonia is a complication of mechanical ventilation, these pneumonias are usually the result of aspiration of pharyngeal secretions and are seldom related to the ventilator circuit. Ventilator circuits do not need to be changed more frequently than weekly for infection control purposes, and the incidence of ventilator-associated pneumonia may be greater with more frequent circuit changes.