ﺑﺎﺯﮔﺸﺖ ﺑﻪ ﺻﻔﺤﻪ ﻗﺒﻠﯽ
خرید پکیج
تعداد آیتم قابل مشاهده باقیمانده : 3 مورد
نسخه الکترونیک
medimedia.ir

The ventilator circuit

The ventilator circuit
Literature review current through: Jan 2024.
This topic last updated: May 12, 2022.

INTRODUCTION — The ventilator circuit refers to the tubing that connects the ventilator to the patient, as well as any devices that might be connected to the circuit.

The basic components of the ventilator circuit and their maintenance are reviewed here. The information in this topic applies to patients who are ventilated through an endotracheal tube, tracheostomy tube, or noninvasive interface. Modes of mechanical ventilation are reviewed separately. (See "Modes of mechanical ventilation" and "Overview of initiating invasive mechanical ventilation in adults in the intensive care unit".)

COMPONENTS OF THE VENTILATOR CIRCUIT — The ventilator circuit refers to the tubing that connects the ventilator to a patient, as well as any device that is connected to the circuit tubing. The most common devices include heaters and humidifiers, filters, suction catheters, and therapeutic aerosol generators (nebulizers and inhalers) (figure 1 and figure 2).

Circuit tubing — The ventilator circuit tubing is generally corrugated plastic (22 mm inside diameter for adults), which has universal connectors (22 mm outside diameter, 15 mm inside diameter) that connect the ventilator to the endotracheal tube (ETT), tracheostomy tube, or noninvasive interface (figure 1). There are two types of circuits commonly used:

Single limb circuit – Two types of single limb circuits are available:

For bilevel ventilators, a single limb circuit is used with a leak port, which is a passive exhalation port for the patient. Rebreathing is possible if flow through the circuit is insufficient to allow flushing. Note that the leak port is essential for exhalation and should never be occluded.

For portable ventilators with an active exhalation valve, a single limb circuit is used with the exhalation valve near the patient. Because the expiratory valve is near the patient, rebreathing is minimized.

Dual limb circuit – Dual limb circuits are used for critical care ventilators; these have inspiratory and expiratory valves. The expiratory valve actively closes during the inspiratory phase and the inspiratory valve closes during the expiratory phase. There is separate tubing for the inspiratory gas and the expiratory gas. In this way rebreathing is minimized. With the dual circuit design, the inspiratory and expiratory valves are typically inside the ventilator.

Characteristics of the ventilator circuit — Dead space and compression volume are important characteristics of the ventilator circuit.

Dead space – Mechanical dead space is that part of the circuit through which the patient rebreathes, and is thus an extension of the anatomic dead space (figure 2). Mechanical dead space decreases alveolar ventilation and increases the partial arterial pressure of carbon dioxide (PaCO2). This is particularly an issue when low-tidal-volume lung-protective ventilation is used [1]. Dead space is increased by adding volume between the Y-piece and the patient interface. Examples of increased mechanical dead space include excessive connecting tubing and passive humidifiers.

Compression volume – When the circuit is pressurized, gas is compressed in the circuit and the volume of the circuit increased due to its compliance. As much as 3 to 5 mL/cm H2O can be compressed in the ventilator circuit. Thus, for a peak airway pressure of 30 cm H2O and positive end-expiratory pressure (PEEP) of 5 cm H2O, about 100 mL of the gas will be compressed in the ventilator circuit and not delivered to the patient during inspiration. For patients ventilated with a small tidal volume, compressible gas volume can greatly affect alveolar ventilation. Fortunately, most modern ventilators determine the compressible volume during the pre-use check and adjust for the effects of compressible volume. Thus, the volume selected and displayed on the ventilator represents the actual delivered tidal volume after correction for the effect of compressible volume. Many portable ventilators, however, do not compensate for compressible volume in the circuit. This might affect tidal volume delivery when patients are transported outside of the intensive care unit, such as for a diagnostic procedure.

Heaters and humidifiers — During mechanical ventilation, inspired gas is must be warmed and humidified [2]. This is necessary to prevent desiccated respiratory secretions, ETT (or tracheostomy tube) occlusion, lower respiratory tract airway occlusion, and consequently atelectasis, during mechanical ventilation (since the ETT or tracheostomy bypasses the normal warming and humidification functions of the upper airway). During invasive mechanical ventilation, the inspired gas should be conditioned to near body conditions. This is typically 100 percent relative humidity at body temperature (ie, 44 mg of water per liter of gas). Careful attention to adequate humidification of the inspired gas is particularly important in patients with coronavirus disease 2019 (COVID-19) [3,4]. To avoid over-humidification, the temperature of gas delivered should be set to the patient's core body temperature. Humidification is also necessary during noninvasive ventilation for patient comfort but is usually not set at body conditions, because the upper airway in intact.

Humidifiers – Active humidification occurs when a humidifier in the ventilator circuit warms and humidifies the inspired gas [2]. Passive humidification uses a device called an artificial nose (heat and moisture exchanger) to trap the heat and humidity in the exhaled gas and delivers that to the patient during the subsequent inspiration (figure 3). Some passive humidifiers also serve as filters, thus reducing circuit contamination from the patient, but the importance of this is unclear.

Choosing among passive or active options is typically based upon institution and clinician preference, since meta-analyses have shown that neither active nor passive strategies result in different rates of ventilator-associated pneumonia (VAP), mortality, or respiratory complications (eg, episodes of airway occlusion, frequency of atelectasis) [5]. Several important differences that should be considered when choosing a humidifier [2,5,6]:

The cost of passive humidification may be less than active humidification

Passive humidifiers are less effective than active humidifiers, which may lead to a greater risk of airway occlusion if not monitored closely

Passive humidifiers have higher resistance to flow, which may be problematic in spontaneous breathing modes (eg, pressure support)

Passive humidifiers increase dead space volume, which can be problematic during low tidal volume ventilation (see "Acute respiratory distress syndrome: Ventilator management strategies for adults", section on 'Low tidal volume ventilation: Initial settings')

Active humidifiers do not need to be changed at regular intervals since they are considered part of ventilator circuit and studies have shown that frequent changes are not beneficial. (see 'Frequency of ventilator circuit change' below).

Passive humidifiers can be safely used for at least 48 hours in most patients and even longer (eg, up to seven days) with better performing devices in patients without significant airway secretions [7]. Patients who potentially require more frequent changes include those with obstructive airways disease, copious airway secretions, or frequent obstruction with secretions. If frequent obstruction of the device is an issue, switching to an active humidifier is appropriate.

Heated circuit – In modern ventilator circuits, heated wires are placed in the inspiratory and expiratory tubing to reduce the formation of condensate when an active humidifier is used (figure 4). When the gas in the inspiratory circuit is warmed to a temperature greater than the humidifier, the absolute humidity remains the same but the decrease in relative humidity results in no condensate in the inspiratory limb. Similarly, when the gas in the expiratory circuit is warmed to a temperature greater than exhaled gas, the absolute humidity remains the same but the decrease in relative humidity results in no condensate in the expiratory limb. On some systems, the user can adjust the gradient between the humidifier and circuit temperatures, whereas other systems do this automatically. An adjustment in heater plate temperature may be necessary for adequate humidification [3]. It is important that the gas delivered to the patient has both an adequate absolute humidity (44 mg/L) and relative humidity (100 percent). A common clinical error is to reduce the water output of the humidifier, which results in a dry circuit, but reduces the absolute humidity delivery to the patient. The result may be drying of secretions and endotracheal tube occlusion [8,9]. To ensure adequate humidification of the inspired gas, the temperature of the gas entering the airway should be 37°C and the temperature differential in the circuit should be adjusted to the point at which condensation forms near the patient's airway. This indicates that gas is fully saturated at body temperature. If no condensate is visible, the clinician has no way of knowing what the relative humidity is, and it might be very low.

A heated circuit reduces the risk that a patient receives a bolus of contaminated circuit condensate into the lungs [10]. It also reduces the inconvenience of disposing of circuit condensate, a process that can contaminate the healthcare provider, which could lead to the provider transmitting infection to other patients. However, the type of humidifier (ie, active or passive) does not affect the rate of VAP [11,12].

Filters — Bacterial filters are sometimes placed at the inlet and the outlet of the ventilator circuit. The inspiratory filter is intended to minimize delivery of microorganisms from the gas supply to the patient, whereas the expiratory filter is intended to minimize contamination of the environment from the exhaled breath of the patient. This is especially important in the setting of highly infectious diseases, such as COVID-19. Filters may also be placed in the expiratory limb during aerosol delivery to minimize the risk of malfunction of expiratory flow sensors due to contamination with aerosol. It is important that expiratory filters are changed after each nebulizer treatment, or every two to four hours when continuous aerosols are administered, because the filter can increase the expiratory resistance for the patient.

Suction equipment — Tracheal secretions are removed during mechanical ventilation by passing a suction catheter through the endotracheal tube. Closed suction systems are used in many hospitals. With a closed system, the suction catheter is part of the ventilator circuit and, therefore, the patient can be suctioned without being disconnected from the ventilator (figure 5). With an open system, the patient is disconnected from the ventilator and then the suction catheter is passed through the ETT. Although neither system is superior to the other in preventing VAP [13], closed systems are generally used since they prevent exposure of staff to contaminated condensate and tracheal secretions [14]. Moreover, there is less respiratory and hemodynamic derangement during closed suctioning because the patient remains connected to the ventilator, and thus PEEP and fraction of inspired oxygen (FiO2) are better maintained.

Since closed suction catheters are considered part of the ventilator circuit, they are not changed routinely, a practice that is supported by numerous studies that have shown that daily changes of suction catheters does not reduce the frequency of VAP or other respiratory complications compared to less frequent changes [15,16]. (See 'Frequency of ventilator circuit change' below.)

Nebulizers and inhalers — Inhaled medications, typically bronchodilators, can be introduced into the ventilator circuit by a pressurized metered-dose inhalator, jet nebulizer, or mesh nebulizer (figure 2). The choice of delivery device is usually determined by institutional and clinician preference. Factors that should be taken into consideration include the following:

Observational evidence reports increased risk of VAP in association with jet nebulizer use [17,18]. This is likely because the jet nebulizer can become contaminated when solution is added to the cup. Also, the nebulizer surface can contaminate the ventilator circuit when it is added.

The risk of contamination might be lower with the pressurized metered-dose inhaler because it is not part of the ventilator circuit; only the stem of the inhaler is introduced into the actuator port. Use of metered-dose inhaler during mechanical ventilation has been reportedly associated with a low risk of VAP [19].

The cost of pressurized metered-dose inhalers has increased with the conversion to hydrofluoroalkane (HFA) propellant. This has resulted in the development of less expensive alternatives, such as mesh nebulizers. The mesh nebulizer is incorporated into the ventilator circuit and has been reportedly associated with a low risk of VAP [19].

Additional details regarding delivery of inhaled bronchodilators during mechanical and noninvasive ventilation are provided separately. (See "Delivery of inhaled medication in adults", section on 'Mechanically ventilated patients' and "Delivery of inhaled medication in adults", section on 'Patients receiving noninvasive ventilation'.)

Other inhaled gases — Gases other than air and oxygen can be delivered during mechanical ventilation. Most common are inhaled nitric oxide (INO) and heliox. Heliox is introduced to the gas inlet of the ventilator (ie, same place that oxygen or air is introduced); note that the ventilator must be compatible with heliox delivery. INO, however, is introduced directly into the ventilator circuit via a specific port/adaptor. (See "Physiology and clinical use of heliox" and "Inhaled nitric oxide in adults: Biology and indications for use", section on 'Administration'.)

Monitoring devices — Monitoring devices might be introduced into the circuit, typically between the Y-piece and endotracheal or tracheostomy tube. They include carbon dioxide monitors (ie, capnography), pressure monitors, and flow sensors (figure 2).

Bag-valve resuscitator — Bag-valve resuscitators are kept at the bedside of mechanically ventilated patients to allow emergency ventilation should there be a ventilator failure. Use of the bag-valve resuscitator should be minimized by using strategies such as closed suction and portable ventilators during patient transport. These devices are often contaminated [20,21]. Although strategies to reduce such contamination have not been studied, it is prudent to cap the patient connection of the bag-valve resuscitator when it is not in use. Similarly, it is prudent to cap the ventilator's Y-piece tubing when the ventilator is disconnected from the patient (eg, during prolonged spontaneous breathing trials). Either single patient use devices should be used, or the device should be cleaned and sterilized between patients.

TROUBLESHOOTING PROBLEMS WITH THE CIRCUIT — Maintenance of the ventilator circuit is crucial for adequate ventilation. The ventilator circuit should be inspected on a regular basis, such as a patient-ventilator system check by a respiratory therapist. Common problems include:

Detachment – The circuit can detach from the patient if not attached snugly, which results in an immediate disconnect alarm on the ventilator.

Gas leaks – Gas leaks can occur at any connection point in the circuit. A leak will result in the ventilator displaying an exhaled tidal volume lower than the volume delivered from the ventilator, prompting the clinician to correct the leak.

Excessive condensate – When an active heated humidifier is used, excessive condensate can accumulate in the circuit. This should be removed aseptically without interrupting ventilation, and water traps might be useful to accomplish this.

Obstructed passive humidifier – If an artificial nose (heat and moisture exchanger) is used for humidification, it can become obstructed with secretions, which may result in a high-pressure alarm on the ventilator.

FREQUENCY OF VENTILATOR CIRCUIT CHANGE — Evidence-based guidelines recommend that ventilator circuits are not changed on a routine basis but only if they malfunction or are visibly soiled, and between patients [7,22]. Ventilator circuits should not be changed routinely for infection control purposes [7,22]. These recommendations are based upon randomized trials and observational data that report no difference in the rate of ventilator-associated pneumonia (VAP) or mortality in association with frequent circuit changes compared with less frequent changes [23-30]. As examples:

One trial that randomly assigned 63 mechanically ventilated patients to have their ventilator circuit changed every 48 hours or not at all reported no difference in the rate of VAP (31 versus 29 percent), mortality, or duration of mechanical ventilation [23].

In another randomized trial of 300 patients, there was no difference in the rate of VAP (25 versus 29 percent) or mortality in patients receiving no circuit changes compared with those receiving changes every seven days [28].

One observational study of 637 mechanically ventilated patients reported an increased rate of VAP among those with frequent circuit changes (every two days) compared with those changed every 7 or every 30 days (12 versus 3 or 6 episodes per 1000 ventilator days).

The same lack of benefit from frequent circuit changes has also been found among patients who are mechanically ventilated in subacute centers [31] and in children [29].

This practice is further supported by evidence which suggests that the ventilator circuit plays a minimal role on the development of VAP:

Aspiration of contaminated pharyngeal secretions is the predominant cause of nosocomial pneumonia, not inhalation of aerosols containing bacteria. Thus, strategies to minimize pooling of secretions above the cuff of the artificial airway are recommended [22].

The microorganisms that colonize the ventilator circuit originate in the patient rather than the circuit [32]. This suggests that the patient contaminates the circuit, rather than the circuit contaminating the patient.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Hospital-acquired pneumonia and ventilator-associated pneumonia in adults".)

SUMMARY AND RECOMMENDATIONS

Definition – The ventilator circuit refers to the tubing that connects the ventilator to the patient (via endotracheal tube, tracheostomy tube, or noninvasive interface), as well as any devices that might be connected to the circuit. (See 'Introduction' above.)

Circuit tubing

Dual limb circuits are used for critical care ventilators and have separate inspiratory and expiratory valves that are typically inside the ventilator (figure 1). Portable and bilevel ventilators use single limb circuits and have an expiratory valve or leak port located near the patient, respectively. (See 'Circuit tubing' above.)

Mechanical dead space and compressed gas can be troublesome issues with the circuit tubing. Most ventilators adjust for the effects of compressible volume such that the volume selected and displayed on the ventilator represents the actual delivered tidal volume (figure 2).

Heaters and humidifiers – During mechanical ventilation, inspired gas is generally warmed and humidified using heated humidifiers or heat moisture exchangers (artificial nose) to prevent desiccation of respiratory secretions (figure 3 and figure 4). The choice of active or passive humidifiers is institution- and clinician-specific. (See 'Heaters and humidifiers' above.)

Other circuit components – Additional equipment that may be part of the ventilator circuit include filters (to capture microbes at the inspiratory or expiratory inlets), suction catheters (open or closed systems for secretion removal), nebulizer or metered-dose inhaler ports (for medication delivery), and monitoring devices (carbon dioxide, pressure, and flow sensors) (figure 2 and figure 5). Bag-valve resuscitators are kept at the bedside to allow emergency ventilation should there be a ventilator failure. (See 'Filters' above and 'Suction equipment' above and 'Nebulizers and inhalers' above and 'Monitoring devices' above and 'Bag-valve resuscitator' above.)

Trouble shooting problems – The ventilator circuit should be inspected on a regular basis and is typically done by a respiratory therapist during routine patient-ventilator system checks. Problems that may be encountered include detachment, air leaks, excessive condensate, and clogged humidifiers all of which can be easily remedied. (See 'Troubleshooting problems with the circuit' above.)

Circuit changes – Changing or replacing the ventilator circuit does not need to be performed on a routine basis but is indicated if it malfunctions or is visibly soiled, and between patients. Ventilator circuits should not be changed routinely for infection control purposes. This approach is based upon randomized trials and observational data that report no difference in the rate of ventilator-associated pneumonia (VAP) or mortality in association with frequent circuit changes compared with less frequent changes. (See 'Frequency of ventilator circuit change' above.)

  1. Hinkson CR, Benson MS, Stephens LM, Deem S. The effects of apparatus dead space on P(aCO2) in patients receiving lung-protective ventilation. Respir Care 2006; 51:1140.
  2. American Association for Respiratory Care, Restrepo RD, Walsh BK. Humidification during invasive and noninvasive mechanical ventilation: 2012. Respir Care 2012; 57:782.
  3. Lavoie-Bérard CA, Lefebvre JC, Bouchard PA, et al. Impact of Airway Humidification Strategy in Mechanically Ventilated COVID-19 Patients. Respir Care 2022; 67:157.
  4. Wiles S, Mireles-Cabodevila E, Neuhofs S, et al. Endotracheal Tube Obstruction Among Patients Mechanically Ventilated for ARDS Due to COVID-19: A Case Series. J Intensive Care Med 2021; 36:604.
  5. Pelosi P, Solca M, Ravagnan I, et al. Effects of heat and moisture exchangers on minute ventilation, ventilatory drive, and work of breathing during pressure-support ventilation in acute respiratory failure. Crit Care Med 1996; 24:1184.
  6. Prin S, Chergui K, Augarde R, et al. Ability and safety of a heated humidifier to control hypercapnic acidosis in severe ARDS. Intensive Care Med 2002; 28:1756.
  7. Hess DR, Kallstrom TJ, Mottram CD, et al. Care of the ventilator circuit and its relation to ventilator-associated pneumonia. Respir Care 2003; 48:869.
  8. Gilmour IJ, Boyle MJ, Rozenberg A, Palahniuk RJ. The effect of heated wire circuits on humidification of inspired gases. Anesth Analg 1994; 79:160.
  9. Miyao H, Hirokawa T, Miyasaka K, Kawazoe T. Relative humidity, not absolute humidity, is of great importance when using a humidifier with a heating wire. Crit Care Med 1992; 20:674.
  10. Craven DE, Goularte TA, Make BJ. Contaminated condensate in mechanical ventilator circuits. A risk factor for nosocomial pneumonia? Am Rev Respir Dis 1984; 129:625.
  11. Lacherade JC, Auburtin M, Cerf C, et al. Impact of humidification systems on ventilator-associated pneumonia: a randomized multicenter trial. Am J Respir Crit Care Med 2005; 172:1276.
  12. Lorente L, Lecuona M, Jiménez A, et al. Ventilator-associated pneumonia using a heated humidifier or a heat and moisture exchanger: a randomized controlled trial [ISRCTN88724583]. Crit Care 2006; 10:R116.
  13. Siempos II, Vardakas KZ, Falagas ME. Closed tracheal suction systems for prevention of ventilator-associated pneumonia. Br J Anaesth 2008; 100:299.
  14. Cobley M, Atkins M, Jones PL. Environmental contamination during tracheal suction. A comparison of disposable conventional catheters with a multiple-use closed system device. Anaesthesia 1991; 46:957.
  15. Stoller JK, Orens DK, Fatica C, et al. Weekly versus daily changes of in-line suction catheters: impact on rates of ventilator-associated pneumonia and associated costs. Respir Care 2003; 48:494.
  16. Kollef MH, Prentice D, Shapiro SD, et al. Mechanical ventilation with or without daily changes of in-line suction catheters. Am J Respir Crit Care Med 1997; 156:466.
  17. Craven DE, Lichtenberg DA, Goularte TA, et al. Contaminated medication nebulizers in mechanical ventilator circuits. Source of bacterial aerosols. Am J Med 1984; 77:834.
  18. Kollef MH, Von Harz B, Prentice D, et al. Patient transport from intensive care increases the risk of developing ventilator-associated pneumonia. Chest 1997; 112:765.
  19. Dubosky MN, Chen YF, Henriksen ME, Vines DL. Vibrating Mesh Nebulizer Compared With Metered-Dose Inhaler in Mechanically Ventilated Subjects. Respir Care 2017; 62:391.
  20. Thompson AC, Wilder BJ, Powner DJ. Bedside resuscitation bags: a source of bacterial contamination. Infect Control 1985; 6:231.
  21. Weber DJ, Wilson MB, Rutala WA, Thomann CA. Manual ventilation bags as a source for bacterial colonization of intubated patients. Am Rev Respir Dis 1990; 142:892.
  22. Klompas M, Branson R, Eichenwald EC, et al. Strategies to prevent ventilator-associated pneumonia in acute care hospitals: 2014 update. Infect Control Hosp Epidemiol 2014; 35:915.
  23. Dreyfuss D, Djedaini K, Weber P, et al. Prospective study of nosocomial pneumonia and of patient and circuit colonization during mechanical ventilation with circuit changes every 48 hours versus no change. Am Rev Respir Dis 1991; 143:738.
  24. Hess D, Burns E, Romagnoli D, Kacmarek RM. Weekly ventilator circuit changes. A strategy to reduce costs without affecting pneumonia rates. Anesthesiology 1995; 82:903.
  25. Kotilainen HR, Keroack MA. Cost analysis and clinical impact of weekly ventilator circuit changes in patients in intensive care unit. Am J Infect Control 1997; 25:117.
  26. Fink JB, Krause SA, Barrett L, et al. Extending ventilator circuit change interval beyond 2 days reduces the likelihood of ventilator-associated pneumonia. Chest 1998; 113:405.
  27. Han JN, Liu YP, Ma S, et al. Effects of decreasing the frequency of ventilator circuit changes to every 7 days on the rate of ventilator-associated pneumonia in a Beijing hospital. Respir Care 2001; 46:891.
  28. Kollef MH, Shapiro SD, Fraser VJ, et al. Mechanical ventilation with or without 7-day circuit changes. A randomized controlled trial. Ann Intern Med 1995; 123:168.
  29. Chu SM, Yang MC, Hsiao HF, et al. One-week versus 2-day ventilator circuit change in neonates with prolonged ventilation: cost-effectiveness and impact on ventilator-associated pneumonia. Infect Control Hosp Epidemiol 2015; 36:287.
  30. Thompson RE. Incidence of ventilator-associated pneumonia (VAP) with 14-day circuit change in a subacute environment. Respir Care 1996; 41:601.
  31. Thompson, RE. Incidence of ventilator-associated pneumonia (VAP) with 14-day circuit change in a subacute environment. Respir Care 1996; 41:601.
  32. Craven DE, Connolly MG Jr, Lichtenberg DA, et al. Contamination of mechanical ventilators with tubing changes every 24 or 48 hours. N Engl J Med 1982; 306:1505.
Topic 1656 Version 22.0

References

آیا می خواهید مدیلیب را به صفحه اصلی خود اضافه کنید؟