Oxygenation Without Intubation

Transcription

1 CHAPTER 2 Irene Permut and Wissam Chatila Oxygenation Without Intubation CHAPTER OUTLINE Learning Objectives Supplying Supplemental Oxygen Devices that Provide Supplemental Oxygen Nasal Cannula Simple Mask Partial-Rebreathing Mask Nonrebreathing Mask Venturi Mask Nasal High-Flow Oxygen Therapy AMBU (Airway Mask Breathing Unit) Bag and Mask Oxygen-Conserving Devices Heliox Continuous Positive Airway Pressure Monitoring Summary Review Questions Answers References Additional Reading LEARNING OBJECTIVES After studying this chapter, you should be able to: Identify different devices for supplying oxygen therapy. Describe the mode of function of different oxygensupplying devices. Select specific devices to deliver oxygen in different patient populations. Adjust the oxygen delivery devices to ensure adequate oxygen supplementation. Oxygen therapy, a lifeline for many critically ill patients, can be delivered in nonintubated patients via several devices. Unlike patients with chronic hypoxemia, the long-term comfort and cosmetics of the patient are not a concern of intensivists; instead, the goal is to ensure adequate oxygen delivery to prevent hypoxemia. Although hypoxemia is often corrected with oxygen therapy, care should be taken to understand the pathophysiology leading to hypoxemia. The appropriate management of hypoxemia should include treatment of the underlying pathology to prevent any complication and progression of the disease. For example, many patients with postoperative atelectasis develop hypoxemia responsive to oxygen therapy. Treatment of postoperative hypoxemia with oxygen supplementation alone, without initiating lung reexpansion measures to treat atelectasis, is insufficient. 1 This chapter covers noninvasive modes of supplying oxygen and does not discuss other means of correcting hypoxemia. 27

2 28 I. Permut and W. Chatila SUPPLYING SUPPLEMENTAL OXYGEN The goal of oxygen supplementation is to ensure adequate oxygenation regardless of the mode of delivery. Do not confuse low-flow devices with low concentration of oxygen supplementation. There are three main components of oxygen supplementation: (1) the control component, which includes regulators (reducing valves that buffer high pressures from bulk oxygen systems to a lower pressure patient point of access) and flowmeters (which control and indicate flow) (Fig. 2-1), (2) the blending of air and oxygen, and (3) the administration of oxygen through devices that include cannulas and masks. 2 Respiratory care therapists are usually responsible to ensure proper functioning of the first two components, but physicians who order oxygen supplementation tend to specify the mode of oxygen delivery; therefore, physicians should familiarize themselves with indications of available devices for oxygen administration. The oxygen delivery devices can be divided into two major groups: low-flow and highflow oxygen systems. 3 Nasal cannulas, simple masks, and reservoir masks are examples of low-flow systems that are used when consistency of the fraction of inspired oxygen (FiO 2 ) delivery is not crucial. Low-flow systems provide supplemental oxygen at a rate that is less than the peak inspiratory flow rate. In contrast, high-flow oxygen systems deliver oxygen at a rate that is above the peak inspiratory flow rate. Therefore, they are capable of delivering up to 40 L/min of conditioned gas and providing a precise and consistent FiO 2 regardless of the patient s breathing pattern. Venturi masks and oxygen tents are examples of high-flow systems. Accordingly, when prescribing oxygen, the desired range of FiO 2 and the patient s ventilatory pattern need to be considered to ensure effective oxygen supplementation. Both low-flow and high-flow systems can deliver a wide range of FiO 2 ; the terms low and high do not reflect the delivered FiO 2 but describe the flow of gas delivered through the system. A detailed description of each device follows in the next section (Table 2-1). DEVICES THAT PROVIDE SUPPLEMENTAL OXYGEN Nasal Cannula The nasal cannula, the most common oxygen delivery system, is used both for hospital inpatients and for outpatients (Fig. 2-2). It consists of two small prongs inserted about 1 cm into each nare through which flows 100% oxygen, with the oxygen flow adjusted by the flowmeter. Although nasal cannulas are well tolerated in the majority of patients, there is a great FIGURE 2-1 A flowmeter that regulates the flow of oxygen from a central source while simultaneously displaying the oxygen flow rate.

3 CHAPTER 2 Oxygenation Without Intubation 29 DEVICE OXYGEN FLOW RATE (L/min) FiO 2 Nasal cannula Up to 0.50 Simple masks Venturi masks a Partial-rebreathing masks ³8 ³0.60 Nonrebreathing masks ³10 ³0.80 High-flow nasal cannula ³15 ³0.80 TABLE 2-1 OXYGEN CONCENTRATIONS FOR LOW- AND HIGH-FLOW DELIVERY SYSTEMS a The final FiO 2 varies according to the oxygen flow and the total gas delivered, which is a function of the diluter jet and flow settings FIGURE 2-2 A nasal cannula used to deliver supplemental oxygen. variability in the final FiO 2 because of admixture with entrained ambient air. The amount of oxygen delivered to the patient depends upon the amount of oxygen supplied as well as the minute ventilation of the patient. Thus, this system is valuable for patients who require up to 40% of uncontrolled oxygen, or those who do not tolerate facemasks. The use of a nasal cannula is not effective in patients who have significant nasal obstruction and are mouth breathers. Flows greater than 6 L/min are discouraged because of drying of the nasal mucosa, crusting of secretions, epistaxis, and septal perforation. However, recently nasal cannulas have been used also in several high-flow delivery systems that could provide adequately humidified oxygen at flow rates up to 40 L/min (see below). Simple Mask Similar to the nasal cannula, the simple mask does not allow precise control of delivered oxygen concentration because of dilution with ambient air that is drawn in and inspired from the exhalation ports (Fig. 2-3). However, the mask can deliver higher FiO 2 (to 55%) with higher flows (7 10 L/min) and produces a good seal around the patient s nose and mouth. Another advantage of the mask compared to the nasal cannula is improved humidification

4 30 I. Permut and W. Chatila FIGURE 2-3 A simple facemask for oxygen delivery that has portholes for expiration (EP). and fewer drying side effects. On the other hand, care should be taken not to order low flows (<5 L/min) when using the simple mask because of the potential for rebreathing exhaled carbon dioxide when the mask dead space is not continuously flushed by flowing oxygen. Partial-Rebreathing Mask Except for a reservoir bag, the partial-rebreathing mask is comparable to the simple mask (Fig. 2-4). The oxygen source directly feeds into the reservoir bag. As the patient exhales, the first third of the exhaled tidal volume returns into the reservoir and the rest dissipates through exhalation ports. The first third of the exhaled tidal volume comes mostly from the anatomic dead space, and therefore has high oxygen and low carbon dioxide concentration. FIGURE 2-4 A partial-rebreathing facemask that delivers high levels of oxygen. The inflatable bag acts as an oxygen reservoir from which the patient can rebreathe high concentrations of supplemental oxygen. The patient expires through expiration ports (EP), as illustrated.

5 CHAPTER 2 Oxygenation Without Intubation 31 When the patient inhales, gas is drawn from the bag, which contains oxygen-rich exhaled gas and supplied oxygen, as well as from the exhalation ports. The partial-rebreathing mask has the potential to deliver up to 60% inspired oxygen concentration as long as a high oxygen flow rate is maintained and the reservoir bag does not collapse. Partial-rebreathing masks are variable performance devices, and therefore, the amount of oxygen delivered is partially dependent on the breathing pattern of the patient. Nonrebreathing Mask Two valves, added on the inhalation and exhalation ports, distinguish the nonrebreathing mask from the partial-rebreathing mask (Fig. 2-5). These two one-way valves allow the patient to inhale oxygen from the reservoir, but prevent the backflow of expired volume into the bag during exhalation and thereby avoid entraining ambient air through the exhalation ports during inspiration. The nonrebreathing mask can deliver close to 100% oxygen when adequate flow is maintained and the mask has a good seal on the patient s face. Manufacturers of nonrebreathing masks avoid placing valves on the two exhalation ports as a precautionary measure in the event of inspiratory valve malfunction, which would interrupt the flow of oxygen (note one exhalation port is covered in Fig. 2-5). To avert potential valve problems, some intensivists make up reservoir masks by adding large deadspaces to simple masks (Fig. 2-6). These reservoir masks, known as tusk masks, still require high flows of oxygen to flush all exhaled air from the mask dead space and minimize the entrainment of the ambient air during inspiration. The nonrebreathing mask can deliver up to 100% FiO 2. FIGURE 2-5 A nonrebreathing facemask. A one-way valve at the inhalation port (IV) prevents expired gases from refilling the oxygen reservoir bag. The presence of a one-way exhalation valve (EV) prevents room air from being inspired during inhalation. FIGURE 2-6 Modification of a facemask with 6-in. tubing substituted for EVs to prevent entrainment of room air during inspiration. The mask is also known as a tusk mask.

6 32 I. Permut and W. Chatila In nonintubated patients, the venturi mask is the only mask that delivers controlled high-flow oxygen concentration. Venturi Mask The venturi mask is an example of a high-flow oxygen delivery system. Oxygen is forced through a short constriction (the venturi valve) which results in increased gas flow based on the Bernoulli principle; the high-velocity flows of oxygen going through the narrow orifice generate a subatmospheric pressure around the stream of oxygen, which in turn entrains a specific proportion of room air (Fig. 2-7). After the gas leaves the valve, there is an increased area causing the pressure to drop and the flow to increase and the air is entrained from either side of the valve. Changes in the patient s minute ventilation do not effect the concentration of delivered oxygen because the mask delivers a constant mixture of supplied oxygen and surrounding air at a flow rate higher than the patient s inspiratory flow. Therefore, the accuracy of delivered oxygen is within 2% of the set FiO 2. Patients with chronic respiratory insufficiency who are at a risk of developing worsening hypercapnia while on oxygen supplementation are good candidates for this mask. Nasal High-Flow Oxygen Therapy Traditional nasal cannulas are unable to safely and comfortably deliver oxygen at flow rates above 6 L/min. This is due to a lack of adequate humidification, which is necessary for ciliary function, to prevent thickening of secretions and to decrease heat loss. Another important a b FIGURE 2-7 (a) A disposable venturi mask that allows more precise control of delivered oxygen. Gas passes through a small opening exit with a high velocity generating subatmospheric pressure that also entrains room air from the side ports. (b) Different diluter jets specify the amount of delivered oxygen and entrained room air mixtures that are used to vary the inspired oxygen concentration. 100% oxygen Entrained air

7 CHAPTER 2 Oxygenation Without Intubation 33 consideration for high-flow oxygen delivery systems is the energy required to heat the inspired gas from ambient to body temperature. The need for the delivered gas to be heated increases as the flow of delivered oxygen increases. There are now several high-flow nasal cannula oxygen delivery systems 4 that can provide adequately humidified oxygen at flow rates ranging between 15 and 40 L/min (Vapotherm and AquinOX ). Because these systems operate at high flows, the oxygen delivery is constant regardless of the patient s minute ventilation. Concerns pertaining to patient exposure to Ralstonia species isolated from the heated humidification system of Vapotherm led to its withdrawal from the market, 5 but it was recently reintroduced. AMBU (Airway Mask Breathing Unit) Bag and Mask Bag and mask ventilation is usually reserved for patients with decompensated respiratory failure, or after cardiopulmonary arrest, while preparing the equipment required for intubation. The majority of patients can be adequately supported with bag-mask ventilation as long as a tight seal between the patient s face and mask is maintained. A variety of masks are available, but a clear mask should always be used to observe for vomiting and potential aspiration. Oxygen-Conserving Devices Oxygen-conserving devices are used mostly in outpatients; these systems are not available in many hospitals and are not suited for the management of acute hypoxemia. There are two main mechanisms for oxygen conservation. One mechanism is based on collecting 100% oxygen during exhalation in a reservoir. The reservoir is either mechanical, such as the nasal reservoir cannula or a pendant reservoir cannula that then empties on inspiration (Fig. 2-8), or anatomic via a small catheter inserted into the trachea (Fig. 2-9). The transtracheal oxygen system uses the proximal trachea as an expanded anatomic reservoir; oxygen flowing into the trachea washes out the anatomic dead space, thereby also reducing the work of breathing. Transtracheal catheters may be effective at treating patients with severe hypoxemia that is refractory to treatment with oxygen via nasal cannula. In addition, they may be concealed with clothing, and may therefore improve compliance, comfort, and functional capacity in comparison to other oxygen-conserving devices. On the other hand, transtracheal catheters require higher maintenance to prevent infection at the site and obstruction of the catheter by dried secretions or life-threatening mucous balls. The second mechanism for oxygen conservation is based on the pulsation of oxygen during the first quarter to one-half of each inspiration. During inspiration, the final portion of inspired air never reaches the alveoli and therefore does not participate in gas exchange. FIGURE 2-8 Nasal cannula capable of delivering high filled oxygen with the use of an oxygen reservoir. The pendant reservoir device serves as a repository of enriched oxygen from which the patient can breathe with each inhalation. The nasal cannula and tubing are larger than conventional nasal cannula to allow a higher flow of inspired gas.

8 34 I. Permut and W. Chatila FIGURE 2-9 Proper placement of a transtracheal catheter for oxygen administration. The transtracheal oxygen delivery system uses the proximal trachea as an anatomic reservoir. To Oxygen Tube Therefore, efficient oxygen delivery should occur during the initial portion of inhalation. With oxygen pulsation devices, as in the pendant reservoir cannula, nasal prongs are used to deliver the oxygen. Heliox Helium is 86% less dense (0.179 g/l) than room air (1.293 g/l). The lower density of helium improves the chances of obtaining laminar flow through the airways. Laminar flow through an airway occurs at low flow rates, whereas turbulent flow occurs at high flow rates. The likelihood of laminar gas flow through an airway is determined by the Reynolds number (Re), which represents the relationship between the airway radius and velocity, gas density, and gas viscosity. 6 Re = (airway radius)(velocity)(density of gas) gas viscosity The lower density of helium lowers the Reynolds number and thus promotes laminar flow through the airways. Heliox decreases the work of breathing in patients with increased airways resistance and improves ventilation. Importantly, heliox does not treat the underlying disease, but can be used as a temporizing measure until other therapies take effect. Indications for heliox include upper airway obstruction, with some suggestions that it might be of benefit in status asthmaticus and chronic obstructive pulmonary disease (COPD) exacerbation. 7 Helium and oxygen mixtures typically come in ratios of 80:20, 70:30, and 60:40 (helium:oxygen). Therefore, this therapy will not be beneficial if an FiO 2 of greater than 40% is necessary. Continuous positive airway pressure (CPAP) is not considered as a mode of noninvasive ventilation. CONTINUOUS POSITIVE AIRWAY PRESSURE CPAP is often confused with noninvasive ventilation (bilevel positive airway pressure [BiPAP ] or pressure support ventilation). Although both modes of ventilatory support can be delivered via nasal or oronasal masks, they have different functions. 8

9 CHAPTER 2 Oxygenation Without Intubation 35 CPAP works by generating a continuous airflow that maintains a continuous positive pressure to the respiratory system during inspiration and expiration. Unlike BiPAP, CPAP does not provide increased pressure during inhalation and therefore does not provide true ventilatory support. However, CPAP may be helpful in improving oxygenation through three mechanisms. First, CPAP serves to prevent airway collapse. Second, by expanding endexpiratory lung volume, CPAP increases functional residual capacity, thus reducing the degree of intrapulmonary shunt caused by both atelectasis and fluid. In addition, through complex heart lung interactions, the applied positive pressure may have favorable hemodynamic effects in patients with compromised cardiac function. CPAP improves left ventricular performance by reducing left ventricular preload and afterload. Obviously, CPAP can be applied in intubated patients in the form of positive end-expiratory pressure (PEEP), but when used in spontaneously breathing nonintubated patients it serves as a pneumatic splint of the airway, which makes it a very effective method to treat obstructive sleep apnea. In the hospital, the role of CPAP is limited to patients with known obstructive sleep apnea and selected patients with decompensated heart failure (hemodynamically stable and cooperative) to prevent intubation by improving oxygenation and decreasing the work of breathing. Despite the above mechanisms of action, oxygen is frequently bled into the apparatus, that is, added to its tubing, to treat hypoxemia. Although CPAP has been shown to reduce the work of breathing in patients with chronic obstructive lung disease, many physicians elect to use noninvasive ventilation such as BiPAP support. BiPAP provides the added advantage of delivering inspiratory support as well as PEEP. While BiPAP can provide ventilatory support to a spontaneously breathing patient, there is also the ability to set a back-up ventilatory rate to ensure continued respiratory effort. However, one should keep in mind that in both CPAP and BiPAP the final oxygen concentration will be uncontrolled because of patient s breathing pattern, mask fit, and, most important, the machine setting. The efficacy of CPAP and BiPAP in improving oxygenation is partially dependent on patient selection. Acute or chronic respiratory failure, acute pulmonary edema, and sleep-related breathing disorders are all clinical settings when it is appropriate to consider the use of CPAP or BiPAP (Chap. 46). However, intubation and mechanical ventilation should be pursued if the patient has failed CPAP or BiPAP, is hemodynamically unstable, or is at high risk of aspiration. Recently, another mode of noninvasive ventilation has been introduced to treat patients with complex form of sleep disordered breathing. These devices (BiPAP AVAPS, and VPAP adapt SV ) adapt to changing breathing patterns of patients with mixed types of apneas and deliver variable pressure support. They perform breath-to-breath analysis, constantly adjusting the delivered bilevel pressures (IPAP and EPAP), in order to deliver a steady minute ventilation. These devices are mostly used in the outpatient setting and have not been evaluated in hospitalized patients requiring treatment for central or mixed sleep apnea syndromes. MONITORING Hospitalized patients requiring oxygen supplementation should be monitored with transcutaneous pulse oximetry to ensure adequate oxygen delivery and oxygenation. However, oxygen saturation is not the only parameter that needs to be closely observed in critically ill patients with impending respiratory failure. 9 Other clinical parameters (unstable vital signs, physical findings such as altered mental status that suggest organ dysfunction) that characterize severe illness dictate the frequency and the intensity of monitoring. A subgroup of patients with chronic hypoventilation, for example obesity hypoventilation and some patients with COPD, may experience worsening respiratory acidosis with supplemental oxygen. These patients are better monitored with arterial blood gases to better assess the level of carbon dioxide retention. It is also important to be aware of other limitations of the transcutaneous pulse oximetry measurements. 10 Patients suffering from various hemoglobinopathies and poisonings, such as carbon monoxide inhalation or cyanide toxicity, can have normal transcutaneous oxygen saturation values but still be severely hypoxemic. Transcutaneous pulse oximetry is effective to monitor adequate oxygenation but can be inadequate in certain subgroups of patients.

10 36 I. Permut and W. Chatila SUMMARY A wide variety of devices are available to deliver oxygen therapy for inpatients. Although the nasal cannula route is most widely used, critically ill patients often require other devices to meet their oxygen needs. Breathing pattern, underlying mechanism of hypoxemia, and tolerability should all be considered when choosing an oxygen delivery device, keeping in mind that the primary goal of management is adequate oxygenation. REVIEW QUESTIONS 1. Which of the following oxygen devices delivers precise FiO 2? A. Partial-rebreathing mask B. Venturi mask C. AMBU bag and mask D. CPAP 2. A patient with COPD on home oxygen, set at 2 L/min and delivered via a nasal cannula, was admitted to the intensive care unit for the monitoring of the upper gastrointestinal bleeding. The patient is comfortable with an oxygen saturation of 95% while on oxygen at 2 L/min via nasal cannula. While the patient is monitored with continuous pulse oximetry, it is recommended to do the following: A. Increase the FiO2 to 4 L/min using the nasal cannula B. Change the nasal cannula to a venturi mask to deliver 30% FiO2 C. Continue the current oxygen setting D. Continue oxygen at 2 L/min but change from nasal cannula to a partial-rebreathing mask 3. A 70-year-old-man, with a past medical history of severe chronic obstructive lung disease on chronic oxygen therapy at 2 L/min via nasal cannula, presents to the emergency room in respiratory distress, diaphoretic, and agitated. He gives a history of progressive dyspnea associated with a worsening productive cough, fevers, and chills. On arrival to the emergency room, his vital signs were blood pressure 150/90 mmhg, pulse rate 130 beats/min, temperature 38.5 C, and respiratory rate 33 breaths/min; his oxygen saturation measured by transcutaneous pulse oximetry was 80%. He was placed on oxygen supplementation at a FiO 2 of 30% delivered with a simple facemask, and he was treated with repeated doses of nebulized bronchodilators. While waiting for the rest of his workup, the patient s transcutaneous pulse oximetry was reading 90% but his breathing was becoming more labored and he was difficult to arouse. What is the most appropriate step in the management of this patient? A. Discontinue the simple mask and place him back on nasal cannula at 3 L/min of oxygen flow B. Keep the simple mask and increase the FiO 2 to 50% C. Change the simple mask to a nonrebreathing mask to try to deliver a FiO 2 of 100% D. Start AMBU bag-mask ventilation and prepare to intubate the patient ANSWERS 1. The answer is B. The most precise delivery devices are the highflow air-entrainment devices such as the venturi mask. The rest are dependent on mask seal and patient ventilatory pattern. 2. The answer is C. Because the patient is hemodynamically stable and there is no evidence of hypoxemia, there is no need to change the oxygen delivery system or FiO The answer is D. The patient is in acute respiratory failure showing deterioration of his clinical status despite aggressive conventional therapy; therefore, he needs to be intubated for ventilatory assistance and protection of his airway. Remember that the primary goal to therapy is to ensure adequate oxygenation. Although the change in his mental status may be related to CO 2 retention, if FiO 2 is lowered he will become more hypoxemic.

11 CHAPTER 2 Oxygenation Without Intubation 37 REFERENCES 1. Magnusson L, Spahn DR. New concepts of atelectasis during general anaesthesia. Br J Anaesth. 2003;91: Ward JJ. Equipment for mixed gas and oxygen therapy. In: Barnes TA, ed. Core Textbook of Respiratory Care Practice. 2nd ed. St Louis: Mosby; Branson RD. The nuts and bolts of increasing arterial oxygenation: devices and techniques. Respir Care. 1993;38: Waugh JB, Granger WM. An evaluation of 2 new devices for nasal high-flow gas therapy. Respir Care. 2004;49: Jhung MA, Sunenshine RH, Noble-Wang J, et al. A national outbreak of Ralstonia mannitolilytica associated with use of a contaminated oxygen-delivery device among pediatric patients. Pediatrics. 2007;119: Hess DR, Fink JB, Venkataraman ST, Kim IK, Myers TR, Tano BD. The history and physics of heliox. Respir Care. 2006;51: Gupta VK, Cheifetz IM. Heliox administration in the pediatric intensive care unit: an evidence-based review. Pediatr Crit Care Med. 2005;6: Mehta S, Hill NS. Noninvasive ventilation. Am J Respir Crit Care Med. 2001;163: Pierson DJ. Goals and indications for monitoring. In: Tobin MJ, ed. Principles and Practice of Intensive Care Monitoring. New York: McGraw-Hill; Hanning CD, Alexander-Williams JM. Pulse oximetry: a practical review. BMJ. 1995;311: ADDITIONAL READING American Association of Respiratory Care. Clinical practice guidelines: oxygen therapy in the acute care hospital. Respir Care. 1991;36: Cairo JM. Administering medical gases: regulators, flowmeters, and controlling devices. In: Cairo JM, Pilbeam SP, eds. Mosby s Respiratory Care Equipment. 6th ed. St. Louis: Mosby; Phillip Y, Kristo D, Kallish M. Writing the take-home oxygen prescription for COPD patients. Document hypoxemia, then aim for a target oxygenation level. J Crit Illness. 1998;13(2): Campkin NTA, Ooi RG, Soni NC. The rebreathing characteristics of the Hudson oxygen mask. Anaesthesia. 1993;48:239. Waugh Jonathan B, Granger Wesley M. An evaluation of 2 new devices for nasal high-flow gas therapy. Respir Care. 2004;49(8): Ho AM, Lee A, Karmakar MK, Dion PW, Chung DC, Contardi LH. Heliox vs air-oxygen mixtures for the treatment of patients with acute asthma: a systematic overview [Review]. Chest. 2003; 123(3): Rees RJ, Dudley F. ABC of oxygen: provision of oxygen at home. Br Med J. 1998;317:935. Jaber S, Fodil R, Carlucci A, et al. Noninvasive ventilation with helium-oxygen in acute exacerbations of chronic obstructive pulmonary disease. Am J Respir Crit Care Med. 2000;161(4 Pt 1): Teschler H, Dohring J, Wang YM, Berthon-Jones M. Adaptive pressure support servo-ventilation: a novel treatment for Cheynestokes respiration in heart failure. Am J Respir Crit Care Med. 2001;164(4): Tassaux D, Strasser S, Fonseca S, Dalmas E, Jolliet P. Comparative bench study of triggering, pressurization, and cycling between the home ventilator VPAP II and three ICU ventilators. Intensive Care Med. 2002;28(9):

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