Part one we will be dealing with various part of Non Invasive ventilation setup. It may also be helpful while making a purchase of NIV masks, their circuits and various other accessories, in PICU
TYPES OF INTERFACES
- Face mask/ Oro- nasal mask
- Nasal mask / Nasal pillow / Nasal prongs
- Total face mask: It covers the entire face, however I have never used one.
| NASAL MASK | FACE MASK |
| Less dead space Less claustrophobia Allow for expectoration, vomiting, oral intake vocalize | Fever air leaks More stable Mean Airway pressure |
THERE IS NO PERFECT MASK
Changing interfaces to find out which is suitable, is in fact a good strategy to improve patient tolerance.
VENTED Vs NON-VENTED MASK
Vented mask
Vented masks have small perforation or leak port to allow the CO2 clearance when the patient exhales.Vents are generally present on the body of mask and sometimes the adapter or elbow is fitted as such to create a gap at joint from where the expired gases can escape. Such adapters can be changed and non- vented mask can be converted into vented mask. (eg.Hamilton )
Vented mask are connected with single limb circuit and used for conventional NIV machine, BiPAP machines. Generally they are not used with mechanical ventilators, as the large leak will result in ineffective mean airway pressures, beside the exhalation vents are not needed due to availability of dedicated expiration limb in double limb circuit (See types of circuit below)
The mask elbow is usually white / transparent for identification of vented mask in most of the brands
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| Vented mask - Note the exhalation vents on body of mask |
Non-vented mask
No provision of ventilation or leak ports, the Co2 clearance depends either on leak valve (see below) in single limb circuit, or exhalation limb in double limb circuit. These mask are usually used with double limb circuit and mechanical ventilators which has dedicated expiration valve.The mask elbow is usually colored blue to “warn” the user that it does not have an anti-asphyxia valve or exhalation ports.
ADAPTERS / ELBOW
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| Standard elbow: is a simple adapter which connects mask to double limb circuit, universally they are blue in color to warn that there is no exhalation mechanism in the mask or elbow. |
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| Vented adapter creates leak when it connects between mask and circuit making it vented just like having exhalation ports in the mask. They are generally white in color and have inbuilt anti-asphyxia valve. |
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| The third type of adapters have an integrated expiration system, which include an anti-asphyxia valve (read below) besides it also has a leak for allowing patient to exhale. If used with non-vented mask it will have half blue and half white/ transparent color. |
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| Some companies like Hamilton, F and P provide Elbows or adapters which can be exchanged with same mask, making the mask vented and non –vented with a option of +/- anti asphyxia valve. |
ANTI-ASPHYXIATION VALVE
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The anti-asphyxia valve also called as non-rebreather valve allows the patient to access room air in case of machine fails to generate sufficient flow for patient.
It is not synonymous with exhalation vent / leak ports and has no role in Co2 washout. When the airflow enters the elbow it pushes a small diaphragm up and the valve closes, thus there is no air leak during inspiration. when there is no flow (Machine failure) the diaphragm is not pushed up and the valve opens to room air preventing asphyxia.
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Vented mask with anti asphyxia valve: Completely transparent adapter.
Non vented mask with anti-asphyxia valve : Half blue half transparent.
Non vented mask without anti-asphyxia valve : Blue
ACTIVE VS PASSIVE CIRCUIT
NIV circuit is active if there is a expiration valve in the circuit which allows the patient to exhale through a separate tubing avoiding the mixing of gases. It utilities a diaphragm which opens up during exhalation and divert the expired gas though separate tubing. Valve is called as true expiratory valve
If the single limb circuit is active and NIV/BIPAP machine has a built-in safety valve for ventilator failure, like in Dragger Carina NIV machine, or respironics trilogy a non-vented mask is used.
If there is no active safety valve in the ventilator or circuit, it is passive circuit, such circuit will be used with vented mask or non vented only if exhalation port (a small inline adapter with hole ) is attached (1)
Inline exhalation valve is a small hole in an adapter which can be attached in the single limb circuit. (Respironics Triology)
SINGLE LIMB CIRCUIT
No separate inspiration and expiration limb, Used in conventional NIV, BIPAP machinesFor CO2 clearance a single limb circuit either will depend on a exhalation valve (True or a leakage port, read above) in circuit or exhalation ports in mask itself (Vented mask)
Thus these circuits are used with vented masks, or if used with non-vented mask, a separate inline leak port or active / true exhalation valve must be present.
DOUBLE LIMB CIRCUIT
These circuits are used with Non-Vented masks and mechanical ventilator having NIV mode.Separate inspiration and expiration limb along with the exhalation valve is present inside the ventilator and therefore exhalation mechanism is not needed in circuit or mask, neither the anti-asphyxia valve is required
SUMMARY
| System | Circuit Type | Mask type | Mask Interface | Co2 removal by | Provision for room air if ventilator fails |
| Active | 2 Limb | Non-vented | Face mask Nasal mask | Separate limb for expiration | Ventilator exhalation valve |
| 1 Limb (Active/True Expiration valve ) | Non-vented | Face mask Nasal mask | True Expiration valve | True Expiration Valve | |
| Passive | 1 Limb with leakage valve | Non-vented | Nasal mask | Leakage Valve in circuit | Mouth |
| 1 Limb with leakage valve | Non-vented | Face mask with anti-asphyxiation valve | Leakage Valve in circuit | Anti-asphyxiation valve | |
| 1 Limb simple | Vented | Face mask with anti-asphyxiation valve | Exhalation vents on mask | Anti-asphyxiation valve | |
| 1 Limb Simple | Vented | Nasal mask | Exhalation vents on mask | Mouth Vents on mask |
References:
- Interfaces and humidification for noninvasive mechanical ventilation. Nava S1, Navalesi P, Gregoretti C. Respir Care. 2009 Jan;54(1):71-84.
- Trilogy accessory guide
- Ventilator modes and settings during non-invasive ventilation: effects on respiratory events and implications for their identification. Thorax. 2011 Feb;66(2):170-8. doi: 10.1136/thx.2010.142661. Epub 2010 Oct 14.
- Non-invasive Ventilation – A century of experience, Frank van Rooyen, Krisztina Soltész
