Humidifiers in anaesthesia and critical care

Humidification of inhaled gases has been standard of care during mechanical ventilation in anaesthesia and intensive care. Active & Passive humidification devices have rapidly evolved. basic knowledge of the mechanisms of action of each of these devices, as well as their advantages and disadvantages, becomes a necessity for anaesthesiologists and intensivists.

1. Humidifiers in AnaEsthesia & intensive care Moderator : DR. MAMTA KHANDELWAL Associate Professor Presenter : DR. TUHIN MISTRY 2 nd year Resident

2. INTRODUCTION Humidification of inhaled gases has been standard of care during mechanical ventilation in anaesthesia and intensive care. Water is intentionally removed from medical gases so that gases delivered from anaesthesia machine are dry to prevent clogging of regulators and other valves.

3. INTRODUCTION( contd …) Tracheal intubation & high fresh gas flows bypass normal humidification by upper airways and expose lower airways to dry(< 10 mg H2O/L), room temperature gases. External humidifiers compensate for the lack of natural humidification mechanisms when the upper airway is bypassed .

4. INTRODUCTION( contd …) Active & Passive humidification devices have rapidly evolved. sophisticated systems composed of reservoirs, wires, heating devices & other elements have become part of our usual armamentarium in anaesthesia & intensive care unit. Therefore, basic knowledge of the mechanisms of action of each of these devices, as well as their advantages and disadvantages, becomes a necessity for anaesthesiologists and intensivists.

5. physics Humidity is the amount of water in vaporous state contained in a gas . Absolute humidity (AH) is the mass of water vapour present in a given volume of gas and it is usually expressed in mg/L . Relative humidity (RH) is the amount of water vapour at a particular temperature expressed as a percentage of the amount that would be held if the gas were saturated.

6. PHYSICS( contd …) Whenever the amount of gas contained in a sample is equal to its water vapour capacity, the RH is 100% and the gas is completely saturated . So,

7. PHYSICS( contd …) Water vapour capacity of a sample will increase exponentially to the temperature. If AH remains constant, RH will decrease whenever the temperature increases (because the denominator increases), and RH will increase when the temperature decreases (because the capacity to hold water vapour decreases).

8. physiology Heat and moisture exchange is one of the most important functions of the respiratory system . C onnective tissue of the nose is: rich vascular system of numerous and thin walled veins . responsible for warming the inspired air to increase its humidity carrying capacity.

9. Anatomic site Nose or mouth Hypopharynx Midtrachea Humidity requirements 50% RH with AH of 10 mg/L at 22°C 95% RH with AH of 28 to 34 mg/L at 29 to 32°C 100% RH with AH of 36 to 40 mg/L at 31 to 35°C Humidity requirements for gas delivery at different anatomic sites in the airway: * * J . M. Cairo, Mosby's Respiratory Care Equipment, Mosby, Elsevier, St. Louis, Mo, USA, 9th edition, 2013.

10. PHYSIOLOGY( contd …) As the inspired air goes down the respiratory tract, it reaches a point at which its temperature is 37°C and its relative humidity is 100%. This point is known as the isothermic saturation boundary (ISB ) * , and it is usually located 5 cm below the carina The respiratory mucosa is lined by pseudostratified columnar ciliated epithelium and with numerous goblet cells . * S . T. Ballard and S. K. Inglis , “Liquid secretion properties of airway submucosal glands,” Journal of Physiology, vol. 556, no. 1, pp. 1–10, 2004.

11. PHYSIOLOGY( contd …) Goblet cells + submucosal glands-  maintain the mucous layer(serves as a trap for pathogens and as an interface for humidity exchange). At the level of the terminal bronchioles, the epithelium turns into a simple cuboidal type with minimal goblet cells and scarce submucosal glands . Hence , the capacity of these airways to carry on the same level of humidification maintained by upper airways is limited

12. PHYSIOLOGY( contd …) After endotracheal intubation upper airway loses its capacity to provide heat and moisture to inhaled gas ISB is shifted down the respiratory tract burden on the lower respiratory tract (not well prepared for the humidification process) delivery of partially cold & dry medical gases potential damage to the respiratory epithelium increased work of breathing, atelectasis, thick & dehydrated secretions, cough and/or bronchospasm

13. PHYSIOLOGY( contd …) During the exhalation process, the expired gas transfers heat back to the upper airway mucosa. As the airway temperature decreases, the capacity to hold water also decreases. Therefore, condensed water is reabsorbed by the mucosa, recovering its hydration. In periods of cold weather, the amount of water condensation may exceed the mucosal capacity to accept water. Therefore, the remaining water accumulates in the upper airway with consequent rhinorrhea .

14. Advantages: Prevention of cilial damage and reduced drying of secretions Microbial filtration Artificial Airway Humidification

15. Disadvantages: Disconnection Overheating Overhydration Circuit resistance, deadspace and circuit compliance changes Infection Interference with other devices Inadequate humidification

16. SOURCES OF HUMIDITY CO 2 absorbent – Absorbent granules contain water Exothermic reaction of CO 2 with absorbent releases water & produce heat. Exhaled gases – Rebreathing in tracheal tube, supraglottic airway device & connections to breathing system -almost half of humidity in expired gases is preserved in this manner

17. SOURCES OF HUMIDITY( contd …) Moistening (Rinsing) breathing tubes and reservoir bag - Rinsing the inside of the breathing tubes and reservoir bag with water before use increases the inspired humidity Low fresh gas flows – conserve moisture with a circle breathing system

18. SOURCES OF HUMIDITY( contd …) Coaxial breathing circuits – When combined with low flows increase humidity more quickly than a system with 2 separate limbs. Not very efficient in terms of heat or humidity improvement. Bain system(coaxial version of Mapleson D) does not meet optimal humidification requirements because of high fresh gas flow required

19. Humidifiers Humidifiers are devices that add molecules of water to gas . C lassified as :- Active humidifiers (presence of external sources of heat and water) Passive humidifiers ( utilization of patient’s own temperature and hydration to achieve humidification in successive breaths )

20. Active humidifiers Act by allowing air passage inside a heated water reservoir . A dd water to gas by passing the gas over a water chamber , through a saturated wick, bubbling it through water, or mixing it with vaporized water . Unlike passive humidifiers, they do not filter respiratory gases Two types – H eated Unheated

21. Heated humidifier

22. HEATED HUMIDIFIERS Incorporate a device to warm water in the humidifier, some also heat inspiratory tube. Humidification chamber – contains liquid water, disposable/ reusable, clear (easy to check water level)

23. HEATED HUMIDIFIERS (contd.) Heat source/ Heating systems – Hot plate element - sits at the bottom of the humidifier Wraparound element - surrounds the humidifier Collar element - sits between the reservoir and outlet Immersion heater - inside water reservoir Heated wire - placed in the inspiratory limb

24. HEATED HUMIDIFIERS (contd.) Figure 1: Heated humidifier and condensation * * R . M. Kacmarek , J. K. Stoller , and A. H. Heuer , Egan's Fundamentals of Rrespiratory Care, Mosby-Elsevier, St.Louis , Miss, USA, 10th edition, 2012.

25. HEATED HUMIDIFIERS (contd.) Temperature monitor – to measure gas temperature at patient end of breathing system Thermostat – Servo-controlled units – automatically regulates power to heating element in response to temperature sensed by a probe near patient connection/ humidifier outlet Nonservo -controlled units – provides power to heating element according to setting of a control, irrespective of delivered temperature

26. HEATED HUMIDIFIERS ( contd …) Controls – most allow temperature selection at end of delivery tube or at humidification chamber outlet Alarms – to indicate temperature deviation by a fixed amount displacement of temperature probe disconnection of heater wire low water level in humidification chamber faulty airway temperature probe lack of gas flow in the circuit Standard requirements - An international and a U.S. standard on humidifiers have been published

27. HEATED HUMIDIFIERS ( contd …) Placed in the inspiratory limb of the ventilator circuit, proximal to the ventilator . After the gas is loaded with water vapour in the reservoir, it travels along the inspiratory limb to the patient’s airway . As condensation of water vapour may accumulate as the surrounding temperature of the inspiratory limb decreases, these systems are used with the addition of water traps, which require frequent evacuation to avoid risk of contamination of the circuit .

28. Figure 2: Humidifier with heated wire in the inspiratory limb* * R . M. Kacmarek , J. K. Stoller , and A. H. Heuer , Egan's Fundamentals of Rrespiratory Care, Mosby-Elsevier, St.Louis , Miss, USA, 10th edition, 2012.

29. HEATED HUMIDIFIERS ( contd …) In circle system, heated humidifier is placed in the inspiratory limb downstream of unidirectional valve by using an accessory breathing tube Must not be placed in the expiratory limb Filter, if used, must be placed upstream of humidifier to prevent it from becoming clogged In Mapleson systems, humidifier is usually placed in fresh gas supply tube

30. HEATED HUMIDIFIERS ( contd …) Humidifier must be lower than patient to avoid risk of water running down the tubing into the patient Condensate must be drained periodically or a water trap inserted in the most dependent part of the tubing to prevent blockage or aspiration Heater wire in delivery tube should not be bunched, but strung evenly along length of tube Delivery tube should not rest on other surfaces or be covered with sheets, blankets, or other materials; a boom arm or tube tree may be used for support

31. HEATED HUMIDIFIERS ( contd …) Advantages – Capable of delivering saturated gas at body temperature or above, even with high flow rates More effective humidification than an HME

32. HEATED HUMIDIFIERS (contd.) Disadvantages – Bulky and somewhat complex Involve high maintenance costs, electrical hazards, and increased work (temperature control, refilling the reservoir, draining condensate, cleaning, and sterilization) Offers relatively little protection against heat loss during anesthesia as compared to circulating water and forced-air warming

33. HEATED HUMIDIFIERS (contd.) Different designs and different techniques for humidification . Classified as :- Bubble Passover Counter-flow Inline vaporizer.

34. HEATED HUMIDIFIERS( contd …) BUBBLE G as is forced down a tube into the bottom of a water container. The gas escapes from the distal end of the tube under water surface forming bubbles, which gain humidity as they rise to the water surface . The smaller the bubbles, the larger the gas-water interface allowing for higher water vapor content.

35. HEATED HUMIDIFIERS( contd …) PASSOVER Gas passes over a heated water reservoir carrying water vapour to the patient . WICK :- Gas enters a reservoir and passes over a wick that acts as a sponge that has its distal end immersed in water . MEMBRANE :- hydrophobic only allows passage of water vapour , precluding liquid water to travel through it.

36. HEATED HUMIDIFIERS( contd …) COUNTER FLOW water is heated outside the vaporizer . water is pumped to the top of the humidifier. It enters inside the humidifier through small diameter pores, and then runs down a large surface area . Gas flows in counter direction. During its passage through the chamber of the humidifier, the gas is moisturized and warmed to body temperature.

37. HEATED HUMIDIFIERS( contd …) INLINE VAPORIZER uses a small plastic capsule where water vapour is injected into the gas in the inspiratory limb of the ventilator circuit immediately proximal to the Y- connecter. Gas heating is supplemented by a small disk heater in capsule . Water is delivered to the capsule by a peristaltic pump housed in a controller. Amount of water is set by the clinician based on minute volume through the circuit. Both temperature and humidity are adjustable and displayed constantly. The proximity to the Y - connection obviates the requirement for heated wires and external temperature probes.

38. UNHEATED HUMIDIFIERS Disposable, bubble-through devices used to increase humidity in oxygen supplied to patients via facemask or nasal cannula. Simple containers containing distilled water through which oxygen is passed and it gets humidified. Maximum humidity that can be achieved is 9 mg H 2 O/L.

39. PASSIVE HUMIDIFIERS Simplest designs are Heat & Moisture Exchangers (HMEs ) A rtificial noses - mimic the action of nasal cavity in gas humidification . Also called as condenser humidifier, Swedish nose, nose humidifier, regenerative humidifier, vapor condenser

40. HME

41. PASSIVE HUMIDIFIERS( contd …) Exchanging medium enclosed in plastic housing Vary in size, shape, dead space – pediatric and neonatal HMEs with low dead space available May have a port to attach gas sampling line for respiratory gas monitor Placed between ET tube and breathing circuit

42. PASSIVE HUMIDIFIERS ( contd …) Figure 4: HME position in ventilator circuit.

43. PASSIVE HUMIDIFIERS( contd …) Figure 5: HME position in ventilator circuit.

44. PASSIVE HUMIDIFIERS( contd …) May increase resistance to airflow not only during inspiration, but also during the expiratory phase . In situations in which administration of aerosolized medications is needed, HMEs need to be removed from the circuit to avoid aerosol deposition in HME filters.

45. PASSIVE HUMIDIFIERS (contd.) May be used for tracheostomised patients May be combined with another source like unheated humidifier, but should not be used with heated humidifier Nebuliser or metered dose inhaler if used, should be inserted between HME and patient, or HME removed from circuit during aerosol treatment Should be replaced if contaminated with secretions

46. PASSIVE HUMIDIFIERS ( contd …) Should be of appropriate size for patient’s tidal volume Connecting more than one in series will improve performance but care should be taken that increase in dead space is not excessive for particular patient (especially small patient) Should be visible and accessible at all times in order to detect contamination or disconnection

47. PASSIVE HUMIDIFIERS (contd.) Indications – To increase inspired heat and humidity during both short and long term ventilation Especially useful when transporting intubated patients - transport ventilators frequently have no means for humidifying inspired gases To supply supplemental oxygen to intubated patient/ patient with a supraglottic airway - by connecting oxygen tubing to gas sampling port

48. PASSIVE HUMIDIFIERS (contd.) Advantages – Inexpensive Easy to use Small, lightweight, simple in design Silent in operation Do not require water/ external energy source/ temperature monitor/ alarms No danger of overhydration / hyperthermia/ burns/ electrical shock

49. PASSIVE HUMIDIFIERS (contd.) Disadvantages – Can deliver only limited humidity Insignificant contribution to temperature preservation Less effective than active humidifiers, specially after intubation lasting for several days Increased dead space  may necessitate increase in tidal volume  increased work of breathing

50. PASSIVE HUMIDIFIERS( contd …) Types : Initial designs/simple HMEs : C ondensers made of metallic elements that had high thermal conductivity . R ecapture only 50% of the patient’s exhaled moisture, provided humidification of 10–14 mg H2O/L, at tidal volumes (VT) ranging between 500 mL and 1000   ml. N ot disposable and created a significant resistance during mechanical ventilation

51. PASSIVE HUMIDIFIERS( contd …) b. Newer designs : Hydrophobic : Condenser is made of a water repelling element ( Hydrophobic membrane) with low thermal conductivity that maintains higher temperature gradients than in the case of simple HMEs . Allow passage of water vapor but not liquid water at usual ventilatory pressures.

52. PASSIVE HUMIDIFIERS( contd …) 2) Combined hydrophobic hygroscopic : A hygroscopic salt (calcium or lithium chloride) is added inside the hydrophobic HME. These salts have a chemical affinity to attract water particles and thus increase the humidification capacity of the HME.

53. PASSIVE HUMIDIFIERS( contd …) 3) Pure hygroscopic HMEs : Wool , foam or paper like material coated with moisture-retaining chemicals ( hygroscopic compartment). During exhalation, vapour condenses in the element as well as in the hygroscopic salts. During inspiration, water vapour is obtained from the salts, obtaining an absolute humidity ranging between 22 and 34 mg H2O/L.

54. PASSIVE HUMIDIFIERS (contd.) Type Hygroscopic Hydrophobic Heat and moisture exchanging efficiency Excellent Good Effect of increased tidal volume on heat and moisture exchange Slight decrease Significant decrease Filtration efficiency when dry Good Excellent Filtration efficiency when wet Poor Excellent Resistance when dry Low Low Resistance when wet Significantly increased Slightly increased Effect of nebulised medications Greatly increased resistance Little effect

55. PASSIVE HUMIDIFIERS( contd …) * American Association for Respiratory Care, R. D. Restrepo , and B. K. Walsh, “Humidification during invasive and noninvasive mechanical ventilation: 2012,” Respiratory Care, vol. 57, no. 5, pp. 782–788, 2012.

56. Neonatal Issues • Increased concern regarding HME dead space • Relatively high HME dead space volume compared to tidal volume could result in hypoventilation • Uncuffed endotracheal tubes – Leak could decrease HME efficiency • Incubator may affect performance of the HME • Heated environment decreases inhaled/exhaled gas temperature gradient

57. heat and moisture exchanging filter (HMEF) Operate based on electrostatic or mechanical filtration . classified into : Pleated Electrostatic filters

58. HMEF( contd …) PLEATED: More dense fibers & less electrostatic charges. Pleated nature of the membrane causes a turbulent air flow, which increases the pathogen’s deposition inside the filter . Function better as barriers to bacterial and viral pathogens than electrostatic filters . However, they confer higher airflow resistance

59. HMEF( contd …) ELECTROSTATIC FILTERS M ore electrostatic charges & less dense fibers. Subjected to an electric field. B acteria & viruses carry electric charges, they get trapped within the electric field of these filters . have larger pores than the pleated membranes

60. Active hme In certain devices, an active heated water source can be added to HMEs converting them from passive to active, increasing their humidification capacity. If the external source of water runs out, these devices will still work as passive HMEs.

61. Active HME( contd …) Several models exist: Booster : heating unit is incorporated between the HME & patient . It is thought that the utilization of this device may increase AH by 2-3 mg/L of H2O more than passive HMEs

62. Active HME( contd …) HME Booster

63. Active HME( contd …) 2. Performer: A metal plate in the middle of the HME, in between two hydrophobic and hygroscopic membranes. H eated by an external source(three sets of temperature to deliver 40°C, 50°C, and 60°C). A water source provides it to one end of the humidifier. The performer is able to deliver AH of 31.9 to 34.3 under normothermic conditions.

64. Active HME( contd …) HME-Performer.

65. Active HME( contd …) 3. Humid Heat: Hygroscopic HME with an external heating source with the water being added at the patient side. It was found to provide an absolute humidity of 34.5 mg  H2O/L. Humid Heat has preset values for temperature and humidity. The only parameter that needs to be set is the value of minute volume of the ventilator, making its use very simple.

66. Active HME( contd …) 4. Hygrovent Gold: Active hydrophobic HME that has an adapter to which a heating element can be inserted and a water line to supply water inside the HME. There is a thermal sensor to avoid over humidification. P rovide an AH of 36.3 mg H2O/L.

67. Active HME( contd …) Disadvantages : Placing a heat source near the patient Higher dead space than passive HMEs

68. AARC Clinical Practice Guidelines The following recommendations are made following the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) scoring system : Humidification is recommended for every patient receiving invasive mechanical ventilation. Active humidification is suggested for noninvasive mechanical ventilation, as it may improve adherence and comfort.

69. AARC Guidelines( contd …) 3. When providing active humidification to patients who are invasively ventilated , it is suggested that the device provide a humidity level between 33 mg H2O/L and 44 mg H2O/L and gas temperature between 34°C and 41°C at the circuit Y-piece, with a relative humidity of 100%. 4 . When providing passive humidification to patients undergoing invasive mechanical ventilation , it is suggested that the HME provide a minimum of 30 mg H2O/L .

70. AARC Guidelines( contd …) 5. Passive humidification is not recommended for non-invasive mechanical ventilation. 6 . When providing humidification to patients with low tidal volumes, such as when lung-protective ventilation strategies are used, HMEs are not recommended because they contribute additional dead space, which can increase the ventilation requirement and PaCO 2 . 7 . It is suggested that HMEs are not used as a prevention strategy for ventilator-associated pneumonia.

71. Monitoring of Humidification Systems The most reliable mean to measure humidity is by using a hygrometer-thermometer system. However, these devices are not always available at the bedside for every patient. Consequently, different surrogate markers have been suggested to monitor humidification levels.

72. Monitoring( contd …) The most popular surrogates are secretion characteristics, visual observation of condensate in tubing system, and requirement for saline instillation . In general, the volume of secretions is directly proportional to degree of humidification. Excessive humidification will increase secretion volume, and suboptimal humidification will lead to crusting, inspissation of secretions, and a decrease in their volume

73. Monitoring( contd …) secretion volume may be altered by administered aerosolized medications, frequency of suctioning, and saline instillation there is still no clear consensus about a universal way to assess for humidity adequacy at the bedside.

74. Humidification Performance HMEs design and performance standards are defined by the International Organization for Standardization (ISO). According to AARC guidelines, HHs should provide an absolute humidity level between 33 and 44 mg H2O/L, whereas HMEs should provide a minimum of 30 mg  H2O/L

75. Humidification Performance( contd …) During anaesthesia, time duration of the bypass of the upper airway is much shorter. Kleeman proposed a minimum level of 20 mg H2O/L sufficient to prevent damage to trachea-bronchial epithilia during 10 hours of mechanical ventilation.

76. Performance( contd …) Combined hydrophobic hygroscopic HMEs should be the first choice if passive humidification is selected, as they have better humidification capacity than the hydrophobic ones Most manufacturers recommend exchanging HMEs every 24 hours in case of long term mechanical ventilation. HMEs are passive devices that require retention of heat to provide effective function, they are deemed contraindicated for hypothermic patients with temperatures lower than 32°C

77. Effect on Ventilatory Mechanics Increase the dead space, which in turn decreases alveolar ventilation and leads to increase in arterial carbon dioxide tension . Hence, in order to keep the same level of alveolar ventilation, tidal volume has to be increased exposing patients to volume-induced lung injury .

78. Effect on Ventilatory Mechanics cont.. In spontaneously breathing patients, the addition of dead space associated with HMEs may increase work of breathing precluding liberation from mechanical ventilation. Prat and colleagues demonstrated a mean of 17 mm Hg decrease in PaCO2 levels in ARDS patients, when heated humidifiers were used instead of HME.

79. Le Bourdellès et al. suggested that although dead space added by HMEs may be trivial, it may adversely affect weaning process in patients with limited respiratory reserve . In addition to the dead space effect, HMEs increased inspiratory and expiratory resistance, which contributed to the development of intrinsic PEEP

80. Take home message Airway humidification represents a key intervention in mechanically ventilated patients . Inappropriate humidifier settings or selection of devices may negatively impact clinical outcomes by damaging airway mucosa, prolonging mechanical ventilation, or increasing work of breathing.

81. Humidifier devices may function passively or actively, depending on the source of heat and humidity . Depending on the clinical scenario, humidifier selection may change over time. Therefore, knowledge of the advantages and disadvantages of each of these devices is essential.

82. REFERENCES Understanding Anaesthesia Equipment (5 th Edition) - Dorsch and Dorsch Clinical Anaesthesiology (5 th Edition) – Morgan Short Textbook Of Anaesthesia (5 th Edition) - Ajay Yadav The ICU Book (4 rd Edition) – Paul Marino

83. THANK YOU